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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
Seb RaePrincipal, London
+44 (0) 7557 095465
Sam ParkDirector, London
+44 (0) 7500 896757
Daniel StevensDirector, London
+44 (0) 7768 992206
Elaine GreigDirector, London
+44 (0) 7981 205753
Jamie FlemingSeniorUnderwriter,London
+44 20 7847 3426
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
Source: wikimedia [8]
Source: wikimedia [9]
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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.
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[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.
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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.
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[Accessed21March2019].
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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/
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farm.[Accessed27March2019].
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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
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