EXPORT CABLE FAILURE - AXIS Capital

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Export Cable Failure 1

EXPORT CABLE FAILUREAXIS RENEWABLES IN PARTNERSHIP WITH

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This document is confidential and is intended for the use and information of the client to whom it is addressed.

TABLE OF CONTENTS

INTRODUCTION1.1 AC vs. DC, Manufacturing and Market Size 5

1.2 Export cable installation 7 EXPORT CABLE FAILURES

2.1 Installation:Failuretomeetcableinstallspecifications 10

2.2 Installation&Operations:Significantcableburialissue 11

2.3 Installation: Electrical connection fault 11

2.4 Manufacture: Manufacturing defect or testing error 12

2.5 Operations: Man-made or natural damage 12

DEFECTS IN OPTICAL FIBRE CABLING3.1 Opticalfibresinpowercables 14

3.2 Riskstopowercablesduetoopticalfibres 16

CABLE CONDITION MONITORING SYSTEMS4.1 Testingoffshorepowercables 17

4.2 Distributed sensing for online monitoring 17

4.3 Future development in export cable condition monitoring 19

COMPARISON TO OIL & GAS POWER CABLES 20

CONCLUSION 21

REFERENCES 22

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CONTACTS

Laurie SmithAssociate, London

+44 (0) 7505 604915

[email protected]

Seb RaePrincipal, London

+44 (0) 7557 095465

[email protected]

Sam ParkDirector, London

+44 (0) 7500 896757

[email protected]

Daniel StevensDirector, London

+44 (0) 7768 992206

[email protected]

Elaine GreigDirector, London

+44 (0) 7981 205753

[email protected]

Jamie FlemingSeniorUnderwriter,London

+44 20 7847 3426

[email protected]

Liam McEneaneyRenewableEnergyUnderwriter,London+44 20 7002 [email protected]

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Dear Broker Partner,

TogetherwithourengineeringpartnerRCG,wehavecommissionedthisreporttolookatissuesofcablingrelated

toOffshoreWindFarms,whichunfortunately,aretheprimarysourceofclaims.Accordingtolossadjuster,Lloyd

Warwick,40%ofallclaimsarecablerelated,producing83%ofclaimscostsinOffshoreWind.Theaimofthisreportis

toattempttogetabetterunderstandingofthisareaandwhatleadstothisexpense.Thisreportwillcoverthetypes

ofcablesused,howthecablesusedcomparetocablesintheOilandGasindustry,theissuesthatleadtofailures,

potentialdefectsandhowcablesaremonitored.

Itrustthatyou’llfindthisreportinterestingandIlookforwardtocontinuingtoworkwithyou.

Jamie Fleming

SeniorUnderwriter

AWORDFROMAXIS

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1.1 AC vs. DC, Manufacturing and Market Size Exportcablesaretheoffshorewindfarm’sconnectionbacktoshore,allowingthefullpowergeneratedbythefarm

tobetransmittedtothelocalgrid.Twogeneralcabletechnologiesexist:alternatingcurrent(AC)anddirectcurrent

(DC).ACispreferredasagridconnectioncanbemadedirectlyintoanexistingornewsubstationwhereasDC

requiresanoffshoreconverterstationplusanonshoreconverterstation,beforeconnectingintoanACsubstation.

LongdistanceelectricalinterconnectorsusehighvoltageDC(HVDC)cablesbecausethehigherlossesonhigh

voltageAC(HVAC)transmissionlinesmeanlongACsystemsarenotviable,buriedcablesbeingmoreonerousin

termsoflossesthanoverheadlines.Earlystudies [1]calculatedthatthebreakevenpointwheretheadditionalcostof

HVDCoutweighedthegreaterlossesincurredbyHVACconnectionsasaround80kminconnectionlength,below

whichACwaspreferableandabovewhichDCwaspreferable.However,windfarmownersareincentivisedtouse

longerdistanceHVACduetotechnologyandconsentingrisk,aswellashavingalternateviewsoncosting.Offshore

cablelengthsnowreach>150km,andtotalcablelengths,includingoffshoreandonshore,areapproaching200km

(Hornseaproject,asextractedfromRCG’sGRIPTMdatabase)withACtechnology;thishowevercanonlybeachievedby

usingintermediatereactorstationsalongthelength.AcomparisonofACandDCcablesisprovidedinExhibit1below.

INTRODUCTION

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Exhibit 1: Comparison of AC and DC submarine cable systems

Onecircuitconstitutesa3-phasecablewiththreepowercoresplusopticalfibresincluded.

Range of voltages:

• Lowvoltage(LV):<30kV

• Mediumvoltage(MV):30-100kV

• Highvoltage(HV):100-220kV

• Extrahighvoltage(EHV):>220kV

Ethyleneandpropylene-basedinsulationispreferred.

One circuit constitutes a pair of single-core cables (in a ‘bipole configuration’),whichmayalsobepackagedtogetherwithanopticalfibrecable.

Twomanufacturedtypes,withvoltagesstartingat100kV:

1. Single-coremassimpregnated(MI)orself-containedfluid-filled

(SCFF),upto500kV.

2. Ethyleneandpropylene-basedinsulation,incl.XLPE,upto320

kVoperationalandupto600kVplanned.

AC Submarine Cables DC Submarine Cables

Regardlessofthetypeofcable,submarinepowercablemanufactureisaslowanddelicateprocess,whereasingle

longpieceofcableisproducedoveranextendedperiod.Duringthistime,itisnecessarytoperformcontinual

monitoringofthecableandthemanufacturingequipmenttomeettheclosetolerancesandhighdegreeofaccuracy

required to ensure a reliable cable product. [5]

RCG’sglobalforecastforexportcablesinstalledperannumupto2023isshowninExhibit2below.HVACcontinues

todominatethecurrentmarketforoffshorewindandwhiletheuseofHVDCtechnologyhasstalledinrecentyears,

itisexpectedtopickupagainintheearly2020s.LVACandEHVACremainsmallpartsofthemarket.Itisseenfrom

Exhibit2thatthechallengetotheindustryofmaintainingthereliabilityofcableswithintheoffshorewindsectoris

exacerbatedbytheincreasinglengthsofcablerequiredtoservicetheindustry;althoughthisisnaturallymitigated

bythegaininexperienceofcablemanufacturingandlayingoverrecentyears.

1 XLPEstandsfor‘Cross-linkedpolyethylene’andHPTEstandsfor‘HighPerformanceThermoplasticElastomer’,i.e.apolypropylene-based solution

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Exhibit 1: Comparison of AC and DC submarine cable systems

Source: [2], [3] Data Sources: [4], RCG GRIPTM (2019)

Onecircuitconstitutesa3-phasecablewiththreepowercoresplusopticalfibresincluded.

Range of voltages:

• Lowvoltage(LV):<30kV

• Mediumvoltage(MV):30-100kV

• Highvoltage(HV):100-220kV

• Extrahighvoltage(EHV):>220kV

Ethyleneandpropylene-basedinsulationispreferred.

One circuit constitutes a pair of single-core cables (in a ‘bipole configuration’),whichmayalsobepackagedtogetherwithanopticalfibrecable.

Twomanufacturedtypes,withvoltagesstartingat100kV:

1. Single-coremassimpregnated(MI)orself-containedfluid-filled

(SCFF),upto500kV.

2. Ethyleneandpropylene-basedinsulation,incl.XLPE,upto320

kVoperationalandupto600kVplanned.

AC Submarine Cables DC Submarine Cables

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Exhibit 2: Historical and forecasted (to 2023) lengths of offshore wind export cable installed globally (excluding the Chinese market)

Source: RCG GRIPTM (2019)

1.2 Export cable installationCableinstallationmethodsaresimilarforbothACandDCsubmarinecables,withtheaimtoprotectboththecables

andtheenvironment,aswellasmeetthecableinstallspecifications.Man-maderiskslikeanchorsandtrawler

fishingnetsmeanthatcablesinshallowerwater(<500m)shouldbeburiedintheseabedforprotection(though

somesurfacelaidcablesdoexist,safetyconcernsforthevessels,aswellaseconomicriskthroughlossofoperation,

meanmostareburied),whilecablesindeeperwatermaybelaiddirectlyontheseabedastheserisksareeliminated

bythedepth.Cableburialdepthisdefinedthroughundertakingacableburialriskassessment,whiletherehave

beenspecialistsinthisactivityformanyyears,andstandardsfromparallelindustrieshavebeenusedinthepast,

therecentTheCarbonTrust(TCT)guidanceformsthebasisofwindindustrypractice[6].

Environmentalbodiesoftenseekdeeperburialduetoconcernsfromaroundinducedelectromagneticfields,but

deeperburialaddssignificantexpenseasfewcableburialtoolsexistthatcanburydeeply,isverydependentonthe

seabedsoilconditions,andresultsinwarmertemperaturesaroundthecablewhichcanresultinalargerandmore

difficult-to-handlecablebeingrequiredtotransmitthesamepower.Shallowburial,between0.8–2m,istherefore

highlyeconomicallyincentivised.Deeperburialupto3mmayberequiredthroughsandwaves,orifthereisa

significantriskoflargeshippinganchoring,althoughthisisunlikelysincelargeshippingisgenerallypassingrather

thanstopping,andnoburialwillpreventdamagefromthelargestshippinginanemergencysituation.Whereburial

isnotpossible,otherprotectionmethodssuchasrockdumping,concretemattressesormetalstructuresmaybe

used.Alloffshorewindfarmstodatehavebeeninstalledin<500mwaterdepth,makingcableburialorother

protectionanecessityinmostcases.

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Cableinstallationrequiresspecialisedshipsorbargestocarrythekilometresofcableandinstallthemaccording

tothespecifications,andadedicatedandexperiencedteamisnecessary.Tosimultaneouslylayandburycables,

ploughs,remotelyoperatedvehicles(ROVs),waterjettrenchersormechanicaltrenchersareuseddependingon

thesoilcomposition,aswellasotherequipmentsuchasexcavators.Whilemostoftheinstallationequipmentis

remotelyoperatedfromthecableshiporasupportship,diversmayalsobenecessary(especiallywhenothercable

protectionmethodsareemployed,suchasthelayingoffrondorconcretemats).

Followingpreparatoryworksandcableload-outontothecableship,a‘shore-pull’iscarriedout,wherethelandfall

endoftheseacableislandedontheshoreandinstalledwithhorizontaldirectionaldrilling(HDD)ortrenching

methods,dependingontheshore’sphysicalandenvironmentalrequirements.Thecableisthenlaidontheseabed,

asdescribedabove,outtothewindfarm,whereuponthecablemustbecuttothecorrectlengthtoallowittolie

properlyontheseabed.Finally,a‘pull-in’operationisperformedtobringthecableendintotheoffshoresubstation

orwindturbine.Atalltimesduringinstallation,thecables’mechanicalpropertiessuchaspullingtensionsandbend

radii are monitored and recorded.

Aftercableinstallationiscompleted,post-installationverificationiscarriedouttoensurethatthecablewasinstalled

correctly,whichisnecessaryfromaprojectduediligenceperspective,aswellasforprospectiveOFTOs.Thisprocess

willincludeelectricaltesting,usuallyaccordingtoIECandCIGREstandards,aswellassurveysassessingthecable

burial/protection.Ifissuesareidentifiedinthesubsequentriskassessment,remedialworksmaybeneeded.

Individualcablesmustbeseparatedonthesea-bedbyasufficientdistance,whichdistancedependsuponthewater

depth.Cablesareoftenlaidslightlysnakingratherthanstraight,toaccommodatesea-bedmovement.Thisalso

meansthatshouldacablerepairberequired,thecablecanbeliftedtothesurface,anadditionallengthadded,and

thecablere-laidwithoutoverlayinganyneighbouringcables.

Exhibit 3: Cable ship Stemat Spirit performing a shore-pull using a plough

Source: wikimedia [7]

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Exhibit 4: Cable ship Nexans Skagerrak in port, with a large central cable tank and specialised handling equipment visible

Exhibit 5: Cable installation barge BoDo Installer under tow



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Exportcablescanexperiencefullorpartialfailuresduringthemanufacture,constructionandoperationsphases

oftheproject,leadingtodelaysthatarelikelytoaffecttheprojecttimelineandresultinsignificantcapitalcostsfor

repairsorreplacements,andlossofrevenueforanoperatingwindfarm.Theexportcableisacriticalcomponent

andafailurewilldelaycommissioningorinterruptoperationsunlessanalternativecableconnectionisavailable.

Repairorreplacementworkisoftencarriedoutbyadifferentspecialisedvesselthantheinstallationcableships

andinvolvesthefollowingsteps:de-burialand/orremovingcableprotections,liftingthecableontothevessel,

repairingorreplacingthedamagedcablesectionandfinallyre-buryingthecable;withalloftheseoperationsbeing

highlyweathersensitive.Toensureconfidencethattheissuehasbeenfixedandthatwateringressintothecable

conductorsisnotaproblem,itisoftennecessarytoremoveasectionaround1kminlengtharoundthedamage

usingthebespokesparerepairjointsandcleancutstothecableandjointingonareplacementpiece.

Exportcablereplacementworkcanhavetimeimplicationsofaround6monthswithcapitalandlossofrevenue

costsofmultiplemillionsofGBP(notcountinganyadditionalsparejointorsparecablemanufacture). [10]

Thissectionlistssomeofthesefailuremodesandassessestheirimpactsandassociatedcostsanddelays.

2.1 Installation:FailuretomeetcableinstallspecificationsCablesaredifficulttohandleandduringinstallation;thecablemustbemanagedaccordingtothemechanical

handlingandphysicalspecificationsgivenbythemanufacturer,whichincludesrequirementssuchaskeeping

thecabletensionwithinthecorrectrange,preventingthecablefrombendingpasttheminimumbendradius,

andothers.Installersdomonitorthecable’sconditioncarefullyduringinstallationbyuseofsubseavideoand

continuouslyrunning‘OTDR’testingofthefibres(seelatersection)butamomentarylapseinthecontinuouslaying

operationscanleadtothefailurebytheinstallertomeetthesespecificationsandcanresultinsignificantdamageto

thecablethatmayormaynotbedetectableduringinstallationorcommissioningandcouldcausethecabletofailprematurely.

Itisalsocriticalthatthegeotechnicaldatafortheseabedsoilsisaccurateandcomprehensiveenoughtoensure

thatthecableinstallerusestheappropriatemethodsandequipment,bothtoensurethermalpropertiesaroundthe

cableandtoavoidphysicaldamageduetohazardssuchassharprocks.Thisgeotechnicaldataistypicallyprovided

totheinstallerbythedeveloperandcanthusbethecauseofsignificantcontractualdisputes.

Damagefromcableinstallationwilltypicallyrequireatleastasectionofthecabletobereplaced,althoughthe

entirecablecouldbeaffected.Theresultingdelaymaybeafewmonthsdependingonwhetherthereisenough

sparecable,orifanewinstallationcontractorisneeded,availableweatherwindowsforinstallation2, etc. In addition

tothedelaycosts,therewillbecapitalcostsontherepair/replacementworksandtheextrasparecable/joints

manufacture(ifneeded).Thesecostsarenottypicallycoveredbytheinstallationcontractorsandoftentherisk

remainstobeassumedbytheprojectdeveloper.

EXPORT CABLE FAILURES

2 Inadditiontorequiringtheappropriatespecialistequipment,offshoreworksareverydependentonagoodweatherbeingavailable,sincehighwindsandlargewavescanpresentunacceptableriskstocomponents,vesselsandcrew.

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2.2 Installation&Operations:SignificantcableburialissueDifficultiesincableburialarecommon,oftenduetoinsufficienciesand/orinaccuraciesinthemarinegeophysical

andgeotechnicalsurveysconductedduringtheprojectplanningphase.Unexpectedcableburialdifficultiesresult

ininstallationcostoverrunsanddelays,althoughthesethemselveswillnotcausecabledamage.Ifacableisleft

unburiedandunprotected,itwouldbemoresusceptiblefordamagefrommaritimeactivities(trawlerfishingand

ships’anchors)andpotentialliabilityfordamagetosuchvessels,butagainwouldnotnecessarilyfail,especiallyif

othermitigationisperformedsuchascommunicatingtheriskstomariners.

Inmoreextremecaseshowever,cableburialandprotectionissuescandirectlyleadtocablefailure:ifasectionof

thecableisleftunsupportedunderneathasaresultoftheseissues,knownasa‘freespan’,theincreasedtensional

loadand/orfatigue-inducingvibrationsonthecablecandamageit.Hydrodynamicscour,wheresupporting

sedimentiscarriedawayfromanobject(cableinthiscase)bywatercurrentsandresultsin‘scourpits’forming

around/underneathit,canleadtothesefreespans.Iftheriskoffreespansisnotproperlymitigatedduring

installation,theywilloftenonlybecomeevidentlater(e.g.duringthecommissioningorpost-installationsurveys).In

theworstcases,cablere-burialremedialworks,suchasbytheapplicationofrock-dumping,concretemattresses,

frondmatsorbywater-jetting,arealwaysexpensive,canposefurtherrisktothecable(andpotentiallyduringdiving

operations),plusmayonlybedoneduringfullcableoutagesresultinginfurtherlostrevenue.

2.3 Installation: Electrical connection faultInterfacingoftheexportcablewiththeelectricalinfrastructureinboththeonshoreandoffshoresubstations(or

converterstationforHVDC)isgenerallydoneasthelastmaininstallationstepbeforetestingandcommissioning.

Theterminationoperations(i.e.makingthefinalelectricalconnections)ofallthepowercableshasallthetypical

highvoltageelectricalrisks(compoundedbythenatureofworkingoffshorewheretheenvironmentisnotasclean

andcontrollableas,say,anonshoresubstationbuilding)meaningthatfaultsandsubsequentdamagetoeither

thecableorsubstationinfrastructureispossible.However,ifcableterminationdamagedoesoccur,therepair/

replacementworksaregenerallynotascomplexasforasubseaelectricalfault,resultinginlowerassociatedcosts

anddelays.Withgoodelectricalsafetypractices,thedamagefromanyincidentscantypicallybelimitedandthe

riskstotheelectriciansthemselveswillbeminimised.Again,lossofrevenuewilloccurifanycableterminationsdo

showfaults.

Exportcablesaretypicallylongandmayexceedthelengthofcablethatcanbecarriedbyasinglevessel,preventing

itfrombeinglaidinasinglepiece.Suchexportcableswillrequirein-fieldcablejointstobeincluded,whichare

inherentlyvulnerablecomparedtotheun-cutlengthsofcableandconstitutepotentialweakpoints.Becausethe

jointsmustbemadeoffshore,theywillbeespeciallysusceptibleasitisimpossibletoachievetheengineering

controlstandardspossiblewithinanonshorecablefacility,althoughexperiencedoffshoreinstallerswillhave

acceptable results.

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2.4 Manufacture: Manufacturing defect or testing errorManufacturingdefectsarerelativelycommonforlonglengthpowercables,sincetheyaremadeinlongcontinuous

sectionsinatime-consumingandpreciseprocess,whereeventhesmallestprocesserrorscanproducedefects

thatcouldleadtoelectricalfaultswithcatastrophicimplicationsiftheyarenotrectifiedbeforethecableisputinto

service.Themainwaythataprojectownercanmitigatemanufacturingdefectsisbyensuringthehigheststandards

ofexperience,qualitycontrolandassuranceprocessesaremaintainedbythemanufacturer.Manyownersrequire

thatthecabledelivereddoesnotincludeanyfactoryjoints,whichmaynotbepossibleifthetotallengthrequired

exceedsthephysicallengthcapacityofthefactory.Thismeansthatneworbespokecabletypesshouldalsobe

avoidedwherepossible.Asageneralpoint,‘qualityisking’whenmanufacturingpowercablesofthelengthsusedin

offshorewind.3

2.5 Operations: Man-made or natural damageThetrendforoffshorewindcables(inter-arrayandexport)isthatmostcabledamageiscausedbymanufacturing

andinstallationissues.ThecableburialreportissuedbytheCarbonTrust’sOffshoreWindAccelerator(OWA)

programmestatedthat80%ofEuropeanoffshorewindfarminsuranceclaimswerecablerelated [6]–ofthese62%

relatedtocabledamageduringconstruction,althoughnotallattributabletocableburialoperations.Inaddition,to

theknowledgeoftheOWApartnersthereisnoevidenceofanchorstrikesand/ordragging,eithertoexportorinter

arraycables,onoffshorewindfarmsoperatinginUKwaters.Thefibre-opticmanufacturingfault(section3.2)was

notknownatthetimeofthisreport’spublication,andwiderEuropeanexperiencewasnotdocumented,however

cablefailureexamplespresentedbyindustryparticipantsappeartofollowthistrend.ThemorerecentORECreport[11]includesmorerecentUKcablefailures,whichagainareasaresultofmanufacturingissues,notthird-party

damage.TheCIGREworkinggroup(B1.57)isexpectedtopublishanupdatetoTB379[12]shortly,whichwillupdate

thisknowledgebase.

Itis,however,interestingtonotethatforfibre-opticcablesglobally,thegreatestrisktooperationalsubmarine

cablesisphysicaldamagefromhumanactivities,includingfromships’anchorsandfishingtrawlernets.Forthese

submarinecablesworldwide,80-90%ofallfaultscanbeattributedtohumanactivities,whichpredominantly

occurinwaterdepthsshallowerthan1,000mandmostlyinlessthan200mwaterdepth[13].Therisksfromhuman

activitiesarelowerincountrieswherebetterriskcommunicationpracticesareestablished,suchasthroughtheKIS-

ORCAprojectinEurope[14].Thus,itmaybethatwhenthewindindustry’steethingproblemsareresolved,human

damagemaybecometheprimarycauseofdamage,ifprotectionsarecompromised.

3 Itshouldbenotedthatthetotalconductorlengthfor3-phaseACcablesistriplethelengthofthecableitself.

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Themainsourceofnaturaldamagetosubmarinecablesisseabedmovementfrommovingsand-waves,subsea

landslides,tidalmovementoreventectonicshifts.Thesecanresultindirectphysicaldamagetothecableorcan

causeittobecomede-buried(seeSection2.2above).Abrasionfromwater-bornematerialsalsoposesaphysical

risk,andtogetherthesehazardsaccountfor10-15%ofall(global)submarinecablefaults[13].

Conditionmonitoring,whichiscoveredinmoredetailinSection4below,iscriticalforeffectiveresponsestocable

failuresduringoperations.Ifcabledamagecanbepre-empted,repair/replacementworkcanbeplannedinadvance

forsignificantlyreducedcapitalanddowntimecosts,butquickresponsestounexpectedcablefailureswillalso

reducethedowntimecosts;somecableownersarepoolingtheirresourcesintoa‘club’whichcanthenofferamuch

fastertimeofresponsethanifprocuredviaasinglecableowner.

4 Thisfiguredoesincludeopticalfibres,whichareinstalledmoregloballyincommunicationscablesandoftenhavelessprotectionthansubmarinepowercables.

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OpticalfibresinpowercablesExhibit6belowshowsa245kVHVACsubmarinecablewithabundleofopticalfibrespackedintothecable.The

fibresareincludedseparatelyoutsidethethreeXLPEinsulatedconductingcoresandtypicallyarewithintheirown

metaltube,whichmaybesurroundedbyfurtherplasticormetalarmouring(similartothearmouringaroundthe

outsideofthecable).Stainlesssteelisthemostcommonlyusedmetalfortheseparts.

HVDCusestwosingle-corepowercablesandtheopticalfibresaregenerallyincludedasaseparatecable,whichis

commonlywrappedtogetherwiththepowercablestosimplifyinstallation,asshowninExhibit7below.

DEFECTSINOPTICALFIBRECABLING

Exhibit 6: Annotated photograph of a (E)HVAC submarine cable

Source: wikimedia [15] & Canadian Copper & Brass Development Association [16]

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Inbothcases,thesteeltubearmouringontheopticalfibrecableprovidesstiffnesstoprotectthefibresfrom

mechanicaldamage(includingduringmanufacture),aswellasfurtherprotectionincaseofwateringress.To

minimisetheimpactofanyindividualfibrehavingafault,manylevelsofredundancyareoftenin-builttothefibre

packagebyitcontainingdoubleortriplethenumbersoffibresactuallyrequiredforthesafecontrolandoperation

ofthewindfarm.Afailureinenoughofthefibreswould,however,meanthereplacementrunningofanewfibre

cablealongthelengthofthecable,regardlessofwhetherthepowercoresthemselvesareaffected.

3.2 RiskstopowercablesduetoopticalfibresItisnotpracticaltoprovideastatementontheoverallreliabilityofopticalfibresusedwithinsubmarinepower

cables,astheydonotexperiencethesameexposureasstandalonefibre-opticcables.

‘ExportCableReliability:DescriptionofConcerns’,aMay2017reportbyTransmissionExcellenceLtdonbehalfof

theOffshoreWindProgrammeBoard(OWPB),statedthattherehavebeensevenpost-commissioningfailuresin

UKoffshorewindACexportcables,andbasedondatafromthecable’sowners,atleastsixofthefailureswould

nothaveoccurred“hadafibreopticcorenotbeenincludedwithinthepowercable”.[11]Thereportproposesthree

possible contributing factors:

1.Theopticalfibrecables,whicharesmallandlight,arepronetoaccidentaldamageduringcablemanufacture;

2.Thedesignandtestingoftheopticalfibresisfocusedontheiropticalperformance,withoutenoughconsideration

oftheeffectsofexposuretohighvoltagesandcurrents,whichmayhappenundersomefaultconditions;

3.Thefibremaybeexposedtolargemagneticfieldsfromtheadjacentpowercoreswhichcouldinducedamaging

electricalcurrentsandvoltagesinthemetaltubearmourusedaroundthefibres.Thesewouldbemorelikelyifan

electricalfaultcausedthemagneticfieldstobehigherthanexpected.

Exhibit 7: Annotated cross-section of a HVDC bipole system showing optical fibre cable

Source: RCG

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Thefirstandsecondpointsaremanufacturingpracticeissuesthatmaybereducedthroughcarefulauditingof

manufacturers,whilethethirdisapotentialdesignissue.InbothACandDCconfigurations,designsymmetry

helpsensurethatthemagneticfieldsgeneratedduringnormaloperationwillbeminimised,reducingthepotential

impactoftheissue(alsoforDCthemagneticfieldsarestatic).AnapproximatecalculationforastandardHVACcable

suggestedthatduringnormaloperation,theinducedvoltagesandcurrentswouldbefartoosmalltoresultinany

significantdamagetothefibres5andisonlylikelytocausealittleheatinginthefibreareaofthecable.

TheaboveconclusionissupportedbyaninquirysubmittedbyRCGtoalargepowercablemanufacturer.The

responsestatedthatdesignersdoconsiderthevoltagesandcurrentsinducedinthestainless-steeltubing,but

thattheassociatedlossesintransmittedpowerareverylow,sothatdesignersdonotseektomitigatethemby

introducingalternativedesigns.Thesmallpowerlosssuggestslittleheatingduetothismechanism,andthecables

arealreadydesignedtomanagetheheatfromtheconductingcoresthemselveswhichwillbemuchlargersources

ofheat.Althoughdamagetothefibresfromthismechanismisveryunlikelyduringnormaloperation,itpossibly

couldresultinasignificantelectricalfaultwhichsubsequentlyaffectsthefibresaswell.

Wedonotbelievethereisenoughinformationonexportcablefailuresoveralltodeterminehowprevalenteach

oftheabovethreefailuremodesis,withtheavailabledatabeinglimitedbythesmallnumberofcasesandthe

commerciallysensitivenatureofthefailures.Ifthethirdissueweresignificant(althoughthisisunlikelyasexplained

previously),theplannedmovetowardsHVDCcablesintheUKandothermarketscouldslightlyreducethefailurerate6,

althoughotherfactorssuchasthedifferencesintestingandmaintenancepracticesarelikelytohavealargerimpact.

5 Inducedvoltages~mVpermetreofcable,producing~mAofcurrent.6Themean‘time’betweenfailuresforHVACcablesisthoughttobearound600kmyears.Definition:foralengthofcableinstalledL[km],thetimeexpectedbeforeafailurewouldbegivenbyT[years]≈(600[kmyears])/(L[km])basedontheempiricalstudiesperformedbyCIGRE(2009)andTransmissionExcellenceLtd.(2017).[11][12]

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4.1 Testingoffshorepowercables‘ConditionMonitoring’referstothecontinuedtestingoftheexportcablesystemafterithasbeencommissioned

andisfullyoperational,whichcanincludemanualorautomatedtestingofelectrical,opticalandotherphysical

conditions,aswellasphysicalsubseasurveys.

Duringcommissioning,typically,afullsetofcabletestingiscarriedouttodemonstratecontinuityandinsultation

integrity,whichcanonlyberepeatedlaterbytakingthecablefullyoutofservice,somethingthatalloperatorswould

wishtoavoid.TheseincludehighvoltageAC‘pressure’tests(eitheratverylowfrequency‘VLF’,orattheresonant

frequencyofthesystem),lineresonanceanalysisandotherpartialdischarge(PD),continuityandresistancetests.

VLFtestsareoftenthesubjectofnegotiationwiththecablemanufacturerandsupplier,becausesomeconsider

thattheycancausedamagewhereasotherscontesttheydonot;theymerelyexacerbateadevelopingproblemand

bringthefailureforward.VLFisgenerallyacceptedaspreferabletoDCtesting,whichisseenasinvasive,meaning

thatsuchtestsareoftenacceptedonceonly.ForDCcablesystems,highvoltageDCtestswillconstitutedirectproof-

of-functiontests,althoughPDandotherelectricaltestswillalsobeperformed.Duringoperationitisnotpossible

toperformthesecontrolledtests(whichwouldrequiredowntime)andanalternativeapproachmustbeadopted:

anoperatingsystemthatdemonstratesperformance,andanelectricalprotectionsystemthatidentifiesinsulation

degradation-relatedfaultsasandwhentheyoccur/escalate.

Continuousmonitoringofthehealthoftheopticalfibresisoftencarriedoutduringinstallation,althoughthecurrent

techniquestendtoyieldlimitedinformation.Thetake-upofconditionmonitoringofcablesduringoperationsis

alsolimited,largelyduetotheincreasedOpExcoststhatareoftenseenasfinanciallyunattractive.Ofcourse,alot

ofcontinuoustestingispresentduringmanufacturingaspartofthecablemanufacturer’squalitycontrolprocess,

althoughthisisnotconsideredconditionmonitoringfromtheperspectiveoftheproject.

4.2 Distributed sensing for online monitoringThegeneralapproachistouseremotelyoperatedsensingequipmentdistributedalongthelengthofthecableto

observe it during operations and provide continuous and non-invasive data collection.

Theopticalfibresincludedinside,oralongwith,theexportcablesenablevarioustypesofdistributedopticalfibre

sensingtomeasuretemperatureandstrainalongthecable(withspatialresolutions~1m).‘Opticaltimedomain

reflectometry‘(OTDR)istheindustrystandardtechniqueforthesemeasurements,althoughfrequencydomain

reflectometryisalsoused(OFDR),andbothmethodsallowforfaultstobelocatedalongthelengthofthecable.

CABLE CONDITION MONITORINGSYSTEMS

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Distributedtemperatureorstrainsensingwithopticalfibreshasbeenimplementedinoffshorewindtocharacterise

andtestexportcableduringcommissioning,tomonitorthemduringoperationsandtodiagnosefaultswhentheyoccur.

OrganisationssuchasTheCarbonTrustandtheUSDoEareknowntobeinterestedindistributedsensingfor

condition monitoring:

• TheCarbonTrusthasanoffshorewindcableconditionmonitoringworkstreamaspartoftheirindustry-

collaborationOffshoreWindAcceleratorprogramme.Acablemonitoringcompetitiontodevelopreal-time

mechanicalcablemonitoringwaslaunchedinJanuary2017.OTDRmethodswereexpectedtoprovesuccessful,

buttheresultshaveyettobeannounced.[17] [18]

• TheCarbonTrustalsolaunchedaresearchprojectITTfor‘FaultLocationandConditionMonitoringinLong

OffshoreCables’inDecember2018,withafocuson‘online’solutionstobeusedwhileawindfarmisoperational.

Theclosingdatewas25thJanuary2019.[19]

• TheUSDoEhasrecentlyawardedseveralmillionUSDoffundstoresearchprojectsaddressingotherissuesfaced

byoffshorewind(ecologicalprotection,etc.)andRCGisawareofinterestwithinthedepartmenttofundcable

conditionmonitoringresearchaswell.[20]

Opticalfibre-baseddistributedsensingisnotabletodirectlysenseelectricalproblemswithinthecableconducting

cores/insulation,suchasanypartialdischarge(PD).However,anyfaultissueswilleventuallybecomeapparentvia

anincreasedtemperature,orstrain,whichmaybedetectedthroughOTDR/OFDRmethods,allowingthelocationof

thefaulttobeidentifiedalongthelengthofthecable.

Itisalsopossibletodetectsuchfaultswithelectromagneticmethods,allowingforpre-emptiverepair/replacement

atallegedlysignificantlylowercosts.CablemanufacturerPrysmianGrouphascommercialisedoneoftheonly

implementationsofthistechnologyunderthetradename‘PRY-CAM’,whichhasbeendevelopedsince2008[21]and

announceditsfirstsaleforanoffshorewindproject–afullcablemonitoringsystemincludedwiththeexportand

arraycablesupply–toEDFRenewablesfortheProvenceGrandLargefloatingoffshorewindprojectinsouthern

France. [22]Thetechnologyusesindividualbattery-poweredsensors(showninExhibit8below)eachwithawireless

electromagnetic sensor to detect local PD occurrences.

ItisnotknownhowrobustthesystemisorwhetherPrysmian’smaincompetitors,suchasNexansandNKT,areworking

onsimilarsystems,howeverwithmoreandmorepotentialmonitoringsolutionscomingintothemarket,itisourview

thatbothCapExandOpExcostswillbetendingtodecrease,withthesystems’effectivenesstendingtoincrease.

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4.3 Future development in export cable condition monitoringAsdowntimeinanoffshorewindfarmcableisextremelyfinanciallydamaging,projectownersandoperators

aimtocreatemaintenanceandrepairinfrastructure(includingtechnicians,vessels,crew,sparecomponents)

thatcanrapidlyrespondtofailuresandotherissues.Forthepowercables,thiswillinvolveelectricalcontractors

specialisedinhighvoltagework,cablemanufacturersandcableshipowners/operators,andistypicallyestablished

separatelyandspecificallyforeachproject.However,asshownwithPrysmian’sfull-scopeofferingfortheProvence

GrandLargeproject,cablemanufacturersareincreasinglylookingtoprovidetheirownconditionmonitoring

andO&Mservices,inasimilarwaytothoseofferedbyturbinemanufacturers;itdoeshoweverremainuncertain

howcost-effectivethesesolutionsare,andwhetherprojectownersarewillingtoinvestinwhatcanbeseenasan

unestablisheddesign.

Marinesurveysplaysomeroleinpost-commissioningconditionmonitoringbuttheyarerelativelyexpensivetocarry

outandaregenerallyonlyusedwhenfurtherinformationisneededaboutastronglysuspectedproblem,suchasa

significantcableburialissuehavingdevelopedsincecommissioning.Similartothepost-laysurveycarriedoutduring

commissioning,thesesurveyscanmeasurethecable’sexactposition,thedepth-of-burial/depth-of-cover,thephysical

conditionsofthecableandcableprotection,whetherthecableissupportedwelloriffreespansectionshaveformed,

etc.Surveydatashouldbeindependentlyvalidated,andrepeatedtoensurehonestandconsistentreporting.

Whilebestpracticeshavelargelybeenestablishedforpre-installationtestingandelectrical&opticalcommissioning,

thereareseveraltechnologygapsforconditionmonitoringduringinstallationandoperationswhichtheindustryis

currentlyseekingtoaddress.Furthermore,whereconditionmonitoringhasbeenimplemented,ithaslargelybeen

doneonaverybespokebasisandtherangeofcommerciallyavailable‘off-the-shelf’solutionsremainslimited.The

continuedcommercialisationanddevelopmentofsubmarinepowercableconditionmonitoringsystemsisexpected

toresultincostreductionsfortheseservices,althoughitisdifficulttoanticipatetimescalesforthesechangesas

newsensingtechnologiesmustfirstbedevelopedbeforebeingbroughttomarket.

Exhibit 8: A Prysmian Group PRY-CAM PD sensor; multiple networked sensors are attached to the cable to form a distributed monitoring system

Source: Prysmian [21]

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Submarinecablesareusedintheoil&gasindustryinmultipledifferentapplications,andthetypesofcablesvary

greatlyasaresult.Alargeportionofthesecablesarereferredtoas‘umbilicals’,whichlinksurfaceandseafloor

equipmenttotransmitcommunicationssignals(forsensingandcontrol),power,hydraulicandchemicalinjection

fluids,aswellasheating.Umbilicalcablesdohaveusesinoffshorewindfarmconstruction,suchasforremote

controllingsubmarineinstallationequipment,buttheyarenotveryrelevanttooperationalwindfarms.Power

umbilicalsaretypicallymediumvoltage(upto36kV).

Offshoreoil&gasplatformshaveenergy-intensiveoperations,withpowerrequirementsofuptoseveralhundred

megawatts(MW)onthelargestinstallations.Thetraditionalsolutionhasbeentousedieselenginesorgasturbines

fuelledbythepetroleumorgasextractedandprocessedbytheplatform,howevertheindustryisincreasingly

consideringthisapproachtobeinefficientandenvironmentallydamaging.Themainalternativeisreferredtoas

‘power-from-shore’,whereadedicatedsubmarinepowercableisusedtotransmitpowerfromtheonshoregridto

theplatform.

ApioneerinthisapproachwasStatoil,whoseGjøafloatingoil&gasplatformlocatedintheNorthSeaispowered

bya100km-long115kVHVACXLPEcable,manufacturedandinstalledbyABBin2010.[23]ABBhavesinceprovided

ACandDCpower-from-shoresolutionstoseveraloil&gasplatforms,ashaveothercablemanufacturerssuchas

PrysmianGroup.ShorterLV,MVandHVACsubmarinecableshavealsobeenusedforpowerdistributionbetween

differentsectionsinoil&gasinstallations,e.g.asaninter-linkbetweentwonearbyplatforms,althoughthis

applicationremainssignificantlysmallerthanHVACcableuseinoffshorewind.

Additionally,floatingoffshorewinddemonstratorshavebeenusedonoffshoreoilandgas,toprovidepower

insteadofon-platformdieselgeneration.Thisprovidesthemutualbenefitofdemonstratingthefloatingwind

technology,whilstprovidingneededpowertogettheremainingdepositsoutofthefield.

Althoughthepowerloadsgeneratedbyanoffshorewindfarmwillbedifferenttothoserequiredbyanoffshore

oil&gasplatform,thecabletechnologieswillberelativelysimilar,andtheinstallationswillhavesimilarassociated

risks.However,oneofthemainnotabledifferencesisthewaterdepthsinwhichthecablesareinstalled.Oil&

gasplatformswillbelocatedwhereveroil/gasfieldscanbefoundandmayoperateinwaterdepthsofhundreds

tothousandsofmetres,whereasalmostalloffshorewindfarms(exceptforthenewerfloatingdesigns)arein

waterdepthslessthan60m.Theriskstosubmarinecablesareknowntovarywithwaterdepthasman-made

activitiessuchasshipsdroppinganchorsortrawlingnetsareonlypresentuptoaround200mbelowthesurface

andinwaterdepthsgreaterthan1,000malmostalldamageto(fibreoptic)cablesisduetonaturalcausessuchas

abrasion,underwaterlandslidesandseismicactivity.Asaresult,agreaterportionofsubmarinepowercablesinthe

offshorewindindustrycanbeexpectedtobeexposedtotheman-madeshallowwaterriskscomparedtothosein

theoil&gasindustry,whichwouldinsteadexperiencemoredamagefromseismicactivityandothernaturalcauses,

althoughafullstudywouldbenecessarytoconfirmthis.Manufacturingandinstallationdamagewouldbeequally

applicabletobothindustries.

COMPARISON TO OIL &GASPOWERCABLES

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• Reliablecablesystemsareessentialtomaintaintherevenuebeinggeneratedinoffshorewindfarms;anditisthis

lossofrevenuewhichisakeydriverinpromptreinstatementoffaultedsystems(forboththedevelopersand

theirinsurers).

• Theoffshorewindindustryhasgainedmoreexperienceandcompetitioninthemanufacturingandlayingof

cables,andmistakesoftherecentpastarebeinglearnt.

• However,theincreasingvolumeofcablesinthisindustrytendstoindicatethenumberoffaultsoccurringwill

remainsteady.

• Thereissome(limited)newevidencethatthemetaltubingwithinwhichfibresareplacedmaybethecauseof

particularfaults;althoughthemanufacturersthemselvesdoubtthis.

• Maintainingthehighestqualityofmanufacturing,handlingandinstallationofmarinecablesisessentialinthe

reduction of faults.

• Cableconditionmonitoringsystemsarebeingdevelopedwhichshouldtendtodrivecostsdownwards,although

thereisnotonecleartechnologywinner.

• Therelativelyshallowwaterinstallationofoffshorewindcablesmaybecomeafactorintheirreliability,something

whichO&Gcablestendtoavoid.

CONCLUSION

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[1]R.Rudervall,J.CharpentierandR.Sharma,“EconomiccomparisonofVSCHVDCandHVACastransmission

systemfora300MWoffshorewindfarm,”EuropeanTransactionsonElectricalPower,vol.20(5),pp.661-671,

2009.

[2]commons.wikimedia.org,“150kV3-PhaseSubmarineCable.jpg,”9April2018.[Online].Available:https://

commons.wikimedia.org/wiki/File:150_kV_3-Phase_Submarine_Cable.jpg.[Accessed29March2019].

[3]commons.wikimedia.org,“Hochspannungskabel400kVQuerschnitt.JPG,”8May2010.[Online].Available:https://

commons.wikimedia.org/wiki/File:Hochspannungskabel_400kV_Querschnitt.JPG.[Accessed28March2019].

[4]Europacable,“Energy,”[Online].Available:https://www.europacable.eu/energy.[Accessed18March2019].

[5]Elanders&Västerås(ABB),“SubmarinePowerCables,”11May2006.[Online].Available:https://library.e.abb.com/

public/36cfa59fed0e355cc1257b130057b279/SubmarinePowerCables.pdf.[Accessed19March2019].

[6] TheCarbonTrust,“CableBurialRiskAssessmentMethodology:GuidanceforthePreparationofCableBurial

DepthofLoweringSpecification,”CTC835,London,2015.

[7]commons.wikimedia.org,“StematSpirit&PloughWIKI.jpg,”28January2011.[Online].Available:https://commons.

wikimedia.org/wiki/File:Stemat_Spirit_%26_Plough_WIKI.jpg.[Accessed19March2019].

[8]commons.wikimedia.org,“Nexans_Skagerrak_cable_laying_ship_in_Horten,_Norway.jpg,”10June2014.[Online].

Available:https://commons.wikimedia.org/wiki/File:Nexans_Skagerrak_cable_laying_ship_in_Horten,_Norway.jpg.

[Accessed19March2019].

[9]commons.wikimedia.org,“BoDo_Installer_7426.JPG,”16September2014.[Online].Available:https://commons.

wikimedia.org/wiki/File:BoDo_Installer_7426.JPG.[Accessed19March2019].

[10] ReinierNagtegaal(ECEOffshore),“Engineering|Consultancy|Equipment:OffshoreCableRepairOperations,”

24October2018.[Online].Available:https://www.hydrographicsocietybenelux.eu/storage/workshops/6/oe18-

offshore-cable-repair-operations.pdf.[Accessed21March2019].

[11]TransmissionExcellenceLtdonbehalfoftheOffshoreWindProgrammeBoard(OWPB),“ExportCableReliability:

DescriptionofConcerns,”May/July2017.[Online].Available:https://ore.catapult.org.uk/app/uploads/2018/02/

Export-Cable-Reliability-Step-1-v7-UPDATE-Jul-17.pdf.[Accessed15March2019].

[12]CIGREStudyCommitteeB1(InsulatedCables),“UpdateofServiceExperienceofHVUndergroundandSubmarine

CableSystems,”TechnicalBrochures,no.379,2009.

[13]M.E.Kordahi,R.K.Stix,R.J.Rapp,S.Sheridan,G.Lucas,S.WilsonandB.Perratt,“GlobalTrendsInSubmarine

CableSystemFaults,”inSubOptic2016,Dubai,2016.

[14]KIS-ORCA,“AboutUs:TheKIS-ORCAProject,”[Online].Available:http://kis-orca.eu/about-us#.XJI4LLinw2w.


[15]commons.wikimedia.org,“Wolfe_Island_Wind_Project_Submarine_Power_Cable.jpg,”9August2012.[Online].

Available:https://commons.wikimedia.org/wiki/File:Wolfe_Island_Wind_Project_Submarine_Power_Cable.jpg.


REFERENCES

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[16]CanadianCopper&BrassDevelopmentAssociation,“WolfeIslandWindProject,”CanadianCopperMagazine,no.

156, 2008.

[17]TheCarbonTrust,“CarbonTrustlaunchescompetitiontoimprovesubseacablingsystemsintheoffshore

windindustry,”10January2017.[Online].Available:https://www.carbontrust.com/news/2017/01/carbon-trust-

launches-competition-subsea-cabling-systems-offshore-wind/.[Accessed25March2019].

[18]TheCarbonTrust,“OffshoreCableConditionAssessmentCompetition:Technicalspecification,”January2017.

[Online].Available:http://www.nsri.co.uk/uploads/owa-c-cms-technical-specification.pdf.[Accessed25March

2019].

[19]TheCarbonTrust,“ITTfortheFaultLocation&ConditionMonitoringinLongOffshoreCables(FLOC)project,”18

December2018.[Online].Available:https://www.carbontrust.com/media/677146/invitation-to-tender-document-

floc.pdf.[Accessed25March2019].

[20]OfficeofEnergyEfficiency&RenewableEnergy,“EnergyDepartmentAwards$6MillioninWindEnergyResearch

Projects,”13March2019.[Online].Available:https://www.energy.gov/eere/articles/energy-department-awards-6-

million-wind-energy-research-projects.[Accessed25March2019].

[21]PrysmianElectronicssrl,“pry-cam.com,”2019.[Online].Available:https://pry-cam.com/en/.[Accessed27March

2019].

[22]PrysmianGroup,“Prysmiantodevelopturn-keyexport&inter-arraycableconnectionsforthefirst66kVoffshore

floatingwindfarm,”19March2019.[Online].Available:https://www.prysmiangroup.com/en/press-releases/

prysmian-to-develop-turn-key-export-and-inter-array-cable-connections-for-the-first-66-kv-offshore-floating-wind-

farm.[Accessed27March2019].

[23]ABBLtd,“World’sfirstpower-from-shoredynamicACcable:Gjøafloatingoilandgasplatform,NorthSea,”March

2011.[Online].Available:https://library.e.abb.com/public/f53ec349b3873a7dc1257781003a31b0/Project%20

Gjoa%20115%20kV%20XLPE%20subm.pdf.[Accessed21March2019].

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RCGisaspecializedexpertservicesfirmdedicatedtotheglobalrenewableenergysector.Weareafirmofpractical

consultantsknownforourtechnicalexpertise,industryforesight,andsleeves-rolled-upapproachtoprojects.From

strategytoimplementation,weservebusinesses,governments,andnon-profitsaroundtheworldwithtechnical,

commercial,andmanagementconsultingservicesforthepublicsector,privateequityandfinancialservicesfirms,

utilities,independentpowerproducers(IPPs),andcontractorsandmanufacturersforland-basedandoffshorewind,

solar,hydrokineticandoceanenergy,andenergy-storagetechnologies.RCGisheadquarteredinLondoninthe

UnitedKingdom,andhasofficesinNewYork,Taipei,Glasgow,SanFrancisco,BarcelonaandAmsterdam.

LondonGilmooraHouse

57-61 Mortimer Street

London

W1W8HS

NewYork433Broadway

6thFloor

NewYork

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