Asset management of submarine cables and lessons learned from a repair

06/03/2018
Auteurs : Jean Charvet
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Asset management of submarine cables and lessons learned from a repair

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REE N°1/2018 Z109 JICABLE HVDC'17 DOSSIER 2 REE N°1/2018 Z 109 Asset management of submarine cables and lessons learned from a repair Jean Charvet, RTE (France), Jean.charvet@rte-france.com Context Service experience of HVDC submarine cables CIGRE brochure TB379 – Update of service experience of HV underground and submarine cable systems, December 2009 – presents a failure rate for HVDC cables of approximately one failure per year per 1000 km of circuit, mostly caused from external damage. However, this figure is to be taken with great care because it is based on rather old service experience between 1990 and 2005, while in the past ten years, a lot of improvements has been achieved in marine engineering and routing, cable design, installation, protection and preventive maintenance. TB379 is currently being updated by CIGRE Working Group B1.57 with more recent data but results will not be published before end of 2018. In the meantime, RTE carried out a similar survey on the following sample: s (6$#SUBMARINELINKSIN%UROPE RANGINGFROMK6TOK6 s3ERVICEEXPERIENCEFROMTO s &AILURESKNOWNFROMPUBLICSOURCES COUNTEDPERCIRCUITONE failure may affect one or two cables) and after commissioning. Results of this survey is that for MI insulation technology, there is an average of less than one failure per year per 3000 km of circuit, while no failure has been reported for 8,0%TECHNOLOGYSOFARONAVERYLIMITEDSAMPLE  %VEN IF THOSE lGURES SEEMS REASSURING A TREND OF decreasing failure rates), it is worth mentioning that HVDC submarine links are being built on increasingly long length, and thus can be more vulnerable to faults. Unavailability and cost of repair 3UBMARINE CABLES GENERALLY DONT NEED ANY PLANNED unavailability, but can suffer from unplanned availabilities due to faults or the need for remedial protection works. Even with low failure rates, the fact that HVDC submarine cable failures takes a long time to repair can lead to significant impact on interconnector business models and security of electricity supply. &ORASINGLEFAULT TWOTOTHREEMONTHSISATYPICALTIME to be considered for repair, excluding hazards, while cost of repair and losses of revenue can be in the order of tens of million euros. &OR A LONG INTERCONNECTOR OF  KM FOR INSTANCE SUP
posing a failure rate of one failure per year per 3000 km would mean that, as an average, 2 to 3 months unavailability CANBEEXPECTEDONAYEARSPERIOD4HISCORRESPONDSTOAN average of 3 to 4% of the time which is significant to impact profitability of the interconnector. KEYWORDS : 3UBMARINECABLES 3ERVICEEXPERIENCE 0REVENTIVE-AINTENANCE 2EPAIR0REPAREDNESS /FFSHORE2EPAIR 3PAREPARTS Provided that submarine cable are correctly designed and installed, failures are rare but do happen on some occasions. Consequent repairs can be very costly and cause long unavailabilities. This article aims to identify the levers to improve reliability of submarine cable assets by limiting occurrence of failures and induced losses. Asset manage- ment policies including preventive maintenance, repair preparedness, and spare parts are described and discussed from a TSO perspective. Finally, lessons learned are shared from repairs managed by RTE on HVDC submarine cables of the IFA2000 interconnector (FR-UK) during the winter 2016-2017. ABSTRACT IFA 2000 experience /N th OF .OVEMBER  FOUR OUT OF EIGHT CABLESOFTHE)&!INTERCONNECTORFAILEDOFFSHORE LEADINGTO-7LOSTCAPACITY!FTERCONSEQUENT MOBILIZATIONOFRESOURCESTOPERFORMREPAIRASQUICKLY as possible, the interconnector recovered 500 MW CAPACITYTWOlRSTCABLESREPAIRED ONth OF&EBRUARY ANDITWASFULLYOPERATIONALREPAIROFTHETWO remaining cables) on the 2nd of March 2017. Lesson learned: > It took slightly more than three months in total to repair four cable damages, > #ONSIDERING THE EXTENT OF WORKS THIS GOOD PER
formance was made possible by hiring two repair vessels and two jointing teams working in parallel. JICABLE HVDC'17 DOSSIER 2 110 ZREE N°1/2018 Design measures Fault causes and preventive design measures Risk mitigation regarding fault occurrence and induced losses starts from the design phase of submarine links. Type of faults and design measures to prevent them are described below. External faults may be caused either by human activities, natural phenomena or a combination of those: s 3EABED MOVEMENTS LINKED TO SEISMIC ACTIVITY OR CURRENTS ANDWAVES s !BRASIONORFATIGUEONNON
BURIEDCABLESANDFREESPANS s )MPACTORHOOKBYANCHORS lSHINGGEARS s $AMAGECAUSEDBYWORKSINTHESEABEDDREDGING CABLEOR PIPELINELAYING EXTRACTIONOFAGGREGATE  s&ALLINGOBJECTS SHIPWRECK s/RDNANCEEXPLOSIONINTHEVICINITY To prevent external faults, the cable route and the level of protection shall be carefully designed depending on the above mentioned risks. Internal faults may be caused by: s %RRORORDEFECTDURINGMANUFACTURINGORASSEMBLYOFJOINTS ANDTERMINATION s -ECHANICALDESIGNPARAMETERSEXCEEDEDDURINGTRANSPORT STORAGEORINSTALLATION s "AD THERMAL ENVIRONMENT LEADING TO EXCEED TEMPERATURE DESIGNVALUES s/VERVOLTAGEOROVERLOADSABOVEDESIGNVALUES 4OPREVENTINTERNALFAULTS ITISCONSEQUENTLYRECOMMENDED to select properly tested materials, have a robust inspection and test plan during every step from design, manufacturing and installation of submarine cables, have a robust thermal DESIGN BASED ON ON
SITE MEASUREMENTS AND PUT IN PLACE proper protections against overvoltage and overloads. Maintenance friendly designs In order to allow an effective preventive maintenance and AQUICKREPAIRTHEFOLLOWINGKEYPOINTSMUSTBECONSIDERED s )NTEGRATIONOF&/UNITINSIDEPOWERCABLES ORALTERNATIVELY BUNDLED ISBENElCIALTOALLOW&/BASEDCABLEMONITORING ANDFAULTLOCATION s #ABLEMUSTBEEASILYACCESSIBLEINCASEAREPAIRISNEEDED this may be contradictory to preventive protection measures AGAINSTEXTERNALTHREATSANDSHOULDBECAREFULLYBALANCED s ,IMITINGTHENUMBEROFDIFFERENTCABLEDESIGNSANDACCES
sories or making sure they are compatible between each other in order to rationalise spare parts storage. Preventive maintenance The purpose of preventive maintenance policies is to de
crease the probability of failure. Cable awareness It is obvious that precise cable position shall be reported on every relevant marine charts. Figure 1: IFA 2000 cable damage. IFA2000 experience 3INCE IT HAS BEEN COMMISSIONED IN  THE submarine part of the interconnector has experienced TWO SIMULTANEOUS EXTERNAL FAULTS IN  AFFECTING four cables, presumably caused by anchors although cables were well buried at approx. 1.5 m in relatively stiff soil, 5 km away from English coast and relatively far from shipping lanes. Lessons learned: > Emergency anchoring is probably more likely to happen in nearshore areas and not inside a shipping lane. IFA2000 experience 3INCE IT HAS BEEN COMMISSIONED IN  THE submarine part of the interconnector has experienced one internal fault in 2003, affecting one cable, caused by mishandling during installation which created a weak point. Lesson learned: > .OT ALL DEFECTS CAN BE DETECTED DURING COMMIS
sioning tests, neither warranty period, which means that controls during all steps from cable design to cable installation are crucial. REE N°1/2018 Z 111 Asset management of submarine cables and lessons learned from a repair It is also recommended to work with fishermen to define good practices when they work in the vicinity of the cables. Moreover, it is also possible to monitor vessels positions and movements in the vicinity of cable routes using real time !)31 DATA!DETECTEDRISKSITUATIONCANTHENLEADTOCONTACT the vessel captain or marine authorities in order to prevent unauthorized activities in the vicinity of the cable. Cable monitoring 3TANDARDPRACTICEFORNEWLYBUILT(6$#LINKSISTOMONITOR TEMPERATURE ALONG THE LINK BY IMPLEMENTING $43 SYSTEMS Development of local hotspots could indicate an internal defect or an unfavourable thermal environment, while local “cold spots” could reveal deburial. )TISALSORECOMMENDEDTOINSTALL$!3SYSTEMSTHATCOULD help locating deburials or external aggressions but experience is still very limited and signals are not easy to interpret. Partial discharge measurement are also sometimes considered but its interpretation can also be tricky. Those systems have a limited range and only a part of the submarine link may be monitored for long interconnectors, although technology is constantly improving. )N ORDER TO ENSURE THE EFlCIENCY OF &/ BASED SYSTEMS BESTPRACTICEISTHAT&/UNITSAREEITHERBUNDLED ORDIRECTLY integrated to each power cable. This sometimes lead to install ANEXTRA&/UNITWHENAPAIROFPOWERCABLEISUNBUNDLED WHICHISOFTENTHECASEATLANDFALLS  The way of interpreting data from those monitoring systems can vary from regular checks with analysis reports to CONTINUOUSCHECKWITHPRE
DElNEDLEVELSOFALARM!LEARNING phase in the first month or years of operation may be necessary to fine tune the interpretation of monitoring data. In case an anomaly occurs and depending on its severity, it may be decided to launch surveys and/or remedial works. 1 AIS: Automatic Identification System, système d’échanges automatisés de messages entre navires par radio VHF qui permet aux navires et aux systèmes de surveillance de trafic de connaître l'identité, le statut, la position et la route des navires se situant dans la zone de navigation (NDLR). Geophysical marine surveys -INIMUM FREQUENCY AND EXTENT OF SURVEYS ARE OFTEN part of regulatory or insurance obligations, which can vary depending on the asset. The data to collect which are project specific generally INCLUDE MULTI
BEAM BATHYMETRY AND SOMETIMES SIDE
SCAN sonar, measurement of cable position and burial depth, environmental monitoring, etc.. Because marine survey operations on long links are very COSTLYACTIVITIES THEBESTPRACTICEISTOADAPTTHEFREQUENCY and the extent of planned surveys depending on risks, notably SEABEDMOBILITYANDEXTERNALTHREATSANCHORS lSHING  Moreover, unplanned surveys may be decided upon occurrence of extreme meteorological event or anomaly detected on monitoring systems. 'REATCARESHALLBETAKENONFORMATOF')3DATA INORDER to be able to compare each survey data from the previous surveys, and make the data usable for potential future works on or next to the link. Repair preparedness The purpose of repair preparedness is to reduce the time for a repair, and thus the induced losses. Organisation and emergency contingency plans %LABORATING AND MAINTAINING AN UP
TO
DATE EMERGENCY contingency plan for each submarine link is a key point for a QUICKRESPONSEAFTERAFAULT Figure 2: AIS monitoring on IFA2000. IFA2000 experience Immediately after fault happened in November  AND BASED ON ITS EXPERIENCE OF EMERGENCY situation, RTE put in place an operational project team involving local personnel from project management and maintenance departments, relying on the support from internal cable expertise and offshore project departments, procurement and legal departments and outsourced marine and legal experts. Lessons learned: > (AVING24%QUALIlEDPERSONNELONBOARDOFREPAIR vessels allowed to handle interfaces between different contractors on board, which were sometimes critical and it surely has saved time and PARTICIPATEDTOQUALITYANDSAFETY > Experience from repairs is valuable to improve contingency plans. JICABLE HVDC'17 DOSSIER 2 112 ZREE N°1/2018 3UCHAPLANWOULDTYPICALLYINCLUDE s $ESCRIPTIONOFINTERNALORGANISATIONTOPUTINPLACEINCLUDING HUMANRESOURCES ROLEANDRESPONSIBILITIES DECISIONMAKING s2EPAIRPROCEDURESFORDIFFERENTPLAUSIBLEFAULTSCENARIOS s,ISTOFRELEVANTCONTACTSANDPROVIDERS s)NTERFACEMANAGEMENT s3AFETY %NVIRONMENTAND2EGULATORYREQUIREMENTS Periodic revision of contingency plans shall be performed ANDITISALSORECOMMENDEDTOPERFORMREGULARCRISISEXER
cises. Fault location &AULTLOCATIONISONTHECRITICALPATHOFAREPAIR)TISGENERALLY performed in two steps: s 0RE
LOCATIONFROMLANDUSING4$2BASEDMETHODSONTHE power cables, s 0IN
POINTINGUSINGMAGNETIClELDORACOUSTICMEASUREMENTS at sea, and/or with fibre optic when available. Reliability and reactivity of those operations is of paramount importance and thus it is recommended either to have an internal expertise or frame agreement with a specialised provider. Marine operations Mobilization of an adapted marine spread to allow the re
pair needs to be done as soon as possible after fault location is confirmed. In addition to vessels that are necessary for fault location and surveys, type of marine vessels to mobilize depends mainly on water depth, and are generally: s*ACK
UPBARGESANDTUGSFORREPAIRSATLANDFALLS s !NCHORED BARGES AND TUGS FOR REPAIRS IN SHALLOW WATERS < 15 m WD s$36OR$0VESSELSFORREPAIRSINDEEPWATERM7$ Because it is very costly to keep in standby all those type of potentially necessary vessels for repairs, it is general practice, upon a failure, to hire vessels that are available on the market, through a specialized broker for example. -OREOVER REPAIROPERATIONSNEEDSPECIlCEQUIPMENTTOBE installed onboard which can vary depending on the situation. 3OMEOFTHECRITICALEQUIPMENTWHICHCANBEPROJECTSPECIlC are listed below. s 4OOLSFORDEBURIALANDRE
BURIAL MAINLYDEPENDINGONTHE type of soil and cable diameter s 4URNTABLEORBASKET MAINLYDEPENDINGONCABLECOILABILITY length and weight of spare and cable MBR s #ABLECHUTEANDQUADRANT MAINLYDEPENDINGONCABLE-"2 IFA2000 experience )N .OVEMBER  PRE
LOCALIZATION AND PIN
pointing of faults on the four cables with 50 m accuracy were completed within 9 days after faults OCCURRENCE AND LATER DOUBLE
CONlRMED BY SURVEYS showing anchor scars on the seabed. 24%HASANINTERNALEXPERTISEINFAULTPRE
LOCATION 4$2 BASED SYSTEMS AND OWNS SPECIlC OFFSHORE PIN
POINTINGFAULTLOCATIONEQUIPMENTMAGNETIClELD based) developed internally and patented. Lesson learned: > (AVING INTERNALLY THE EQUIPMENT READY FOR MOBILISATIONTOGETHERWITHREGULARLYTRAINEDMAIN
tenance teams proved to be very efficient. Figure 3: Offshore pin-pointing equipment. Figure 4: Pin-pointing based on magnetic field measurements. Figure 5: one of the vessels hired for IFA2000 cable repairs, at mobilization site. REE N°1/2018 Z 113 Asset management of submarine cables and lessons learned from a repair s 4ENSIONER MAINLY DEPENDING ON CABLE WEIGHT AND WATER depth (AVINGPRE
CONTRACTUALARRANGEMENTSORFRAMEAGREEMENT FORMOBILIZATIONOFMARINESPREAD PERSONNELANDEQUIPMENT with a specialized contractor is a common practice in order to save time for negotiations and engineering after occurrence of a fault. Jointing operations +NOW
HOWOFSPECIALIZEDJOINTINGTEAMSISAKEYPOINTFOR a successful and reliable repair, especially in very high voltage ranges. Spare parts storage The purpose of spare part storage is to make sure that reliable spare materials of the cable system is immediately available in case a repair is needed without waiting for remanufacturing. Quantity of spares is project specific and mainly depends on: s2ISKSANDFAILURESSCENARIOSTOCOVER s7ATERDEPTH s 0RESENCEOFAREASWHEREJOINTINGWILLHAVETOBEAVOIDEDFOR EG($$ANDPOSSIBLYROCKBERMS  s ,EADTIMESANDMINIMUMQUANTITIESTORElLLTHESTOCKAFTER ITISUSED s(AZARDSTOCONSIDER 3TORAGESITEISUSUALLY s .EARBYAQUAYINAPORTWITHDIRECTANDPERMANENTACCESS TOSEA s )NACONTROLLEDANDSECUREAREA AIREDANDPROTECTEDFROM UV and rain. Conclusion Lessons learned from submarine cable repair experience makes possible to improve asset management policies. 3HARINGOFSERVICEEXPERIENCEANDCOLLABORATIONFORMORE standardization of repair solutions must be encouraged. Glossary AIS:!UTOMATIC)DENTIlCATION3YSTEM DAS:$ISTRIBUTEDTEMPERATURE3ENSING DTS:$ISTRIBUTED!COUSTIC3ENSING DP: Dynamic Positioning DSV:$IVING3UPPORT6ESSEL FO:&IBRE/PTIC GIS:'EOGRAPHIC)NFORMATION3YSTEMS MBR: Minimum Bending Radius MI: Mass Impregnated TDR:4IME
$OMAIN2EmECTOMETRY XLPE:#ROSSED
LINKED0OLYETHYLENE#LOSEAND2ETURN IFA2000 experience RTE and National Grid Interconnector Ltd share a frame agreement for mobilization of marine spread. Considering that failures happened on two distinct locations for each pair of cables, it has been decided to work as much as possible in parallel in order to save time. Two repair vessels and one support vessel were hired within one month and ready before the jointer teams. The support vessel was dedicated to prepare CABLESCABLECUTS DEBURIALS EXPERTISEOFDAMAGES checks and tests on cables, sealing ends) while the two others were dedicated to jointing operations. Lessons learned: > Having a frame agreement made possible to hire ANDMOBILIZEACONSEQUENTmEETWITHINASHORTTIME IFA2000 experience Because of the extent of the repairs to be done and limited availabilities of jointing teams, RTE contracted those operations to 2 different suppliers who were both competent to perform the cable JOINTINGOPERATIONSONTHE24%)&!SUBMARINE CABLES-)TECHNOLOGY BUTSTILLNEEDEDREHEARSALON a piece of spare cable before going offshore. Those operations appeared to be on the critical path of the repair, as marine spread was ready before jointing teams. Lessons learned: > Having the possibility to install compatible joint from a different supplier than the original cable was beneficial and saved time. IFA2000 experience Tests confirmed that spare cable that was stored for more than thirty years in cable tanks were still in good condition. Two cable joints from the spare parts have been used for training of jointers, prior to perform the offshore repairs. Lesson learned: > Regular inventory and maintenance on the spare parts is valuable.