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RECOMMENDED PRACTICE

DET NORSKE VERITAS

DNV-RP-F202

COMPOSITE RISERS

OCTOBER 2010

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The electronic pdf version of this document found through http://www.dnv.com is the officially binding version Det Norske Veritas

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FOREWORD

DET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification andconsultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carriesout research in relation to these functions.

DNV service documents consist of amongst other the following types of documents:

Service Specifications. Procedual requirements.

Standards. Technical requirements. Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:

A) Qualification, Quality and Safety Methodology

B) Materials Technology

C) Structures

D) Systems

E) Special Facilities

F) Pipelines and Risers

G) Asset Operation

H) Marine Operations

J) Cleaner EnergyO) Subsea Systems

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Recommended Practice DNV-RP-F202, October 2010

Changes Page 3

MOTIVES

No design code for Fibre Reinforced Plastic, often called com-posite structures, exists today except for some special applica-tions like FRP pipes, pressure vessels and ships.

The realisation of even simple designs of FRP structures tendsto become a major undertaking due to the lack of applicabledesign standards. It is DNVs impression that the lack of agood FRP guideline is one of the major obstacles to utilise FRPstructurally in a reliable and economical way.

For this reason DNV started a JIP to develop a guideline forcomposite risers directly linked to the newly developed Off-shore Standard for Dynamic (metal) Risers, in response torequest by the industry to develop a specific standard for thisimportant application.

Upon termination of the JIP, the members participating i.e.ABB, Conoco, FMC Kongsberg Subsea, Gurit Suprem,Kvrner Oilfield Products, Norsk Hydro, Statoil, Timetagreedthat DNV shall transform the resulting project report into a

DNV Recommended Practice.The new DNV Recommended Practice is indexed: DNV-RP-F202 Composite Risers, and has a contents layout as shownoverleaf.

CHANGES

General

As of October 2010 all DNV service documents are primarilypublished electronically.

In order to ensure a practical transition from the print schemeto the electronic scheme, all documents having incorporatedamendments and corrections more recent than the date of thelatest printed issue, have been given the date October 2010.

An overview of DNV service documents, their update statusand historical amendments and corrections may be foundthrough http://www.dnv.com/resources/rules_standards/.

Main changes

Since the previous edition (May 2003), this document has beenamended, most recently in April 2009. All changes have beenincorporated and a new date (October 2010) has been given asexplained under General.

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Page 4 Changes

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Contents Page 5

CONTENTS

Sec. 1 General................................................................... 7

A. General....................................................................................7A 100 Introduction.......................................................................7

A 200 Objectives .........................................................................7A 300 Scope and application .......................................................7A 400 Other codes ....................................................................... 9A 500 Structure of the RP ........................................................... 9

B. Normative References ..........................................................10B 100 Offshore Service Specifications......................................10B 200 Offshore Standards ......................................................... 10B 300 Recommended Practices.................................................10B 400 DNV Rules......................................................................10B 500 DNV Standards for Certification

and Classification notes .................................................. 10B 600 Other (external) references .............................................11

C. General Definitions (see DNV-OS-F201)............................11C 100 Definitions ......................................................................11C 200 Verbal forms used...........................................................11

D. General Abbreviations and Symbols(see DNV-OS-F201).............................................................11

D 100 Abbreviations and symbols.............................................11

E. Definitions for Composite Risers .........................................11E 100 Definitions ......................................................................11

F. Abbreviations and Symbols forComposite Risers..................................................................12

F 100 Symbols and abbreviations .............................................12F 200 Ply and laminate co-ordinate systems............................. 13

Sec. 2 Design Philosophy and Design Principles......... 14

A. General..................................................................................14

A 100 Objective.........................................................................14A 200 Applicability ...................................................................14

B. General Safety Philosophy ...................................................14B 100 General............................................................................14

C. Design Format ......................................................................14C 100 General............................................................................14C 200 Failure types....................................................................14C 300 Reliability based design ..................................................14C 400 Design by testing combined with analysis......................14

Sec. 3 Design Input - Loads .......................................... 15

A. Introduction ..........................................................................15A 100 Introduction..................................................................... 15

B. Product Specifications ..........................................................15B 100 General function or main purpose of the riser ................15

C. Division of the Product or Structure into Components,Parts and Details ...................................................................15

C 100 Levels of division............................................................15

D. Phases ................................................................................... 15D 100 Phases..............................................................................15

E. Safety and Service Classes ...................................................15E 100 Safety classes ..................................................................15E 200 Service classes ................................................................ 16

F. Loads ....................................................................................16F 100 General............................................................................16

F 200 The sustained load effect ................................................ 16F 300 The fatigue load effects...................................................17

G. Environment .........................................................................18G 100 General............................................................................18G 200 Effects of the environment on the material properties ...18

Sec. 4 Analysis Methodology ........................................ 19

A. General..................................................................................19A 100 Objective......................................................................... 19

B. Combination of Load Effects andEnvironment .........................................................................19

B 100 General............................................................................ 19B 200 Fundamentals..................................................................19B 300 Load effect and environmental conditions

for ultimate limit state..................................................... 19B 400 Load effect and environmental conditions

for time-dependent material properties........................... 19B 500 Load effect and environmental conditions

for fatigue analysis.......................................................... 20B 600 Direct combination of loads and moments.....................20

C. Analysis Procedure for Composite Risers ............................20C 100 General............................................................................ 20C 200 'Global - Local' procedure...............................................20C 300 Global procedure with response surface.........................20

C 400 Fatigue and long term analysis for composite risers ......22

D. Local Analysis ......................................................................22D 100 General............................................................................ 22D 200 Input data ........................................................................ 22D 300 Analysis types.................................................................22D 400 Local linear analysis with degraded properties .............. 22D 500 Local progressive analysis.............................................. 23

E. Analytical Methods...............................................................23E 100 General............................................................................ 23E 200 Assumptions and Limitations .........................................23E 300 Link to Numerical Methods............................................23

F. Local Finite Element Analysis..............................................23F 100 General............................................................................ 23F 200 Modelling of structures general ...................................23F 300 Software requirements ....................................................24F 400 Execution of analysis......................................................25F 500 Evaluation of results .......................................................25F 600 Validation and Verification ............................................ 25

G. Local Dynamic Response Analysis ......................................25G 100 General............................................................................ 25

H. Impact Response...................................................................25H 100 General............................................................................ 25

I. Thermal Stresses...................................................................25I 100 General............................................................................ 25

J. Swelling Effects....................................................................25J 100 General............................................................................ 25

K. Buckling................................................................................25K 100 General............................................................................ 25K 200 Buckling analysis of isolated components...................... 26K 300 Buckling analysis of more complex elements

or entire structures .......................................................... 26

L. Partial Load-Model Factor....................................................26L 100 General............................................................................ 26L 200 Connection between partial load-model factor

and analytical analysis .................................................... 26L 300 Connection between partial load-model factor

and finite element analysis..............................................27L 400 Connection between partial load-model factor

and dynamic response analysis........................ ............... 27

Sec. 5 Design Criteria for Riser Pipes ......................... 28

A. General..................................................................................28A 100 Objective......................................................................... 28A 200 Application ..................................................................... 28A 300 Pressure testing ...............................................................28A 400 Limit states...................................................................... 28

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B. Load Effects.......................................................................... 29B 100 Design load effects..........................................................29B 200 Load effect factors ..........................................................29B 300 Load model factors..........................................................29

C. Resistance .............................................................................29C 100 Resistance factors............................................................29C 200 Geometrical parameters ..................................................30C 300 Material strength .............................................................30

C 400 Resistance model factors.................................................30C 500 System effect factor ........................................................30

D. Ultimate Limit State .............................................................30D 100 General............................................................................30D 200 Bursting ..........................................................................31D 300 Liquid tightness - leakage...............................................31D 400 Buckling .........................................................................31D 500 Propagating buckling ......................................................33D 600 Wear and tear ..................................................................33D 700 Explosive decompression................................................33D 800 Chemical decomposition - corrosion ..............................33D 900 Displacement controlled conditions................................33

E. Fatigue Limit State ...............................................................33E 100 General............................................................................33

E 200 Cyclic fatigue ..................................................................33E 300 Stress rupture...................................................................33E 400 Factors for static and dynamic fatigue analysis ..............33

F. Accidental Limit State..........................................................34F 100 General............................................................................34F 200 Resistance against fire.....................................................34F 300 Resistance against dropped objects - impact...................34F 400 Impact testing ..................................................................34F 500 Evaluation after impact testing .......................................34

G. Serviceability Limit State.....................................................34G 100 General............................................................................34

H. Special Considerations .........................................................34H 100 Interference .....................................................................34H 200 Unstable Fracture and Gross Plastic Deformation ..........34

Sec. 6 Connectors and Liners....................................... 35

A. General..................................................................................35A 100 Objective .........................................................................35A 200 Definition of joint............................................................35

B. Connector Designs................................................................ 35B 100 Functional requirements..................................................35B 200 Design and qualification considerations .........................35

C. Composite - Metal Connector Interface ...............................35C 100 General............................................................................35C 200 Limit states......................................................................35

D. Inner Liner ............................................................................36D 100 General............................................................................36

D 200 Mechanical performance.................................................36D 300 Autofretage......................................................................36D 400 Liner buckling.................................................................36D 500 Liner composite interface................................................37D 600 Liner to end connector interface .....................................37D 700 Wear and tear .................................................................37

E. Outer Liner............................................................................37E 100 General............................................................................37E 200 Mechanical performance.................................................38E 300 Blow out of outer liner ....................................................38

F. Joints of Materials or Components - general aspects ...........38F 100 Analysis and testing ........................................................38F 200 Qualification of analysis method for other load

conditions or for scaled joints .........................................38

F 300 Multiple failure modes....................................................39F 400 Evaluation of in-service experience................................39F 500 Laminated joints..............................................................39F 600 Adhesive joints................................................................39F 700 Mechanical joints ............................................................39

G. Test Requirements ................................................................39G 100 General............................................................................39G 200 Axial/ pressure test of riser with composite metal

interface...........................................................................40G 300 Cyclic fatigue testing for end fittings and composite

metal interface.................................................................40G 400 Stress rupture testing for end fittings and composite

metal interface.................................................................40G 500 Inner liner test requirements ...........................................41G 600 Specimen geometry - Scaled specimen...........................41

Sec. 7 Materials ............................................................ 42

A. General..................................................................................42A 100 Objective .........................................................................42A 200 Material Description .......................................................42

B. Fabrication ............................................................................42B 100 Objective .........................................................................42B 200 Material Description .......................................................42

Sec. 8 Documentation and Verification....................... 43

A. General..................................................................................43A 100 Documentation and verification .....................................43

Sec. 9 Operation, Maintenance, Reassessment,

Repair.................................................................. 44

A. General..................................................................................44A 100 Objective .........................................................................44

B. In-service Inspection, Replacement and Monitoring............44B 100 General............................................................................44B 200 Inspection methods .........................................................44

C. Reassessment........................................................................44C 100 General............................................................................44

D. Repair....................................................................................44D 100 General............................................................................44D 200 Repair procedure.............................................................44D 300 Requirements for a repair................................................44D 400 Qualification of a repair..................................................44

E. Maintenance.......................................................................... 45E 100 General............................................................................45

F. Retirement.............................................................................45F 100 General............................................................................45

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Sec.1 Page 7

SECTION 1

GENERAL

A. General

A 100 Introduction

101 This Recommended Practice (RP) document gives crite-ria, requirements and guidance on structural design and analy-sis of riser systems made of composite materials exposed tostatic and dynamic loading for use in the offshore petroleumand natural gas industries.

102 The major benefits in using this RP comprise:

provision of riser solutions with consistent safety levelbased on flexible limit state design principles

application of safety class methodology linking accept-ance criteria to consequence of failure

provision of state-of-the-art limit state functions in a Loadand Resistance Factor Design (LRFD) format with relia-bility-based calibration of partial safety factors

guidance and requirements for efficient global and localanalyses and introduction of a consistent link betweendesign checks (failure modes), load conditions and loadeffect assessment in the course of the global and localanalyses

allowance for the use of innovative techniques and proce-dures, such as reliability-based design methods.

103 The basic design principles and functional requirementscomply with state-of-the-art industry practice.

A 200 Objectives

201 The main objectives of this RP are to:

provide an international RP of safety for composite risersutilised for drilling, completion/ workover, production/injection, or transportation of hydrocarbons (import/export) in the petroleum and gas industries

serve as a technical reference document in contractualmatters, and

reflect the state-of-the-art and consensus on acceptedindustry practice and serve as a RP for riser design andanalysis.

A 300 Scope and application

301 This RP provides the design philosophy, loads and glo-bal analysis aspects valid for risers made of composite materi-als. The RP applies to all new built riser systems and may beapplied to modification, operation and upgrading of existingrisers.

302 The risers covered in the RP can be jointed or continu-ous. Bonded rubber risers and risers with un-bonded load bear-ing structures are not included. Applications are production,drilling and injection risers, as well as choke and kill lines.

303 Composites are fibre reinforced plastics. The fibresshould have a higher modulus than the surrounding polymericmatrix material. The matrix may be thermoset or thermoplas-tic.

304 Composite risers have typically internal and external lin-ers around the main pipe section. Any material may be chosen

for the liners, as long as long term performance of the linerscan be demonstrated. Standards related to chosen liner material

shall be used to document liner performance. Additionalrequirements to liners and interfaces are given in Sec.6.

305 Composite risers have typically metal end flanges. Anymaterial may be chosen for the flanges, as long as long termperformance of the flanges can be demonstrated. Standardsrelated to chosen flange material shall be used to documentperformance of the flanges. Additional requirements to endflanges are given in Sec.6 (composite metal interface).

306 The scope covers design, materials, fabrication, testing,operation, maintenance and re-assessment of riser systems.Aspects relating to documentation, verification and qualitycontrol are also addressed. The main purpose is to cover designand analysis of top tensioned and compliant composite risersystems operated from floaters and fixed platforms. The RPapplies for permanent operation (e.g. production and export/import of hydrocarbons and injection of fluids), as well as fortemporary operation (e.g. drilling and completion/ workoveractivities).

307 This RP is applicable to structural design of all pressurecontaining components that comprise the riser system. Othercomposite components can be designed according to DNV-OS-C501.

Guidance note:

Most composite risers of today consist of metallic or polymericliners within the composite pipes. The purpose of the liners is toprevent leakage of the riser, while the composite pipes are theload carrying part of the riser system. This RP covers risers with(and without) liners as well as riser connectors and other risercomponents such as tension joints and stress joints.

---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

308 There are, in principle, no limitations regarding floatertype, water depth, riser application and configuration. How-ever, for novel applications where experience is limited, spe-cial attention shall be given to identify possible new failuremechanisms, validity/adequacy of analysis methodology and

new loads and load combinations.Guidance note:

Composite risers are novel applications and it shall be docu-mented that the global load effects can be predicted with sameprecision as for conventional riser systems. This may typicallyinvolve validation of computational methodology by physicaltesting.

As an alternative, an appropriate conservatism in design shouldbe documented.

Procedures of DNV-RP-A203 Qualification of new technol-ogy should be considered.


309 Examples of typical floater and riser configurations areshown schematically in Fig. 1. Examples of some typical com-ponents/important areas included in typical riser systems areillustrated in Fig. 2.

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Page 8 Sec.1

Figure 1Examples of typical riser configurations and floaters

Figure 2Examples of riser components

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Sec.1 Page 9

A 400 Other codes

401 This RP shall be used in combination with the standardsfor dynamic risers and submarine pipeline systems denotedDNV-OS-F201 and DNV-OS-F101, respectively. This RPshall not be used as a stand-alone document. The RP is alsorelated to the offshore standard for composite componentsdenoted DNV-OS-C501. The limit state design checks for thisRP and DNV-OS-F201 and DNV-OS-F101 are similar, butdue to differences in the governing failure modes and prevail-ing uncertainties, some differences in safety factors exist.

402 Where reference is made to codes other than DNV doc-uments, the valid revision shall be taken as the revision thatwas current at the date of issue of this RP unless otherwisenoted, see list under B600.

403 The framework within DNV riser standards and RPs isillustrated in Fig. 3.

Figure 3Framework for DNV riser standards and RPs

404 This RP provides specific aspects related to compositerisers, including material description, local analysis and designcriteria. General design philosophy, loads and global analysisaspects valid for all riser materials are covered by the DNV-

OS-F201. The present RP document subscribes, for consist-ency, to the safety philosophy and analyses methodology setforward by this standard.

A 500 Structure of the RP

501 This RP is organised as follows:

Section 1 contains the objectives and scope of the RP. It fur-

ther introduces essential concepts, definitions and abbrevia-tions.

Section 2 contains additions to the fundamental design philos-ophy and design principles in DNV-OS-F201.

Section 3 in DNV-OS-F201 contains a classification of loadsinto pressure loads, functional loads and environmental loads.Important internal pressure definitions are given. This RP con-tains additional aspects that should be considered for compos-ite risers. In particular the description of long term loads andenvironments.

Section 4 in DNV-OS-F201 contains the framework for globalanalysis methodology. This RP provides some additions to thecombination of long term loads and concentrates mainly on thelocal analysis of composite risers.

Section 5 contains acceptance criteria for the riser pipe forULS, SLS, ALS and FLS. This includes a definition of resist-ance and load effects and safety factors for explicit limit states.It provides links to DNV-OS-C501 for specific composite fail-ure criteria.

Section 6 contains the fundamental functional requirementsfor connectors and liners. It also provides test requirements forthese components.

Section 7 contains requirements for materials. They are iden-tical to the requirements in DNV-OS-C501.

Section 8 contains requirements for documentation and verifi-cation of the riser system. They are identical to the require-ments in DNV-OS-F201.

Section 9 contains basic requirements for operation and in-service operations in addition to DNV-OS-F201.

502 The close relationship between this RP and DNV-OS-F201 and DNV-OS-C501 is shown in Fig. 4.

DNV-RP-F202

DNV-OS-F201

Deep water risers

STEEL

Design Philosophy

Loads

Analyses

DNV-OS-F101

Material

Testing

Installation

DNV-RP-F201

TITANIUM

Material

Testing

Design Criteria

COMPOSITE

Materials

Local Analysis

Testing

Design Criteria

DNV Rules

FLEXIBLES

Other

RPs

CN

Guidelines

Rules

DNV-OS-C501

Composite

Components

DNV-RP-F202

DNV-OS-F201

Deep water risers

STEEL

Design Philosophy

Loads

Analyses

DNV-OS-F101

Material

Testing

Installation

DNV-RP-F201

TITANIUM

Material

Testing

Design Criteria

COMPOSITE

Materials

Local Analysis

Testing

Design Criteria

DNV Rules

FLEXIBLES

Other

RPs

CN

Guidelines

Rules

DNV-OS-C501

Composite

Components

DNV-OS-F201

Deep water risers

STEEL

Design Philosophy

Loads

Analyses

DNV-OS-F101

Material

Testing

Installation

DNV-OS-F101

Material

Testing

Installation

DNV-RP-F201

TITANIUM

Material

Testing

Design Criteria

COMPOSITE

Materials

Local Analysis

Testing

Design Criteria

DNV Rules

FLEXIBLES

Other

RPs

CN

Guidelines

Rules

DNV-OS-C501

Composite

Components

DNV-OS-C501

Composite

Components

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Figure 4Relationship between this RP, DNV-OS-F201 and DNV-OS-C501

B. Normative References

B 100 Offshore Service Specifications

101 The following Offshore Service Specifications shall beused:

DNV-OSS-301 Certification and Verification of Pipe-lines.

B 200 Offshore Standards

201 The following Offshore Standards shall be used:

DNV-OS-F101 Submarine Pipeline Systems DNV-OS-F201 Dynamic Risers DNV-OS-C105 Structural Design of TLPs by the LRFD

Method DNV-OS-C106 Structural Design of Deep Draught Float-

ing Units DNV-OS-C501 Composite Components.

B 300 Recommended Practices

301 The following Recommended Practices shall be used:

DNV-RP-B401 Cathodic Protection Design

DNV-RP-C203 Fatigue Strength Analysis of OffshoreSteel Structures

DNV-RP-C205 Environmental Conditions and Environ-mental Loads

DNV-RP-F101 Corroded Pipelines DNV-RP-F104 Mechanical Pipeline Couplings DNV-RP-F105 Free Spanning Pipelines DNV-RP-F106 Factory applied Pipeline Coatings for Cor-

rosion Control DNV-RP-F201 Design of Titanium Risers DNV-RP-O501 Erosive Wear in Piping Systems

B 400 DNV Rules

401 The following Rules shall be used:

Rules for Certification of Flexible Risers and Pipes Rules for Planning and Execution of Marine operations.

B 500 DNV Standards for Certification and Classifica-

tion notes

501 The following Standards for Certification and Classifi-cation notes shall be used:

No. 1.2 Conformity Certification Services, Type Approval

1 General

2 DesignPhilosophy

9 Operation,Repair, Maint.,Reassessment

5 Design Criteriafor Riser Pipes

4 AnalysisMethodology

3 Design InputLoads

8 DocumentationVerification

7 Materials

6 ConnectorsLiners Joints

DNV-RP-F202

Composite

Risers

6 FailureMechanisms and

Criteria

4 Materials

10Component

testing

1 General

2 DesignPhilosophy

9 Operation,Repair,Maint.,Reassessment

5 Design Criteriafor Riser Pipes

4 AnalysisMethodology

3 Loads

8 DocumentationVerification

7 Materials

6 Connectorsand Components

DNV-OS-F201Dynamic Risers

DNV-OS-C501Composite

Components

Content of DNV-RP-F202 Composite Risers

and links to other DNV standards

...

...

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No. 7 Ultrasonic Inspection of Weld Connections No. 30.4 Foundations No. 30.6 Structural Reliability Analysis of Marine Struc-

tures

B 600 Other (external) references

601 The following other references shall be used:

BS 7910 Guide on methods for assessing the acceptabilityof flaws in fusion welded structures

API RP1111 Design, Construction, Operation, and Main-tenance of Offshore Hydrocarbon Pipelines (Limit StateDesign)

API RP2RD Design of Risers for Floating Production Sys-tems (FPSs) and Tension-Leg Platforms (TLPs)

EUROCODE 3 Design of steel structures - Part 1.1: Gen-eral rules and rules for building

ISO/FDIS 2394 General Principles on Reliability forStructures

ISO/CD 13628-7 Petroleum and natural gas industries -Design and operation of sub-sea production systems - Part7: Completion/ workover riser systems

Guidance note:

The latest revision of the referenced documents applies. The lat-est revision of the DNV documents may be found in the publica-tion list at the DNV website www.dnv.com.


C. General Definitions (see DNV-OS-F201)

C 100 Definitions

101 The general definitions are identical to and as found inDNV-OS-F201.

C 200 Verbal forms used

201 shall= indicate requirements strictly to be followed inorder to conform to this RP and from which no deviation is per-mitted.

202 should= indicate that among several possibilities oneis recommended as particularly suitable, without mentioningor excluding others, or that a certain course of action is pre-ferred but not necessarily required as other possibilities may beapplied subject to agreement.

203 may= indicate a course of action permissible withinthe limits of the RP.

204 "agreement" and or "by agreement" = agreed in writingbetween the manufacturer or contractor, and the purchaser

(unless otherwise indicated).

D. General Abbreviations and Symbols(see DNV-OS-F201)

D 100 Abbreviations and symbols

101 The general abbreviations and symbols are identical toand as found in DNV-OS-F201.

E. Definitions for Composite Risers

E 100 Definitions

101 Angle-ply laminate: symmetric laminate, possessingequal plies with positive and negative angles.

102 Anisotropy: material properties varying with the orienta-

tion or direction of the reference co-ordinate.

103 Characteristic load: reference value of a load to be usedin the determination of the load effects. The characteristic loadis normally based upon a defined fractile in the upper end ofthe distribution function load.

104 Characteristic resistance: the nominal value of thestructural strength to be used in the determination of the design

strength. The characteristic resistance is normally based upona definedfractile in the lower end of the distribution functionfor resistance.

105 Constituent: in general, an element of a larger grouping.In advanced composites, the principal constituents are thefibres and the matrix.

106 Cross-ply laminate: special laminate that contains only0 and 90 degree plies.

107 De-lamination: separation or loss of bonds of plies (the2-D layers) of material in a laminate.

108 Environmental conditions: environmental exposure thatmay harm or degrade the material constituents.

109 Environmental loads: loads due to the environment,such as waves, current, wind, ice, earthquakes.

110 Fabric: planar, woven material constructed by interlac-ing yarns, fibres or filaments.

111 Failure criterion: criterion to define or identify whenfailure has occurred, usually expressed as an inequality in thegoverning variables, e.g. load greater than resistance.

112 Failuremechanism: a mechanism of failure is the under-lying phenomenon at the material level that determines themode of failure. Depending on its level of severity a mecha-nism of failure can lead to various failures. Failure mecha-nisms are specific to material type.

113 Failure mode: state of inability to perform a normalfunction, or an event causing an undesirable or adverse condi-

tion, e.g. violation of functional requirement, loss of compo-nent or system function, or deterioration of functionalcapability to such an extent that the safety of the unit, person-nel or environment is significantly reduced.

114 Failuretype: failure types are based on safety margin,intrinsic to a given failure mechanism. A distinction is madebetween catastrophic and progressive failures, and betweenfailures with or without reserve capacity during failure.

115 Fibre Reinforced Plastic (FRP): a general term poly-meric composite reinforced by fibres.

116 Fibre: single filament, rolled or formed in one direction,and used as the principal constituent of woven or non-wovencomposite materials.

117 Filament: the smallest unit of a fibrous material. Thebasic units formed during drawing and spinning, which aregathered into strands of fibre. It is a continuous discrete fibrewith an effective diameter in the range of few micrometersdepending on the source.

118 Functional requirement: a functional requirement isdefined as a requirement that the global structure has to fulfil.

119 Glass Fibre Reinforced Plastic (GRP): general term pol-ymeric composite reinforced by glass fibres.

120 Homogeneous: descriptive term for a material of uni-form composition throughout. A medium that has no internalphysical boundaries.

121 Inspection: activities, such as, measuring, examination,

testing, gauging one or more characteristic of a product or aservice, and comparing the results with specified requirementsto determine conformity.

122 Interface: boundary or transition zone between constitu-ent materials, such as the fibre/matrix interface, or the bound-

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Page 12 Sec.1

ary between plies of a laminate or layers of a sandwichstructure. Boundary between different materials in a joint. Aninterface can also be the area where two components or partstouch each other.

123 Lamina: same as ply.

124 Laminae: plural of lamina

125 Laminate: layers of a plies bonded together to form asingle structure. Also the process to build a laminate.

126 Laminate ply: same as ply.

127 Layer: a single layer of reinforcement (see also defini-tion for ply)

128 Liner: the thin wall/pipe (usually made of metal) that isapplied within the composite pipe of most composite risers.The purpose of the liner is to avoid leakage of the riser.

129 Local analysis: detailed analysis of parts of the riser sys-tem, e.g. critical cross-sections, connectors and joints. Thelocal analysis should provide stresses and strains on the plylevel.

130 Matrix: the cured resin or polymer material in which the

fibre system is imbedded in a ply or laminate.131 MCI: metal composite interface

132 Monolithic structure: laminate consisting uniquely ofcomposites materials except core materials; also called single-skin structure.

133 Off-axis: not coincident with the symmetry axis; alsocalled off-angle.

134 On-axis: coincident with the symmetry axis; also calledon-angle.

135 Orthotropic: having three mutually perpendicularplanes of material symmetry.

136 Ply: basic building block of a laminate with orthotropic

properties. Reinforcement surrounded by a matrix. Severallayers of reinforcement may form a ply. Several plies form alaminate.

137 Reinforcement: a strong material embedded into amatrix to improve strength, stiffness or impact resistance.

138 Roving: a number of strands, tows, or ends collected intoa parallel bundle with little or no twist.

139 Strand: normally an untwisted bundle or assembly ofcontinuous filaments used as a unit, including slivers. Tows,ends, yarn and so forth, sometimes a single filament are calleda strand.

140 Stacking sequence: a description of the orientation ofplies in a laminate.

Guidance note:

The term stacking sequence is also often used to describe theorder riser joints are mounted to make up an entire riser. It shouldbe clear from the context which definition is valid.


141 Warp: the direction along which yarn is orientated lon-gitudinally in a fabric and perpendicularly to the fill yarn.

142 Weft: the transversal threads of fibres in a woven fabricrunning perpendicular to the warp.

F. Abbreviations and Symbols forComposite Risers

F 100 Symbols and abbreviations

101 The symbols, abbreviation subscripts etc. given in

Table F1 to Table F5 are used.

Table F1 Definitions of symbols for variables

Variables

1,2,3 ply, laminate, or core local co-ordinate system , 1 beingthe main direction

a half crack length

ai scalarAi,j matrix A components

[A] extensional stiffness matrix

b width

C swelling agent concentration coefficient

COV coefficient of variation, i.e., standard deviation overmean

D flexural rigidity

e core width

{e} expensional strain field

E modulus of elasticity

ei general expensional strain

f correction factor scalar

G shear modulusG strain energy release rate

h height of boxed beam

H anisotropy factor

I 2nd moment of area

k scalar

K stress intensity factor

l length

m surface mass

M moment

N in-plane load

Qi,j matrix Q components

[Q] stiffness matrixR Resistance or Radius of pipe

S shear stiffness, local or global structure response

SCF stress concentration factor

Si,j matrix S components

[S] transformed compliance matrix

t thickness

T transverse load, temperature

U strain energy

u, v, w displacement in (x, y, z)

V volume fraction

x, y, z global co-ordinate system

failure criteria function

ratio between quantiles in the marginal distributionsand extreme-value distributions

thermal expansion coefficient

loading mode factor

thermal swelling coefficient, or boundary conditionsfactor

direct strain, i.e. 1 in the main direction

strain to failure

{} strain field

shear strain

F partial load factors

FM partial load and resistance factor

M partial resistance factorsRd partial model factor, resistance component

Sd partial model factors, load component

mean value

Poissons ratio, i.e. major12, minor21

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Sec.1 Page 13

F 200 Ply and laminate co-ordinate systems

201 Local co-ordinate system and symmetry planes in anorthotropic bi-directional ply is shown in Fig. 5.

Figure 5Local co-ordinate system and symmetry planes in an orthotropicbi-directional ply

ply angle

density

direct stress, i.e. 1 in the main direction, or standarddeviation

strength, or stress to failure

{} stress field

shear stress, i.e. 12

(or12

sometimes)

angular velocity

Table F2 Definitions of subscripts

Subscripts

b bending effects

ben bending

c compression

core core

corrected value corrected by using a correction factor

cr critical

d design

Delam delaminationE(n) time curve

face face

Fiber fiber

i effects due to in-plane size of sandwich beam

ip effects due to in-plane size of sandwich panel

k characteristic value

Matrix matrix

max maximum

meas measured value

min minimum

nom nominal

ply ply

ref mean of the measured values

Shear shear

sl shear-loaded

SLS serviceability limit state

t tension

tc core thickness effects

typ typical value

ULS ultimate limit state

Table F3 Definitions of superscripts

Super-scripts

maximum direct or shear stress in the structure/com-ponent

^ direct or shear stress of material at failure

* elastic or shear modulus of damaged face or core

nl non-linear lin linear

0 initial

1 final

top top face

bottom bottom face

Table F4 Definitions of sub-subscripts

Sub-subscripts

lin linear limit

y, vz, w

x, uO

23

1

23

1

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Page 14 Sec.2

SECTION 2DESIGN PHILOSOPHY AND DESIGN PRINCIPLES

A. General

A 100 Objective101 The purpose of this section is to present the safety phi-losophy and corresponding limit state design format applied inthis RP.

102 The design philosophy and design principles are thesame as stated in the DNV-OS-F201. This RP refers to thisstandard and addresses additional issues that are relevant forcomposite risers.

A 200 Applicability

201 This section applies to all risers that are to be built inaccordance with this RP.

B. General Safety Philosophy

B 100 General

101 The general safety philosophy as described in the DNV-OS-F201, is also applicable for composite risers.

102 The following issues are addressed in DNV-OS-F201:

safety objective systematic review fundamental requirements operational considerations design principles quality assurance and quality system.

C. Design Format

C 100 General

101 The design objective is to keep the failure probability(i.e. probability of exceeding a limit state) below a certainvalue. All aspects described in the DNV-OS-F201 are alsoapplicable for composite risers.

102 The following issues are addressed in DNV-OS-F201:

safety class methodology design by LRFD-method reliability based design

design by testing.103 Additional requirements specific for composite risersare given below.

C 200 Failure types

201 Composite materials can fail in different ways than met-als. The safety factors given in this RP are linked to failuretypes that are modelled by the design criterion. Failure typesare based on the degree of pre-warning intrinsic to a given fail-ure mechanism. A distinction is made between catastrophicand progressive failures, and between failures with or withoutreserve capacity during failure. The failure types for each fail-

ure mechanism described in this RP are specified for eachdesign criterion.

The specification is based on the following definitions: failure type 'ductile' = corresponds to ductile failure mech-

anisms with reserve strength capacity. In a wider sense, itcorresponds to progressive non-linear failure mechanismswith reserve capacity during failure. The design criteriondescribes the onset of the failure process, e.g. it is based onthe yield point and not the ultimate strength, even thoughit is used to describe total failure

failure type 'brittle' = corresponds to brittle failure mecha-nisms. In a wider sense, it corresponds to non-stable fail-ure mechanisms.

202 The different failure types should be used under the fol-lowing conditions for materials that show a yield point.

The failure type 'ductile' may be used if the design criterion isapplied to the yield point , and: ult > 1.2 yield and ult > 2yield where ult is the ultimate strength at a strain ult , andyield is the yield strength at a strain yield .

The failure type 'ductile' may be used if onset of damage ismodelled, but extensive damage is needed to cause failure, e.g.for the onset of matrix cracking, when failure is related to leak-age and only a substantial number of cracks causes leakage.

In all other cases, the failure type 'brittle' shall be used.

C 300 Reliability based design

301 As an alternative to design according to the formatsspecified and used in this RP, a recognised structural reliabilityanalysis (SRA) design method may be used. All requirementsgiven in DNV-OS-F201 shall be followed.

302 As far as possible, target reliability levels shall be cali-brated against existing riser designs that are known to haveadequate safety. If this is not feasible, the target safety levelshall be as given in Table C1. The values are nominal valuesreflecting structural failure due to normal variability in loadand resistance but excluding gross error.

C 400 Design by testing combined with analysis

401 Testing may be performed as described in DNV-OS-F201. Additional guidance and requirements are given inSec.4, Sec.5 and Sec.6.

Table C1 Target annual failure probabilities PFT for ULS, FLS

and ALS

Failure consequence

Failure type Lowsafetyclass

Normalsafetyclass

Highsafetyclass

Ductile failure type (e.g. as forsteel)

PF = 10-3 PF = 10

-4 PF = 10-5

Brittle failure type (base casefor composite)

PF = 10-4 PF = 10

-5 PF = 10-6

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Sec.3 Page 15

SECTION 3DESIGN INPUT - LOADS

A. Introduction

A 100 Introduction101 The offshore standard DNV-OS-F201 Sec.3 contains aclassification of loads into - pressure loads, functional loadsand environmental loads. Important internal pressure defini-tions are given. All these are also relevant for composite risers.

102 This RP contains additional aspects that should be con-sidered for composite risers. In particular the description oflong term loads and environments.

B. Product Specifications

B 100 General function or main purpose of the riser

101 The general function or the main purpose of the riser andits main interactions with other components and the environ-ment shall be specified in the product specifications.

102 The design life in service should be specified in theproduct specifications.

Guidance note:

E.g., the riser will work as a production riser for a deep waterfield of 1500 m for 25 years.


C. Division of the Product or Structure into

Components, Parts and Details

C 100 Levels of division

101 The following levels of division of the riser (product orstructure) are used in this RP:

riser (structure / product) sub-structure / sub-product components parts details.

102 The riser can be divided into sub-products or sub-struc-tures, each of which may belong to different safety classes.

103 The riser can be divided into components correspondingto the same safety class but may be subject to different func-tional requirements.

104 Each component can be divided into parts and each partinto details.

Guidance note:

Structure = riser

Sub-structure = The riser can be divided into to sub-structurescorresponding to different safety classes, e.g. parts of the riserunderneath the platform and parts far away from the platform.

Components = the riser could be constituted of an inner liner, anouter shell and the connectors (flanges). The liners function is tokeep the riser tight, whereas the shells function is to carry thepressure loads. The two components have different functional

requirements. The connector caries all loads and transfers theloads into the main body of the riser.

Parts and details = Different design approaches and design solu-tions may be used for the different parts and details.


105 A structure or substructure is an independent part forwhich a safety class can be defined. Components, parts and

details are part of a structure or substructure. Failure of any ofthese components, parts or details shall be seen in combinationwith each other.

106 The interfaces between parts, components or structuresshould be considered carefully. Interfaces shall be analysed asa part itself if they belong to a continuous structure. If the inter-faces are physical interfaces, the requirements of Sec.6 D andF shall be considered.

D. Phases

D 100 Phases

101 The design life of the riser shall be divided into phases,i.e. well-defined periods within the life span of the product.

102 All phases that could have an influence on the design ofthe riser shall be considered.

103 As a minimum, the construction phase and the operationphase shall be considered. However, it may be convenient tosplit the design life into more detailed phases as shown inTable D1.

104 Spooling should be considered as part of the construc-

tion phase if relevant for the riser solution.105 A decommissioning phase may be specified in somecases.

106 The duration of each phase should be specified. Espe-cially, the lifetime in service shall be specified.

E. Safety and Service Classes

E 100 Safety classes

101 The riser can be divided into sub-structures, each ofwhich may belong to different safety classes.

102 For each sub-structure the safety classes, as described inDNV OS F201 Sec.2 C, shall be specified and documented.

103 The safety class of a riser or its sub-structures maychange from one phase to another during the life of the riser.

Table D1 Design phases

Manufacturing Construction

Fabrication / Assembly

TransportHandling

Storage

Installation

Testing

Commissioning

Operation Operation

MaintenanceBOP handling

Repair

Retrieval / re-circulation Post-operation

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Page 16 Sec.3

E 200 Service classes

201 The riser may be divided into sub-structures, each ofwhich may belong to different service classes.

Guidance note:

Service classes may be used to discriminate between parts of ariser system with different maintenance requirements. For exam-ple, some parts of a riser system, which are less accessible, could

be designed for a lower maintenance frequency.---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

F. Loads

F 100 General

101 Loads for composite risers are as specified in DNV-OS-F201. Loads and deformations are categorised into fourgroups:

pressure (P) loads functional (F) loads

environmental (E) loads accidental (A) loads

102 All the load cases shall be described separately for eachphase during the design life of the structure.

103 Long term loads need special considerations. The effectof permanent loads like top tension shall be considered andfatigue loads shall be known in terms of mean loads and ampli-tude. More details are given below.

F 200 The sustained load effect

201 The sustained load effect value should be used for thedetermination of time-dependent material properties asdescribed in DNV-OS-C501 Sec.4.

Guidance note:In general, it would be very conservative to determine the timedependent degradation of material properties under long-termloads by using the characteristic load effect value (i.e. extremeload effect value). The sustained value is defined in this RP as anaverage load effect value over the lifetime of the product.


202 Sustained load values are defined over an observationperiod, which can correspond to the entire design life of theriser or to a part of that design life. This observation periodshould be divided into several time intervals. Time intervalsshould not be chosen shorter than 1 hour. The maximum lengthof a time interval depends on the load variations. Variations inmagnitude of the load within a time interval shall not be larger

than half the absolute load amplitude during the total observa-tion period.

203 Load effects are divided, according to their variationwith time, into:

permanent load effects; effects likely to act or be sustained

throughout the design life and for which variations in mag-nitude with time are negligible relative to their mean val-ues; or load effects which are monotonically in - ordecreasing until they attain some limiting values

variable load effects; effects which are unlikely to actthroughout the specified design life or whose variations inmagnitude with time are random rather than monotonicand not negligible relative to their mean values.

204 The sustained value of permanent load effects shall cor-respond to their characteristic value, the 99% quantile in thedistribution of the annual extreme value.

205 The sustained value of variable load effects is defined asthe mean value of the effects over the time interval. The sus-tained value Ss during the time interval to is determined suchthat the corresponding total duration above Ss is a portion= 0.5 of the exposure period ts. See Fig. 1:

Figure 1Sustained value of a variable load effect

206 The sustained value of the stress or strain fluctuations(load effect fluctuations) shall be specified within each obser-vation period for each time intervals.A 'table' of the following form should be established.

207 The sustained value of a load effect over an observationperiod may conservatively be chosen as the maximum value ofthat load effect during the observation period.

208 The sustained conditions should be considered for fail-ure mechanisms or material property changes governed or

influenced by long-term load effects.Guidance note:

For example, thesustainedload effect value shall be used for thecalculation of creep and for stress rupture.


Exposure time (duration) Sustained value

ts Ss

i

si tt

t ime

Load effec t S

Sust a inedvalue S s

e x p o s u re p e ri o d t s

t1 t2 t3

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Sec.3 Page 17

Figure 2Division into time intervals and definition of sustained values Ssi for different load effect cases

F 300 The fatigue load effects

301 All load effect fluctuations, e.g. stress or strain fluctua-tions, imposed during the entire design life, shall be taken intoaccount when determining the long-term distribution of stressor strain ranges. All loads as given in F100 and all phases shallbe included and both low-cycle fatigue and high-cycle fatigueshall be considered.

302 Fatigue may be analysed for load effects in terms ofeither stress or strain. Strain is preferred for composite lami-nates.

303 The characteristic distribution of load effect amplitudesshould be taken as the expected distribution of amplitudesdetermined from available data representative for all relevantloads. This is a long-term distribution with a total number ofstress/strain cycles equal to the expected number of stress/strain cycles over a reference period such as the design life ofthe structure.

304 For fatigue analysis, the mean and amplitude of thestress or strain fluctuations shall be specified. A 'table' of thefollowing form should be established.

As an alternative to the presentation in table format, the fatigueloads can be presented on matrix form with one row for eachmean strain, one column for each strain amplitude, and numberof cycles as the entry of each matrix element; i.e.

Matrix representation of rain-flow counted strain amplitudedistribution.

Guidance note:The history of mean and amplitude of stress should be estab-lished on discrete form by a rain-flow analysis.

A minimum resolution of the discrete stresses has to be definedbefore the stress history is established.

Note that for the fatigue analysis the history of mean stress/ strainand amplitude is needed. In a non-linear analysis, the mean mayshift relative to the amplitude during the transfer from appliedload to load response.

If the time duration of some cycles is long or if the mean value isapplied over a long time, these loads may have to be consideredfor sustained load cases (stress rupture) as well.

Degradation is a non-linear, history-dependent process. If differ-ent load and environmental conditions can cause different degra-

dation histories, all relevant load combinations shall beconsidered.


305 Based on the material properties, in particular the char-acteristic S-N curve and the magnitude of its slope parameter,it shall be assessed whether the bulk of the fatigue damage willbe caused by several thousand or more stress cycles from thecharacteristic stress distribution, or if it will be caused by onlyone or a very few extreme stress amplitudes from this distribu-tion. In the former case, the natural variability in the individualstress amplitudes can be disregarded as its effect on the cumu-lative damage will average out, and the partial load factor canbe set equal to 1.0. In the latter case, the natural variability inthe few governing extreme stress amplitudes cannot be disre-garded and needs to be accounted for by a partial load factorgreater than 1.0. If no detailed analysis of the load factor canbe made, the same factors as those given for static loads shallbe used.

S

t

S s

S

t

S s 1

S s 2

S

t

S s

S

t

S s 1

S s 2

t 1 t 2

S

t

S s 1

S s 2

t 1 t 2

Number of cycles Mean load Amplitude

N S A

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Page 18 Sec.3

G. Environment

G 100 General

101 The term environment designates in this RP the sur-roundings that impose no direct load on the product.

Guidance note:

Environment can be chemicals, temperature. The environmentshould not be confused with environmental loads as defined infollowing interactions should be considered:


102 The environment may impose indirect loads on thestructure, e.g. thermal stresses or swelling due to moistureuptake. This should be considered as a load effect and shouldbe calculated according to the relevant parts of Section 4.How-ever, the environment is generally considered for itseffect on the degradation of material strength or change of elas-tic properties.

103 The following aspects should be considered when eval-uating the effect of the environment on local volume elementsin a structure:

direct exposure possible exposure if protective system fails exposure after time exposure after diffusion through a protective layer exposure after accident exposure after degradation of a barrier material, or any

material.

Guidance note:

The most common environments to be considered are given in

Table G1.

104 The time history of all quantities that characterise envi-ronmental conditions (e.g. temperature, humidity) should bedocumented for each phase during the design life of the struc-ture.

105 The time history of all environments should be docu-mented for the entire life of the product. Time histories andcharacteristic values should be established according to thesame principles as described for loads in Sec.4 B.

106 Different environmental values are defined in this RP:

the characteristic value the sustained value

Guidance note:

The definition of the different load values is summarised

in Table G2, for further details the definitions presented in rele-vant chapters shall be used.

For example: when considering temperature as an environment,the following values can be defined:

- sustained environmental value corresponding to the averagetemperature

- extreme environmental value corresponding to the maximumtemperature

- accidental environmental value corresponding to a fire situa-tion

- fatigue environmental values corresponding temperature fluc-tuations imposing thermal stress fluctuations in the material.


107 The notion of fatigue value for the environment is not

considered in this chapter. If the environment imposes indirectfatigue loads on the structure the loads and their resultingstresses should be considered according to Sec.4 B, e.g. cyclicthermal stresses, stresses from waves and currents etc.

108 Different types of loads and environment shall be com-bined. Depending on which load and environment values arecombined, different load and environmental conditions aredefined. These different load and environmental conditionsdefine the different design cases to be considered. Thesedesign cases are described in Sec.4 B.

G 200 Effects of the environment on the material prop-

erties201 All possible changes of material properties due to theeffect of the environment should be considered.

Guidance note:

The following interactions should be considered:

- temperature: variation of the mechanical properties (stiffness,strength)

- exposure to water (salinity / corrosion, marine fouling)- exposure to humidity

- exposure to chemicals- exposure to UV- exposure to other radiation

- erosion.---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

202 The degradation of material properties caused by envi-ronmental conditions is described in DNV-OS-C501 Sec.4.

Table G1 Common environments

Natural Temperature internal and external

Temperature variations

Temperature gradients

UV radiation (if above the water line)

Moisture

Sea water

Animals (e.g. shark bites

Functional Transported or contained fluids and chemicals

Temperature internal and external

Pressure internal and external

Oil spill

Cleaning materials

Paint solvents

Accidental chemicals

Fire

Process gas leaks

Service induced shocks

Accidental high pressure steam

Table G2 Load Values

Designation Definition To be used for

Characteris-tic value

Extreme value with returnperiod of 100 years

Check of UltimateLimit States

Sustainedvalue

Average value over a longperiod

Long-term degra-dation of materialproperties

Fatiguevalue

Only for loads

Accidentalvalue

See DNV-OS-F201

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Sec.4 Page 19

SECTION 4ANALYSIS METHODOLOGY

A. General

A 100 Objective101 The purpose of this section is to provide an overview ofthe analysis methodology for composite risers.

102 Global analysis shall be performed as described inDNV-OS-F201.

103 All phases identified in Sec.3 D shall be analysed.

B. Combination of Load Effects andEnvironment

B 100 General

101 The fundamental approach to combine load effects isdescribed in DNV-OS-F201.

102 Combined loading in DNV-OS-F201 is described foracceptance criteria that can be used directly with respect toapplied forces and moments. If such acceptance criteria can befound (see C300), the same methods as in DNV-OS-F201 canbe used. Otherwise, the procedures described in C200 shall beused.

103 If the local load effect is linearly proportional to theactual load, loads may be combined directly instead of com-bining load effects. See also DNV-OS-F201 Appendix C onhow to combine loads for non-linear systems.

B 200 Fundamentals

201 The combination and severity of load effects and/orenvironmental conditions should be determined taking intoaccount the probability of their simultaneous occurrence.

Guidance note:

For example, a severe wave climate producing a large wave loadis usually accompanied by a severe vessel offset producing largeaxial loads or bending moments.


202 Load effects and/or environmental conditions, which aremutually exclusive, should not enter together into a combina-tion, e.g. ice load effects and wave load effects in a riser envi-ronment.

203 All directions of load effects are to be taken as equally

probable, unless data clearly show that the probability ofoccurrence is different in different directions, or unless loadeffects in a particular direction is particularly critical.

204 Permanent load effects and permanent environmentalconditions shall be taken into consideration in all combinationsof load effects and environmental conditions. When combinedwith other load effects or environmental conditions, their char-acteristic values shall be included in the combination.

205 The following load effect and environmental conditionsare defined in this RP:

load effects and environmental conditions for ultimatelimit state

load effects and environmental conditions for time-

dependent material properties load effects and environmental conditions for fatigue anal-ysis.

206 Table B1 summarises the load and environmental condi-tions that should be considered for the determination of the

time-dependent material properties and those that should beused for the design checks during all phases of the life of the

product, e.g., installation, transport, operation, etc.

B 300 Load effect and environmental conditions forultimate limit state

301 At any time during the design life of the structure itshould be documented that the structure can fulfil its functionalrequirements for:

all characteristic load effect values combined with all sus-tained environmental values

all sustained load effect values combined with all charac-teristic environmental values.

302 When environment and load effect are fully-correlated,their characteristic values shall be combined.

303 The combination of characteristic load effects and envi-ronment should be determined such that the combined charac-teristic effect has a return-period of 100 years.

Guidance note:

A method to determine the 100-years combined effect of severalload effects and environments is described in this chapter. It isbased on the so-called Turkstras rule.


304 When several stochastic load effect and/or environmen-tal conditions occur simultaneously, the extreme combinedeffects of the associated stochastic processes are required fordesign against the ultimate limit state. Each process is charac-

terised by a characteristic value. The characteristic values areto be factored and combined to produce a design effect. Forthis purpose, a (limited) number of possible load effect and/orenvironmental condition combinations are considered. Themost unfavourable combination among these shall be foundand will govern the design.

305 The most unfavourable relevant combinations shall bedefined for every point in time during the design life.

Guidance note:

In most cases, the most unfavourable relevant combinations arethe same over the entire design life. However, in some cases con-ditions may change with time, which may in turn cause changesin the relevant combinations.


B 400 Load effect and environmental conditions fortime-dependent material properties

401 The sustained load effect values or the fatigue loadeffect values (when relevant) and the sustained environmental

Table B1 Combinations of load and environmental conditions to

be considered for the determination of material degradation

and for design checks

Loads

Characteristicvalue

Sustainedvalue

Fatiguevalue

Environ-ment

Characteristicvalue

ULS checkFully correlated

onlySee B302

ULS checkNot fullycorrelatedSee B304

Sustainedvalue

ULS checkNot fully corre-

latedSee B304

Materialdegrada-

tionSee B400

Fatigueanalysis

SeeB500

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Page 20 Sec.4

values should be used for the determination of time-dependentmaterial properties as specified in Sec.3 F200.

B 500 Load effect and environmental conditions forfatigue analysis

501 The fatigue load effects should be combined with thesustained environmental values for the fatigue analysis asspecified in Sec.3 F300.

B 600 Direct combination of loads and moments

601 The combination of load effects and environments asdescribed above should be used to obtain the load effects, i.e.,local stresses and strains.

602 If transfer functions and structural analysis are linear,loads or moments can be combined by the procedures givenabove instead of the load effects.

C. Analysis Procedure for Composite Risers

C 100 General

101 The global analysis of the riser system shall be per-formed the same way as described in DNV-OS-F201. Detailedlocal analysis should be applied for connectors/ joints andother critical parts of the riser system.

102 Risers made of composites possess a complex behaviourdue to the fact that the development of failure in compositematerials usually involves a sequence of failure mechanisms(e.g. matrix cracking, de-lamination and fibre failure), each ofwhich leads to local change of material properties.

103 Due to the large number of failure mechanisms and thefact that local effects are crucial for most failure modes relatedto composite structures, it is extremely difficult to establishanalytical acceptance criteria on a global level for all failure

modes. Therefore, local analysis should be extensively used inthe evaluation of failures for composite risers. A method toobtain global acceptance criteria by numerical analysis isgiven in 300.

104 The development of local failure mechanisms, with cor-responding local degradation of material properties, may resultin decreased values for the global stiffness parameters. Thismay affect the overall global behaviour (e.g. displacements,bending moments and effective tension) of the riser system.Thus, the parameters that serve as boundary conditions for thelocal analysis may be modified.

105 In the following two analysis procedures for compositeriser, systems are recommended. The principal differencebetween the methods is the level on which the failure criteria

(or limit states) are evaluated. Another obvious difference,which follows from the prior, is the order in which the globaland local analysis is conducted. Other analysis procedures maybe found in DNV-OS-C502 Sec.9.

C 200 'Global - Local' procedure

201 In order to evaluate the limit states one first performsglobal analysis of the entire riser system. The resulting globalload effects (e.g. effective tension, bending moment, thermalloads and internal or external overpressure) serve as boundaryconditions for the forthcoming local analysis.

202 Based on the load effects from the global analysis, localanalysis, which leads to local load effects (stresses and strains),is now conducted.

203 The local load effects resulting from the local analysisare finally applied in the local acceptance criteria (or failurecriteria) in order to detect possible failure mechanisms of theriser components.

204 If the local investigations are performed by progressive

failure analysis (E300), it is possible to detect a sequence of(acceptable) failure mechanisms that may happen prior to thefinal (unacceptable) failure mechanism (often fibre failure).Let us assume that the local analysis predicts the presence ofmatrix cracking somewhere in the riser (and that matrix crack-ing is accepted), which in turn leads to reduced riser stiffness.This local reduction of stiffness may influence the overallbehaviour of the riser system. Therefore, in certain cases it may

be necessary to repeat the global analysis (with degraded mate-rial properties where relevant). Then, the presence of addi-tional failure mechanisms should be investigated through anew local analysis. This iterative procedure (between globalanalysis (with degraded material properties) and detailed localfailure analysis) should be performed until no new failuremechanism is observed (acceptable design) or until a crucialfailure mechanism is predicted (unacceptable design).

Guidance note:

The change of axial stiffness due to local degradation mecha-nisms is usually small and does not influence the global loads onthe riser system. In such cases, the global (static and dynamic)analysis does not need to be repeated although the local analysisdemonstrates that (acceptable) failure mechanisms occur. A con-servative approach should be chosen for the simplified analysis.


C 300 Global procedure with response surface

301 As an alternative to the global local procedure pre-sented in 200, a procedure may be used that requires extensivelocal analysis to be conducted prior to the global failure analy-sis. The local analysis is used to establish response surface thatcan be used in subsequent global analysis.

302 A riser system is a relatively simple structure on a globalscale. Usually, the riser pipes contain a large number of iden-tical pieces of composite pipes that are all connected with thesame type of connectors or joints. In other applications contin-uous riser pipes, with constant properties along the pipe, may

be used. In all these situations, the following procedure forevaluation of failure may be advantageous.

303 Prior to the global analysis of the riser system, globallimit states (on the form gmax = 1) are established by perform-ing local failure analysis (of the pipes as well as the connec-tors/joints) for a large number of combinations of global loadeffects (bending moments, effective tension and internal orexternal overpressure). The global limit states are representedas surfaces in a space/ co-ordinate system with bendingmoments, effective tension and internal/external overpressurealong the axes. The surfaces are obtained by interpolating acollection of points (load cases) from the local analysis thatsatisfies gmax = 1. Such global limit states may be establishedfor several kinds of (local) failure mechanisms.

304 After these initial local investigations, the rest of theriser analysis may be performed on a global level.

305 If the initial local investigations are conducted by pro-gressive failure analysis (D500) global limit states may beestablished for a wide range of (local) failure mechanisms. Inthis way, an iterative procedure may be adopted. In the firststep (after having established the limit states) global analysis isperformed with initial (non-degraded) stiffness properties. Letus assume that a limit state (corresponding to a non-crucialfailure mechanism) is exceeded in certain global elements.Then the stiffness properties in those elements should bereduced (according to the observed local failure mechanism)and the global analysis should be repeated. This iterationshould continue until no new limit state is exceeded (accepta-ble design) or until a crucial limit state is exceeded (unaccept-able design).

Guidance note:

The change of axial stiffness due to local degradation mecha-nisms is usually small and does not influence the global loads onthe riser system. In such cases, the global (static and dynamic)

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analysis does not need to be repeated although the local analysisdemonstrates that (acceptable) failure mechanisms occur.


Guidance note:

Example of a global failure criterion. The global failure criterionshould be established for a small section of the riser that repeats

itself along the length of the string. Typically such a sectioncould be a riser joint of about 15m length consisting of a pipe sec-tion with two end fittings. A joint could also be modelled byestablishing two separate response surface, one for the pipe sec-tion and one for the joint. For a long continuous riser a globalfailure criterion would typically only be established for the pipesection. The two joints would be investigated individually.

The loads and a riser section are shown schematically in Fig. 1.Typically a section is analysed for the following loads:

pressure = P axial load = A moment = M

torsion = T

Figure 1

General loading conditions for a riser pipe


Guidance note:

The axial load can be defined as effective axial load, i.e., Theaxial load without the axial end cap load caused by the pressure,or it can be defined as the absolute axial load. Which choice ismade is a matter of convenience, but it is important to use a con-sistent approach. Torsion can often be neglected for metal risers.However, even small torsional loads may cause damage in acomposite riser, depending on the particular layout and jointgeometry.

The selected section of the riser should now be analysed for allpossible combinations of: P, A, M and T.

For each combination a stress analysis of the section is carriedout and all relevant failure criteria are checked at all places of the

section. The relevant failure criteria are at least fibre failure andbuckling, but other criteria like matrix cracking may have to beconsidered. Which criteria should be considered is described inSec. 5.

Once all combinations of: P, A, M and T have been analysed afour dimensional failure envelope can be defined for that sectionof the riser.

To make the example more specific, just a riser pipe section isdescribed in the following part. The same type of arguments canalso be used for joints or a combined pipe-joint analysis. Thelaminate of the pipe has a 0/90 orientation with the same numberof fibres running in the hoop direction as in the axial direction.

A typical failure envelope for such a laminate is shown in Fig. 2.

Figure 2Simple schematic of a failure envelope of a 0/90 laminate


Guidance note:

If the pipe is put under internal pressure, the fibres in the hoopdirection see twice as much stress as the fibres in the axial direc-tion (since we have the same number of fibres running in bothdirections). The burst pressure will be related to the maximumstress the fibres can take in the hoop direction, provided the lam-inate is thin and we have the same stress in the hoop fibres

through the thickness (a condition that is often not fulfilled forcomposite risers). The calculation gives point P1 in the globalfailure envelope on the pressure axis.

This is shown in Fig. 3 for a two dimensional P versus A, failurecriterion.

Figure 3Global failure envelope for pressure and axial force


Guidance note:

If the riser is exposed to additional effective axial loads thestresses in the axial fibres will increase. The strength of the axialfibres has to be large enough to carry the applied axial load plusthe end cap load from the pressure. This gives points P2 and P3in the global failure criterion. P2 describes the maximum axialload under maximum pressure. P3 the maximum axial load with-

out internal pressure. Ignoring Poissons effects and interactionsbetween the fibres, the failure envelope is given by lines betweenP1, P2 and P3.

Under external pressure, collapse is defined by a buckling crite-rion. The collapse pressure is shown as P4. If we assume that thecollapse pressure is not effected by an axial load, P5 indicates themaximum external pressure and maximum axial load combina-tion.

Many risers are not exposed to compre

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