OTC 23336

Offshore Pipeline Trenching In Arctic Regions: A Unique Concept “The Arctic Subsea Bucket Ladder Trencher” N. Vaartjes - Van den Berg, IHC Merwede B.V.; J. Frederiks, Shell Global Solutions International B.V.; S. Miedema, Delft University of Technology; P. Swart, Shell Global Solutions International B.V.; R. Plat, IHC Merwede B.V.

Copyright 2012, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2012. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

Pipelay in Arctic offshore regions comes with a number of challenges. The pipelay process includes safe burial of

pipelines, while minimizing the disturbance to the natural environment is key priority. Deep trenching will be necessary,

resulting from deep keeled ice features that can gouge the soil and present a threat to the pipelines. In addition, a fast trenching

solution should be obtained to enable the pipelay operation to be carried out in the short open water season. So far, this can

hardly be carried out by conventional equipment.

This paper describes a deep and fast trenching method suitable for Arctic conditions. The concept is approached from a

dredging industry perspective since the huge soil volumes to be excavated relate to practices common in the dredging industry.

The views of operating and dredging companies are integrated, leading to a promising conceptual solution for Arctic

trenching.

The purpose of the design case was created to discuss several Arctic trenching challenges to find a conceptual solution for

deep and fast trenching (6m) in the Arctic. The case was about trenching through stiff clay in which backpack size boulders

are present, while keeping in mind a high required trenching speed (75m/hr) and the desire to keep the spoil to remain as in-

situ as possible to minimize the impact on the environment.

For this base case The Arctic Subsea Bucket Ladder Trencher that is presented in the paper is a promising method. This

Arctic Trencher (AT) has the following characteristics:

• Precise excavation possible;

• Highly capable of operating in stiff clay;

• Relatively low cutting power required;

• Capable of excavating without compromising the in-situ soil;

• Boulder handling possible up to the size of the buckets;

• Trenching possible in one operating season;

• Proven method in dredging industry: expertise readily available.

Based on the initial base case design conditions it was concluded that the presented design case has an excellent potential

for successful trenching in the Arctic. Although some uncertainties remain, the authors are continuing development of the

concept to further assure its feasibility. This paper shows that it is worthwhile to include the joint effort of field operator,

pipelay/trenching operator and manufacturer of trenching equipment, to tackle the challenges when the difficulties of trenching

operations increase.

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Nomenclature A Adhesive force on the blade kN

C Cohesive force on shear plane kN

K1 Grain force on the shear plane kN

K2 Grain force on the blade kN

N1 Normal grain force on shear plane kN

N2 Normal grain force on blade kN

S1 Shear force due to internal friction on the shear surface kN

S2 Shear force due to soil/steel friction on the blade kN

v Cutting velocity m/s

W1 Force resulting from pore under pressure on the shear plane kN

W2 Force resulting from pore under pressure on the blade kN

α Blade angle rad

β Angle of the shear plane with the direction of cutting velocity rad

hi Thickness of the layer cut m

Introduction As the age of technically easy access to Oil and Gas is coming to an end, the global demand for energy is growing at an

ever increasing rate. This results in a necessity to develop new oil and gas areas such as the Arctic Region. It is estimated that

20% of the world’s remaining oil and gas reserves are to be found in the Arctic; a remote area characterized by an extreme and

hostile environment. Many technological improvements are needed before an economical and safe solution for oil and gas

field developments is available. [Frederiks, 2011] One of the areas for technical improvement is the trenching of offshore

pipelines.

A major risk for offshore pipelines in the Arctic is caused by deep keeled ice features capable of gouging the seafloor. One

of the options to protect pipelines from ice gouging is to bury them to a sufficient depth below the seabed to prevent damage

from direct ice gouging and sub-gouge soil deformations (Figure 1). Future burial depth requirements in the Arctic depending

upon location may be much deeper than what is commonly experienced to date. [Frederiks, 2011]

As Arctic development interest moves further away from shore beyond the fast ice zone, installation and burial of pipelines

during the open water period is required. Since the open water period in the Arctic varies between 2-4 months, only a very

small working season remains. This implies that relative fast trenching and corresponding high production volumes are

required. These production volumes relate to quantities typical for dredging.

This paper describes a concept for deep and fast trenching, capable of meeting the Arctic challenge. The paper presents an

integrated solution considering operating and dredging company experience.

Trenching challenges

In spite of harsh environmental challenges, Arctic Oil and Gas development seems to be promising. Operators desire safe

procedures in the field and efficient development solutions. During construction of the pipeline system, given the short

operating windows, it is important to realize that the entire section of pipeline laid in the season, should be buried in the same

season. This is in order to avoid potential damage to an exposed pipeline section by ice. Avoiding potential damage from ice to

the pipeline system during construction requires a trenching system and operation with a high reliability and minimized risk on

down time (as often is the case).

The sequence of pipelaying and trenching is of great significance for the pipelay operator and the execution of the project.

Furthermore, the complexity of the Arctic Trenching (AT) system depends on the pipelay sequence. Both pre trenching and

post trenching modes, present advantages and disadvantages for pipelay and trenching respectively as indicated in Table 1.

Gouge Depth

Burial Depth

Water Depth

Pipeline

DriftingIce ridge

Sub Gouge Deformation

Figure 1 Ice gouging and the potential risk for a pipeline. In relation to a buried pipeline the ice gouging is divided into two zones. Zone 1 is directly created by the gouging ice feature. Zone 2 represents the sub-gouge deformation zone. If a pipeline is buried within these regions and a gouge event takes place, the ice feature could cause severe damage either directly or indirectly. [Frederiks, 2011]

OTC 23336 3

Generally speaking, it can be alleged that post trenching is favored for the pipelay operation and pre trenching is preferred for

the trenching operation.

A large challenge is presented by the soil characteristics. It might be that the seabed consists of hard or stiff soils, and that

boulders can be encountered along pipeline routes, which could limit the trencher to move forward or cause damages to the

equipment. It may be possible to detect boulders during pre-survey of the route to be removed in advance of pipelay activities,

but this is not very reliable. [Frederiks, 2011], [USGS, 1979], [Blanchet, 2002]. The unevenness of the seabed at the shallow

water locations, where gouging occurs most frequently, presents a hazard to pipelay and trenching operations as well.

Consequently, these geotechnical complexities should be taken into account in the design of an AT.

While seeking for an optimal Arctic trenching solution, this paper is a summary of investigating the possibility of pipelay

and trenching during the open water season. The idea of an optimal Arctic trenching solution would be to perform trenching

independent of location (water depth, soil characteristics, bathymetry etc.). However, due to the variety in characteristics of

locations this will not be possible. Therefore, a design case with specified data is introduced.

Table 1 Advantages and disadvantages for pre and post trenching operations respectively [Frederiks, 2011]

Advantages Disadvantages

More freedom in the pipeline installation for the pipelay

vessel.

Risk of damage to the pipeline.

Post

trenching Lower excavation volume. Narrower trench possible; no

issues regarding the tolerances of the lay vessel.

You are committed to a one season strategy since the pipeline is

already installed on the seabed.

Easier excavation. No pipeline interfering with the

trenching operation.

Accuracy requirements to the pipe lay vessel. Touchdown

monitoring could help in this respect.

A two season strategy might be possible as well,

although you might need to do some extra trenching in

the second season.

Less flexibility for the pipe layer. No option to deviate somewhat

from the original pipe route.

Risk of trench collapse and fill up in between the operations of

trenching and pipelay. This might involve extra trenching.

Pre

trenching

More trench options possible. Most post-trench systems

usually are also capable of performing pre-trench

operations. The other way around will be more difficult.

Trenching might become onto the critical path of the pipe lay

operation. In Arctic regions there is a very tight schedule due to

the short open water and working season.

Design case, Input data Depending on climate and environment, different areas present different challenges. Therefore, in this section a basic

design case is defined. The design case is used as a basis for the conceptual design of an AT.

The design case was set by assuming boundaries and values for the following issues:

1. Trenching depth

2. Soil characteristics, including boulder size estimates

3. Trencher speeds

4. In-situ soil backfill as much as possible

1. At some locations the ice gouging can be up to 50m wide and in extreme, but unlikely cases 5m deep. A lot of

research has already been carried out to improve the estimates on pipeline burial depth. For now, it is estimated that burial

depths of about 5m (Top of Pipe) and trench depths of approximately 6m, are required to provide for sufficient protection.

2. In many places on Arctic locations, one can find a remolded, softer top layer of deposit on top of a stiff layer of clay.

For this research a layer of about 2m of remolded soil on top of about 400 kPa stiff clay layer is considered. For the boulders

which the trenching system should be capable of handling, the size of a backpack (0.5m diameter) is used as the estimate. This

implies that any larger boulder(s) will require a separate trenching solution or should be worked around.

3. Looking at the pipelay projects in total, it is believed that trenching and backfill within one season is very beneficial.

Costs of execution of projects in the Arctic are already high, thus extension of the operation over multiple years could make

the projects economically not feasible. Therefore, for the design of the AT one has to bear in mind that the speed in which

trenching can take place is a decisive issue.

This means that the time frame which is left to excavate these quantities of soil is dependent on the duration of the open

water season, the mobilization and demobilization time for offshore installation vessels and some margins to cope with

insecurities in these estimates and unforeseen events. This results in the fact that for a relatively unfavorable case the total time

for pipelay and trenching is estimated to be only about 60 days. Depending on the selected pipelay mode (post trenching or pre

trenching), this might entail an expected 30-40 days for the trenching part. A required trencher speed of 75 m/h (while

4 OTC 23336

excavating to full depth) is assumed. This is based on estimates for e.g. waiting on weather, the length of the pipeline route,

mob- and demob times of the trencher and breakdown.

4. When handling the dredged spoil, maintaining the backfill material with in-situ density could work favorably for the

insulation of the pipeline and the stability of the pipeline in the trench once backfilled. Furthermore, it could work beneficial

for environmental regulations to leave the excavated spoil intact as much as possible. For these reasons, it should be carefully

considered whether jetting or hydraulic transportation, is a good method for use in the Arctic. Therefore, excavation methods

for which the dredged spoil can maintain its in-situ density (as much as possible) are assumed to be more preferable.

Summarizing: the generic Arctic trenching case is about deep trenching through stiff clay of about 400 kPa in which

boulders are present, while keeping in mind a tight operation schedule; high excavation volumes/trenching speeds; working in

the challenging Arctic environment and maintaining a high reliability to assure that the project can be carried out in a safe and

efficient way.

Screening study Once the input was defined, a screening study was performed to come up with an optimal excavation method for the AT

considering the profile and width of the trench, pipeline handling, and the topic of multi-passing.

The bottom width (Figure 2) of a trench is dependent on the lay mode and pipeline diameter. For post trenching, the

required bottom width is just slightly larger than the pipeline diameter, because the accuracy of the vessel does not have to be

accounted for. This makes that the bottom width for post trenching can be considerably smaller than for pre trenching. In this

research a value of 1.5 m has been suggested for post trenching, while for pre trenching the values have been estimated in the

region of 3-5m. It should be mentioned that these values are dependent on the concerned pipeline diameter. From Table 1 it

can be observed that in general the advantages and disadvantages for post trenching will be better than for pre trenching. A

major reason for this is the possibility that the trench (in pre-trenching) is already collapsed before the pipe is laid; and

accordingly the lack of guarantee that the pipeline is sufficiently buried for the entire route. Moreover, the benefit of post

trenching has been assumed, because generally speaking a trenching tool that is capable of post trenching will require minimal

soil displacement, thus minimal environmental impact. However, it has been indicated that by using a post-trench method the

project is committed to a one season strategy.

Considering the desire to keep the footprint and the involved excavation volumes to a minimum, it will be better to

excavate in a box shape. In addition, it is preferred to look at the possibilities for a post trencher. And thus, it has been

proposed to investigate the viability of a box shaped trench instead of a v-shaped trench. In Figure 2 the parameters referring

to the shape of the trench have been depicted. The trench depth was taken at 6m and due to the stiff clay layer the common v-

trench angle that was used was 35°. In the graph of Figure 3 the comparison between calculated excavation volumes has been

depicted for some characteristic soil profiles. Compared to a box shape a complete v-shape takes up to six times the amount of

excavated volume, which is considerably more. Therefore a box shaped trench is the most efficient way of excavating. Taking

into account the estimated trench speed and trench profile the required production rate is estimated at 1000m3/hr.

However, the box cut should also be achievable referring to soil properties and risk of fill up before the pipe is lowered to

the bottom of the trench. Therefore the amount of free span of the pipeline is important. If one knows the amount of free-span

and assumes a value for the overall productivity of the trencher one can calculate how long the trench has to be stable for the

pipeline to touch down on depth. The deduction has been made, that for a trench depth of around 6m and pipeline

characteristics of a reference Arctic project, the pipeline’s free span will vary between 60-90m (Figure 5). This estimate is

based on the assumption that the pipeline will be filled with water and no concrete coating is added. Taking into account the

required estimates on trenching speeds, it is presumed that the trench only has to remain open for a few hours.

Some basic calculations (GeoStudio 2007, Figure 6) showed that it is likely that a box cut trench will stay open for long

enough taking into account the free span of the pipelines and estimated speed of the trencher. However, on locations where the

top layer is very weak, parts of this layer could collapse and fall into the trench. Perhaps, an extra tool to remove this soil

before or after the trencher has past can be a solution for this problem. For the moment, this remains a topic for further studies.

In addition it must be noted that if the soil at location would consist of sand instead of clay, it will most likely not be possible

to excavate in a box-shape.

Other important topics of trenching are pipeline handling and multi-passing of the trencher. In case the pipeline is post

trenched it should be assured that the pipeline will stay intact during the trenching operation. In response to this, it would be

preferred if multi-passing is not needed. This way multiple handling of the pipeline can be avoided. Therefore, a post trencher

which trenches to depth with one pass is assumed favorable.

Concluding, post trenching in a box shape is a good possibility for the considered case. This way, the lowest soil volumes

will need to be excavated, which results in a minimum footprint and analogous cutting power consumption. However, the

amount of overburden soil which might fall into the trench presents a complexity that should be dealt with. Of course one

should bear in mind that the design process has a cyclic origin and that therefore the applicability of a box shape is dependent

on e.g. the soil conditions, the selected excavation method, the pipeline handling process and the way the trencher is deployed.

OTC 23336 5

Concept Selection Using in-house knowledge, old and new methods for offshore soil removal were assessed on their applicability. To find a

new concept, one can take a look into the options that have already proven its value in the excavation and dredging industry.

From a dredging approach one could think of the following trenching options:

• Cutter Suction Dredgers

• Clamshell or Grab Dredgers

• Backhoes or Front Shovels

• Trailer Suction Hopper Dredgers

• Bucket Ladder Dredgers

• Bucket Wheel Dredgers

• Jetting

• Ploughing

• Mechanical Trenchers

Figure 5 Slope stability output GeoStudio 2007 [Frederiks, 2011]

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7

Exc

av

ati

on

vo

lum

e [

m3/m

]

x [m]

Post-trench (1.5m bottom) Pre-trench (touchdown monitoring) Pre-trench (no touchdown monitoring)

Figure 3 Estimated soil volumes required per m trench, trench depth 6 m. [Frederiks, 2011]

dt

wt

x�

Figure 2 The shape of the trench relative to x [Frederiks, 2011] dt [m] Trench depth wt [m] Bottom width

x [m] Variation of the height of the ‘V’ shaped profile

Φ [°] Angle of the slope

Free Span

Touch Down

Trench

Depth

Figure 6 The pipeline’s free span length [Frederiks, 2011]

Figure 4 Pipeline plough systems IHC Engineering Business: the PL3 and BPL3 systems designed for Saipem allow products to be buried up to a depth of 2.5m and then subsequently backfilled, in water depths up to 1500m. The maximum tow force of the PL3 is 400t. [Photo courtesy of IHC Merwede B.V.]

Φ

6 OTC 23336

None of these options can immediately provide for a good post trenching solution for the desired design requirements.

Nevertheless, one of the oldest methods, the bucket ladder principle, emerged as a method with relatively good prospects for

trenching in Arctic waters, taking into account the high production volumes, the required trencher speeds, involved soil

conditions and boulders and the requirement with respect to in-situ soil.

The principle of the bucket ladder dredger is that buckets serve as cutting and carrying mechanism for the soil and the

boulders (Figure 7). Main advantage of this method is that it has proven itself as a precise dredger (with the ability to excavate

in a box shape), highly capable of operating in stiff clay. From this experience it also can be stated that a relatively low cutting

power will be required for the excavation of large volume flows compared to other trenching options. It has been mentioned

that hydraulic transportation for the excavation and transportation might not be preferable. Actually, for bucket ladder

dredgers, the spoil can remain in the in-situ position comparatively well and the turbidity plume that is created will be

minimal. In addition, boulders can be handled up to the size of the buckets on the ladder and can be transported out of the

trench.

Another method that was considered is ploughing (Figure 4), which uses a tool that is already on the market for specific

trenching operations. A plough excavates the soil in a v-shape, shown in Figure 3 at the right end of the graph. Although

ploughing could work perfectly up to depths of about 2.5m for the right soil type, the disadvantage of ploughing is that

extrapolation of this method to 6m of depth leads to huge soil volumes (in the range of 60-81 m3/m, Figure 3) that would have

to be excavated. In addition to the fact that ploughs are not capable of dealing efficiently with stiff clay, this excavation

method would in this case result in excessive power consumption and impact to the environment.

This indicates that the technology of ploughing might not be the best solution to apply on the Arctic trenching case, but

that the bucket ladder method is promising for use in the Arctic Area. However, it will need some adjustments before it is fit

for use as and AT. Therefore, some key components referring to the implementation of this method will be reflected on in the

next section.

Conceptual design: “The Arctic Subsea Bucket Ladder Trencher” It has been proposed to use one of the most proven methods of dredging, bucket ladder dredging, as a solution for the

Arctic trenching challenge. The method is not immediately fit for use and therefore in this section is reflected on several

design issues which have to be touched upon before the bucket ladder principle can be implemented. These are for example the

way the buckets can be emptied under water and the size determination of the buckets itself. Also other important issues, e.g.

cutting transportation mechanism and the pipeline handling integration, which are relatively independent on the excavation

mechanism, have to be reflected on.

Figure 9 shows the principle of the proposed concept(s), the ‘compact ladder’ and ‘triangular ladder’. The buckets serve to

cut the soil and then transport it out of the trench, while fixed on a transportation belt. After the spoil has been transported out

of the trench, the buckets will be emptied, the spoil will be collected and afterwards the process will begin from start again.

This will all be done subsea. It should be guaranteed that the buckets are empty before the process restarts, which is difficult to

achieve subsea. The difference between the compact and triangular ladder origins from the attempt to incorporate all functions

of the trencher in one tool while maintaining its reliability, as has been explained in more detail below.

Compact ladder versus triangular ladder

For an AT it would be desirable if the emptying process, spoil collection area, transportation process and drive system can

all be incorporated in one tool, which is able to operate completely subsea. The compactness of the system is important, but

the consistency of the system should not be affected by the integration attempt. Therefore the idea arose to break up the

location of drive system and actuation wheel from the emptying and spoil collection area. Sketches of either a ‘compact ladder

concept’ or ‘triangular ladder concept’ are shown in Figure 9.

The principle behind the idea of a compact ladder is based on

collecting the spoil inside the actuating wheel to decrease the size

and weight of the system. To enhance the reliability of the system

the triangular ladder concept has been proposed. For that proposal

the spoil collection is kept out of the actuating wheel, which

potentially lowers the risk for the drive system to be affected by

the cuttings or damaged by boulders. Another advantage of the

triangular ladder concept could be that there is more space and

time for the buckets to be emptied.

At the moment in-house researchers and developers are

examining the potential of both options in order to find the most

feasible concept, in combination with a bucket emptying

mechanism and subsequent spoil collection and transportation

system.

Trencher

movement

Bucket chain

rotation

Cutting

edge

Buckets

Actuating

wheel

Following

wheelX

Z Y

Bucket

cutting and transporting

Bucket

emptying

Figure 7 Subsea bucket ladder principle [Frederiks, 2011]

OTC 23336 7

Bucket emptying process

The main difference of subsea emptying of the buckets compared to emptying in dry air is that you can only take the

submerged weight of the soil into account. By comparison, this slows down the emptying process drastically. The emptying

process is an important issue as it will have an influence on the design of the buckets, e.g. whether bottomless buckets will be

used and whether they should be hinged to the chains, but also on the general arrangement for the different parts of the

trenching tool.

In order to be able to guarantee that the buckets will be emptied before cutting restarts, it is proposed to include a scraper to

the design. A key selection criterion will be the choice for a stationary or dynamic concept of the scraper. Various options for

both concepts have been taken into account. One of the examples of a stationary scraper has been depicted in Figure 8. Both

designs have advantages and disadvantages, but an advantage of this stationary version (specialized for use on a bucket ladder

AT) would be that there are lesser movable parts in the system. Obtaining a method for a reliable subsea emptying process,

and therefore also the design of the scraper, are essential components of the design which have to be carefully reflected on.

Therefore, the final selection cannot be presented yet. However, the topic has been taken into consideration and for the

remainder of this paper the example of a stationary scraper will be taken into account. [Frederiks, 2011]

Cutting transport process

After the cuttings have been cleared from a bucket they must be transported out of the working region of the trencher. A

few options have been considered to transport the cuttings to the sides of the AT and for now it is believed that a screw

conveyor provides for the best prospects. Referring to other projects it can be said that the advantages of a screw conveyor (or

auger system) are that the system can be classified as simple and robust which is capable of transporting high density fluid

mixtures. Therefore, it is believed, that in this way the cuttings can be kept their in-situ condition the best, even after

transportation. Additionally shaft-less augers normally provide for a method in which high density fluid mixtures and large

solid materials incl. boulders can be transported without jamming. As a result, presently the opinion is that this could provide

for a viable initiative. In the conceptual sketch of Figure 9, the proposed idea is shown. [Frederiks, 2011]

Pipeline handling

In case it is desired that the pipeline is left completely untouched, the thoughts are in the direction of a ‘twin ladder

method’ (Figure 10, left) that excavates underneath the pipeline. However, if the intactness of the pipeline can be guaranteed

one of the ideas is to make use of a ‘single ladder method’ (Figure 10, right) that excavates the trench next to the pipeline and

then guides the pipeline into the trench. [Frederiks, 2011]

The idea behind the twin ladder method is to cut the trench underneath the pipeline without touching the pipeline, by tilting

the trenching tool underneath the pipeline to cut a trench. This implies that there will always be a wedge of soil in the trench

that is not cut (indicated in blue in Figure 10, left), which presents a disadvantage to the system. By using two identical ladders

of half the needed trench width it is alleged that this wedge can be kept to a minimum. Potential solutions can be created by

excavating a deeper trench than required and store this extra estimated 1.5 m2 inside the trench; or by designing an extra third

system that is going to take care of this extra soil. Potential solutions for this topic are still under consideration. The twin

ladder method for the triangular ladder has not been depicted here, but it can also be made suitable for use with triangular

ladders. [Frederiks, 2011]

Bucket chain

rotation

Screw

conveyor Actuating

wheel

Following

wheel

X

Z Y

Rotating

back plate

Figure 9 Two potential arrangements for the bucket ladder and spoil transportation system. Left: Compact ladder concept. Right: Triangular ladder concept [Frederiks, 2011]

Scraper

Figure 8 IHC Merwede patented scraper on bucket wheel dredge with bottomless buckets. [Drawing courtesy of IHC Merwede B.V.]

8 OTC 23336

Slightly tilted ladders

Uncut soil

Figure 10 Left: Twin ladder method: two inclined ladders dig a trench underneath the pipeline. The blue wedge of soil in the lower right corner indicates the amount of uncut soil using this method. Right: Single ladder method: trench is dug next to the pipeline and subsequently pipeline is guided into the trench. [Frederiks, 2011]

The single ladder method draws back from onshore pipeline burial methods. The trench is excavated next to the pipeline

and afterwards the pipeline will be picked up and guided into the trench. Only one ladder will be needed, decreasing the

amount of movable parts, while gaining in robustness of the system. The single ladder method is not a proven method for

offshore pipeline trenching yet. Physically forcing and guiding the pipeline into the trench has not been done before. It is

alleged that more research will be needed before this single ladder method can be implemented for offshore use.

Modeling

The self weight of the AT adds to the forces the buckets will conduct to the soil. Consequently, the deployment of the

bucket ladder in an inclined manner will add to the efficiency of the AT up to a certain level. By creating a model of the AT

concept its feasibility can be checked. In a dimensioning model incorporating the basic dimensions of the buckets and ladder

the input for the determination of the cutting forces and the power model of the system was verified. The most important

parameters are the minimal cutting velocity vc and the thickness of the layer cut hi. By using the cutting model that includes

these parameters as given in Appendix A, the required cutting power for excavation can be acquired and the trench speeds of

the bucket ladder could be derived. The modeling which was carried out showed cutting forces and required power for cutting

and transportation are well within boundaries and trench speeds up to 75m/hr can be achieved. These results show that it is

worthwhile to continue with the development of the AT.

Conclusion The objective of this work was to find a conceptual solution for deep and fast trenching (6m) in the Arctic. The design case

requirements includes the ability to trench through stiff clay in which backpack size boulders are present, while keeping in

mind a high required trenching speed and the desire to keep the spoil to remain as in-situ as possible.

To minimize the excavation volumes for trenching through stiff clay the chosen concept is a post trenching system that is

capable of excavating in a box cut using a bucket ladder principle. This concept seems a promising method because it has the

following characteristics:

• Proven method in dredging industry;

• Precise excavation possible;

• Highly capable of operating in stiff clay;

• Relatively low cutting power required;

• Capable of excavating without compromising the in-situ density soil requirement;

• Boulder handling possible up to the size of the buckets;

• Trenching possible in one operating season;

• Minimal environmental impact due to the minimal soil displacement;

• Proven method in dredging industry: expertise readily available.

Based on the initial base case design the presented design case shows excellent potential for successful trenching in the

Arctic. Although some uncertainties remain, the authors are continuing development of the concept to further demonstrate its

feasibility. The modeling which was carried out showed cutting forces and required power for cutting and transportation are

well within industry boundaries.

Based on the above, “The Arctic Subsea Bucket Ladder Trencher” is considered to be a promising concept for offshore

pipeline trenching for the Arctic offshore.

OTC 23336 9

Epilogue Out-of-the-box and innovative thinking is required for finding solutions that will address the extreme pipeline trench

depths and excavation volumes needed for upcoming development locations. For concept optimization it is recommended that

all parties (i.e. field operator, pipelay/trenching operator and manufacturer of trenching equipment) are included in the decision

making process with regards to the design of trenching equipment. Maximum value will be derived from this joined up

thinking (Figure 11).

Figure 11 Incorporate all parties in the decision making process concerning Arctic trenching

References Blanchet, D. et Al., “Pipeline trenching in permafrost: a review”, Proceedings of IPC’02, 4th International Pipeline

Conference, Alberta, Canada.

Frederiks, J. “A conceptual design for an Arctic Subsea Pipeline Trencher”, Thesis Report, Delft University of

Technology, October 13, 2011 (Confidential)

Jukes, P. et Al., “Arctic and harsh environment pipeline trenching technologies and challenges, OTC 22083, 2011 Houston,

Texas

Hatamura, Y., & Chijiiwa, K. (1975). Analyses of the mechanism of soil cutting, 1st report. Bulletin of JSME, vol. 18, no.

120 , pp. 619-626.

Hatamura, Y., & Chijiiwa, K. (1976). Analyses of the mechanism of soil cutting, 2nd report. Bulletin of the JSME, vol. 19,

no. 131. , pp. 555-563.

Hatamura, Y., & Chijiiwa, K. (1976). Analyses of the mechanism of soil cutting, 3rd report. Bulletin of the JSME, vol. 19,

no. 139. , pp. 1376-1384.

Hatamura, Y., & Chijiiwa, K. (1977). Analyses of the mechanism of soil cutting, 4th report. Bulletin of the JSME, vol. 20,

no. 139. , pp. 130-137.

Hatamura, Y., & Chijiiwa, K. (1977). Analyses of the mechanism of soil cutting, 5th report. Bulletin of the JSME, vol. 20,

no. 141. , pp. 388-395.

Miedema, S. (1992). New developments of cutting theories with respect to dredging, the cutting of clay. WODCON XIII.

Bombay, India: World Dredging Association (WODA).

Miedema, S. (2009). New Developments Of Cutting Theories With Respect To Dredging, The Cutting Of Clay And Rock.

WEDA XXIX/Texas A/M 40. Phoenix, Arizona, USA: WEDA/TAMU.

Miedema, S. (2010). New Developments of Cutting Theories with respect to Offshore Applications. ISOPE (p. 8). Beijing,

China.: ISOPE.

Miedema, S. (2012). The Cutting of Sand, Clay & Rock. Lecture notes, Delft University of Technology, Delft, The

Netherlands.

Field operator

Manufacturer

of trenching

equipment

Pipelay/

Trenching

operator

10 OTC 23336

Appendix A: Clay Cutting Hatamura and Chijiiwa [1975], [1976], [1976], [1977] and [1977] distinguished three failure mechanisms in soil cutting.

The "shear type", the "flow type" and the “tear type”. The "flow type" and the "tear type" occur in materials without an angle

of internal friction. The "shear type" occurs in materials with an angle of internal friction like sand. A fourth failure

mechanism can be distinguished [Miedema (1992), (2012)], the "curling type", as is known in metal cutting. Although it seems

that the curling of the chip cut is part of the flow of the material, whether the "curling type" or the "flow type" occurs depends

on several conditions. The curling type in general will occur if the adhesive force on the blade is large with respect to the

normal force on the shear plane. Whether the curling type results in pure curling or buckling of the layer cut giving obstruction

of the flow depends on different parameters.

Figure 12 illustrates the curling type, the flow type mechanism as they might occur when cutting clay or rock, the tear type

and the shear type mechanism as they might occur when cutting clay or rock (the tear type) or cutting sand (the shear type). To

predict which type of failure mechanism will occur under given conditions with specific soil, a formulation for the cutting

forces has to be derived. The derivation is made under the assumption that the stresses on the shear plane and the blade are

constant and equal to the average stresses acting on the surfaces. Figure 13 gives some definitions regarding the cutting

process. The line A-B is considered to be the shear plane, while the line A-C is the contact area between the blade and the soil.

The blade angle is named α and the shear angle β. The blade is moving from left to right with a cutting velocity vc. The

thickness of the layer cut is hi and the vertical height of the blade hb. The horizontal force on the blade Fh is positive from right

to left always opposite to the direction of the cutting velocity vc. The vertical force on the blade Fv is positive downwards.

Since the vertical force is perpendicular to the cutting velocity, the vertical force does not contribute to the cutting power,

which is equal to:

c h cP F v= ⋅= ⋅= ⋅= ⋅ (Equation 1)

In clay the cutting processes are dominated by cohesion and adhesion (internal and external shear strength). Because of the

φ=0 concept, the internal and external friction angles are set to 0. Gravity, inertial forces and pore pressures are also neglected.

This simplifies the cutting equations. Clay however is subject to strengthening, meaning that the internal and external shear

strength increase with an increasing strain rate. The reverse of strengthening is creep, meaning that under a constant load the

material will continue deforming with a certain strain rate.

Figure 12 The Curling Type, the Flow Type, the Tear Type and the Shear Type.

OTC 23336 11

Under normal circumstances clay will be cut with the flow mechanism, but under certain circumstances the curling type or

the tear type may occur.

The Forces

The most common failure mechanism in clay is the Flow Type as is shown in Figure 12, which will be considered first.

The Curling Type and the Tear Type may occur under special circumstances and can be derived from the equations of the

Flow Type.

Figure 14 illustrates the forces on the layer of soil cut. The forces shown are valid in general. The forces acting on this

layer are:

1. A normal force acting on the shear surface N1 resulting from the effective grain stresses. 2. A shear force C as a result of pure cohesion ττττc. This force can be calculated by multiplying the cohesion c/cohesive

shear strength ττττc with the area of the shear plane.

3. A force normal to the blade N2 resulting from the effective grain stresses.

4. A shear force A as a result of pure adhesion between the soil and the blade ττττa. This force can be calculated by

multiplying the adhesion a/adhesive shear strength ττττa of the soil with the contact area between the soil and the blade.

The forces acting on a straight blade when cutting soil, can be distinguished as:

• A force normal to the blade N2 resulting from the effective grain stresses.

• A shear force A as a result of pure adhesion between the soil and the blade ττττa. This force can be calculated by

multiplying the adhesive shear strength ττττa of the soil with the contact area between the soil and the blade.

These forces are shown in Figure 14. If the forces N2 and S2 are combined to a resulting force K2 and the adhesive force

and the water under pressures are known, then the resulting force K2 is the unknown force on the blade. By taking the

horizontal and vertical equilibrium of forces an expression for the force K2 on the blade can be derived.

Pure clay under undrained conditions follows the φ=0 concept, meaning that effectively there is no internal friction and

thus there is also no external friction. Under drained conditions clay will have some internal friction, although smaller than

sand. The reason for this is the very low permeability of the clay. If the clay is compressed with a high strain rate, the water in

the pores cannot flow away resulting in the pore water carrying the extra pressure, the grain stresses do not change. If the grain

stresses do not change, the shear stresses according to Coulomb friction do not change and effectively there is no relation

between the extra normal stresses and the shear stresses, so apparently φ=0. At very low strain rates the pore water can flow

out and the grains have to carry the extra normal stresses, resulting in extra shear stresses. During the cutting of clay, the strain

rates, deformation rates, are so big that the internal and external friction angles can be considered to be zero. The adhesive and

cohesive forces play a dominant role, so that gravity and inertia can be neglected. With the relations for the cohesive force C,

the adhesive force A and the adhesion/cohesion ratio r (the ac ratio r):

(((( ))))ic h w

Csin

λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅====

ββββ

(Equation 2)

(((( ))))ba h w

Asin

λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅====

αααα

(Equation 3)

Figure 14 The forces on the blade in clay.

Figure 13 Clay cutting definitions.

12 OTC 23336

b

i

a hr=

c h

⋅⋅⋅⋅

⋅⋅⋅⋅ (Equation 4)

The horizontal Fh and vertical Fv cutting forces can be determined according to:

(((( )))) (((( )))) (((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))

bi

h i

sin sina h wc h wsin sin r

C sin A sin sin sin sin sinF c h w

sin( ) sin( ) sin

α βα βα βα βλ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅⋅ α + ⋅ β + ⋅⋅ α + ⋅ β + ⋅⋅ α + ⋅ β + ⋅⋅ α + ⋅ β + ⋅

⋅ α − ⋅ β β α β α⋅ α − ⋅ β β α β α⋅ α − ⋅ β β α β α⋅ α − ⋅ β β α β α= = = λ ⋅ ⋅ ⋅ ⋅= = = λ ⋅ ⋅ ⋅ ⋅= = = λ ⋅ ⋅ ⋅ ⋅= = = λ ⋅ ⋅ ⋅ ⋅

α +β α + β α +βα +β α + β α +βα +β α + β α +βα +β α + β α +β (Equation 5)

(((( )))) (((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))(((( ))))

(((( ))))

bi

i

cos cosa h wc h wcos cos r

C cos A cos sin sin sin sinF c h w

sin sin( ) sinνννν

α βα βα βα βλ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅λ ⋅ ⋅ ⋅⋅ α − ⋅ β − ⋅⋅ α − ⋅ β − ⋅⋅ α − ⋅ β − ⋅⋅ α − ⋅ β − ⋅

⋅ α − ⋅ β β α β α⋅ α − ⋅ β β α β α⋅ α − ⋅ β β α β α⋅ α − ⋅ β β α β α= = = λ ⋅ ⋅ ⋅ ⋅= = = λ ⋅ ⋅ ⋅ ⋅= = = λ ⋅ ⋅ ⋅ ⋅= = = λ ⋅ ⋅ ⋅ ⋅

α + β α + β α + βα + β α + β α + βα + β α + β α + βα + β α + β α + β

(Equation 6)

Finding the Shear Angle

There is one unknown in the equations and that is the shear angle β. This angle has to be known to determine cutting

forces, specific energy and power. The shear angle ββββ is determined by the case where the horizontal cutting force hF is at a

minimum, based on the minimum energy principle:

(((( )))) (((( )))) (((( )))) (((( )))) (((( )))) (((( )))) (((( )))) (((( ))))(((( ))))(((( )))) (((( )))) (((( ))))

2 2 2

h2 2 2

2 r sin cos sin sin sin sin 2 sin r sinF0

sin sin sin

⋅ ⋅ β ⋅ β ⋅ α + β ⋅ α − α ⋅ α + ⋅β ⋅ α + ⋅ β⋅ ⋅ β ⋅ β ⋅ α + β ⋅ α − α ⋅ α + ⋅β ⋅ α + ⋅ β⋅ ⋅ β ⋅ β ⋅ α + β ⋅ α − α ⋅ α + ⋅β ⋅ α + ⋅ β⋅ ⋅ β ⋅ β ⋅ α + β ⋅ α − α ⋅ α + ⋅β ⋅ α + ⋅ β∂∂∂∂= == == == =

∂β∂β∂β∂β α + β ⋅ α ⋅ βα + β ⋅ α ⋅ βα + β ⋅ α ⋅ βα + β ⋅ α ⋅ β (Equation 7)

For very hard clay, with shear strength c 400 kPa==== , a layer thickness of ih 0.1 m==== and a blade height of bh 0.2 m==== , Figure

15, Figure 16 and Figure 17 give the values of the shear angle ββββ , the horizontal cutting force hF and the vertical cutting force

Fv for different values of the adhesion/cohesion (ac) ratio r and as a function of the blade angle.

Figure 17 The vertical cutting force as a function of the blade angle and the ac ratio r (c=400kPa).

Figure 15 The horizontal cutting force as a function of the blade angle and the ac ratio r (c=400kPa).

Figure 16 The shear angle as a function of the blade angle and the ac ratio r.

OTC 23336 13

Specific Energy

In the dredging industry, the specific cutting energy Esp is described as: the amount of energy, that has to be added to a

volume unit of soil (e.g. clay) to excavate the soil.

The dimension of the specific cutting energy is: kN/m² or kPa for sand and clay, while for rock often MN/m2 or MPa is

used. For the case as described above, cutting with a straight blade with the direction of the cutting velocity vc perpendicular to

the blade (edge of the blade), the specific cutting energy Esp is:

h c hsp

i c i

F v FE

h w v h w

⋅⋅⋅⋅= == == == =

⋅ ⋅ ⋅⋅ ⋅ ⋅⋅ ⋅ ⋅⋅ ⋅ ⋅ (Equation 8)

The cohesion c can be related to the SPT value of the clay by a factor 6, further, the strengthening λ factor will have a

value of about 2 at normal cutting velocities of meters per second, this gives:

c 2 6 SPT 12 SPTλ ⋅ ≈ ⋅ ⋅ = ⋅λ ⋅ ≈ ⋅ ⋅ = ⋅λ ⋅ ≈ ⋅ ⋅ = ⋅λ ⋅ ≈ ⋅ ⋅ = ⋅ (Equation 9)

Now a simplified equation for the specific energy Esp is found by:

2 2

sp

sin ( ) r sin ( )E 12 SPT

sin( ) sin( ) sin( )

α + ⋅ βα + ⋅ βα + ⋅ βα + ⋅ β= ⋅ ⋅= ⋅ ⋅= ⋅ ⋅= ⋅ ⋅ α + β ⋅ β ⋅ αα + β ⋅ β ⋅ αα + β ⋅ β ⋅ αα + β ⋅ β ⋅ α

(Equation 10)

Figure 18 shows the specific energy Esp and the production Pc per 100 kW installed cutting power as a function of the SPT

value.

0 10 20 30 40 50 60 70 80 90 100

0

600

1200

1800

2400

3000

3600

4200

4800

5400

6000

Specific energy

SPT

kP

a

r = 0.25 r = 0.50 r = 1 r = 2 r = 4

1 10 100

10

100

1000

10000

100000

Production per 100 kW

SPT

m3

/ho

ur

r = 0.25 r = 0.50 r = 1 r = 2 r = 4

Specific energy and production for a 60 degree blade in clay.

Figure 18 Specific energy and production in clay for a 60 degree blade