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Page 1: DNV-RP-F401: Electrical Power Cables in Subsea Applicationsrules.dnvgl.com/docs/pdf/DNV/codes/docs/2012-02/RP-F401.pdf · DET NORSKE VERITAS AS Recommended Practice DNV-RP-F401, February

RECOMMENDED PRACTICE

The electronic

DNV-RP-F401

Electrical Power Cables in Subsea Applications

FEBRUARY 2012

DET NORSKE VERITAS AS

pdf version of this document found through http://www.dnv.com is the officially binding version

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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, andcarries out research in relation to these functions.

DNV service documents consist of among others 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 MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset OperationH) Marine OperationsJ) Cleaner Energy

O) Subsea Systems

© Det Norske Veritas AS February 2012

Any comments may be sent by e-mail to [email protected]

This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document, and is believed to reflect the best ofcontemporary technology. The use of this document by others than DNV is at the user's sole risk. DNV does not accept any liability or responsibility for loss or damages resulting fromany use of this document.

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Recommended Practice DNV-RP-F401, February 2012Changes – Page 3

CHANGES

General

This is a new document.

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Recommended Practice DNV-RP-F401, February 2012 Contents – Page 4

CONTENTS

1. Introduction............................................................................................................................................ 52. Scope........................................................................................................................................................ 52.1 Application................................................................................................................................................52.2 Applicable standards.................................................................................................................................52.3 Terminology..............................................................................................................................................53. Requirements.......................................................................................................................................... 63.1 General construction requirements ...........................................................................................................63.2 Conductor..................................................................................................................................................73.3 Electrical insulation of core – breakdown strength...................................................................................73.4 Screen/sheath for prevention water exposure to the insulation system ....................................................73.5 Water blocking..........................................................................................................................................83.6 Degassing..................................................................................................................................................83.7 Longitudinal gas barrier ............................................................................................................................83.8 Armour......................................................................................................................................................93.9 Anchoring of armour.................................................................................................................................93.10 Radial compression – load carrying capacity ...........................................................................................93.11 Flexibility/Compliance .............................................................................................................................93.12 Bending radius ........................................................................................................................................103.13 Coiling.....................................................................................................................................................104. References............................................................................................................................................. 10Appendix A. Qualification with Respect to Fatigue................................................................................... 11A.1 Limitations ............................................................................................................................................. 11A.2 Definitions.............................................................................................................................................. 11A.3 Input data ............................................................................................................................................... 12A.4 Pre-test straining .................................................................................................................................... 12A.5 Qualification principles.......................................................................................................................... 12A.6 Qualification based on components ....................................................................................................... 13A.7 Qualification of complete cable cross section ....................................................................................... 14A.8 Electrical verification tests..................................................................................................................... 15Appendix B. Test Methods – Fatigue Loading of Complete Cables......................................................... 17B.1 General................................................................................................................................................... 17B.2 4-point-bending...................................................................................................................................... 17B.3 Bending against template....................................................................................................................... 17Appendix C. Fatigue Testing Detection Techniques .................................................................................. 19C.1 Metallic materials................................................................................................................................... 19C.2 Plastic materials ..................................................................................................................................... 20Appendix D. Estimation of Fatigue Design Curves – Least Squares Method ......................................... 22

Appendix E. Estimation of Fatigue Design Curves - Incomplete Observations of Number of Cycles to Failure ......................................................................... 23

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Recommended Practice DNV-RP-F401, February 2012 Sec.1. Introduction – Page 5

1. IntroductionThis Recommended Practice is to be used as a supplement to ISO 13 628-5 /1/ with regards to electrical powercables. This ISO standard does not give requirements to such cables on a detailed level. This RP coversadditional requirements for power cables being submerged in seawater at large water depths and/or beingexposed to dynamic excitation, e.g. when suspended from floating production units.

The RP is intended to be used together with /1/. In case of conflict between the ISO standard and this documentthe ISO standard shall prevail.

It is a pre-requisite that power cables are designed and fabricated according to existing IEC standards.

2. Scope

2.1 ApplicationThe RP covers electrical power cables, as single cables or integrated in an umbilical in an application coveredby ISO 13 628-5 /1/.

The RP covers cables which comply with IEC 60 502-1 /2/ and IEC 60 502-2 /3/.

The RP applies to cables used for AC power transmission. DC cables are not covered.Guidance note:Examples of single cables may e.g. be power supply to direct electrical heating systems for pipelines, main powersupply from shore to floating production units, power supply from floating units to subsea installation etc.

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Guidance note:The following definition of an umbilical is given in /1/: “group of functional components, such as electric cables,optical fibre cables, hoses, and tubes, laid up or bundled together or in combination with each other, that generallyprovides hydraulics, fluid injection, power and/or communication services”.

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2.2 Applicable standardsPower cables shall comply with the following standards unless otherwise stated:

— IEC 60 502-1. Power cables with extruded insulation and their accessories for rated voltages from 1 kV(Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Part 1: Cables for rated voltages of 1 kV (Um = 1,2 kV) and 3kV (Um = 3,6 kV) /2/

— IEC 60 502-2. Power cables with extruded insulation and their accessories for rated voltages from 1 kV(Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Part 2: Cables for rated voltages from 6 kV (Um = 7,2 kV) upto 30 kV (Um = 36 kV) /3/

— IEC 60 228. Conductors of insulated cables /4/.

and the following recommendations:

— Electra 189: Recommendations for Testing of Long AC Sub-marine Cables with Extruded Insulation forSystem Voltage Above 30(36) to 150(170) kV /5/.

2.3 TerminologyThe terminology used in the document follows the terminology specified in /2/ or /3/:

AC: Alternating current.Armour: Covering consisting of a metal tape(s) or wires, generally used to protect the cable from

external mechanical effects.Note: In this RP Armour is used for the components providing the longitudinal strength and

stiffness to the cable.Conductor: Part of a cable which has the specific function of carrying current.Conductor screen: Electrical screen of non-metallic and/or metallic material covering the conductor.Core: Assembly comprising a conductor with its own insulation (and screens if any)DC: Direct current.Insulation screen: Non-metallic, semi-conducting layer in combination with a metallic layer applied on the

insulation.Insulation: Assembly of insulating materials incorporated in a cable with the specific function of

withstanding voltage.Oversheath: Non-metallic sheath applied over a covering, generally metallic, ensuring the protection of

the cable from the outside.

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Recommended Practice DNV-RP-F401, February 2012 Sec.3. Requirements – Page 6

Screen: Conducting layer or assembly of conducting layers having the function of control of theelectric field within the insulation.

Sheath: Uniform and continuous tubular covering of metallic or non-metallic material, generallyextruded. (North America: jacket)

Strand: One of the wires in a stranded conductor.In addition the term Water blocking is used for powder, tape, grease, compound, yarn or glue applied under asheath or into the interstices of a conductor to prevent water migrating along the cable.A Barrier sheath, IEC 60 050-461 /6/, having the function of protecting the insulation and its screen fromoutside contamination may be specified by the purchaser.

3. Requirements

3.1 General construction requirements

3.1.1 Insulation systemThe insulation system shall consist of a fully bonded true triple extruded XLPE system (extrusion of conductorscreen, insulation and insulation screen simultaneously). The insulation screen is not required for cablesaccording to /2/. The use of other insulation system(s) is the subject of agreement between manufacturer andpurchaser.

3.1.2 ConductorA joint of the entire conductor cross section of the conductor shall not be located in a dynamic part of a cable,i.e. parts of the cable not resting on the seabed or otherwise prevented from motion.

3.1.3 ArmourCables shall be balanced with respect to torsion. Un-balanced designs may be used subject to agreementbetween manufacturer and purchaser. Test methods and acceptance criteria may have to be modified for un-balanced designs.

3.1.4 Thermal conditionsCable routing and installation method (e.g. burial, rock dumping, guide tubes etc.) may reduce the heattransport from the cable. Ancillary equipment like bend stiffeners may act as thermal insulators on the outsideof the cable reducing the heat transport from the cable. Hence, the cable system shall be designed to meet theworst case thermal loads. The temperature shall not exceed the thermal limitations for any materials in thepower cable.

3.1.5 Longitudinal static strength of cableThe conductor and sheath(s) or screen(s) shall not be taken into account when assessing the longitudinalcapacity of the cable cross section. The strain in the conductor and sheath(s)/screen(s) shall be limited by thestrain in the load carrying elements in the cable cross section.For applications where it can be shown that it is acceptable that the conductor contributes to the longitudinalcapacity of the cable, e.g. at smaller water depths, the load carrying capacity of the conductor may be taken intoaccount. In such a case it shall be demonstrated that failure due to creep or any other failure mechanism willnot occur.A joint of the entire cable cross section shall not be located in dynamic part(s) of a cable, i.e. parts of the cablenot resting on the seabed or otherwise supported.

3.1.6 Fatigue strength of cable.Cables exposed to dynamic excitation (e.g. cables suspended between floating installations and the sea bottom,cables exposed to vortex induced vibrations) shall be qualified with respect to fatigue as specified in thisdocument. For static applications (e.g. cables resting on the sea bed) such qualification is not mandatory.However, dynamic effects during installation shall be considered.

3.1.7 Hydrostatic strengthThe components in the cable cross section shall, when the complete cross section is subjected to an externalhydrostatic pressure not smaller than the larger of 3.5 MPa or the pressure corresponding to the maximum waterdepth multiplied by a factor of 1.25, not exhibit any damage that may impair its capacity with respect tomechanical loads. Casings (including seals) for cable joints or terminations shall, when subjected to an external hydrostaticpressure not smaller than the larger of 3.5 MPa or the pressure corresponding to the maximum water depthmultiplied by a factor of 1.5, not exhibit any damage or leakage. The effect of external hydrostatic pressure on the electrical properties of the cable is handled elsewhere.

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Recommended Practice DNV-RP-F401, February 2012 Sec.3. Requirements – Page 7

3.2 Conductor

3.2.1 Static strength of conductor

The conductor shall be supported in the longitudinal direction of the cable such that failure due to creep isprevented. This shall be confirmed by calculations based on adequate test data or data available in literature.The evaluation of creep shall consider the effect of service temperature on the rate of creep. Variations in theservice temperature shall be considered or a conservative approach chosen.

Guidance note:For cables suspended in large water depths the self-weight of the conductor may induce unacceptable creep in theconductor if the conductor is not supported.

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3.2.2 Fatigue strength of conductorConductors in cables exposed to dynamic loading shall be qualified with respect to fatigue. A procedure for thequalification of power cables with respect to fatigue is given in Appendix A.A failed strand may have a detrimental effect on the conductor screen. It is recommended that failure of onestrand is used as fatigue failure criterion. Alternative failure criteria may be agreed upon between purchaserand manufacturer.The safety factor on fatigue life, determined by calculation or testing, shall not be smaller than 10 unlessotherwise agreed between manufacturer and purchaser.

Guidance note:Electra 189 includes tests for cable joints for submarine cables. These tests are however considered sufficient for staticapplications only with respect to demonstrating sufficient fatigue strength. Depending on the application, further testsneed to be carried out.

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3.3 Electrical insulation of core – breakdown strengthCable insulation for wet applications shall be qualified in accordance with CENELEC HD 605 S2 /7/ Sec.5.4.15.

It is recognised that these tests are carried out using tap water at a small water head (for practical purposes thespecified tests can be considered carried out at atmospheric pressure) and at a temperature of 40ºC. The cableswill be exposed to sea water, a significantly higher hydrostatic pressure and temperatures within a wide range,the maximum temperature exceeding 40ºC. An evaluation of the significance of the actual service conditionswith respect to the test conditions shall be carried out and the test conditions modified accordingly.For dry applications, i.e. where the cable cross section includes a sheath preventing radial water transport tothe insulation and where this sheath is duly qualified in accordance with this document, the qualificationspecified above need not be carried out.

Guidance note:Dynamic response of the cable is assumed not to have any adverse effect on the capacity of the insulation with respectto breakdown strength. It is, however, advised that research in the matter is consulted when becoming available.

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3.4 Screen/sheath for prevention water exposure to the insulation system

3.4.1 General

Sheaths for prevention or slowing down of radial water transport are not prescribed in cables conforming toIEC 60 502-1 and 2, /2/ and /3/. The requirement for a metallic sheath is the subject of agreement betweenmanufacturer and purchaser. Herein such a sheath is referred to as a Barrier sheath /6/.

3.4.2 Static strength/overstraining of screen/sheathIn suspended cables the barrier sheath shall be supported in the longitudinal direction of the cable such thatfailure due to creep is prevented. Alternatively it shall be demonstrated by analysis based on adequate data oncreep that creep failure will not occur. The evaluation of creep shall consider the effect of service temperatureon the rate of creep.

Guidance note:The hydrostatic pressure on the cable may be sufficient to support the barrier sheath if the coefficient of friction to theadjacent components in the cross section and the load carrying capacity of these components are sufficiently large.

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3.4.3 Buckling

If not properly supported, bending of a barrier sheath may induce local buckling, particularly for combinations

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Recommended Practice DNV-RP-F401, February 2012 Sec.3. Requirements – Page 8

of thin materials made from material with a large Young modulus (e.g. metallic tubes or foils). Local bucklingshall be prevented.

3.4.4 Corrosion of sheath/screen

The material in the barrier sheath shall be chosen such that it has the sufficient resistance to corrosionconsidering the service environment: exposure to sea water, temperature. There shall be no penetration of thesheath due to corrosion (holes, pits, cracks etc.) during the service life of the cable.

3.4.5 Fatigue strength of barrier sheaths – global loads

Barrier sheaths in cables exposed to dynamic loading shall be qualified with respect to fatigue. A procedure forthe qualification of power cables with respect to fatigue is given in Appendix A.

Penetration of the sheath, e.g. a crack, hole etc., shall constitute failure. The safety factor on fatigue life,determined by calculation or testing, shall not be smaller than 10 unless otherwise agreed betweenmanufacturer and purchaser.

3.4.6 Fatigue strength of barrier sheath – thermal effects/radial expansion

Repeated thermal expansion/contraction of components inside of the sheath may induce fatigue stress in thecircumferential direction. The number of load cycles may be small, but the circumferential strain induced inthe sheath may be relatively large. The number and magnitude of the load cycles shall be specified by thepurchaser as well as the relevant service temperatures.

A satisfactory fatigue life of the sheath shall be demonstrated by recognised methods taking into accountdeformation in the non-linear regime of the materials. The design with respect to fatigue shall be based onfatigue design curves or by direct testing on cable samples. Fatigue design curves shall be determined by testingat the strain levels that are relevant and be expressed as fatigue life in number of load cycles vs. strain range.Damage accumulation shall be carried out in accordance with recognised methods.

The fatigue life may alternatively be determined by testing of samples of cable by exposing the cable to heatingcycles while applying the relevant external pressure.

Penetration of the sheath, e.g. a crack, hole etc., shall constitute failure. The safety factor on fatigue life,determined by calculation or testing, shall not be smaller than 10 unless otherwise agreed betweenmanufacturer and purchaser.

3.5 Water blocking

3.5.1 Water blocking

Block against water transport along the conductor and the interstice between screen and external sheath shallbe fitted.

The type of water block is subject to agreement between purchaser and manufacturer.

3.5.2 Longitudinal water blocking along conductor

The cable shall be tested in accordance with and comply with the requirements given in /5/ Sec.4.8.3 –Conductor penetration test. The test shall be carried out with sea water, preferably with artificial sea wateraccording to recognized standards or according to agreement between purchaser and manufacture. The testpressure shall not be lower than the maximum hydrostatic pressure in operation.

3.5.3 Longitudinal water blocking between screen and external sheath

The cable shall be tested in accordance with and comply with the requirements given in /5/ Sec.4.8.3 – Outersheath penetration test. Alternative requirements may be agreed upon between manufacturer and purchaser.

The simultaneous effect of an external hydrostatic pressure on the external sheath on the water transport alongthe cable may be considered and included in the test procedure.

3.6 DegassingCables shall be de-gassed as part of the manufacturing process. The de-gassing procedure is subject toagreement between the purchaser and manufacturer.

Guidance note:There is at present no generally accepted procedure for degassing.

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3.7 Longitudinal gas barrierThe use of gas blocks, continuous or intermittent, is the subject of agreement between manufacturer andpurchaser.

The design and fabrication of the cable shall aim at minimising the volume of trapped voids inside the cable.

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Recommended Practice DNV-RP-F401, February 2012 Sec.3. Requirements – Page 9

3.8 Armour

3.8.1 Static strengthThe allowable utilization factors for load carrying elements for longitudinal loads are specified in Table 3-1.The utilisation factor shall be taken as the ratio of the applied load to the lesser of the specified minimum yieldstrength and 90% of the specified minimum ultimate tensile strength of the steel material in the armour.For armour of other material than steel the utilisation factors are subject to agreement between the purchaserand the manufacturer.

3.8.2 Fatigue strengthArmour in cables exposed to dynamic loading shall be qualified with respect to fatigue. A procedure for thequalification of power cables with respect to fatigue is given in Appendix A.The safety factor on fatigue life, determined by calculation or testing, shall not be smaller than 10.

3.9 Anchoring of armour

3.9.1 Static strengthThe utilisation factor of anchoring of armour in end terminations shall comply with Table 3-1. The utilisationratio shall be calculated as the applied load divided by the capacity of the termination.

3.9.2 Fatigue strengthArmour anchors subjected to significant fatigue loading shall be qualified with respect to fatigue strength bytesting. The safety factor on fatigue life, determined by calculation or testing, shall not be smaller than 10.

3.10 Radial compression – load carrying capacity

3.10.1 Radial compression – load casesAll radial loads on the cable cross section shall be considered. The design load cases shall include, but not belimited to:

— hydrostatic pressure— installation loads, e.g. clamping forces from caterpillar, temporary hang-off— contact forces in chutes— loads from clamps for anchors, buoyancy modules etc.— support reactions, e.g. over mid-water arches.

3.10.2 Radial compression – allowable load/stress/strainThe manufacturer shall specify the allowable compression loads, short term and long term, and/or allowablecompression strains as relevant, for each of the types of loads identified in accordance with 3.10.1.The manufacturer shall specify radial compression creep data enabling the proper design of clamps with respectto possible relaxation of clamp forces due to creep. Creep data shall reflect the service temperatures the cablewill experience.Compression may lead to damage of the semi-conducting screen when compressed on the conductor. Thisfailure mode shall be considered when determining the allowable compression force.

Guidance note:The maximum allowable compression force/strain may have a large impact on the choice of installation method,installation equipment, design of ancillary equipment like clamps etc. A clear specification of the allowables istherefore important at an early stage. The design of the cross section may be the subject to an iterative process betweenmanufacturer and purchaser.

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3.11 Flexibility/CompliancePossible requirements to the flexibility of the cable shall be clearly stated by the purchaser.The manufacturer shall specify the flexibility or the bending stiffness of the cable, maximum and minimum,for different temperatures as agreed with the manufacturer. The flexibility depends on the curvature of thecable. The flexibility should as a minimum be stated for the minimum bending radius specified by themanufacturer for installation and service.

Table 3-1 Armour utilisation factorsUtilisation factor

Normal operation 0.67Installation 0.78Abnormal operation 1.00

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Recommended Practice DNV-RP-F401, February 2012 Sec.4. References – Page 10

3.12 Bending radiusPossible requirements to the allowable minimum bending radius of the cable, during installation and operation,shall be clearly stated by the purchaser.The manufacturer shall specify the minimum allowable bending radius for different temperatures for storage,installation and operation.

3.13 CoilingCables which during manufacture, storage or installation may be subjected to coiling shall be qualified inaccordance with /8/.

4. References/1/ ISO 13628-5. Petroleum and natural gas industries - Design and operation of subsea production systems -Part 5: Subsea umbilicals. 2009./2/ IEC 60 502-1. Power cables with extruded insulation and their accessories for rated voltages from 1 kV(Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Part 1: Cables for rated voltages of 1 kV (Um = 1,2 kV) and 3 kV(Um = 3,6 kV). 2009./3/ IEC 60 502-2. Power cables with extruded insulation and their accessories for rated voltages from 1 kV(Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Part 2: Cables for rated voltages from 6 kV (Um = 7,2 kV) up to30 kV (Um = 36 kV). 2005./4/ IEC 60 228. Conductors of insulated cables. 2004./5/ Electra 189. Recommendations for Testing of Long AC Sub-marine Cables with Extruded Insulation forSystem Voltage Above 30(36) to 150(170) kV. /6/ IEC 60 050-461. International Electrotechnical Vocabulary - Part 461: Electric cables. 2008./7/ CENELEC HD 605 S2. Electric cables - Additional test methods. 2008./8/ Electra 171. Recommendations for Mechanical Tests on Sub-marine cables./9/ DNV RP-C203. Fatigue Design of Offshore Steel Structures.

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Recommended Practice DNV-RP-F401, February 2012 App.A Qualification with Respect to Fatigue – Page 11

APPENDIX A QUALIFICATION WITH RESPECT TO FATIGUE

A.1 LimitationsThis section covers qualification with respect fatigue loading in the high cycle regime.Due to the limited strain levels normally expected in service, well known insulation and oversheath materialswith large strain to failures need not be subjected to a qualification procedure. The following materials areconsidered to have adequate fatigue strength with respect to mechanical damage:

— low density thermoplastic polyethylene (PE)— high density thermoplastic polyethylene (HDPE)— cross-linked polyethylene (XLPE)— ethylene propylene rubber (EPR),

unless significantly modified with fillers, additives or similar.

For other insulation materials a qualification with respect to fatigue shall be carried out. The qualificationprogram will be subject to agreement in each particular case.

A.2 Definitions

C Curvature. C = 1/ρCmax The maximum curvature during one deformation cycle = 1/ ρminCmin The minimum curvature during one deformation cycle = 1/ ρmaxρ Bending radius. The bending radius shall be assigned a negative and a positive value when bending

occurs to either side of a straight line, see Figure A-1

Figure A-1 Definitions of bending radius sign

ρstatic The bending radius in the static equilibrium position of the cableρmax The maximum bending radius during one deformation cycleρmin The minimum bending radius during one deformation cycleε Strainεmax Maximum strain during one deformation cycleεmin Minimum strain during one deformation cycle

Rρ Curvature ratio. Rρ =

Rε Strain ratio. Rμ = εmin/ εmaxN Number of deformation cycles

D Accumulated fatigue damage

k Number of strain/stress blocksni Number of cycles in strain block i

ρ > 0ρ < 0

Umbilical/cable

min

max

/1

/1

ρρ

=k

i i

i

N

nD

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Recommended Practice DNV-RP-F401, February 2012 App.A Qualification with Respect to Fatigue – Page 12

Ni Number of cycles to failure at strain range for strain block im Slope of fatigue curve.

A.3 Input dataA detailed drawing of the cable cross section shall be available as well as specifications of all materials andcomponents included in the cross section. The specification shall be given on a level of detail that is sufficientto carry out the qualification.

A.4 Pre-test strainingThe cable and its components will be subjected to operations during which it may be deformed in ways thatmay have an effect on the fatigue properties. Examples of such operations are:

— cold work during closing and compacting of conductor— forming during cable manufacturing (e.g. metallic sheaths, screen or armour wire)— bending during reeling operations during manufacturing of the cable— bending when manufacturing the umbilical— bending during installation.

The effect of these operations on the mechanical properties shall be represented and/or simulated in arepresentative manner, on cable samples or the individual components prior to testing.

A.5 Qualification principles

A.5.1 Qualification method

The different components in the cable cross section may be qualified separately, ref. A.6. For the subsequentqualification of the complete cable cross section a reliable model of the cable cross section shall be established.This model shall as a minimum describe how the local response of the components is dependent on the globalresponse of the cable, how the different components interact in the cable cross section, how relevant materialparameters scale with size, where relevant, etc. Important factors are friction between the components.Calculation tools and assumptions for this purpose will in such a case also be subject to qualification.

Alternatively, qualification can be carried out on a complete cable cross section, i.e. all tests are carried outon samples of the complete cable as delivered from the manufacturer, ref. A.7. (In some cases it may besufficient to carry out testing on complete cores, i.e. when it can be demonstrated that the components outsidethe core do not have any effect on the test results.)

Irrespective of the approach that is chosen a number of electrical verification tests shall be carried out, ref. A.8.

For the mechanical tests relatively short samples may be used. For the electrical tests the length of thespecimens shall be as specified in the specified standards. Hence, excess cable length may have to be includedin the mechanical test specimens to carry out the subsequent electrical tests.

Guidance note:Existing calculation tools applied to umbilicals might be possible to use to calculate the response of some of thecomponents in the cross section, e.g. conductor, screen wire, armour wire.

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A.5.2 Load conditions

The fatigue load effect due to bending of the cable may be specified in terms of bending radius, i.e. adeformation. Some of the materials, e.g. copper, may be deformed non-linearly. For such materials fatigue testsand results should be performed in controlled strain and presented as strain vs. number of load cycles.

A.5.3 Load effect ratio – mean stress/strain

A cable will be subjected to a static curvature on which a dynamic curvature is superimposed and in additiona tensile load. Consequently stress or strain ratio may vary from application to application and along the cable.Stress/strain ratio shall be reflected in the qualification.

A.5.4 Temperature

Test conditions shall reflect the range of the service temperature whenever the temperature has a significantinfluence on the properties. Alternatively, generally accepted methods for modifying material properties dueto temperature effects may be applied.

Fatigue testing may heat the test specimens, particularly at higher test frequencies and/or if the specimen hassome form of insulation, e.g. when testing a complete core or cable cross section. Further, the cable containsvisco-elastic materials and some materials may be deformed into their non-linear regime, leading to furthergeneration of heat. This shall be considered when carrying out the tests. Control of the test temperature shallbe established.

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Recommended Practice DNV-RP-F401, February 2012 App.A Qualification with Respect to Fatigue – Page 13

A.6 Qualification based on components

A.6.1 Qualification testing

A.6.1.1 ConductorTest shall be carried out as uni-axial tensile tests on individual conductor strands (single wires). A fatiguedesign curve shall be established as specified in A6.2. The failure criterion is broken strand. Due to therelatively strong non-linear behaviour of copper the tests may have to be carried out in controlled strain.The possible effect on the fatigue strength of wear and/or fretting between the strands shall be considered.Fretting damage may occur due to the sliding between strands when the core is bent. The amount of frettingdepends also on the compressive force between the strands, due e.g. to external hydrostatic pressure, fitting ofclamps for buoyancy modules of anchors. It shall be demonstrated that wear/fretting is not significant withrespect to the fatigue strength. Otherwise alternative test methods shall be used, e.g. fatigue testing of thecomplete conductor, e.g. in a completed core of cable.

A.6.1.2 Solid metallic tube sheathQualification can be based on small scale testing of samples taken from the tube. A fatigue design curve shallbe established as specified in A6.2. The failure criterion is broken specimen.Test specimens shall be cut in the longitudinal direction of the tube and shall include the whole thickness. Anarrow gauge section may be included in the specimen.Test specimens shall also include longitudinal weld(s), if any. Test shall be carried out on both base materialand welds.

A.6.1.3 Sheath – other configurationsSome types of sheaths, e.g. based on metallic foils, are not possible to fatigue test as individual components.Qualification of such sheaths should be based on testing of cores or complete cable cross sections. Alternativelytesting could be carried out with the foil adhered to adjacent plastic layer for stabilisation of the foil, ref. A6.1.2.Special attention shall be given to adhesive bonds used in foils. Such bonds will be subjected to a shear loadwhen the cable is bent. The effect of the fatigue shear stress shall be considered.Other configurations of e.g. solid closed corrugated profiles may or may not be suitable to qualify based onsmall scale testing. It may be impossible from a practical point of view, or the geometry may not lend itself toreliable analysis of local stress/strain response. Such configurations should be tested either as a singlecomponent or as part of the complete cable cross section depending on the possibility to generate reliable testresults.

A.6.1.4 Longitudinal armour, wire screenSteel tensile armour will normally operate in the linear elastic regime. Fatigue testing can therefore be carriedout in load control and fatigue design curves can be presented based on stress range. A fatigue design curveshall be established as specified in A6.2. For other materials, e.g. copper screen wires, fatigue curves determined in load control may not be representative.For such materials the requirements may have to be adapted, but shall follow the same principles.Testing shall include welds for joining armour wire, if used in the dynamic section of the cable.

A.6.2 Fatigue design curve

A.6.2.1 Fatigue testingTesting shall be carried out at minimum three different stress/strain range levels. For the lowest stress/strainrange it shall be aimed at obtaining fatigue lives of at least of the order of 2×106 load cycles. For the higheststress/strain range it shall be aimed at obtaining fatigue lives of the order of 104 load cycles.A minimum of 5 valid test results shall be obtained for each stress/strain range level.A suitable method of gripping the specimens should be developed so that the specimens do not sustain damagethat may have an effect on the fatigue life. Alternatively, specimens failing in the gripping area may be acceptedif the fatigue design curve is based on these results.The tests shall preferably be carried out at the R-ratio(s) and at the mean strain(s) that is relevant based on theloading conditions. If the strain range to be tested is completely or partly on the compression side testing maybe impossible due to problems with the stability of the specimens.

A.6.2.2 Presentation and analysis of fatigue test resultsResults shall be presented as stress or strain range, as applicable, versus number of cycles to failure. The resultsshall be presented numerically and in plots that present the results in a representative manner. R-values, pretension and specimen temperature shall be stated as well as any observations that may be relevantfor the evaluation of the results.

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Recommended Practice DNV-RP-F401, February 2012 App.A Qualification with Respect to Fatigue – Page 14

Fatigue design curve(s) shall be estimated based on the data using the least squares method (ref. Appendix D).The design curve shall be given at the mean minus 2 standard deviations in log(N), i.e. representing a 97.5%probability of survival.

For data for which complete information about the number of cycles to failure is not available (e.g. when testingcomplete cross sections) the method given in Appendix E may be applied. The design curve shall represent a97.5% probability of survival. It is also referred to /9/ that gives a procedure for how to analyse data of this type.

If the number of tested stress levels is sufficiently large the mean and design curves may be divided in morethan one regression line, thus increasing accuracy.

A.7 Qualification of complete cable cross section

A.7.1 Qualification testing

The qualification program shall address as a minimum the items given in A6.

Testing shall be conducted to establish fatigue design curves (SN-curves) for the different components listedin A6 for which satisfactory fatigue life can not be demonstrated by alternative means.

Failure of the cable cross section is defined as failure of at least one of the components.

Guidance note:For a cross section with a solid metallic sheath it may be the case that the sheath will be the component with theshortest fatigue life by a significant margin. If this can be demonstrated by other means than testing of the crosssection, fatigue testing of the remaining components may not be necessary. Similar evaluations may be relevant forother components.

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

A.7.2 Fatigue design curve

A.7.2.1 Test method

The test methods shown in Appendix B may be used. Other test methods providing reliable results may beaccepted.

Testing shall be carried out for realistic R-values, including the effect of pre-tension. Alternatively generallyacceptable methods for converting test results for R-values different from the test conditions may be used.

Dynamic axial tension shall be considered.

If 4-point-bending is used, due regard to the proper control of the bending radius shall be established, seeAppendix B.

The length of the gauge section shall be at least 6 times the outer diameter of the cable, in any case not smallerthan 400 mm. The length of the specimens and the configuration of the test fixture shall be such that end effectsare eliminated in the gauge section.

A.7.2.2 Number of tests

A fatigue design curve shall be established. The curve shall be given as allowable number of load cycles vs.stress/strain/curvature range, as applicable. The curve shall be based on testing of at least three different strain/curvature range levels. For the lowest range it shall be aimed at obtaining fatigue lives of at least 2×106 loadcycles. For the highest range it shall be aimed at obtaining fatigue lives of the order of 2×103 load cycles. Non-metallic materials may require testing at additional strain levels.

A minimum of 5 valid test results shall be obtained for each strain/curvature range level.

If tests are carried out at more strain levels the design curve may be divided in more than one straight segment,thus increasing the accuracy.

A.7.2.3 Detection of failure

A clear definition of failure shall be stated when reporting the results. This definition may be based on theacceptance criteria of the component in question, A5, but may alternatively have to be defined based on whatcan be detected during the tests. For metallic materials failure may be defined as e.g. breakdown of the stiffnessof the specimen, the observation of a macroscopic fatigue crack, the breakage of a core strand etc.

Detection techniques shall be established for all the relevant acceptance criteria. Detection techniques arediscussed In Appendix C. Where non-destructive detection techniques that provide a reliable indication of thepoint of failure are not available conservative measure of the fatigue life of the specimens must be applied.

A.7.2.4 Presentation of test results and analysis

For the presentation and analyses of the results, see A6.2.2.

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Recommended Practice DNV-RP-F401, February 2012 App.A Qualification with Respect to Fatigue – Page 15

A.8 Electrical verification tests

A.8.1 General

There exists limited service experience from the use of LV and MV cables in subsea dynamic applications. Inorder to ensure that all possible failure modes are covered it is also required that cable samples shall undergostandard electrical tests followed by a prescribed fatigue load (in the form of a full scale bending test) followedby the same standard electrical tests.

The following tests shall be carried out irrespective of which qualification alternative is chosen: testing of thewhole cross section or testing of components.

A.8.2 Test method – electrical verification tests – long specimens

The tests include verifying the electrical and some mechanical properties prior to and after the test sample beingsubjected to a prescribed fatigue load.

Pre-fatigue electrical tests

The following tests, Table A-1, shall be carried out prior to fatigue loading.

Application of fatigue loading

Subsequent to the Pre-fatigue electrical tests the specimens shall be subjected to dynamic loading. The methodsshown in Appendix B may be used for applying dynamic deformation to the specimens, with due regard to thecontrol of the curvature.

The length of the gauge section (the part of the specimen exposed to dynamic loading) shall be at least 6 timesthe outer diameter of the cable, in any case not smaller than 400 mm. The length of the specimens and theconfiguration of the test fixture shall be such that end effects have been eliminated in the gauge section. Thelength of the specimens shall also be in accordance with the requirements of the specified IEC test standards.

All specimens shall be subjected to the same deformation in terms of range of curvature, strain range etc.

The curvature/strain range shall be based on the application for which cable cross section is to be qualified. Thefatigue load shall be described by a histogram or similar giving the curvature range and corresponding numberof cycles. The effective curvature ΔCeff shall be calculated according to the following equation:

The fatigue loading is thus given by the two parameters:

— the effective curvature range ΔCeff

— the total number of load cycles

The fatigue load that the cable will be exposed to in service can thus be represented by the expression:

The fatigue load used in the test shall be given by the following equation:

The range of curvature and the number of cycles in the test can in principle be chosen arbitrarily as long as theequation above is fulfilled. However, Ntest shall not be taken smaller than 50 000.

Table A-1 Pre-fatigue Electrical TestsType of test Test specification Acceptance criteriaConductor resistance IEC 60 502-2 Sec. 16.2

IEC 60 502-1 Sec. 15.2IEC 60 502-2 Sec. 16.2IEC 60 502-1 Sec. 15.2

Partial discharge IEC 60 502-2 Sec. 16.3 IEC 60 502-2 Sec. 16.3Voltage IEC 60 502-2 Sec. 16.4

IEC 60 502-1 Sec. 15.3IEC 60 502-2 Sec. 16.4IEC 60 502-1 Sec. 15.3

(A1)

(A2)

(A3)

m

k

i

mii

eff

N

CnC

/11

)(=

Δ⋅=Δ

NC meff ⋅Δ )(

NCNC mefftestmtest ⋅Δ⋅≥⋅Δ )(10)(

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Recommended Practice DNV-RP-F401, February 2012 App.A Qualification with Respect to Fatigue – Page 16

In the preceding tests more than one slope m of the fatigue curve may have been obtained, e.g. it may vary fromcomponent to component. The test condition shall be determined based on the most conservative choice of m.Post-fatigue electrical testsThe following tests, Table A-2 shall be carried out subsequent to fatigue loading.

A.8.3 Number of testsAt least 5 specimens shall be tested as described above.

A.8.4 Electrical verification testsAll specimens in the electrical verification test shall pass the acceptance criteria specified in A.8.2.For applications where the fatigue load has not yet been established the qualification can be based on aspecified fatigue load ΔCeff ⋅ N. The cable is then qualified for fatigue loads that do not exceed the specifiedfatigue load.

Table A-2 POST-FATIGUE ELECTRICAL TESTSType of test Test specification Acceptance criteriaElectricalConductor resistance IEC 60 502-2 Sec. 16.2

IEC 60 502-1 Sec. 15.2IEC 60 502-2 Sec. 16.2IEC 60 502-1 Sec. 15.2

Partial discharge IEC 60 502-2 Sec. 16.3 IEC 60 502-2 Sec. 16.3Voltage IEC 60 502-2 Sec. 16.4

IEC 60 502-1 Sec. 15.3IEC 60 502-2 Sec. 16.4IEC 60 502-1 Sec. 15.3

Mechanical/visualVerification of cross section The dimensions of all components in the

cross section shall be verified 1)All measurements to be within specified tolerances.

Visual inspection of conductor In fatigue loaded area No visual defectsVisual inspection of sheath In fatigue loaded area No visual defectsDye penetration examination of sheath

In fatigue loaded area No defects

Visual inspection of armour wires

In fatigue loaded area No visual defects

1) The following shall be measured:

— diameter or weight per unit length of conductor— inner diameter and thickness of conductor screen— inner and outer diameter of insulation— inner diameter and thickness of insulation screen— wire dimensions of screen— inner diameter and thickness of sheath— wire dimension of armour— inner diameter and thickness of external sheath.

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Recommended Practice DNV-RP-F401, February 2012 App.B Test Methods – Fatigue Loading of Complete Cables – Page 17

APPENDIX B TEST METHODS – FATIGUE LOADING OF COMPLETE CABLES

B.1 GeneralTwo methods for fatigue testing are considered applicable for testing of complete cable cross sections:

— 4-point-bend testing— bending against template.

These two methods are described in the following sections.Alternative methods for applying a fatigue deformation and/or for detecting failure may be accepted subject toa qualification of the reliability of the method(s) to impart a realistic fatigue deformation and/or to detectfailures(s).

B.2 4-point-bendingThe principle for the method is shown in the Figure B-1. The method can be used on both short and longspecimens, but for practical reasons the gauge section will have to be relatively short.The test set-up can be used for controlled displacement and controlled load and for different R-values.The curvature of the cable can in principle be determined based on beam theory provided the length of thespecimen is long enough to eliminate end effects. Curvature as a function of displacement of the moving yokeshall be calibrated. This can be accomplished by instrumenting one or more specimens with e.g. strain gaugesin order to obtain strain level at various positions as a function of yoke displacement. Effects of creep shall bespecially considered.For long and or slender configurations 4-point-bending may not give sufficient control of the bending radius,particularly when a tensile preload in the cable is used. Bending against templates may be necessary in suchcases.

Figure B-1 4-point bending

B.3 Bending against templateThe principle for the method is shown in the figure B-2 and B-3. The method may for practical reasons not besuited for long specimens.The specimen is bent against preformed templates thus controlling the curvature directly. The test frequencymay be restricted to lower values than for 4-point-bending. Other arrangements using templates are possible.The test method can be used for controlled displacement only. Varying R-values can be applied by modifyingthe shapes of the templates.

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Recommended Practice DNV-RP-F401, February 2012 App.B Test Methods – Fatigue Loading of Complete Cables – Page 18

Figure B-2 Bending against former

Figure B-3 Bending against former

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Recommended Practice DNV-RP-F401, February 2012 App.C Fatigue Testing Detection Techniques – Page 19

APPENDIX C FATIGUE TESTING DETECTION TECHNIQUES

C.1 Metallic materialsMetallic materials are/may be used in the following components:

— conductor core— sheath— screen— armour.

A number of different techniques for detecting defects are available, depending on the component in question.Detection techniques are discussed below. Common for all or most of the techniques is that they are impossibleto automate or that automation will require a significant effort.

C.1.1 Visual inspection – dissectionDestructive detection methodVisual inspection from the outside of components inside the specimen during the course of the test is inprinciple impossible, except for the external sheath. Visual inspection can be carried out after the specimen hasbeen taken apart for access to the component of interest, but this will prohibit further testing of the specimen.Visual inspection can therefore be used for verifying that a defect has not been formed after a certain numberof cycles, but it will not give any indication of the remaining fatigue life. Or, it can be used to verify that adefect has been formed, but not exactly at what number of load cycles. These uncertainties could in principlebe mitigated if a reasonably reliable way of back calculating defect growth rates is available. Such methodsmay be included in the basis for the qualification.The result from using visual inspection is illustrated in principle in Figure C-1 below. The cases where a defectis found or no defect is found upon inspection are shown, the arrows indicating the uncertainty in determiningthe “true” number of cycles to failure. Results of this type will increase the uncertainty in determining a SN-curve.

Figure C-1 Presentation of fatigue test results

C.1.2 Structural breakdown/Reduction of stiffnessNon-destructive detection methodStructural breakdown or a measurable reduction of the specimen stiffness is indicative of failure of one or moreof the load carrying components, e.g. a solid tube sheath or armour wires. The method is most sensitive for the component giving the largest contribution to the stiffness of the crosssection, e.g. a solid metal tubular sheath. It is relatively less sensitive to failure of armour or screen wires. Such

Examination: no defect

Examination: defect

∆ε

N

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Recommended Practice DNV-RP-F401, February 2012 App.C Fatigue Testing Detection Techniques – Page 20

wires are helically wound and relatively numerous such that a fairly large number of wires have to fail in closeproximity before a detectable change in stiffness occurs.Similarly, it is questionable whether failure in the core can be detected by this method.For early detection of fatigue cracks calibration of the detection method is probably required, i.e. the stiffnessas a function of crack size. The sensitivity to small cracks may be low.Longitudinal cracks, e.g. in longitudinal weld seams, may not be possible to detect by this method due to theirrelatively small effect on the stiffness.

C.1.3 Electrical resistanceNon-destructive detection methodFatigue crack growth in metallic materials will eventually increase their electrical resistance. However, thesensitivity of this technique may not be sufficiently high for many cable cross sections.Currents applied to the sheath may pass through the semi-conducting layers under the sheath giving no or verylittle appreciable increase in resistance as the length of the damage in the sheath is short, further reducing thesensitivity of the method. Whether the sensitivity of this method is sufficient or not has to be determined fromcase to case.The resistance of wire screens may not increase appreciably until a large portion of the wires are broken in onecross section. The resistance of armour wire could be used as a detection method, provided the individual armour wires areisolated from one another.

C.1.4 Eddy currentNon-destructive detection methodMay be developed for use for detection of fatigue cracks in metal sheaths and screens and armour wires.Depending on the level of development of the tools early detection of fatigue cracks may be possible. Asignificant amount of development and calibration work may be necessary.Eddy current can not be used, or is difficult to use, for components inside metallic screens or sheaths.The method may require a significant investment for automating the inspection during testing.

C.1.5 X-rayNon-destructive detection methodX-ray is a well developed technique for detection of defects in metallic materials. However, the ability of thetechnique to discover defects in complete cables, in terms of sensitivity and discrimination between thedifferent components, has to be demonstrated in each individual case.The method is not possible to automate.

C.1.6 LeakageNon-destructive detection methodMay tentatively be used for detection of through-wall defects on solid metal sheaths by applying compressedair on the inside of the sheath. The method requires free passage of air inside and outside of the sheath. Thismay not be possible to achieve for all cross section designs.

C.2 Plastic materialsBased on the discussion above the detection techniques that are considered possible to use are listed in the tablebelow. Development of detection techniques will form an important part of the qualification process. Wheresuitable detection techniques are not available alternative means of determining the fatigue strength in aconservative manner may be used.

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Recommended Practice DNV-RP-F401, February 2012 App.C Fatigue Testing Detection Techniques – Page 21

Table C-1 Defect detection techniques for metallic materialsComponent Failure mechanism Possible detection techniqueCore Fatigue crack growth Visual inspection – dissection

Electrical resistance(X-ray)

Sheath Fatigue crack in sheath – circumferential crack

StiffnessLeakageEddy currentElectrical resistance(X-ray)Visual inspection – dissection

Fatigue crack in sheath – longitudinal crack

LeakageEddy current(X-ray)Visual inspection – dissection

Screen, wire Fatigue crack growth Eddy currentElectrical resistance(X-ray)Visual inspection – dissection

Armour Fatigue crack growth Eddy currentElectrical resistance(X-ray)Visual inspection – dissection

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Recommended Practice DNV-RP-F401, February 2012 App.D Estimation of Fatigue Design Curves – Least Squares Method – Page 22

APPENDIX D ESTIMATION OF FATIGUE DESIGN CURVES –

LEAST SQUARES METHOD

SN-type fatigue design curves can be expressed on the following form:

where:

N : predicted number of cycles to failure for strain range ΔεΔε : strain rangem : inverse slope of SN-curvea : intercept of log N axis by SN-curve

Using the least squares method the constants in the regression curve can be estimated by the followingequations:

where n is the number of data points/test results (log Ni; log Δεi) and:

Ni : number of cycles to failure in test iΔεi : strain range in test i

The standard deviation s of log N is given by the following equation:

A fatigue design curve design curve can then be defined by the following equation, based on a 97.5%probability of survival:log N = log - m log Δεwhere:log = log a - 2⋅s.

log N = log a - m log Δε (D1)

(D2)

(D3)

(D4)

Δ−Δ

Δ−⋅Δ=

n n

ii

n n n

iiii

n

NNnm

1

2

1

2

1 1 1

log)(log

loglogloglog

εε

εε

n

mNa

n n

ii Δ−= 1 1

logloglog

ε

[ ]2/1

1

2

1

)log(loglog

Δ−−=

n

maNs

n

ii ε

ā

ā

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Recommended Practice DNV-RP-F401, February 2012 App.E Estimation of Fatigue Design Curves - Incomplete Observations of Number of Cycles to Failure – Page 23

APPENDIX E ESTIMATION OF FATIGUE DESIGN CURVES -

INCOMPLETE OBSERVATIONS OF NUMBER OF CYCLES TO FAILURE

The procedure below applies to situation where the exact numbers of cycles to failure for the test specimensare not known.SN-type fatigue design curves can be expressed on the following form:

N : predicted number of cycles to failure for strain range ΔεΔε : strain rangem : inverse slope of SN-curvea : intercept of log N axis by SN-curve

For simplicity the equation above is written as:

where:

x = log Δεy = log NA = log aB = - m

In case of SN-curves it is often assumed that A is Normal distributed with a constant standard deviation equalto σA. y is then also Normal distributed with a constant standard deviation σA and a median value given by E2.The distribution density function for y is thus given by:

with the cumulative probability function given by:

Assume that the test observations consist of knf of SN-data (Xnfi;Ynfi) where no failure was observed oninspection (i.e. the fatigue life is longer than the number of cycles applied in the test) and kf of SN-data (Xf;Yf)where failure was observed on inspection (i.e. the fatigue life is shorter than the number of cycles applied inthe test).The likelihood function for a sample including knf observations of non-failed specimens and kf observationsof failed specimens is then given by:

Since there is no way to determine E5 analytically, the estimators for A, B and σA may be found by maximisingE5 numerically.

log N = log a - m log Δε (E1)

y = A + Bx (E2)

(E3)

(E4)

(E5)

−−−=

2

2

2 2

)(exp

2

1)(

AA

y

BxAyxf

σπσ

= dxxfxF yy )()(

[ ]∏ ∏ −⋅=nf fk k

fiynfiy YFYFL )(1)(

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