Survey methodology for offshore wind farms and

integration of a DTS-based cable monitoring system

Academiejaar 2020-2021

Lucas VANMAELE

FACULTEIT INDUSTRIËLE INGENIEURSWETENSCHAPPEN

CAMPUS BRUGGE

Promotor: prof. dr. ir. J. Peuteman

Co-promotoren: ir. B. De Baere ing. B. Reynaert

Masterproef ingediend tot het behalen van de

graad van master of Science in de industriële

wetenschappen: Energie,

afstudeerrichting Elektrotechniek

met optie Hernieuwbare energie

© Copyright KU Leuven

Zonder voorafgaande schriftelijke toestemming van zowel de promotor(en) als de auteur(s) is overnemen, kopiëren,

gebruiken of realiseren van deze uitgave of gedeelten ervan verboden. Voor aanvragen i.v.m. het overnemen en/of

gebruik en/of realisatie van gedeelten uit deze publicatie, kan u zich richten tot KU Leuven Campus Brugge,

Spoorwegstraat 12, B-8200 Brugge, +32 50 66 48 00 of via e-mail iiw.brugge@kuleuven.be.

Voorafgaande schriftelijke toestemming van de promotor(en) is eveneens vereist voor het aanwenden van de in

deze masterproef beschreven (originele) methoden, producten, schakelingen en programma’s voor industrieel of

commercieel nut en voor de inzending van deze publicatie ter deelname aan wetenschappelijke prijzen of

wedstrijden.

i

Acknowledgements

This work has been written as a master thesis at KU Leuven, Campus Bruges in the area of

industrial engineering: energy. This master thesis was made possible by an internship at

Norther NV, the operator of an offshore wind farm in the North Sea. Here I would like to thank

all the people who assisted me during the internship period and during the editing of the master

thesis.

J. Peuteman (Promotor, KU Leuven), your dedication and work ethic is something few can

match. It was an honour to have you as my promotor throughout this master thesis. I could

always ask questions which were always answered promptly. I would like to thank you for your

fantastic follow-up of the master thesis. Even before a contract was signed, you helped in every

way you could. Your constructive feedback on the draft versions was very much appreciated!

B. De Baere (Co-promotor, Norther), I would like to thank you from the bottom of my heart for

the support you have given me throughout the academic year. Without your help and input,

this master thesis would not have been the same. By seeing how you deal with certain

technical business matters, I have learned a lot about how these business matters can be

addressed in a proper way. Throughout the internship, I have grown as a person and engineer

and I owe this mainly to you!

B. Reynaert (Co-promotor, Norther), I would like to thank you for the opportunity you gave me

to do an internship at Norther! The BoP package proved to be a very interesting and versatile

package within the operational team. Working on a larger project with colleagues and an

external party was a very enriching experience. During the actual internship period, I could

learn more about the other branches within the operational team. Throughout the academic

year, I have learned a lot about the fascinating sector of offshore wind energy and there is still

a lot more to discover.

T. Aelens (Norther), although we did not have to work together on the project specifically, the

insights and knowledge you shared were fascinating! My view of how the world operates has

also changed because of your anecdotes.

Furthermore, I would like to thank the other employees of Norther for the help they gave me,

the chats and the interesting stories. I felt welcome within the team and there always was a

welcoming atmosphere. To the entire Norther team, it was an honour to do my master thesis

within this incredible team!

A word of appreciation must go to the team of Marlinks and especially Roel. Thank you for the

time you took to explain certain processes and principles as well as the smooth cooperation.

To the surveyor, Jeroen, thank you for the time you took to explain how you conduct surveys

and the very relevant side information. Your explanations gave me a clear view on the systems

that could be used to determine the cable depth of burial and recent developments in the world

of surveys.

Last but not least, I would like to thank my family and friends for the moral support throughout

the entire master year and the preceding academic years. A special thank you goes to my

parents and brother for their extensive support throughout my academic path.

ii

Samenvatting

Deze masterproef werd als project uitgevoerd bij Norther, de uitbater van een offshore

windturbinepark op de Noordzee. Het park bestaat uit 44 Vestas V164 8.4 MW windturbines

en een transformatorstation die allemaal op een monopile fundering staan. Het maximaal

toegelaten injectievermogen van het park op het hoogspanningsnet bedraagt 350 MW. Het

hoofddoel van het project is de integratie van een kabelcontrolesysteem op basis van het DTS-

principe. Het kabelcontrolesysteem moet in staat zijn om de temperatuur, alsook de

begraafdiepte van de kabels, te bepalen.

Om de berekening van de begraafdiepte te kunnen controleren werd eerst onderzoek gedaan

naar de klassieke methoden om deze begraafdiepte te bepalen. Deze klassieke methoden zijn

op basis van surveys, waarbij een schip het traject van de kabels afvaart en een scan van de

zeebodem doet. Het onderzoek moest uitwijzen of de gebruikte methode voldoende accurate

resultaten kon opleveren en als er alternatieven mogelijk zijn. Hierbij kon worden uitgewezen

dat de MBES (Multi Beam Echo Sounder) methode voldoende performant is, maar ook dat er

nog alternatieven ontwikkeld worden die zonder externe referentiepunten de begraafdiepte

van de kabel kunnen bepalen.

DTS staat voor Distributed Temperature Sensing, de bepaling van de temperatuur over een

lange afstand. Dit wordt binnen het project verwezenlijkt door het opmeten van de temperatuur

in de glasvezelkabels. De glasvezelkabels lopen langsheen de 3 fasegeleiders van de

hoogspanningskabels, waardoor de temperatuur binnenin de hoogspanningskabels kan

worden afgeleid. Het werkingsprincipe is gebaseerd op lichtpulsen die in de glasvezelkabels

worden gestuurd. Door microscopische imperfecties in deze kabels zal licht reflecteren naar

de bron. Het gereflecteerde licht wordt met een optische ontvanger gecollecteerd. Vervolgens

kan dit weerkaatste licht geanalyseerd worden. Door de invloed van temperatuur zullen

bepaalde componenten binnen het spectrum van het gereflecteerde licht verschuiven in het

frequentiedomein of veranderen in intensiteit. Door deze eigenschappen correct te vertalen

kan men de temperatuur van de glasvezelkabels meten.

Daarnaast wordt met informatie over de samenstelling en opbouw van de zeekabels voor elk

type zeekabel een kabelmodel opgemaakt. Deze modellen bevatten alle parameters om

effecten op de kabels dynamisch te kunnen modelleren. Via deze modellen wordt de

temperatuur van de glasvezelkabel omgezet naar een kerntemperatuur van de geleiders. Met

deze kerntemperatuur van de geleiders en informatie van de stroom die door de kabel vloeit

kan een verdere analyse gedaan worden van hoe zwaar de kabel belast wordt. Door

omgevingsfactoren ook in rekening te brengen kan men tot een RTTR (Real Time Thermal

Rating) komen. Hiermee kan worden bepaald in welke mate de kabel meer of juist minder mag

belast worden met een te transporteren vermogen. Daarbij zijn ook voorspellingen voor

toekomstig toegelaten stromen of voorspelde temperaturen voor de komende uren mogelijk.

De laatste uitbreiding hierop laat toe om een berekening van de begraafdiepte van de kabels

uit te voeren. Voor deze uitbreiding worden nog meer factoren in rekening gebracht en wordt

de begraafdiepte bepaald door een iteratief rekenproces.

De milieuvergunning van Norther vereist dat er jaarlijks een rapport wordt opgemaakt die de

begraafdiepte van de exportkabel beschrijft. Deze kabel transporteert het geproduceerde

vermogen van het windturbinepark naar land. Het doel van het project is om de klassieke

survey methode te vervangen door de berekende begraafdiepte van de kabel. Daarvoor moet

iii

de methode voor de bepaling van de begraafdiepte worden voorgelegd aan de

verantwoordelijke overheidsinstanties. Tot op heden is dit proces nog niet afgerond aangezien

de eerste resultaten van de nieuwe methode voor de bepaling van de begraafdiepte tegen het

einde van de zomer van 2021 verwacht worden.

Door onderzoek te doen op ruwe survey data van een MBES werd meer inzicht verworven in

scans van de bodem. Hieruit is een zijproject gevormd waarbij een tool werd ontwikkeld die de

integriteit van de scour protection (specifiek ontworpen en geplaatste lagen met rotsblokken)

rondom de monopile fundering nagaat. Door stormen kunnen deze beschermende lagen

aangetast worden waardoor ze niet meer voldoen aan de minimumeisen. De tool kan helpen

bij het bepalen of er al dan niet reparatiewerken aan deze beschermende lagen moeten

worden uitgevoerd.

iv

Abstract

This master thesis was carried out as a project at Norther, the operator of an offshore wind

farm in the North Sea. The wind farm consists of 44 Vestas V164 8.4 MW wind turbines and a

transformer station, all of which are built on a monopile foundation. The maximum allowed

power injection of the wind farm into the high-voltage grid is 350 MW. The main objective of

the project is the integration of a cable monitoring system based on the DTS principle. The

cable monitoring system must be able to determine the cable temperature as well as the cable

depth of burial along the entire length of the cables.

In order to be able to verify the calculation of the burial depth using the DTS system, research

of the classic methods for depth of burial determination was conducted. These classical

methods are based on surveys. For these surveys, a vessel travels along the cable route and

scans the seabed topography. The research had to show whether the currently used method

could provide sufficiently accurate cable depth of burial results and if alternatives were

possible. The conclusion on the surveys is that the MBES (Multi Beam Echo Sounder) survey

method is sufficiently accurate for regular check-up surveys when reference data is available.

Besides the MBES method, an alternative method which can determine the cable depth of

burial, without external reference points, is still being developed.

DTS is the abbreviation of Distributed Temperature Sensing, the determination of temperature

over a long distance. Within the project, this is achieved by measuring the temperature in the

fibre optic cables. The fibre optic cables run along the three-phase conductors of the high

voltage cables, which allows to measure the temperature within the high voltage cables. The

operating principle is based on light pulses that are sent into the fibre optic cables. Due to

microscopic imperfections in these fibre optic cables, light will reflect back to its source. The

reflected light is captured by an optical receiver. Subsequently, this reflected light can be

analysed. Due to the influence of temperature, certain components within the reflected light

spectrum will change in the frequency domain or change in intensity. By translating these

changes correctly, the temperature of fibre optic cables can be measured.

With information about the composition and structure of the submarine cables, a cable model

is built for each type of cable. These models contain all the parameters to dynamically model

effects on the cables. Using these models, the temperature of the fibre optic cable can be

converted into the core temperature of the conductors. With this core temperature of the

conductors and information of the current flowing through the cable, a further analysis can be

done on the cable load. By taking environmental factors into account, the RTTR (Real Time

Thermal Rating) of a cable can be determined. This can be used to determine whether the

cable can transport more or less power. Predictions of the allowed currents or temperatures

for the next few hours are also possible. The last extension of the DTS system allows to

perform a calculation of the depth of burial of the cables. For this extension, even more factors

are taken into account and the burial depth is determined by an iterative calculation process.

Norther's environmental permit requires an annual report describing the depth of burial of the

export cable. This cable transports the produced power from the wind farm to shore. The aim

of the project is to replace the classical survey method with the calculated depth of burial of

the cable. To request this change, the method for determining the cable depth of burial and

the results of the determination must be submitted to the responsible governmental authorities.

v

To date, this process has not yet been completed as the first results of the new method for

determining the cable depth of burial are expected by the end of summer 2021.

By conducting research on raw survey data from a MBES, more insight was gained into the

topographic scans. From this, a side project evolved, where a tool was developed to check the

integrity of the scour protection layers around the monopile foundation. Storms can damage

these protective rock layers. As a result, these rock layers may no longer meet their minimum

design requirements. The tool can help determine whether repair work needs to be carried out

on these protective rock layers after a storm event.

Keywords: Offshore surveys, Distributed Temperature Sensing (DTS), Real Time Thermal

Rating (RTTR), cable Depth of Burial (DoB), Scour protection.

vi

TABLE OF CONTENTS

Acknowledgements ............................................................................................ i

Samenvatting ..................................................................................................... ii

Abstract ............................................................................................................ iv

List of abbreviations ........................................................................................ ix

List of figures .................................................................................................... x

1 Introduction .............................................................................................. 1

1.1 Norther .............................................................................................. 1

1.2 Project ............................................................................................... 1

1.3 Scope ................................................................................................ 3

2 Research ................................................................................................... 5

2.1 Surveys ............................................................................................. 5

2.1.1 History of maritime surveys ...................................................... 5

2.1.2 Survey methods in the offshore wind industry .......................... 8

2.1.3 Survey methods for determining the cable depth of burial ........ 9

2.1.4 Cable depth of burial determination using survey technologies21

2.1.5 Cable depth of burial survey with ROV ................................... 24

2.1.6 Cable depth of burial determination in the Norther OWF ........ 26

2.1.7 General conclusion of traditional DoB determination .............. 27

2.2 Distributed Temperature Sensing .................................................... 27

2.2.1 Rayleigh scattering................................................................. 28

2.2.2 Raman scattering ................................................................... 28

2.2.3 Brillouin scattering .................................................................. 30

2.3 Real Time Thermal Rating .............................................................. 31

2.4 Optical Time-Domain Reflectometer ................................................ 34

3 Cables ..................................................................................................... 37

3.1 Cable information ............................................................................ 37

3.1.1 Export cable ........................................................................... 38

3.1.2 Infield cables .......................................................................... 41

3.2 Heat losses in cables ...................................................................... 42

3.3 Fibre optic cables ............................................................................ 47

3.3.1 Fibre optic operation .............................................................. 47

vii

3.3.2 Fibre optic connectors ............................................................ 48

3.3.3 Fibre optic splices .................................................................. 49

3.4 Cable entry and CPS....................................................................... 50

4 Preliminary works .................................................................................. 55

4.1 Design and engineering .................................................................. 55

4.1.1 RC thermal cable model ......................................................... 55

4.1.2 Transport of electrical power .................................................. 57

4.2 Installation and testing ..................................................................... 59

4.2.1 Factory Acceptance Test........................................................ 59

4.2.2 Offshore works ....................................................................... 61

4.2.3 Problems along the way ......................................................... 65

5 DTS system calibration and data processing ...................................... 66

5.1 DTS temperature calibration............................................................ 66

5.2 Real Time Thermal Rating .............................................................. 67

5.3 Depth of Burial ................................................................................ 67

5.4 Site Acceptance Test ...................................................................... 71

6 Environmental permit and DTS ............................................................. 72

6.1 Setup of the document .................................................................... 72

6.2 Case studies ................................................................................... 73

6.2.1 Soil conditions ........................................................................ 73

6.2.2 Cable burial ............................................................................ 74

6.2.3 Cable crossings ..................................................................... 74

6.2.4 Cable paths ............................................................................ 78

6.2.5 DoB result verification method................................................ 79

6.3 Decision tree ................................................................................... 82

6.4 Outcome of the document ............................................................... 83

7 Scour protection integrity analysis ....................................................... 84

7.1 General info .................................................................................... 84

7.2 Scour protection integrity ................................................................. 86

7.3 Determining storm scenarios ........................................................... 87

7.4 Analysis tool .................................................................................... 89

8 Conclusion.............................................................................................. 94

References ....................................................................................................... 96

Attachments .................................................................................................. 102

viii

Attachment A Interview with a surveyor ...................................................... 1

Attachment B Cable irrelevant length determination .................................. 6

Attachment C Fibre optic splice plan ........................................................... 7

Attachment D Python scripts for DoB verification ...................................... 8

Attachment E Storm scenario determination ............................................... 9

Attachment F Scour Protection Analysis Tool .......................................... 11

ix

List of abbreviations

AC Alternating Current

APC Angled Physical Contact

BMM Beheerseenheid van het Mathematisch Model van de Noordzee

BoP Balance of Plant

CPS Cable Protection System

DC Direct Current

DoB Depth of Burial

DTS Distributed Temperature Sensing

FAT Factory Acceptance Test

GNSS Global Navigation Satellite System

HVAC High Voltage Alternating Current

HVDC High Voltage Direct Current

ICCP Impressed Current Cathodic Protection

LAT Lowest Astronomical Tide

MBES Multi Beam Echo Sounder

MP Monopile

MSBL Mean Seabed Level

OHVS Offshore High Voltage Station

OTDR Optical Time-Domain Reflectometer

OWF Offshore Wind Farm

PES Parametric Echo Sounder

ROV Remotely Operated Vehicle

RSBL Reference Seabed Level

RTK Real Time Kinematic

RTTR Real Time Thermal Rating

SBP Sub Bottom Profiler

TP Transition Piece

UPC Ultra Physical Contact

UPS Uninterruptible Power Supply

UXO Unexploded Ordnance

WTG Wind Turbine Generator

XLPE Cross-Linked Polyethylene

x

List of figures

Figure 2-1: Historical survey methods [3] .............................................................................. 6

Figure 2-2: Wire drag survey method [4] ............................................................................... 6

Figure 2-3: Single beam echosounding ................................................................................. 7

Figure 2-4: Multibeam echosounding ..................................................................................... 8

Figure 2-5: Comparison between different survey technologies [6] .......................................10

Figure 2-6: Movement of a vessel on water [9] .....................................................................11

Figure 2-7: Temperature, salinity and pressure influence on sound velocity [10] ..................13

Figure 2-8: Multibeam echosounder survey method [11] ......................................................14

Figure 2-9: Multibeam echosounder survey result [13] .........................................................15

Figure 2-10: Multibeam survey raw data visualisation ..........................................................15

Figure 2-11: Multibeam survey interpolated data visualisation ..............................................16

Figure 2-12: Multibeam survey topographical view ...............................................................16

Figure 2-13: Multibeam survey topographical view including raw data ..................................17

Figure 2-14: Sub bottom profiler geological profile and horizons [14] ....................................17

Figure 2-15: Sub bottom profiling survey methods [16] .........................................................18

Figure 2-16: Sub bottom profiler echo print example [17] .....................................................19

Figure 2-17: Sub bottom profiler exposed and buried pipe example [18] ..............................19

Figure 2-18: Sub bottom profiler survey path ........................................................................20

Figure 2-19: Magnetometry survey method [20] ...................................................................21

Figure 2-20: Sub bottom imager measurement result [28] ....................................................25

Figure 2-21: 217 mm x 136 mm HVDC cable [27] ................................................................26

Figure 2-22: Wavelength spectrum of backscattered light [30]..............................................28

Figure 2-23: Temperature measurement using DTS [34] ......................................................29

Figure 2-24: Thermal step response of the Norther export cable at nominal load [37] ..........32

Figure 2-25: Cable installation vessel ...................................................................................33

Figure 2-26: OTDR measuring principle of operation [38] .....................................................35

Figure 2-27: OTDR block diagram [38] .................................................................................35

Figure 2-28: OTDR trace with different kinds of connections [38] .........................................36

Figure 3-1: Overview of infield cables in the Norther wind farm ............................................37

Figure 3-2: Export cable armour types over cable length (based on [39]) .............................38

Figure 3-3: Export cable installation overview [40] ................................................................38

Figure 3-4: Submarine export cable cross section [41] .........................................................39

xi

Figure 3-5: Cross section of the Norther 1600 mm² export cable ..........................................41

Figure 3-6: Cross section of the Norther 800 mm² infield cable ............................................42

Figure 3-7: Heat losses in power cables [42] ........................................................................43

Figure 3-8: Heat distribution in the export cable (figure courtesy of Marlinks) .......................46

Figure 3-9: Fibre optic cable composition [46] ......................................................................47

Figure 3-10: Multimode and single mode fibre optic principle [47] ........................................47

Figure 3-11: Backscatter in fibre optic cable [48] ..................................................................48

Figure 3-12: Difference between UPC and APC connectors [50] ..........................................49

Figure 3-13: From cable hang-off to cable burial (figure not to scale) ...................................50

Figure 3-14: Cable hang-off on the cable termination platform of a WTG .............................52

Figure 3-15: Norther cable protection system spare parts ....................................................53

Figure 3-16: Cable starting point determination ....................................................................54

Figure 4-1: Cable model - export cable drawing [37] ............................................................55

Figure 4-2: Marlinks thermal RC cable model [37] ................................................................56

Figure 4-3: Comparison between FEM model and RC-ladder model thermal response [54] .57

Figure 4-4: WTG configuration and power measurement .....................................................58

Figure 4-5: Current and voltage distribution along a string of infield cables ..........................59

Figure 4-6: FAT setup [55] ....................................................................................................60

Figure 4-7: FAT temperature repeatability test at 50°C [55] ..................................................60

Figure 4-8: DTS cabinet - electrical panel .............................................................................61

Figure 4-9: Fibre optic string and loop ..................................................................................62

Figure 4-10: OTDR trace with different kinds of connections [38] .........................................63

Figure 4-11: String A OTDR test result .................................................................................63

Figure 4-12: Front of the Omnisens DTS interrogator and optical switch ..............................64

Figure 4-13: Back of the Omnisens DTS interrogator and optical switch ..............................64

Figure 4-14: Cracked fibre optic connector ...........................................................................65

Figure 5-1: Calculation of the temperature coefficient of the fibre optic cables [56] ...............67

Figure 5-2: Subsea power cable example [58] ......................................................................68

Figure 5-3: Marlinks DoB determination algorithm [59] .........................................................68

Figure 5-4: Marlinks least square error method [61] .............................................................69

Figure 5-5: Example of k-value and DoB determination [61] .................................................70

Figure 5-6: Comparison between measured and calculated temperature [54] ......................71

Figure 6-1: Thermal resistivity of the soil in the Norther OWF [62] ........................................73

Figure 6-2: Infield cable I1 regular burial section and cable entry - side view [63] .................74

xii

Figure 6-3: Third party assets within the Norther OWF .........................................................75

Figure 6-4: Infield cable I1 crossing with pipeline – map view ...............................................75

Figure 6-5: Infield cable I1 crossing with pipeline – cross-sectional view [63] .......................76

Figure 6-6: Infield cable I1 crossing with pipeline - topographical view [63] ..........................76

Figure 6-7: Temperature profile along infield cable I1 ...........................................................77

Figure 6-8: Temperature profile along the Norther export cable ............................................78

Figure 6-9: Infield cables rock dumps [64] ............................................................................78

Figure 6-10: Comparison between two depth of burial datasets ...........................................80

Figure 6-11: Detailed comparison between two depth of burial datasets - part 1 ..................81

Figure 6-12: Detailed comparison between two depth of burial datasets - part 2 ..................81

Figure 6-13: Provisional decision tree ...................................................................................82

Figure 7-1: flows leading to scour process [66] .....................................................................84

Figure 7-2: Scour hole formation [68] ...................................................................................85

Figure 7-3: Scour protection design [69] ...............................................................................86

Figure 7-4: Falling apron design [70] ....................................................................................86

Figure 7-5: Falling apron principle for monopile foundations [71] ..........................................87

Figure 7-6: Analysis tool - start screen .................................................................................89

Figure 7-7: Analysis tool - input dialog ..................................................................................90

Figure 7-8: Analysis tool - visual result newer VS older survey data .....................................91

Figure 7-9: Analysis tool - visual result newer survey data VS rock design ...........................92

Figure 7-10: Analysis tool - visual result older survey data VS rock design...........................92

Figure 7-11: Analysis tool - numerical results .......................................................................93

1

1 INTRODUCTION

This master thesis is a continuation of a master thesis [1] that has been realised by another

KU Leuven student in academic year 2019-2020. The wind farm operator Norther wants to

install a DTS system (Distributed Temperature Sensing) in their wind farm. Last year, a tender

was set up allowing potential contractors to apply and submit their services. During the contract

negotiations, some preparatory work has been done.

During academic year 2020-2021 the project was continued. By the end of the summer of 2020

a contract with an installer of DTS systems was signed, the cooperation between Norther and

the contractor could start. This is also the beginning of this master thesis.

1.1 Norther

Norther is a cooperation between stakeholders i.e. Norther is owned by Elicio (50%), Eneco

(25%) and Diamond Generating European Limited (Mitsubishi Corporation) (25%). In March

2020, Eneco was acquired by a consortium including Mitsubishi corporation. Thus, there are

now two shareholders with each 50% of the shares. The Norther OWF (Offshore Wind Farm)

is one of the largest commissioned and operational wind farms of the Belgian North Sea. The

park consists of 44 Vestas V164 8.4 MW wind turbines and an offshore high voltage station

(OHVS) where the voltage is transformed to 220 kV AC. The wind turbines are divided into 11

strings of 4 turbines each. The 11 strings are all connected to the OHVS. The OHVS is

connected to the 220 kV export cable that connects the wind farm to the Belgian high voltage

grid and transports the generated power to shore. All wind turbines and the OHVS use a

monopile foundation. The park has a total power capacity of 370 MW and is allowed to inject

350 MW into the high voltage grid.

Most Belgian wind farms, including Norther, export their power to the Stevin high voltage

substation. The high voltage cables arrive at the beach of Zeebrugge where they are buried a

few metres below the soil and connected in the “beach pit”. Cables are connected from the

beach pit to Elia’s “Stevin high voltage substation” where the power is injected into the Belgian

high voltage grid. Elia is the high voltage grid operator in Belgium.

1.2 Project

The temperature of the cables in the Norther offshore wind farm was not monitored before the

DTS project. The current flowing through the cable was the only parameter Norther could

monitor to estimate the thermal load of the cable.

Based on the current in the cable and information of the cable manufacturer, Norther has an

estimate of the allowed load of the cables. As wind speed is not constant over time, the energy

production of wind turbines will change accordingly. The cables in the wind farm were designed

for these dynamic loads, implying they cannot transport the full 350 MW continuously. The

cable insulation may not exceed the temperature of 90°C. Above that threshold, the projected

lifespan of the insulation would shorten significantly. Before the realisation of the DTS project,

the current, combined with alarms for high current, was the only parameter available to

2

estimate the electrical and thermal load of the cables. This estimate is rather inaccurate,

implying a better monitoring system is desirable.

Another aspect of the cable that must be monitored is the depth of burial (DoB). It is important

that the cables are buried sufficiently deep in order to protect them from external factors. It

would be catastrophic for an offshore wind farm operator if the cable was dragged along by an

anchor or a fishnet because the cable wasn’t buried deep enough. Not only human interference

can cause problems with too shallow burial depths. Due to the sea currents and tides, the

cable could move, exposing it to mechanical loads and stresses. Under any circumstance, the

cable must remain at least 1 metre below the sea floor. For the ‘Scheur’ shipping lane crossing,

a DoB of 3 metres is required. This DoB has to be verified at least once a year and this is

currently done by surveys that scan the seafloor. A sufficiently large burial depth is a

requirement of the environmental permit issued by the government and must be met in order

to keep the permit to operate the wind farm. When the cable appears to be buried less than 1

metre below the surface or even exposed, a 3-month time window is given to resolve this

problem. This is solved by dumping rocks on the cable.

Traditionally, the cable DoB (Depth of Burial) is measured with a survey vessel equipped with

a multibeam echo sounder (MBES). Using this approach, it may take weeks to get a depth of

burial measurement of all cables, depending on weather conditions. It is important to mention

that these surveys are expensive to conduct. More information on offshore surveys can be

found in section 2.1.

It would be a great improvement to monitor the temperature along the cables and the DoB of

the cables in a cost-efficient way. A DTS system can provide a solution for monitoring both

values.

A number of surveys could be performed for the same price as the cost of the implementation

of a DTS system (including the tool to determine the depth of burial). This means the amount

of required surveys could be reduced when the DoB monitoring using the DTS system is

accepted by the governing authorities. The aim is to reduce the required amount of surveys by

80% to 90% after two years of successful DoB monitoring using the DTS.

DTS is the abbreviation of Distributed Temperature Sensing, meaning a temperature is

measured over a certain distance. The DTS system will use laser pulses in the fibre optic

cables to determine the temperature along the fibre optic cable every two metres. Due to

imperfections in the fibre structure, some parts of the pulsed light will be reflected. The DTS

system measures both the frequency spectrum and time of the reflected (backscattered) light.

The frequency spectrum of the reflected light can be translated to a certain temperature value.

The time lag gives an indication of the location where the light is reflected (time lag multiplied

by the speed of light provides the distance this light has travelled). Thus, the temperature at

known locations along the cable can be determined. A DTS system measures the temperature

over a distributed path. A more elaborate explanation on the DTS working principle is given in

section 2.2. Using this temperature, combined with information derived from a cable model, a

conversion from fibre temperature to temperature of the cable conductor cores can be done.

This is done via an RC cable model (with thermal resistors and capacitors).

RTTR (Real Time Thermal Rating) is also included in the package. With RTTR, the ampacity

rating of the cable will be dynamic rather than static. This implies the allowed current in the

conductor can vary depending on the thermal load of the cable. More information on RTTR

3

can be found in section 2.3. From this temperature data and information of the heat distribution

of the soil, the data can be converted to the depth of burial data along the length of the cable.

A DTS system monitors the temperature inside the cable almost in real time (updates every 2

to 3 minutes) and provides an update of the depth of burial every two weeks. This means that

when the cable’s DoB is too shallow, Norther will have at least 5 DoB measurements from the

DTS system in the time frame they would have to fix the shallow burial depth. Since the seabed

is quite dynamic, sand dunes may cover a more exposed area in a few weeks, so the problem

could be solved in a natural way and no rocks have to be dumped on that part of the seafloor.

This once again results in a cost saving.

The benefit of using a DTS system is that it provides more information about the cable more

frequently. The DTS system could have a critical role in the decision whether to produce more

power or let the cable cool down at any given time. This approach will result in a longer lifespan

of the cable and possibly avoiding additional investments on a long-term basis.

When a correct depth of burial is measured using the DTS method, the required amount of

classical surveys could be reduced significantly. After the installation of the DTS system and

DoB monitoring using the DTS system, the multibeam surveys will be done for two more years.

The results of that reliable survey can be compared to the results of the DTS system and if

required, adjustments to the DoB parameters of the DTS system can be made.

During academic year 2019-2020 another KU Leuven student [1] set up a tender for such a

DTS system with the help of Norther. Based on this tender, different parties could submit their

services. With strict selection criteria and deadlines, three potential contractors remained after

a few selection rounds. Out of these three, one was awarded the contract which has been

signed in the summer of 2020. Since then, the contractor (Marlinks) and Norther have been

working together to prepare the required documents and share relevant information implying

Marlinks could start the engineering of this system (e.g. working on the required models).

This project will provide two important advantages for Norther. The first is a more elaborate

monitoring of the temperature along the cables and the second is a more frequent and cost-

effective way of measuring the depth of burial of the cables in the wind farm. When a problem

can be detected in an early stage, additional preventive maintenance can be done in order to

remain operational and the system will pay itself back.

1.3 Scope

The scope of this project is the installation and the calibration of the DTS system implying it

can be used as the primary method for DoB determination. Therefore, a calibration report has

to be delivered to Norther which proves that the system works as intended. With information

of the calibration of the system, a formal document can be sent to the BMM (Beheerseenheid

van het Mathematisch Model van de Noordzee). Via the document, the BMM can evaluate the

use of the DTS system in the Norther OWF as the primary method of DoB determination. The

use of the system has to be evaluated and compared to the survey data and should at least

deliver comparable results. If the system proves to measure accurately enough for the BMM,

they can decide to approve the use of the system and the scope of the project is fulfilled.

Once the project is completed, Norther will have a real time temperature measurement along

the cable, including RTTR, with multiple updates every hour. New information on the depth of

4

burial of the cable will be given in a timeframe of every two weeks. With the traditional check-

up surveys, it is virtually impossible to get the DoB information in this timeframe.

During the first semester, research was done to check if the traditional survey methods that

are used to determine the depth of burial were good. Since the new system has to be

calibrated, the reference depth of burial measurement has to be done accurately. During the

research it was important to look for improvements.

Besides researching what survey methods are used and if any improvements could be made,

some support for the Norther team was given. This includes reading reports and checking if

the information it contains is correct. Comments and/or questions were sent to the person that

made the report or document. Furthermore, attending meetings with contractors and writing

meeting minutes; checking the electrical drawings that were made for the DTS cabinet and

sending comments to the responsible person; sending information and updates to the

contractor when required; helping the contractor get the right information of the wind farm; ….

During the second semester, the task of providing the contractor with information continued as

well as supervising the work of the contractor. Research was done to calculate the length of

certain cable sections from as-built data (discussed in section 3.4). Additionally, a method to

verify the correlation of two DoB datasets was created (discussed in chapter 6). Besides the

DTS project, a side project evolved from the need to gain more insight in survey data and

analysing raw survey data. This project is the development of a tool to analyse the integrity of

the scour protections (discussed in chapter 7).

5

2 RESEARCH

As mentioned before, the DTS system has to be calibrated. First, the temperature has to be

calibrated. In the next step, the DTS-based DoB determination has to be calibrated. In order

to check and adjust the DTS system DoB determination, reference data is required. This

reference data is the traditional DoB determination method by using surveys. Since the data

has to be as accurate as possible, different survey methods were investigated and can be

found in the first part of this chapter.

In the second part of this chapter, the working principle of DTS systems will be explained. The

third part of this chapter will discuss the functionality of an RTTR dynamic ampacity rating. To

end, the general principle of an OTDR test will be explained.

2.1 Surveys

In this section, the traditional method of checking the depth of burial of the cables will be

discussed. Surveys can be done for multiple purposes. Before laying the cable, one could do

a magnetometry area scan to check whether there are shipwrecks in the area. During the cable

burial procedure, some survey methods can be combined to check the depth of burial of the

cable and check the seabed depth. After everything is installed, a check-up survey of the

seabed can be conducted to check if everything is still in place. After a short general

introduction on surveys, the discussed survey methods will be focused on determining the

depth of burial (DoB) of cables.

2.1.1 History of maritime surveys

The first type of surveys [2] were conducted from the time of the ancient Egyptians until about

the 1900’s. The survey method was the so called “lead line method”.

The lead line method used a rope with depth marks along the length and at the end of the rope

a lead weight was added. The lead line was lowered into the water and the end sank to the

bottom. The depth of the water could be read from the depth markers that were above water.

This resulted in a single point depth measurement that was quite accurate. However,

determining the position was done with a sextant and that approach was less accurate.

A similar type of survey used a so-called sounding pole. The basic principle was that a stick

was lowered from a boat into the water. On the stick, depth markings were made and by

reading where the water level is, the depth could be determined. The use of a lead line and

sounding pole are illustrated in Figure 2-1.

6

Figure 2-1: Historical survey methods [3]

Another method that was frequently used in the past was a so-called wire drag survey (Figure

2-2). With this type of survey, objects in a wide corridor could be detected down to a certain

depth. A wire with weights and buoys was hung between two boats. The two boats sailed

parallel to each other. When an object stuck out higher than the line, the buoy would be lifted

upwards and the crew could note the potential hazard and its approximate location. This

method was useful for detecting objects a few metres deep. No information regarding the

sediment or type of object detected was recorded with these methods.

Figure 2-2: Wire drag survey method [4]

It is clear that these methods were very time consuming, required hard work and in the end

they were not very accurate. These methods are deemed unusable in today’s standards.

Through the years, a lot of technological evolutions have shaped a different method of

surveying. In the 1930’s echosounders gained popularity. With echo sounding technology,

transducers emit a sound signal towards the seabed and the reflected signal gets measured.

Using echosounders, the depth of the seafloor with respect to the water surface could be

measured. This method is based on the time of flight of the sound signal propagating trough

7

the water. To get correct measurements, the speed of sound through the medium must be

known. This is elaborated in section 2.1.3.

With these echosounders, fewer crew members could work more rapidly. A disadvantage of

these simple echosounders is that only the part of the seabed where the vessel passed over

is measured. The rest of the surface is unknown. In Figure 2-3 a simple representation of such

a survey is given. A vessel passed in an area and sailed in parallel lines. Along these lines the

depth is measured and known. The area in-between the green lines can be guessed but will

always remain unknown.

Figure 2-3: Single beam echosounding

When using a single beam echosounder, the shallowest object or seabed surface will reflect

the soundwave first. This first reflection is the one measured by the sensor. This means that if

a rock is on the bottom of the seabed, this rock will reflect the sound first and the depth of the

seafloor will be the distance between the sea level and the rock. [2]

By using more modern equipment like a multibeam echosounder (MBES), also the area in

between paths of the vessel can be mapped. A multibeam echosounder sends a wide array of

beams in a swath of 90° to 170° towards the seafloor (wider than the vessel itself) and

measures the reflected signals. This will result in a wider scanned area. Figure 2-4 illustrates

this wider swath from the MBES compared to the single beam echosounder. Thanks to the

wide scanned area, some structures can be identified. Shipwrecks for example get

surveyed/examined using multibeam echosounders.

8

Figure 2-4: Multibeam echosounding

The swath of a multibeam echosounder typically is about 2.5 to 3 times the water depth. This

may vary depending on the selected swath angle. Modern scans attach GNSS coordinates to

the bathymetric data implying an exact position is determined for the scanned point. External

positioning factors like pitch, heave and yaw are compensated using sensors.

After a survey, data gets processed and cleaned up. Different representation modes are

possible. If the purpose of the scan was the creation of a nautical chart, the representation

mode would be different compared to scanning the seafloor when mapping a shipwreck for

archaeological purposes. A 3D representation or a section of the seabed compared to a

reference level may be required.

Surveys used to be done primarily for dredging applications. A plan can be made before the

actual works start i.e. an optimal schedule can be made to provide the most cost-effective

solution for the client and the contractor. Surveys are frequently used in offshore industries

such as gas and oil platforms to do some maintenance check-ups and to verify if the pipes are

laid correctly in the first place.

2.1.2 Survey methods in the offshore wind industry

In the offshore wind industry, surveys are also conducted to determine a good position for a

wind farm. When considering the situation in the North Sea, sand banks are a popular area to

install wind turbines. Before building the actual farm, multiple surveys are done to ensure it is

possible to build a farm at the desired location. Furthermore, if there are any hazards in the

park, they must be dealt with in an adequate manner. When designing the positions of the

turbines and the trajectories for the cables, some changes may be necessary to deal with these

hazards. During the installation of submarine cables, surveys are done to have a good

reference of the positions of the cables. After installation of the OWF, surveys are carried out

to check how the equipment is faring in the sea and if the foundations are sufficiently stable.

The surveys conducted in wind farms today are more than monitoring the depth of burial of the

cable. Multiple surveys are done to ensure all necessary objects are detected. Furthermore,

9

visual inspection surveys are done using an ROV (Remotely Operated Vehicle). These surveys

are done to ensure proper health of structures in the sea. These structures are monopiles

(MP’s) and transition pieces (TP’s) that are completely or partially submerged in the seawater.

Offshore structures are quite prone to corrosion since they are in a corrosive environment.

TP’s are painted in yellow and the paint forms a protective layer between the corrosive

seawater and the steel. When problems are detected, they can be fixed before they escalate

into bigger problems. In order to protect the submerged structures against corrosion, an

Impressed Current Cathodic Protection (ICCP) system is installed.

"Corrosion is a natural process of materials, usually metals, moving towards their lowest

possible energy state, resulting in a spontaneous reaction between the material and its

environment which results in the degradation of that material. The objective with cathodic

protection is to suppress the electrochemical reaction taking place. Under normal corrosive

conditions, current flow from the anode results in a loss of metal at the anodic site which results

in the protection of the metal at the cathodic site. Protection can be provided by making the

structure you wish to protect cathodic, using two methods.

The first method relies on sacrificial anodes. When two dissimilar metals are immersed in

seawater, the metal with the lowest electrical potential will suffer the greatest corrosion. For

example, the corrosion rate of mild steel can be controlled by connecting it to zinc as it will

then become the anode and corrode. In this example, the zinc anode is referred to as a

sacrificial anode because it is slowly consumed (corrodes) during the protection process.

Another use of zinc as a sacrificial anode is when steel is coated with the zinc, either in the

form of galvanization or metallisation or in a paint which contains high levels of active zinc.

The second method uses an impressed current system. A foundation can be made cathodic

by using a direct current source. An impressed current is applied in the opposite direction to

cancel out the corrosion current and convert the corroding metal from anode to cathode. The

negative terminal of DC is connected to a pipeline or structure to be protected. The anode is

kept in the water to increase the electrical contact with its surrounding environment." [5] This

will prevent the iron particles from leaving the steel structure, resulting in the structure to remain

intact. These cathodic protection systems are tested, using saturated calomel electrodes.

A very large disadvantage of all these surveys is their cost. Offshore works are expensive, and

so are the special sensors and equipment used during surveys. More risk is involved and it

takes longer to do certain tasks compared to the same tasks onshore. In offshore industries,

being able to work often depends on the weather conditions. High winds and high waves make

it hard or sometimes even impossible to go out on the sea. For ROV operations for example,

the maximal wind speed is about 10 m/s and the maximal wave height is 0.7 to 0.8 metres.

2.1.3 Survey methods for determining the cable depth of burial

Most survey technologies use sonars with sound waves or sound beams. A soundwave is

propagated from the ship and reaches the seafloor. The seafloor reflects a wave towards the

source. A sensor or transducer detects the reflected sound waves. The time between the

propagation and reception will be measured to determine the distance from the transducer to

the seafloor or objects that reflected the sound wave. When scanning different points on a line

next to each other, an array of data is obtained with different distances to the seafloor in an X-

Y grid. Since most surveys are done in combination with GNSS data, the X-Y data is the

position of the measured data.

10

Echosounders can either be a single beam or multibeam. The single beam method sends a

single beam along a parallel route and obtains an approximation of the sea depth in that area.

The single beam method isn’t very accurate when the measured wave is reflected from

materials which absorb sound waves (such as mud). Only an approximation of the depth is

possible in that situation. With multibeam echosounders, multiple beams are sent out

simultaneously and capture a wider area of the seafloor. The differences are illustrated in

Figure 2-5.

Figure 2-5: Comparison between different survey technologies [6]

2.1.3.1 Considerations for surveys

Before immersing in the different technologies, it is important to emphasise the importance of

correct position determination of the surveyed area and the technique used to obtain that

position.

Position determination

Contrary to the well-known regular GNSS (Global Navigation Satellite System) that can be

found in cars and mobile devices, the position determination of surveys demands a higher

accuracy. A regular GNSS is accurate to a few metres, while surveys require an accuracy in

the centimetre range. For this purpose, a GNSS is used in combination with an RTK (real time

kinematic) to do real time corrections. This allows for a centimetre level accurate position [7].

This method uses several satellites simultaneously. In order to obtain these accurate positions

using RTK, connection to the internet is required.

For the surveys of the Norther offshore wind farm, generally an RTK is used to determine the

exact position of the vessel. This RTK has a maximum offset of 4 to 5 centimetres and in

normal conditions the offset is only 2 to 3 centimetres. For the accurate position determination

four possibilities are used depending on the position [8].

Close to shore, the Flemish Positioning Service (FLEPOS) is used to obtain an RTK. This

service is intended for onshore position determination, but can be used for near shore position

determination. The service offers position accuracies in the centimetre range. The vessel

connects to two onshore GNSS position stations with RTK (Ostend and Zeebrugge for

example). The vessel itself forms the third point to obtain a triangle. The position of the vessel

11

is determined using triangulation. It is interesting to note that a network base station is present

on the Blighbank (about 40 km away from shore) that allows for RTK position determination.

When further away from shore without internet connection, the position compensation can be

determined using a UHF (Ultra High Frequency) connection to base stations along the coast.

These base stations are located in Ostend and Zeebrugge among other locations. In order to

receive these UHF signals, a UHF antenna must be present on the vessel. Alternatively,

DGNSS (Differential Global Navigation Satellite System) can be used. With DGNSS, position

corrections are done using a base station with a precisely known location that calculates the

differential correction of location and time to a satellite, resulting in a more accurate position

determination.

When an accurate position of offshore locations has to be determined, without the possibility

to connect to base stations, postprocessing software can be used. Using this software, 24

hours after the survey is conducted, the accurate positions can be determined. This

postprocessing software obtains a more accurate position by adjusting the actual ephemerides

(paths the satellites orbited earth) at the given recording time. This compensates for the

uncertainty of the real time GNSS signal at the measured location. The mentioned position

determination methods all offer inaccuracies in the centimetre or decimetre range. The

accurate positions can be linked to the measured depths or scanned areas.

Other possibilities exist with varying accuracies. These systems and techniques are position

dependent since each system has its own positioning satellites orbiting the earth over certain

areas.

Compensations for movement and position of the vessel

A lot of corrections are required to compensate for movement of the vessel. Depending on the

position of the sensor, this may influence the measured data.

In Figure 2-6, the difference between the situation of a boat at rest or in motion is illustrated.

The boat could be resting in still water or squat when accelerating or cruising. Unlike

measurements on land, boats will move constantly so a good reference is required. A

difference in the angle is noticeable in the three situations. This would result in a false scan if

this remained uncompensated.

Figure 2-6: Movement of a vessel on water [9]

12

The movement of the vessel is accurately monitored using a motion sensor. The motion of the

vessel can cause errors in the surveyed data, but these errors are rather limited due to the

accuracy of the motion sensor. The inaccuracies caused by the movement sensor itself are

about one or two hundredths of a degree. In shallow water the impact of these inaccuracies is

nearly negligible. However, when working in deeper waters, this movement can cause

inaccuracies of a few centimetres. The aim is to have a maximal deviation of 5 centimetres

over the total survey, so the movement inaccuracies have to be kept at a minimum. It is

important to know the surveys are conducted in both directions. This allows to compensate for

position offsets, which gives more measuring points per m² and functions as a double check

for the data. These specific items were discussed in an interview [8] with the surveyor that

provides the surveys of the Norther OWF. The interview can be found in Attachment A.

The compensations include roll, heave and pitch. The roll is the sideward roll motion of a

vessel. The heave aims to compensate for heave of the sea i.e. for the vertical motion. The

pitch is the angle the front or rear end is positioned relative to the centre of the vessel. These

factors must be compensated in order to link the correct positions to the measured data.

To illustrate the effects of the movement of a vessel, one could think of it as illuminating a

surface with a flashlight. The goal would be to know what area is illuminated, given the

compensated distance from the flashlight to the surface and direction the flashlight is pointing

in. When holding the flashlight in a different angle (with the same centre), the illuminated area

will be different. This is similar to pitch and roll of a vessel. When the flashlight is moving away

from the surface, the distance to the surface increases, implying a larger surface is illuminated.

This is similar to the effect heave has on the echosounder. The distance to the seafloor is

increased, implying a wider area is scanned. If heave, roll and pitch were to be left

uncompensated, the depth of the seabed would be determined incorrectly and the

interpretation of the area scan would be wrong.

Another compensation is required because the transducer is mounted under the vessel and

not on the water surface. This implies the depth of the transducer should be corrected to a

reference depth. The position of the transducer relative to the position of the positioning system

receiver should also be compensated for. Since these positions are fixed, a fixed offset can be

used.

A last factor that has to be taken into account is the tide. Depending on the tide, the water level

may be different, resulting in different measured depth values at different times. In order to

compensate for this phenomenon, the LAT (Lowest Astronomical Tide) reference is used. This

eliminates the difference and standardises the reference level for future surveys. [2]

Sound propagation through water

The speed of sound through the water column is not constant due to temperature, salinity and

pressure. The change of these parameters as a function of depth is visualised in Figure 2-7.

According to [10], the sound velocity through water is about 1500 m/s. “The speed of sound in

water increases with increasing water temperature, increasing salinity and increasing pressure

(depth). The approximate change in the speed of sound with a change in each property is:

• Temperature 1°C = 4.0 m/s

• Salinity 1 PSU (Practical Salinity Unit) = 1.4 m/s

• Depth (pressure) 1 km = 17 m/s” [10]

13

Figure 2-7: Temperature, salinity and pressure influence on sound velocity [10]

The correct propagation speed is especially important in multibeam surveying as the beams

follow a complex path, visualised in Figure 2-8, and will have to be compensated.

There are two aspects to the sound velocity. The first is a regular sound velocity sensor that

measures the sound velocity in the water. With this device, the speed of sound through the

seawater is measured by determining the time of flight. A sound pulse is emitted on one side

of the sensor and it will be measured at a known distance away from the source. The device

measures the time it took the sound to travel from the source to the sensor. The time of flight

is known and the distance is known, implying the speed of the traveling sound can be

determined. The accuracy of such a device is around 0.02 m/s, which is very accurate

considering the speed of sound through water is about 1500 m/s [10]. This value is monitored

constantly. After all, due to the differences in temperature and salinity, the sound velocity may

deviate based on the location and moment of the measurement.

The second aspect of checking the sound velocity in the water is to measure the sound velocity

profile of the entire water column. With this second measurement, a probe is lowered into the

water and the sound velocity of the water column is measured. Different water layers can be

obtained when a stream of water from rivers is evacuated into the sea by floodgates. It is very

important to detect these changes to obtain good measurements. Throughout a survey day,

the sound velocity profile is measured multiple times.

2.1.3.2 Multibeam echosounders

Multibeam echosounders are used for geophysical surveys and use multiple narrow beams

that are positioned next to each other. One beam gives a single depth measurement over a

certain width, so a single multibeam measurement gives a line of depth measurements. This

results in a few hundred depth measurements per second. The width of the scanned area, also

known as the swath, can be 2.5 to 3 times as wide as the distance to the seafloor. These

surveys are ideal for deeper waters. The swath and accuracy are dependent on the operational

frequency. A lower frequency will result in a wider but less accurate swath. Its important to

choose the most optimal frequency to obtain a wide swath with good horizontal accuracies.

Multibeam echosounders create a lot of data and are harder to analyse than the single beam

echosounder data. A lot more computing power is required to process these surveys. At

present these multibeam surveys are the most common among submarine topographic

surveys, even though the data is complex to analyse.

14

Similar to the single beam echosounders, compensation of roll, heave and pitch is required.

Because of the fan-shaped beam, the corrections are more complex. For optimal results, the

motion sensor should be placed as close as possible to the centre of the vessel. Location data,

the heave, the roll and the pitch information are linked to the measured multibeam-fan.

With multibeam echosounders, the sound velocity of the water is monitored continuously and

used in real time for MBES compensations. Information of the sound velocity profile is taken

periodically and is critical because most of the beams won’t have a straight path but will bend

off. This must be taken into account to avoid the measured depths and positions will be wrong.

In Figure 2-8, these bending beams are visualised. If improperly compensated, the beams

could curl inwards or outwards at the bottom, resulting in bad survey data.

Using the multibeam echosounder, good accuracies are obtained. The global accuracy should

be within 5 to 10 centimetres of the actual situation. The surveyor aims to get a maximum

offset of 5 centimetres. When the uncertainty is larger, they will check what is wrong and fix

the issue before continuing. This means the maximal offset in the X, Y and Z direction should

be within 5 centimetres of the actual position.

Figure 2-8: Multibeam echosounder survey method [11]

Multibeam echosounder data can be processed to obtain high resolution 3D images [12] or

plots [2] of the seafloor. The level of detail can be impressive as can be seen in Figure 2-9.

15

Figure 2-9: Multibeam echosounder survey result [13]

An actual raw data file of an area in the Norther wind farm was visualised to gain more insight

in the Multibeam scan method. On Figure 2-10 the seafloor topography with small sand dunes

is visible. The large downward spikes are from the jack-up vessel, used for the installation of

the monopile. The blue lines represent the raw data and the colormap is the result of an

interpolation of the data.

Figure 2-10: Multibeam survey raw data visualisation

When omitting the original data and only plotting the interpolated data, this results in Figure 2-

11. With the 3D colormap, the surface of the seabed is clearly shown.

16

These figures clearly show that the multibeam survey only shows a topographical view, and

no layers of the soil. In order to present the data for the chosen purpose, some clean-up is

required.

Figure 2-11: Multibeam survey interpolated data visualisation

When looking from above, a topographical view of the seabed and area is obtained, as can be

seen in Figure 2-12. On the figure, the yellow area in the centre is the scour protection of a

monopile. This is a zone to protect the foundation of the monopile. The line towards the scour

protection is the incoming infield cable that is covered by a rock berm. The blue zones are the

holes from the offshore jack-up installation vessel.

Figure 2-12: Multibeam survey topographical view

17

When looking from above (different area) and including the raw data, the survey path can be

identified. Due to the interpolation, non-existing data is shown in the heatmap. It is important

to note the uncertainty concerning the area which is not surveyed. In Figure 2-13, three corners

should be omitted since they were not surveyed.

Figure 2-13: Multibeam survey topographical view including raw data

2.1.3.3 Sub bottom profilers

A sub bottom profiler does not only measure the depth of the seabed. This is a geophysical

scan that detects different soil layers. Sound pulses are emitted and penetrate the soil. An

image (Figure 2-14) of the geological profile of the soil layers is obtained. The pulses will travel

through the different layers of the soil and reflect differently when a different geology layer is

detected. The different layers are called horizons. A horizon can be a change of soil material

like the change from a sand layer to a layer of clay or rock. The angular change of the soil can

be detected as well. This is more relevant for archaeological research. For example, detecting

areas where a river used to flow, which are now filled with mud and sand. Using a sub bottom

profiler, the original riverbed can be detected as can clearly be seen on Figure 2-14. For cable

laying activities, this information is relevant to plan how the cable can be buried in the soil and

to estimate the time it will take to bury the cable.

Figure 2-14: Sub bottom profiler geological profile and horizons [14]

18

Sub bottom profilers can be split up into different categories [15]. These different systems are:

the non-linear parametric sub bottom profiler, the Pinger, the Chirper and the Sparker/Boomer.

Some of these techniques are illustrated in Figure 2-15.

The non-linear parametric sub-bottom profiler emits two signals simultaneously. This

system is also known as the parametric echosounder (PES). Both signals have a slightly

different frequency (for example: 100 kHz and 110 kHz) and the difference in frequency

generates another signal that has a lower frequency. This results in accurate measurements.

The specifications of such systems are as follows: a common frequency range of about 100

kHz; a vertical penetration depth of under 100 metres; a vertical resolution smaller than 0.05

metres.

A Pinger is mounted directly under a vessel and emits and receives a vertical signal. The

frequency range is typically between 3.5 to 7 kHz. The penetration depth is between 10 and

50 metres. The vertical resolution is about 0.2 metres.

A Chirper (CHIRP = Compressed High Intensity Radar Pulse) has a common frequency

range of 3 to 40 kHz and a penetration depth of less than 100 metres. The vertical resolution

is about 0.05 m. A chirper is towed behind a vessel and emits and receives vertical signals.

A Sparker or Boomer is towed behind the vessel and emits signals under a certain angle. A

receiver array is required to capture the reflected signals. These signals will show different

layers of the ground. The specifications for a boomer system are as follows: a common

frequency range of 500 Hz to 5 kHz; a penetration depth between 30 to 100 metres and a

vertical resolution of 0.3 to 1 metre. The specifications of a Sparker system are: a common

frequency range of 50 Hz to 4 kHz; a penetration depth up to 1000 m in ideal conditions; a

vertical resolution of over 2 metres.

Figure 2-15: Sub bottom profiling survey methods [16]

The result of such a survey is a vertical profile of the soil. The scanned area has to be selected

carefully. The resolution of the result depends on the frequency used to scan the soil. A higher

frequency will result in a higher resolution result. However, using these higher frequencies, the

penetration depth is smaller, so a trade-off has to be made between losing resolution and

gaining range. Note that less penetration depth is not favourable in most cases since the image

might not contain all relevant layers, deeming the survey useless.

19

On Figure 2-16, an example is shown of the echo print of a parametric sub-bottom profiler

(SES-2000). On the horizontal axis, the travelled distance is visualised. On the vertical axis,

the profile depth is visualised. This profiler can penetrate up to 40 metres depending on

sediment type and noise. The vertical resolution is about 5 centimetres.

Figure 2-16: Sub bottom profiler echo print example [17]

This specific profiler can be combined with a side scan extension, so side scan profiles (images

of the seafloor) can be made simultaneously. Since the primary frequency of the side scan

sonar is about 100 kHz, the frequency at which the sub-bottom profiler works should be altered

to a different frequency to avoid interference.

On Figure 2-17, an example of a buried and exposed pipe is shown. The principle is similar for

subsea cables. The main difference will be the size of the object that is detected because in

general, pipes are bigger than power cables.

Figure 2-17: Sub bottom profiler exposed and buried pipe example [18]

20

This implies the cable can be detected using a sub bottom profiler (SBP). The SBP will detect

the different soil layers by sending an acoustic signal towards the soil and receiving the

reflected signal. The hard and dense cable will reflect the signal as well, but this will be shown

as a deviation in the soil layers. Figure 2-17 clearly illustrates this deviation. The cone of

detection of a SBP is quite wide, so it is less accurate than a narrower beam multibeam

echosounder. The results of both measurements are completely different. The MBES will result

in an area of the seafloor and the SBP will show the different soil layers (including the cable).

The biggest disadvantage of the SBP is the fact that the cable must be crossed in order to be

detected (see Figure 2-18). Unlike the MBES surveys, SBP can’t be surveyed in the

longitudinal direction. Consequently, if every metre over a trajectory of 10 kilometres has to be

surveyed, the survey vessel needs to cross the cable 10 000 times. This would be very

expensive and very time consuming. The precision of such a SBP is about 10 to 15 centimetres

due to the wide swath. A SBP will show a profile of the seabed and solid items will show as a

hyperbola. Using software, the hyperbolas can be extracted automatically. More information

on sub bottom profiling can be found in [12], [15] and [19].

Figure 2-18: Sub bottom profiler survey path

2.1.3.4 Marine magnetometry

Magnetometry has a different working principle compared to the acoustic techniques that use

sound waves. With magnetometry, a difference in the magnetic field will be detected. This

difference is induced by the proximity of an object containing iron. This method is commonly

used to detect shipwrecks. Similar to a side scan sonar, a fish is towed behind the vessel.

Since most boats contain iron, it is important for the towing vessel to be at a certain distance

from the tow fish to reduce possible interference. After all, the goal is to scan the seafloor, not

the towing vessel. The tow fish is lowered into the water, close to the sea floor and at a

considerable distance away from the towing vessel, so only scanned objects are detected.

The earth’s magnetic field will be disturbed by an iron-containing object, like the shipwreck in

Figure 2-19. The disturbance will show as a large anomaly in the detected magnetic profile.

Magnetometers are used for surveys such as UXO (Unexploded Ordnance) surveys to detect

ammunition. Magnetometer surveys can also detect anchors, shipwrecks and pipelines. After

conducting such a survey, a decision will be made whether to avoid the object or remove the

object considering the cost and potential risk. One could argue that a side scan sonar or

multibeam survey can be used to detect these objects, but 90% of these items are fully

submerged under the seafloor [12].

21

One could use a magnetometer to determine if a cable is present at all, but a magnetometer

cannot be used to determine the depth of burial of a cable, unless the cable was magnetised.

Figure 2-19: Magnetometry survey method [20]

2.1.4 Cable depth of burial determination using survey technologies

First, some initial measurements are taken once at the site. For instance, the sound velocity is

measured using a sound velocity sensor. This sound speed measurement is necessary to

determine the exact measured depths of the survey. Since these values can vary over the

whole wind farm, multiple of these measurements are taken.

The depth of the seafloor can vary because of the dynamic environment. Once a cable is

buried, it is considered to stay in place. The seabed consists of several layers. A study with

survey data of the last 15 to 20 years was carried out before installation of the wind farm to

determine what layers could move and their respective depth. Based on that study, a reference

seabed level (RSBL) was determined. This RSBL is the depth of a stable layer wherein the

cable is buried. When the top layers of the seafloor move around, some parts may become

more shallow or deeper and those will ultimately change the measured depth of burial. This

implies that the cable is considered to be at a constant depth to a fixed reference and the depth

of burial may increase or decrease over time due to movement of the top layers.

Typical steps of an offshore survey [21]

1) Checking and calibration of the equipment

It is important to check the equipment to ensure it is working correctly before arriving

on site.

22

2) Mobilise equipment

This step is mostly checking all necessary equipment is onboard the vessel.

3) Setting up the equipment

In this step the equipment is set up once the vessel arrives on the test site. The

GNSS (Global Navigation Satellite System) receivers are set up and deployed. This

GNSS is a very accurate position receiver (centimetre level precision). The GNSS

has two main parts, a receiver and a processor. The processor does a conversion,

so the measured position is translated to world coordinates. The data of the GNSS

will be coupled to the measured data implying a measured point can be assigned

to a precise position.

4) Measuring sound velocity in water

In order to calibrate the transducer on site, the velocity of sound in the water has to

be measured. It is important that this measurement is done correctly as it is critical

for the acoustic surveys. With a sound velocity sensor, the speed of sound through

water is measured over a known distance, for example 10 cm. The velocity of the

sound can be determined by measuring the time it took for the signal to arrive at

the receiving end of the sensor. The sound velocity in water depends on factors like

salinity and water composition. This sound velocity will then be used in data for the

multibeam survey for example. If this measured sound velocity value is incorrect,

all multibeam data will be incorrect.

5) Start of the survey

This is the part where the vessel will sail a pre-determined route to perform the

survey, for example a bathymetric survey (survey of the seabed depth or submarine

topography). This measured depth is combined with the previously mentioned

GNSS data in order to link the measured data to a precise location.

6) Checking the data before leaving the site

Before all survey equipment is hauled back on board and cleaned for storage, it is

important that the data is checked on site. “Was every part of the pre-determined

area actually scanned?” is a key question in this step. If an area was not scanned

properly or only scanned partially, it is easier to go back while you are still on site

and survey the area. This prevents the need for the surveyor to go back and set up

all the equipment again. This check is carried out by the responsible surveyor.

7) Demobilisation

After the surveyed data is checked and verified, all equipment can be retrieved from

the water or test position and the equipment can be shut down. All equipment

should be cleaned with fresh water if possible, in order to prevent the equipment

from corroding. After everything is thoroughly cleaned, the equipment can be stored

and is ready for the next survey.

8) Data processing

The raw data has to be processed in order to analyse and present it in an optimal

way for its purpose. This is done by the so-called offline surveyor. This person will

check the raw data for faulty signals and filter or clean up where needed. The next

23

step is to process the raw data so the surveyed areas can be charted, either in 2D

topographical, 2D sectional or 3D. The data can be presented in numerous ways,

whichever is the most convenient for the purpose.

Multi Beam Echo Sounder (MBES)

A typical multibeam echo survey will scan the top layer of the seabed and compare it to a

reference level e.g. the LAT (Lowest Astronomical Tide) to measure the depth of the sea. Since

the regular multibeam echosounder will not measure the depth of burial of a cable directly, the

as-laid or as-built data is critical in this comparison. Imagine the cable is buried at a depth of -

24 m (LAT) on a specific location. A next survey is done once the cable is buried and the

seabed depth on that location equals -22.4 m (LAT). This means the cable is buried 1.6 m

below the surface of the seabed. The next survey that is conducted is another MBES survey,

measuring the seabed level. Imagine on the same location as previously described, the mean

seabed level (MSBL) equals -22m (LAT). This means the cable is now buried 2 metres below

the seabed surface. This change of burial depth may originate from sand dunes which are

moved on top of the cable.

The bottom detect resolution of a multibeam echosounder mentioned in datasheets [22] is the

margin in which the seafloor can be detected in a controlled environment i.e. a laboratory test.

This resolution will be expressed in millimetres and will have a value of e.g. 3 mm. This means

the vertical accuracy of the echosounder itself is +1.5mm or -1.5mm. This margin is not the

resolution of the depth of burial of the cable since the required compensations for vessel

movement and position cause more uncertainties.

Using the multibeam echo sounder, the vessel may not exceed a maximal speed to ensure a

full coverage of the area. This maximal speed is about 11 knots [23] for example. 11 knots

equals to 5,7 metres per second. Let’s assume the survey vessel only goes 5 metres per

second to ensure proper coverage. This means over a single trajectory of 25km, the scan

would take 5000 seconds, which equals about an hour and a half. If several of these scans

have to be conducted in order to have a proper scan area, this scan time can easily double or

triple.

This seems to be quite fast. Therefore realistic survey campaign data was looked up. There, it

became clear that such a campaign takes significantly longer. Including the mobilisation, it took

about 36 hours (or about two days) to completely scan a 26 km trajectory. This survey was

done in both directions to ensure the quality of the measured results. This means one kilometre

of surveying in both directions takes about one hour and a half. The vessel may sail at a speed

of 5 to 6 knots to have the optimal equilibrium between quality of the data and the progress

(speed) of the survey itself. This is much slower than the 11 knots of the theoretical example.

The difference is also due to the fact more measurements have to be done during the survey,

like the sound velocity in the water for example.

Sub Bottom Profiler (SBP)

The reason SBP is not commonly used for check-up surveys is because it is very labour

intensive. The maximal survey speed for a SBP is 2 to 3 knots and the cable has to be crossed

in order to measure the location of the cable. Comparing this to the speed of a MBES, it is

clear that the MBES can be used at double the speeds. This results in less time on the sea,

thus resulting in a cheaper survey with the MBES. SBP can be used for spot checks along a

24

cable trajectory, but would it be financially irresponsible to survey the whole trajectory of a

cable with a DoB value every few metres, using the zig-zag SBP pattern.

When considering a new SBP-technique, where it is possible to sail along the cable instead of

crossing it, the survey speed is once again decreased to about 1 to 1.5 knots. This is

significantly slower than the MBES. The technique uses a single transducer and multiple

receivers. With the received signals, the position of the cable can be calculated. It should be

noted that this method is still being developed at the time of writing and such a survey is more

expensive to conduct. With this SBP method, one pass after good calibration would be

sufficient. The uncertainty in the X, Y and Z direction is about 15 centimetres. Consequently,

the method is less precise than a MBES. However, in a case where the cable is expected to

have moved, a SBP can be used to determine its reference position. For SBP, it would be

more interesting to scan a few locations (spot checks) along the trajectory of the cable. These

locations can give a good indication on how the cable moves, if it moves at all and provide a

verified reference position and depth.

2.1.5 Cable depth of burial survey with ROV

Some manufacturers of survey equipment offer dedicated cable tracking systems. These

systems can measure the DoB of cables in offshore wind farms. Such a system is mounted on

an ROV implying the range of operation is more limited compared to the acoustic scanning

techniques (mostly due to the state of the sea and weather conditions). These systems [24]

provide accurate survey data with vertical accuracies of about 5 cm or 5% uncertainty. The

position of the ROV can be determined using a USBL (ultra-short baseline) position tracking

system. A USBL transponder is mounted on the ROV and the transceiver is mounted under

the survey vessel. The distance and angle from the vessel to the ROV are used to determine

the relative position of the ROV to the absolute position of the survey vessel.

However, using an ROV makes this a slow process. The speeds described are around 2 knots

which is about 1 metre per second. Scanning a similar trajectory of 25 km would take 25000

seconds, if the system was used continuously. This means surveying the 25km trajectory

would theoretically take about 7 hours. When considering the difference between the

previously mentioned realistic and theoretical speed of the MBES survey, this ROV survey will

probably take a few days to conduct.

2.1.5.1 Cable tracker

Cable trackers [25] can either be passive tone detectors, passive gradiometers or active pulse

inducers. A tone detector detects the cable frequency. A passive gradiometer measures a

difference or change in the magnetic field of the cable. An active pulse inducer uses

electromagnetic pulses and induces a current flowing in the cable, generating a magnetic field

that can be measured.

Since the power cables of most wind farms are transporting AC with a 50 Hz frequency, the

tone detector system can be used when the cable is actively in use. HVDC (High Voltage Direct

Current) cables don’t have such a frequency i.e. no tone is emitted. This tone detection system

doesn’t work with cables that are out of service. HVDC cables can be tracked using this tone

detecting technique, but this requires the cables to be out of nominal operation during the

survey. Another signal is sent in the cable i.e. an AC signal goes through the HVDC cable and

a tone is emitted. It should be noted that HVDC cables do emit a few harmonic frequencies (in

the range of 1 to 2 kHz) which implies alternative sideband frequencies can be tracked.

25

An electromagnetic field of a HVAC (High Voltage Alternating Current) cable consists of an

electric field and a magnetic field. The passive gradiometer detects a magnetic field that is

emitted by the cable. The shielding used in the submarine cables provide a good shielding of

the electric field, but the magnetic field is not shielded, implying it can be detected. When using

the method that detects a magnetic field, a problem occurs with cable crossings. These

crossings give a distortion of the magnetic field resulting in an error. The error can and will be

detected. [25]

The active pulse induction method can only work with cables that are not in use. Another signal

is injected in the cable implying the current and resulting magnetic field are known. When

measuring the emitted magnetic field of the cable, several sensors (coils) will be used to

determine the location of the cable.

For the Norther offshore wind farm, the cables were magnetised during loadout of the cables

(moving the cable from the harbour to the cable drum on the installation vessel) before they

were layed on the seabed. Soon after a cable was buried, the magnetic field emitted from the

cable was measured using a passive gradiometer. The data from this passive gradiometer was

used to determine the as-laid depth of burial of the cables. This as-laid depth of burial is the

reference depth of the cable that is currently used. This implies there is still an uncertainty in

the actual depth of burial of the cable. The precision of the depth of burial depends on the

precision of the passive gradiometer. For the Norther offshore wind farm this accuracy is 5 cm

or 5% of the depth.

2.1.5.2 Sub bottom imager (SBI)

A sub bottom imager [26] is an active acoustic device that is fitted on an ROV. Acoustic signals

are emitted and the response is measured. These scans give a 3D acoustic image of the

sediment below the seafloor. The survey has a swath of about 5 metres and can go on for

several kilometres in distance.

An exemplary result is given in Figure 2-20. This is part of a survey along a dual core HVDC

(High Voltage Direct Current) cable with outside dimensions of 217 mm x 136 mm [27]. This

implies, this method can also be used to detect HVDC in active or inactive state. In the centre

of the image it is clearly visible that the cable isn’t buried deep enough and requires more

coverage.

Figure 2-20: Sub bottom imager measurement result [28]

The cable used for this sub bottom imager measurement is visualised in Figure 2-21.

26

Figure 2-21: 217 mm x 136 mm HVDC cable [27]

2.1.5.3 Conclusion on ROV DoB surveys

Generally, DoB surveys using an ROV have a less accurate vertical resolution than the

previously described acoustic methods (echosounders) that don’t use an ROV. The method

may be less accurate, yet it is necessary to determine how deep the cable is actually buried.

The two methods have a different scope. The echosounders typically measure the depth of

the seafloor, while the ROV techniques measure the actual depth of burial of the cables.

Since these ROV surveys take significantly longer to conduct than an acoustic echo survey

and they require more equipment, these are expensive operations. In theory, the multibeam

survey can be conducted about 2.5 times faster and can take place in worse weather

conditions. Since the cable is considered to stay in its position, it would be financially

irresponsible to conduct these expensive ROV-based cable depth of burial surveys after the

as-laid depth is already determined. Submarine topographical surveys prove to be a more cost-

effective way to determine the depth of burial after the reference position of the cable is known.

2.1.6 Cable depth of burial determination in the Norther OWF

During the installation, the BoP (Balance of Plant) contractor magnetised the cable and buried

it in the seafloor. Next, the cable position was surveyed and determined using an ROV-

mounted passive gradiometer system. This system measured the magnetic field emitted from

the cable. Based on the measurement, the as-laid depth of burial of the cable could be

determined. For future reference, a multibeam echo survey was conducted implying this could

be referred to as the as-laid seabed depth. Since the cable is assumed to stay in its position,

only the top layer of the seabed will vary. Due to the dynamic behaviour of the seafloor, the

cable coverage will increase or decrease. When the wind farm operator wants to know how

deep the cable is buried, a new MBES survey is conducted and the change in the seafloor will

be monitored. When the seafloor becomes more shallow, the cable coverage will increase,

resulting in a deeper cable depth of burial. When the seabed lowers, the cable coverage will

decrease, resulting in a more shallow cable depth of burial. This approach implies the update

of the depth of burial is done using multibeam echo sounder surveys that scans the seafloor.

The changes of the seafloor due to the current, weather conditions and natural effects will bury

the cable deeper or more shallow.

27

2.1.7 General conclusion of traditional DoB determination

Based on the research results, one can conclude that the method with the multibeam

echosounder, which is currently used, is sufficient for updates on the cable depth of burial.

Some locations could be measured using a SBP if a more precise location and DoB is required.

Survey techniques are still being developed. Consequently new methods could become the

new standard if they prove to be more efficient than the techniques currently in use.

2.2 Distributed Temperature Sensing

In this section the general working principle of DTS systems will be explained. For a more

elaborate explanation on various DTS systems, please see the thesis [1] of the KU Leuven

student who worked on the project during academic year 2019-2020.

Within DTS technology, several different approaches are possible. The most popular

approaches use Raman or Brillouin scattering. The Norther DTS is based on the Brillouin

scattering principle. However, it still is interesting to describe the most popular methods [29].

When a laser sends a light pulse through a fibre optic cable, most of the light is transmitted to

the other end of the cable. When a fibre optic installation is commissioned, the optical fibre

quality has to be tested (by the means of an OTDR test, described in section 2.4). Due to

imperfections in the material, the light will be partially reflected to its source. This causes

attenuation of the transmitted signal. When a fibre optic cable is bent or sections were welded

together, this will cause additional attenuation relative to the severity of the situation. In case

fibre optic connections are made through connectors, additional attenuation can be expected.

The attenuation due to connectors are more or less standard values depending on the type of

connector.

These attenuations imply less light reaches the end of the optical fibre and this light is

backscattered to the light source. When light gets backscattered in a fibre optic cable, it is

captured by the optical detector. The time between the transmission of the pulse and the

reflection to the optical detector can be used to determine the distance from the incident light

source to the position of the backscattered light. The backscattered light contains a Rayleigh

component, visualised in Figure 2-22. This component has the same wavelength (same colour)

as the incident light. Besides the Rayleigh component, also a Raman and a Brillouin

component will be present. The Raman and Brillouin wavelengths are split into shorter

wavelength Anti-Stokes and longer wavelength Stokes components. Since wavelengths can

easily be converted to frequencies with equation (1),

𝑓 =𝑐

𝜆 (1)

With

𝑓 Frequency [Hz]

𝑐 Speed of light (depending on medium) [m/s]

𝜆 Wavelength of light [nm]

some literature describes these components in the frequency domain. When considering

measurement equipment, the shift in wavelength is often referred to as a frequency shift.

28

Figure 2-22: Wavelength spectrum of backscattered light [30]

For every component, a technique to measure temperature (and in some cases strain) was

developed. With Raman scattering, only temperature can be monitored. However, it must be

noted that the techniques that allow for strain and temperature measurement are slightly

different. This implies it is not possible to measure both strain and temperature at the same

time. However, attempts have been made to combine these measurements. This is further

described in section 2.2.3 and [31].

Combining the position determination of the backscatter along the cable with the temperature

determination from a backscattered signal implies that for every backscattered light pulse a

temperature can be measured and it can be linked to a known distance from the light source,

in this case, the DTS interrogator. This allows the principle of distributed temperature sensing

through a fibre optic cable using a DTS interrogator with one of the following principles.

2.2.1 Rayleigh scattering

Rayleigh scattering is the least popular technique to obtain a temperature measurement. This

technique is based on optical frequency domain reflectometry (OFDR). For an application in a

nuclear (test)reactor [32], this technique was tested and used. A high spatial resolution1 of one

centimetre was necessary to monitor the temperature along a limited trajectory (in the order of

metres). The test distance was 25 metres, which is very different from the kilometre-trajectories

found in offshore wind farms. The technique also has a high sensitivity along this limited path.

The Rayleigh scattering type of DTS is not applicable for temperature measurement along

cable trajectories in offshore wind farms. The technique will not be discussed more elaborately

as it is irrelevant for the scope of this project.

2.2.2 Raman scattering

With Raman scattering, the temperature can be measured by the difference in intensity of the

anti-stokes component compared to the stokes component. This is due to the fact that the anti-

stokes component of the Raman scattering is temperature dependent. A temperature

1 The spatial resolution is defined as the distance between two consecutive measured points.

29

measurement is given for every location where light is reflected. According to [29], the

approach was developed in the 1980s.

“Both Brillouin and Raman Scattering produce photons at longer and shorter wavelengths than

the incident light. Photons which are emitted with less energy are referred to as red shifted,

Stokes Scattering. Photons which are emitted with more energy are referred to as blue shifted,

Anti-Stokes Scattering. This shift is a function of the incident light and the material they interact

with. For Raman sensing, the Anti-Stokes signal is strongly temperature dependent, whereas

the Stokes signal is weakly temperature dependent.

As temperature increases, more Anti-Stokes photons are produced than Stokes photons for

any given interaction. If a reference temperature is known and the system is calibrated for the

optical fibre being used, then a ratio of the Anti-Stokes to Stokes intensities can yield the

temperature at any given location. The simplified Anti-Stokes/Stokes equations solved for

temperature at a given location, 𝑇(𝑍), are shown below.” (in equation 2) [33]

𝑇(𝑍) = 𝑇𝑟𝑒𝑓 ∗ [1 + ∆𝛼𝑧

ln (𝐶𝑆𝑡𝑜𝑘𝑒𝑠

𝐶𝐴𝑛𝑡𝑖−𝑠𝑡𝑜𝑘𝑒𝑠)

+ ln (

𝐼𝑆𝑡𝑜𝑘𝑒𝑠(𝑧)𝐼𝐴𝑛𝑡𝑖−𝑠𝑡𝑜𝑘𝑒𝑠(𝑧)

)

ln (𝐶𝑆𝑡𝑜𝑘𝑒𝑠(𝑧)

𝐶𝐴𝑛𝑡𝑖−𝑠𝑡𝑜𝑘𝑒𝑠(𝑧))

] (2)

With

𝑇(𝑍) Temperature along the fibre at distance z [K]

𝑇𝑟𝑒𝑓 Reference temperature [K]

𝑧 Location along the fibre

∆𝛼 Differential attenuation between Stokes and Anti-Stokes backscatter

wavelengths [m-1]

𝐶𝑆𝑡𝑜𝑘𝑒𝑠 Constant relating to sensitivity of IStokes to temperature [-]

𝐶𝐴𝑛𝑡𝑖−𝑠𝑡𝑜𝑘𝑒𝑠 Constant relating to sensitivity of IAnti-Stokes to temperature [-]

𝐼𝑆𝑡𝑜𝑘𝑒𝑠(𝑧) Intensity of Stokes band as a function of location

𝐼𝐴𝑛𝑡𝑖−𝑠𝑡𝑜𝑘𝑒𝑠(𝑧) intensity of Anti-Stokes band as a function of location

This will result in a temperature profile along the cable, as shown in Figure 2-23.

Figure 2-23: Temperature measurement using DTS [34]

30

2.2.3 Brillouin scattering

Brillouin frequency shifts can occur under the influence of temperature and strain. Generally,

only one of the quantities can be measured at once. Since the cable is already in position and

the fibre is embedded in a loose tube, twisted in a helix and in a gel, the influence of strain on

the frequency shift is very minimal (in the order of nanometres). Additionally, DSS (Distributed

Strain Sensing) requires a higher monitoring frequency for measurements, implying

inaccuracies due to strain should be minimal and barely noticeable in the temperature

measurement using a lower monitoring frequency. The scope of this project is to measure the

temperature and not the strain on the cable. The strain would be more interesting to monitor

during installation of the cables.

As described earlier, the combined measurement of strain and temperature is not a standard

implementation, but attempts have been made. In 1998, the simultaneous measurement of

distributed temperature and strain was tested and is described in the IEEE journal of quantum

electronics [31]. The paper concludes that it is possible to determine both factors

simultaneously. However, for the Norther application the specifications and setup used for the

simultaneous measurement, mentioned in the paper, would not be acceptable. The spatial

resolution of the temperature is too large to detect possible hotspots in the cable.

The paper describes the general Brillouin frequency shift as follows: “When a highly coherent

pulse of light propagates through an optical fibre, part of its energy is backscattered due to a

nonelastic interaction with the acoustic phonons of the medium. This Brillouin scattering

produces both frequency down-shifted Stokes light and up-shifted Anti-Stokes light depending

on whether a phonon is generated or annihilated. The Brillouin frequency shift is given by

equation (3).

𝑣𝐵 = 𝜔𝐵

2𝜋=

2 ∗ 𝑛 ∗ 𝑣𝐴

𝜆𝐿 (3)

With

𝑣𝐵 Brillouin frequency shift

𝜔𝐵 Angular Brillouin frequency shift [rad/s]

𝑛 Refractive index of the fibre core

𝑣𝐴 Longitudinal acoustic velocity for the fibre glass

𝜆𝐿 Free-space wavelength of the pump light

The equation describing the linear variance of the Brillouin frequency shift as a function of

strain and temperature is described by equation (4).

𝛿𝑣𝐵 = 𝐶𝑣𝜀 ∗ 𝛿휀𝑧 + 𝐶𝑣𝜃 ∗ 𝛿𝜃 (4)

With

𝛿𝑣𝐵 Brillouin frequency shift [MHz]

𝐶𝑣𝜀 Coefficient of strain [MHz/µm]

𝛿휀𝑧 Difference in (longitudinal) strain [µm]

𝐶𝑣𝜃 Coefficient of temperature [MHz/K]

𝛿𝜃 Difference in temperature [K]

31

The effect described in this equation (4) has been the basis of strain and temperature sensors.”

[31]

For 𝜆𝐿 = 1.55 µ𝑚(= 1550 𝑛𝑚) the coefficients which relate to the strain and temperature in the

equation result in 𝐶𝑣𝜀 = 0.048 𝑀𝐻𝑧/µ𝑚 and 𝐶𝑣𝜃 = 1.1 𝑀𝐻𝑧/𝐾 according to [31]. From the

experience of Marlinks, the value for the temperature coefficient may vary from 𝐶𝑣𝜃 = 1 𝑀𝐻𝑧/𝐾

to 𝐶𝑣𝜃 = 1.4 𝑀𝐻𝑧/𝐾. This implies it is very important to measure this coefficient in order to

properly calibrate the DTS system for the used fibre optic cables.

2.3 Real Time Thermal Rating

RTTR is the abbreviation of Real Time Thermal Rating. This calculation method determines

the cable rating (ampacity) in real time. The rating of a cable tells how much current the cable

is allowed to carry in order to remain within the temperature limits for the cable. In international

standards, the cable rating is determined at a certain ampacity. This ampacity would typically

be allowed for an unlimited amount of time without degradation of the cable insulation.

With RTTR, a more dynamic approach is taken. The RTTR calculation has multiple input

parameters as well as the historic load of the cable. Such input parameters are the current

load of the cable, i.e. the amount of power being transported, the temperature of the fibre inside

the cable (from the DTS system), the cable and soil parameters (to determine how difficult or

easily the cable will be able to dissipate the developed heat), … . The result of this calculation

is the thermal behaviour of the cable at a specific location and at a moment in time, related to

its (maximum) ampacity. The dynamic rating of a cable can be calculated with parameters from

the cable datasheet, information of the cable temperatures and the thermal resistance of the

soil. Equation (5) from the export cable ampacity calculations [35] shows this relationship.

𝐼 = √∆𝜃 − 𝑊𝑑 [0.5 𝑇1 + 𝑇2 + 𝑛 (𝑇𝑓 + 𝑇3 + 𝑇4)]

𝑅 𝑇1 + 𝑅 (1 + 𝜆1) 𝑇2 + 𝑛 𝑅 (1 + 𝜆1) 𝑇𝑓 + 𝑛 𝑅 (1 + 𝜆1 + 𝜆2) ( 𝑇3 + 𝑇4) (5)

With

𝐼 Cable current carrying capacity [A]

∆𝜃 Conductor temperature rise above ambient temperature [K]

𝑊𝑑 Dielectric loss [W/m]

𝑛 Number of cores [-]

𝑇𝑓 Thermal resistance of the Fillers (Armour bedding) [K*m/W]

𝑇1 Thermal resistance between Conductor and Sheath [K*m/W]

𝑇2 Thermal resistance of Anti-Corrosion Sheath of each core [K*m/W]

𝑇3 Thermal resistance of outer covering (Serving) [K*m/W]

𝑇4 External thermal resistance at Burial Depth [K*m/W]

𝑅 AC resistance of the conductor at maximum operating temperature [Ω/m]

𝜆1 Loss factor of Sheath and Screen [-]

𝜆2 Loss factor of Armour [-]

32

Most of these factors are mentioned in section 3.2. The formulas for the maximally allowed

currents for different types of cables and in various conditions can be found in the IEC 60287

standard [36].

As mentioned before, the allowed cable temperature depends on the insulation material. For

cross-linked polyethylene (XLPE) insulated cables, the temperature limit equals 90°C. Cables

in offshore wind farms are often designed with a dynamic load i.e. with historic wind data and

production models. The production models often use a high-load case to create some safety

margin in the design. In wind energy, power production is highly dependent on the wind and

windspeeds. This implies there are periods of low wind speeds and low energy production, but

there are also periods where there is a lot of wind, resulting in high power production of the

wind farms. Consequently, the cable could be cooled down during periods with little to no

power production. Imagine the cable temperature is vastly below this temperature threshold

due to low wind speeds. Suppose the next few days windspeeds are expected to be over the

nominal windspeeds for the wind turbines in the wind farm. This will result in nominal power

production for these next few days.

Nominal power production will result in heating of the cable. The thermal step response of the

Norther export cable is visualised in Figure 2-24. In order to heat the Norther export cable up

to nearly steady state temperature, about 10 days of constant nominal power production are

needed [37]. The actual steady state temperature is obtained after about 50 days of nominal

power production. These are very long periods and the occurrence of such long wind-rich

periods is very low. The thermal inertia of the cable prevents the cable from overheating.

Figure 2-24: Thermal step response of the Norther export cable at nominal load [37]

Using RTTR, the ampacity of the cable could be calculated dynamically while the wind farm is

operational. This implies that if the cable has cooled down enough, instead of the rated 1000

A current, 1500 A could be allowed for a short period of time. Once the cable has heated up

and reaches a temperature threshold of e.g. 80°C, the ampacity could be reduced to 700 A.

This approach allows the cable to cool down or it allows to decrease the rate by which the

33

temperature increases. Once the cable temperature has decreased or stabilized sufficiently,

the RTTR tool could suggest an ampacity of 1200 A is allowed again.

This is an interesting approach to determine ampacity ratings in offshore wind farms. Taking

the variable power production and the slow heating of the cable into account, one could

conclude cables in offshore wind farms could be slightly under-dimensioned and monitored

using RTTR. In some cases it might be more cost-effective to implement an RTTR system and

rely on an under-dimensioned cable.

The cost of the cable itself is only a small part of the cost to install a wind farm. However, if a

smaller cable is used, this could lead to a single trip with a cable installation vessel (visualised

in Figure 2-25) instead of two trips. This is a factor that could reduce costs during the

installation and it could make the necessary difference to be able to finance a project. The DTS

system with RTTR will provide the required continuous monitoring for the years to come.

Figure 2-25: Cable installation vessel

Notice however, this technique is not used in most wind farms. If a project developer requires

investment from a financial institution and they want to use this method because it could reduce

the costs for the transport and installation of the cable, the financial institution will have to be

convinced by the reliability the RTTR system. Moreover, for the cables using nominal ampacity

rated dimensioning (based on historic wind speed data and expected production of the wind

farm) it will be easier to be certified by standards. This nominal ampacity rated dimensioning

results in a guarantee of being reliable in all conditions, which is of interest to financial

institutions and insurance companies.

34

Using a slightly under dimensioned cable, relying on the dynamic rating (allowed ampacity)

during operation, would result in an even less predictable production schedule. Working with

such a method requires grid compatibility and grid operators are not in favour of using

unpredictable power plants.

However, this dynamic temperature monitoring approach could be useful in the future. It

provides more information on the condition of the cable. For current projects, this could help

operators to verify the margin they have on the cable design of the cable manufacturer. Using

this approach, they rely less on data specified by the manufacturer of the cable, but they use

actual measurements where external conditions are taken into account.

Another aspect is insurance. Most insurance claims in offshore wind farms are due to cable

failures. Some wind farms do not take an insurance policy from an insurance company, but

use the funds to implement their own insurance. For example a certain wind farm has two

export cables, both working on partial load in the nominal situation. If one of the cables fails,

they can rely on the remaining cable and about 60% of the produced power can still be

transported to shore. This remaining cable will be used to its maximal limit. RTTR could help

to determine the maximal allowed current in the cable, based on the dynamic parameters,

allowing the operators to maximise the production without risking an overload of the cable and

without the risk of a complete standstill due to one cable failure.

For the Norther case, the cable is dimensioned based on historic wind speed data and

expected production of the wind farm over the course of one year. A conservative depth of

burial and conservative soil parameters were taken into account, resulting in extra margin. The

RTTR tool will show how the cable handles the transport of power. This could help to determine

if it would be more interesting to reduce power output of the wind farm for a short period of

time. Such decisions have to be made if the power purchaser has to reduce power injection,

due to lower power usage than expected.

Using this approach, the cable would have the opportunity to cool down, allowing a higher

current to flow after the cable cooled down. This works in both ways. If the RTTR shows the

cable can handle an extra unexpected load, this could be used to increase the injected power

in the grid by the wind farm. This avoids the use of a fossil fuel power plant.

The monitoring system provides an additional benefit when there is a hotspot in the cable i.e.

the DTS system is able to warn the operators. This allows operators to take the appropriate

actions before a real problem occurs and the cable needs to be repaired. Without the

distributed temperature measurement in the cable, detection of hotspots is almost impossible.

2.4 Optical Time-Domain Reflectometer

OTDR is the abbreviation of Optical Time-Domain Reflectometer. The OTDR is an

optoelectronic test instrument used to measure the attenuation of fibre optic cables. An OTDR

test verifies that the measured fibre optic channels do not have excessive signal attenuation

over their length. The OTDR can also be used to detect where problems occur along the length

of the fibre optic cable. The general purpose of an OTDR is to send light pulses with a known

intensity through a fibre optic cable. Within the fibre optic cable, imperfections will cause a

backscatter of the light pulse. This is visualised in Figure 2-26.

35

Figure 2-26: OTDR measuring principle of operation [38]

The returning signal is measured and the difference between both intensities can be described

as the optical attenuation. Some attenuation is allowed, but there is an optical budget of 10 dB

per sensing channel that may not be exceeded for the Norther case. The internal workings of

an OTDR can be found in Figure 2-27. The pulse generator generates a pulse with known

properties. This pulse is converted into a light pulse by the light source (a laser in the case of

a single mode fibre). This light is sent into the fibre optic cable. The reflected light returns into

the OTDR and is bent towards the optical receiver. The time delay of the outgoing pulse

compared to the received backscattered part of the pulse is used to determine the position of

the backscatter along the fibre optic cable. The intensity and composition of the backscatterd

light is used to determine the attenuation at that particular location.

Figure 2-27: OTDR block diagram [38]

In Figure 2-28, the effect of certain connections and physical alterations along the cable are

shown. All connections will cause attenuation of the light signal. In the OTDR trace, it is clear

36

that the signal dips down after a connection and the signal has a higher attenuation compared

to a section of fibre optic cable.

Figure 2-28: OTDR trace with different kinds of connections [38]

37

3 CABLES

Within the Norther offshore wind farm, three different types of cables are used. These cables

will all have different heat losses depending on the load. For each type of cable, Marlinks will

develop cable models [37] based on the cable specifications. To gain more insight in these

models, the physical composition of such cables will be described in this chapter. Next, a

general approach to investigate heat losses in underground/submarine power cables will be

discussed. Since the DTS system is based on fibre optic cables, an introduction to the use of

fibre optic cables will be given. Finally, an analysis to determine certain section lengths of the

cable will be explained. These are the parts of cable that are not buried in the soil.

3.1 Cable information

The cables used in the Norther project will be described as infield cables and export cable.

The infield cables are the cables used to connect the strings of WTG’s (Wind Turbine

Generators) to the OHVS. The export cable is the high voltage cable that transports power

from the OHVS to shore. All cables are XLPE insulated, meaning the maximum tolerable

temperature is 90°C. If this maximum temperature is exceeded, this will deteriorate the

insulation and decrease the life expectancy of the cable. Figure 3-1 shows an overview of the

Norther OWF and the string configuration of the cables.

Figure 3-1: Overview of infield cables in the Norther wind farm

38

3.1.1 Export cable

The Norther OWF transports its produced power via a 3x1600 mm² aluminium conductor cable.

Over the total length of the cable, the cable passes through two shipping routes. When crossing

these routes, the minimal required depth of burial can be deeper than one metre. The chance

of accidental damage in those areas is larger because of the marine traffic. A deeper depth of

burial implies the heat generated by the cable will be more difficult to dissipate. Therefore a

different type of armour is used for these crossings [35]. The following types of export cable

and armour are used:

- Al, 3x1600 mm², XLPE insulated, 220 kV stainless steel (STS)

- Al, 3x1600 mm², XLPE insulated, 220 kV galvanised steel (GS)

The export cable is 26.2 km long. As visualised in Figure 3-2, the two types of armour are

interchanged based on the crossings with shipping lanes. The “Westpit” shipping route is

crossed near the OHVS. Over the length of this crossing, the stainless steel armour cable is

used. For the sections where the cable is buried without crossings of these shipping lanes, the

galvanised steel cable is used. Around the middle of the cable, the “Scheur” shipping lane is

crossed. In this shipping lane, the cable must be buried 2 metres deeper than elsewhere.

An overview of the export cable and the armour types can be found in the figures below. Figure

3-2 shows the lengths of the cable and their respective armour type. The shipping routes are

also added in this figure to give a clear view of the overlap of the armour type and shipping

routes. Figure 3-3 visualises the path the cable follows from the OHVS (KP 0) to the landfall

(KP 26.2) and beach pit.

Figure 3-2: Export cable armour types over cable length (based on [39])

Figure 3-3: Export cable installation overview [40]

The export cable is composed of several different layers. In the datasheet of the cable, the

different names of these layers are given. On Figure 3-4, these layers are visualised. It must

39

be noted that this figure is not to scale. The actual cable is vastly different in scale. The XPLE

insulation is thicker and the wire armour is quite a lot smaller on the actual cable.

Figure 3-4: Submarine export cable cross section [41]

The composition of the cable will be described layer by layer.

1. Conductor

The conductor is made out of aluminium. The core of the aluminium conductor is

circular in shape. Around this core, keystone-shaped aluminium strands are added

tightly, the further away from the core, the more rectangular these strands are. This

forms a circular aluminium conductor. The conductor strands are filled with a water

blocking compound for longitudinal water sealing.

2. Conductor screen

The conductor screen is made from an extruded semi-conducting compound.

3. Insulation

This insulation is composed of an extruded cross-linked polyethylene (XLPE). This is

the material that can withstand temperatures up to 90°C. If this temperature limit is

exceeded, the lifespan of the insulation and thus the cable will be reduced.

4. Insulation screen

The insulation screen is made from an extruded semi-conducting compound, just like

the conductor screen, and is firmly bonded to the insulation.

40

5. Water blocking layer

The water blocking layer is a semi-conducting swelling tape. This water blocking layer

is applied to prevent longitudinal water penetration. When water gets into the cable, the

tape swells, blocking more water from entering the cable.

6. Metallic sheath

The metallic sheath is an extruded lead-alloy sheath. This sheath provides a path for

the charging and short-circuit current.

7. Anti-corrosion sheath

The anti-corrosion sheath is made of an extruded semi-conducting high density

polyethylene (HDPE) and is applied over every core.

8. Filler

The filler material is a polymer, formed into a profile to house the conductors and keep

them in place. This filler also houses and protects the fibre optic cables.

9. Binder tape

A relatively thin polymeric binder tape is added around the power- and fibre optic

cables, combined with the filler. This tape binds and holds these parts together.

10. Armour bedding

The armour bedding is a propylene yarn with bitumen. The armour wires will be

wrapped over this bedding.

11. Wire armour

The wire armour is either galvanized steel wires, polyamide coated stainless steel wire

or stainless steel with bitumen. This armour wire is added over the armour bedding and

protected by the serving. The strength of the armour wires is also used to clamp and

fasten the cable on the offshore cable hang-offs, so the cable cores can be separated

and connected.

12. Serving

The serving is the polypropylene yarn, with bitumen and black and yellow stripes. Such

stripes are also added on the infield cables. These stripes are used to visually

differentiate between the different types of cables.

13. Optical fibre unit

The optical fibre unit contains a total of 48 single mode optic fibres. These fibres are

mainly used for data communication to shore. One pair of these single mode fibre optic

cables is used for the DTS measurement application.

Figure 3-5 visualises the actual cross section of the Norther export cable. As mentioned earlier,

the cross sections shown on the figure from the cable datasheet are not to scale.

41

Figure 3-5: Cross section of the Norther 1600 mm² export cable

3.1.2 Infield cables

Figure 3-1 visualises the layout of the Norther OWF and the infield cables. The figure clearly

shows two types of infield cables are used. These types are:

- Al 3x400 mm², XLPE insulated, 33 kV

- Al 3x800 mm², XLPE insulated, 33 kV

The 800 mm² cables are used to connect the first WTG to the OHVS. The other infield cables

are smaller, with a 400 mm² conductor section. The infield cables have a similar composition

compared to the export cable (the export cable composition is explained in section 3.1.1).

However, the infield cables have circular stranded conductors. The other layers are very

similar.

The 800mm² infield cable is shown in Figure 3-6. Here, a single multicore fibre optic cable is

embedded.

42

Figure 3-6: Cross section of the Norther 800 mm² infield cable

3.2 Heat losses in cables

An important factor to take into consideration when talking about power cables is the generated

heat (Joule losses) due to the load. As a result of current flowing through the conductors, heat

will be generated. This heat has to be managed, to be sure the temperature does not to exceed

the limit of 90°C in the case of XLPE insulated cables (used in the Norther OWF). If the

temperature limit is exceeded (for an extended time), it will result in a shorter life expectancy

or premature failure of the cable. There are four sources of heat losses. The first source

consists of the copper- or joule losses (𝑄𝑐) in the conductor, which are a direct result of current

flowing through a conductor with a certain resistance. Furthermore, there are iron losses in the

sheath (𝑄𝑠) and in the armour (𝑄𝑎). Finally, the insulation of the conductor causes dielectric

losses (𝑄𝑑). These losses are described in IEC 60287 and give a good insight in these different

sources. Figure 3-7 shows these different heat losses in the power cable. The calculations

were taken from [1], [42] and the international IEC 60287 standard [36].

43

Figure 3-7: Heat losses in power cables [42]

1. Conductor losses

The copper- or Joule losses in the conductor for three-phase cables are the heat losses due

to the current passing through a cable with a certain resistance. This resistance is the AC

resistance. In datasheets, the DC resistance is often mentioned. To calculate the AC

resistance from the DC resistance, the skin2 (λs) – and proximity3 (λp) effect factors are needed.

The calculations for these factors are described in the IEC 60287 standard [36]. The AC

resistance can then be calculated using equation (6). It must be noted however that the

conductor resistance is temperature dependent.

𝑅𝐴𝐶 = 𝑅𝐷𝐶 ∗ (1 + 𝜆𝑠 + 𝜆𝑝) (6)

With

𝑅𝐴𝐶 AC resistance of the conductor at maximum operating temperature [Ω/m]

𝑅𝐷𝐶 DC resistance of the conductor at maximum operating temperature [Ω/m]

𝜆𝑠 Skin effect factor [-]

𝜆𝑝 Proximity effect factor [-]

2 The skin effect of a conductor is the tendency of an AC current to flow into the outer skin of the conductor. This effect limits the effective cross-section of the conductor, increasing the effective resistance of the conductor where the current flows through.

3 The proximity effect is the tendency of the currents of two or more conductors to attract or oppose each other. This results in the currents flowing more towards the outer skin of the conductor along a more concentrated path. This effect limits the effective cross-section of the conductor, increasing the effective resistance of the conductor where the current flows through.

44

The effect of the temperature on the DC conductor resistance, as described in the IEC 60287

standard, can be denoted by equation (7). A higher temperature will result in a higher

resistance, implying less power can be transported by the cable since the extra heat will

decrease the maximum allowed current in the cable.

𝑅𝐷𝐶(𝑇) = 𝑅𝐷𝐶(𝑇0) ∗ (1 + 𝛼20 ∗ (𝑇 − 𝑇0)) (7)

With

𝑅𝐷𝐶(𝑇) DC resistance of the conductor at maximum operating temperature

[Ω/m]

𝑅𝐷𝐶(𝑇0) DC resistance of the conductor at 20°C [Ω/m]

𝛼20 Constant mass temperature coefficient at 20°C [-]

𝑇 Maximal operating temperature of the cable (90°C for XLPE insulated

cables such as been used in the Norther OWF) [°C]

𝑇0 Reference temperature (20°C) [°C]

If the AC resistance of the cable is known, these heat losses in the conductor can be calculated

with the following equation (8).

𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) = 3 ∗ 𝑅𝐴𝐶 ∗ 𝐼2 (8)

With

𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) Conductor losses for the 3 phases [W/m]

𝑅𝐴𝐶 AC resistance of the conductor [Ω/m]

𝐼 RMS current flowing through the conductor [A]

2. Iron losses (shield and armour losses)

The iron losses in the shield and armour are in fact hysteresis and eddy current losses. The

shield, referred to as metallic sheath in the cable datasheet, is made from a lead-alloy metal.

The armour wires are made out of steel (containing mostly iron).

Alternating current in the power cable will cause a time-varying magnetic field. Hysteresis

losses are due to the change in magnetic poles during the magnetisation of a metal. This

change requires some work and causes heat. The change in magnetisation, following the path

of a B-H curve, will occur each time the current changes polarity. This implies the hysteresis

losses are proportional with the frequency.

Eddy currents are current loops in a stationary conducting material, caused by a changing

magnetic field perpendicular to the stationary conducting material. This changing magnetic

field due to the alternating current induces eddy currents in the stationary metallic sheath and

armour. These eddy currents are short circuited by the metallic sheath and armour, causing

heat losses.

These losses are calculated as a factor of the conductor losses. The factors for hysteresis and

eddy current losses are described in the IEC 60287 standard. These factors can be calculated

with equation (9) and equation (10).

45

𝜆1 = 𝜆1′ + 𝜆1′′ (9)

𝜆2 = 𝜆2′ + 𝜆2′′ (10)

With

𝜆1 Loss factor shield/sheath

𝜆1′ Loss factor eddy current losses in the shield/sheath

𝜆1′′ Loss factor hysteresis losses in the shield/sheath

𝜆2 Loss factor armour4

𝜆2′ Loss factor eddy current losses in the armour

𝜆2′′ Loss factor hysteresis losses in the armour

The shield losses can then be calculated with equation (11). The armour losses can be

calculated with equation (12).

𝑄𝑠ℎ𝑖𝑒𝑙𝑑 = 𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) ∗ 𝜆1 (11)

𝑄𝑎𝑟𝑚𝑜𝑢𝑟 = 𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) ∗ 𝜆2 (12)

With

𝑄𝑠ℎ𝑖𝑒𝑙𝑑 Shield/sheath losses [W/m]

𝑄𝑎𝑟𝑚𝑜𝑢𝑟 Armour losses [W/m]

𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) Conductor losses for the 3 phases [W/m]

𝜆1 Loss factor shield/sheath [-]

𝜆2 Loss factor armour [-]

3. Dielectric losses

“Dielectrics (insulating materials for example) when subjected to a varying electric field, will

have some energy loss. The varying electric field causes small realignment of weakly bonded

molecules, which lead to the production of heat. The amount of losses will increase as the

voltage level rises. For low voltage cables, the loss is usually insignificant and is generally

ignored. For higher voltage cables, the loss and heat generated can become important and

needs to be taken into consideration.

Dielectric loss is determined based on the loss tangent or tan delta (tan δ). In simple terms,

tan delta is the tangent of the angle between the alternating field vector and the loss component

of the material. The higher the value of tan δ the greater the dielectric loss will be.” [43]

The formula for calculating the dielectric losses is a function of the phase voltage and is

denoted by equation (13).

4 For the export cable with stainless steel armour type, the loss factor of the armour (𝜆2) equals 0. This implies no armour heat losses occur with this armour type. Therefore, this type of armour is used with the deeper burial depths (passing under shipping lanes) where heat dissipation is more difficult.

46

𝑄𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 (3𝑝ℎ) = 3 ∗ 𝜔 ∗ 𝐶 ∗ 𝑈𝑝ℎ𝑎𝑠𝑒2 ∗ tan(𝛿) (13)

With

𝑄𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 (3𝑝ℎ) Dielectric losses [W/m]

𝜔 Angular frequency [rad/s]

𝐶 Capacitance of the cable [F/m]

𝑈𝑝ℎ𝑎𝑠𝑒 Phase voltage [V]

tan(𝛿) Loss factor insulation [-]

4. Total heat losses

When combining all these heat losses, the heat distribution of the export cable looks like the

distribution of Figure 3-8. It is clear that the conductors have the highest temperature. The fibre

optic cables are being heated up as well, but the temperature is vastly different from the

conductor temperatures. With the DTS system, only the temperature of the fibre optic cable

will be measured. In order to know the temperature of the conductor cores, a conversion from

fibre optic cable temperature to conductor temperature is needed. This conversion is done by

calculating the conductor temperature with an RC cable model (described in section 4.1.1).

Figure 3-8: Heat distribution in the export cable (figure courtesy of Marlinks)

Excessive heat in cables should be avoided as it decreases the life expectancy of the cable

insulation. A temperature increase of 10°C [44] will result in the life expectancy of the cable’s

insulation being halved. A temperature decrease of 10°C will result in double the life

expectancy of the cable insulation. When a cable is buried deeper, the heat dissipation will be

more difficult. This results in higher temperatures of the cables implying burying the cables

unnecessarily deep could have negative consequences on the life expectancy of the cable’s

insulation. The aim is to keep the life expectancy of the cable as high as possible, without

disrupting the normal operation.

47

3.3 Fibre optic cables

In order to understand the working principle of this measurement method, it is important to

know how a fibre optic cable works. A brief introduction to fibre optics [45] will be given in this

section. Since fibre optic splices are used in the wind farm, a brief explanation on those splices

will be given as well.

3.3.1 Fibre optic operation

Fibre optic cables are thin strands of optically pure glass. Through the glass, light pulses can

be sent over very long distances. This allows for fast communication networks since the light

pulses travels at the speed of light. Notice the glass core itself is too brittle to use as a cable.

So the glass core is surrounded by a cladding which is surrounded by a protective buffer

coating. Figure 3-9 shows the composition of a fibre optic cable with the core, the cladding and

the coating.

Figure 3-9: Fibre optic cable composition [46]

The core transports the light pulses in the fibre optic cable from end to end. The cladding

reflects light back into the fibre core, since the light does not travel along a straight path. The

buffer coating protects the fibre optic cable from external influences.

Fibre optic cables are usually made into multicore cables (with the exception of fibre optic

patch cords). This combines multiple fibre optic cables and covers them by an outer jacket. In

fibre optic cables, there are two types. The first type is a single-mode fibre and the other type

is a multi-mode fibre. The physical difference between these two cable types is the diameter

of the cores (single-mode fibre: 9 µm diameter, multi-mode fibre: 62.5 µm diameter). The

difference in core diameter is shown in Figure 3-9. With single mode (SM) fibre optics, the light

source is a laser that emits infrared light. With multi-mode (MM) fibre optics, infrared light with

a shorter wavelength is emitted from an LED (Light Emitting Diode). According to [45], plastic

cores also exist. These plastic cores have a diameter of 1 mm and transmit visible red light

originating from an LED. Multimode fibre optics is used for shorter distance data

communication (the maximum distance is 2 km), while single mode fibre optics is used for

longer distances and can be used for tens of kilometres without too much signal attenuation.

Figure 3-10: Multimode and single mode fibre optic principle [47]

48

The light does not travel through the cable in a straight line. As mentioned before, the cladding

reflects the light back into the core. Visualised in Figure 3-10, the light follows a zig-zag pattern

to reach the end of the cable. This principle is called total internal reflection. The cladding does

not absorb the light, implying the light could, in principle, travel infinitely long distances.

However, the glass contains microscopic impurities (exaggeration shown in Figure 3-11).

Consequently, light is backscattered to the source. Fibre optic cables are characterised by

their loss per unit of distance (attenuation). This will eventually lead to a light signal that is too

weak to receive at the end of a long fibre optic cable, implying fibre optic cables that are

infinitely long would not work. However, this backscattered light is essential for the DTS

system. The optical receiver will analyse this backscattered light spectrum and determine the

Brillouin frequency shift.

Figure 3-11: Backscatter in fibre optic cable [48]

Considering data communication, fibre optics show several advantages over regular copper

cables. Glass fibres are less expensive compared to copper communication cables. The fibres

are only a few microns in diameter. Copper wires are much thicker resulting in less individual

cables inside a large multi core cable of the same diameter compared to a fibre optic multi-

cable. Long copper wires will consume a part of the signal. Therefore, sensor cables are

preferably kept short in order to reduce cable losses. Thinking about copper based ethernet

cables, they may only be 100 m long in order to be able to function at the required speed and

have proper signal quality. With fibre optics, the signal has less degradation over long

distances and the limits for fibre optic lengths are much greater compared to copper wires. A

large advantage is that the optical signal is not altered in any way due to the magnetic and

electric field of the power cables, contrary to regular data communication using copper wires.

The downsides for fibre optics are mainly the more complex generator and receiver.

Additionally, when a fibre optic cable is bent too far and it breaks, it has to be welded. With

copper, one could bend the wire too tight and the wire would still not break. The copper wire

would only break after multiple of these tight bends due to metal fatigue. This implies one

should handle fibre optic cables more carefully compared to copper cables.

For the purpose of this project, the backscattering based measurement principle is not possible

with regular copper cables. The advantages and properties of fibre optic cables (especially the

distance) allow for this method of temperature measurement.

3.3.2 Fibre optic connectors

For single mode fibre optic communication, two connector types [49] are important. More

precisely the UPC (Ultra Physical Contact) and the APC (Angled Physical Contact) connectors

are important. The difference between both connectors is the finish of the endface. The UPC

connectors have a straight edge, with rounded corners. These corners serve their purpose for

better core alignment. UPC connectors will cause more reflections, returning the incident light

49

back to the source. APC connectors will reflect towards the cladding. APC connectors have an

angled end face, reducing reflections and thus resulting in less return losses (less light is

reflected back to the source). APC connectors are green and UPC connectors are blue.

Figure 3-12: Difference between UPC and APC connectors [50]

For measurement applications, losses and reflections have to be reduced as much as possible.

APC connectors have less reflection losses implying they are preferred for (DTS)

measurement applications. Manufacturers of such measurement equipment will allow a certain

attenuation over the entire fibre optic cable length (e.g. 10 dB), where they can guarantee the

proper functioning of the device. When realizing data communication, the type of connector

does not really matter, as long as the connectors are compatible.

During the construction of the wind farm, two turbines have been equipped with UPC

connectors instead of APC connectors. It was uncertain whether the DTS system would

function as intended with the UPC connectors. The OTDR test revealed that the losses are

acceptable. More information on the OTDR test results is given in section 4.2.2.3. If the losses

caused by these UPC connectors is too large, it would be necessary to replace the UPC

connectors by APC connectors. Alternatively, welding both fibres to each other could be

another option. Replacing the connector would be done either by a fuse connection or a fast

connector. With a fuse connector the fibre optic cable has to be welded to a connector. With a

fast connector, the fibre optic cable can be inserted and fastened in the connector. This change

was not carried out in the Norther OWF, since the optical losses were within the optical budget

of the DTS system.

3.3.3 Fibre optic splices

Within the Norther OWF, fibre optic splices are used. Splices [51] can cause insertion loss

which results in less light propagating through the fibre optic cable after a splice. Therefore, it

is interesting to know what kinds of splices exist and if there are any compromises by using

them.

The first kind of splice is a mechanical splice. Here, optical fibres are aligned and held in

place in an assembly. Index matching fluid is used to optimize matching the refractive index of

the optical materials. These are permanent or non-permanent splices that have a typical

splicing attenuation of 0.3 dB.

50

The second method is fusion splicing. This method will use heat to permanently join two fibre

ends together. This type of splice is also known as a fibre optic weld. In this process, a fusion

splicer is used. After checking if both surfaces of the fibre optic cables are sufficiently clean,

the splicer will move both fibres towards each other. Next the splicer will heat the connection

point using an electrical arc. After cooling down, the two fibre ends are joined together. Fusion

splices cause a typical attenuation of 0.1 dB, which is very low.

Finally, splices can be made with connectors. This approach is very useful to quickly connect

and disconnect or re-route fibres. In the Norther OWF, a lot of connectors are used in splice

boxes of wind turbines. This allows for easy OTDR measurements after installation of the

cables and it is useful for fast changes within the fibre optic network. When considering

connector splices, several different types exist. Within the Norther OWF, UPC and APC type

connectors are used. UPC connectors cause more attenuation with about 0.25 dB per

connector. APC connectors cause less attenuation at about 0.1 dB per connector.

3.4 Cable entry and CPS

The cable entering the monopile, starting from a buried cable and ending in the cable hang-

off, is built up of several sections. On Figure 3-13, a simplified version of the cable entry is

visualised. Please note this figure is not to scale.

Figure 3-13: From cable hang-off to cable burial (figure not to scale)

When performing a MBES survey, the cable is surveyed from the location where it exits the

monopile. The length from the cable hang-off to the cable entry hole of the monopile will be

51

calculated to determine the same starting location of the cable along the fibre optic cable for

the DTS-based DoB determination. This length will be referred to as the hang-off to KP0 length.

Unburied parts of the cable will have more opportunity to cool down due to the water flow

around the cable entry, but these parts are encased by the CPS (Cable Protection System).

Consequently, these factors will result in more uncertainty of the DoB determination using the

DTS system compared to buried parts of the cable. This lead to the decision that no alarms

should be raised along the section from the cable hang-off to the end of the CPS. This length

will be referred to as the hang-off to end of CPS length. To calculate both sections, the length

of the cable along these trajectories has to be known. These positions along the cable are

marked on Figure 3-13 and highlighted in yellow.

To determine these lengths, a study was conducted. Information was gathered from as-built

documents to determine these irrelevant lengths. The lengths of the cables inside each

monopile from the hang-off to the cable entry holes were calculated. Since every location is

built at a different depth, the monopile for every location is different. This implies that the length

of the cables inside each monopile is different for every location. For the CPS, several design

cases exist. These CPS lengths can be added to the hang-off to KP0 length as previously

described. These lengths have been calculated for every location and a report [52] with these

lengths has been written. The report can be found in Attachment B.

The DTS system measures the length of fibre optic cable, using the same principle as an

OTDR. The cable hang-off is defined along the fibre optic cable using the measured

temperature and anomaly caused by the fibre optic splice. Combining the hang-off location

along the fibre optic cable with the calculated hang-off to KP0 and hang-off to end of CPS

lengths ensures that the sections of cable in the monopile and unburied sections of cable

(protected by the CPS) can be determined.

With these irrelevant lengths known, Marlinks can determine a more appropriate starting point

for the start of the RTTR and depth of burial calculation. Specific zones can be configured in

the DTS system software so they can be excluded from alarm triggers.

Before looking at an example, the trajectory of the cable will be explained. Starting from the

top, the cable cores and fibre optic cables are separated at the cable hang-off on the cable

termination platform. The hang-off is a steel structure that has a flange to clamp the armour

wires of the power cable. The cable physically hangs from the armour wires of the cable

implying no forces act upon the cable cores or fibre optic cables. The cable hang-off in a wind-

turbine is visualised in Figure 3-14. Here, the separation of the conductor cores and fibre optic

cables is clearly visible. The meshed surface is the top of the cable termination platform. The

metal sections where the cables come out of are the cable hang-offs.

52

Figure 3-14: Cable hang-off on the cable termination platform of a WTG

Going further down, the cable goes through an I-tube where it is straightened and can’t move

around. After the I-tube, the cable hangs inside the monopile. Every monopile has several

cable entry holes above each other. For the first three monopiles of a string, these sets of holes

are made on two orientations to accommodate the two entering cables. These holes are the

barrier between the inside and outside of the monopile for the cables. During the cable

installation, a specific entry hole has been chosen and used for the cable entry.

In order to calculate the length of cable from the hang-off to the cable entry hole, the following

lengths have to be taken into account:

- The height of the steel hang-off piece

- The LAT height of the of the cable termination platform

- The LAT depth of the cable entry hole

- A compensation for the bend in the cable

53

The cables are not entering the monopile unprotected. For the entry of the cable into the

monopile, a cable protection system (CPS) is used. This CPS offers protection for the part of

the cable that is not buried and would otherwise be exposed to the elements. The CPS is

several tens of metres long and consists of a regular cable protection array (CPA) and

articulated pipe. The CPA protects the cable from the cable entry hole of the monopile, as well

as the rough rocks of the monopile scour protection (more information on scour protection can

be found in chapter 7). The CPA parts are the orange and black parts visualised in Figure 3-

15. The CPS ends with an articulated pipe of several metres long (yellow parts on Figure 3-

15). When the cable is buried in the ground, no more external protection is necessary. This

implies the unprotected power cable has direct contact with the soil.

Figure 3-15: Norther cable protection system spare parts

In the Norther OWF, rocks were dumped on the CPS between the cable entry hole and the

burial of the cable in the seabed. Since those sections of the cable are not buried in the soil,

no alarm for the depth of burial should be raised for these sections.

In order to be able to compare the measured depth of burial of the DTS to the measured depth

of burial from the surveys, the appropriate length of the cable should be determined. In the

survey data, the start of every cable burial is the location where the cable exits the monopile.

The aim is to get a similar starting location along the cable for the measured survey depth of

burial and calculated depth of burial from the DTS system.

By analysing the temperature profiles generated by the DTS system, the hang-off location can

be determined. In order to determine the cable entry-hole along the cable length, some as-

built documentation was consulted.

54

Let’s look at an example that illustrates the determination of these lengths. The cables entering

WTG D2 have been chosen to give this example. Cable D2 is routed from WTG D1 and enters

the monopile of WTG D2. From the as-built data that was gathered, the length of the cable

from the hang-off to the cable entry hole of cable D2 in the monopile is calculated at 36.41 m.

The length of cable from the cable hang-off to the end of the CPS is calculated at 55.67 m.

From the DTS system configuration, the lengths and corresponding temperatures can be

derived. The hang-off location along the fibre optic cable was determined by Marlinks. The

hang-off of cable D2 along the fibre optic cable is determined at 2799 m and denoted by “Cable

D2 hang-off” in red on Figure 3-16. This position along the fibre optic cable and the

corresponding temperature profile can be found in Figure 3-16. Please note that the

temperature spikes are from the fibre optic connections. When deducting the cable length

inside the monopile from the hang-off position, 2799 m – 36.41 m results in the cable entering

the monopile at about 2762.59 m. This position has been marked in blue by “Cable D2 entry”.

The position of the start of the cable burial (and the end of the CPS) can be calculated by 2799

m – 55.67 m, resulting in the position of 2743.33 m. This position is marked in blue by “Cable

D2 burial”.

The second cable entering the monopile is cable D3, connecting WTG D2 to WTG D3. Here

the length of the cable from the hang-off to the entry of cable D3 in the monopile is calculated

at 35.81 m. The length of cable from the cable hang-off to the end of the CPS is calculated at

55.07 m. The mentioned lengths should now be added to the cable hang-off position, in order

to obtain the same starting point as the survey depth of burial determination. On Figure 3-16,

the hang-off location of cable D3 is chosen at 2811 m and denoted by “Cable D3 hang-off” in

red. This results in 2811 m + 35.81 m = 2846.81 m to be the cable entry position. This position

is marked in blue as “Cable D3 entry”. The end of the CPS would be at 2811 m + 55.07 m =

2866.07 m along the cable, which cannot be indicated on Figure 3-16.

Figure 3-16: Cable starting point determination

55

4 PRELIMINARY WORKS

Before the DTS system could be installed on the offshore high voltage station (OHVS),

preliminary works were required. This approach minimises the offshore workload and it also

minimizes possible issues in the future. Design and engineering as well as installation and

testing had to be done. During the design and engineering cable models were developed and

the proper input parameters for the calculations were determined. The installation and testing

includes the factory acceptance test of the DTS system, making changes to the fibre optic

network of the wind farm and testing it, as well as preparing the DTS cabinet for the installation

of the DTS interrogator and optical switch.

4.1 Design and engineering

A substantial part of the design work is related to the cable report [37]. The goal is to

dynamically calculate the thermal behaviour of the Norther submarine cables. Marlinks wanted

to make models of the cables and therefore they needed specific parameters of the three types

of cables used in the Norther OWF. Most parameters were provided via datasheets and

specific parameters were asked to the cable manufacturers directly. Cable models for all three

Norther submarine cables were made by Marlinks. These cable models include all necessary

parameters to dynamically calculate their thermal behaviour. For the export cable, the cross

section of the model can be found in Figure 4-1.

Figure 4-1: Cable model - export cable drawing [37]

4.1.1 RC thermal cable model

“The thermal model, developed by Marlinks, is based on a modified version of the international

IEC 60287-1-1 standard. The equivalent thermal circuit is shown in Figure 4-2. The model is

56

defined by the conductor temperature 𝑇𝑐, the fibre temperature 𝑇𝑓, the armour temperature 𝑇𝑎

and the boundary temperature 𝑇𝑏 . The thermal resistances 𝑅 and thermal capacitances 𝐶

define the thermal behaviour of the system. 𝑅1 and 𝐶1 are the thermal properties between

conductor and fibre, 𝑅2 and 𝐶2 between fibre and armour and lastly, 𝑅3 and 𝐶3 are the thermal

properties between armour and boundary.” [37] These thermal resistance and thermal

capacitance values have been determined by Marlinks and added in the cable report [37]. The

model of the IEC standard does not take the temperature of the fibre optic cables into account.

The Marlinks model includes this temperature and that is an important difference. The thermal

RC-model will be used to calculate the conductor core temperature based on the measured

fibre temperature. The model mentioned in the IEC standard does not take the cancelling effect

of armour currents due to the helical laying pitch of the cores and armour into consideration,

resulting in an overestimation of the heat losses of the armour [53]. The model developed by

Marlinks should give a more accurate estimation of the heat losses and temperatures in the

cables.

Figure 4-2: Marlinks thermal RC cable model [37]

The heat sources shown in Figure 4-2 (Q1 and Q2) represent the heat losses in the cable. The

heat losses are a combination of the conductor-, iron- and dielectric losses. These losses

depend on the distributed temperature and voltage as explained in section 3.2. Equation (14)

and equation (15) represent these heat sources.

𝑄1 = 𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) + 𝑄𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 (3𝑝ℎ) + 𝑄𝑠ℎ𝑖𝑒𝑙𝑑 (3𝑝ℎ) (14)

𝑄2 = 𝑄𝑎𝑟𝑚𝑜𝑢𝑟 (3𝑝ℎ) (15)

With

𝑄1 Three-phase heat losses from source 1 [W/m]

𝑄2 Three-phase heat losses from source 2 [W/m]

𝑄𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟 (3𝑝ℎ) Conductor heat losses [W/m] (7)

𝑄𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 (3𝑝ℎ) Dielectric heat losses [W/m] (12)

𝑄𝑠ℎ𝑖𝑒𝑙𝑑 (3𝑝ℎ) Sheath heat losses [W/m] (10)

𝑄𝑎𝑟𝑚𝑜𝑢𝑟 (3𝑝ℎ) Armour heat losses [W/m] (11)

57

“The temperatures in the cable also depend on the depth of burial (DoB), the thermal

conductivity (𝑘) of the soil and the sea water temperature (𝑇𝑆). With this knowledge, the thermal

resistances and capacitances are calculated by Marlinks, using a finite element model (FEM).

To obtain this result, a 2D cable drawing is made, as shown in Figure 4-1 for the 3x1600 mm²

cable.” [37]

The FEM method is only used to evaluate the RC model. Using the FEM model as the

calculation model would require a lot of computational power. The thermal RC model is used

for calculations to obtain the conductor core temperature (𝑇𝑐). Figure 4-3 shows the correlation

between the FEM model and the thermal RC model. The RC model and the FEM model deliver

nearly identical results. This implies it is more efficient to use the thermal RC-ladder model for

the temperature calculations. The correlation between the measured temperature of the DTS

system and the calculated model temperatures will be clarified in chapter 5.

Figure 4-3: Comparison between FEM model and RC-ladder model thermal response [54]

4.1.2 Transport of electrical power

On every wind turbine, the generated power is measured before the transformer at a voltage

level of 640 V. The general setup is visualised in Figure 4-4. In order to calculate the heat

losses along the cable, the current and voltage distribution along the transport cables, after the

transformer, should be calculated.

To obtain the current flowing in the transport cable, the measured active and reactive power

before the transformer is converted to the power after the transformer. However, in order to

obtain the active and reactive power from after the transformer, the absorbed active and

reactive power of the transformer should be calculated, as it is included in the total active and

reactive power before the transformer. The absorbed active and reactive power can be

obtained from the measured reactive power before the transformer, to obtain the active and

reactive power transported by the cable. According to the cable report [37], the consumed

reactive power of the WTG transformer at full load (943 kVAr) is almost eight times higher than

the consumed active power (123 kW) at full load.

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Figure 4-4: WTG configuration and power measurement

The active power and reactive power of the transport cables allow to calculate the current at

every wind turbine injection point. They also allow to calculate the corresponding heat losses

along the cables. Equation (16) is given in the cable report [37] and is used to approximate the

consumed reactive power of the transformer.

𝑄𝑇 = 𝑄0 + %𝑧

100∗ (

𝑆𝐿𝑜𝑎𝑑

𝑆𝑇𝑅)

2

∗ 𝑆𝑇𝑅 (16)

With

𝑄𝑇 Total reactive power consumed by transformer [VAr]

𝑄0 Reactive power consumed by shunt magnetizing reactance of transformer (no

load) [VAr]

%𝑧 Transformer percentage short circuit impedance [%]

𝑆𝐿𝑜𝑎𝑑 Transformer apparent load power [kVA]

𝑆𝑇𝑅 Transformer rated apparent power [kVA]

After obtaining the injected power and current in the 33 kV infield cables, the load along the

cable is calculated and visualised in Figure 4-5. Consequently, the voltage and current along

the cable can be derived. This results in a voltage and current value along the cable. The

current along the length of a cable is nearly constant and the voltage may drop about 50 V

from the start of an infield cable to the end of an infield cable. Please note 50 V is about 0.15%

of 33 kV, hence the voltage drop might seem significant but is in fact very limited.

The produced power of the wind farm is measured on the OHVS before it is transported to

shore via the export cable. The high voltage load data can be used to calculate the heat

generation of the export cable.

59

Figure 4-5: Current and voltage distribution along a string of infield cables

4.2 Installation and testing

4.2.1 Factory Acceptance Test

For large projects, specialised equipment is often required. Since these devices may not be

off the shelf types of equipment, no standard test procedure is done after the production, hence

a FAT is carried out in the presence of the customer. FAT is the abbreviation of Factory

Acceptance Test. This means the manufacturer will test the DTS system at the factory. During

the FAT, tests are carried out to ensure the precision and repeatability of the DTS system. If

test criteria are met, the system receives a pass for the test. This test may carry contractually

agreed terms of payment, depending on the agreements.

For the Norther case, the DTS system was tested and a report was written by the manufacturer.

Since the representatives of Marlinks and Norther were unable to attend this test in person

due to travel restrictions, a meeting was organised to discuss the FAT-report with the

manufacturer.

Three tests were conducted and each test will be explained in this section. An overview of the

test setup is shown in Figure 4-6. The first test verified if all materials were present and listed

on the bill of material. This test was passed.

The second test dealt with monitoring the temperature along fibre optic cables. The DTS

interrogator has a single optical laser and a single optical receiver. In order to measure all

channels, the light is sent in and received from the different channels by internal switches in

the DTS interrogator and optical switch, i.e. the system sequentially switches between the

channels. This implies that testing the temperature along a fibre optic cable on two channels

60

is more than enough to test the systems capabilities for measuring along long fibre optic

cables. All other channels can be tested using a shorter length fibre optic cable to check the

proper sequential working. In this second test, two long fibre optic cables were submerged in

a thermal bath with a known temperature. The thermal bath is filled with water and has an

average temperature of 20°C to 60°C. The temperature along the two long fibre optic cables

could be determined and the DTS system sequentially switched to the remaining channels,

measuring the temperature along those fibre optic cables. Consequently, this test was passed.

Figure 4-6: FAT setup [55]

The third test verified the precision and repeatability of the DTS system. For this test, five

measurements were performed sequentially on each of the two channels that were submerged

in the thermal bath. The measured values should be as close as possible to the reference

temperature (50°C for the first fibre optic cable) and all five measurement results should be

close to each other. The result of the repeatability test of the Norther DTS system is visualised

in Figure 4-7. Here it is clear that all temperatures have a very limited deviation from the 50°C

reference temperature and the results are very similar for all 5 tests. During this third test, the

measurement time was recorded. Every measurement had to finish within a certain timeframe.

Figure 4-7: FAT temperature repeatability test at 50°C [55]

61

According to the test report [55], all tests were passed. The test report and meeting gave good

insights about the working principle of the system and how it can be configured.

4.2.2 Offshore works

Before installing the DTS interrogator and optical switch, some work had to be carried out to

accommodate the installation. On the OHVS, the last preparations in the DTS cabinet had to

be done. On the turbines, the loop of every string had to be completed and tested allowing the

DTS system to measure the temperature as intended.

4.2.2.1 DTS cabinet

The DTS cabinet itself and the appropriate cables were installed based on information provided

by a previous master thesis of academic year 2019-2020 [1]. A proposal for the fibre optic

configuration was prepared. In order to reduce the possibility of further optical losses, some

changes were made to the setup. A splice panel using connectors would be replaced by a weld

box, since fibre optic welds cause less attenuation compared to splices with connectors.

The appropriate provisions and safety equipment still had to be installed. This includes the

power outlets for the measurement equipment as well as a switch for the redundant power

supply, fed by two separate UPS’s. This equipment had to be mounted to an electrical panel.

After this work was completed, the DTS interrogator and optical switch could be installed

offshore. On Figure 4-8 the installed electrical panel is visualised. At the other side of the

cabinet, the DTS interrogator and optical switch are installed. On the bottom of the figure, the

back side of these devices (with the power cord and fibre optic cables) is shown.

Figure 4-8: DTS cabinet - electrical panel

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4.2.2.2 Loop completion

For every infield cable, two fibre optic cables are reserved for the DTS measurement

application. On the first three turbines of a string, these two fibre optic cables are connected

to the next turbine. On every fourth turbine in a string, the ends of both fibre optic cables arrive.

The goal is to be able to connect fibre optic strings together. Therefore the loop of the two fibre

optic cables must be completed. On Figure 4-9 a fibre optic string connection is visualised.

The loop itself is circled in green.

Figure 4-9: Fibre optic string and loop

In order to complete the fibre optic loops, a short fibre optic cable has to bridge both ends of

each fibre optic string. This will be done by visiting these turbines and installing the short fibre

optic patch cable. As a preparation for these works, the fibre optic splice plan for the DTS

system was changed to have the loops completed with the additional components added and

connected. The full fibre optic splice plan can be found in Attachment C.

4.2.2.3 Optical Time-Domain Reflectometer

In Figure 4-10, the effect of certain connections and physical alterations along the cable are

shown. As explained before (in section 3.3.2 and 3.3.3), all connections will cause attenuation

of the light signal. In the OTDR trace, it is clear that the signal dips down after a connection

and the signal has a higher attenuation compared to a section of fibre optic cable. The UPC

connectors will cause a spike in the OTDR trace due to backscatter caused by the straight

edge. In Figure 4-10, this effect is similar to the illustrated “connector pair” case. Meanwhile,

the APC connectors will have a slope which is comparable to a “fusion splice” on Figure 4-10.

The attenuation of a UPC connector is higher than the attenuation of an APC connector.

63

Figure 4-10: OTDR trace with different kinds of connections [38]

Figure 4-11 shows an OTDR trace of one of the strings in the Norther OWF. The fibre optic

cables are made into a loop (from the OHVS to all turbines of a string and returning along a

second fibre optic cable, back to the OHVS), causing a mirrored effect in the OTDR trace. In

string A and string D of the Norther OWF, one of the turbines has different connectors, which

cause more attenuation. On WTG-A1, LC/UPC connectors were used instead of the less

attenuating LC/APC connectors. The spikes in the trace originate from these more attenuating

connectors. Since the ongoing and returning fibre both have the same type of connector, the

spikes can be found at both ends of the trace. These loops are made to accommodate for

multiple strings to be combined and the returning fibre can be plugged into the DTS interrogator

or optical switch. The return signal of the fibre is used internally and provides more information

to the device. The exact use of this return signal is only known to the DTS manufacturer.

Figure 4-11: String A OTDR test result

In OTDR analysis software, these spikes are referred to as reflections, whilst the regular

attenuation drops are referred to as splice losses. If this channel does not meet the optical

budget specification (10 dB per channel), a change to a better connector can be done and

another OTDR test can be conducted on this channel. After the OTDR tests were completed,

these particular strings with the UPC connectors met the optical budget. Since these particular

UPC connectors are the only more attenuating connectors in this entire string, the result is

acceptable. For the 11 kilometre trajectory, the loss is about 0.352 dB/km. No changes had to

be made in order to reduce these optical losses. Due to the low optical losses, multiple strings

can be connected and still meet the optical budget specification. There were some concerns

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regarding the added reflections from the UPC connectors. These reflections could blind the

photo detector of the DTS interrogator. Results from the field tests of the Norther DTS system

revealed that there were no problems with these connectors and their reflections.

4.2.2.4 Installation of the DTS system

The DTS system is installed in the DTS cabinet on the Norther OHVS. All fibre optic loops were

connected to the DTS interrogator and its optical switch. Since everything was prepared,

installing the system was ‘plug and play’. After installing and connecting the device, Marlinks

verified that all systems worked properly. After the system was successfully installed offshore,

the calibration of the system could commence. This calibration will be explained in chapter 5.

Figure 4-12 shows the front of the DTS interrogator and the optical switch. On Figure 4-13, the

back of the DTS interrogator and optical switch, as well as the power connections and the fibre

optic connections can be seen.

Figure 4-12: Front of the Omnisens DTS interrogator and optical switch

Figure 4-13: Back of the Omnisens DTS interrogator and optical switch

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4.2.3 Problems along the way

Initially, the project aimed for the DTS to be operational by the summer of 2020, but the project

realisation required more time than expected. The contract signing was shifted to the end of

the summer of 2020. The collaboration between Marlinks and Norther started at the beginning

of September 2020. Due to several setbacks, the installation of the DTS system took quite a

bit longer than anticipated. This was largely due to weather conditions.

4.2.3.1 Weather conditions

By the end of October and start of November 2020, most of the engineering work was finished.

Now the offshore works could start when a good weather window was available. However, due

to weather conditions at that time of the year, the DTS system could not be installed as soon

as originally planned. Some works had to be carried out before the system installation and that

got delayed week after week since weather conditions did not allow offshore works to be

carried out. This lead to a further delay in the project of about 2 months.

4.2.3.2 Fibre issues discovered by the OTDR test

When testing the fibres using the OTDR, some fibre optic loops appeared to be shorter than

expected. After measuring in both directions, it seemed like there was a fibre break situation

in these loops. Upon inspection of the connections on the WTG’s, a split in one of the

connection joints, visualised in Figure 4-14, was discovered. This lead to an incomplete loop

as the connector had slid down and interrupted the circuit. The connector was replaced and

after repeating the test, the loop passed the test.

Figure 4-14: Cracked fibre optic connector

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5 DTS SYSTEM CALIBRATION AND DATA PROCESSING

After the installation of the DTS system on the Norther OHVS at the end of February 2021, the

system had to be calibrated. The temperature calibration process will be described in this

chapter. Using this calibrated temperature, the RTTR of the cable can be calculated. When

these systems are fully operational, the SAT (Site Acceptance Test), where the proper

functioning of the systems is demonstrated, can be conducted.

When a sufficient amount of (historic) temperature data is collected, the cable depth of burial

calibration can be performed. Depending on the results, some alterations to the calculation

may have to be made. After the first calibrated results of the DTS-derived cable depth of burial

are available, they can be compared to the reference depth of burial results from the MBES

survey.

5.1 DTS temperature calibration

In order to calibrate the temperature measurement of the DTS system, standalone temperature

sensors were mounted close to the cable hang-off on the OHVS. Temperature sensors were

also mounted in some WTG’s. These temperature sensors logged the temperature over one

or multiple days. It is important to monitor the behaviour of the backscattered light of the DTS

system over a wide range of temperatures to calibrate the temperature measured by the DTS

system.

The temperature measured by the standalone sensors is correlated to the Brillouin frequency

shift, measured by the DTS system. As already mentioned in section 2.2.3, the coefficient of

temperature can vary from 1 MHz/K to 1.4 MHz/K depending on the fibre optic cable properties.

Marlinks measured a high temperature and low temperature case with a reference

thermometer and determined the frequency shift due to this temperature difference with the

DTS system. Since the temperature at the cable hang-offs is relatively stable, external heating

elements were used to provoke a large temperature difference. The results are visualised in

Figure 5-1. This data allows for a calculation of the temperature coefficient using a regression

line. The slope of the line is the temperature coefficient. The temperature coefficient for the

fibre optic cables used in the Norther OWF is determined at 1.113 MHz/K. This value can be

used as the 𝐶𝑣𝜃-value in equation (4). As mentioned before, the temperature coefficient is

greater than the coefficient of strain. Consequently, the effects of strain will be hardly

noticeable in the temperature measurement.

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Figure 5-1: Calculation of the temperature coefficient of the fibre optic cables [56]

This temperature coefficient as well as the offset/intercept of the temperature can be set up in

the DTS system software. The exact alterations in the DTS system with these parameters are

unknown, since this is specialised knowhow of DTS manufacturers. The basic working

principle of the DTS-based temperature determination is well known and has been explained

in section 2.2. The accuracy of such a system is about 1°C.

5.2 Real Time Thermal Rating

Marlinks will provide Norther with an RTTR software module with two operating modes. The

first mode is entirely based on international standards. This will give an indication on the cable

rating using fixed parameters. The second mode should be more accurate as it takes all cable

details and the depth of burial from their DoB calculation into consideration. RTTR starts with

calculating the conductor temperature, based on the measured fibre temperature. Next, hot-

spot alerts will be given and the maximum ampacity for the next 6, 12 and 24 hours are

calculated. In a last step, simulations will be carried out to calculate the thermal behaviour of

the cable, based on various load profiles [57].

5.3 Depth of Burial

The length of the buried cable is different from the absolute length of the fibre optic cables and

the power cores. This difference is due to the helical shape of the twist in the fibre optic and

power cables. The length of the fibre optic cables is approximately 3% longer than the lay

length5 of the cables. In Figure 5-2 the helical twist in a subsea cable is clearly visible.

5 The lay length of a cable is the distance of the cable in a straight line. The pitch or spiral/helical twist causes the fibre optic cables and conductor cores to be longer than the straight line length of the cable.

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Figure 5-2: Subsea power cable example [58]

To determine the DoB, the Marlinks calculation model needs input parameters. These input

parameters can be found in the scheme on Figure 5-3. The input parameters include the

produced power of the wind turbines to determine the load of the cables, the Marlinks RC cable

model to determine the conductor core temperature and various other parameters. All the

parameters are used to determine the DoB with an iterative calculation process.

Figure 5-3: Marlinks DoB determination algorithm [59]

The depth of burial is determined via an algorithm that calculates the temperature at the fibre

location. In order to calculate this temperature at the fibre location, the heat generation in the

cable itself and the heat propagation in the soil have to be determined. The heat generation in

the cable is dependent on the load. The active and reactive power as well as the voltage for

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every wind turbine and for the export cable is provided by Norther via a REST API 6

(Representational state transfer Application Programming Interface). This way, Marlinks can

download the required load data and use this information as an input parameter. Additionally,

a cable model is used to dynamically calculate the effect of the load on the cable. This cable

model has already been described in section 4.1.1. Based on that information, the heat

generation in the cable can be determined.

In order to calculate the heat propagation in the soil, the temperature of the sea has to be

known. Therefore the sea water temperature is imported from “Meetnet Vlaamse Banken” [60].

This is a public database of the Flemish government with maritime data. This temperature is

used as the 𝑇𝑎𝑚𝑏 value of the RC-ladder model of Figure 4-2.

The deph of burial determination algorithm needs the cable and soil thermal properties as input

parameters. Marlinks uses a thermal RC soil model, validated by lab experiments [54], to

determine the thermal properties of the soil at a specific location. The single value of heat

generation in the cable and an array of possible heat propagations in the soil (within boundary

values) are combined. With these values and the cable RC model, the temperature at the fibre

location is calculated iteratively with the same heat generation in the cable and another value

for the heat propagation in the soil for each iteration. These calculated values are compared

to the actual measured temperature at the fibre location along the cable. All possible

combinations are calculated and a score is given for every possibility. The k-value and DoB

value of the combination with the lowest score, i.e. the best fitting result, will be saved. The

least square error method using the temperature at the fibre location is visualised in Figure 5-

4. When the score equals zero (or the score is minimised), the calculated temperature (closely)

matches the measured temperature. The two input parameters (k-value and DoB) for this

location are now determined.

Figure 5-4: Marlinks least square error method [61]

6 The REST API is a secure communication link that allows specific data to be downloaded and imported from a server via an automated request. This way, the specifically requested data can be used for the data processing.

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The method of DoB determination visualised in Figure 5-4 uses 20 DoB values and 20 k-

values. This implies 400 calculations have to be done for every measured location. The DTS

system measures the temperature of the cable every 2 metres. Consequently, a DoB

calculation can be performed on the same interval. For every temperature value, the k-value

and the DoB value are determined. An interpolation of the k- and DoB values results in a k-

value and a DoB value determined for every metre of cable. Figure 5-5 shows an example of

these results.

Figure 5-5: Example of k-value and DoB determination [61]

For the Norther export cable (approximately 26.2 km long), about 13100 measured

temperature values are used. Every temperature value is used in the iterative 400-point

calculation. Consequently, over 5 million calculations are done to determine the k-value and

the DoB value along the export cable. Marlinks also interpolates the 13100 calculated k-values

and DoB values, adding an extra value in between the 2-metre space. Using this method, the

k-value and DoB value are determined for every metre of cable, i.e. 26200 datapoints per

parameter.

It should be noted that Marlinks is working on a fitting algorithm that should significantly reduce

the required calculation time. The fitting algorithm would use neighbouring values as an

optimisation instead of the rigid 20x20 values. The calculation model would try a few points

first and then proceed to calculate the local minimum. At the time of writing, this feature is not

implemented yet. However, updates of the Marlinks software will provide the Norther system

with this feature when it becomes available.

When comparing the measured temperature by the DTS to the calculated temperature of the

calculation model (the closest values), the results are very similar and follow the same pattern.

Figure 5-6 illustrates this correlation with the measured fibre temperature in orange and the

calculated fibre temperature in blue. This implies the method of calculating the cable

temperature works fine and the k-value and the DoB value are reliable. The visible differences

may originate from load peaks since the load of the cable is the most important factor of the

cable temperature.

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Figure 5-6: Comparison between measured and calculated temperature [54]

As mentioned before, the DoB value will be calculated for every metre of cable. On a two week

basis, Norther will receive an update on this DoB value. The vertical accuracies of the DoB

determination are described in [61] and are as follows: 15-20 cm on 0 m up to 0,5 m; 20-25

cm on 0,5m up to 1 m; 25-30 cm on 1 m+ and beyond. All these accuracies depend on the

environment i.e. the soil type.

5.4 Site Acceptance Test

The site acceptance test (SAT) for the DTS system consists of verifying two protocols. First,

the installation and calibration protocol was checked, followed by the overall commissioning

protocol. After the successful SAT in the beginning of May 2021, Norther officially took over

the DTS system.

The installation and calibration protocol starts with a visual inspection of the installed

equipment on the Norther OHVS. Proper installation of the equipment, as well as labelling is

checked. Next, the electrical installation is checked. Here the electrical connections are

checked for proper connection and the power fallback switch for both UPS systems is tested.

The next verification is the network test. In this test, the network parameters are verified and

the access to the network devices was tested. Furthermore, the OTDR test results and fibre

optic connections were checked after all repair works were completed. The attenuation of all

strings were within the optical budget. Finally, the calibration of the DTS system itself was

verified for all locations.

The overall commissioning protocol started by explaining the temperature calibration process

and its results. Next, the measured temperatures were compared to the temperatures

visualised in the Marlinks UI (user interface). The RTTR function was tested using simulated

values. Finally, the Marlinks UI was checked to verify if all required information is shown.

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6 ENVIRONMENTAL PERMIT AND DTS

As part of Norther’s environmental permit, the cables of the Norther offshore wind farm have

to remain buried under a minimal burial depth (1 metre in most cases). These burial depths

are currently monitored by surveys, as described previously in section 2.1. These check-up

surveys are done once a year. In the ministerial decree, it is stated that the method of

monitoring the burial depth of the cables may be changed if the method is equivalent. Therefore

an approval of the minister is necessary to change the cable depth of burial monitoring method.

In order to ask the governing authorities if the cable burial depth monitoring method may be

changed to the DTS system method, an application explaining the idea was submitted by the

end of 2018. After the approval from the BMM, Norther could start a tender procedure where

developers of such systems could apply. This work was carried out during academic year

2019-2020 by the previous thesis student and is described in the master thesis [1] of that

student. Currently, the system is installed on the OHVS. A document explaining the working

principle and the first results of this DTS-based depth of burial determination needs to be set

up for the BMM. The DTS-based DoB calculations will be compared to the latest reference

survey data to substantiate the results.

6.1 Setup of the document

An introduction to the use of the DTS system and its benefits are described in this document.

The document will also contain case studies, showcasing the performance of the DTS

monitoring method. Moreover, the difference between the survey-monitoring method and the

DTS monitoring method will be described.

In order for the DoB calculation to provide reliable results, about three months of temperature

data is required. The temperature monitoring of the cable started in the beginning of May 2021.

Consequently, the first DoB results are expected to be available by the end of the summer.

Therefore, an initial body of the document will be set up and all preparations will be done.

These preparations include a tool to compare the survey results to the calculated DoB results

from the DTS system.

When reliable DoB data is available from the DTS system, Marlinks will provide a calibration

report where the performance of the system is demonstrated. In order to substantiate these

results, Norther will verify the results using an independent method. This method will compare

the results of both depth of burial datasets. A visual representation of both profiles, as well as

the correlation coefficient between the two datasets will be available. The independent

verification method gives Norther the flexibility to compare every zone they wish to compare.

The final version of the document is expected to be finished after the first results of the DoB

calculation are sufficiently stable and the results are reliable, i.e. the calculated results match

the surveyed results.

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6.2 Case studies

In order to obtain relevant case studies, a few different scenarios should be checked. As an

introduction to how these cases should be chosen, different conditions within the Norther OWF

will be described. Soil conditions, specific burial zones and crossings will all have an effect on

the heat dissipation of the cable, hence the case studies should reflect these different

conditions.

6.2.1 Soil conditions

Within the Norther OWF, several different cable burial cases exist. Figure 6-1 visualises the

zones within the concession and their particular thermal soil characteristics i.e. the thermal

resistivity of the soil. For the DoB calculation these parameters will be useful as they will be

the indicative value for the calculated k-value. Moreover, the thermal resistivity will have an

impact on the heat dissipation in the soil. When a cable is buried deeper in the soil, it will be

harder to dissipate the heat in the soil.

Figure 6-1: Thermal resistivity of the soil in the Norther OWF [62]

The values mentioned in Figure 6-1 imply the difference in thermal resistivities within the wind

farm is rather limited. When cables are buried in the soil with a depth of burial of about 1 to 2

metres, they are often buried in a clay layer (depending on the location). Generally, the

composition of the soil where a cable is buried will be very similar along the cable trajectory.

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6.2.2 Cable burial

In most cases, the cable is buried in the soil. In these cases, the DoB calculation should

perform optimally. Such a case is visualised on Figure 6-2, to the left side of “KP 3.200”.

Sections along the cable corridor where this is not the case, such as cable crossings, cable

inside the CPS, cable entry in the monopile are different cases. One of these cases (cable

entry in the monopile) is visualised on the far right in Figure 6-2. Here, the cable rests on the

scour protection rock layers (described in chapter 7) and are covered by a rock berm.

Figure 6-2: Infield cable I1 regular burial section and cable entry - side view [63]

Typical for these cases compared to the buried case, is a cooling effect due to increased water

flowing through the cavities of the present rock berm or along the cable in freespan.

6.2.3 Cable crossings

The wind farm of Norther is the closest to the coast of all Belgian wind farms in the North Sea.

This implies all cables of other wind farms have to pass next to or through the Norther OWF.

As a result, a lot of crossings are present in the Norther wind farm. Besides crossings with

cables of other offshore wind farms, pipelines run through the concession zone. On Figure 6-

3, the borders of the Norther OWF have been highlighted in yellow. All coloured lines are third

party assets such as cables from other wind farms or pipelines. The Norther WTG’s are

included on this figure. Crossings of these third party assets with the infield cables of the

Norther OWF are inevitable.

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Figure 6-3: Third party assets within the Norther OWF

6.2.3.1 Cable and pipeline crossings

In order to protect all cables and pipelines, crossings will receive special attention. In Figure 6-

4, such a crossing is visualised. In this case, the interconnector pipeline (between Zeebrugge

and Bacton (UK)) is buried along the path where an infield cable of the Norther OWF is

installed. The pipeline is represented by the purple dotted line and the infield cable is

highlighted in yellow. The cable crossing is highlighted in orange.

Figure 6-4: Infield cable I1 crossing with pipeline – map view

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On Figure 6-5 the cross-sectional view of the crossing with the pipeline is shown. The

interconnector pipeline has a diameter of slightly over 1 metre. The cable is laid on top of a

rock bed and protected by more rocks on top of the cable, to comply with the 1 m DoB

requirement. This implies the cable is not buried in the soil, but rather protected by a rock berm.

Figure 6-5: Infield cable I1 crossing with pipeline – cross-sectional view [63]

The top view of the crossing is visualised in Figure 6-6. The top part of the figure shows the

rock layer before the cable was installed. Here, it is clear that the section where the pipeline

effectively crosses is equipped with a rock bed on top of the pipeline. The trench where the

cable will be buried in the soil can be clearly distinguished. On the bottom part of the figure,

the rock layer has been installed on top of the cable. It is clear that the rock protection on top

of the cable is not a confined zone, but rather a long zone along the length of cable within 50

metres on either side of the crossing.

Figure 6-6: Infield cable I1 crossing with pipeline - topographical view [63]

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The as-laid cable length for cable I1 is about 3250 metres. At about 900 metres along infield

cable I1, the “Concerto” telecommunication cable is crossed. At 1950 metres along the infield

cable, the interconnector pipeline is crossed. At these crossings, a temperature drop can

clearly be identified on the Marlinks user interface, visualised in Figure 6-7. For both crossings,

a rock berm is used to protect the Norther infield cable. The rocks allow for water to flow

through the cavities. Consequently, the cable can cool down along these rock berms, resulting

in temperature drops along the infield cable. Due to movement of the soil, the cavities of the

rock layers will be filled by sand, forming a more solid mass, resulting in a different soil

composition over time.

Figure 6-7: Temperature profile along infield cable I1

In the case studies for the BMM, such a crossing is an interesting case to check whether the

DTS-based depth of burial calculation is done correctly.

6.2.3.2 Shipping lane crossing

The Norther export cable has a deeper depth of burial requirement along the Scheur shipping

lane crossing. This crossing is located at about 15 kilometres along the export cable. Due to

the deeper depth of burial requirement of 3 metres instead of the regular 1 metre along the

Scheur shipping lane crossing, the heat in the cable is more difficult to dissipate. This results

in a higher cable temperature. Similar to the infield cable crossing, this shows its effect in the

temperature profile along the cable. In Figure 6-8, the temperature profile along the export

cable is visualised. At the 15 kilometre mark along the export cable, an increase in the cable

temperature can be clearly identified.

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Figure 6-8: Temperature profile along the Norther export cable

6.2.4 Cable paths

On Figure 6-9 the sections where rocks have been placed to protect the cables are highlighted

in green. These zones can either be cable crossings or zones where the cable was not buried

deep enough.

Figure 6-9: Infield cables rock dumps [64]

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6.2.5 DoB result verification method

Marlinks aims to get a DoB value for every metre of cable, just like the surveyor.

The DoB calculation is designed to provide this data. However, it should be noted that the fibre

optic cable inside the power cable has a helical twist. This results in a longer fibre optic cable

than the lay length of the cable itself. Since the surveyors only use the external length of the

cable (the lay length), the calculation from the fibre optic cable may suggest the cable is longer.

This additional length is about 3% of the lay length of the cable and will be compensated. This

compensation will use the cable lay length and map the total fibre length on this lay length.

This way, both lengths will correspond to the same KP-points.

In order to verify the depth of burial calculation, Norther will compare the survey data, delivered

by the surveyor, to the DoB calculation, provided by Marlinks. This comparison is done using

a tool, coded in python, where both datasets can be imported and visualised. A correlation

coefficient is calculated hence a numeric result is determined. The scripts for the general and

detailed analysis can be found in Attachment D.

First, the tool can be used to compare the results of both DoB determination methods. Here,

the goal would be to have the DTS-derived DoB data match the survey data as close as

possible, i.e. a high correlation coefficient. Next, the tool can be used to analyse changes in

the cable depth of burial. No reliable DTS-derived DoB calculation data was available at the

time of writing. To illustrate the comparison, an example using two datasets from surveys will

be given. Please note a larger difference is expected from this comparison since the surveys

were taken with a year in between.

The export cable survey of November 2019 has been compared to the survey from October

2020. (MSBL is the abbreviation for Mean Seabed Level and TOC is the abbreviation of

Top Of Cable). The top of cable value is the LAT depth that was surveyed after the cable was

buried. In order to measure this, the cable was magnetised before it was laid on the seafloor.

After the cable was buried, an ROV could measure the depth of burial of the cable by

measuring the magnetic field emitted by the magnetised cable. The BoP contractor then

processed this data and determined the depth of burial of the cable with reference to the

seabed. A MBES survey was conducted after the ROV survey in order to have a reference

measurement of the MSBL at the time of cable installation. The cable is expected to stay in

position, with the layers moving on top of the cable. Consequently, an uncertainty could be

created over a long time since the reference position of the cable could change.

When analysing both datasets, visualised in Figure 6-10, it is evident that the “Scheur” shipping

lane (at around 15 km of the export cable trajectory) shows a large difference. This may be

due to dredging works. In order to have the flexibility to exclude certain parts or to do a more

detailed analysis on specific sections, an extra comparison can be done. These detailed

analysis can be found in Figure 6-11 and Figure 6-12.

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Figure 6-10: Comparison between two depth of burial datasets

The correlation coefficient along the entire length of the cable equals 90.5%, which is quite

good since the section of the shipping lane has a large anomaly. When comparing the same

data from the sections before and after the shipping lanes separately, the correlation

coefficient changes to 69.1% for the first part (Figure 6-11) and to 83% for the second part

(Figure 6-12). This may seem incongruous, but when splitting the sections, the overall margin

becomes smaller, resulting in a smaller correlation coefficient. Additionally, these are the

square correlation coefficients, implying the differences are amplified. Over longer sections,

with more margin, the correlation will show less deviation.

The analysis shows there is a good correlation and the situation has generally improved

slightly from 2019 to 2020. Some sections have become more shallow than before, but some

sections are buried deeper than before. This may originate from the movement of sand dunes.

As mentioned before, a more detailed analysis can be done. Here, the two datasets will be

visualised and the correlation coefficient will be given. Moreover, an extra feature has been

implemented. Now, the most shallow depth of burial value will be indicated and the depth of

burial value will be displayed. Figure 6-11 shows a short section along the cable (about 3

metres) did not meet the 1 metre depth of burial requirement in October 2020, however, the

analysis indicates the most shallow value is 0.99m. Consequently there is no big problem along

this section of the cable, but it would be advised to keep an eye on the section for future

reference. Along the same length of cable, the cable did meet the requirement in 2019. In

2019, the most shallow part was buried with 10 centimetres of margin to the limit.

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Figure 6-11: Detailed comparison between two depth of burial datasets - part 1

The second part of the cable, after the “Scheur” shipping lane is visualised in Figure 6-12.

Comparing the two datasets shows this section changed similarly to part 1 of the cable. Here,

in 2019, 7 metres of cable did not meet the requirement and in 2020, two more metres did not

meet the requirement. Generally, the lines follow each other, but at about 17500 m, the cable

was buried about a metre more shallow in 2019 compared to the same area as of 2020.

However, the section from about 18500 m to 21500 m was buried deeper in 2019. This change

may be due to the movement of sand dunes.

Figure 6-12: Detailed comparison between two depth of burial datasets - part 2

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6.3 Decision tree

When the cable DoB appears to be too shallow, action is required. Norther has a time window

of 3 months to fix the sections where the DoB is too shallow. This means they will have about

5 updates from the Marlinks DoB calculation within the 3 month period. When a section is

buried too shallow, Norther has to keep an eye on this section. If certain sections do not show

any improvement, action will be required in order to fix these problems. In order to judge

whether action is required, a decision tree can be used. A provisional setup can be found in

Figure 6-13 below.

Figure 6-13: Provisional decision tree

Different deficits will require different degrees of action. The tree first verifies whether the cable

is buried within the limit. If a section is within the limit, but only with a small margin, it should

be monitored to see if the situation stabilizes, improves or deteriorates. If only a short section

of the cable exceeds the limit, the previously mentioned action could be repeated. If a longer

section of cable exceeds the limit, it should be monitored more closely. Depending on the

gravity of the situation, action could be required sooner. The exact implementation of this

decision tree should be reviewed after obtaining the first depth of burial results. Over the next

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few months, the Norther team could change or add a lot of factors described in the decision

tree based on the analysis of the first results.

6.4 Outcome of the document

After analysing the data during a few months or years, seasonal variances may occur. Here,

the cable could be uncovered after a storm season, but due to movement of the top layers,

these zones could be covered back up after few months. At the time of writing, there are a lot

of unknown factors that could have to be taken into account for this document. Based on the

first results, the appropriate case studies can be chosen.

After the verification of the results by all parties involved, the next step would be to either

approve the use of the system when it proves to work correctly or more calibration work could

be required. It is important to mention a verification survey is required one year after the

implementation of the DTS-based DoB calculation. If the calculated and surveyed results prove

to be similar or better than the year before, the use of the system can be approved by the BMM

for the rest of the operational period of the Norther offshore wind farm.

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7 SCOUR PROTECTION INTEGRITY ANALYSIS

This chapter of the master thesis is dedicated to a side project that evolved from the need to

gain more insight in survey data. As mentioned before, a study was carried out to determine if

the used survey method for DoB determination was the most optimal and cost-effective method

available. In this process, visualising raw survey data was one of the things carried out in order

to gain more insight in the data. Taking these results, an idea was created to analyse the

integrity of the scour protections around the monopiles of the wind turbines. As a part of the

environmental permit, the scour protections must be monitored after storm events.

7.1 General info

“Scour is a process which takes place when a fixed structure is placed in a marine or fluvial

environment with an erodible bed. As a consequence of the structure, the water flow pattern

is disturbed. This can lead to flow pattern changes.” [65] These flows are visualised in Figure

7-1.

Figure 7-1: flows leading to scour process [66]

“When a vertical circular pile is placed on the bed in a steady current, the flow will undergo

substantial changes: (i) a horseshoe vortex is formed in front of the pile; (ii) a vortex flow pattern

(usually in the form of vortex shedding) is formed at the lee-side of the pile; and (iii) the

streamlines contract at the sides of the pile.

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In addition, a downflow forms as a consequence of flow deceleration in front of the pile. If the

bed is erodible, the overall effect of these changes generally is the increase of sediment

transport, resulting in local scour around the pile. Extensive scour around the pile may reduce

its stability, thus leading to its failure.” [66]

When left unmonitored, a scour hole, as visualised in Figure 7-2, around the monopile

foundation may be formed, reducing the structural stability of the soil where the monopile is

placed. Numerical simulations have been carried out in [67] to study these influences on the

development of scour.

Figure 7-2: Scour hole formation [68]

The scour protection around a monopile foundation is a layer of rocks that is placed in a well-

designed, sliced cone-shaped form. These rock layers prevent the sand and soil layers

underneath from moving and eventually eroding the area around the monopile.

However, this protective layer will go through an erosion process similar to the scour process

of the sand around the monopile, although a lot slower. The rocks that are placed there can

move around a little due to the dynamic behaviour of the seabed and the flows around the

monopile. The design of these layers is as such that it accommodates for initial settlement of

the rocks due to regular flow. It is important for these rocks to stay within their minimal design

limits. If necessary, appropriate action is required in order to repair the scour protection layers.

This ensures the layers remain within their minimal design limits. The scope of this analysis

tool is to check changes in the levels of the rock layers. Consequently, the dynamic behaviour

of the soil around the monopiles due to currents and waves will not be studied further. More

information on this dynamic behaviour can be found in [65] and [66].

The scour protection layout itself is designed [69] by an independent technological institute.

The scour protection is built up of a filter layer on the seabed around the monopile and an

armour layer on top of that filter layer. Depending on the depth of the location and monopile

diameter, the diameter of the filter and armour layer and their respective thicknesses are

chosen. In Figure 7-3 a sectional view of such a scour protection design can be found.

The filter layer has a diameter of 31 to 36 metres and a thickness of 35 centimetres. The

armour layer has a diameter of 22 to 24 metres and a thickness of 60 to 80 centimetres.

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Figure 7-3: Scour protection design [69]

7.2 Scour protection integrity

Due to currents of the sea and the movement of sand dunes, the protecting rock layers can

move a little as well. These scour protection layers have to remain within their minimal design

limits. If a design limit is exceeded, an intervention will be required to fix the deficit. As

mentioned before, the rocks are meant to move a little and settle. Due to the slow erosion

process around the scour protection, more rocks will move and settle. This erosion and

settlement process is called falling apron behaviour and is visualised in Figure 7-4.

Figure 7-4: Falling apron design [70]

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The scour protection is designed in a way to allow for seabed lowering and falling apron

behaviour at the sides of scour protections. The falling apron behaviour visualised in Figure 7-

4 shows the rock layers are designed to move. When the soil around the scour protection

erodes, the rocks of the scour protection will also erode and settle in order to stabilize and

protect the soil and slope around it. More information on these falling aprons can be found in

[70]. In the case of these scour protections for the Norther OWF, the falling apron will take the

shape as visualised in Figure 7-5. Similar to the rock layers of the cable crossings, the cavities

in between the rocks of the scour protection layers could be filled by sand over time. This

phenomenon provides a more solid and stable mass after its initial settlement.

Figure 7-5: Falling apron principle for monopile foundations [71]

7.3 Determining storm scenarios

The integrity of the scour protection does not have to be monitored continuously, but rather

after the occurrence of a significant storm. This significant storm is described as a 10-yearly

storm event or any other storm event where certain parameters are exceeded. Harsh

conditions and violent sea states will lead to a change in the submarine topography. The

seafloor may change more during or after a storm rather than during a calm sea state period.

To check whether there was a severe sea state, leading to potential damage to the scour

protection layers, a calculation method is used. This calculation method is based on the

significant wave height in the area of the wind farm. A calculation is performed to check the

Keulegan-Carpenter (Kc) number7 of a certain condition. If this number is higher than a

predetermined value (2.7 for the Norther case) or if measured significant wave heights

exceeded a certain height (5.4 metres for the Norther case), an analysis of the scour

protections is required.

7 “The Keulegan-Carpenter number is a dimensionless quantity describing the relative importance of the drag forces over inertia forces for bluff objects in an oscillatory fluid flow.” [75] For the purpose of this calculation, it is used to describe the severeness of a sea state.

88

“The Keulegan-Carpenter number Kc is important in the description of scour, since it is a way

to express the relative amplitude of orbital motion to the pile diameter. For scour caused by

waves this is the main parameter required to determine the equilibrium scour depth. This

quantity is described by equation (17).

𝐾𝑐 = 𝑈𝑚 ∗ 𝑇𝑃

𝐷 (17)

With

𝐾𝑐 Keulegan-Carpenter number [-]

𝑈𝑚 Maximum value of the undisturbed orbital velocity at the bed [m/s]

𝑇𝑃 Peak wave period [s]

𝐷 Pile diameter [m]

A small value of Kc means that the amplitude of the orbital motions from a wave is small

compared to the pile diameter.” [65]

A calculation example will be given here. First, the highest significant wave height of a certain

period must be known. Therefore, a script has been written to determine this height, based on

wave data which will be imported. The highest value of the height will be isolated. This value

will be used throughout the analysis. The wave data can be acquired from publicly available

databases or from privately owned weather stations in offshore wind farms.

The analysis starts by defining this significant wave height as the parameter 𝐻𝑠. According to

data from “Meetnet Vlaamse Banken” [60], on 25/09/2020 a significant wave height of 6.01m

was measured on the Thorntonbank south buoy (implying 𝐻𝑠 = 6.01 𝑚).

This 𝐻𝑠 value will be used as the most severe storm of 2020. Next, the peak wave period is

calculated using equation (18) provided in the calculation sheet [72].

𝑇𝑝 = 14.3 ∗ √𝐻𝑠

𝑔 = 14.3 ∗ √

6.01 𝑚

9.81 𝑚𝑠2⁄

= 11.22 𝑠 (18)

Here 𝑔 = 9.81 𝑚𝑠2⁄ is the gravitational acceleration.

The depth of the sea at this measured location is about 18 metres. This value will now be noted

as ℎ𝑡𝑜𝑡 (ℎ𝑡𝑜𝑡 = 18 𝑚).

The diameter 𝐷 of the monopile equals 7.8 metres (𝐷 = 7.8 𝑚). However, this is the best case

scenario for this calculation since Norther has monopiles with a diameter of 7.4 m and 7.6 m

as well.

Next, the kind of wave theory is determined in order to calculate the wave characteristics.

When the wave characteristics are known, the orbital velocities can be calculated. With the

orbital velocities and the aforementioned parameters, the Keulegan-Carpenter number can be

calculated using equation (17).

The result of the analysis is a surface Keulegan-Carpenter number. This number will be an

indication on the roughness of the sea for the scour protection with the selected significant

wave height. In this case, the result is a Kc of 3.55. This Kc number is higher than 2.7, implying

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the integrity of the scour protections needs to be checked and analysed. For the full calculation

of this value, information can be found in Attachment E.

7.4 Analysis tool

After a storm event, where certain parameters are met, the scour protections need to be

analysed. The analysis is done using a MBES survey around the monopile area. This implies

surveys will still be conducted in offshore wind farms, for other purposes. The new data can

be compared to an older dataset of the surveyed area. If there seems to have been a significant

lowering of the scour protection layers, this could be taken into account for possible repair work

on other scour protections. The second check is whether the scour protection is within its

design limits. If (one of) the scour protection layers seems to be out of its design limits, a

detailed analysis of this location’s deficiencies is required. This consists of checking if there is

sufficient material in the vicinity of the area that is out of the design limits.

This tool can greatly benefit the operational team of Norther, to perform evaluations in case a

severe storm event occurs. Before this tool was developed and created, no such in-house

check of the scour protection was possible. The tool can be found in Attachment F.

The functionality of the tool will be illustrated in this paragraph. The tool is made in Matlab and

compiled to an executable file (.exe). The end user will have to install Matlab runtime software

in order to use the tool. This runtime can be downloaded free of charge from the Mathworks

website [73]. Upon opening the executable tool, a window with instructions (Figure 7-6) will

pop up.

Figure 7-6: Analysis tool - start screen

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After clicking the “Run analysis” button, an input dialog window will be presented to the user.

Here, the user can specify the files he or she wants to analyse as well as their references for

user output use. The input dialog window (Figure 7-7) will pop up and example names are

given. The user can change these names to the ones he or she wants to use.

Figure 7-7: Analysis tool - input dialog

The user will be asked for three datafiles. The first datafile is the latest MBES survey of the

scour protection. The second datafile is the previous MBES survey of the scour protection.

These two files will be compared to each other. For this comparison, the user can specify a

visual offset so the change of the surrounding area of the scour protection can also be

visualised. The third datafile is the design datafile. Here the user can choose the desired

reference data he or she wants to compare the measured data to. In this case, the design of

the rock dump was chosen. This is not the minimal required height of the scour protection

layers. The user is able to import a design file with these minimal layer dimensions. The user

can choose a gridsize, specifying the amount of required datapoints to use for the analysis.

More datapoints will result in a more accurate representation, but will take a longer computation

time.

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A .csv file with information of the foundations is also required. This datafile contains the centre

positions of the monopiles. This is not hardcoded, allowing to change specific parameters if

necessary. Such changes could include faults in as-built documentation, or changes due to

updates of documents the datafile is based on. The user can specify a reference to each of

the datafiles. This reference will be used in the titles of the resulting graphs. The user must

specify the required location to analyse. The centre position of this location will be taken from

the previously mentioned .csv file.

When everything is specified, the user can click on “OK” and the analysis will run. If the

datafiles are out of bounds for the chosen location, an error will pop up and notify the user.

The user can check if the correct files were chosen and run the analysis again with the correct

datafiles. The result of the analysis will contain three figures where the changes are visualised

and a window with numerical results will be presented.

The first figure (Figure 7-8) shows the change of the seabed around the scour protection (area

is user specified) of the most recent file compared to the previous file.

All values mentioned can be considered in metres. The visualised scour protection area is the

difference between the surveyed area after the scour protection layers were installed, without

the cables and the area after the monopile and cables were installed. In future comparisons

this difference will be minimized because both surveys will contain the same features.

Figure 7-8: Analysis tool - visual result newer VS older survey data

The second figure (Figure 7-9) and third figure (Figure 7-10) show the difference between the

latest and the previous file, compared to a design or reference file.

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Figure 7-9: Analysis tool - visual result newer survey data VS rock design

Figure 7-10: Analysis tool - visual result older survey data VS rock design

Based on these results, originating from April 2020, everything remains within the acceptable

boundaries concerning the rock dump design. On Figure 7-9 there are some red zones around

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the centre of the monopile. Possibly these red zones originate from backscatter from the MBES

i.e. these values were not actually measured. This does not mean this area should be

neglected. In fact, this makes it very hard to judge the actual result since the true origin of this

difference is unknown.

The numerical results are visualised in Figure 7-11. For every figure, a numerical result is

calculated. This calculation includes the minimum, the maximum and the mean change in

seabed level around the scour protection for the three cases.

Figure 7-11: Analysis tool - numerical results

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8 CONCLUSION

The survey methodology of offshore wind farms combined with the implementation of a

distributed temperature sensing (DTS)-based cable monitoring system was studied in the

present master thesis. Norther wanted to install a DTS system to monitor the temperature and

real time thermal rating (RTTR) of their submarine power cables. Additionally, Norther wants

to determine the depth of burial (DoB) of these cables using the same system. Since the DoB

determination of the cables using the DTS system has to be calibrated to a reference DoB,

research was done to ensure the most cost-effective method for reference measurements is

used. The reference measurements are conducted using offshore surveys.

The current state of offshore surveys proves that the multibeam echosounder (MBES) survey

method is the most cost-effective way to scan the surface of the seafloor. When considering

check-up surveys for cable DoB determination, MBES surveys are the most suitable survey

methodology if reference data is available. Sub bottom profiling (SBP) can be used for spot-

checks on certain locations. The SBP will detect the cable position and its depth instead of

calculating the change of the top layer. That way, no reference from a previous survey is

required and changes in the position and the actual DoB can be detected. This implies the

level of uncertainty of the surveyed DoB will be reduced. In the future, SBP-based surveying

may become the new market leader if it proves to work in the longitudinal direction of the cable.

However, this technique is still under development and therefore expensive to conduct. Based

on this observation, SBP is currently not the standard cable DoB determination method.

The main subject of the master thesis is the implementation of the DTS-based cable monitoring

system. For this implementation, the design and engineering of cable models and preparation

of the data processing were done in October and November of 2020. The cable models allow

for a simulation of the thermal behaviour of the cables, based on the measured input

parameters. In the meantime, the electrical and fibre optic wiring diagrams for the DTS cabinet

were made. When weather conditions allowed, offshore works were carried out. These

offshore works included finalising the work on the DTS cabinet and completing the fibre optic

loops of the fibre optic circuit. Next, the attenuation of the fibre optic loops was examined. The

tests revealed some issues in the loops. By replacing some faulty connectors, the loops passed

the test and were ready for the temperature measurement of the DTS system.

The DTS system was installed on the Norther OHVS at the end of February 2021. After the

DTS installation, the calibration of the temperature measurement could start. In this phase, the

measured temperature of the DTS system was calibrated to match a temperature measured

by a reference thermometer. Additional measurements were done to ensure the correct

temperature coefficient was determined. Three months after the offshore installation, in early

May 2021, the take-over of the temperature measurement and RTTR calculation took place.

The cable temperature measurement and RTTR calculation will give the operators of the wind

farm more insight in the thermal load of the power cables.

An additional challenge presented itself as the length of a cable that hangs in the monopile is

not buried and should therefore not be monitored. In order to determine the sections of the

cable that are hanging in the monopile or sections that are not buried, as-built information was

used. These sections were deemed irrelevant as they do not require alarm triggers. The

lengths of cable from the cable hang-off to the reference starting point and end of the CPS

95

were calculated. For the take-over, the adjusted cable lengths were implemented as well as

the temperature alarms for the buried sections of cable. For the DTS-derived cable DoB, these

same sections will be used to determine whether an alarm should be raised on a location

where the cable DoB is too shallow i.e. the cable coverage is insufficient.

In mid-May of 2021, a reference DoB measurement using a MBES survey was conducted for

the Norther export cable. Since the take-over at the beginning of May 2021 until the end of the

summer, the DTS system will collect temperature data. After this initial data collection, the DoB

results of the DTS system can be calibrated to the reference depth of burial, determined by the

survey.

When the first DTS-based DoB results are available, Norther will be able to determine the

appropriate case studies where the performance of the system can be demonstrated to the

BMM. One year later (in the summer of 2022), a verification survey will be conducted to

substantiate the results of the DTS-based DoB determination. If the results of both

measurements closely match their corresponding reference DoB measurement, the

performance of the DoB determination can be substantiated. After presenting such results to

the BMM, they can approve the use of the system as the main DoB determination method for

the rest of the operational period of the Norther offshore wind farm.

While studying raw survey data, a side-project evolved into the development of a tool to

analyse the integrity of the scour protections around the monopiles. This tool allows Norther to

perform in-house checks of the integrity of their scour protection layers after a severe storm

event. The tool uses MBES data from the survey around the monopiles. A first check is the

comparison of the latest survey data to the previous survey data. Here the change of the scour

protection layers can be monitored over time. The second check will allow the user to analyse

the difference from the actual (surveyed) situation to a design reference. This design reference

can either be the rock dump design or the minimal design of the scour protection layers. This

last design has yet to be made as a dataset, but Norther aims to make this by the summer of

2021.

96

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Attachments

Attachment A Interview with a surveyor

Attachment B Cable irrelevant length determination

Attachment C Fibre optic splice plan

Attachment D Python scripts for DoB verification

Attachment E Storm scenario determination

Attachment F Scour Protection Analysis Tool

A. 1

ATTACHMENT A INTERVIEW WITH A SURVEYOR

Interview with a surveyor_V6.pdf

A. 2

A. 3

A. 4

A. 5

A. 6

ATTACHMENT B CABLE IRRELEVANT LENGTH DETERMINATION

NRT-BOP-DTS-CAL-20210309_04_final.pdf can be made available upon request.

A. 7

ATTACHMENT C FIBRE OPTIC SPLICE PLAN

NRT-BOP-DTS-DWG-00003.pdf can be made available upon request.

A. 8

ATTACHMENT D PYTHON SCRIPTS FOR DOB VERIFICATION

CompareDoB_3.py can be made available upon request.

CompareDoBSection_3.py can be made available upon request.

A. 9

ATTACHMENT E STORM SCENARIO DETERMINATION

144147-VOWP-ENG-TN-1029-Technical-Note-One year Scour rock stability check rev3 -

Matlab equations.pdf

A. 10

A. 11

ATTACHMENT F SCOUR PROTECTION ANALYSIS TOOL

ScourProtectionAnalysisTool_12.1.zip can be made available upon request.

FACULTEIT INDUSTRIËLE INGENIEURSWETENSCHAPPEN CAMPUS BRUGGE Spoorwegstraat 12

8200 BRUGGE, België tel. + 32 50 66 48 00

iiw.brugge@kuleuven.be www.iiw.kuleuven.be