DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION 2013
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Contents
1 Foreword 32 Key Findings 43 Introduction 64 Pipelines In the North Sea 7 4.1 Pipeline Types 7 4.2 PipelineConfiguration 9 4.3 PipelineInventoryandFunction 10 4.4 PipelineAncillary/AssociatedEquipment 115 DecommissioningRegulationsforPipelines 13 5.1 Overview 13 5.2 NotificationofDisusedPipelines 136 DecommissioningOptions 15 6.1 SelectionofDecommissioningOptions 15 6.2 Pipelines 15 6.3 AncillaryEquipment 187 Safety 19 7.1 Overview 19 7.2 ShortTermOperational HealthandSafetyChallenges 19 7.3 LongTermHealthandSafetyChallenges 208 Environmental Impact 21 8.1 Overview 21 8.2 EnvironmentalImpacts 219 MonitoringandLiability 2410 CostofPipelineDecommissioning 2511 PipelineDecommissioningtoDate 2612 Technology 27 12.1 Overview 27 12.2 PipelineCleaning 27 12.3 TrenchingandBurial 27 12.4 DeburialandDredging 30 12.5 SubseaCutting 30 12.6 Lifting 32 12.7 ReverseInstallationMethods 32 12.8 MattressRecovery 3413 RecyclingandReuse 36 13.1 Recycling 36 13.2 Reuse 3614 PublicConsultation 37 14.1 RequirementsforConsultation 37 14.2 StatutoryConsultees(UKCS) 37 14.3 ConsultationProcess 3715 References 38
Appendix A:TableofDecommissionedPipelines IntheNorthSeaRegion 39
Appendix B:CaseStudiesofPipelineDecommissioningProjects 44
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Abbreviations
CA ComparativeAssessmentEIA EnvironmentalImpactAssessmentDECC TheDepartmentForEnergyAndClimateChange,UKGovernmentHSE HealthAndSafetyExecutive,UKJNCC JointNatureConservationCommittee,UKMPA MarineProtectedAreaNORM NaturallyOccurringRadioactiveMaterialPLUTO PipelinesUnderTheOceanPWA PipelineWorksAuthorisationUKCS UnitedKingdomContinentalShelf
Definitions Pig PipelinemaintenancetoolusedforcleaningorinspectingtheinsideofapipelinePiggy-back A smalldiameterpipelinewhich isphysicallyattached toa largerdiameterpipelineusing
strapstofacilitateitsinstallationand/orlongtermprotectionPipelinespan Asectionofpipelinewhereseabedsedimentshavebeenerodedorscouredfromundera
pipeline,resultinginanunsupportedsectionofpipeS-lay Pipelineinstallationmethodforlargerdiameterpipelines
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1 Foreword
ThefirstmajoroffshorepipelineconstructionprojectintheUKwasthePipeLinesUndertheOcean(PLUTO1)projectwhichinstalled1,000milesofpipelinebetweenthesouthcoastofEnglandandFrancetoprovidefuelfortheinvasionofFranceduringWorldWarII[Ref.1].Inthemoderneraofoilandgasproduction,thefirstpipelineswereinstalledin1966totransportgasfromBP’sWestSolefieldtoareceivingterminalatEasingtonon the Lincolnshire coast. Since then, inexcessof 45,000kilometresofpipeline,umbilical and cablehasbeeninstalledacrosstheNorthSearegiontoenablethegatheringanddeliveryofhydrocarbonstoreceivingfacilitiesandend-usersacrossEurope.
Naturally, in amature province such as theNorth Sea,when fields reach the end of their economic life,sectionsofthetransportationinfrastructurebecomeredundantandmustbedecommissioned.TheprocessofdecommissioningredundantNorthSeaassetshasbeenongoingsincetheearly1990swiththedecommissioningoftheCrawfordfieldbyBHP.Sincethenpipelineinfrastructurehasbeendecommissionedatamodestratewhensystemsaredeemedtohavenofutureeconomiclife,andnoalternativeusecanbefound.
Thisreportaimstoprovideanoverviewofthedecommissioningperformedtodateofpipelinesandtheirassociated infrastructure. ItseekstocoverallareasoftheNorthSea, includingthe IrishSeaandWestofShetland,underthejurisdictionoftheUK,Norway,Denmark,theNetherlandsandGermany,althoughitisnotedthatdatafromsomeareasismorelimitedthanothers.
IntheUKandNorway,thedecommissioningofoilandgas-relatedpipelinesisconsideredonacase-by-casebasis,usingtheComparativeAssessment(CA)processtodeterminethebestoptionfordecommissioning.Thisenablestheparticulardiameter,lengthandconfigurationofindividualpipelinestobetakenintoaccountwhenconsideringdecommissioningoptionsagainst thecriteriaofsafety,environmental impact,costandtechnicalfeasibility.
ThedocumentdrawsonresearchperformedbyOil&GasUKovertheperiodfrom2010to2013,alongwithpubliclyavailableoilandgasindustrydata.ItprovidesapictureofthescaleofpipelineinfrastructureintheNorthSea,andtheindustry’sachievementsindecommissioningpartsofthatinfrastructure.Italsohighlightsthetechnicalcapabilitiesand limitationsthat impact thedecommissioningoptionsavailable toownersofpipelinesystems.
ThereportisdesignedasareferenceforindustryandothersinterestedinthedecommissioningofpipelineinfrastructureintheNorthSeaRegion.
11942and1943sawthedevelopmentofthefirstflexiblepipeline,the3inch‘Haiscable’;andthefirstrigidreeledpipe,the3inchsteel‘Hamelpipe’whichwereinstalledacrosstheEnglishChannel.TheserevolutionarypipelinedesignsprovidedtheallieswiththecapacitytotransportonemilliongallonsoffuelperdaytonorthernFrance.
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2 Keyfindings
Experience to-date
• SincetheWestSolegasexportpipelinewasinstalledin1966,anestimated2,500individualpipelines,umbilicalsandpowercableswithalengthinexcessof45,000kilometreshavebeeninstalledintheNorthSearegion,includingtheEastIrishSeaandWestofShetland.
• Thepipelineinventoryismadeupofrigid(steel)pipelinesandflexibleflowlines,andvariesindiameterfrom2inchestoamaximumof44inches.ThelongestpipelinecurrentlyoperatingintheNorthSearegionisFranpipe,withadiameterof42inchestransportinggas840kilometresfromDraupnerEintheNorwegianNorthSea,toDunkirkinFrance.
• Lessthan2percentoftheNorthSeapipelineinventoryhasbeendecommissionedsofar.Ofthepipelineswhichhavebeendecommissioned,80percentarelessthan16inchesindiameter.Halfofthelargerdiameterpipelines(16inchesorgreater)decommissionedtodatewereremoved:thesewereallunder1kilometreinlengthandinfieldpipelines.
• Somepipelines, in particular large diameter trunklines, represent important infrastructurewhichprovidesthemeansoftransportingcurrentoilandgasproductionbetweenfacilitiesandtoshore.Thisinfrastructurealsoprovidesopportunitiesforfuturedevelopmentofhydrocarbonsreserves,orstorageofcarbondioxideorgasinthebasin.ThisisakeyreasonwhythereiscurrentlyonlylimitedexperienceofdecommissioningsuchpipelinesintheNorthSea.
Processes:
• UndercurrentregulationsacrosstheNorthSea,pipelinedecommissioningiscarriedoutonacase-by-case basis, with the decommissioning option selected for each pipeline, umbilical and cableconfirmedbydetailedCA.
• HealthandsafetyisadominantfactorinanyCA,withthefocusbeingonminimisingrisksinthelongtermtootherusersofthesea,andintheshorttermtothosecarryingoutthedecommissioningoperations.
• An Environmental Impact Assessment (EIA) is prepared to support all pipeline decommissioningplans.Potentialenvironmentalimpactsarereasonablywellunderstoodfortheshorterlengthinfieldpipelines,andmitigationmeasureshavebeenestablishedtominimisetheeffectsduringandafterdecommissioning. At present, due to the limited experience of decommissioning larger diameterpipelinesintheNorthSea,itisdifficulttoquantifytheenvironmentalimpactofsuchdecommissioning.
• In themajority of decommissioning cases, it has been demonstrated that the best option is toleaveapipelineinplace,eitherontheseabed,orleftburiedbelowtheseafloor.Thisapproachiscomplementedbywhateverremedialactionisdeemednecessarytofurtherreduceanyriskstootherusersofthesea,forexamplethecuttingandremovalofexposedpipelineends.
• Anumberoftoolshavebeenproveninthecuttingofsteelandflexiblepipelines,includingthosewithmultiplecoatings,concrete,anti-corrosionlayersandinsulation.Cuttingcanbetimeconsuming,andcanleadtoextendedriskexposuretodiversifmultiplecutsareundertakensubsea.
• If removal is identifiedasthebestoption,reversereelinghasbeenusedasameansofremovingsmallerdiameterrigidsteelandflexibleflowlines.OntheUKContinentalShelf(UKCS),45kilometresofsmalldiameterpipelinesareknowntohavebeenremovedusingthethismethod.Althoughithasonlybeenusedforasmallnumberofpipelines,reelvesselshavebeenadaptedinthepasttoreversetheirnormalinstallationmodetoremovepipelines.
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• ThereisverylimitedexperiencegloballyofremovingpipelinesusingareversaloftheS-layinstallationmethod.Anumberoftechnicalchallengesexistintheapplicationofthismethodtolargediameterageingpipelines,andinparticularconcrete-coatedtrunklines.Theseissuesrelatetotheintegrityoftheconcreteweightcoatingandthesteelpipewallitselfaftermanyyearsofservice,bothofwhichwouldbesubjecttohighforcesduringrecovery.
• The reverse S-laymethod of pipeline recovery during a decommissioning programme has neverbeenusedintheNorthSeaandcannotbeconsideredproven,particularlyintheapplicationtolargediameterconcretecoatedpipelines.
• Overall,acase–by-caseapproachisconsideredappropriateforpipelinedecommissioning.AspartoftheCAprocessthewidevariationinpipelinetype,diameter,length,integrityandin-placeconditionareexamined.Whensafety,environmentalandcostconsiderationsarealsotakenintoaccount,thebestdecommissioningoptionforeachpipelinecanbeidentified.
Cost and reuse:
• EstimatingthecostsofdecommissioningthetotalpipelineinventoryintheNorthSearepresentsanon-goingchallengefortheindustry.Factorssuchaslimitedexperience,technicalunknowns,integrityuncertaintiesandthesignificantvariationinpipelineconfigurationsmakeitverydifficulttoforecastcostswithanyrealdegreeofaccuracy.
• Reuseopportunities for rigidsteelpipelinesrecoveredbythereversereelingprocessare limited.Subjectingapipetomultiplecyclesofplasticdeformationduringboththereelingandreversereelingprocesseswouldlikelycompromiseitsintegrity.
• Rigidsteelpipelinescanberecycledalongwithsomeofthecoatingsthatmaybeappliedtothem.Likewise,flexiblepipelines,umbilicalsandpowercablescanbeprocessedtoseparatetheirmetallicandplasticcomponentsandthenrecycled.
• Potential opportunities may exist for the reuse of flexible pipelines and umbilicals if their postrecoveryintegritycanbeconfirmed.
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3 Introduction
InOctober2010,Oil&GasUKinitiatedtheDecommissioningBaselineStudytocompiledata,experiencesandlessonslearnedonthedecommissioningofNorthSeaoilandgasinfrastructure.AmajoroutputfromthisworkwastheOil&GasUKreport‘TheDecommissioningofSteelPiledJacketsintheNorthSeaRegion’whichwaspublishedinOctober2012.
In addition to providing this visibility on recent work performed in the decommissioning of steel piledjackets,theDecommissioningBaselineStudyalsoprovidedsignificantinsightintothedecommissioningof oilandgaspipelines.
The networks of pipelines currently installed in the North Sea collectively provide the transportationinfrastructurethatallowsNorthSeaoilandgasproductiontobedeliveredtohostplatformsortoshore.Inmanycases,theexistenceofnearbypipelineinfrastructurehasleddirectlytotheeconomicexploitationofmarginalfields,whichwouldotherwisebeconsidereduneconomic.Suchopportunitiesremainakeyfactorinthetimingofanypipelinedecommissioning.
Asfieldshavereachedtheendoftheireconomiclife,specificpartsofthepipelinesystemnaturallybecomeredundant,andwithnopotentialfutureuse,theyareavailabletobedecommissioned.OilandgaspipelinedecommissioninghasbeentakingplaceintheNorthSeasincetheearly1990s,whentheCrawfordpipelineswere decommissioned. Since then, pipeline decommissioning has continued at a modest rate and onlywhen all potential re-use options for the infrastructure, including new field developments, have been carefullyconsidered.
ThisreporthasbeencompiledusingtheoutputfromtheDecommissioningBaselineStudyandadditionaldatafromtheindustrytoestablishareferenceonpipelinedecommissioningintheNorthSea.Itprovidesanoverviewofthepipelineinventoryandthedecommissioningperformedtodate.Italsoincludesasummaryof the applicable regulations, health and safety and environmental challenges, and an overview of thetechnologyavailable,itsapplicationsanditslimitations.
Unlessnotedotherwisethegeneralreferenceto‘pipelines’throughoutthisdocumentreferstotrunklines,rigidflowlines,flexibleflowlines,umbilicalsandpowercables.
Oil&GasUKwouldliketoacknowledgethevaluablecontributionmadebythefollowinggroupsandorganisationsinthepreparationofthisdocument:TheDecommissioningBaselineStudyJIPSponsors2,PremierOilPlcandAtkinsLimitedinthepreparationofthisdocument.TheauthorswouldparticularlyliketothankthemembersofOilandGasUK’sDecommissioningSteeringGroupTaskGroup2fortheirsignificantcontribution.
2ApacheNorthSea,BPExplorationOperatingCompanyLimited,CNRInternational(UK)Limited,ConocoPhillipsUKLimited,DONGE&PAS,FairfieldEnergyLimited,MarathonOilDecommissioning ServicesLimited,MobilNorthSeaLLC,ShellUKLimited,StatoilAS,TalismanEnergy(UK)Limited, TotalE&PUKLimited,VentureNorthSeaGasLimited
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4 Pipelines in the North Sea
Oil andgasproduction involves the transportationofmanydifferentfluidsunderdifferent conditions, invaryingwaterdepthsandoceanographicenvironments.Thishas led toa rangeof typeofpipelinebeinginstalledacrosstheNorthSea.Thissectionprovidesadescriptionand inventoryofthedifferenttypesofpipelinecurrentlyinstalledandoperationalintheregion.
4.1 Pipeline TypesFigure1providesahighlevelcategorisationofthetypesofpipelinesinoperationintheNorthSeaRegion.
Figure 1 Pipeline Category Descriptions
Pipeline Description1
Typical Dimensions1
Applications PrimaryMaterialsofConstruction
AdditionalCoatings
Trunklines Upto44inchesdiameter,upto840kilometreslong
Majorexportinfrastructureforoilandgas
Carbonsteel Anti-corrosion2 coatingplusconcreteweightcoating3
Rigidflowlines Upto16inches,diameter,lessthan50kilometreslong
Infieldflowlinesandtie-inspools
Carbonsteelorhighspecificationalloy
Polymeranti-corrosioncoating
Flexibleflowline Upto16inchesdiameter,upto10kilometreslong
Infieldflowlinesandtie-inspools
Carcassofhighspecificationalloysandpolymerlayers;alloyend-fittings
Polymerexternalcoatings
Umbilical Between2and8inchesdiameter,upto50kilometreslong
Chemical,hydraulicandcommunicationdistribution
Thermoplasticpolymerorhighalloysteeltubes;wirearmouredprotection
Polymerexternalcoatings
PowerCables4 Between2and4-inchesdiameter;upto300kmlong
Powerdistributionbetweenandwithinfields
Coppercoreswithwirearmouredprotection
Polymerexternalcoatings
Notes1.Pipelinedescriptionsandtypicaldimensionsreflecttheiruseinthisdocument:othersourcesmaydifferin
theapplicationofthisterminology.2.Anti-corrosioncoatingsusedforthesepipelinesinclude:coal-tarenamel,bitumenandfusionbondedepoxy.3.Concreteweightcoatingsusuallyincludereinforcingwireorbars.4.Powerdistributioncablesareoftenincludedinanumbilicalstructure.
4.1.1 TrunklinesTrunklinesaremajorelementsofinfrastructuretransportinglargequantitiesofoilorgastoonshorereceivingfacilities.Trunklinesaccountfor18percentofthetotalnumberofpipelinesand63percentofthetotalpipeline length in theNorthSea inventory.Typically thesepipelinesareownedandoperatedbya singleoperator,orgroupofoperators,andtransportproductionfromanumberoffieldsonbehalfofthedifferentfieldowners.SuchpipelinesincludesomeofthelongestintheNorthSea,oftenhavingdiametersinexcessof
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30inches,withthelargestbeing44inchesindiameter.ThelongesttrunklinecurrentlyoperatingintheNorthSearegionisFranpipe,(seeSection2).
Largediametertrunklinesareinstalledutilisingthe‘S-lay’pipelaymethodfromaspecialistlay-vessel.Thisinvolvesweldingsectionsofpipetogetheronthedeckofthevessel,thenloweringthepipelinetotheseabedasacontinuousstringofpipe,asthevesselmovesforward.Thisprocesscancontinueformanykilometres,subjectonlytothesupplyofpipesectionsandsuitableweatherconditions.
4.1.2 RigidflowlinesFlowlinesaresmallerdiameter,shorterpipelinesusuallyassociatedwithasingleoilorgasfield.Socalled‘rigidflowlines’aremanufacturedfromcarbonsteelorahighperformancesteelalloy,withadditionalcoatingsprovidingcorrosionprotection,andinsomecasesinsulation.Thesepipelinesaccountforapproximatelyhalfofthetotalnumberofpipelinesand27percentofthetotallengthintheNorthSeapipelineinventory.Rigidflowlinesusuallytransportoilandgasbetweensubseainfrastructuretoahostplatformforprocessing.Theycanalsobeused to transport injectionwater to subseawells forpressuremaintenancepurposes. Thesepipelinesaretypicallylessthan16inchesindiameterandaremostofteninstalledbythereelingmethod.Thisinvolvesfabricatingtherequiredlengthofpipelineonshorebeforereelingthesteelpipearoundalargedrumonaspecialistreel-shipfortransportationtothefield.Theendofthepipelineisanchoredintherequiredlocationand thepipeunreeledas thevesselmovesalong theproposedpipeline route. The steelpipe isstraightenedasitisdeployed.
Flowlinesmaybeasshortas10metreslongwheninstalledbetweenasubseawellandmanifold(asocalledtie-inspool),butinmanycasestheyarealotlonger.Forexample,Total’sNUGGETSfieldN4wellislinkedtotheAlwynplatformviaflowlinestotalling67kilometresinlength(Ref2].
4.1.3 FlexibleFlowlinesFlexibleflowlineshavethesameapplicationasrigidflowlines,butaremanufactureddifferently.Insteadofhavingaconventionalhomogeneoussteelwalltocontainthefluid,thewallofaflexibleflowlineismadeupofcompositelayersofsteelwireandpolymersheathing,eachprovidingadifferentfunctioninthestructureofthepipewall.Collectivelytheselayersprovidetheflexibilityinthepipeline.
Unlikearigidflowline,whichisterminatedbyweldingonastandardendflangeatanappropriatelocation,flexible flowlines have specially made ‘end-fittings’ which are connected to each end of the pipeline atmanufactureandcannotbeeasilyremovedtoadjustthelengthon-site.Thesetypesofpipelineareinstalledusingavesselequippedwithalargecarousel,oftencapableofinstallinganumberofsimilarflowlinesinthesamecampaign.
Thepreferenceforusingeitherarigidorflexibleflowlineforagivenapplicationisdrivenbymanyfactorsincludingspecificdesignrequirements,installationconstraints,costorschedule.
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4.1.4 UmbilicalsandPowerCablesUmbilicals are commonplace in subsea developments, providing chemical injection, hydraulic andcommunicationsupporttowells.Theyaremadeupofabundleof‘cores’,eachofwhichmaybeuptooneinch in diameter, transporting chemicals or hydraulic fluid. The bundle also often contains fibre-optic orinstrumentationcableslinkingthesubseacontrolstothehostfacility.Theoutersheatharoundthebundleofcoresisprotectedbywirearmouring,givingtheappearanceofasinglecablewithanoutsidediameterofanythingupto8inches.Umbilicalsaretypicallyinstalledalongsideflowlinesystemsusingsimilarequipmenttothatusedtoinstallflexiblepipelines.Likeflowlines,umbilicalsareroutinelytrenchedbelowtheseabedlevel.
Power cables have a similar structure and installationmethod to umbilicals except they carry dedicatedpowertoasubseasystemorbetweenplatformfacilities.Oftenapowercablewillbeincludedinanumbilical.Wheninstalledseparatelytheyareprotectedinthesamewayasumbilicals,usinganexternalsheathmadeupofwirearmoursandinstalledinatrenchbelowseabedlevel.
4.2 PipelineConfigurationOfparticularsignificancewhenconsideringthedecommissioningofapipelineisitson-bottomstatus,post-installation.Thedesignprocessforanewpipelinedetermineswhetheritisinstalledrestingontheseabed,inanopentrenchcutintheseabed,orinstalledinatrenchandthenburiedusingseabedsoiltoalevelbelowthesurrounding levelof theseabed.Anyof theseconfigurations,orvariationsof them,maybespecifiedat installation.However,changescanoccurduringthe lifeofthepipelineduetotheactionofwavesandcurrentsonseabedsediments,orfromaccidentalinterferencebyotherusersofthesea,e.g.fishinggear,anchors,etc.
Thevariouspipelineon-bottomconfigurationscanbegeneralisedintothefourcategoriesshowninFigure2below.
Figure 2 Pipeline Configuration on the Seabed (Source Atkins)
Fullyexposedonunmodifiedseabed.(Somenaturalsettlement/embedmentmayhaveoccured.)
Fullyorpredominantlyexposedintrench:crownofpipebelowambientseabedlevel.
Crownofpipe–max25%diameter–exposedafternaturalbackfilloftrench:topofpipebelowambientseabedlevel.
Fullyburiedbyeithernaturalorploughedbackfill: topofpipebelowambientseabedlevel.
(A)
(B)
(C)
(D)
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Theas-designedburialstatusofapipelineisdrivenbyanumberoffactors.Theremaybearequirementtoprotectapipelinefromnearbyoilandgasoperationalactivities,suchasaroundplatforms,ortoreduceriskstoandfromotherusersoftheseasuchasfishermanoranchorsfrommooredvessels.Insomecasesitmaybenecessarytolowerapipelineintotheseabedinareasofhighon-bottomcurrentstoensureitslong-termstability.Inothercasesburialandbackfillingofthetrenchmayassistininsulatingthepipelineforoperationalreasons,orbyprovidingresistancetoupheavalbucklingofthepipe.
Afterinstallationofapipeline,whetheritistrenchedornot,theseabedarounditmaymoveundertheactionofwavesandcurrents.Overtimethismayleadtoanunburiedpipelinebeingburied(socalledself-burial),oraburiedpipelinebecomingexposed,potentiallyleadingtospanning.(Thishappenswhereseabedsedimentshavebeenerodedorscouredfromunderasectionofpipeline,whichthenbecomesunsupported.)Incaseswhereapipelinebecomesexposed,thedegreeofexposuremayvaryalongitslengthfromfullyexposedontheseabed(Figure1,exampleA),tofullyburiedinabackfilledtrench(Figure1,exampleD).
Theon-bottomconfigurationofapipelineismonitoredoveritslifetimesothatwhenthedecommissioningplanisprepared,itsburialhistorycanbeusedtoassistindeterminingthepreferredmethodofdecommissioning.
4.3 PipelineInventoryandFunctionAspreviouslydetailed,around45,000kilometresofpipelineshavebeeninstalledintheNorthSeaRegionsince1966.Figure3illustratestheaggregatedlengthofpipelinesbydiameterinstalledandtheirrangeofservice.
Figure 3 Pipelines Installed in the North Sea by Diameter and Service (Source Xodus)
0.0
500.0
1000.0
1500.0
2000.0
2500.0
3000.0
3500.0
4000.0
4500.0
5000.0
Aggr
egat
e Pi
pelin
e Le
ngth
(km
)
OD (inches)
Aggregate Pipeline Lengths by Service
Unknown/Other
Methanol
Mixed Hydrate
Water
Condensate
Chemical
Oil
Gas
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Figure3showsthatasignificantproportion(63percent)ofthetotallengthofallpipelinesinstalledhaveadiameterinexcessof16inchesandmaybeconsideredas‘trunklines’asdefinedinTable1.Aswouldbeexpected,suchpipelinestransportmainlyoilandgas.Itcanalsobeseenthatthemanykilometresofsmallerdiameterpipelinescarryamuchwiderrangeofproductsfromoilandgastowaterandchemicalstoassistintheproductionofdifferenthydrocarbonstreams.
4.4 Pipeline Ancillary/Associated EquipmentIn addition to the pipelines themselves, there are twoother groups of associated equipment,which areusuallydealtwithinthesamedecommissioningplanasapipeline.Theseareconcretemattresses,includinggroutbags,andpipelinecrossings.
4.4.1 MattressesConcretemattresseshavebeenusedextensivelyintheNorthSeatoprovideprotectionand/orstabilitytosubseapipelinesandumbilicals,includingthejumperspoolsthatfacilitatethetie-instoplatforms,manifoldsandwellheads.Theyhavealsobeenusedasaneffectiveinterventiondevicefortherectificationofpipelinespans.FlexiblemattressesaretypicallymanufacturedbyjoiningdifferentshapesofconcreteblockstogetherwithpolypropyleneorKevlarrope.
Oldermattressesinstalledinthe1970sweremadefrombitumenoraggregatepouredintomattressbags.Theuseofbitumenmattressesstoppedintheearly1980s. TheexactnumberofconcretemattressesintheNorthSeaisnotreadilyavailable.Oil&GasUKestimatessuggestthatbetween35,000and40,000mattresseshavebeendeployedonandaroundoilandgassubseainfrastructuresinceoperationsbeganintheNorthSea.
4.4.2 CrossingsTheneedtocrossotherpipelinesalongadesignatedpipelinerouteisinevitableinawelldevelopedbasinsuchastheNorthSea.Manycrossingshavebeenconstructedovertheproductivelifeofthebasinwithincreasingnumbersbeingrequiredastheregionmatures.Pipelinecrossingsaresignificantintermsofdecommissioningbecauseofthedirectimpacttheycanhaveondecommissioningoperationsandtheoptionselected.Ifapipelinewhichisbeingdecommissionedcrossesoriscrossedbyanotheroperatingpipeline,thesectionofthepipelineatthecrossingwillbeleftinsituuntilsuchtimeastheoperatingpipelineisalsodecommissioned.Thismeansthatadecommissionedpipeline iscutsomedistanceawayfromanycrossingsalong its length, typicallynocloserthan50metresfromtheoperatingpipeline.Ineachcasetheresponsibilityforthedecommissioningofthesectionsleftinplaceneedstobeestablishedbetweentheparties.Unlessotherwiseagreed,thefinancialresponsibilitywill remainwith theownerof the respectivepipelines. There aremanyways that a pipelinecrossingmaybeconstructed.TheexamplesshowninFigure4illustratehowaconfigurationchangesdependingonburialstatusandwhattypicalpipelineconfigurationsmustbeconstructed.
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Figure 4 Examples of Pipeline Crossing Configurations (Source Atkins)
Crossings are usually constructed using concretemattresses, grout-filled bags or bespoke cast concretestructures.Theremustbeaminimumclearancebetweentwopipelinesatacrossingof300millimetres,andaconcretemattressusuallyprovidesthisclearanceandprotectionbetweenthetwopipes.Theothermaterialsusedtoconstructthecrossingdependontherequiredheightofthecrossingpipeline.Smallercrossingscanbebuiltfromasmallnumberofmattressesand/orgroutfilledbags.Largercrossingsarerequiredforlargerdiameterpipelinesandareoftenmadeusingpurpose-builtcastconcretesections.
Thisshortensinstallationtimeswhencomparedtotheplacementoflargenumbersofmattressesorgroutbags.Inmanycasescrossingsareburiedinrockdumptoprovideprotectiontothecrossing,andtoreduceanypotentialsnagginghazards.
Exposedpipelinecrossesburied3rdpartypipeline–removalreadilyachieved:insituburialnotfeasible
Exposedpipelinecrossesexposed3rdpartypipeline–removalsubjectto3rdpartyapproval,andsignificantmaterialtoberemoved:insituburialnotfeasible
3rdpartypipelinecrossesburiedpipeline–notanissuesincepipelinecan remainburied
3rdpartypipelinecrossesexposedpipeline–sectionofpipeline‘trapped’ifcrossingpipeline is live
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5 DecommissioningRegulationsforPipelines
5.1 OverviewAlthoughanumberofinternationaltreatiesgovernthedisposalofwasteatsea,includingthemanagementofdecommissionedoffshorestructures,therearenointernationalregulationsorguidelines,relatingspecificallyto the decommissioning of pipelines. At present, pipeline decommissioning is covered within nationallegislation.
IntheUK,thePetroleumAct1998[Ref3]outlinestherequirementsforownersofinstallationsandpipelinestoobtainapprovalfortheirdecommissioningprogrammefromtheSecretaryofState.Thedecommissioningprogrammeshouldcontaindetailsofcostandproposalsforremovalanddisposal.ItmustbesupportedbyanEIAandissubmittedtotheDepartmentforEnergyandClimateChange(DECC).
Pipelinesshouldbethesubjectofaseparatedecommissioningprogrammeunlesstheyarelocatedwithinthesamefieldasotherequipmentorinstallationstobedecommissionedatthesametime.
Inadditiontotheapprovalofthedecommissioningprogrammeforapipeline,thefollowingmayalsoberequired:• ConfirmationthattherequirementsoftheCoastProtectionAct1949Section34PartIIhavebeensatisfied• FulfilmentofnotificationrequirementsfortheHealthandSafetyExecutive(HSE)underregulation22
ofthePipelineSafetyRegulations1996[Ref7]• Anyenvironmentalconsentsorpermitsrequiredduringdecommissioningactivity• Disposalofmaterialsonshoremustcomplywithrelevanthealthandsafety,pollutionpreventionand
wasterequirements/permits
IfpartortheentirepipelineistoberemovedorthedecommissioningprogrammewouldresultinachangetoanypartoftheTableAinformationintheoriginalPipelineWorksAuthorisation(PWA)thenaPWAVariationwouldalsoberequired.
Iftheapproveddecommissioningprogrammeforapipelinecontainsproposalsfortheplacementofassociatedmaterialsontheseabedsuchasrockdump,thenalicencemustbeobtainedundertheMarineandCoastalAccessAct2009[Ref4]inEnglandandWalesortheMarine(Scotland)Act2010[Ref5].
InNorway,pipelinesandcablesarenotspecificallyreferredtoinChapter5Decommissioning,ofthePetroleumAct1996.Theyare,however,coveredbyaseparateWhitePaper47(1999–2000),‘DisposalofPipelinesandCablesontheNorwegianContinentalShelf’.
5.2 NotificationofDisusedPipelinesIntheUK,theownerofapipelinemustnotifytheDECCwhenapipelinereachestheendofitsoperationallife.Undercertaincircumstances,thismaybebeforeotherfacilitiesinthesamefield.InsuchcasestheDECCmayconsiderthedeferralofdecommissioningforthepipelineuntiltheendofthewholefieldlife.
SomepipelinesmayrepresentimportantUKCSinfrastructureandprovidethemeansforfuturedevelopmentofhydrocarbonsreserves,orstorageofcarbondioxideorgasinthebasin.Toallowforthefuturereuse,thedecommissioningofsuchpipelinesmayalsobedeferred.
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Thedeferralofpipelinedecommissioningtotheendoffieldlifeorforpossiblereuseiscarriedoutunderthe‘InterimPipelineRegime’(IPR).TheDECCwillsendthepipelineowneraDisusedPipelineNotificationformrequestingdetailsonthestatusofthepipeline.TheDECCwillconsultwithothergovernmentdepartmentsandthen issuea letteroutliningtheconditionsunderwhich it ispreparedtodeferdecommissioningtoaspecifieddate.Ifreuseofthepipelineisconsideredviable,thensuitableandsufficientmaintenanceofthepipelineisrequiredoftheowner.
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6 DecommissioningOptions
6.1 SelectionofDecommissioningOptionsAsnotedpreviously,pipelinedecommissioningintheUKisregulatedbytheDECCandguidanceisprovidedintheDECCGuidanceNotes[Ref6].Inaddition,thePipelineSafetyRegulations1996[Ref7]providerequirementsforthesafedecommissioningofpipelines.Thereareanumberofoptionsforthedecommissioningofoffshorepipelines,andtheseareevaluatedbycomparativeassessmentinaccordancewiththeDECCGuidance.
Optionsforthedecommissioningofpipelines,mattressesandpipelinecrossingsaredescribedbelow.Thesedescriptionspresentthetechnicaloptionsfordecommissioning:theCAprocesswouldalsotakeaccountofsafety,environmentalandsocietal impactandcost inorder todeterminetheoptimumdecommissioningoptionforaspecificpipelineandassociatedinfrastructure.
6.2 PipelinesWhendevelopingtheoptionsfordecommissioningapipeline,theprimaryoptionscanbegroupedintosub-optionsofeitherleaveinsituorremoval.Typicallytheyaresummarisedas:• Leaveinsitu–minimalintervention• Leaveinsitu–minorintervention• Leaveinsitu–majorintervention• Removalbyreversereeling• RemovalbyreverseS-lay• Removalbycutandlift
6.2.1 LeaveInSituIn theUK, theDECCprovidesguidanceonpipelines, includinganypiggy-backpipelineorumbilicalwhichcannoteasilybeseparated,whichmaybecandidatesforinsitudecommissioning.ThecaseshighlightedbytheDECCare:• Thosewhichareadequatelyburiedortrenchedandwhicharenotsubjecttodevelopmentofspans
andareexpectedtoremainso• Thosewhichwerenotburiedortrenchedatinstallationbutwhichareexpectedtoself-buryovera
sufficientlengthwithinareasonabletimeandremainsoburied• Thosewhereburialortrenchingoftheexposedsectionsisundertakentoasufficientdepthandis
expectedtobepermanent• Thosewhicharenottrenchedorburied,butwhichneverthelessarecandidatesforleavinginplaceif
theCAshowsthattobethepreferredoption(e.g.trunklines)• Thosewhereexceptionalandunforeseencircumstancesduetostructuraldamageordeterioration,
orothercause,meanstheycannotberecoveredsafelyandefficiently
Thevarioussub-optionsofinterventionpriortoinsitudecommissioningofpipelinesaredescribedbelow.Inallcases,pipelinesarecleanedtoanappropriatelevelaspartofthedecommissioningoperations.
6.2.1.1 MinimalInterventionForapipelinethatwastrenchedandburiedatinstallationandcanbeshowntohaveremainedburiedalongitslengthoveritslifetime,theoptiontodecommissionthepipelineinsitumayrequireminimalintervention.After cleaning, apipeline isusually leftfilledwith seawaterwith theends leftopen to the sea.Potentialsnagginghazardsatthepipelineendswouldberemovedtocompletethedecommissioningplan.Thiswouldrepresent a ‘minimal intervention’ decommissioning option and may include cases where a pipeline isexpectedtoself-buryovertime.
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6.2.1.2MinorInterventionInadditiontothetasksdescribedaboveforastableburiedpipeline,theremaybeaneedforselectedremovalorremedialburialofshortsectionsofpipelinealongitslength,whichcouldpresentapotentialhazardtootherusersofthesea.Thiscouldincludesectionsofpipelineswhichlieontheseabedbetweenthetrenchandtheformerlocationofasubseastructure.Likewise,sectionsofpipelinethathavebecomescouredandformedspansmayalsoberemovedaspartofadecommissioningplan.
Underthesecircumstances,sectionswouldtypicallyberemovedbysubseacuttingandliftedtothesurfacebyasuitablyequippedsupportvessel.Otheroptionsavailableareremedialtrenchingofexposedsections,orusingrock-dumptoremovethesnagginghazard.
ForatrunklinewhichisinstalledontheseabedandwhereCAhasshownthatinsitudecommissioningisthebestoption,similarminorinterventionmayberequired.Thiscouldinvolverectificationworksonsectionspronetoscourandthedevelopmentofspans,andmanagementofthepipelineends.
6.2.1.3MajorInterventionApipeline initially installedontheseabed,orwhichwasoriginallytrenchedmayhavesignificantsectionsthathave required interventionover its lifetime. In thesecircumstances, thepreferredoptionmaybe todecommissionthepipelineinsituandcarryoutmajorinterventionworks,ratherthancompleteremoval.Aftercleaningandremovalofthetie-insateachend,thepipeline,orsignificantsectionsofit,maybetrenchedbelowthesurroundingseabedlevel.Alternatively,significantsectionsmayberemovedbyutilisingthecutandliftorreverseinstallationmethods.
Whereapipelineistrenched,thedepthoftrenchingisdeterminedbytheneedtoremoveanyhazardstootherusersofthesea,takingaccountseabedandsoilconditionsandotherdeterminingfactors.AtypicaltargetdepthsuggestedbytheDECC[Ref6]is0.6metrestothetopofthepipe.
6.2.2 RemovalFor small diameterpipelines, flexibleflowlines andumbilicalswhich are installedon the seabedandnottrenched,theDECCguidance[Ref6]isthattheseshallnormallyberemoved.Formoresignificantremovalsthefollowingsectiondescribestheoptions.
6.2.2.1 ReverseReelingForpipelineswith adiameterof 16 inchesor less,which arenot concrete coated, apossiblemethodofremoval isbyareversalofthereelinginstallationprocess.Reeling istheinstallationmethoddescribedinSection4.1.2andhasbeenusedextensivelyacrosstheNorthSeaforbothrigidandflexibleflowlines.
Theinstallationofrigidpipelinesbythereelingmethodreliesontheplasticdeformationofthepipewallduring installation to ensure the reeled pipelinewill subsequently lie straight on the seabed.When theprocessisreversedfortheremovalofapipeline,thepipeisreeledontothespecialistreelvesselandisonceagainplasticallydeformedsothatitsitsontherecoveryreel.Thelengthofpipelinethatcanberecoveredislimitedbythesizeandcapacityofthereel.Oncethepipelineisonthereelitistakentoashore-basedfacilityandremovedbyreversingtheprocessonceagain.
Duetothenatureofthereelingandunreelingprocess,itisunlikelythatarigidpipelinerecoveredusingthismethodcouldbereused.Themultiplecyclesofplasticdeformationof thepipelinewallcouldpotentiallycompromiseitslongtermintegrity.Thesteelfromrecoveredrigidpipelinesisrecycled.
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Thismethodisalsousedintherecoveryofflexibleflowlines.Thestructureofthewallofaflexibleflowlinemeansitdoesn’texperiencethesamedeformationcyclesastherigidpipelineduringthereelingandunreelingprocess.Multiplereelingandunreelingcyclesshouldnot,therefore,compromisethelongtermintegrityofaflexibleflowline.Intheory,suchpipelineshavethepotentialforreuseifasuitableapplicationisfound.Itis,however,theresponsibilityoftheend-usertodemonstratetheintegrityofarecoveredflowline(seeSection13.2).
6.2.2.2 ReverseS-layLargerdiameterandconcretecoatedtrunklinesaretypicallyinstalledusingtheS-laymethodasdescribedinSection4.1.1.AlthoughithasneverbeenusedbeforeintheNorthSea,apotentialremovalmethodisthereversalof theS-lay installationprocess.Thismethod isoftenconsidered in theCA fordecommissioningpipelinesinexcessof16inchesdiameterand/orconcretecoated.
ThismethodwouldinvolverecoveringapipelineendtothedeckofaspecialistS-layvessel.Thevesselwouldthenmovealongtherouteofthepipeline,stoppingatsuitablepointswhereacutwouldbemadetoremoveasectionofpipefromtherecoveredpipelinestringonthedeckofthevessel.Thesesectionswouldthenbetransferredtoasuitabletransportationbargeforonshorerecycling.
Althoughtherehavebeensomeexamplesoftheapplicationofthismethodintheshallowwater(lessthan24metreswaterdepth)oftheGulfofMexico,anumberofsignificanttechnicallimitationscurrentlyexistwhichprecludeitslargescaleapplication,i.e.:• Hightensionforceswouldneedtobeappliedtothepipelineduringrecoveryfromthevesseltensioner
systemtotheoutersurfaceoftheconcreteweightcoattobringthepipeontothedeckandholditinplaceforcutting.Theintegrityofagedconcreteweightcoatingcannotbeassuredandwouldneedtobecarefullyassessedtoconfirmthatthenecessarytensioncouldbegenerated,withouttheconcretecoatingdisintegratingandthecontrolofthepipelinebeingcompromised.
• This tension would also be applied into the steel wall of the pipeline and after many years ofoperation,theintegrityofthepipewallalongitslengthunderthehighrecoveryloadswouldneedtobeconfirmed.
• Thereisthepotentialforverylargequantitiesofmaterialstoberecoveredduringthedecommissioningofalargediametertrunkline.Thereisnoestablishedsupplychain/disposalrouteforthequantitiesofconcrete,steelandanti-corrosioncoatingswhichwouldbetakenonshoreduringamajorpipelineremovalcampaign.
6.2.2.3 CutandLiftAnotherpossiblemethodusedfortheremovalofpipelinesectionsistheso-called‘cutandlift’method.Thiscanbeusedforanydiameterorlengthofpipeline.Thisistheprocesswherebyapipelineiscutintosectionssubseabydiver-operatedcuttingtoolsorusingremotelyoperatedcuttingequipment,andthesectionsarethenrecoveredtoasurfacevesselusinganon-boardcrane.
Thisoptionhasbeenwidelyusedforremovingshortersectionsofpipe,eitherfortheremovalofashortpipeline in its entirety, or when discrete sections are being removed under a decommissioning plan. Itis usually thepreferred removal option for short sections of pipe,when it is impractical or prohibitivelyexpensivetomobilisemajorremovalequipment.
Mostsignificantly,thecutandliftmethoddoescreategreaterriskstothepersonnelcarryingouttheoffshoreoperations,especiallydivers.Ithasthereforebeenpreferabletolimitthatriskexposurebyavoidingextensiveoffshorecutandliftprogrammes.
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6.3 AncillaryEquipment
6.3.1 MattressesThe DECC guidance on mattresses and grout bags is that they should be removed from the seabed atdecommissioning. The guidancedoes, however, recognise that in some circumstances itmight be betterfor badly degradedmattresses tobedecommissioned in situ. In such circumstances, a CA is required todemonstrate that the best decommissioning option has been chosen. It is common practice to removemattressesandgroutbagsduringthedecommissioningofapipelineandassociatedtie-ins,withoutaseparateCAbeingperformed.
Thefeasibilityofremovaldependsmainlyontheageofthemattress,anditsburialstatus.Bitumenmattressescan be difficult to recover as they can break up when lifted. Similarly, older block-type mattresses candisintegrateduringrecoveryduetothedegradationofthepolypropyleneropeholdingtheblockstogether.Insuchcircumstances, therisktopersonnelperformingthedecommissioning increasesandtheoperatormayrequestpermissiontodecommissionsuchmattressesinsitu.
Somemattressesarefittedwithfrondstopromotesedimentdepositionafterdeployment.These,andothermattresses,canbecomeburiedovertime,andundersuchcircumstancestheoperatorcouldrequestthattheyaredecommissionedinsitu.
TechnologyoptionsfortheremovalofmattressesaredescribedinSection12.8.
6.3.2 Crossings,GroutBagsandConcreteFormworkIf a pipeline being decommissioned crosses other operational pipelines, it is usual practice to leave theconstructedcrossing inplaceuntilallpipelinesaredecommissioned.Thisavoidsunnecessaryrisks tothe‘live’infrastructure.Thisrepresentsadeferralofthedecommissioningworks.
Asforallpipelineinfrastructure,operatorsarerequiredtoconsideralloptionsfordecommissioningacrossing.Anyproposaltoleaveallorpartofacrossinginsitumustbesupportedwithevidencedemonstratingthereasonswhythisispreferred.Suchreasonsmayincludesufficientburial,impracticalitiesorsafetyconcernswithremoval,oranyotherexceptionalcircumstance.Manycrossingsarerockdumpedforprotection,whichmaybeavalidreasonwhythecrossingshouldbeleftinsitu.
Formworkusedtoconstructlargercrossingsisinstalledusingdedicatedliftingpad-eyesorslings,builtintotheconcretestructure.Thefeasibilityofremovingsuchitemsbyreversingthisprocessrequiresconfirmationoftheintegrityoftheliftpoints.Undersuchcircumstancesitmaybedemonstratedthatdecommissioninginsituispreferred.
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7 Safety
7.1 OverviewIntheUKundertheSafetyCaseRegulations[Ref.8]andpriortoanydecommissioningworkbeginning,theSafetyCaseforaninstallationmustbeupdatedandsubmittedtotheHSE.TheSafetyCaseforaninstallationwill include thoseelementsof apipeline that are safety critical and in closeproximity toan installation,for example isolation valves. The Safety Case must demonstrate that the proposed decommissioningarrangementsreducetherisktopeopletothelowestlevelthatisreasonablypracticable.
ThenotificationrequirementsunderthePipelineSafetyRegulations1996[Ref7]mustalsobefulfilled.Theseregulations ensure that a pipeline is designed, constructed andoperated safely, andprovide ameansofensuringpipelineintegrity,therebyreducingriskstopersonnelandtheenvironment.Undertheregulations,pipelinesshouldbedecommissionedinsuchamannerthattheydonotbecomeadangertopeople.Offshore,theextentoftheobligationtoremoveapipelinewilldependonthediameterofthepipeline,itslocationontheseabed,itsstabilityandthelocalsubseaconditions[Ref7].
Safetyisparamountandintegraltoallphasesofdecommissioningprojects,andsoformsakeypartoftheCAofthepipelinedecommissioningoptions.IntheCA,safetyistypicallyconsideredontwodifferenttimescales:• Thehealthandsafetychallengesthatmayposearisktopersonnelduringdecommissioningoperations
intheshortterm• Thehealthandsafetychallengesthatmayposearisktootherusersoftheseainthelongterm
7.2 ShortTermOperationalHealthandSafetyChallengesThemainhealthandsafetychallengesthatmayposearisktopersonnelduringdecommissioningoperations,arecommontoallpipelinedecommissioningoptions.However,thoseoptionswhichrequiretheleastintervention,andthereforetheuseoffewervesselsandoffshoreworkers,mayrepresentalowerrisktopersonnel.
Themainhealthandsafetychallengesareasfollows:• Lifting – the potential for large numbers of vessel-based lifts and the uncertainties surrounding
structural integrity of an aged pipe section, concrete mattresses, or the lift points of concreteformwork
• Diving–significantdiverinterventionmayberequiredtosupportextensivesubseacuttingandliftingoperations
• Hazardoussubstances–residualmaterialswithinpipelinessuchasmethanol,chemicalsfromumbilicalcores,waxdeposits,hydrocarbonsorNaturallyOccurringRadioactiveMaterial(NORM)scaling
• Integrity–hiddenflawsand structuraldegradation in the steelpipewallor concrete coatingsofagedpipelines,orauxiliaryequipmentsuchasgroutbagsormattresseswhichwerenotdesignedforremovalaftermanyyearsinservice
• Highlevelsofactivity–therearemanyworkersatallstagesofadecommissioningproject,onshoreandoffshore,potentiallyworkinginadynamic,constantlychangingenvironment
• Poorweather–thisextendsthedurationofoffshoretasksbyprohibitingwork,andincreasesthenumberofman-hoursrequiredoffshore
• Marinegrowth–managementofwasteandodour
Anumberoftechniquesareemployedtoreduceand/ormitigatetheriskstopersonnel.Thosemethodswhichhaveprovensuccessfulincluderegularupdatingofworkplansandemergencyproceduresthroughouttheproject;permit-to-worksystems;safetyinitiativessuchasgoodquality‘toolboxtalks’,sharingofexperienceandlessonslearned;andtechnologyimprovementsandtraining.
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7.3 LongTermHealthandSafetyChallengesTheinsitudecommissioningoptionsforpipelinespotentiallyposealongtermhealthandsafetychallengeintheformofsnaggingriskstootherusersofthesea.Atparticularriskarefishermenwhousedemersalbottomtrawlinggearoffshore,orfixedgearfisheriesnearshore.Asnaggingriskonadecommissionedpipelinemaybecausedby:• Pipelinespansduetoseabedscourunderapipeline• Exposedpipelineends• Longtermridgesintheseabedfromtrenchingoperations• Exposedpipelinecrownsduetodeburialofpipelines• Unevendegradationofexposedpipelinesovertime• Anchorscarsormounds• Steepsidedrockdumpprofiles
Anumberofinitiativesareemployedtoreducethepotentialsnaggingrisktofishermen.Immediatelyafterdecommissioningoperationsarecomplete,debrisontheseabedisremoved,andtypically,trawlsweepsbyfishingvesselswithchaintrawlsarecarriedoutalongthedecommissionedpipelinecorridor.Thishelpstoidentifyanypotentialsnagginghazards,whichcanthenbemanaged.
Theendsofdecommissionedpipelines,orcutsectionsofpipelines,poseapotentialrisktofishermen.Thiscanbereducedwithremedialmeasuressuchastheplacementofrockdumporgroutbagsattheendstoroundthemoff,andcreateanover-trawlableprofile.Earlyconsultationwiththefishingindustryassistsinestablishingthemostappropriateremedialmeasurestoreduceorremovethehazard.
Ownersofpipelinesdecommissionedinsituwillcarryoutregularsurveystomonitorandinspecttheconditionofthepipeline.ThedetailsofdisusedpipelinesarereportedtotheHydrographicOfficeandrecordedintheFishSAFEdatabase[Ref9].ThisdatabasecontainsinformationonalloilandgasinfrastructureontheUKCS.Itisprovidedtofishermentwiceayearasanoverlaytotheiron-boardnavigationalplotters.TheprovisionofdatathroughtheFishSAFEprojectisfundedbytheoilandgasindustry.FurtherdetailsoninteractionwiththefishingindustryduringoperationsanddecommissioningcanbefoundintheOil&GasUKFisheriesLiaisonOfficerGuidelines[Ref10].
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8 Environmental Impact
8.1 OverviewIntheUK,thedecommissioningprogrammeforapipelinemustbesupportedbyaCAoftheoptionsand anEIA.
TheCAhelps select thebest decommissioningoptionby comparing eachonbasis of complexity, safety,economicsandimpacttotheenvironment.
TheEIAidentifiesthelikelyenvironmentalandsocietalimpactsofdecommissioningactivities,andproposesmitigationmeasurestoavoid,orreducetoacceptablelevels,anysignificanteffects.TheEIAalsoassessescumulativeimpactsaswellasthosethathavethepotentialtoaffectMarineProtectedAreas(MPAs).
TheregulationsoftheNorwegianPetroleumActof1996alsorequirethatanEIAiscarriedoutaspartofthepreparationfordecommissioninginfrastructureassetsincludingpipelines.
Thepotential environmental impacts andareas formitigation that are considered aspart of theEIA arehighlightedbelow.Thepotentialsignificanceoftheenvironmentalimpactisrelatedtothelengthofpipelinetobedecommissioned.
8.2 Environmental Impacts
8.2.1 GaseousEmissions/EnergyUsageGaseousemissionsmaycausealocalreductioninairqualityandcontributetowiderclimatechangeprocesses.EmissionsofprimarilyCO2,butalsosmallerquantitiesofCO,NOx,SOxandVOC,aregeneratedduringthecombustionoffuelbyvesselsusedforcutting,liftingandtransportationofrecoveredpipelines.Emissionswouldalsobegeneratedthroughtheproductionofnewrawmaterialssuchassteel, toreplaceanequalquantityofmaterialinpipelineswhicharedecommissionedinsitu.
Emissions can be calculated from industry standard data, and project-specific estimates of likely fuelconsumptionbyvesselsandthereplacementoflostmaterialwillbeincludedintheEIA.
8.2.2 Discharges to SeaDischargeofsewageandfoodwaste,ballastwaterandtreatedbilgewatermayoccurduringvesseloperations.Thesewouldcauselocalisedandtransientdeteriorationinwaterquality,butposenoreallong-termhazardstobirds,fish,benthosorplankton.
Any chemicals that are used to clean and flush pipework during decommissioning are strictly controlledthroughtheOffshoreChemicalRegulations2002[Ref11].Pipeworkisflushedthroughtheexistingprocessingroutetotheonshoreterminalortransportedbyshuttletankerashore.
All pipelines are cleanedbeforedecommissioning, however, there is a possibility that a small amountofresidualdepositswillremainontheinsideofthepipe.Asapipelinewhichhasbeendecommissionedinsitudegrades,thereisapossibilitythatsuchdepositsontheinsideofthepipelinewillbreakdownandbereleasedintothewatercolumn.Anysuchreleasewouldbeverygradualandanyimpactwouldbehighlylocalised.
AlldischargestoseaduringdecommissioningoperationsarepermittedactivitiesthatareregulatedbyDECC.
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8.2.3 Underwater NoiseUnderwater noise is generated from vessel operations, particularly from the use of dynamic positioningsystems,aswellasfromcuttingandseabedexcavationworks.Thishasthepotentialtocausedisturbancetoanymarinemammalsincloseproximitytothedecommissioningoperations.
The potential disturbance to marine mammals will be assessed in the EIA process. Decommissioningoperations followthe JointNatureConservationCouncil (JNCC)guidanceonmitigatingthe impactofanynoise.MarineMammal Observers are used on board vessels andmitigationmeasures may include notcommencingoperationsuntiltheareaisshowntobeclearofmammals,andperhapsasoft-starttonoisyoperations.
8.2.4 PhysicalDisturbancetotheSeabedDecommissioningoperationsmayresultinlimiteddisturbancetotheseabedaroundpipelines.Forexample,sedimentcouldbedisturbedtoenableaccessforcuttingandlifting,forpipelineburialthroughjetting,orthrough the placement of remedialmaterials such as rock dump. Each of these operationswould resultin various degrees of physical disruption to the seabed, localised sediment re-suspension and potentialsmotheringofbenthicanimals.
Theextentofphysicaldisturbanceislikelytobesimilarorlessthanthatcausedduringinstallation,andwouldoccurinnarrowcorridorsalongtherouteofthepipeline.ThepotentialimpactwouldbeassessedthroughtheEIAprocess.Recoveryratesforbenthiccommunitiesarelikelytobeveryrapid.
Anyadditionalmaterialsplaced,suchasrockdump,willhaveaverysmallfootprintontheseabedandmayprovideadditionalhardsubstratumwhichcanbecolonisedbymobileandencrustingorganismcommunities.ThelongtermeffectoftheintroductionofsmallareasofsubstratumintopartsoftheNorthSeawithnaturallysandyormuddyseabedsisnotfullyunderstoodatpresent,andiscarefullyconsideredintheEIAandbytheregulators.
8.2.5 WasteandNORMInsomecasestheselecteddecommissioningoptionforapipelineistobringthepipelineonshorefordisposal.It is likelythatthemajorityofapipelinewouldberecycled.Theremaybesomematerials thatwouldbeconsignedfordisposal(suchassomeplastichoses).Thismayincludetheresidualcontentsofpipelineswhichmayhavebuiltupduringtheiroperationallife;forexample,waxydeposits,oilysludgesorNORMscale.Allresidualcontentswouldberecoveredanddeposedof inaccordancewithcurrentpermitrequirementsatlicensedsites.
Disposalofmaterialstolandfillwillreducethefuturecapacityforsuchdisposal,whichmayresultinalandfillresourceissueinfutureasmoreinfrastructureisdecommissioned.
The transportation toanonshore facilityand subsequentdisposalofpipelinesmaycausedisturbance tolocalcommunitiesthroughnoise,odourfrommarinegrowth,dustandincreasedtraffic.ThepotentialimpactdependsonthelocationofthesiteandvolumestobeprocessedandwillallbeassessedwithintheEIA.
8.2.6 MetalsSacrificialanodesareusedasprotectionstructuresonpipelinestoreducecorrosionandmaintainintegrityduringitsoperationallife.Theseanodesaremadefromzincoraluminium-zinc-indiumandmaycontaintraceamountsofmercury,copper,cadmiumorlead.
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Astheanodesdepleteovermanyyears,there isapossibilitythattraceamountsofmetalscouldmigratethroughthesedimentandinsomecircumstancescouldbeaccumulatedbysomemarinespecies.Theimpactofsuchmetalsdependsontheratesatwhichtheydissolve,migratethroughthesediment,anddissolveinthewatercolumn,andthedegreetowhichtheyarebioavailable.Italsodependsonphysicalfactorssuchaswaterdepth,temperature,oxygenlevelsandflowoverthesurfaceofthepipeline.Thisis,however,likelytobeanimpactoflowsignificance.
Thelevelsoflead,cadmiumandmercuryfoundinsedimentintheNorthSeahasbeenfallingsince1990,andinputsfrompipelineanodesareconsideredtobeinsignificantincomparisonwithothersourcessuchasriverineandcoastalindustries[Ref12].
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9 MonitoringandLiability
ResidualliabilityfordecommissionedoffshoreinfrastructureisdeterminedintheUKbythePetroleumAct1998[Ref3],wherebytheliabilityforanystructuresleftinsiturestswiththefacilityownerinperpetuity.NorwegianlegislationislessdefinitiveandfutureliabilityisagreedbetweenthefacilityownerandtheState,andmaybeassumedbytheStatebasedonanagreedfinancialcompensation.
Thus,ifapipelineisdecommissionedinsituintheUK,thereremainsaliabilityonitsownertomonitoritsconditionandtoensureitremainssafeforotherusersofthesea.GuidancefromtheDECC[Ref6]providesforapost-decommissioningsurveyalongthepipelinecorridor,typicallyextendingto100metreseithersideofthepipelinealignment.
Asecondsurveyistypicallyperformedayearlaterfromwhichthestabilityoftheremnantinfrastructure isconfirmed.Anenvironmentalsurveyisalsoperformedpost-decommissioningduringwhichsamplesarecollectedforanalysis.Thefuturemonitoringplanforthesite isagreedwiththeregulatorthrougharisk-basedapproach.InNorway,pipelinesthatarelaidontheseabedaresurveyedafterdecommissioningusuallywithanROV.Environmental surveys (chemical,physical andbiota) are carriedoutat the site twiceafteroperationofthepipelineshasceased.
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10 CostofPipelineDecommissioning
Asnoted,themethodsusedfordecommissioningpipelinesacrosstheNorthSeaarebasedonacase-by-caseevaluationofoptionsusingtheCAapproach.Withatotalinventoryinexcessof2,500individualpipelinesandatotallengthof45,000kilometres,determininganoverallcostofdecommissioningthisinventorywithevenamodestdegreeofaccuracyrepresentsasignificantchallenge.Anumberofotherfactorsemphasisethecomplexityofthetask:• With limitedactualpipelinedecommissioningexperience, there isminimalcostdataavailable for
benchmarking• Some of the principle methods being considered for decommissioning large diameter pipelines
are unproven and hence the actual cost of applying these methods is yet to be determined (i.e.reverseS-lay)
• An execution model, which seeks to realise economies of scale by combining decommissioningcampaignsacrossgroupsoffieldsandoperators,hasyettobedetermined
Oil&GasUKseekstoprovideindicativecostsbasedonoperatorprovideddatainitsannualDecommissioningInsightSurvey[Ref13],andthiswork iscontinuing.Furthercollaborativeeffort isunderwaytodevelopacredibleanddetailedcostmodelforthetotalcostofpipelinedecommissioning.
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11 Pipeline Decommissioning to Date
ThepipelinesdecommissionedintheNorthSeatodateanddescribedassuchinpublicallyavailablesourcesarepresentedinAppendixAandsummarisedinFigure5.
Figure 5 Pipelines and Umbilicals Decommissioned in the North Sea To Date (2013)
Pipeline Description
Diameter Range
Numberof Pipelines
TotalLength(km)Estimated1
Trunklines 16to32inches 17 62Flowlines(RigidandFlexible)
upto14inches 123 692
Umbilicals and PowerCables
upto8inches 20 79
Totals - 1601 833
Note1:Thisdataisfrompublicallyavailableresources,anddiameterandlengthdataisnotavailableforallpipelines/umbilicalsdecommissionedsofar.
Case studies for the Shelley, NorthWest Hutton and Tristan NW pipeline decommissioning projects areincludedinAppendixB.TheseprojectsillustratetherangeofprojectsperformedtodateintheNorthSeaRegion.
Figure5showsthatpipelinedecommissioningintheNorthSeaRegionisstillataveryearlystage.Lessthan2percentofthetotallengthoftheexistingNorthSeapipelineinventoryhassofarbeendecommissioned.
Ofthetotalnumberofpipelineswhichhavebeendecommissioned80percentarelessthan16inchesindiameter.Halfofthelargerdiameterpipelinesalreadydecommissionedwereremoved.Thesewereallunderonekilometreinlengthandinfieldpipelines.Theremainingpipelineshavebeenleftinsitu.Thelongestlargediametertrunklinetobedecommissionedsofaristhe35kilometrePiperAtoClaymore30incheexportline,whichwasdecommissionedinsitu.
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12 Technology
12.1 OverviewWhen evaluating a preferred option for decommissioning a pipeline and its associated equipment, theavailability and track record of technology provides the context for the other key CA criteria of safety,environmentalimpactandcost.Keytechnologyareasinpipelinedecommissioningare:• Pipelinecleaning• Trenching,burialandde-burial• Subseacutting• Lifting• Reverseinstallationmethods• Mattressremoval
Thecurrentstatusoftechnologyintheseareasisnotedinthefollowingsections.ExamplesofthepipelinedecommissioningprojectscompletedtodateareprovidedinAppendixB.
12.2 Pipeline CleaningPriortodecommissioning,apipelinewillbedepressurizedandanyhydrocarbonsremoved.ItisthenpurgedofitscontentsandcleanedinaccordancewiththePipelinesSafetyRegulations[Ref7].Thismayinvolvetheuseofpigs,whicharepipelinemaintenancetoolsusedforcleaningorinspectingtheinsideofapipeline.
Whetherapipelineisremovedordecommissionedin-situ,itisthoroughlycleanedtoensurethatpollutantsare not released to the environment in unacceptable quantities. For a pipeline decommissioned in-situ,thepipelineiscleanedtominimisepotentialcontaminationofthemarineenvironmentbydischargeofanyresidualhydrocarbonsfromthepipelineasitdegradesovertime.
Cleaningandpurgingiscarriedoutfollowingcessationofproduction,pipelinesystemdepressurisationandremovalofbulkhydrocarbons.Thecleaningprogrammeisdevelopedbasedonthespecificneedsofeachsystem,butatypicalprogrammemayinclude:• Chemicalcleaningtodetachhydrocarbonresiduefromthepipewall(usingbulksurfactantsorgelpigs)• Bi-directionalmagneticcleaningtoremoveferrousdebris• Bi-directionalbrushcleaningtoremoveotherloosedebris• Bi-directionaldisccleaningpigtoscrapetheremainingsoftermaterialfromthepipewall
Bi-directionalmagneticcleaningandbrushcleaningmayrequiremultiplepassesuntilthelineisattherequiredcleanlinessi.e.thewaterqualityemergingwiththepigsiswithinallowablecontaminantlevels.Dependingontheconditionofthepipeline,andthecleaningscheduleadoptedduringoperation,thecleaningprogrammeatdecommissioningmayincludefoampigsorspecialistmechanicalcleaningpigs.
12.3 Trenching and BurialThetechnologyfortrenchingandburialofpipelinesiswellestablished.Anumberofcontractorsofferarangeoftrenchingtoolscapableoftrenchingandburyingpipelinesofvariousdiametersinallsoiltypes.
Technologyexistsforpost-laytrenchingandburialofpipelines,andforremedialburial,thistechnologyisreadilyapplicabletodecommissioning.Thereislimitedexperienceofpipelinesbeingburiedspecificallyfordecommissioninginsitu.Itwas,however,thepreferredoptionfordecommissioningthe20inchoilexportpipelineinBP’sNorthWestHuttondecommissioningprogramme.SimilarlytheFrigg-Oseberg‘Frostpipe’oilpipelinewasapprovedforselectiveburialofexposedsectionsalongitsroute.
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Therearethreemaintypesoftoolincommonuseonsubseapipelines:• Jettingmachine• Cuttingmachine• Plough
Theapplicabilityofeachtrenchingmethodtoaburialoperationwilldependonanumberoffactors,mostgenerally the sizeof thepipeline and the type and strengthof soil. Figure 6 gives a general viewof theapplicabilityofthetypesoftoolsavailable.Therearealsohybridtoolsavailablethatcombine jettingandtrenchingfunctionstocoverawiderrangeofsoilconditions.
Figure 6 Trenching Method Suitability (Source Atkins)
Jettingsystemsvaryfromcomplexexcavatorstosimpletrenchingsleds.Jettingtoolscanworkinsand,siltandmediumclay.Jettingexcavatorscanalsobeusedindeburialoperationstoremovenon-cohesivematerials,includingrockdump.
Jettingproduceswideshallowtrenches in loosesand,andthereforemaynotprovidesufficientburial fordecommissioning.Indensersandsandweakercohesivesoilsthetrenchshapeiswelldefined.
J E T T E R S
P L O U G H SC U T T E R S
Denisty Very loose
Very Soft Soft Firm Sti� Hard RockStrength
ASSETSELECTION
COHESIONLESS SOIL - SAND
COHESIVE SOIL - CLAY
Loose Medium Dense Dense Very Dense Cemented
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Cuttingtrenchersareessentiallythesameas jettrenchingvehiclesbutusemechanicalmeansofcreatinganopentrenchsuchaschaincutters,wheels,disks,etc.Thesoil iscutunderthepipeandthematerial isentrainedusingadredgepumpsystemandejectedtothesideofthetrench. Atrenchingploughoperatesbybeingpositionedastridethepipelinewiththecuttingshareopen.Thepipelineispickedupbyforeandaftgrabscreatingaspaninthepipeline.Rollersareclosedaroundthepipetosupporttheloadduringburialprocessandtheshareisclosedbeneaththepipe.Therigidpipelineis loweredintoa‘V’cuttrench,formedbymechanicaldeformationoftheseabedbythepipelineploughastheploughis pulledforward.
Theexcavatedtrenchmaterialisdepositedinbermsonbothsidesoftheformedtrenchandcanberemovedoncompletionofthetrenchingpassbyabackfillingprocess.Sometrenchingploughsexistthatcanbeusedtobackfilltrenchesonasecondpass.Otherwise,aseparatebackfillploughwillbeusedincombinationwiththetrenchingplough.
Figure 7 Pipeline Trenching Plough
This image is reproduced with kind permission from Deep Ocean.Further use of this image, of any kind in any format, must first have written consent from Deep Ocean.
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12.4 DeburialandDredgingThereareanumberofcontractorsofferingMassFlowExcavators(MFEs)capableofdredginganddeburialoperationsduringpipelinedecommissioning,aswellaslocalpipelineburial.MFEscanbeemployedinmostsoilconditions,andarecapableofexcavatingrockdump.
MFEsworkusingrotatingpropellerstocreateahighspeed,lowpressureaeratedcolumnofwatertofluidisetheseabedmaterialforeitherburialordeburialoperations.AMFEduringdeploymentisshowninFigure8.
Figure 8 Mass Flow Excavator
This image is reproduced with kind permission from Reef Subsea.
Further use of this image, of any kind in any format, must first have written consent from Reef Subsea.
12.5SubseaCuttingThere are several differentmethods and types of equipment for cutting pipelines subsea as part of thedecommissioningprogramme.Themaintypesofcuttingequipmentfallintothefollowingcategories:• AbrasiveWaterJet• DiamondWireCutting• ReciprocatingCutting• HydraulicShears
Theseareall termed‘coldcuttingtools’andcanbeoperatedbydivers,mountedtoanROV,oroperateddirectly fromonboardavessel. Socalled ‘hot’ cutting toolsalsoexist,but thesehave limitedapplicationduringdecommissioningandarenotdiscussedfurtherhere.
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In all pipeline-cuttingapplications, the cuttingdevicemust gain access aroundall orpartof thepipelinecircumference,andthismayrequiredredgingunderneaththepipelinetopositionthetool. Inadditiontopotentialaccessconstraints,theselectionoftheappropriatetoolforcuttingpipelinesectionswilldependonthesizeofthepipelineandthecoatingsappliedtoit.Inallcases,toolselectionwillbebasedonfindingthesafestapproach,whichminimizestherisksposedtopersonnelbytheoperation.
Abrasivewaterjetisacommonlyusedsubseacuttingmethodandconsistsofahigh-pressurejetofwaterandsand/gritmix,whichisdirectedontotheitemtobecut.Typically,theabrasivewaterjetwilloperateat10,000-15,000psi.Thismethodisveryversatile,andduetotherelativelysmallcuttingheadtheprocesscanbeusedwhereaccessisrestricted.Insometools,thewaterjetiscapableofcuttingthroughbothsidesofthepipesimultaneously,andhenceneedonlyrotatehalfwayaroundthepipetocompleteacut.Othersrequireaccessaroundthecompletecircumference.
Diamond wire cutting tools are commonly used on pipeline decommissioning and intervention projectsaround theworld. A diamondwire cuttingmachine consists of a continuous loopof diamond-encrustedwiremountedonapulleysystemwhich isdriveneitherhydraulicallybyahydraulicpowerunitonboardthedeploymentvessel,orsubseabyaworkclassROV.Theycanalsobedrivenelectricallywhenworkingatgreaterwaterdepths.Aswithabrasivewaterjetting,dredgingofpipelinesontheseabedmayberequiredtopositionthetoolatthelocationtobecut.
Dependingonthenumberandtypeofcutsbeingmade,adiamondwiremayrequireregularreplacement.Thisisanexpensiveandtime-consumingoperation,whichisperformedonthedeckofthesupportvessel.Forlargerdiameter,concretecoatedpipelines,thediamondwiremayneedreplacingaftereverycut,whichwouldlimititsapplicabilityinamajorcutandliftoperationonalongdistancetrunkline.
Therearetwomaintypesofreciprocatingtool:thebandsawandtheguillotine.Bothtoolsuseaserratedsteelblade:theguillotinecutterusesareciprocatingmechanismto‘slice’downintothepipewithaback-and-forthmotion,whilethebandsawhasacontinuousflexiblebladedrivenroundanumberofpulleys.Bothtoolsareclampedtothepipeinordertoperformthecut.
Theguillotinecuttercancutamaximumdiameterof32inchesandcanbefullyROVcontrolled.Likewise,thebandsawiseitherdiverorROVcontrolled,buttoolingupto48inchescutdiameterisavailable.Forbothtools,thespeedofcuttingandthelifespanofthebladesaredependentonthematerialsbeingcut.Largediameterconcretecoatedpipewillbeparticularlyslowtocutandwillleadtothehighestbladeconsumption.Thismakesthesecuttersalessattractiveoptionforlong-distancetrunklines.
Hydraulicshearsaretraditionallyusedonshoreoronthedeckofanoffshoreplatform.Thesearemountedontheboomofanexcavatoranddismantlefacilitiesusingthe‘piece-small’method.Recentlyhowever,theyhavebeenusedforsubseaapplication,andalsoinmajorprojectssuchasthedecommissioningoftheNorthWestHuttonplatform.
For subseaapplication theshear is suspended fromthevessel craneandplaced inposition foreachcut.Although the tool has had limited use subsea, its simple operation could make it suitable for makingmultiple cuts along longpipelineswithout recovery to thedeck for replacementof consumables, etc.Atpresentthetoolisonlydeployablewithdiverassistance,butitisunderstoodthatanROVoperabledeviceis underdevelopment.
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Onedisadvantageof thehydraulicshear is that itdoesnotproducea ‘clean’cut,whichmayrepresentahazardtopersonnelduringhandlingandmaymakethehandlingofcutpipesectionsthemselvesdifficult.Itisnotedthatthereisnopublishedevidenceofshearsbeingusedforpipelinedecommissioningtodate.
12.6 LiftingThereisaneedduringthe‘cutandlift’processofdecommissioningtoliftthecutpipelinesectionsfromtheseabedtoatransportationvessel.Thisisperformedusingroutineliftingtechniques,butwillusuallyrequirediversupport.Asnotedabove,somecuttingtechniquesproduceacoarsecut,whichcaninfluencehowtheliftingequipmentisattachedtothepipesection.Thiscanslowdowntheliftingoperationsandmayhaveasignificantimpactonthedurationofliftingoperationsforlonglengthsofpipeline.
12.7 ReverseInstallationMethods
12.7.1 Reverse ReelingReversereelingistheprocessbywhichrigidorflexiblepipelinescanberecoveredfromtheseabedbyreelingthemfromtheseabedusingaspecialistreelvessel.TheprocessisdescribedinSection6.2.2.1.
For rigidpipe, therearea limitednumberof specialist reelvesselsavailable fromthe leading installationcontractors.Thesevesselsareusuallyengagedininstallationactivities,butcanbeandhavebeenadaptedtorecoverpipelinesaspartofadecommissioningproject.VesselssuchasTechnip’sApacheII(Figure9)andSubsea7’sSevenNavica(Figure10)arecapableofperformingthiswork.
Figure 9 Apache II Reeling Vessel
This image is reproduced with kind permission from Technip. Further use of this image, of any kind in any format, must first have written consent from Technip.
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Figure 10 Seven Navica Reeling Vessel
This image is reproduced with kind permission from Subsea 7. Further use of this image, of any kind in any format, must first have written consent from Subsea 7.
Flexibleflowlinesandumbilicalsalsorequirespecialistequipmenttocarryouttherecoveryoperationbutmorevesselsareavailabletoperformthiswork.
Althoughtheuseofthesevesselsforbothrigidandflexiblepipelinerecoveryhasnotbeencommon,bothmethodshavebeenusedindecommissioningprojectsandcanbeconsideredproven.
12.7.2 Reverse S-layTheprocessbywhichpipelinescouldberemovedatdecommissioningbythereverseS-layprocessisdescribedinSection6.2.2.2.Asnoted,thisisnotanoperationthathasbeencarriedoutintheNorthSea,althoughitisunderstoodthatthereissomeexperienceofremovingshortlengthsofsmalldiameterpipelinesusingthismethodinshallowwaterintheGulfofMexico.
ExamplesofpipelayvesselsutilisingtheS-laymethodforinstallationofpipelinesareSaipem’sCastoroSeiandAllseas’Audacia(Figure11).
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Figure 11 Dynamically Positioned Pipelay Vessel Audacia
This image is reproduced with kind permission from Allseas.
Further use of this image, of any kind in any format, must first have written consent from Allseas.
ThesevesselsareexamplesofhighspecificationinstallationvesselsthathaveoperatedregularlyintheNorthSeainstallinglargediameterpipelines.Neitherhasbeenusedtoremovelonglengthsofpipelineaspartofadecommissioningproject.
Ingeneral,reverseS-layhasnotbeenusedforthedecommissioningoflargediameterpipelines,howevera number of issues have been identified regarding the feasibility of the process. These are noted in Section6.2.2.2.
FurtherstudyisnecessarybeforethereverseS-layprocesscanbeconsideredfeasiblefordecommissioninglongdistancelargediameterpipelines.
12.8 MattressRecoveryTherecoveryofmattressesisadiverandvessel-intensiveoperation,withthetimetakentoperformtheworkverymuchdependentontheageandconditionofthemattressesbeingrecovered.
Asyet,noestablishedtechniqueortechnologyhasbeenuniversallyadoptedformattressrecovery.Itislikelythatthemajorityofnewermattresses(iethoseinstalledwithinthelast10years)willhaveblockslinkedwithpolypropyleneorKevlarropes.The lifting loopscouldbeexpectedtobe ingoodcondition,althoughthiswouldhavetobeconfirmed,andthesemattressescouldberecoveredtosurfaceusinghandlingframesand/orspeedloaders.Speedloadersarealiftingarrangementdeployedontheseabedontowhichanumberofmattressescanbeliftedusingliftingframesbeforethespeedloaderisrecoveredtothesurface.
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Foroldermattresses,orformattressesthathavebrokenup,aconventionalgrabtoolcanbedeployedtorecoverthemattresspiecesdirectlytotheservicevessel,ortoabasketsubseaandthenrecovered.
Areviewofmattressrecoveryprojectsto-dateshowsthatitispossibleinsomecasesfornewermattressestoberecoveredinlessthananhour.Foroldermattresses,whichmayhavebeensubseafor20+years,therecoverytimecanbeupwardsof12hourspermattress.
Forspecificdecommissioningprogrammes,samplemattressescanberecoveredduringthepre-commissioningsurveystoreviewtheirconditionandtoconfirmtherequiredrecoverymethod,orthefeasibilityofreusingthemattresseswithinthefield.
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13 Recycling and Reuse
13.1 RecyclingWhenlengthsorsectionsofsteelrigidpipelinesarerecoveredaspartofadecommissioningprogramme,thesteelitselfisrecycledusingaprovensupplychain.Allsteelpipelineshaveananti-corrosioncoatingandoftenhaveinsulationcoatingsapplied.Wherepossiblethesecoatingsareremovedandrecycled,otherwisetheyaresenttolandfill.
Forrecoveredflexibleflowlines,umbilicalsandpowercables,themetallicendfittingscanberemovedandrecycled,orinsomeinstancesreused.ThisisdescribedintheShelleyCloseOutReport[Ref.14].Themetallicelementsofthecarcassofflexibleflowlines,andthewiresusedinarmouringlayersinumbilicalsandpowercables,canalsoberecoveredusingspecialistequipmentandthenrecycled.Suchprocessesseparateouttheplasticmaterialsfromthedifferentlayers,whichcanthenberecycledifpossible.
Adecommissioningprojecttypicallyachievesrecyclingorreuseratesinexcessof95percentoftherecoveredmaterials,andinsomecasesupto98percent.Similarratescanbeachievedforpipelinedecommissioningprojects,dependingonthevolumeandtypeofnon-recyclablecoatingsrecoveredwiththepipelines.
13.2 ReuseIntegrityisakeyissuewhenconsideringthereuseofpipelinesorpipelinematerials.Forrigidsteelpipelines,recoveredinasinglelengthbythereversereelingprocess,thepipewallwillhavebeensubjecttosignificantreverse cycle plastic deformation during its original deployment and then recovery process. This cansignificantlyaffectthelongtermintegrityofthepipestructureandwouldruleitoutforreuse.Hence,nosteelpipelinerecoveredinthiswayhasbeenreused.
Itwouldbepossibletodemonstratethatadecommissionedpipelineleftinsitucouldbereusedforalternativeserviceandoperatingconditionsandthisisregularlyconsidered.
Flexible flowlines, umbilicals and power cables are readily recovered by reverse reeling as part of adecommissioningprogramme.Suchmaterialscantheoreticallybereused,butprovingthattheintegrityofthecomplexmulti-layeredstructureofsuchcomponentshasnotbeencompromisedduringthehandlingandoperationalprocessisdifficult,andoftenrecyclingistheonlyrealisticoption.
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14 PublicConsultation
14.1 RequirementsforConsultationIntheUKthereisastatutoryrequirementforoperatorstoconsultwithstakeholderswhomaybeaffectedby decommissioning proposals under section 29(3) of the PetroleumAct 1998 [Ref 3]. This includes thedecommissioningofoilandgaspipelines.
InNorwaythecessationofproductionplanrequiresaseparateimpactassessmentprogrammetobeprepared.Thisensures thepublicareproperly informedandprovidesvariousstakeholderswith theopportunity toexpressopinionsandinputsintothescopeandexecutionoftheproject.
14.2 StatutoryConsultees(UKCS)AnnexHoftheDECCGuidanceNotes[Ref6]specifiesthoseorganisationsthatshouldbecontactedaspartof statutory stakeholder consultation. These are Global Marine Systems, Northern Ireland Fishermen’sFederation,ScottishFishermen’sFederationandTheNationalFederationofFishermen’sOrganisationsUK.AnnexEandSection6.14oftheGuidanceNotesalsoidentifyGovernmentdepartmentswitharelevantroleandtowhomcopiesofadraftdecommissioningprogrammemustbesent.
14.3 ConsultationProcessOn theUKCS statutory consultation starts at the point atwhich a draft decommissioning programme issubmittedtotheDECC.Aperiodof30daysfortheconsultationisstatedintheDECCguidelines.
Decommissioning proposals are announced by placing a public notice in appropriate national and localnewspapersandjournals,andbyplacingdetailsontheInternet.Thisnoticeindicateswherecopiesofthedraftdecommissioningprogrammecanbeviewedandtowhomrepresentationsshouldbesubmitted.
TypicallytheprogrammeisavailabletodownloadfromtheInternetwithhardcopiesavailableforinspectionattheoperator’soffices.Theresultsofconsultationsarereportedinthedecommissioningprogrammewhenitissubmittedforfinalapproval.
FurtherguidancecanbefoundintheGuidelinesonStakeholderEngagementforDecommissioningActivitiesontheOilandGasUKwebsiteat:http://www.oilandgasuk.co.uk/knowledgecentre/decom_guidelines.cfmInNorwaytheplanforcessationofproductionmustbepresentedbytheoperatortwotofiveyearsaheadofthecessationofproduction.Theplanforcessationissubjecttopublichearing.
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15 References
1 OperationPluto,A.C.Hartley,ProceedingsoftheInstitutionofMechanicalEngineers1946154:433,DOI:10.1243/PIME_PROC_1946_154_054_02
http://pme.sagepub.com/content/154/1/433.citation
2 TotalNUGGETSFieldFactsheet http://www.uk.total.com/pdf/activities/NuggetsFactSheet.pdf
3 UKPetroleumAct1998 http://www.legislation.gov.uk/ukpga/1998/17/contents
4 MarineandCoastalAccessAct2009 http://www.legislation.gov.uk/ukpga/2009/23/contents
5 Marine(Scotland)Act2010 http://www.scotland.gov.uk/Topics/marine/seamanagement/marineact
6 DECCGuidanceNotesDecommissioningProgrammes https://www.og.decc.gov.uk/regulation/guidance/decomm_guide_v6.pdf
7 PipelineSafetyRegulations1996 http://www.hseni.gov.uk/l82_a_guide_to_the_pipelines_safety_regulations_1996.pdf
8 OffshoreSafetyCase http://www.hse.gov.uk/offshore/safetycases.htm
9 FishSAFEproject http://www.fishsafe.eu/en/home.aspx
10 FisheriesLiaisonOfficerGuidelines http://www.oilandgasuk.co.uk/knowledgecentre/Fisheries.cfm
11 OffshoreChemicalRegulations2002 http://www.legislation.gov.uk/uksi/2002/1355/contents/made
12 OSPARCEMPAssessmentReport2011 http://www.ospar.org/documents/dbase/publications/p00563_cemp_2011_assessment_report.pdf
13 Oil&GasUKDecommissioningInsightReport http://www.oilandgasuk.co.uk/knowledgecentre/market_information.cfm
14 ShelleyCloseoutReport https://www.gov.uk/oil-and-gas-decommissioning-of-offshore-installations-and-pipelines
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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AppendixA:TableofDecommissionedPipelinesintheNorthSeaRegion
Country Operator Pipeline Type
Location Length Diameter Fluidtype InstallationDate
DecommissioningOption
From To km inches
NL GDFSuez L10-K L10-B/L10-A(s) 5.8 11 1984
NL GDFSuez K12-A/L10-A K12-E 3.9 2 1986
NL GDFSuez K12-E K12-C 6.3 11 1986
NL GDFSuez L10-S1 L10-AP 11.5 7 1988
NL GDFSuez K12-E L10-S1 4.6 4 1988
NL GDFSuez L14-S1 L11a-A 6.0 7 1990
NL GDFSuez K12-S1 K12-BP 4.9 7 1991
NL GDFSuez K11-B K12-C 16.1 14 1995
NL NAM K11-FA-1 K8-FA-1 6.0 7 1978
NL NGT L11-A NGT-pipe 11.8 11 1990
NL Taqa P15-B P15-C 3.4 10 1985
NL Taqa P15-B P15-C 3.4 6 1985
NL Taqa P15-C P15-B 3.4 6 1985
NL Taqa P15-B P15-C 3.4 4 1985
NL Total K5-EN/C K5-D 2.7 12 1997
NL Total L4-PN L4-A 11.4 10 1999
NL Total K4-BE K4-A 8.0 10 2000
NL Unocal Q!-Helder-B Q1-Helder-AW 1.8 9 1986
NL Unocal Q!-Helder-B Q1-Helder-AW 1.8 9 1987
NL Unocal Q1-Haven-A Q1-Heder-AW 5.8 9 1989
NL Wintershall K13-B K13-AP 9.2 10 1977
NL Wintershall K13-D K13-C 3.5 10 1978
NL Wintershall P12-C P12-SW 6.9 8 1990
NL Wintershall P14-A P15-D 12.6 10 1993
NL Wintershall P2-NE P6-A 38.2 10 1996
NO ExxonMobil Odin FriggTCP2 26.0 20 Gas
NO GasscoAS FriggTCP2 VesterledT _ 32 Gas
NO Total FriggTCP2 OsebergA 82.0 16 Oil
NO Total Trunkline DP2 TCP2 0.7 26 Gas Removal
NO Total Trunkline DP2 TCP2 0.7 26 Gas Removal
NO Total Flowlines DP2 TCP2 0.7 4 Condensate Removal
NO Total Flowlines DP2 TCP2 0.7 8 Chemical Removal
NO Total Power DP2 TCP2 0.7 3 Power Removal
UK _ Ardmore toSal2 1.4 _
UK _ Ardmore toSal2 1.4 _
UK Apache FortiesE FortiesA 4.0 6 Oil 1986
UK Apache FortiesD FortiesC 3.6 20 Oil 1975
UK BHP Flowlines Douglas Lennox 12 Gas Leaveinsitu-minorintervention
UK BHP Flowlines Esmond Forbes 10 Gas Leaveinsitu-minorintervention
UK BHP Flowlines Esmond Gordon 12 Gas Leaveinsitu-minorintervention
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Country Operator Pipeline Type
Location Length Diameter Fluidtype InstallationDate
DecommissioningOption
From To km inches
UK BHP Flowlines Esmond Forbes 2 Gas Leaveinsitu-minorintervention
UK BHP Flowlines Esmond Gordon 12 Gas Leaveinsitu-minorintervention
UK BP FlexibleFlowlines
Don Don 4 Oil Removal
UK BP FlexibleFlowlines
Don Don 4 Water Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.8 10 Oil Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.8 8 Oil Removal
UK BP Umbilical Schiehallon&Loyal
Schiehallon&Loyal
0.7 8 Removal
UK BP Umbilical Schiehallon&Loyal
Schiehallon&Loyal
0.7 8 Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 10 Oil Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 10 Oil Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 10 Oil Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 8 Oil Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
1.4 8 Gas Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
2.9 12 Water Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 8 Oil Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.8 10 Oil Removal
UK BP Flowlines NinanTee NorthWestHutton
10 Gas Leaveinsitu-minimalintervention
UK BP Trunkline NorthWestHutton
CormorantA 20 Oil Leaveinsitu-majorintervention
UK BP Trunkline Hutton(TLP) NorthWestHutton
<0.5 12 Oil Leaveinsitu-minimalintervention
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 10 Removal
UK BP Risers Schiehallon&Loyal
Schiehallon&Loyal
0.7 10 Oil Removal
UK BP RigidFlowlines
Don Thistle 17.3 8 Production Leaveinsitu-minimalintervention
UK BP RigidFlowlines
Don Thistle 13.1 8 Water Leaveinsitu-minimalintervention
UK BP Umbilical Don Thistle 17.7 3 Chemical Leaveinsitu-minimalintervention
UK BP FlexibleFlowlines
Don Don 4 Water Removal
UK BP FlexibleFlowlines
Don Don 4 Oil Removal
UK BP FlexibleFlowlines
Don Don 4 Oil Removal
UK BP Flexible Flowlines
Schiehallon&Loyal
Schiehallon&Loyal
1.9 12 Removal
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Country Operator Pipeline Type
Location Length Diameter Fluidtype InstallationDate
DecommissioningOption
From To km inches
UK BP Flexible Flowlines
Schiehallon&Loyal
Schiehallon&Loyal
4.5 _ Removal
UK BP Flexible Flowlines
Schiehallon&Loyal
Schiehallon&Loyal
2.0 10 Oil Removal
UK BP Flexible Flowlines
Schiehallon&Loyal
Schiehallon&Loyal
0.1 10 Oil Removal
UK BridgeEnergy
Flowlines TristianNW TristianNW 15.5 6 Production Leaveinsitu-minorintervention
UK BridgeEnergy
Umbilical TristianNW TristianNW 15.3 4 Leaveinsitu-minorintervention
UK CNRI Flowlines Staffa Ninan 9.6 9 Oil Removed
UK ConocoPhillips Hutton(TLP) NWHutton 8.0 12 Oil Leaveinsitu
UK ConocoPhillips Maureenplatform
ALC 24 Oil Leaveinsitu
UK ConocoPhillips Maureenplatform
Moira 10.0 6 Oil Removal-reversereel
UK ConocoPhillips Maureenplatform
Moira 10.0 2 Gas Removal-reversereel
UK ConocoPhillips Maureenplatform
Moira 10.0 4 Removal-reversereel
UK Hamilton Moira Maureen* 10.1 7 Oil
UK Hess FlexibleFlowline
Fife Fife 2.0 6 Production Leaveinsitu-minimalintervention
UK Hess FlexibleFlowline
Fife Fife 1.7 7 Water
UK Hess FlexibleFlowline
Fife Fife <0.1 4 Water Removal-reversereel
UK Hess Flowlines Fife Fife _ Gas Removal-reversereel
UK Hess Flowlines Fife Fife _ Chemical Removal-reversereel
UK Hess FlexibleFlowline
Fergus Fergus 7.4 7 Leaveinsitu-majorintervention
UK Hess Flowlines Fergus FPSO 5.6 7 Oil Leaveinsitu
UK Hess Flowlines Fergus Fergus _ Chemical
UK Hess RigidFlowline
Flora Flora 8.0 8 Production Leaveinsitu-minimalintervention
UK Hess RigidFlowline
Flora Flora 7.0 3 Gas Leaveinsitu-minimalintervention
UK Hess RigidFlowline
Flora Flora 8.4 8 Water Leaveinsitu-minimalintervention
UK Hess Flowlines Flora Flora 9.5 6 Chemical
UK Hess FlexibleFlowline
Fife Fife 2.0 6 Production Removal-reversereel
UK Hess RigidFlowline
Flora Angus 18.8 8 Oil 2002 Leaveinsitu
UK Hess RigidFlowline
Angus Flora 10.4 3 Gas 2002 Leaveinsitu
UK Hess Flowlines Ivanhoe/RobRoy
Claymore 40.0 14 Oil 1990 Leaveinsitu-minorintervention
UK Hess Flowlines Ivanhoe/RobRoy
Tartan 23.0 8 Gas 1990 Leaveinsitu-minorintervention
UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Production
UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Oil 2000 Removal
UK Hess Flowlines Ivanhoe RobRoy 1.6 5 2000 Removal
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Country Operator Pipeline Type
Location Length Diameter Fluidtype InstallationDate
DecommissioningOption
From To km inches
UK Hess Flowlines Ivanhoe RobRoy 1.6 4 Gas 2000 Removal
UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Water 2000 Removal
UK Hess Flowlines RobRoy Ivanhoe/RobRoy
1.6 4 Chemical 2000 Removal
UK Hess Flowlines RobRoy RobRoy <0.1 4 Power Removal-reversereel
UK Hess Flowlines RobRoy RobRoy <0.1 2 Gas Removal-reversereel
UK Hess Flowlines RobRoy RobRoy <0.1 4 Water Removal-reversereel
UK Hess Flowlines RobRoy RobRoy <0.1 _ Chemical Removal-reversereel
UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Oil 2000
UK Hess Flowlines Ivanhoe RobRoy 1.6 5 2000
UK Hess Flowlines Ivanhoe RobRoy 1.6 4 Gas 2000
UK Hess Flowlines Ivanhoe RobRoy 1.6 8 Water 2000
UK Hess Flowlines RobRoy Ivanhoe 1.5 _ Chemical 2000 Removalofdynamicsections
UK Hess Flowlines Ivanhoe Ivanhoe <0.1 4 Power Removal-reversereel
UK Hess Flowlines Ivanhoe Ivanhoe <0.1 2 Gas Removal-reversereel
UK Hess Flowlines Ivanhoe Ivanhoe <0.1 4 Water Removal-reversereel
UK Hess Flowlines Ivanhoe Ivanhoe <0.1 _ Chemical Removal-reversereel
UK Hess Umbilical Flora Angus 10.5 1 Chemical Leaveinsitu-minimalintervention
UK Maersk _ 9 Oil
UK Maersk _ 2 Fibre
UK Nexen SCOTT SCOTT 1.7 _
UK Oryx Flowlines Ninian HuttonTLP 8.6 8 Gas 2000
UK Perenco Flowlines Welland Thames 17.5 16 Production Leaveinsitu-minimalintervention
UK Perenco Flowlines Thames Welland 17.2 3 Chemical Leaveinsitu-minimalintervention
UK Perenco Flowlines Welland Welland 8.0 8 Gas Leaveinsitu-minimalintervention
UK Perenco Flowlines Welland Welland 5.8 8 Gas Leaveinsitu-minimalintervention
UK Perenco Flowlines Welland Welland 3.9 8 Gas Leaveinsitu-minimalintervention
UK Perenco Umbilical Welland Welland 8.0 4 Leaveinsitu-minimalintervention
UK Perenco Umbilical Welland Welland 5.8 4 Chemical Leaveinsitu-minimalintervention
UK Perenco Umbilical Welland Welland 4.2 4 Chemical Leaveinsitu-minimalintervention
UK Premier Umbilical Shelley FPSO 2.4 3 Power Removal-reversereel
UK Premier Flowlines Shelley FPSO 2.0 8 Production Leaveinsitu-minimalintervention
UK Shell BrentSouthUTA
TOBS-2 _ _ Condensate
UK Shell BrentSouthUTA
TOBS-1 _ _ Condensate
UK Shell BrentSouthWI2
TO OP4 _ _ Water
UK Shell Flowlines LemanAP LemanCD 8 Chemical
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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DatainthistablewasobtainedfrompubliclyavailablesourcesviaDECCandDEAL.Insomeinstancethefulldata-setrequiredforindividualpipelineswasnotavailable.
Country Operator Pipeline Type
Location Length Diameter Fluidtype InstallationDate
DecommissioningOption
From To km inches
UK Shell LemanBT LemanBP 4
UK Shell Flowlines IndeM IndeJ 3.4 12 Gas 1985 Leaveinsitu-minorintervention
UK Shell Flowlines IndeJD IndeM 3.7 3 Condensate 1985 Leaveinsitu-minorintervention
UK Shell Flowlines IndeK IndeN 2.4 10 Gas 1987 Leaveinsitu-minorintervention
UK Shell Flowlines IndeK IndeN 2.4 3 Condensate 1987 Leaveinsitu-minorintervention
UK Shell Trunkline IndeJ IndeAT 3.9 20 Production 1971 Leaveinsitu-minorintervention
UK Shell Trunkline IndeK IndeAT 9.1 24 Production 1972 Leaveinsitu-minorintervention
UK Shell Flowlines IndeL IndeJ 3.2 16 Gas 1977 Leaveinsitu-minorintervention
UK Shell Flowlines Statfjord BrentSouth 10 Oil Notinuse
UK Silverstone Flowlines TristianNW DavyNUI 15.5 6 Gas Leavein-situ-minimalintervention
UK Talisman Trunkline PiperA Piper/Claymore 35.2 30 Oil
UK Talisman Trunkline Tartan MCP01 18 Gas
UK Talisman Claymore 2.0 14 1999
UK Talisman ClaymoreSpur 7.3 30 2000 Leavein-situ-minimalintervention
UK Total Power TP1 FP 0.5 3 Power Removal
UK Total Power TP1 FP 0.5 4 Power Removal
UK Total Power CDP1 TP1/QP 0.5 3 Power Removal
UK Total TP1 FP 0.5 3 Removal
UK Total TP1 FP 0.5 4 Removal
UK Total Risers MCP01TalismanPipeline 0.5 18 Removal
UK Total Trunkline MCP01FriggPipeline 0.5 32
UK Total Trunkline MCP01VesterledPipeline 0.5 32
UK Total AlwynNorth 4.7 6 Water 2000
UK Total Umbilical CDP1 TP1/QP 0.5 8 Chemical Removal
UK Total Trunkline CDP1 TP1/QP 0.5 26 Gas Removal
UK Total Trunkline CDP1 TP1/QP 0.5 26 Gas Removal
UK Total Flowlines CDP2 TP1/QP 0.5 4 Condensate Removal
UK Total Trunkline TP1 FP 0.5 24 Gas Removal
UK Total TP1 FP 0.5 2 Removal
UK Total TP1 FP 0.5 2 Removal
UK Venture Umbilical Kittiwake Kittiwake _ 3 Chemical Removal-Cutandlift
UK Venture FlexibleFlowlines
Kittiwake Kittiwake _ 8 Production Removal-Cutandlift
UK Venture Flowlines Kittiwake KittiwakeSal 3.0 8 Oil 2005 Removal-reversereel
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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NorthWestHutton
Operator BP
Water depth 143metres
Pipelines 10inch12.6km OilImport
20inch,12.8km OilExport
Field Description
The NorthWest Hutton field is located in Block 211/27a of the UKCS. The facilities at NorthWestHuttoncomprisedasteelplatform,wellsandpipelinesthatwereinstalledtoproducehydrocarbonsandassociatedproductsfromtheNorthWestHuttonreservoir,discoveredin1975.
The North West Hutton facilities were installed and commissioned between 1981 and 1983. TheplatformisoperatedbyAmoco(UK)ExplorationCompany,onbehalfofAmocoUKPetroleumLimited,asubsidiaryofBPplc.BPown25.8percentofthefield,andtheotherownersareCIECOExplorationandProduction(UK)Limitedwith25.8percent,EnterpriseOilU.K.Limitedwith28.4%andEssoExplorationandProductionUKLimitedwith20percent.
TheNorthWestHuttonfieldbeganproductionin1983,andbythetimeofcessationofproductioninJanuary2003thefieldhadproducedsome126millionbarrelsofoil.
Pipeline Decommissioning Execution
DecommissioningoftheNorthWestHuttonpipelinestookplacein2011and2012.
The 10 inch, concrete coated, gas import pipeline carried gas from the Ninian Tee to the NorthWestHuttonplatform.Thepipelinewastrenchedat installationandremainedburiedat thetimeofdecommissioning,andtheCAofoptionsconcludedthatthepipelineshouldbedecommissionedinsitu.Thepipelineendsandancillaryequipment,suchasmattresses,wereremovedandrecoveredtoshoreforrecyclingordisposal.
At installation, the20 inch,concretecoatedoilexportpipelinewas laidontheseabedbetweentheNorthWestHuttonandCormorantAlphaplatforms.Toremoveanypotentiallong-termriskstootherusersofthesea,thedecommissioningoptionselectedusingCAwastotrenchandburythepipelinebelowtheseabed.At threecrossingsalong the lengthof thepipeline, sectionsof thepipelinewereremovedandrecoveredtoshore,withthepipelineendsremaininginthetrench.Allmattressesusedtoprotectthepipelinewererecoveredtoshoreforrecyclingordisposal.
AppendixB:CaseStudiesofPipelineDecommissioningProjects
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Shelley
Operator PremierOil
Water depth 92-96metres
Pipelines 8inch2.0km,Production 3.5inch,2.4km,
ControlUmbilical
Field Description
TheShelleyfieldislocatedinBlock22/02band22/03aoftheUKCS,approximately192kilometresfromthenortheastcoastofScotlandand32kilometresfromtheUK/Norwaymedianline.
TheShelleyfacilitiesconsistedoftwoproductionwellswithXmastreesandfishing-friendlyprotectivestructures,andasubseaproductionmanifold insideaprotectionstructure.TheseweretiedbacktotheSevanVoyageurFPSObya2.02kilometrelongtrenchedandrockdumpedrigid8inchproductionpipelineanda2.42kilometretrenchedelectro/hydrauliccontrolumbilical.Bothofthesewerelocatedina10metrewidecorridorbetweentheproductionmanifoldandtheFPSO.
Thefieldbeganproducing inAugust2009,but reservoirperformancedidnotmeetexpectations socessationofproductionoccurredinJuly2010.TheShelleyfacilitiesweredecommissionedduring2010and2011,anddetailscanbefoundinthepubliclyavailabledecommissioningclose-outreport.
Pipeline Decommissioning Execution
Fromacomparativeassessmentofdecommissioningoptions,exposedsectionsof the trenchedandrockdumped8inchproductionpipelinewereremoved,withthepipelinecutasclosetotherockdumpbermaspossible. The remaining trenchedand rockdumped sectionsweredecommissioned in situ.Priortoremovaloperations,theproductionpipelinewasflushed,achievingoilinwaterconcentrationoflessthan30partspermillion.
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Theexposedsectionsoftherigidproductionpipelinewerecutusingahydraulicallypoweredsubseasaw,withtheresultingshortsectionsprotrudingfromtherockdumpprotectedfromsnaggingusinggrout bags. The cut sections of pipewere returned to shore for recycling. The 2.42 kilometre longelectro-hydraulicumbilicalwasremovedinsectionsandreturnedtoshorefordisposal.Theumbilicalwascutusingahydraulicguillotinecutteronthedeckoftherecoveryvessel.Theshorttie-insectionsofflexiblepipeateachendofthepipelinewererecoveredtoshoreforrefurbishmentandpotentialreuse.Allmattresseswererecoveredtoshoreandreusedasfoundationmaterialsbyalocalfarmer.
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
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Tristan NW
Operator BridgeEnergy (FormerlySilverstone EnergyLimited)
Water depth 25-35metres
Pipelines 6inch15.5km, Production
4inch,15.3km, ControlUmbilical
Field Description
The NorthWest Hutton field is located in Block 211/27a of the UKCS. The facilities at NorthWestHuttoncomprisedasteelplatform,wellsandpipelinesthatwereinstalledtoproducehydrocarbonsandassociatedproductsfromtheNorthWestHuttonreservoir,discoveredin1975.
The North West Hutton facilities were installed and commissioned between 1981 and 1983. TheplatformisoperatedbyAmoco(UK)ExplorationCompany,onbehalfofAmocoUKPetroleumLimited,asubsidiaryofBPplc.BPown25.8percentofthefield,andtheotherownersareCIECOExplorationandProduction(UK)Limitedwith25.8percent,EnterpriseOilU.K.Limitedwith28.4%andEssoExplorationandProductionUKLimitedwith20percent.
TheNorthWestHuttonfieldbeganproductionin1983,andbythetimeofcessationofproductioninJanuary2003thefieldhadproducedsome126millionbarrelsofoil.
Pipeline Decommissioning Execution
AdetailedCAwasperformedto identifythebestoptionfordecommissioningthe14.8kilometre lengthof trenchedandburiedproductionpipelinewithpiggy-backedumbilical.Optionswere comprehensivelyassessedandcomparedonthebasisoftheirsafetyrisk,environmentalimpacts,CO2emissions,technicalfeasibility and cost. This assessment indicated that thebestoptionwouldbe to remove the sectionsofpipelineandumbilicallyingontheseabedateitherendoftheline,andleavethecombinedpipelineandumbilicalinitspresenttrench,buriedbynaturalsedimentandareasofspotrockdump.
Theproductionwell,umbilicalandproductionpipelinewereflushedandcleaned,withallproductionfluidsdisposedofviatheDavyNormallyUnmannedInstallation(NUI),or,fortheumbilical,tosea.
TheinfrastructureattheTristanNWFieldwasthencompletelyremovedtoshore,withtheexceptionofthe14.8kilometretrenchedsectionofthepipelineandpiggy-backedumbilical,itsexistingcoverofspotrockdump,andtherockdumppreviouslyprotectingthe250metrecoilofumbilicalattheTristanNWwellhead.
Thisequipmentwasdecommissionedinsituinaccordancewiththeapproveddecommissioningplan.Allmaterialsrecoveredtoshorewererecycled,andnothingwassenttolandfill.
DECOMMISSIONING OF PIPELINES IN THE NORTH SEA REGION
page 48
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