锚泊

171APPENDIXJ—MOORINGSTRENGTHANDFATIGUEANALYSISEXAMPLESJ.1StrengthAnalysisExampleThefollowingproblemillustratestheproceduresfortheanalysisofamooringsystemusingthreeanalysismethods:a.Quasi-staticanalysis.b.Timedomaindynamicanalysis.c.Frequencydomaindynamicanalysis.Theexampleisintendedtoillustratetheprincipalstepsintheanalysisandtogiveguidanceparticularlyonthedynamicanalysisprocedure.Themethoddescribedhereisnotuniqueanditmaybenecessarytomodifysomeofthestepsintheproceduretoaccommodatespecificsoftware.J.1.1MOORINGSYSTEMDESCRIPTIONThesystemtobeanalyzedis:a.Asemi-submersiblewitha10point,36°symmetricpat-tern,asshowninFigureJ.1.b.312in.K4chain5.100ftoutboard,breaktestload=1838kips.c.Initialtension=280kips.d.Mooringlinedescription.1.Diameter=312in2.Elasticstretch(AE)=123.4lbs/ft3.Weightinair=107.2lbs/ft4.Frictioncoefficient=1.0(withmudline)5.Breaktestload=1,838kips6.Linemass=3.84slugs/ft7.Tangentialaddedmass=0.25slugs/ft8.Normaladdedmass=0.51slugs/ft9.Dragcoefficient=1.210.Dragdiameter=7in.(25nominaldiameter)FigureJ.1—MooringConfigurationîíïïðçèéêëìCopyrightAmericanPetroleumInstituteReproducedbyIHSunderlicensewithAPILicensee=SingleBuoyMoorings/5938767100NotforResale,01/05/200600:59:47MSTNoreproductionornetworkingpermittedwithoutlicensefromIHSPDF文件使用pdfFactoryPro试用版本创建:a.Waterdepth=1,233ftb.Significantwaveheight=55.8ftc.Peakspectralperiod=17.49secd.JONSWAPspectrumwithapeakednessfactor3.3e.Windvelocity(1min.)=100ktf.Surfacecurrentvelocity=3.1ktg.Quarteringdirection(SeeFigureJ.1)h.Designstormduration=3hoursi.Wind,wave,currentcolinearWindloadingonthevesselcanbeaccountedforintwodifferentprocedures:a.Windloadisconsideredtobeappliedstaticallytotheves-sel.Inthisproceduretheoneminuteaveragewindspeedisappliedonthevesselasastaticload.b.Thedynamiceffectsofwindareconsideredbycombiningasteadywindforcewithafluctuatingwindcomponent.Theone-houraveragewindspeedisappliedstaticallytothevessel.Awindgustvelocityspectrumisdefined.Theapproachdescribedinitemahasbeenusedinthepresentexample.J.1.3MEANLOADCOMPUTATIONThemeanloadscanbederivedfrommodeltestdataorcom-puted.Thecomputedmeanloadsonthevesselareasfollows:Wind(1minuteaverage)680kipsCurrent378kipsSteadywavedrift70kipsTotalmeanload1,128kipsJ.1.4VESSEL/FAIRLEADMOTIONSBothwavefrequencyandlow-frequencymotionsarerequiredinallanalyses.Ingeneraltheresponseamplitudeoperators,andphases,inthethreelineardirections(heave,surge,sway)andthethreeangulardirections(roll,pitch,yaw)mustbederived.Thederivationofthesedatarequireshydro-dynamiccomputerprogramsormodeltestdata.Anysuitablereferencepoint,usuallythevessel’scenterofgravity,canbeusedtodefinethemotions.Thelow-frequencymotionscanbecomputedfromhydro-dynamiccomputerprograms,modeltestdata,ordesigncurves.Thecomputedrootmeansquare(rms)low-frequencymotionforthisexampleis0.97ft.Thevesselmotionsatthereferencepointmustbetrans-formedtotheendorfairleadofthelinetobeanalyzed.Theprocedurerequiredvarieswiththetypeofanalysis.J.1.4.1Quasi-StaticAnalysisInaquasi-staticanalysis,onlysurgeinthequarteringdirectionisconsidered.Heaveisignored.ThevesselmotionRAOsandphasescanbetransformedintothequarteringdirectionandconvolutedoverthesea-statespectrumtopro-ducermslineendmotion.Thecomputedwavefrequencyrmsmotioninthisexamplewas8.6ft.Toestablishthedominantfrequencyresponse,comparethemaximumwavefrequencymotionwiththemaximumlow-frequencymotionasfollows:maximumwavefrequencymotion:3.72x8.6=32.0ftmaximumlow-frequencymotion:3.03x0.97=2.9ftwherethefactors3.72,3.03representa1in1,000and1in100wavemaximumrespectively,typicalfor3-hourstormconditions.Wavefrequencydominates.Hencethedesignconditionis:maximumwavefrequencymotion+significantlow-frequencymotion=32.0+2x0.97=33.94ftNotethefactors3.72and3.03areapproximatevaluestypicallyusedfora3-hourstorm.MoreprecisevaluescanbeobtainedbyEqua-tions5.5to5.8.J.1.4.2FrequencyDomainAnalysisTocomputelineendmotionsinthefrequencydomain,thevesselRAOsandphasesinthesixdegreesoffreedommustbetranslatedtothefairleadlocation.Ingeneral,onlythemotionsintheplaneofthelineareofinterest.Thelineendmotionsinthehorizontalandverticaldirectionsintheplaneoftheline,andthephases,betweenthemarecomputedforeachfrequency.Itisextremelyimportanttoretainallphaseinformationtothispointasthedynamicbehaviorofthelineisheavilyinfluencedbythetangentialmotionorstretchingoftheline.ThelineendmotionsusedinthisexamplearegivenincomplexforminTableJ.1foreachfrequency.Thestan-CopyrightAmericanPetroleumInstituteReproducedbyIHSunderlicensewithAPILicensee=SingleBuoyMoorings/5938767100NotforResale,01/05/200600:59:47MSTNoreproductionornetworkingpermittedwithoutlicensefromIHSPDF文件使用pdfFactoryPro试用版本创建