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OTH93395DRAGANCHORSFORFLOATINGSYSTEMSAuthorsPSincockandNSondhiMSLEngineeringLtdTechnologyTransferCentreSilwoodParkBuckhurstRoadAscotBerkshireSL57PWHSEBOOKSHealthandSafetyExecutive-OffshoreTechnologyReportCrowncopyright1993ApplicationsforreproductionshouldbemadetoHMSOFirstpublished1993ISBN0717606422ThisreportispublishedbytheHealthandSafetyExecutiveaspartofaseriesofreportsofworkwhichhasbeensupportedbyfundsformerlyprovidedbytheDepartmentofEnergyandlatelybytheExecutive.
NeithertheExecutive,theDepartmentnorthecontractorsconcernedassumeanyliabilityforthereportsnordotheynecessarilyreflecttheviewsorpolicyoftheExecutiveortheDepartment.
Results,includingdetailedevaluationand,whererelevant,recommendationsstemmingfromtheirresearchprojectsarepublishedintheOTHseriesofreports.
BackgroundinformationanddataarisingfromtheseresearchprojectsarepublishedintheOTIseriesofreports.
CONTENTSInstallationandrecovery5.
3Handlingondeck5.
2Mechanicaldamage5.
1ANCHORHANDLINGDIFFICULTIES5.
Corrosionprotection4.
8Inspectionandrepairbetweendeployments4.
7In-positioninspectionprocedures4.
6Recoveryprocedure4.
5Post-installationholdingcapacitytests4.
4Piggybacking4.
3.
2General4.
3.
1Installationmethod4.
3Pre-installationbottomsurvey4.
2Sizingofanchors4.
1.
3Choiceofdraganchortype4.
1.
2Useofdraganchors4.
1.
1Anchorselection4.
1CURRENTDEPLOYMENT4.
Codesandregulations3.
5Manufacturers'data3.
4Proprietarydata3.
3Publishedtechnicalliterature3.
2Introduction3.
1REVIEWOFPUBLISHEDDATAANDINFORMATION3.
CURRENTLYEMPLOYEDDRAGANCHORTYPES2.
INTRODUCTION1.
GLOSSARYSUMMARYPage1REFERENCESANDBIBLIOGRAPHY9.
CONCLUSIONS8.
ANCHORFAILURES7.
EFFECTOFANCHORDRAGGINGONSUBSEQUENTHOLDINGPOWER6.
2SUMMARYThisdocumentpresentsthefindingsofastudyofDragAnchorsforFloatingSystems.
Theobjectiveofthisstudywastoreviewthecurrentuseofdraganchorsoffshore.
Thestudyestablishedcurrentpracticewithrespecttodraganchorusage,includinganchorselection,deploymentandrecoverymethods,surveyandinspection,handlingdifficulties,andanchorfailures.
Informationwasgatheredonanchorshapes,dimensions,fabricationmethods,reliabilityandholdingpower.
Informationforthestudywasobtainedbytwoprimarymethods.
Thefirstmethodwasasubstantialliteraturesurvey,whichencompassedtechnicaljournals,conferenceproceedings,designcodesandguidelines,andmanufacturer'sdata.
Thesecondmethodofgatheringdatawastocirculatequestionnairestoorganisationsintheoffshoreindustrywithsignificantexperienceofspecifyingandusingdraganchors,andthentofollowupthesequestionnairesbyinterviews.
Aconsiderablequantityofdatawasgatheredbythesetwomeans.
Thisdataispresentedindetailinthereport.
Anumberofimportantsafetyandreliabilityrelatedconclusionsaredrawnfromtheinformationcollectedandcollatedinthereport.
Theauthorswishtoextendtheirthankstoallthoseindividualsandorganisationswhocontributedtotheinformationsurvey.
Theauthorsalsowishtoexpresstheirgratitudeforthegrantingofpermissionstoreproducecopyrightmaterialinthisreport.
3GLOSSARYStabilisingbar.
Stock:Increaseinholdingcapacityofanewlyinstalledanchorduetotimedependenteffectsinthedisturbedsoiladjacenttotheanchor.
SoakIn:Componentoftheanchorwhichtransmitsthetensionforceintheanchorlinetotheflukes.
Shank:Theratioofthelengthoftheanchorlinetotheverticaldistancefromthefairleadtotheseabed.
Scope:Adeviceusedforanchordeploymentandrecovery.
Aringjustlargeenoughforthemooringlinetopassthrough,andwhichcanbepulledalongthemooringlineandontotheanchorshankbyawiretothesurface.
Ringchaser:Anchorconnectedintandemwiththemainanchortoincreasetotalholdingpower.
Piggyback:Awire,permanentlyattachedtotheheadoftheanchor,whichisusedtoassistdeploymentandrecoveryoftheanchor.
Oncetheanchorisdeployedthefreeendoftheanchorisattachedtoasurfacebuoy.
Pendantwire:Holdingpower/anchormass(orweight).
Holdingefficiency:HighHoldingPoweranchor.
HHPanchor:Pipesinbowsofshipsthroughwhichtheanchorchainsrun.
Hawsepipe:Componentoftheanchorwhich,byvirtueofitsprojectedsurfacearea,generatesmostoftheholdingcapacityoftheanchor.
Fluke:Anchorstoragearrangementsonboardavessel.
Anchorrack:41.
INTRODUCTIONThisdocumenthasbeenpreparedbyMSLEngineeringLimitedandreportsthefindingsofastudyonDragAnchorsforFloatingSystems.
Theobjectiveofthisstudy,whichwascommissionedbytheOffshoreSafetyDivisionoftheHealthandSafetyExecutive,wastoundertakeastate-of-the-artreviewofpresentdaypracticesfordraganchors.
Floatingoffshorestructuresandvesselsaresubjectedtolargeenvironmentalforces,andmostrelyonpermanentortemporarymooringstowithstandtheseforces.
Someofthesefloatingsystems,forexampleFloatingProductionSystems(FPSs)andSingleBuoyMoorings(SBMs)arerequiredtoremainonstationforlongperiods,oftenseveralyears.
Otherfloatingsystems,forexampleMobileOffshoreDrillingUnits(MODUs),usemooringswhichmustnotonlywithstandtheseverestweatherconditions,butmustalsobecapableofbeingrapidlyandeconomicallydeployedandrecovered.
Mostofthesefloatingsystemsemploydraganchorsasafundamentalpartoftheirmooringsystems.
Theperformanceofdraganchorsthereforehasagreatinfluenceonthereliability,integrityandoperationalsafetyoffloatingsystems.
IntheNorthSea,themostfrequentoffshoredeploymentofdraganchorsisbyMODUs.
Morethan60MODUsarecurrentlyinoperationintheNorthSea,indicatingtheconsiderablesignificanceofdraganchorstooffshoreoperations.
Anchorsforoffshorestructuresmaybedividedintofourcategories:draganchors,deadweightanchors,pileanchorsandembeddedplateanchors.
Draganchorsgeneratetheirholdingpowerbyembeddingintheseafloorwhenpulledhorizontally,mobilisingtheshearstrengthofthesoiltoresistthepullingforce.
Deadweightanchorsdependprimarilyontheirownmasstoprovideholdingcapacity,whilstpileanchorsgeneratetheirholdingpowerbymobilisinglateralearthpressureandskinfrictioninthesurroundingsoil.
Likedraganchors,embeddedplateanchorsgaintheirholdingcapacityfrommobilisingtheshearstrengthofthesoil,butaredistinguishedfromdraganchorsbynotbeingselfembedding.
Thisstudyhasestablishedcurrentpracticewithrespecttodraganchorusage,includinganchorselection,deploymentandrecoverymethods,mooringarrangements,surveyandinspectionprocedures,handlingdifficultiesandanchorfailures.
Informationhasbeengatheredonanchorshapes,dimensions,manufacturers,fabricationmethods,reliabilityandholdingpower.
Informationforthisstudyhasbeenobtainedbytwoprimarymethods.
Thefirstmethodwasasubstantialliteraturesurvey,whichincludedtechnicaljournals,conferenceproceedings,designcodesandguidelines,andmanufacturers'data.
Thiswassupplementedbycitationandkeywordindexsearches.
Thesecondmethodofgatheringdatawastocirculatequestionnairestoorganisationsintheoffshoreindustrywithsignificantexperienceofspecifyingandusingdraganchors,andthentofollowupthesequestionnairesbyinterviews.
Theorganisationsinterviewedrepresentedawidecrosssectionoftheindustryandincludedsixoperators,twodrillingcontractors,onecertifyingauthority,amarineequipmentsupplier,amarinecontractorandamooringconsultancy.
InformationwasalsoreceivedfromtheUniversityofManchester.
5Thecommentsmadeduringtheinterviewsbythevariousorganisationsarereproducedherein,withoutmodificationoramplification,andwithMSLEngineeringactingstrictlyinthecapacityofaneutralobserverandreporter.
Therefore,theviewscontainedhereinareattributablesolelytothevariousorganisationsinterviewed.
Theseviews,togetherwiththefindingsfromareviewofpublicliterature,areusedinalatersectiontodrawaseriesofconclusions.
Thefindingsofthestudyarereportedasfollows.
Section2presentsabriefoutlineofthedevelopmentofmoderndraganchortypesandintroducessomebasicterminology.
Arepresentativerangeofdraganchortypesaredescribedandillustrated,withparticularemphasisontypescommonlyusedbytheoffshoreindustry.
Section3presentstheresultsoftheliteraturesurvey.
Publishedtechnicalliterature,designcodesandguidelinesandmanufacturers'dataarereviewedindetail.
Attentionisalsodirectedtothescopeandextentofproprietarydata,wherethisisknown.
Basedoninformationgatheredfromtheinterviewswithoffshoreorganisations,anextensivereviewofcurrentanchordeploymentpracticesispresentedinSection4.
Topicscoveredincludeanchorselection,bottomsurveys,installationandrecovery,post-installationprooftests,in-positioninspection,inspectionandrepairbetweendeployments,andcorrosionprotection.
Sections5,6and7arealsobasedontheinformationgatheredfromtheinterviews.
Section5discussesanchorhandlingdifficulties.
TheeffectofdraggingonthesubsequentholdingpowerofanchorsisexaminedinSection6,andSection7discussesanchorfailures.
ConclusionsarepresentedinSection8.
ReferencesaretabulatedinSection9.
62.
CURRENTLYEMPLOYEDDRAGANCHORTYPESThereareseveraltypesofdraganchorincommonusebytheoffshoreindustry.
Thesearepartofamuchlargerrangeofdraganchortypeswhichareemployedbymerchantships,navalships,fishingboatsandpleasurecraft,aswellasforspecialpurposessuchaspermanentinshoremoorings.
Figure1showsanchorsofthetypesmosttypicallyusedoffshore,plusanumberofotheranchortypesofinterestwithrespecttodesignorapplication.
Notethatnoneoftheanchorsisshownembedded.
Thestockanchorisanolddesign,familiarastheanchorfittedtomany19thcenturysailingships.
Abar(the"stock")fittedthroughtheanchor'sshankclosetotheshacklestabilisestheanchor,sothattheanchorgenerallyembedsirrespectiveofitsorientationonarrivalattheseabed.
Onthestockanchortheflukes,thesurfaceareasofananchorwhichcontributetothegreaterpartofitsholdingpower,arecarriedontwocurvedarmssetatrightanglestothestock.
Onlyoneofthesearmsembedsandbecausetheflukeshaveasmallarea,thestockanchorhasalimitedholdingpowerforitsmass.
Thestockanchorisnolongerusedonlargevesselsandisincludedhereonlyforinformation.
Howeverfoldingformsofthisanchorarestillusedonmanysmallfishingboatsandpleasurecraft.
Thestocklessanchorwasinventedin1821,andgainedpopularityattheturnofthecenturybecauseoftheeasewithwhichitcanbestowedinthehawsepipesofsteelships.
Theheadoftheanchor,towhichareattacheditsflukes,ispivotedabouttheshankintheplaneoftheflukes.
Projectionsontheheadoftheanchor,knownastrippingpalms,areintendedtocatchinthesoil,rotatingtheheadsothattheendsoftheflukespointdownwardsintothesoilandtheanchorcommencestoembed.
Becausetheanchorissymmetrical,itwillembedwhicheversidefirstcomestorestontheseabed.
Stocklessanchors,differingindetailandmanufacturedunderavarietyofcommercialnamesincludingHall,Baldt,UnionandSpek,arestillwidelyusedonmerchantshipsandNavalvessels.
Howevertheirpoorstabilitywhenembeddingandlowholdingpowerforagivenmass,havelimitedtheoffshoreuseofthistypeofanchor.
Stocklessanchorshavebeenfurtherdevelopedbythelateralextensionofthecrowntoformshort,broadstabilisers,intendedtoimprovethestabilityoftheanchor.
TheStokestypeistypical:othersimilartypesincludetheAdmiraltyAC14design.
TheseimprovedstocklessanchorsareusedonbothmerchantshipsandNavalvessels.
TheAC14andStokesimprovedstocklessanchordesignsarerecognisedbyCertifyingAuthoritiesasHighHoldingPower(HHP)anchors.
TobeacceptedasaHHPanchor,ananchordesignmustdemonstrateaholdingpoweratleasttwicethatofastandardstocklessanchorofthesamemass,inarangeofseabedconditions.
TheDanforth(Meon)articulatedstockanchorwasdevelopedin1939andothersimilararticulatedstockdesigns,suchastheUSnavyLWTandtheStatoweredevelopedsubsequently.
Inthistypeofanchortheheadandflukesoftheanchorarepivotedabouttheshank,asforthestocklessanchor.
Howeveralongbarisfittedthroughtheheadoftheanchor.
Thisbarperformsthedualfunctionsofpivotpinandstock,andisoftenreferredtoasthestabilisingbar.
Theanchorissymmetricalandwillembedwhicheverwayupitarrivesattheseabed.
7Articulatedstockanchorshavebeenandareverywidelyusedoffshore,mostnotablybyMobileOffshoreDrillingUnits(MODUs).
Theirholdingefficiency(ratioofholdingcapacitytoanchorweight)istypicallyaround8inclayand10insand(NCEL[1972])forananchorof35,000lbs(15.
9tonnes)mass.
Theequivalentfiguresforastocklessanchorofthesamemassare2and4.
AninterviewedmooringcontractorindicatedthatformostMODUs,anchorsofbetween30,000lbs(13.
6tonnes)and40,000lbs(18.
1tonnes)massarefairlystandard.
TheStevin,Boss,DeltaandBrucesingleshankanchorsweredevelopedwiththeintentionofobtainingimprovedperformanceoverDanforth/LWTtypeanchors.
TheStevinanchorwasfirstusedindredgingoperations,andsubsequentlyforoffshoreapplications.
TheflukesoftheStevinanchorarearticulatedaboutthestock.
TheDeltaanchorhasasingleflukefixedtotheshankandwillonlyembedifitarrivesattheseabedinthecorrectorientation.
ItfollowsthattheDeltaanchormustbeloweredbyasecondlineinadditiontotheanchorline,makingitnecessarytoemployanAnchorHandlingVessel(AHV)orananchorbargetoassistinlayingthisanchor.
TheBruceanchorhasasinglecrow'sfootshapedflukefixedrigidlytoacrankedshank.
Howeverduetotheshapeofitsflukeitisself-righting,thatisifdroppedontheseabedup-side-downitwillturnupright,embeddingasitdoesso,whentensionisappliedtotheanchorline.
Morerecentlyhigherperformanceanchortypeshavebeendevelopedforoffshoreuse.
ThesehigherperformanceanchorsincludetheBrucetwinshank,StevprisandFlipper-Deltatypes.
Availableinlargesizes,andwithclaimedhighefficiencies,theseanchorsofferthepossibilityofveryhighholdingcapacities.
Thetermhigherperformanceanchorshasbeenadoptedhereasagenerictermfortheserecentlydevelopedtypesofanchor.
TheBrucetwinshankhasthesamearrangementoffixedflukewithself-rightingcapabilityastheBrucesingleshankanchor.
TheanchorchainattachedtotheBrucetwinshankanchorshowninFigure1isfittedwithBrucechaindepressors,whichareintendedtoaugmenttheperformanceoftheanchor.
TheStevsharkisaStevprisanchorwithmodificationsforveryhardsoilconditions.
TheStevinandStevsharkarenon-self-rightingdesignswithfixedflukesandthereforetheassistanceofanAHVisessentialwhenlayingtheseanchors.
TheheadandflukesoftheFlipper-Deltaanchorarepivotedaboutitsshank.
Largetrippingpalmsandstabilisingsurfacesareattachedtotheheadinanopenbox-likearrangement.
Theanchorissymmetricaland,liketheDanforth/LWTtypeanchors,willembedwhicheverwayupitarrivesattheseabottom.
Asummaryoftheanchortypescarriedasoriginalequipmentonthevesselslistedinthe1990-91Lloyd'sRegisterofOffshoreunitsisrepresentedinTable1.
Thissummaryoffittedanchortypesmustbetreatedwithsomecaution,asthetypeof8anchorisfrequentlynotreportedwithexacitude,LWTforexamplebeingusedasagenericnameforanyarticulatedstockanchor.
Also,asreportedbelow,onsomeMODUsthefittedcomplementofanchorsarerarelyused,beingsubstitutedforbyhigherperformanceanchorsbroughtoutonAHVstothesitewheretheMODUistobemoored.
Table1Summaryofanchortypes(1990-91OffshoreRegister)1004121002595Totals13120Stato27256Vicinay314382Stevfix210384Stevpris4154112Offdrill6255127Baldt4185130Moorfast8317194Bruce9379245Stevin145914373Danforth156316384FlipperDelta3213031788LWT%Number%NumberUnitsAnchorsTYPEHeretheconventionhasbeenusedofapplyingthetermarticulatedstockanchortodenoteageneraltypeofanchor,ratherthanfollowingthewidespreadpracticeofusingLWTasagenericname.
ItshouldbeappreciatedwhenreadingtheinformationpresentedinSections4to7thatitwassometimesnotclearfromtheinterviewswhetherLWTwasbeingusedasaspecificoragenericterm,andconsequentlyithasbeennecessarytoexercisejudgementintheseinstances.
Fourothertypesofanchor,theMushroom,Clump,KiteandSingleFluke,areshownforinformationinFigure1.
Thefirsttwomayoccasionallybeencounteredoffshore.
TheMushroomanchor,whichisfabricatedfromeithersteelorreinforcedconcrete,isusedforlightshipsandotherpermanentmoorings.
Clumpanchorsaremadeofconcreteandrepresentacrossbetweendraganddeadweightanchors,theirholdingcapacitycomingfromacombinationoftheirweightandtheirtendencytoembedwhendragged.
Clumpanchorsarealsousedforpermanentmoorings.
KiteanchorsandSingleFlukeanchors,suchastheNavySingleFlukeshowninFigure1,havenotbeenusedoffshoretoanysignificantextent.
TheKiteanchorismainlysuitableforuseinmud.
Ithasbeenstatedthatwhenusedinthesesoils,theKiteanchorcangenerateholdingcapacitiesuptoonehundredtimesitsownweight.
Singleflukeanchorsareusedforpermanentmoorings,buthavelimitedholdingcapacities.
93.
REVIEWOFPUBLISHEDDATAANDINFORMATION3.
1INTRODUCTIONInformationforthisstudyhasbeenobtainedbytwoprimarymethods.
Thefirstmethodwasasubstantialliteraturesurvey,whichincludedtechnicaljournals,conferenceproceedings,designcodesandguidelines,andmanufacturer'sdata,andwhichwassupplementedbycitationandkeywordindexsearchesAconsiderableamountofinformation(inexcessoftwentyreferences)relatedtodraganchorshasbeenfoundfromthetechnicalliterature.
Themajorityofthisinformationwasfoundfromasimplesearchofconferenceproceedings,technicaljournalsandoffshoreindustrypublications,carriedinmostcasesbackto1982.
Thefollowingwereincludedinthissearch:-ConferenceProceedingsOffshoreTechnologyConferenceBehaviourofOffshoreStructuresOffshoreMechanicsandArcticEngineeringOffshoreEngineeringInternationalSymposiumonOffshorePetroleumEngineeringFloatingStructuresandOffshoreOperationsTechnicalJournalsJournalofOffshoreMechanicsandArcticEngineeringAppliedOceanResearchJournalofOffshorePetroleumEngineeringOffshoreIndustryPublicationsNorthSeaNewsletterOffshoreEngineerThesewerefollowedupbyasearchbasedonthereferencesquotedinthepapersfoundfromthisinitialsimplesearch.
AkeywordsearchusingtheLondonScienceReferenceLibraryscientificandtechnicalindexyieldedafurthertworeferences.
Finallyacitationindexsearchwasconducted,basedonthepreviouslydiscoveredreferences,andextendingbackto1985.
Thiscitationindexsearchyieldedtwonewreferences.
ThescopeofworkandresultsofeachofthetechnicalreferencesissummarisedinSection3.
2.
Throughinterviewswithseveraloperators,itisapparentthatsomeproprietarydataondraganchorperformanceexists,outsideoftestsconductedbyanchormanufacturers.
Alsosomeresearchandexperimentationisunderwayorrecently10completedintheUSA.
However,becauseoftheproprietarynatureofthisdata,thefindingsandresultsarenotknown,andonlybriefdetailsarepresentedbelow.
Areviewofthedraganchormanufacturer'sdatahasbeenconductedfromacross-sectionofmanufacturerswithintheUKandTheNetherlands.
ThisispresentedinSection3.
4.
Manufacturersgenerallyprovidedthefollowingdatafortheiranchors:dimensions,proofloadsandholdingpowerinarangeofseafloorconditions.
Themostsignificantcodesandregulationsrelatedtodraganchorshavebeenreviewed.
Theseinclude:-DEnGuidanceNotes(4thedition)StatutoryInstrumentsAPIRecommendedPracticeDnVRulesLloyd'sRegulationsSomearecomprehensive,butothersgiveonlyoutlineguidance.
DetailedreviewsarepresentedinSection3.
5.
3.
2PUBLISHEDTECHNICALLITERATUREResultsofanexperimentalstudyofthebehaviour,duringembedment,ofStock,Stockless,DanforthandDeltaanchorsarediscussedbyKlaren[1971].
ThestudywasparticularlydirectedtowardstheDeltaanchor.
Bothmodelandfullscaleembedmenttestswereconducted,theformerinawaterfilledtank,thelatterinheRotterdamandEuroportdocks.
Photographstakenduringthemodeltestsarepresentedtoillustratetheembedmentofeachanchortype.
KlarenstatesthattheDanforthanchorhasahighholdingpoweringoodholdingground.
Howeverhesuggeststhatitcaneasilybeobstructedbydebris.
Hestatesthattheflukeswillnotdiginonahardseabed,andthatinclay,theDanforthembedsverydeeply,withtheresultthattheforcerequiredtobreakouttheanchorissometimesaslargeasitsholdingpower.
TheDeltaanchorisstatedbyKlarentohaveahighholdingefficiencyof15,whichisachievedinmosttypesofsoil.
HealsosuggeststhattheDeltaanchorembedswithinaveryshortdistance,andhashighstability.
ThebreakoutforcefortheDeltaanchorisclaimedbyKlarentobesmall.
Modeltestswerealsoundertakenwithpiggybackedanchors.
Klarenstatesthatthesetestsshowedthatifthereisenoughslacktoallowthemainanchortoembed,itwillstarttorotateandbreaklooseimmediatelythepiggybackanchorstartstoofferresistance.
Alternatively,ifthepiggybackanchorstartstoembedbeforethemainanchor,theflukesofthemainanchorwillnotbeabletoopen.
Klarenrecommendsthat,ifitisnecessarytouseapiggybackarrangement,thebetteranchorshouldalwaysbeusedastherearanchor(presumedtomeanthepiggybackanchor).
11VandenHaak[1972]reviewsthecharacteristicsofanumberoftypesofanchor,includingtheLWT,Stevin,Danforth,DeltaandStatotypes.
PerformancedatafortheseandotheranchortypesissummarisedinFigure2.
Hesuggeststhatgoodtypesofholdinggroundarenormalclay,hardsandandsoftlime.
Helistsfinegravelandcoarsesandaspoorholdingground,andcoarsegravel,shells,softsand,hardclay,stoneandrockasextremelypoorholdingground.
VandenHaakdiscussesindetailthebreakingoutofanchors,withparticularreferencetodredgeroperations.
ResultsofseveralseriesofperformancetestsofMODUanchorsarereportedbyBeck[1972,1974].
Thefollowingdesignsweretested;anarticulatedstockanchorsimilartotheLWT,articulatedstockanchorssimilartotheStatotype,andtheBOSSanchor.
(TheBOSSanchordesignissimilarinappearancetoanarticulatedstockanchor,withtheexceptionthatthetwoflukesaremergedtoformasinglefluke,whicharticulatesononlyonesideoftheshank).
TestswereconductedintheGulfofMexicoandalsoonabeachandinalake.
Thetestsitesencompassedarangeofseabedconditionsfromverysoftmudthroughtofirmmud,andalsostickyclayandhard,finegrainedsand.
Severaldifferentsizesofanchorweretested,rangingfromundertwokilogramsupto10tonnes.
Inthetestsmentionedabove,Beckobservedseveraldifferenttypesofanchorbehaviour,whichareillustratedinFigure3.
TheLWTtypeanchorswerefoundtobepronetotheflukesfailingtotripinmud,(B)ofFigure3.
ThisproblemdidnotaffecttheStatotypeanchors.
Whenballingupoccurs,(C)inFigure3,alargeballofmudbecomestrappedbetweentheshankandtheanchorflukes,causingtheanchortoloseholdingpower.
BeckstatesthatitwasfoundthatballingupcouldoccurfortheLWTtypeanchorsatanystageastheyembedded.
TheStatotypeanchorswerefoundonlytoballupaftertheyhaddevelopedsubstantialholdingpower.
TheresultsofeachofthetestseriesaredescribedindetailbyBeck.
TheperformanceoftheBOSSanchorwasfoundtobegenerallybetterthanthatoftheLWTandStatotypeanchors.
ItwasnotedthattheLWTtypeanchorhadanerraticperformanceinmud,whilsttheStatotypeanchorfailedtoembedproperlyinhardsand.
Table2summarisestheperformanceofeachanchortypeinGulfofMexicomudbottoms(type1andtype2aretheLWTandStatotypes,respectively).
Figure4showsadirectcomparisonofperformanceatasoftmudtestsiteintheGulfofMexico.
Table2AnchorperformanceinGulfofMexicomudbottoms(fromBeck,1974)34.
023.
015.
3Averageholdingpowerforalltests,kips0106Numberofball-ups006Numberoffailurestodigin131517TotaltestsinmudBossType2Type1800lbclassanchorsSPE1974Testswereconductedwithvariousflukeanglesforeachanchortype.
Inmud,theperformanceoftheBOSSanchorwasfoundtobestronglydependentonthefluke12angle,asmaybeseenfromFigure5.
Beckconcludesthat,foreachoftheanchortypestested,improvedholdingpowerinsoftmudcanbeobtainedbyincreasingtheflukeangle.
Beckfurtherconcludedthatinsand,theanchorstestedfailtoembedproperlywhentheflukeangleexceedsacriticalvalue.
Thiscriticalvalueisusuallyinexcessof34oformediumgrainedsand.
Howeverforhardfinegrainedsandsthiscriticalanglemaybelessthan30o.
Klaren[1973]discussestheresultsofanexperimentalinvestigationofpiggybacking.
Thetestswerecarriedoutindrysand,usingtwomodelDeltaanchorsandamodelDanforthanchor.
Themodelanchorshadmassesof0.
35kg,0.
36kgand0.
30kg,respectively.
Anumberofdifferentpiggybackingarrangementswereinvestigated,including:-ThetwoDeltaanchorspiggybackedtogether.
TheDanforthasthemainanchor,andaDeltaanchorasthepiggybackanchor.
ADeltaanchorasthemainanchor,andtheDanforthanchorasthepiggybackanchor.
WhenaDeltaanchorwasusedasthemainanchor,twoattachmentpointsforthechaintothepiggybackanchorwereinvestigated.
Thefirstwasapadeyeonthetopoftheshank,neartotheanchorshackle,andthesecondwasapadeyeontheheadoftheanchor.
Afirstsetoftestswasconductedwiththeangleofthetensionforceonthemainanchortothehorizontalintherange5oto15o.
Asecondsetoftestswasthenconductedinwhichthisanglewas0o.
KlarenstatesthatwhenthelinefromthepiggybackanchorwasattachedtothepadeyesontheheadsoftheDanforthorDeltaanchors,thetensionforcefromthepiggybackanchorcausedthemainanchortorotateandbreakout.
Henotesthatifthemainanchorhasarticulatedflukes,thentheflukeswillsubsequentlybeconstrainedtoremainparalleltotheshank,andthemainanchorwillnotembedagain.
WhenaDeltaanchorwasusedasthemainanchor,andthelinefromthepiggybackanchorwasattachedtothepadeyeonthetopoftheshankoftheDeltaanchor,boththemainandthepiggybackanchorsembeddedfully.
Theholdingpowerofthepiggybackarrangementwasfoundtobeequaltothesumoftheholdingpowersofthetwoindividualanchors.
ThispiggybackarrangementisshowninFigure6.
Klarenrecommendsthatapadeyeshouldonlybefittedontopoftheshankiftheanchorhasafixedfluke.
Hesuggeststhatforarticulatedflukeanchorsthereisadangerthatalineattachedatthispointontheanchorwouldbecomeentangled.
Puech[1978]presentstheresultsoftwoseriesofanchortests.
Thefirstseriesoftestswasintendedtoinvestigatetheinfluenceofparameterssuchasflukeareaandshankshapeonanchorperformance,andmadeuseof0.
1tonneanchorswithsimplifiedconfigurations.
Thesecondseriesoftestswasconductedtocomparetheperformanceofanumberofcommerciallyavailableanchors.
Forthissecondsetoftests,0.
1tonneanchorsofthefollowingtypeswereused:GSarticulatedstockanchor,Hallstocklessanchor,Stevin,Bruce,Flipper-Delta,DeltaTriple.
13TheanchortestsmentionedabovewereconductedattwocoastalsitesnearBrest.
Thefirstsiteisacalcareoussandybeach,havingmediumgrainsize,whichiscoveredbyashallowdepth(lessthan1.
0m)ofwateratlowtide.
Thesecondsitewasatidalmudflatwhichdriesatlowtideandiscoveredby1.
0mto2.
0mofwaterathightide.
Themudhasasurfacecohesionofabout50g/cm2andacohesiongradientofabout30c/cm2/m.
ThefindingsofthefirstsetoftestsaresummarisedinTable3.
TypicalresultsfromthetestsofcommercialanchortypesareshowninFigure7.
Table3Effectsofbasicparametersonanchorbehaviour(fromPeuch1978)xxxFlukeroughnessxxxxx,CuMechanicalpropertiesofsoilxDBurialdepthxPullinganglexxáFluke-shankanglexStocks,stabilisersxSShanklengthxBearingsurfaceofshankxxFlukesurfaceareaxL/FratioorflukespacingxTrimmingpalmareaxxDistributionofmasses-rotationaxispositionHoldingPowerStabilityBurialPenetrationOpeningMainfunctionsFundamentalparametersOTC1978Peuchconcludesthatstocklessandarticulatedstockanchorsarepoorlysuitedtooffshoreuse.
HefurtherconcludesthatStevin,BruceandFlipper-DeltaanchorsarewellsuitedtothetemporarymooringofbargesandMODUs,byvirtueoftheirhighreliabilityandstabilitywhendragged.
ForthepermanentmooringofFPSshesuggeststhatspecialanchortypes,suchastheDeltaTriple,canbeusedtoobtaintherequiredhighholdingcapacities,butthatthehandlingofthesespecialtypesisproportionatelymoredifficult.
Valent,Taylor,AtturioandBeard[1979]considerpossibleanchorstrategiesforOceanThermalEnergyConversion(OTEC)structures.
Withreferencetodraganchors(theyalsoconsiderotherformsofanchor)theirmaintopicistheextrapolationofholdingpowerandotherdataforStato,BruceandDorisanchorstotheextremelylargeanchorsizeswhichwouldberequiredforanOTECstructure.
Valentetalalsogiverecommendationsfortheminimumspacing,(b)betweenpiggybackedanchors,andbetweenanchorssitedinparallel,toachieveaholdingpowerequaltothesumoftheholdingpoweroftheindividualanchors(Figure8).
14Theratioof(b)toembedmentdepthispresentedasafunctionofsoilshearstrengthforclayseabeds(Figure9)andasafunctionofsoilfrictionangleforsandseabeds(Figure10).
Valentetalderivedtheirrecommendationsfor(b)fromconsiderationofdataforgroupsofembeddedplateanchors.
Theycautionthatinrealitythetotalcapacityofagroupofanchorswillalsodependonthewayinwhichtheanchorsareinstalledandconnected.
UraandYamamoto[1979]proposeanapproximatemethodofanalysingthebehaviourofanchorsbeingdraggedthroughanon-cohensivesoil.
Bywayofexample,thismethodwasappliedtoastocklessanchor.
Theirmethodmakesuseofempiricallyderivedexpressionsfortheforcesactingonindividualcomponentsoftheanchor.
Theseexpressionswereobtainedbymeasuringtheforcesoneachcomponentindependently.
Mostoftheanchorcomponents,forexampletheflukeandshank,wereidealisedasrectangularplates.
Theforcemeasurementsforthesecomponentswerethenaccomplishedbydraggingrectangularplatesofvariousaspectratiosthroughatankofdrysand.
UraandYamamotoclaimthatexperimentalresultsfromdrysandtestscanbeappliedtorealanchorsinfloodedsandiftheeffectivedensityofthesandisfullyconsidered.
Theyalsosuggestthatthedraggingspeedhasverylittleeffectonthebehaviourofanchors,andcanbedisregarded.
Theresultsoftheabovementionedexperimentsaresummarisedasasetofvectorintegralexpressionsforforcesandmomentsonrectangularplates.
Theseexpressionsaresimilartotheexpressionsforforcesandmomentsonplatesduetohydrostaticpressure,exceptthatpressureisreplacedbyfrictiontangentialtotheplatesurface.
Thetangentialfrictionisafunctionofthenormalforcecoefficientandtheeffectivefrictionangle.
Thelateralforceandassociatedmomentareadditionallyafunctionofthelateralforcecoefficient.
Empiricalexpressionsforthenormalandlateralforcecoefficientsandtheeffectivefrictionanglearederivedfromthetestdata.
Fromtheexpressionsforforcesandmomentsonindividualcomponentsoftheanchor,UraandYamamotoderivetheequilibriumequationforthestocklessanchor.
Anexpressionforthemaximumholdingpowerisinturnderivedfromtheequilibriumequation,asareexpressionsforequilibriumanchorinclinationandembedmentdepth.
Comparisonsaredrawnbetweenexperimentalmeasurementsofmaximumholdingpower,embedmentdepthandinclinationofa0.
92kgmodelanchorandthevaluespredictedbythederivedexpressions.
Modificationofthederivedexpressions,bytheintroductionofempiricallyderivedfactorstoaccountforinteractioneffectsbetweenthecomponentsoftheanchor,isfoundtosignificantlyimprovethecorrelationbetweentheexperimentalandnumericalvalues,asmaybeseenfromFigure11.
Conditionsforstability/instabilityoftheanchorarealsoestablishedfromtheequilibriumequation.
Twoinstabilitiesareconsidered,liftingoftheheadandrollingabouttheshank.
Liftingoftheheadisfoundtodependlargelyonthelengthoftheshank.
Rollinstabilityisshowntobedependentonthestabilisingmomentgeneratedbytheflukesandthestabilisingfins.
Itissuggestedthatabsoluterollstability(whichensuresthattheanchorwillrotateabouttheshanktoitscorrectorientationwhateverangleitstartsat)canbeachievedbyattachingapairofflat-facedstabilisingfinsperpendiculartotheflukes.
15Dove[1980]reviewsthehistoricaldevelopmentofringchasersystems,anddiscussestheirusefordeploymentandrecoveryofanchors.
InhishistoricalreviewDovefirstdescribesextemporisedchasingdevices,whichwereusedforrecoveringanchorswhendifficultieswereencounteredwithapendantwiresystem.
Hethenreviewsthedevelopmentoftheseearlychasingdevicesintopermanentlyinstalledringchasersystems,whichareusedinsteadofpendantwiresonmanyMODUs.
Themethodofusingringchasersfortheinstallationandrecoveryofanchorsisdescribedandillustrated(Figure12)byDove.
Hepresentsanexampletoshowthecostbenefitsofapermanentringchasersystemoverabuoyedpendantwiresystem,foraMODUcapableofdrillinginupto1000(305metres)feetofwater.
Hethengoesontodiscusstheadvantagesofringchasersystemsoverpendantwiresforverydeepwaterdrilling(inexcessof305metres).
Dovenotesthatifpiggybackanchorsareused,thenapendantwiresystemmustbedeployed,becausethechasercannotbepassedoverthemainanchor.
Hesuggeststhatchaseablechaindepressors,placedonthemooringlineinboardofthemainanchor,mayprovetobeanacceptablealternativetopiggybackanchors.
Dove[1981]firstreviewsexistingdeepwatermooringtechniquesandthengoesontoconsiderfuturedevelopmentsindeepwatermooringtechnology.
Hestatesthatforpermanentmooringoflargefloatingstructures,specialisedtypesofprefabricatedanchors,suchastheHookanchor,DeltaTripleanchorandDorisanchorarethepreferredoption.
Hesuggeststhatthedifficultyofinstallingthesespecialisedanchortypesisnotgenerallyappreciated,andthatthecostoftheinstallationequipment(forexamplelargeconvertedderrickbarges)issuchthatdraganchorswillofferlittleornocostsavingsoverpileanchorsfortheseapplicationsDovealsocommentsthatthereliabilityofdraganchorsonthedepth,shearstrengthandhomogeneityofthesoilinadditiontothestabilityoftheanchor.
Hestatesthataccuratepositioningofeachanchorinamooringsystemisveryimportantforlargepermanentlymooredstructures,becausethemooringlinelengthsarecriticalforsuchstructures.
Henotesthataccuratepositioningofdraganchorsis,however,almostimpossible,asthedragdistancebeforetheanchorfullyembedsisveryvariable.
Inlocationswherethesoilconditionsarenotwellknown,orwheretheseabedisrockyorsloping,Doveconsidersthereliabilityofdraganchorstobequestionable.
UraandYamamoto[1981]applytheanalyticalproceduresoutlinedinUraandYamamoto[1979]todesignnewtypesofstocklessandsingleflukeanchorshavingabsoluterollstability.
Therollstabilityofthesenewdesignsisdemonstratedexperimentally.
Taylor[1981]reportstheresultsofaseriesofUSNavyfieldtestsofStocklessandStatoanchors.
HealsomakesmentionoftestsofMoorfast,Hook,StevinandBruceanchorsconductedconcurrentlywiththeStocklessandStatoanchortests,butincludesnodetailsorresultsofthesetests.
ThreesizesofStocklessanchor5000,9000and20000pounds(2.
3,4.
1and9.
1tonnes)andthreesizesofStatoanchor,1000,3000and6000pounds(0.
5,1.
4and2.
7tonnes)weretested.
ThetestswereconductedatthreesitesSanDiegoHarbour,IndianIslandandGuam.
16ThetestprogrammefortheStocklessanchorsincludedanumberoftestsonpairsofanchorsinvariouspiggybackarrangements(Figure13).
Piggybackanchorsareanchorsconnectedintandemwiththemainanchor,withtheintentionofincreasingtheoverallholdingpowermobilisedonagivenmooringline.
Forthepiggybackteststheflukesoftheanchorswereweldedintheopenposition,topreventtheflukesofthemainanchorbeingclosedbytheloadofthetandemanchor,andbecausetheflukesofStocklessanchorsfrequentlydonotopeninsoftmud.
InthearrangementshowninFigure13c,thedistancebetweenthemainandpiggybackanchorswassufficienttoallowthetwoanchorstobeinstalledseparately.
Theseafloorconditionsatthetestsiteswereasfollows:afewinchesto7'(2.
1metres)ofmoderatetohighplasticityclayoverlyingmediumdensitycorallysandGuamhighlyplasticnormallyconsolidatedclay,withashearstrengthof1.
4psi(0.
01N/mm2)at20'(6.
1metres)belowtheseafloorIndianIslandfirsttestsitefinesand,secondtestsite1'to3'(0.
3to0.
9metres)ofsoftsiltoverdenseoverconsolidatedclayeysandSanDiegoHarbourOnehundredandthirteenanchortestswerecompleted,distributedbetweenthesefourtestsites.
Alimitednumberoftheseresultsarepresentedintabularform.
Graphsarepresentedshowingarangeofparametersasfunctionsofanchordragdistance,derivedfromthetestsonsingleanchors.
Tayloralsopresentsgraphsofloadagainstdragdistance,derivedfrompiggybackanchortestresults.
ThesegraphsincluderesultsforallthreepiggybackarrangementstestedatthefirstSanDiegotestsite,Figure14(test1/21crowntoshackle,test1/22groundingtoshackle,test2/20shanktoshackle)andfortwoofthepiggybackarrangementstestedatIndianIsland,Figure15(test11shanktoshackle,test12crowntoshackle).
AlbertsenandBeard[1982]reviewthecurrent(1982)statusofhighcapacitydrag,pile,embeddedplateanddeadweightanchors.
Thegeneralcharacteristicsandtheadvantagesanddisadvantagesofeachanchortypearediscussed,andseveraltypesareillustrated.
Itisstatedthatdraganchorswilloftencontinuetoprovidenearmaximumloadresistanceeventhoughtheanchorcontinuestoslipinthesoil.
ResultsarepresentedofUSNavytestsoftheholdingefficiency(ratioofanchorholdingcapacitytoactualanchorweight)foranumberofdraganchortypesintwosoiltypes(Table4).
Acautionisgiventhatthisdataissitespecificandthatperformancecouldvaryforotherseafloorconditions.
Itisalsonotedthatthebehaviourofdraganchorsiserraticinlayeredseafloors.
Testresultsshowingthattheembeddedlengthofanchorchainaddssignificantlytotheholdingcapacityoftheanchorarepresented.
ForaStatoanchorinmudtheanchorchainisfoundtohavecontributed50%ofthetotalholdingcapacity.
Albertsenetalstatethattheflukeanglehasamajorinfluenceontheholdingpowerofananchor,andthattheoptimumanglesformudandsandare50oand30oto35o,respectively.
17Table4Efficienciesofdragembedmentanchorsintwotypesofseafloors(fromAlbertsenandBeard,1982)12:1-750750BruceTwinShank-25:11,3201,320Bruce18:112:11,2601,230Hook20:1-2,2002,200Stevmud-29:12,5602,200Stevdig15:131:11,4101,410Stevfix15:118:13,5003,000Stato3:18:15,9505,000NavyStocklessMud/SiltSandActualNominalefficiencyanchorweight(lbs)AnchortypeAseriesofexperimentsontheholdingcapacityofcircularplateanchorsembeddedinsandaredescribedbySutherland,FinlayandFadl[1983].
Anapproximateexpressionfortheholdingcapacityofplateanchorsisderivedfromtheseexperimentalresults.
Bothshallowanddeeplyburiedplatesareconsidered,theburialdepthhavingastronginfluenceonthefailuresurfaceofthesoilattheultimatecapacityoftheanchor.
Twoorientationsoftheplateareconsidered,paralleltoandinclinedtothesoilsurface.
Theloadontheanchorwasappliedperpendicularlytotheplaneofthecircularplate.
Measurementsofanchorload,anchordisplacementandsoilsurfacedisplacementweremadeforeachexperiment.
Thesoilfailuresurfacewasrecordedbymeansoftracerlayersofcementpowderdepositedatseveraldepthsinthesand.
Therelativedensityofthesandwasfoundtostronglyinfluencetheultimatecapacityanddisplacementbehaviouroftheplateanchors.
Sutherlandetalalsoobservedamarkedreductionintheholdingcapacityofshallowanchorsaftertheultimatecapacityhadbeenreached,particularlyindensesand.
Theysuggestthatthiseffectcouldbeexplainedbydilitancyeffects.
Thisreductioninholdingcapacitywasnotobservedfordeeplyburiedanchors.
Fromtheexperimentallyobservedshapeforthesoilfailuresurface,anapproximateexpressionfortheholdingcapacityofadeeplyburiedinclinedplateanchorinsandisderived.
Includedinthisexpressionaretheeffectsoftherelativedensityofthesand,andthedifferencesinholdingcapacityofshallowanddeepburialanchors.
ThesoilfailuresurfaceandloadsassumedinderivingtheexpressionareshowninFigure16.
Expressionsfortheholdingcapacityofhorizontalandshallowburiedplatesarederivedbysimplificationoftheexpressionforthedeeplyburiedinclinedplate.
VoldandEie[1983]reporttheresultsoffieldtestsoffourtypesof3tonnedraganchorsintheNorwegianTrench.
TheanchorstestedwereaStevin,aFlipper-Delta,aBruceMark2andaBaldtstocklessanchor.
GeotechnicaldataforthesoilinthetestareaareshowninFigure17.
Theupper8mofthesoilconsistedofnormallyconsolidatedclayofhighplasticitywithanundrainedshearstrengthincreasingwith18depthfromapproximately2.
5kN/m2to12.
5kN/m2.
Theflukeanglewassetto45ofortheStevinandBaldtanchors,50ofortheFlipper-Deltaanchorand13oto16ofortheBruceanchor.
Instrumentationoneachanchorincludedapairofinclinometerstomeasureanchororientation,aloadcellintheanchorshackle,adepthgaugeandatrailingwiredeviceintendedtorecordthedistancetheanchorwasdragged.
TwoAHVswereemployedtocarryoutthefieldtests.
Thefirst,whichwasmooredtoabuoyanchoredbya12tonneanchor,providedthewinchtopullthetestanchors.
ThesecondAHVwasusedtolowerthetestanchorstotheseabed,andtorecoverthemafterthetests.
Theanchorswereinstalledandrecoveredusinga57mmdiameterpendantwireattachedtothenoseshackleoftheanchor.
A2400mlong57mmdiameterwirecablewasusedtopullthetestanchors.
Thewaterdepthwas300m.
Twelvetestswereconductedinwhichtheanchorswereloweredslowlytotheseabed,thenpulleduntiltheydragged.
Nomentionismadeofa"soaking-in"period.
Readingsweretakenfromalloftheinstrumentationinthesetests.
TheresultsofthesetestsaresummarisedinTable5.
Thefollowingobservationsweremade:-theStevinanchorexhibitedgoodpenetrationandholdingcapacity,buthadatendencytorollwhendragging(ieitwasnotperfectlystable)theFlipper-Deltaanchorexhibitedgoodpenetrationandholdingcapacity,andwasstablewhendraggingtheBruceanchorpenetratedmuchlessandgeneratedonlyonequarterofthemeasuredholdingcapacityoftheStevinorFlipper-Deltatypes,butwasquitestablewhendraggingtheBaldtanchorgeneratedaholdingcapacityhalfthatoftheStevinandFlipper-Deltatypes,butwasveryunstablewhendragging,rollingthrougha29oangleabouttheshank.
Figure18showstensionattheanchorshackle,embedmentdepthandinclinationplottedagainstdraglengthmeasuredinparticulartests,foreachofthefouranchors.
19Table5Summaryofinstrumentedtests(fromVoldandEie,1983)37c)39c)-13130180/270150/245145/240124737c)814-170/270--Baldt1149386.
09130210/280190/260180/26010128-7100120/220110/21090/2009085.
08130110/220100/21090/200Bruce8-38-11130110/210105/20090/2007048-19-470/620--Delta61148-1675460/540430/530370/490Flipper-564610.
81265400/500390/490210/3004-2330-9-330/460--35232-1780420/540380/500210/350Stevin220c)3810.
614-470/620--1Transv.
Long.
(m)(min)(m)Maxi-mum50mdrag25mdragFinalinclination(deg)Max.
penetr-ationb)DurationofdragMax.
Dragdist.
Anchorlinetension(kN)atanchorshackle/atwincha)AnchorTypeTestNo.
OTC1983Notes:a)Fortestsinwhichdragometerdidnotfunction,onlymaximumtensionsaregiven.
b)Penetrationreferstodepthofanchorshanktopbelowseabed.
c)Ahighernumericalinclinationvaluethanthefinalonewasrecordedduringthetest.
Afurthersetoftentestswereconductedinwhichtheanchorswereloweredmuchmorequicklytotheseabed,andforwhichtheinstrumentationpackwasremovedfromtheanchors.
Table6summarisestheresultsofthesetests.
IntwoofthefourtestsontheStevinanchorinthissetoftests,theStevinanchorloststabilityandholdingforce.
Voldetalascribethistothependantlinefoulingtheflukesoftheanchor.
Inoneofthissecondsetoftests,theBruceanchorgeneratedamaximumcapacitymorethandoublethatseenintherestofthetestsontheBruceanchor.
Thishighercapacitywasonlyachievedafterprolongeddraggingoftheanchor,duringwhich580moftheanchorcablewashauledin.
VoldetalsuggestedthattheincreaseincapacityresultedfromtheBruceanchorembeddingmoredeeply,afterdraggingforalongperiodinastalledattitude.
Twoapproachestoextrapolatingtheexperimentaldatatolargeranchorsareconsidered:conventionalapproach,usingtheempiricalexpressionholdingcapacity=c1.
(anchorweight)bwherec1andbareempiricalcoefficientsdependentonanchortypeandsoilconditionsgeotechnicalapproach,usinganexpressionbasedonananalogytotheendbearingcapacityofapileholdingcapacity=c2.
A.
su20whereAandsuare,respectively,theprojectedareaoftheanchorflukesandtheundrainedshearstrengthofthesoil,andc2isanempiricalcoefficient.
Table6Summaryofdroptests(fromVoldandEie,1983)500192802248022300Baldt2158041490b)233506220Bruce14350142101350027570Delta2056021560Flipper-1951026500183603to11230decr.
to130a)173309to18400decr.
to140a)Stevin164501459015Totallengthofanchorlinepulledinduringtest(m)Durationofdrag(min)Max.
anchorlinetensionatwinch(kN)AnchorTypeTestNo.
OTC1983Notes:a)Tensionlossapparentlycausedbyfoulingofpennantlinearoundanchor.
b)Maximumtensionreachedafter35minutes.
Fromtheseexpressions,thesizeofStevinorFlipper-Deltaanchortogiveaworkingcapacityof1500kNiscomparedwiththeanchormanufacturersrecommendationsfortheseanchortypes.
Theconventionalapproachindicatesaweightof34tonnesor19tonnes,dependingonwhetherbisassumedtobe0.
75or1.
0.
Nocommentismadeontheassumptionsbehindthesevaluesforb.
ThegeotechnicalapproachandthemanufacturersdatafortheStevinanchorindicateananchorweightof18to19tonnes.
hemanufacturersdatafortheFlipper-Deltaanchorsuggestaweightof15tonnes.
Puech[1984]givesacomprehensiveassessmentofdraganchorsinhisbook,underthefollowingmainchapters:-historyofanchorshigh-capacityanchoringsystemskinematicsofanchorsholdingpowerofanchorsanchoringsitesurveyschoiceofananchoranchoringtestsItisconsiderencethatthisreferenceisaveryusefulgeneralintroductiontodraganchors.
Zumwait[1986]discussesdeepwatermooringoperationsonthecontinentalslopeoftheGulfofMexico.
Henotesthatthemooringofrigsin600to2000feet(180-61021metres)waterdepthsontheGulfofMexicocontinentalslope,isnowaroutineoperation.
Recentimprovementsinmooringproceduresandtechnologywhichhavebeenboughtaboutbytheneedtomoorinthesedeeperwatersarediscussedunderanumberofheadings.
SuggestingthatthesafetyrecordofMODUswithrespecttomooringshasbeengood,withonlyonemooringfailureincidentbeingsignificantenoughtobereported,Zumwaitgoesontostatethatanchorlinefailureswhichdelaymooringoperationsbutdonotbecomesafetyhazardswillgenerallynotbereported.
ZumwaitreportsthattheseafloorontheGulfofMexicocontinentalslopeisaweakHoloceneunconsolidatedclaylayer,whichoverliesfirmerPleistoceneeraclay,sandandgraveldeposits.
InsomeareascoralisfoundbeneaththeHoloceneclaylayer.
henotesthatananchor'sholdingcapacityintheseconditionsisprimarilydependentonthecohesivenessoftheseabedsoil:wherethebottomisverysoftclayorsilt,theanchormayhavetopenetrateinexcessof100feet(30metres)toachieveapercentageofitsnormalholdingcapacity.
Estimatedholdingcapacitiesforarangeofanchorsinsoftsiltorclay,andinsands(statedtobeoflesserinterestforGulfofMexicoconditions)arepresentedinFigure19.
Zumwaitnotesthattoavoidapplyingupliftforcesonanchors,longanchorlinesarecommonlyused.
Hesuggeststhattheselongscopeshavetheredeemingfeaturethattheycontributeanadditionalholdingcapacitywhichforchainisabout55%oftheimmersedweightofthechain.
Referringtotheuseofpiggybackanchors,hestatesthatthelengthofanchorlinebetweentheprimaryandpiggybackanchorsisusuallytwotothreetimesthewaterdepth,toallowthemtobehandledseparately.
Headdsthatuptothreepiggybackshaveoccasionallybeenusedonasinglelineinverypoorholdingground.
IndiscussingtheuseofpermanentchainchasersZumwaitpointsoutthatthemaximumloadingonthechaseranditspendantwirewilloccurwhendislodgingtheembeddedanchor.
Hestatesthattheforcerequiredtodothiscanexceedthemaximumhorizontalforcetowhichtheanchorhasbeensubjectwhilstinposition.
ThepreferredspecificationsforAHVsintendedfordeepwaterGulfofMexicomooringworkarelisted.
ThepreferreddeckequipmentspecificationforsuchvesselsissummarisedinFigure20.
Dutta[1988]discussesanumericalmethodfordeterminingtheconfigurationofamooringchainwithanembeddedattachmentpointsubjecttoahorizontaltensionforceatitsfreeend.
Hisnumericalmethodusesafinitesegmentapproach,inwhichthechainismodelledasaseriesoflinesegmentsconnectedbyfrictionlessnodes.
Allforces,bothexternalandinternal,actonthechainnumericalmodethroughthesenodes.
Thenumericalmodelprovidesamethodofdeterminingthetensionattheembeddedattachmentpointforagiventensiononthefreeendofthechain.
Thenumericalprocedureisvalidatedbycomparisonwithpublishedexperimentaldata.
Thedesign,installationandtestingofatemporarymooringsystemfortheGullfaksACondeepconcreteplatformisdescribedbyRraasandHagen[1989].
TheyexplainthataneightlinemooringsystemwasrequiredtoholdtheGullfaksplatforminpositionwhilstitwascompletedatadeepwatersiteontheNorwegiancoast.
Fiveoftheselinesweretakentoshore.
Theremainingthreelinesweresecuredbydrag22anchorsdesignedtowithstandamaximumpullof15000kNeach.
Rraasetalnotethattherewasacompletelackofrulesregulationsandexperiencerelevanttothedesignandinstallationofanchorshavingaholdingcapacityofthismagnitude.
ThesoilconditionsatthesitesofthethreedraganchorsaredetailedinTable7.
Rraasetalstatethatadecisionwasmadetocarryoutsmallscalefieldtests,using3tonneanchors,attheplannedanchorlocations.
Theyindicatethattwoanchormanufacturerswereinvolvedinthesmallscaletestprogramme,andthattheanchortypesfinallychosenweretheStevprisandStevsharkdesignsproducedbyVryhofAnkers.
Theothermanufacturerisnotidentified.
Resultsoftestsatallthreesitesaregiven(Figure21),butitisnotcleartowhichdesignsofanchortheseresultsrefer.
Theonly3tonneanchorforwhichdimensionaldataisgivenisaStevprisanchor.
Itisstatedthatthefluketoshankanglewassetto32oforsandand50oformud.
ThetestanchorswereinstalledusinganAHV.
Theanchorlinewasthenpulledataspeedoflessthan5m/minute.
Thetensioningoftheanchorlinewasstoppedat40%andat70%ofthemaximumpredictedholdingcapacityoftheanchor.
Whenthemaximumholdingpoweroftheanchorwasreached,thepullingoftheanchorlinewasstoppedfor15minutes,andtheholdingcapacitywasrecordedagain.
Rraasetalnotethatfortheanchoratline4location,theanchorbecamestuckintherock;theloadreached1500kNbeforethetestwasstopped.
Table7Soilconditionsatthethreelocations(fromRraasandHagen,1989)0.
2-0.
6Grainsize(mm)10-1510-15Sensitivity402828Angleofinternalfriction(degrees)22-2732-4540-45Watercontent(%)8[0-3]22[-8.
5]35[-9]50[-10]10[0-7.
5]15[-8.
5]20[-10]35[-15]Shearstrength(kN/m)[depthinmetres]407to1140Depthtorock(metres)mediumuniformsandsoftclayverysoftclaySoiltype202102299Waterdepth(metres)743Lineno.
OTC1989The3tonneStevprisanchorwasalsosubjecttomechanicalstrengthtests.
ThesetestsaredescribedindetailbyRraasetal,whonotethattheanchorwasfoundtofailbybucklingoftheshankmemberswhensubjecttoa1412kNloadappliedasapointloadatthefluketips.
Rraasetalreportthatbasedonthefieldtests,togetherwiththemechanicaltestdataandtheresultsofastrengthanalysisoftheproposedanchors,Stevprisanchorsof65and40tonneswerechosenforlinelocations3and7respectively,andaStevsharkanchorof60tonneswaschosenforlocation4.
Theypresentdatashowingthe23tensioningtoproofloadofthethreeanchors(Figures22and23).
Foreachoftheanchorstensioningtoproofloadwascarriedoutoveranumberofdays.
Thethreeanchorswererecoveredafteruse.
Theanchoratlineposition3isnotedbyRraasetaltohaverequiredanupliftforceofapproximately11000kNtodislodgeit.
Apreviousattempttomovethisanchorhadfailed,andasteadyverticalpullwassubsequentlyputonitforaday.
Rraasetalstatethattheanchorsattheothertwositeswereeasytoretrieve.
DuttaandDegenkamp[1989]presenttheresultsofanexperimentalinvestigationoftheembedmentofmooringchainsduringchaintensioning.
Althoughtheirworkisdirectedprimarilytowardschainsattachedtopileanchors,theysuggestthattheirfindingsarealsoapplicabletomooringchainsconnectedtodraganchors.
Theexperiments,whichwerecarriedoutatasmallscale,wereconductedbyattachingoneendofananchorchaintoafixedpointbeneaththesoilsurface.
Ahorizontalpullforcewasthenappliedtothefreeendofthechain,causingmoreofthechaintobecomeembeddedastheembeddedpartofthechainwasdisplacedfromaverticalposition.
TheexperimentalapparatusisshownschematicallyinFigure24.
Threedifferentsizesofchainandtwodifferentsaturatedclayswereused.
Thechainswerestudlessandhadnominaldiametersof6.
4mm,9.
5mmand16mm.
Oneofthetwoclayswasverysoft,havinganundrainedshearstrengthof4.
52kN/m2.
Theotherclaywasfirm,havinganundrainedshearstrengthof33.
9kN/m2.
Thefollowingparameterswererecordedasthetensiononthefreeendofthechainwassteadilyincreased:-Forces/chaintensionatthefixedattachmentpoint.
Embeddedchainlength.
Chainangleatthefixedattachmentpoint.
Figure25showsthevariationofchaintensionanangleattheattachmentpointwithhorizontalpullforce,forthesoftclay,whenthefixedattachmentpointwas1.
008mbelowthesoilsurface.
Figure26comparesthehorizontalandverticalchainforcesobtainedinthesoftclaywiththatobtainedinthehardclay,forfixedattachmentpointdepthsbelowthesoilsurfaceof0.
135mand0.
407m(chaindiameter6.
4mm).
Duttaetalconcludedthatthechaintensionatthefixedattachmentpointvariedlinearlywiththehorizontalpullforce,whereasthechainangledecreasednon-linearlywithincreasingpullforce.
DegenkampandDutta[1989]describeanumericalmodelfortheconfigurationofamooringchainattachedtoapointbelowthesurfaceofasoftclayseabed.
Theirnumericalmodelemploysanincrementalintegrationtechniquetoderivetheequilibriumequationsforthechain.
Theeffectivewidthofthechain,whichisusedincalculatingsoilresistanceinbearingandfriction,wasdeterminedfromthemodeltestsreportedinDuttaandDegenkamp[1989].
TheproposednumericalmodelisappliedtodeterminethetensionloadattheembeddedattachmentpointsofthemooringsofaCatenaryAnchorLegMooring(CALM)buoyinstalledin77mofwaterintheAdriatic.
24Oneanchormanufacturerhasproducedananchormanual(VandenHaak[1990])whichinadditiontodescribingthemanufacturer'sproducts,givesadviceonanchorhandlingprocedures,soilsurveysandanchorsizing.
Abriefreviewofanchordesignrequirementsispresented,asisasummaryofchasertypesandhandlingmethods.
3.
3PROPRIETARYDATACertifyingauthoritiesarealmostinvariablycloselyinvolvedwiththeinstallationandtestingofthemooringsofstructuresintendedtoremainontheirmooringsonalongtermbasis(suchasFPSsandloadingbuoys).
Asaresulttheyholdaquantityofproprietarydatafrominstallationtestsofthesemooringsystems.
Withinthescopeofajointindustryfundedproject,afieldtestprogrammewithaplannedtestmatrixofsome30testswasrecentlycompletedintheGulfofMexico.
BruceFFTSIV,StevprisandVicinayNorshoreanchors,in2tonneand7tonnesizes,wereplannedtobetested.
Itisunderstood,however,thatanumberofexperimentalproblemswereencounteredandthatasaresult,onlyoftheorderofonequarteroftheplannedmatrixoftestswereactuallycompleted.
TheplanneddeliverablefromthisJIPwasaprogramtocomputeanchorburial,dragdistanceholdingpowerandanexpressionforholdingpowerbasedonanchorgeometry.
AcentrifugetesttechniquehasbeenemployedbyoneoperatorintheUStocarryoutaseriesofsmallscalemodeltestsondraganchorsinsoftclays.
Thistechnique,haspreviouslybeenappliedtotheexperimentalinvestigationofarangeofothersoilmechanicsproblems.
3.
4MANUFACTURER'SDATAInformationintheformofproductcataloguesanddatasheetswassoughtfromfiveanchormanufacturerschosentobearepresentativecross-sectionoftheEuropeananchormanufacturingindustry.
Theinformationwhichfollowsisdrawnfromtheinformationreceivedfromthesemanufacturersinreply.
Theanchorsdiscussedherearethosewhichareofparticularrelevancetooffshoreuse.
Severalmanufacturersalsoproduceothertypesofanchors,includingtypesintendedprimarilyforyachts,smallcommercialboats,fishfarming,inshorepermanentmoorings,ornavaluse.
IsiahPreston(UK)manufacturertheMeonMk3anchorforoffshoreuse.
ThisparticulardesignofanchorwaspreviouslyknownastheDanforthanchor.
TheMeonMk3anchorisapprovedasaHHPanchorbyLloyd's,GermanisherLloydandBureauVeritas,eachallowinga25%weightreductionrelativetoastandardstocklessanchor.
TheAmericanBureauofShipping(ABS)allowa20%weightreduction.
Figure27showsthemanufacturer'sperformancedataforthisanchor,whichismanufacturedinarangeofweightsupto50,000lbs(22.
68tonnes).
Theflukeangleissetto32o.
IsiahPrestonalsoproducetheStokesHHPstocklessanchor.
Theflukeangleisnormally40o,butthismaybesettoadifferentvalueduringmanufacture.
Themanufacturer'sperformancedataforthisanchorareshowninFigure28.
Thelargestcataloguedanchorofthistypeis50,000lbs(22.
68tonnes)butlargersizesarestatedtobeavailable.
TheStokesanchorisapprovedasanHHPanchor(25%reductionin25weight)byLloyd's,GermanisherLloyd,BureauVeritas,RegistroItalianoNavale(RINa)andDetNorskeVeritas(DnV).
TheABSallowsa20%weightreduction.
BruceInternational(UK)manufacturetwotypesofanchorinsizesappropriateforoffshoreuse.
ThesearetheFFTSMk4,andtheTS,bothofwhichareofweldedconstruction.
TheFFTSandTSanchorsaremanufacturedinweightsofupto60tonnesand25tonnes,respectively.
Theflukeangleofbothtypesmaybeadjusted.
OntheTSanchorthisadjustmentisachievedbymeansofaboltarrangement,whilstontheFFTSplainpinsareused.
Bothanchorsareself-righting.
Figure29showsmanufacturer'sperformancedatafortheFFTSanchor.
AnkerAdviesBureau(TheNetherlands)manufacturetheFlipper-Deltaanchorforoffshoreuse.
Flipper-DeltaanchorsarestatedtobeprooftestedasHHPanchorsinaccordancewithcertifyingbodyrequirements.
Anchorswithweightsofupto75tonnesarecatalogued.
Theflukeangleisnormallysetto36o,butcanbeincreasedto50oforverysoftclayandmudconditions,andreducedto28oforhardseabedssuchasrockorcementedsand.
Adjustmentoftheflukeangelisachievedbycuttingoutorweldinginsmallspacingpieces.
Performancechartsarenotgiven,itissimplyrecommendedthattheanchorweightshouldbe10%ofthemaximumholdingloadrequired.
VryhofAnker(TheNetherlands)producethreeanchorsforoffshoreuse,theStevin,StevprisandStevsharktypes.
Allthreearestatedtobecertifiedbyarangeofcertifyingauthorities,includingLloyd's,Germanisher,Lloyd,BureauVeritas,RINa,ABSandDnV.
TheStevinanchorhashingedanchorflukesandcanbedroppedwithouttheassistanceofanAHV.
Weightsofupto30tonnesarecatalogued.
Themanufacturer'sperformancechartforthisanchorisshowninFigure30.
TheStevprisanchorhasfixedanchorflukes,theangleofwhichcanbeadjustedto50oformudand32oforsand.
Adjustmentisfacilitatedbymovingapairofplainpins,whichareheldinplacebyweldedkeeperplates.
Themanufacturer'sperformancechartfortheStevprisanchorisshowninFigure31.
Weightsofupto65tonnesarecatalogued.
TheStevsharkanchorisanadaptationoftheStevprisanchorforuseinverysoilsandsomerockseabeds.
Theanchorisfittedwithaserratedshank,andisreinforced,particularlyattheflukepoints.
OtherdetailsofthisanchoraresimilartothosefortheStevprisanchor.
VlaardingenOost(TheNetherlands)produceDanforthanchorswithamaximumsizeof30000lbs(13.
61tonnes).
Theyalsomanufactureseveraltypesofstocklessanchors,includingBaldtanchorsofupto35000lbs(15.
87tonnes)andHallanchorsofupto29tonnes.
Theseanchorsarestatedtomeettherequirementsofanumberofcertifyingbodies,includingLloyd's,GermanisherLloyd,BureauVeritas,RINa,ABS,andDnV.
Performancedataarenotgiveninthismanufacturer'scatalogue.
3.
5CODESANDREGULATIONSDEnOffshoreInstallations:GuidanceonDesign,constructionandcertification.
4thedition,1990.
26Section30:FloatingInstallationsThissectionrequiresthataninvestigationoftheplannedoperatingsitebemade,toascertainwhetherthefloatingstructurecanoperatesafelyatthesite,beforethestructureismovedtothesite.
Forananchoredplatformtheinvestigationistoincludeanappropriatesurveyoftheseabed.
Section32:StationKeepingItisrequiredthatmooringsshouldaccommodatethefailureofonelinewithoutcausingconditionsintheotherlineswhichwouldendangerthesecurityoftheanchors.
Inaddition,theselectionofanchorstrengthandtypeshouldbebasedonconsiderationofthemooringpatterntobeused,theexpectedseabottomconditionsandtheanticipatedenvironmentalconditions.
Anchorsaretobetestedandmarkedinaccordancewith"AnchorsandChainCablesRules1970(SI1970/1453").
TheOperationsManualistoincludedetailsoftheholdingpowerandweightoftheanchors,plusaplanshowingatypicalmooringarrangement.
Formobileunitstheoperationsmanualisalsotoincludeproceduresforthedeploymentandrecoveryofmooringsandrequirementsforpre-tensioning.
AnchorsandChainCablesRules1970(SI1970/1453)ThisStatutoryInstrumentspecifiesthemechanicalproofloadtobeachievedbyanchorsofagivenweight(seebelow).
Topass,ananchormustnot,intheopinionoftherepresentativeoftheCertifyingBodywitnessingthetests,showmaterialdeformation,flaworweaknessafterthetest.
Forastocklessanchortheweightoftheanchoristoincludeboththeweightoftheanchoranditsshackle.
Forastockanchortheweightoftheanchoristoincludetheweightoftheanchoranditsshacklebutexcludetheweightofthestock.
Ifaspecialdesignofanchorhasbeenacceptedbyacertifyingauthorityasbeingaseffectiveasaconventionalanchorofgreaterweight,thenattherequestofthepersonseekingthecertificationoftheanchor,thisspecialanchordesignmaybetestedataproofloadappropriatetoananchorofgreaterweight.
Thespecialanchordesignisnottobetestedataproofloadgreaterthantheproofloadforananchorweightmorethanonethirdgreaterthantheactualweightoftheanchor.
Anchorswhichpasstheprooftestmustbemarkedwiththenameofthecertifyingauthorityandthenumberofthetestcertificate.
Thetestcertificateistoincludedetailsofthedimensionsandtypeofanchor,theproofload,thenameofthetesthouseandthenameofthecertifyingauthority.
27Lloyd'sRegisterRulesandRegulationsfortheClassificationofMobileOffshoreUnits,1989Part2,Chapter2,HighHoldingPoweranchorsandSpecialQualityChainCableTherulesandregulationsstatethatcastandforgedanchorcomponentsandplatematerialforanchorsaretobefabricatedincompliancewithLloyd's"RulesfortheManufacture,TestingandCertificationofMaterials",(seebelow).
ProofloadsforHighHoldingPower(HHP)anchorsarespecifiedforanchorsof50kg(testload29.
5kN)to36000kg(testload2730kN).
ForHHPanchorswithamassgreaterthan35000kgtheproofloadistobe:Proofload,(kN)=2.
452x(massofanchorinkg)2/3butnotlessthan2730kN.
Eachanchortobeusedonaplatformmustbesubjecttoaprooftest.
Oncompletionofthetestaninspectionisrequiredandmustshowtheanchortobefreefromsignificantdefects.
Inparticularthepermanentsetoftheanchormustnotexceed20mm.
TherulesinthischapteraredirectedprimarilytowardsHHPanchors.
Howevertheproofloadsspecifiedmayalsobeappliedtoordinaryship-typestocklessanchorsbyusingtheproofloadspecifiedforananchorhavingamassthreequartersthatoftheactualmassoftheshipanchor.
AnchorsmayalsobeprooftestedtohigherloadsthantabulatedintheRules,providedthattheanchordesignhasbeenespeciallyapprovedforhigherloads.
Markingofthecertifiedanchoristoincludeeithertheletters"Lloyd's"or"LR",heprooftestloadand,whenappropriate,theletters"HHP".
Also,eachmajorcomponentoftheanchorwhichiseithercastorforgedistobemarkedassuch.
Part3,Chapter4MooringEquipmentThischapterdealswithtemporarymooringequipment,whichisdefinedasmooringequipmentforuseduringvoyagesorchangesofposition.
CompliancewiththeRulesfortemporarymooringequipmentisnotcompulsory,butvesselswhichdocomplyareawardedaclassmarksignifyingtheircompliance.
TheRulesnotethatmooringequipmentintendedprimarilyforpositionalmooringwillonlybeacceptedasadditionallyfulfillingtherequirementsfortemporarymooringiftheequipmentarrangementissuchthatitcanbeefficientlyusedinthissecondrole.
TosatisfytheClassregulationsregardingtemporarymooringaminimumoftwoanchorsarerequired,arrangedsothattheycanquicklybedropped.
Themassoftheseanchorsisdeterminedbyanexpressiondependentonthedisplacementofthevesselandthesurfaceareaofthevesseloverwhichwindforcesact.
ItisassumedthattheanchorswillbeHHPanchors.
However,anchorswhichmustbelaidtherightwayup,orwhichrequiretheflukeanglestobealteredfordifferentsoilconditionsarenotacceptable.
28ForananchordesigntogainapprovalasaHHPanchor,itmustgenerateaholdingcapacityatleasttwicethatgeneratedbyastandardstocklessanchorofthesamemass,testedatthesametimeinfieldtestsconductedatsea.
Thefieldtestsmustbecarriedoutonatleastthreetypesofseabed,preferablysoftmudorsilt,sandorgravel,andhardclay.
Theanchorlinemusthaveascopeofatleastsix,preferablyten,andthesamescopeistobeusedforboththepotentialHHPanchorandthestandardstocklessanchor.
Theanchorlineusedforthetestsmaybeeitherchainorwire.
Ifapprovalissoughtforarangeofsizesofanchor,aminimumoftwosizesofanchormustbefieldtested.
Thelargerofthesetwotestanchorsmustbeatleastonetenthofthemassofthelargestanchorforwhichapprovalisdesired.
Thesmallerofthetwotestanchorsmusthaveamassofatleastonetenthofthelargertestanchor.
HHPanchorsarerequiredtoembedandremainstableforpullsuptotwicethecapacityoftheequivalentweightstandardstocklessanchor,regardlessoftheorientationatwhichtheyfirstsettleontheseabedwhendroppedfromahawsepipeoranchorrack.
Part6,Chapter7RulesfortheClassificationofPositionalMooringSystemsandThrusterAssistedMooringSystemsThischapterpresentsrulesforpositionalmooringequipment.
Positionalmooringisdefinedasthedeploymentofanarrayofanchorlines,toenableavesseltokeepstationataparticularsite.
Asfortemporarymooringequipment,compliancewiththerulesforpositionalmooringequipmentisnotcompulsory,butvesselswhichcomplyareawardedaclassmarksignifyingtheircompliance.
Theoperationsmanualforthevesselmustincludeinstructionsonlayingandpre-tensioningthemooringsystem,adjustingthetension,andtheproceduretobefollowedintheeventofafailureofthemooringsystem.
Thepositionalmooringsystemistobedesignedtowithstandenvironmentalconditionswithareturnperiodequaltoorgreaterthanfiftyyears,thevesselbeinginsurvivalmode.
Themooringlinesarerequiredtohavesufficientscopetoensurethatupliftforcesarenotexertedontheanchorsinthiscase,evenwhenoneofthemooringlineshasfailed.
Itisstatedthattheanchorsusedinthepositionalmooringsystemmustbesufficientinnumber,holdingpowerandmechanicalstrength,tofulfiltheirintendedfunction.
Itisstressedthattheresponsibilityofthevesselownerstoensurethattheholdingpoweroftheanchorsissufficientforeachparticularsite.
Theanchorsusedforpositionalmooringaretobeofanapproveddesign,fabricatedinaccordancewithPart2,Chapter2,asdiscussedabove.
Themechanicalproofloadappliedtotheanchorsistobeeitherhalfoftheminimumbreakingstrengthofthemooringlinetowhichtheanchoristobeattached,ortheproofloadspecifiedinPart2,Chapter2,whicheveristhegreater.
29Lloyd'sRegisterRulesfortheManufacture,TestingandCertificationofMaterials,1984Lloyd's"RulesandRegulationsfortheClassificationofMobileOffshoreUnits"makereferencetothefollowingchaptersoftheMaterialsRules:-Chapters3.
2and3.
3.
Thesechaptersdefinetherequiredmechanicalpropertiesandtestproceduresforplatesteel.
Chapters4.
1and4.
2.
Thesechapterslaydownrequirementsforthemanufactureandtestingofsteelcastings.
Chapters5.
1and5.
2.
Thesechaptersspecifythemechanicalpropertiesandtestingproceduresforsteelforgings.
DnVRulesfortheClassificationofMobileOffshoreUnitsPart3,Chapter2SpecialDesigns,EquipmentandStabilityThischaptersetsoutrequirementsfortemporaryandemergencymooringequipment.
Temporarymooringequipmentisdefinedasequipmentforuseinharboursandshelteredanchorages.
Emergencymooringequipmentisdefinedasequipmentforuseinbadweatherconditionsduringtransitmovementsofthevessel,andistobecapableofpreventingthevesselfromdriftinginanuncontrolledmannerinbadweather.
Therequirementsfortemporaryandemergencymooringequipmentmayusuallybefulfilledbyequipmentfittedforpositionalmooringpurposes.
Fortemporaryoremergencymooringthevesselmustbefittedwithatleasttwoanchors.
Thesemuststarttofallundertheirownweightwhenthebrakesoftheanchorwinchesarereleased.
Theminimumsizerequirementsfortheseanchorsisdeterminedfromanexpressioninvolvingthedisplacementofthevesselandtheprojectedareaofthesurfacesofthevesselexposedtothewind.
Theanchorsizerequirementsarebasedonstocklessanchors,buteitherstockedorHHPanchorsmaybeusedinstead.
Thefollowingpointsarenoteworthy:-Forstocklessanchorsthemassoftheheadsmustnotbelessthan60%ofthetotalspecifiedanchormassoftheheadsmustnotbelessthan60%ofthetotalspecifiedanchormass.
Ifstockedanchorsareusedtheirmass,notincludingthestock,mustbeatleast80%ofthemassspecifiedforstocklessanchors(thestockitselfmustbe25%ofthemassofthetestoftheanchor).
IfHHPanchorsareused,theirmassmustbeatleast75%ofthemassspecifiedforstocklessanchors.
Ifwireanchorlinesareused,theanchormassmustbeincreasedbyatleast25%.
Theanchorsizerequirementsarebasedontheuseoftwoanchors.
Threeorfouranchorsmaybeusedinstead,inwhichcaseeachanchormaybeonethirdoronefourth,respectively,ofthetotalmassofthetwoanchors.
MechanicalprooftestingisspecifiedforbothordinaryandHHPanchors.
Thenecessaryproofloadistabulatedforanchormassesbetween2.
2tonnesand304.
8tonnes.
Forstocklessanchorstheproofloadistobechosenbasedonthetotalmassoftheanchor,butforstockedanchorstheproofloadistobebasedonthemassoftheanchorminusitsstock.
ForHHPanchorstheproofloadisbasedonanominalmasswhichis4/3ofthetotalmassoftheanchor.
Theproofloadisappliedontheflukesatapositionwhichis2/3ofthedistancefromthecrownoftheanchortothetipofthefluke.
Topassanchorsmustdisplaynoindicationofdefectsaftertheprooftest.
IfthedesignofanchortobeusedhasnotpreviouslyreceivedtypeapprovalasanHHPanchor,strengthcalculationsfortheanchoranditsshacklemustbesubmittedforapproval.
TherequirementswhichananchordesignmustsatisfyinordertogaintypeapprovalasanHHPanchoraredetailed.
Theserelatetoboththebehaviourandfieldtestingoftheanchordesign,andareessentiallythesameastheapprovalrequirementsforHHPanchorssetoutinPart3,Chapter4oftheLloyd'sRulesforMobileOffshoreUnits.
TheDnVrulesdohoweverdifferfromtheLloyd'sRuleswithrespecttothetypeandlengthofanchorlinetobeusedinthetest.
DnVrequirethattheanchorlineistobechainofthediameterspecifiedinequipmenttableslistedinthechapter.
DnValsospecifythatifthepullonthetestanchorsismeasuredbyreferencetothepropellerrevolutionsoftheAHV,theminimumwaterdepthinwhichthefieldtestsmaybeconductedis20m.
AnchorswhichhavebeensatisfactorilyprooftestedaretobemarkedwithDnV'sstamp,theanchormass,thetestcertificatenumberanddateoftest,andwhenappropriate,theletters"HHP".
Material(plate,forgings,castings)requirementsarespecifiedwithinPart2oftheRules.
Part6,Chapter2PositionMooringThischapterlaysdownrequirementsforpositionalmooringequipmentwhichapplytobothsinglepointandspreadmooredvessels.
Allanchorsmustbeprooftested.
Theprooftestprocedureisthesameasfortemporarymooringequipment,discussedinPart3,Chapter2oftheRules.
Howevertheproofloadisrequiredtobe50%oftheminimumbreakingstrengthoftheanchorlinetowhichtheanchorwillbeattached.
Anchorsmustbedesignedsuchthatadditional(piggyback)anchorsmaybeattached.
Anchorsandanchorshacklesmustbedesignedtowithstandaloadequaltotheminimumbreakingstrengthofthestrongestanchorlinethatwillbeused.
Foranchordesignswhichhavenotpreviouslyreceivedtypeapproval,strengthcalculationsmustbesubmitted.
TypeapprovalforanHHPanchordesignmustbeobtainedbyfollowingtheproceduresoutlinedinPart3,Chapter2oftheRules.
Part2,Chapter1SteelandIron31Castanchorcomponentsaretobemadeto"specialquality"specification.
TherequirementsforspecialqualitycastingsincludespecifiedminimumCharpytestresultsinadditiontothetensilestrengthandyieldstressspecificationsfor"normalquality"castings.
APIRecommendedPracticeforAnalysisofSpreadMooringSystemsforFloatingDrillingUnits,RP2P,1987Mooringdesignisbasedonenvironmentalcriteriawhichhavea99.
9%probabilityofnotbeingexceededinanaverageyear.
Itisstatedthatinordertoprovideinformationfromwhichanchorperformancemaybeestimated,bottomsoilconditionsattheintendeddrillingsiteshouldbeinvestigated.
Thepossibilityofpiggybackingbeingrequiredwhentheseabedisverysoftisnoted.
Itisalsosuggestedthatpileanchorsmayhavetobeusedinsteadofdraganchorswhentheseabedisveryhard.
Thefactorsinfluencinganchorholdingpowerarelistedunderheadingsofanchortype,bottomsoilconditionsandanchorbehaviourduringdeployment.
Itisnotedthatpredictionofanchorholdingpowerisdifficult,becauseoftheconsiderablevariabilityinthesefactors.
Further,itissuggestedthattheexactholdingpowercanonlybedeterminedaftertheanchorisdeployedandtested.
Whenattemptingtopredicttheanchorholdingpowerforanewsite,itisrecommendedthatwhereverpossiblereferenceshouldbemadetoperformancedataforthegivenanchortypeinsimilarbottomsoilstothoseatthenewsite.
Graphsofholdingcapacitiesforseveraltypesofanchorinsandandmudsoilsaregiven,whichmaybeusedwhenmorespecificdataisnotavailable(Figures32,33reproducedfromAPIRP2P).
ThesegraphsarebasedonUSNavytestdataanditisnotedthatthedesignofsomeoftheanchorscoveredhasbeenmodifiedsincethetestswereconducted.
AcautionisalsogiventhatperformancedatafromothersourcescanvarysignificantlybothfromtheUSNavydataandbetweeneachother.
Theholdingcapacityoftheembeddedlengthofmooringlineadjacenttotheanchorisexpressedas:Holdingcapacity(Newtons)=f.
L.
wWherefisthecoefficientoffrictionbetweenthemooringlineandtheseabottomandL,ware,thelengthofmooringlineincontactwiththeseabed(m)andthesubmergedunitmassofthemooringline(kg/m),respectively.
ThecoefficientsoffrictionforchainandwiremooringlinesaergiveninTable8,asreproducedfromAPIRP2P.
Table8Coefficientoffrictionforchainandwirerope(fromAPIRP2P:Analysisofspreadmooringsystemforfloatingdrillingunits,1987)0.
250.
6Wirerope0.
71.
0ChainSlidingStartingThelengthsofthemooringlinesaretobesuchthattheyapproachtheseabedatatangenttothebottom,whentheplatformisatitsmaximumpredictedoffset.
32APIDraftRecommendedPracticeforDesign,AnalysisandMaintenanceofMooringforFloatingProductionSystemsRP2FP1,1991Withrespecttodraganchors,theprovisionsofthisdraftrecommendedpracticeareessentiallysimilartotheguidanceondraganchorspresentedinAPIRP2P.
ThefollowingmodificationsandadditionstotheprovisionsofRP2Pshouldbenoted:-Mooringdesignistobebasedon100yearreturnperiodenvironmentalcriteria,althoughscopeismadeformodifyingthereturnperiodwherethiscanbejustifiedbyariskanalysis.
ThegraphsofanchorcapacityinsandandmudincludedinAPIRP2Phavebeenrevised(Figures34and35asreproducedfromAPIRP2FP1).
Draganchorsaretobedesignedwithafactorofsafetyof1.
5forthemaximumstormloadandallmooringlinesintact,andafactorofsafetyof1.
0whenonelineisbroken.
APIRecommendedPracticeforIn-ServiceInspectionofMooringHardwareforFloatingDrillingUnits,RP2I,1987Proceduresfortheinspectionofmooringequipmentonfloatingdrillingunitsaredetailed.
ItisstatedthatsomeoftheseproceduresarealsoapplicabletothemooringsofotherfloatingstructuressuchasFloatingProductionSystems,pipelayingbargesandcranebarges.
Visualinspectionofanchorsisrecommended,directingparticularattentiontotheweldsandcorners,andtoareasofhighstress.
Visualinspectionofanchorshacklesissuggested,supplementedbyMagneticParticleInspection(MPI)oftheinsidebendregionandthecircumstancesoftheholesfortheshacklepin.
WhentheinspectionisconductedoffshoreitissuggestedthattheanchoriseitherhauledonboardanAHVorliftedontothedeckoftheplatformbycrane.
APIRecommendedPracticeforQualificationTestingofSteelAnchorDesignsforFloatingStructures,APIRP2M,1980Thisrecommendedpracticedetailsqualification(typeapproval)testsfordraganchors.
Thetestproceduresarestatedasbeingsuitableforconventionalanchorswithflukes,shank,stockandpadeye.
Qualificationtestingisonlynecessaryforoneanchorofeachdesign.
Thefirsttestconsistsoftheapplicationofaproofloadtotheanchorflukes.
Theproofload,sufficienttoproduceamomentatthepivotof184000Nm/tonnetimestheanchorweightintonnes,isappliedbypullingontheanchorshackle,withtheanchorflukessupportedateithertheirtipsorattwothirdsofthewayalongtheirlength(Figure36asreproducedfromAPIRP2M).
Anumberofstraingaugesaremountedatprescribedpositionsontheanchor.
Theanchorisdeemedtohavefailedtheprooftestifoneofthestraingaugesshowsastraininexcessof0.
001mm/mmbeforetheproofloadisreached,oriftheanchorexhibitsanypermanentdeformationorcrackingattheconclusionofthetest.
33Theanchorshackleandcrownpadeyearetestedbyapplyingatensionloadtotheanchorbymeansoftheshackleandpadeye.
Valuesforthistensionloadarespecifiedforanchorsof9.
07tonnesto13.
61tonnes.
Theanchorisdeemedtohavefailedthistestifanypermanentdeformationorcrackingisapparentattheconclusionofthetest.
Anchorstocks,whenfitted,aretestedbysupportingtheanchoratpointsathirdofthewayfromtheendofeacharmofthestock,whilstapplyingatensionloadtotheshackleoftheanchor.
Proofloadsarelistedforanchorsof9.
07tonnesto13.
61tonnes.
Thefinaltestconsistsofhammeringthesuspendedanchortocheckforinclusionsorvoidsinthemetal.
Aspartoftheinformationgatheringexercise,interviewswereheldwithseveralorganisationsincludingoperators,drillingcontractorsandcertifyingauthorities.
Anumberofcommentswerereceivedwithrespecttocodesandregulations.
Fromtheproceedingsections,itisnotedthatthecodesandregulationsgivedetailedmethodsandrulesformaterialandmechanicaltestingofdraganchors,andproceduresforclassificationofnon-standardanchors.
However,littleornoguidanceisavailableforanchorspecification/selectionproceduresforparticularsites,andforanchorhandlingactivities.
Thelackofguidanceinthisareawasconfirmedbythoseinterviewed.
Someofthoseinterviewedexpressedopinionstoleavethespecificationofanchortypestocertifyingauthoritiesandnotdefineanchorusageinguidelines,wheresitespecificassessmentswouldbeinappropriateanddifficulttoimplement.
Itwasconfirmedthroughmanyinterviewsthattherewerenoguidelinesforhandlinganchorsoffshore,andadrillingcontractornotedthatmuchreliancewasplacedonpreviousexperience.
344.
CURRENTDEPLOYMENT4.
1ANCHORSELECTION4.
1.
1UseofdraganchorsAllthoseinterviewedstatedthatMODUs,cranebarges,flotelsandpipelaybargesalmostinvariablyusedraganchors.
Draganchorsarechosenfortheseapplicationsinpreferencetoothertypesofanchorbecauseoftheirmobility(easeofdeployment/recovery).
Itwasalsoremarkedthatjackupsusedraganchorbasedtemporarymooringstoassisttheminpositioningcorrectly,preparatorytojackingupclosetoajacket,orwhenre-enteringanexistingwellhead.
FloatingProductionSystems(FPSs),SingleBuoyMoorings(SBMs)andstoragebuoysmaymakeuseofeitherdraganchorsorpileanchors.
Bottomconditionsarealwaysimportantindeterminingwhichischosen:pileswillbeusedifthesoilisconsideredtobetoosoftforlongtermmooringusingdraganchors.
Costconsiderationsareanotherfactorinfluencingthedecisionbetweendraganchorsandpiles.
Draganchorsweresaidtobemoreexpensivetobuythanpiles,butmuchcheapertoinstall.
ItwascommentedthattheforSBMsthechoicebetweendraganchorsandpilesdependsonthewaterdepthandconsiderationofthelengthoftheSBMistobeinplace:forshortterminstallationsdraganchorswouldalwaysbechosen.
Themooringconsultantsinterviewedstatedthattheywouldalsorecommendpileanchorsforanystructureintendedtobemooredinthesamepositionfor"prolongedperiods".
DeadweightanchorshavebeenusedforatleastonestoragebuoyintheNorthSea.
Thepersonsinterviewedwerenotawareofthereasonswhydeadweightanchorswerechoseninpreferencetodraganchorsforthisbuoy.
4.
1.
2ChoiceofdraganchortypeSeveralorganisationsstatedthatahighproportionofMODUshadbeenfittedwitharticulatedstockanchorswhenbuilt,andstillretainedtheseanchorsasnormalequipment(theLWTarticulatedstockanchorwasmentionedinparticularasbeingacommonfittingonUSbuiltMODUs).
However,althougharticulatedstockanchorsareverycommonequipmentonMODUs,boththeirholdingpowerandtheirdurabilitywerewidelycriticised(seebelow).
TheconsensuswasthatthemodernBruce,StevprisandFlipper-Deltatypesareallsatisfactoryanchors,withaperformancenormallywellinexcessofthearticulatedstockanchors.
WhenaMODUisengagedtoworkatagivensiteorinagivenarea,oneoftwodifferentapproachesmaybetakenindecidingwhattypeofdraganchorstheMODUwillusewhencarryingoutthework:-ThefirstapproachistousewhateveranchorsarecarriedontheMODU(typicallylowperformancearticulatedstocktypes)acceptingthatitmaybenecessarytopiggybackoneormoreanchorsontosomeorallofthemooringlinesinordertoachievetherequiredholdingpower.
ExtraanchorsforpiggybackingaretakentothenewworksiteonthedecksoftheAHVswhichare35toassistinmooringtheMODU.
Typically,atleastfourextraanchorswillbetakenoutspeculativelytoanynewworksite.
Whenthenewworksiteisknowntobeonbadholdingground,asmanyaseightextraanchorsmaybetakenout.
ThesecondapproachistodecidefromthestarttosubstitutehigherholdingpoweranchorsforthearticulatedstockanchorsfittedtotheMODU.
Highperformanceanchors,usuallyoftheStevpris,BruceorFlipper-Deltatypes,arethentakenouttothenewworksiteonthedecksoftheAHVsandattacheddirectlytothemooringlinesoftheMODU,inplaceofitsfittedanchors.
TheapproachadoptedisdependentontheoperatortowhomtheMODUiscontracted,andontheproximityoftheanchorpositionstopipelines.
ItwassuggestedthatseveraloperatorsarepreparedtoacceptthearticulatedstockanchorscarriedonthemajorityofMODUs,withthelikelihoodofhavingtopiggybacksomeorallofthem,possiblywithmorearticulatedstockanchors.
Further,itwasstatedthatlargeoperatorsaremuchlessinclinedtoacceptarticulatedstockanchorsandmayinsistonhighperformanceanchorstosubstitutetheanchorscarriedontheMODU.
Alternatively,theyaydecideontheprovisionofhighperformanceanchorsasbackupanchorsforpiggybacking.
Whichevercaseapplies,thesupplyofthenecessarypiggybackorhighperformanceanchorsisnormallytheresponsibilityoftheoperator.
Someoperatorsowntheirownstocksofanchorsforthispurpose.
Therequiredanchorsareotherwisehiredfrommarineequipmentsuppliers.
Thepurchase(orhire)priceoftheBruce/Stevpris/Flipper-Deltatypeanchorsisusuallyhigherthanthatofarticulatedstockanchors.
ItwascommentedbyonesourcethatthishigherpriceisusuallyoffsetbythereductioninMODUdowntimeduringachangeinworksite,becausethehighperformanceanchorsdonotnormallyneedtobepiggybacked.
Howeveranothersourceconsideredthatitwasalwayscheapertopiggybacklowperformanceanchors.
Piggybackingisusuallyavoidedwhentheanchorpointsareclosetopipelines.
Instead,atypicalactionwouldbetosubstitutelargehighperformanceanchorsforthelowperformanceanchorsfittedtotheMODU,forexample40tonneFlipper-Deltasmightbeusedtoreplace20tonneLWTs.
Withrespecttothecommentsreportedabove,itshouldbeunderstoodthatthetypeofanchorcarriedbyaMODUisnotusuallyanimportantfactortoanoperatorwhoisconsideringwhethertohiretheMODU.
AnumberofreasonswereadvancedastowhyarticulatedstockanchorshadbeenfittedasoriginalequipmentonmanyMODUs,despitethewidespreaddissatisfactionwiththistypeofanchor.
ThemostfrequentlymentionedwasthelowerpurchasecostofarticulatedstockanchorscomparedtomoremoderntypessuchastheBruce,StevprisorFlipper-Deltaanchors.
WithreferencetoUSbuiltMODUs,itwassuggestedthatarticulatedstockanchorshadprovedtobesatisfactoryforGulfofMexicoconditions.
ThereisanextremereluctancetocompletelyreplacetheanchorsequippingaMODUbyhigherperformanceanchors,becauseoftheveryhighcostsinvolved.
Inadditiontotheactualcostofthereplacementanchors,thefittingofanewtypeofanchoris36likelytorequiretheanchorrackingarrangementstoberevised,ataskneedingdockyardfacilities.
BruceanchorsarecommonlyusedforSBMs,althoughrecourseisalsomadetootheranchortypes.
Thechoiceofanchortypeisdependentonpriceandavailability,butisinfluencedbyageneralpracticeofoverdesigningtheanchorsforanSBM.
AnchorsforFPSswilltypicallybehighperformancetypes,becauseofthecostimplicationsifthereisaproblemwithobtainingtherequiredholdingcapacityatinstallationtime.
4.
1.
3SizingofanchorsTheOperatorsManualforaMODUincludesdetailsoftheapprovedoperatingenvelopeforthatMODU.
Beforemovingtoanewsiteitistheresponsibilityofthedrillingcontractor(MODUowner)toensurethattheconditionsatthenewsitefallwithintheoperatingenvelope.
Thiswillbeconfirmedbyawarrantysurveyor,whowillgivelocationapprovalforthenewsite.
Theoperatorisresponsibleforprovidingthedrillingcontractorwithsufficientsiteinformationtoenablethedrillingcontractortoobtainlocationapprovalfromthewarrantysurveyor.
Thewarrantysurveyorshouldusethesiteinformation,particularlythebottomsurveydata,todecidewhatmaximumpost-installationtesttensionisrequiredforeachanchor.
Howeveronesourcesuggestedthatitisquestionablehowmuchattentionispaidtothebottomsurveydata,anfeltthatthereisatendencytosimplyaskforahighpost-installationtestload.
Theprecedingcommentsonthepost-installationtestloadshouldbeconsideredinconjunctionwithSection4.
4below.
Aquasi-staticmooringanalysisisusuallyconsideredtobeadequateforMODUs.
WhenaMODUistoworkatseveralsitesinthesamegeneralarea,themooringanalysisisbasedontheassumptionofastandardmooringpatternand50yearstormconditionsforthegeneralareainwhichtheMODUistodrill.
Inasite-specificanalysis,separateanalysesarecarriedoutforeachworksite,usingtheconditionspeculiartothatparticularsite.
Whetherarea-specificorsite-specificanalysesarecarriedoutwhentheMODUistoworkatunobstructedsites,isdependentontheoperatorandonthedrillingcontractor.
Asite-specificmooringanalysisisalwayscarriedoutforaMODUwhichistomoorinasensitiveposition,suchasclosetoajacketoroverpipelines.
ItwasstatedthatsomeoperatorstakeanactiveinterestinensuringthatthemooringperformanceofaMODUisadequatefortherequiredpurpose,whereasotheroperatorsconsiderthatitisthetaskofthedrillingcontractortoensurethatthemooringsystemwillbeadequate.
Thelackofindependent,reliableanchorholdingefficiencydatawaswidelyremarkedon.
Itwasnotedthattheavailabledataislimitedtoasmallamountofpublisheddata,themostimportantpartofwhichcomesfromUSNavytests(NCEL[1987]),andtothemanufacturersowntestdata.
Muchofthepublisheddataisquiteold,andconsequentlyrelatestoarticulatedstockanchorsortohigherholdingpoweranchorsofdesignswhichhavenowbeensuperseded.
Anumberoffurtherproblemswiththe37publisheddatawerehighlightedbyoneoperator(theseobservationsfollowarecentreviewofthedatabaseofpublishedtestinformationundertakenbythisoperator):-Almostallpublishedtestdatahavebeenobtainedusinganchorsofverymuchlessermassthantheanchorsusedoffshore.
Thisisdonetoreducetestcosts:minimisingthesizeofthetestanchorreducesthewinchcapacityrequiredtopulltheanchortofailure.
Thefewlargeanchorstestedhavenotbeenpulledtofailure.
Similarly,mostpublishedanchortestshavebeendoneinverysoftsoils,becausethisminimisesthewinchcapacityrequiredtopulltheanchortofailure,andtherebyreducesthecostofthetest.
ThesetestsareinappropriatetothegreaterpartoftheNorthSea,becauseonlytheWitchground,whichconstitutesjust10to15%oftheareaofthenorthernNorthSea,consistsofverysofttosoftclay.
TheremainderofthenorthernNorthSeaisstifftoverystiffclay,with,insomeareas,anoverlyinglayerofsand.
TheseabedinthesouthernNorthSeaissand.
Anumberoftestsofanchorsinsandhavebeendonebypullingatestanchoralongpartiallysaturatedsandonabeach.
Thisgivesanerroneousresultbecausethedilationeffectsaredifferentwhenthesandisfullysaturated.
Theholdingcapacityisactuallylikelytobegreaterwhenthesandisfullysaturated,becausethesanddilationforcescanthenbefullymobilised.
Dataonthesoilsinwhichanchortestswereconductedisgenerallyverybadlyreportedinthetestliterature.
Inmanyinstancesinappropriatetestswereusedtoquantifythesoilproperties.
Theoperatorfurthersuggestedthatasaminimum,thefollowingdatawasrequiredtoquantifythesoilinwhichananchortestisconducted:-forclaysshearstrengthvariationwithdepthforsandsgrainsizeanalysisangleoffrictionfromshearboxtestsconepenetrationtestsTheoperatorcommentedthattopredicttheholdingcapacityofa30tonneanchorinstiffnorthernNorthSeaclay,fromthepublishedtestdataavailableatthetimeoftheirreview,wouldhaveinvolvedatenfoldextrapolationwithrespecttomass,andasevenfoldextrapolationwithrespecttosoilstiffness.
Severaloftheorganisationsinterviewedstatedthattheymadeuseofthemanufacturer'sdatatosizeanchors.
Howeverthemajorityoftheseorganisationsexpressedalackofconfidenceinthisdata.
Thefollowingwerementionedasreasonableworkingestimatesofholdingefficiency,8forarticulatedstockanchorsand15forhighperformanceanchors.
Itwassuggestedthattheactualvalueforhighperformanceanchorsisinexcessof20.
Easeofhandlingwasalsomentionedasaconsiderationwhensizinganchors.
ThusoriginalequipmentMODUarticulatedstockanchorswilltypicallybechosentohavemassesintherange30,000lbs(13.
61tonnes)to40,000lbs(18.
14tonnes),becauseanchorswhicharebiggerthanthisbecomedifficulttohandleondeck.
38OneoperatorindicatedthattheirusualapproachtoimprovingtheanchorholdingpowerofaMODUfittedwitharticulatedstockanchors,whichwillbeofasizealreadyapprovedbyacertifyingauthority,istosubstitutehigherperformanceanchorsofthesamemassasthearticulatedstockanchors.
TheCertifyingAuthorityisthenapproachedtogetareaapprovalforthenewanchors.
Flotelsandtender-assistunitswillgenerallybemooredclosetojacketsandasaconsequencewillhavebeenthesubjectofasite-specificmooringanalysis.
Theirmooringsarearrangedsothatbyhaulinginandpayingoutappropriatelines,theycanmoveawayfromthejacketwhenenvironmentalconditionsdemands.
Thestand-offdistancebetweenjacketandsemisubmersiblewhichcanbeachievedinthiswaywilltypicallybe600feet(180metres)forflotelsandsimilarunits,and1000feet(305metres)frocranevesselsandotherlargesemisubmersibles.
Jackupsaretypicallyfittedwithrelativelylowperformanceanchors.
Whenajackupisbeinginstalled,tugsusuallyprovidebackuptothejackup'sownmoorings.
However,despitethis,thelowperformanceanchorsonjackupsarefrequentlysubstitutedbyhigherholdingpowertypes,particularlywhenthejackupistobepositionedclosetoajacket.
Furthertothis,oneoperatormentionedthattheyarekeentoencouragejackupownerstoimprovethemooringequipmentonjackups.
AninterviewedclassificationsocietystatedthattheywouldusuallyrequiretheloadsonaSBMtobedeterminedbymodeltests.
ThemooringanalysisforanFPSwillalmostinvariablybedynamic.
InrespectoftheanchorperformancedatausedtodesignFPSmoorings,theClassificationSocietysuggestedthatthebestsolutionwouldbetotestaslightlysubsizeanchorofthechosentypeontheactualsite.
HowevertheycommentedthatitwasveryrarethatthislevelofinformationwasforthcomingfromtheoperatorseekingcertificationoftheFPS,andthatoftennotevenperformancedatafromanequivalentsiteismadeavailable.
4.
2PRE-INSTALLATIONBOTTOMSURVEYThegeneralpracticeistocarryoutabottomsurveyforeachnewsiteatwhichaMODUistowork.
Theinformationgatheredwilltypicallyincludeseabedsoiltype,waterdepthcontourlines,andanyobstructionsordebrisontheseabed.
Oneoperatoralsomentionedmakingchecksforshallowgasaspartofthebottomsurvey.
Thebottomsurveyistheresponsibilityoftheoperator,andisoftencarriedoutinconjunctionwithgeotechnicalsurveysofthearea.
Insomeinstances,seabedinformationisextractedfrompreviousgeotechnicalsurveys.
Seabeddataarealsoexchangedbetweenoperators.
Itwassuggestedbyoneintervieweethatthediligencewithwhichtheseabedsurveyiscarriedoutvariesfromoperatortooperator.
Theinformationfromtheseabedsurveyshouldbeanimportantinputtoanchorselectionandsizingactivitiesdiscussedintheprecedingsection.
Inaddition,thefindingsofthesurveymayprompttheflukeangleoftheanchorstobechanged,orthemooringpatterntobemodified.
Severaltypesofanchormakeprovisionfortheflukeangletobealteredbytheuser,dependingonthetypeofsoilinwhichtheanchoristobeused.
Howeverthe39majorityoftheorganisationsinterviewedstatedthattheyrarelyorneveraltertheflukeangle,preferringtoleavetheangleatageneralcompromisesetting.
Itwassuggestedthattheseabedconditionsareoftenlocallyvery"patchy",andsoitisnotstraightforwardtodeterminethetypeofsoilananchorwillenter.
Themooringpatternwillbemodified,bychangingthelengthordirectionofcertainmooringlines,inordertoavoidobstructionsorseabeddebris,andsometimestoavoidbadholdinggroundThecertifyingauthoritystatedthatforanFPSitwouldexpectthepre-installationseabedsurveytoinclude,asaminimum,severalboreholesandasidescansonarsurvey.
4.
3INSTALLATIONMETHOD4.
3.
1GeneralMODUsandflotelsrequiretheassistanceofAHVstoinstalltheirmooringspread.
AtypicaloperationsequenceisshowninFigure25.
TosavetimesomeMODUswilllaythefirstanchoroftheirmooringspreadthemselves.
Somesupportvesselsareabletolaytheircompletemooringspreadwithoutassistance,bymanoeuvringontheirthrusters.
Theinterviewedmarinecontractorscommentedthattoselflayananchor,theanchorshouldbeloweredwiththeanchorwinchunderpower.
Theystatedthatsimplyreleasingthebrakeonthewinchandlettingtheanchordropdoesnotimprovethelikelihoodoftheanchorbiting,asispopularlybelieved.
Moreover,simplyreleasingtheanchorinthismannerislikelytoresultincoilsoftheanchorlinelandingontopoftheanchorandbecomingentangledwithit,andcanalsoresultinmechanicaldamagetotheanchor(particularlytothestockofarticulatedstockanchors).
TheyalsosuggestedthatananchorlaidbyanAHVismuchmorelikelytoembedsatisfactorilythanaselflaidanchor.
Thisisbecausetheorientationoftheanchorwhenitreachestheseabed,willbebetter.
AnchorslaidwiththeassistanceofAHVsarelaidusingeitheraringchaserorapendantwiresystem.
Whicheversystemisused,eachanchoristakenouttoitsdesignatedpositionbytheAHV.
Theanchoristhenloweredtotheseabedbyeitherthewireattachedtotheringchaser,orthependantwire.
Ifaringchaserisbeingused,itisthentakenbacktotherigbytheAHV,whichpullstheringbackalongthemooringlinebythelineattachedtoit.
Ifapendantwireisbeingused,thefreeendofthewireisattachedtoabuoy,andleftinposition.
ChainmooredMODUsoperatingintheNorthSeaaremoreoftenequippedwithpermanentringchasersthanpendantwires.
Itwassuggestedthatringchasersareliabletocausedamagetowiremooringlines,andapendantwiresystemisthereforemoresuitableforwiremooredMODU's.
Ifnoproblemsareencountered,afullMODUanchorspreadcanberuninsixhours.
However,ifproblemswithachievingadequateholdingcapacityareencountereditmaytakedaystofullyinstalltheanchorspread.
40ThreeAHVswillnormallybeusedtoinstallthemooringsofaMODUorflotelFourAHVsareusedforsomeflotelswhichhavecombination(chain/wire)moorings.
Forcertainself-poweredMODUs,twoAHVsareconsideredtobesufficient,butthisisunusual.
TheAHVsusedforMODUmovesrangeinsizefromaround8500HP(100tonnebollardpull)toaround12,000HP(160tonnebollardpull).
ThebiggerAHVshavetwoadvantages.
FirstlytheyareabletopulloutagreaterlengthofchainfromtheMODU;asmallerAHVmaybeunabletoovercometheresistanceofthelengthofchainontheseabed,whentheanchorlinesarelong.
Secondly,biggerAHVsaremorestable,makinganchorhandlingondeckeasier.
SmallerAHVsareconsideredadequateforthesmallmooringsystemsofjackups.
TheanchorsforSBMsaretypicallyloweredintopositionbyAHVs,usingpendantwiresontheanchors,beforetheSBMisinstalled.
Aftertheanchorhasbeenplacedontheseabed,thependantwiresareremovedbydivers.
4.
3.
2PiggybackingMODUsrelyveryheavilyonthepossibilityofpiggybackinganchors.
Theanchorsusedforpiggybackingaretypicallyarticulatedstocktypes.
ItwasnotedthatthelargemarketinpiggybackanchorrentalimpliesthatmanyMODUsdonothaveadequateanchors.
Piggybackinganchorsisatimeconsumingprocedure.
Forthisreasonpiggybackingisusuallyonlyresortedtoafterithasbecomeclearthatthemainanchorwillnotbecapableofholdingthepost-installationtesttensionwithoutassistance.
Thefollowingwassuggestedasausefulruleofthumb.
Ifananchordrags500feet(150metres)duringthepost-installationtensioningwithoutachievingtherequiredholdingcapacity,thentheanchorisrerun.
Iftheanchorstillfailstoholdafterhaulingin500feet(150metres)thenapiggybackanchorisadded.
Ifthecombinedholdingpowerofthemainandpiggybackanchorsisstillinsufficienttoresistthepost-installationtesttension,asecondpiggybackanchorisadded.
Onsomeoccasionsevenmorepiggybackanchorsmaybeadded(seeSection5).
Thelengthofthelinebetweenthemainandpiggybackanchorsischosentobesufficienttoallowthepiggybackanchortobeworkedon,onthedeckoftheAHV,withoutdisturbingthemainanchor.
Typicalvaluesusedforthelengthofthislineincludethewaterdepthplus50m,andthewaterdepthplus20%.
Buoyedpendantwireshavebeenusedonallpiggybackanchors,asitisnotfeasibletouseringchasers.
Itwascommentedthatwhilstpiggybackingworks,inthatitgenerallyincreasesthetotalholdingpower,itisnotclearhowitworks,astheadditionofapiggybackanchorisbelievedtoreducetheholdingpowerofthemainanchor,bypreventingitfromembeddingproperly.
41ItwasalsocommentedthatFlipper-Deltatypeanchorsshouldnotbepiggybacked,becausethepadeyeforpiggybackingissituatedontheheadoftheanchor,andtheloadofthepiggybackanchorwillpulltheflukesclosed.
4.
4POST-INSTALLATIONHOLDINGCAPACITYTESTSAfteradraganchormooringsystemhasbeeninstalled,themooringlinesareusuallysubjecttoatensiontestload,toembedtheanchorsandtesttheirholdingcapacity.
Thistestisnormallywitnessesbytheoperator'srepresentativeandonoccasion,bythewarrantysurveyors.
OnMODUs,flotelsandsimilarvesselsthetesttensionisappliedbytensioningdiagonallyoppositemooringlinesagainsteachother.
Occasionally,ahighertesttensionisappliedtoacriticalanchorbytensioningitagainsttwoorthreeoftheotheranchorsofthevessel'smooringsystem.
Totensionpre-installedmoorings,suchasthoseforanSBM,specialtensioningequipmentisavailablefromsomeanchormanufacturers.
ForMODUsthepost-installationtensiontestisstandardpractice.
However,therearevariationsbothinthewaythistensionisapplied,andthebasisforthemagnitudeoftheprooftension.
Thesefactorsaredependentonwhichoperator/drillingcontractorisinvolved.
OnsomeMODUstheprooftensionisappliedimmediatelyaftertheanchorshavebeeninstalled.
Onothers,theprooftensionisappliedonlyaftertheMODUhasballasteddown.
Therearetwopointstonoteinrespectofthisdifferenceinprocedure.
Firstly,ballastingdowntakesapproximatelysixhours,duringwhichtimetheanchorswill"soak-in"tosomeextent.
Secondly,ifprooftensioningiscarriedoutbeforetheMODUisballasteddown,anyanchorwhichfailstoholdcanbebroughtbacktotheMODUforinspection.
Athirdapproachtoapplyingtheprooftensionistodeliberately"soak-in"theanchorsforfourtosixhourswithsometensionon,beforeapplyingthefulltesttension.
Theprooftensionisgenerallyappliedforaperiodoffifteenminutes.
Variousbasesforthemagnitudeoftheprooftensionwerequotedbytheorganisationsinterviewedincluding,forchainmoorings:thestallloadoftheanchorwinches,statedtobeintherange350to400kips(1555to1780kN)ThefiguredefinedintheoperatingmanualfortheMODU,usuallyaround350kips(1555kN)33%ofthebreakingloadofthemooringsystemandforwiremoorings:33%ofthebreakingloadofthemooringwire(380kips)(1690kN).
Thecertifyingauthoritycommentedthatthebasisforthetensiontestloadisavexedquestion,andthatthetensiontestloadisneverashighasthedesignultimateload.
Consequently,asthe100yearstormconditionsareapproached,itisnevercertainhowtheanchorswillperform.
Thespecificationofatensiontestloadwhichislessthanthedesignultimateloadisdefendedontwocounts.
Firstly"soak-in"will42improvetheperformanceoftheanchorssubsequenttothetensiontest.
Secondly,thereisabeliefthatanchorsarebetterabletowithstandoccasionalhighloadsthansteadytensions.
Thecertifyingauthoritystatedthattheywouldlikeanchorstobetestedtothe100yearstormload.
However,theypointedoutthatdifficultiescouldbeexperiencedinattemptingtoimposesuchhightesttensions.
(AMODUwasdamagedbyanattempttoimposesuchatensiontestloadwhichwas50%ofthebreakingloadofitsmooringlines).
4.
5RECOVERYPROCEDUREMODUsandflotelsrequiretheassistanceofAHVstorecovertheiranchors.
Therecoveryproceduredependsonwhethertherigisequippedwithringchasersorbuoyedpendantwires.
Torecoverananchorusingaringchaser,thelineattachedtothechaserispassedfromtherigtotheAHV.
TheAHVthenpullsthechaseralongthemooringlineouttotheanchor.
Whenthechaserreachestheanchor,theAHVliftstheanchorbythewireattachedtothechaser.
Therigcanthenhaulinthemooringline.
Torecoverananchorfittedwithabuoyedpendantwire,theAHVpicksupthebuoy,andliftstheanchorbypullingonthependantwire.
Therigthenhaulsinthemooringline.
Evenwhenitisfittedwithringchasers,aMODUorflotelwillnormallyresorttousingbuoyedpendantwiresifithastomoornearoroverpipelines.
Thisisdonebecausewhenusingaringchasertorecoverananchor,thereisalwaysthepossibilityofpickingupabightofchainwiththechaser,andtherebydraggingtheanchoroverapipeline.
UsinglargerAHVsanchorrecoverycanbeachievedmorerapidly.
TakeninconjunctionwiththeadvantagesoflargerAHVsforanchorinstallation,thereisamoveintheindustrytowardsemployinglargerAHVsforrelocationofMODUs.
TheanchorsofanSBMaretypicallyrecoveredbydetaching,inturn,eachoneoftheSBM'smooringchains,andpassingtheendtoanAHV.
TheAHVthenretrievestheanchorbyhaulingonthemooringchain.
4.
6IN-POSITIONINSPECTIONPROCEDURESTheinterviewedcertifyingauthoritystatedthattheywouldbestronglyinfavourofROVsurveysbeingconductedonnewlyinstalleddraganchormooringsystemsasageneralpractice.
Theynowinsistonpost-installationinspection,byROVordiver,oftheanchorsofanyFPSwhichtheyareaskedtocertify.
Thereasonfortheirinsistenceonpost-installationinspectionforFPSsisthatrecentlytheyhavebeeninvolvedintwoorthreeinstallationsinwhichtheanchorsfailedtoembedcompletelyinsandseafloors,andwerethereforevulnerabletosubsequentscour.
ForapplicationsotherthanFPSs,in-positioninspectionismostusuallylimitedtomonitoringthetensionsinthemooringlinesofthevesselsorstructure.
43Thegeneralopinionwasthattheaccuratemonitoringofanchorlineloadswasimportant.
Theprimaryreasoncitedforitssignificancewastheneedtobalanceloadsbetweenmooringlinesduringstorms.
TwooftheoperatorsinterviewedconsideredthatthetensionmonitoringequipmentonMODUsismostusuallyofverylowaccuracy.
Reinforcingthisview,oneofthedrillingcontractorsinterviewedsaidthattheyhadputconsiderableeffortintoimprovingthetensionmonitoringsystemsoriginallyfittedtotheirMODUs.
AthirdoperatorwasoftheopinionthatthechaintensiongaugesonMODUsareaccuratetowithin10%.
Thisoperatorchecksthecalibrationofthechainloadgaugesagainstwinchmotorpowerduringthepost-installationtensioningofanchors.
4.
7INSPECTIONANDREPAIRBETWEENDEPLOYMENTSAllcommentsregardingtheinspectionoftheanchorsofMODUsbetweendeploymentswereinaccord.
Inspectionisvisual,andiscarriedoutonthedeckoftheAHVbythecrewoftheAHV.
Inspectioniscarriedoutbothwhentheanchorsaredeployedandwhentheyarerecovered.
SomevisualinspectionisalsoperformedfromthedeckoftheMODU,usingfieldglasses,whentheanchorshavebeenrecoveredontotheirracks.
TheinterviewedmarinecontractorstatedthattheiranchorswerealwayssubjectedtoaClassificationSocietycheckbeforere-use.
ItshouldbenotedthatthisintervieweeusuallydeploysanchorsforlongerperiodsthanwouldbethecaseforatypicalMODU.
Oneoperatorreportedcarryingoutregularthreeyearlyinspectionsoftheirstockofbackupanchors.
Intheseinspections,visualinspectionoftheanchorissupplementedbyNDTinspectionoftheanchorshackles.
Belowarepresentedcommentsregardingwearandteardamage,asmightbediscoveredbyroutineinspectionbetweendeployments.
Dataregardingdamagecausedbyhandling,anddamagesufficienttocausefailureoftheanchor,areincludedinSections5and7respectively.
Thecommentsregardingwearandteardamageweremixed:-Onedrillingcontractorandoneoperatorstatedthatdamagetoanchorswasrare,damagetothemooringsystembeingchieflyconfinedtothechainsandwires.
Twooperatorsandthemarinecontractorreportedoccasionaldamage,mostusuallytotheflukesandstocksofarticulatedstocktypeanchors.
Problemswhichcouldbedirectlyascribedtowearwereonlyreportedbytwooftheinterviewees.
Theproblemswereheavywearoftheblockslimitingmaximumflukeangle,ovalisationoftheholeintheshankfortheanchorshackle,wearoftheanchorshacklepin,anddistortionofstocksandflukes.
Singleinstancesofbentshanksonanarticulatedstockanchorandononeofthehigherperformanceanchortypeswerealsoreported.
Itwassuggestedthattheproblemofwearandteardamagewasworstforpipelaybarges,onwhichtheanchorsaremovedveryfrequently.
44Damageattheweldsoffabricatedanchorsisoccasionallyencountered.
Thisdamagewasascribedtodroppingtheanchorfromtoogreataheightabovetheseafloorinexcessof50feet(15metres).
Theinterviewedmarinecontractorstatedifananchorisrepairable,therepairissubcontractedtoaspecialisedengineeringrepaircompany.
TheClassificationSocietyisthenaskedtore-certifytheanchor.
Themarineequipmentsupplierconsidereddamagetoanchorstobefairlyunusual,andusuallytobeconfinedtobentanchorflukes.
Thesetendtohappenmostoftenwhenanchoringinboulderclay.
Itwasstatedthattorepairbentflukesonanchorsisunusual,andtheanchorshouldforpreferencebereplaced.
Alternatively,forsometypesofanchor,newflukescanbeobtainedfromtheanchormanufacturer.
TheystatedthattheyarereluctanttorepairanchorsandwillonlydosotoCertifyingAuthorityapproval.
Theysuggestedthatinthepast,someanchorusershavetriedtocarryouttheirownrepairstoanchors,tosavemoney.
Theythinkthatsuchrepairsareprobablynolongerundertaken.
ThesecommentsshouldbereadinconjunctionwithSection7.
Oneoftheoperatorsindicatedthatanyoftheirstockofbackupanchorswhichwasseverelydamagedwasscrapped.
Twelveanchorshadbeenscrappedinthelasttwoyears,mainlyofthearticulatedstocktype.
4.
8CORROSIONPROTECTIONAllpartiesinterviewedagreedthatcorrosionwasnotanissue.
Themarineequipmentsuppliercommentedthatanchorsgenerallylastalongtime,unlessmechanicallydamaged.
Bywayofexampletheycitedastocklessship'sanchorintheirpossessionwhichalthoughadmittedlynowlittleused,isstillingoodconditiondespitebeingatleast46yearsold.
Typicalpracticeistooccasionallyshotblastandrepaintanchors.
Onships,spareanchorsareoftensimplyrepaintedovertheoldpaint,withnopriorcleaningdown.
Themarineequipmentsuppliercommentedthatasaresult,suchanchorsfrequentlyhaveanextremelythickcoatingofpaint.
455.
ANCHORHANDLINGDIFFICULTIES5.
1MECHANICALDAMAGESeveralcommentsweremadeconcerningmechanicaldamagetoanchorsduringinstallationandrecovery.
Inthemain,thesecommentsweredirectedtowardsarticulatedstockanchors,thestocksofwhichwerestatedtobeeasilybent.
Occasionalinstancesofflukesbeingbent,particularlyinboulderclay,werealsoremarkedon.
Oneoperatorsaidthattheyexperiencedoneortwoinstancesayearofanchorsbeingdamagedduringinstallation.
Articulatedstockanchorswerealsosaidtobepronetofallingapartwhenhitbyringchasersduringtheanchorrecoveryoperation.
5.
2HANDLINGONDECKArticulatedstockanchorsaregenerallyfoundtobesimpletohandleontheAHVsdeck.
Somedifficultieswerenotedwithhandlingcertainofthehigherperformanceanchors.
OnetypehasbeenfoundtobedifficulttopullontothedeckofanAHV.
Anothertypepresentsproblemsontwocounts.
Firstly,becausethisanchorstandsonitsfluke,theshankisataconsiderableheightabovethedeckoftheAHV,whichmakestheattachmentofthemooringchainadifficultoperation.
Thisproblemisoftenovercomebyleavingatailofchainattachedtotheanchor.
Secondly,onolderpatternsofthistypeofanchor,theshankisattachedtotheflukebyfourbolts,whichmustberemovedtoaltertheflukeangle.
Thisoperationwasstatedtobeveryawkwardandrequiredperfectweatherconditions.
Itwasfurtherstatedtobemucheasiertochangetheflukeangleonanchorsforwhichtheadjustmentismadebycuttingoutorweldinginpackingblocks.
OperationswhichrequiretheAHVcrewtomovearoundananchoronthedeckoftheAHVgenerallyrepresentasafetyproblem,becauseofthepossibilityoftheanchormoving.
Duringcertainstagesoftheanchorlayingandrecoveryprocedures,theAHVcrewmustbeondecktohandlethewire.
HoweverthereshouldbenoneedforthecrewtoleavetheprotectionofthesafetybarriersonthedeckoftheAHVduringthestagesofthelayingandrecoveryprocedureswhenthewireisunderhightension.
5.
3INSTALLATIONANDRECOVERYVeryfrequentlywheninstallingamooringsystem,itisfoundthatoneortwooftheanchorsdragwhenthetesttensionisfirstapplied.
Althoughthismayreflectpoorholdingconditions,itisalsostronglydependentontheskillofthemasteroftheAHV:Hemaynotplacetheanchorontheseabedcorrectly.
Hemayapplytoomuchtensiontothechaserwire(whenrecoveringthechaser)andthuslifttheanchor.
Hemaysnagthechaserontheanchororthechain.
Thenwhentryingtoreturnthechasertotherighewilldragtheanchorbacktowardstherig.
46Hemayletthechaserwiregotooslack.
Theringchaserwillthensitonthechainattooobliqueanangle,whichislikelytomakeitjam.
Theanchorwillthenbedraggedbacktowardstherig.
Whentheholdingconditionsarepoor,itisoftenfoundthattheprooftensioncanbeapproachedwhentheanchorisfirsttensioned,butthatthetensioncannotbeheld.
Insteadthetensiongraduallyfallsback.
Typicalprocedure,ifananchorfailstoholdonfirstapplyingthetesttension,isto:-(i)re-installtheanchorIfthisfails,(ii)re-installatadifferentbearingfromtherig,Iftheanchorstillfailstohold,(iii)piggybacktheanchor.
AlmostinvariablyitisarticulatedstockanchorsratherthanthehigherperformanceBruce,StevprisandFlipper-Deltatypeswhichmustbepiggybacked.
Thefrequencywithwhichpiggybackingisfoundtobenecessaryishigh.
OneoperatorhadpiggybackanchorsontenoutoftwelveMODUsoffshoreatthetimeoftheinterview,andconsideredthatonatypicalMODUmove,onetofiveanchorswouldhavetobere-installedanduptofourpiggybackanchorswouldbeneeded.
Anotheroperatorestimatedthat20%ofallanchorssetrequirepiggybacking.
Itisnotunusualformorethanonepiggybackanchortobeputonanindividualmooringlineinordertoachievetherequiredprooftension.
ItisalsonotuncommonforseveralofaMODU'smooringlinestobepiggybacked.
ForoneMODU,operatingonanareaofhardclayoverlaidwithhighlyplasticclay,itwasfoundnecessarytoputfivepiggybackanchorsonasinglemooringline(inadditiontothemainanchor)inordertoachievetherequiredtesttension.
AnotherMODUoperatinginpoorholdingconditionsneededatotalofthirtyeightanchors(mainanchorsplugpiggybackanchors).
Piggybackedanchorsystemsareawkwardtoinstallandrecover.
Oneproblemmentionedwasthattensioningthepiggybackedmooringlinecancauseothermooringlinestodrag.
Theseinturnthenhavetoberesetanditmaytakedaystofullyinstallthecompletemooringsystem.
Itwassuggestedthatdifficultiescanbeexperiencedinrecoveringanytypeofanchor,andthatperseveranceisrequired.
Problemswhichmayoccurinclude:-thechaserwiremaybreak,achainlinkmaybreak,andtherebylosetheanchor,asaresultofaccidentalapplicationofabendingloadonthelinkwiththechaser,theringchasermaysnagontheshacklesattheendofthechain,andthereforefailtoslideontotheanchorshank.
47Particulardifficultieshavebeenexperiencedinrecoveringthehigherperformanceanchors.
Onsomeoccasionspullsof100tonneshavebeenneededtobreak-outtheseanchortypes.
Certainofthefixedflukedesignsofhigherperformanceanchorwerementionedasbeingthemostdifficulttorecover.
Twosolutionsweresuggestedforrecoveringoneofthesefixedflukeanchortypes.
Thefirstwastoapplyasteadytensiononthechaserwireforanhour.
Thesecondwastoensurethatthechaserispulledrightdowntotheangleintheshank,andthenusealongchaserwiretopullatalargeangletotheanchor.
Mooringchainscansometimesbeverydifficulttopullout,particularlyincertainblocksintheNorthSeawheretheseabedcomprisesisverystickymud.
Intheseareas,350kip(1555kN)tensionshaveonoccasionbeenrequiredjusttohaulinchain.
Theembeddingoflonglengthsofmooringlinesalterstheresponseofrigsoperatingintheseareas,requiringringchaserstobesentdownthemooringlineseverytwoweekstofreethechains.
Mushroomanchorsandsingleflukestockanchorswerestatedtobeverydifficulttouse.
486.
EFFECTOFANCHORDRAGGINGONSUBSEQUENTHOLDINGPOWERTheoccurrenceofanchordraggingwasreportedbyseveraloftheinterviewees.
Theseverityoftheanchordragging,anditseffectonthesubsequentholdingpoweroftheanchor,differedfromreporttoreport.
Therewasalsosomedivergenceinthefrequencywithwhichanchordraggingwasreportedtooccur.
ThecommentsmadebyintervieweesweredirectedtowardsMODUsandflotelsOneintervieweestatedthatduringstormsanchorsdragshortdistances(oftheorderofafewmetres).
Mooringlinetensionsarerestoredafterthestormbyhaulinginashortlengthofthemooringlines.
OtherintervieweesreportedthatoninfrequentoccasionsthetensioninoneofaMODU'smooringlineswilldripmarkedlyduringorimmediatelyaftersevereweather.
Thetensionmayforexamplefallfromaround130tonnestoaround90tonnes.
Anotherorganisationcommentedthatsuchincidentsareveryrareandthattensioncanusuallyberecoveredbyhaulingintheslackmooringline(typicallyaround100feet(30metres)oflinemustbehauledin).
However,yetanotheroftheorganisationsinterviewedstatedthatonceortwiceayear,withinafleetofnineortenMODUs,alossoflinetensionoccurswhichcannotberecoveredbyhaulinginonthemooringline.
Theanchormustthenbehauledbacktotherigandreinstalled.
Itwasclaimedbyanotherorganisationthatonceananchorstartstodrag,itwillcontinuetodoso.
AcommentregardingthefrequencyofanchordraggingincidentswasthatitisoccasionallynecessarytoretensionthemooringlinesofaMODU,subsequenttopost-installationtensioning,butthatthereisalessthan10%probabilityofhavingtodothisduringtheperiodofanyparticulardeployment.
ConcerninganincidentinwhichaMODUlostsomeofitsmooringlinesandwasdrivenoffstationbytheseverestormofDecember1990,itwasstatedthattheMODU'sanchorshaddragged100feet(30metres)undera1000kips(4450kN)tension.
WhentheMODUreturnedtostation,thedraggedanchorswerefoundtobestillcapableofsustainingthepost-installationtesttension.
Dataonanchordraggingincidentssufficientlyserioustocompromisestation-keepingisdiscussedfurtherinSection7.
497.
ANCHORFAILURESArticulatedstocktypeanchorsarepronetocomingapartbecauseoffailureofthestock.
Thestockfrequentlybecomesbentontheseanchors.
Thismayinitselfbesufficienttocausetheanchortofail.
Ifitisnot,straighteningofthestockcanmakeitbrittleandlikelytosnapatalaterdate.
Theproblemisexacerbatedonmanyarticulatedstockanchorsbythewayinwhichthestockislocatedintheheadoftheanchor.
Afixedringonthestocksitsatonesideofthehead.
Alooseretainingwasherisplacedattheotherside,andheldinplacebyablockweldedtothestock.
Thestockmayberemovedinorderforittobestraightened,foranewstocktobesubstituted,ortoallowabentfluketobestraightened.
If,whenthestockisreplaced,theblockisnotweldedonadequately,theblockcanbeknockedoffiftheanchorfallsontheendofthestock.
Theanchoristhenlikelytofallapart.
Iftheweldisextendedcompletelyroundthewasher,thestockismadebrittleandwillsubsequentlysnap,againcausingtheanchortocomeapart.
Theinterviewedmarineequipmentsuppliersprovidelessthanonereplacementstockamonthforarticulatedstockanchors.
Dataonanchordraggingincidentswhicharesufficientlyserioustocompromisestationkeeping,arelimited,andusuallyquitevague.
Opinionsvariedastothefrequencyofseriousanchordraggingincidents,fromveryraretomanycases.
Thecertifyingauthorityinterviewedhasreceivednoreportsofdraggingofanchorsonthevesselswhichitclassifies,butisawareofasmallnumberofseriousanchordraggingincidentswhichhaveoccurredtovesselsclassedbyothercertifyingauthorities.
Specificexamplesofseriousdraggingincidentsincludethefollowing:Aflotelwhichdraggedallofitsanchorsduringastorm.
Itisbelievedthisresultedfrominsufficientpenetrationofitsanchors.
Asupportvesselwhichapparentlydraggedallofitsanchors.
Itisbelievedthatthisvesselwasfittedwithhighperformanceanchorsatthetime.
TheincidentmentionedinSection6inwhichaMODUwasdrivenoffstation.
AnexampleofapiggybackedMODUanchordragging200feet(60metres)underatensionofaround500kips(2225kN).
Themooringsofcraneandpipelaybargesarenotdesignedforstormconditions.
Itwassuggested,however,thatthereisatendencyforthesevesselstocontinuetoworkforaslongaspossibleastheweatherworsens,withtheresultthattheyexperiencedraggedanchorsandbrokenmooringwires.
Insomeareas,suchastheFortiesfield,anchorsholdwellundersteadytension,butthesoilbecomesmorefluidwhensubjecttoshockloads.
Anchorscanpulloutinstormsasaresultofthisfluidisation.
MODUsinconfinedpositionssometimeshaveproblemswithanchorsdraggingacrosspipelines.
Thesituationcanarisethattheweathersidemooringlinescannotbe50adjustedtotheextentdesiredbecauseoftheconfinedspace,whilsttheleesidemooringlinescannotbeslackenedoffbecauseoftheirproximitytopipelines.
518.
CONCLUSIONSThefindingsofadetailedassessmentofcurrentpracticesfordraganchorshavebeenpresented.
Theinvestigationshaveyieldedasignificantnumberofinterestingaspectsfromwhichthefollowinggeneralconclusionshavebeendrawn:Availableexperimentaltestresultsondraganchorsaredated,andbasedonsmallscaleanchors.
TheydonotreflectpresentdayhighholdingpowerrequirementsoftheNorthSeaoffshoreindustry.
Furthermore,testshavebeenconductedinclaysandmudswhicharegenerallyinappropriatetotheseabedconditionsthatexistintheNorthSea.
Inmanyinstances,poortestingmethodologyhasbeenadopted.
Thereforeapplicationofavailabletestdatarequireslargemagnitudeextrapolationwithrespecttosizeofanchorandstrengthofsoil.
Ingeneral,codesandregulationsareconsistentwitheachother,butsomecoverthesubjectofdraganchorsinaverypiecemealanddisjointedmanner.
AlthoughthecodesprovideguidanceonmechanicalprooftestingofanchorsandratingofHHPanchors,noexplicitguidanceisprovidedwithrespecttoanchorhandlingorleveloftensionloadforpost-installationprooftensiontests.
PiggybackinganchorsisverycommonpracticeintheNorthSea.
Surprisinglyhowever,therearenosetproceduresorguidelinesforpiggybacking.
TheperformanceofarticulatedstockanchorsismarginalforapplicationsintheNorthSeaandsincemanyMODU'soperatingintheNorthSeaareequippedwitharticulatedstockanchors,piggybackingofanchorsisveryfrequentlynecessarytoachievepost-installationtesttensionloads.
Sometimes,higherperformanceanchorsaredeployedinsteadofpiggybackingarticulatedstockanchors.
However,thehireorpurchasecostofhigherperformanceanchorsissignificantlygreaterthanthatofarticulatedstockanchors.
Thepermanentreplacementofavessel'sarticulatedstockanchorsbyhigherperformanceanchorsrequirescostlymodificationstotheMODU'sanchorrackingsystem.
Adetailedsearchoftechnicalliteraturehasrevealedthatvirtuallynonumericalanalyseshavebeenconductedfordraganchorsandalmostallinvestigationshavebeenexperimentalinnature.
Inadequateinformationexiststoallowasystematicselectionofadraganchorforaparticularsite.
Muchrelianceisplacedonmanufacturer'sdataandNCELtestdata.
TheNCELdataisnotapplicabletomanyofthemorerecentanchortypes.
Manufacture'sdataprovideguidanceonthesettingofflukeanglesforcertainseabedconditions,andthemanufacturer'sanchorperformancedataarebasedonthesesettings.
However,ingeneralitwassuggestedthatwhentheseanchorsaredeployed,thesettingrequirementsareoftendisregarded,andanaverageflukeangleadopted.
Pre-installationsurveystaketheformofseabedcontourplots,detectionofobstructions,forexamplepipelines,andsoil-typeidentification.
Onlyoneorganisationaddedthatshallowgasdetectioniscarriedout.
52Inmostcases,anAHVisrequiredtopositiontheanchorsontheseabed.
Theeasewithwhichtheanchorscanbehandleddependsonthetypeandsizeoftheanchor,thesizeoftheAHVandtheskillsofthemasteroftheAHV.
Theconsensuswasthatcertainofthehigherperformanceanchortypespresentthegreatestdifficultywithrespecttoanchorhandling.
Higherperformanceanchorsarefrequentlydifficulttorecover,andeitherverylarge,orlongdurationapplicationofloadsmaybenecessary,todislodgethemfromtheseabed.
Ifproblemswithachievingadequateholdingcapacityareencountereditmaytakedaystofullyinstalltheanchorspread.
Itwasnotedthatin-positioninspectionproceduresareappliedonlyinthecaseofFPSs.
Inmostothercases"inspection"isbasedonlinetensionalone.
Ingeneral,theanchorsareonlyvisuallyinspectedbetweendeployments.
Damage,ifany,usuallyresultsfromtheinstallationorrecoverystages.
Corrosionofthesteelisnotaproblem.
Certifyingauthoritiesholdconsiderableproprietarydatathroughtheirinvolvementincertificationofpermanentlymooredsystems.
AjointindustryfundedprojecthasrecentlycompleteddraganchortestsintheGulfofMexico.
Althoughthetestsconsideredarticulatedstockandhigherperformanceanchors,thetestshavebeencarriedoutinsoilconditionsunrepresentativeofsoilconditionsintheNorthSea.
Someresearchhasbeendoneonsmallscaletestsondraganchorsusingthecentrifugemethod.
Thismethodusesaccelerationforcestomodelthebehaviouroftrue-sizedraganchors,andisclaimedtoprovideusefulinformationwithrespecttofailuremechanisms.
Conflictingviewswereexpressedontheholdingpowerofananchorafterdragging.
Someorganisationsstatedthatholdingpowerwasrecoveredafterdragging,butothersfoundthattotallossoftensionresulted.
Anumberofanchorfailureshavebeenreported.
Mostoftheserelatetoarticulatedstockanchors,whicharepronetofallapartfromfailuretoadequatelylocatetheretainingwasheronthestock.
Alternatively,failuremayoccurduetoexcessiveweldingtolocatetheretainingwasher.
53ThisstudyhasconcludedthatdraganchorsarewidelydeployedintheNorthSeaoffshoreindustry.
Thefindingsofcurrentdaypracticesintheuseofdraganchorsrevealthatfurtherinvestigationsinsomeareasarenecessarytoensurethesafeandreliableuseofdraganchors.
549.
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56Figure1aTypicalanchortypes(afterVandenHaak[1990],VandenHaak[1972],alsomanufacturersdatasheetsforBruce,StevprisandFlipper-Deltaanchors)57Figure1bTypicalanchortypes(afterVandenHaak[1990],VandenHaak[1972],alsomanufacturersdatasheetsforBruce,StevprisandFlipper-Deltaanchors)58Figure2Holdingpowerofvariousanchors(fromVandenHaak,1972)59Figure5Effectsofflukeangleonholdingpowerinmud(fromBeck,1974)SPE197460Figure7bComparativetestsinmud:typicalholdingpower-distanceofdragrecords(fromPuech,1978)OTC197861Figure10Minimumspacingofembeddedanchorflukesfornon-cohesivesoilstorealise100%groupefficiency(fromValentetal,1979)ReprintedwithpermissionfromOceanEng.
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6,Valentetal,copyright1979,PergamonPressLtd62Figure11Maximumholdingpowerratio,inclinationandembedmentdepth(fromUraetal,1979)6364Figure16Deepinclinedanchor,forcescontributingtoupliftresistance(fromSutherladnetal,1983)Reprintedwithpermission,copyrightTaylorandFrancis,Washington,D.
C.
65Figure17Geotechnicaldataobtainedfromgravitycores(fromVoldandEie,1983)OTC198366Figure18aTensionatanchorshackle,embedmentdepthandinclination,versusdraglength(fromBoldandEie,1983)OTC198367Figure18bTensionatanchorshackle,embedmentdepthandinclination,versusdraglength(fromVoldandEie,1983)OTC198368Figure19aAnchorchainsystemholdingcapacityatthemudline(fromZumwait,1986)OTC198669Figure19bAnchorchainsystemholdingcapacityatthemudline(fromZumwait,1986)OTC198670Figure21Testresultsfromsmallscaleanchors(fromRraasandHagen,1989)OTC198971Figure23Testresultsfrom60and65tonneanchorinstallationsatlocationNo.
3andNo.
7(fromRraasandHagen,1989)OTC198972Figure25Variationofchaintestingandchainangleattheattachmentpointwithpullforce(fromDuttaandDegenkamp,1989)OTC198973Figure26Effectsofchangingundrainedshearstrengthofclayonchainforcesattheattachmentpoint(fromDuttaandDegenkamp,1989)OTC198974Figure27Manufacturer'sperformancecurveforMeonMk3anchor75Figure28Manufacturer'sperformancecurveforStokesHHPstocklessanchors76Figure29Manufacturer'sperformancecurveforBruceFFTSMk4anchorinsandandmud77Figure30Manufacturer'sperformancecurveforStevinanchor78Figure31Manufacturer'sperformancecurveforStevprisanchor79Figure32Anchorchainsystemholdingcapacityatthemudlineinhardsoils(sandandstiffclay)(fromAPIRP2P:Analysisofspreadmooringsystemsforfloatingdrillingunits,1987)80Figure33Anchorchainsystemholdingcapacityatthemudlineinsoftsoils(siltandclay)(fromAPIRP2P:Analysisofspreadmooringsystemsforfloatingdrillingunits,1987)81Figure34Anchorchainsystemholdingcapacityinsand(fromAPIRP2FP1:Design,analysisandmaintenanceofmooringforfloatingproductionsystems,draft1991)82Figure35Anchorsystemholdingcapacityinmud(fromAPIRP2FP1:Design,analysisandmaintenanceofmooringforfloatingproductionsystems,draft1991)83Figure36Methodofproof-testpullfromAPIRP2M:Qualificationtestingofsteel84