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FrontiersinOffshoreGeotechnics:ISFOG2005–Gourvenec&Cassidy(eds)©2005Taylor&FrancisGroup,London,ISBN041539063X311INTRODUCTIONInthefirstphaseofthisstudyanattemptwasmadetoestablishabaselineofdataandpredictionmethodsforVLAswhichcouldserveasa‘jumpingoffpoint’forthesecondphaseofresearchtobeaimedatclarify-ingtheuncertaintiesinvolvedinanalysisanddesignandidentifyingwaystoreducetheseuncertainties.InparticularPhaseIfocusedon(1)identifyingandcol-lectingapplicablereferences,(2)developingacollec-tionofsummariesofpredictionmethodsrelevanttoVLAinstallationassessmentandholdingcapacityesti-mation,and(3)developingadatabaseofactualVLAapplicationsaswellasrelevantfieldtestsandexperi-ments.Thefocusofthefieldteststudiesisthebehav-iorofverticallyloadedplateanchors.Howeverwehaveincludeddataonmoreconventionaldraganchorsaswell.Thiswasdonesince,despitedifferencesinanchorgeometryandconfiguration,themechanicsofdragembedmentareverysimilarwhetherananchorisaconventionaldraganchororadragembeddedVLA.Further,themostchallengingaspectofadragembed-dedVLAisitsinstallationandtheassociateduncer-taintiesofanchorplacement.Inthesecondphaseofthestudythepredictionmethodswereevaluatedtotheextentpracticableusingseveraltechniques.Theprimarymeansofevaluationisacomparisonofpredictionsofhypotheticalcasesamongvariouspredictionmethodsandpredictionsusingthemethodswith‘groundtruth’datafromeitherfieldorexperimentaltestsormorerigorousnumericalanalyses(finiteelementmodeling(FEM)results).Specificallywecarriedoutthefollowing:1SelectedsimplifiedmethodswereusedtopredictthebehaviorofhypotheticalcasesforbothinstallationVerticallyloadedplateanchorsfordeepwaterapplicationsJ.D.MurffOffshoreTechnologyResearchCenter,TexasA&MUniversity,CollegeStationTexas,USAM.F.Randolph&S.ElkhatibCentreforOffshoreFoundationSystems,UniversityofWesternAustralia,Perth,AustraliaH.J.KolkFugroEngineers,Leidschendam,TheNetherlandsR.M.RuinenVryhofAnchors,KrimpenadIJssel,TheNetherlandsP.J.StromdetNorskeVeritas,Oslo,NorwayC.P.ThorneUniversityofSydney,Sydney,AustraliaABSTRACT:Thispapersummarizestheresultsofanindustrysponsoredstudyonthedesignandanalysisofverticallyloadedplateanchors(VLAs)embeddedinsoftclays.PhaseIofthestudyfocusedoncollectionofref-erences,predictionmethods,anddataonactualapplications,fieldtests,andexperimentalstudiestoestablishabaselineofexperienceandunderstanding.InPhaseII,thecurrentpracticesforpredictingtheinstallationper-formanceandcapacityofVLAswereevaluated,includinganassessmentoftheirsimplicity,completeness,sen-sitivity,practicality,andgenerality.Researchtopicswiththepotentialforimprovingcurrentpracticewereidentified.Thebasisoftheevaluationwasacomparisonofpredictionsofhypotheticalcasesamongvarioussimplifiedmethodsandpredictionsusingthesemethodswith‘groundtruth’datafromeitherrigorousnumer-icalanalyses(FEMresults)orfield/experimentaltestswhereavailable.Mackman-02.qxd7/23/0514:15Page31andcapacityandthesepredictionswerecomparedtoassessvariabilityinherentamongthemethodsforwelldefinedinput.2Resultsofsimplifiedpredictionmethodswerecomparedwithselectedfield/experimentalmeas-urementstakenduringinstallationstoassessthequalitativeagreementaswellasthebiasanduncer-taintiesinpredictionsandthecalibrationsrequiredtoimproveagreement.3AseriesofFEManalyseswasconductedandtheresultswereusedtodevelopamethodforestimat-inganchorholdingcapacityundermultiaxialload-ing.Resultsofsimplifiedpredictionmethodswerecomparedwiththeinterpretednumericalanalyses(FEM)toassessbiasanduncertaintiesinpredic-tionsaswellassensitivitytomethodassumptions.Whiletheresearchproposedhereinisnotintendedtoincludethedevelopmentofnewpredictionmethods,itdoesidentifyareaswherenewdevelopmentsmightservetoimproveandextendcurrentprocedures.2PHASEI2.1Backgroundofverticallyloadedanchors2.1.1PrinciplesofVLAoperationAverticallyloadedanchortypicallyderivesitshold-ingcapacityfromalargebearingplate(calledafluke).DuringinstallationofaVLA,loadisappliedtotheplatethroughanattachedanchorlinebyvariousmeanssuchasthroughaconnectingrigidbar(shank)orthroughaharnessorbridle.Theanchorisplacedontheseafloorsuchthat,astheanchorispulledalongthebottom,itpenetratesthesoil.Initially,theanchordivesmoreorlessparalleltothefluke,eventuallyrotatingsuchthatthetargetpenetrationdepthisachieved.Variousmethodsarethenusedto‘activatetheanchor’,i.e.orienttheanchorflukesothatitbecomesperpendiculartotheanchorlineforce.Forexample,Figure1aisaphotographofaVLAcalledtheBruceDennlaanchor,whileFigure1bshowsaschematicofaVryhofStevmantaanchor.Forboththeseanchors,theshankorbridlearrangementisdesignedtorotateafterplacementsothattheanchorlineloadisapproxi-matelyperpendiculartothefluke.Conventionalmooringlinesarecatenariesbetweenthevesselandtheseafloorsuchthattheanchorlineentersthesoilhorizontally.Fordeepwaterapplica-tions(300m),thiscanresultintheanchorbeingover1000mfromthevessel.Owingtosoilresistance,theanchorlinetakesonareversecurvaturebelowthemudline,suchthatthelineimposessomeverticalloadcomponentontheanchor,asshowninFigure2.Historically,draganchorshavebeenusedfortemporarymooringsystemsalthou