Pages from Floating Structures, A Guide for Desig

4Staticsandstability4.1SummaryThischapterdealswiththestaticandstabilityaspectsoffloatingstructures,i.e.ships,semi-submersibles,TLPsandSparbuoys.Afterabasicdiscussionanddefmitionsofthemainparameters,thesimplifiedlinearapproachtostabilityisoutlinedinSection4.3.Thisapproach,basedonstiffhessinrollandpitchforsmallangles,isquitesuitableforfonnulatingmanyaspectsofthestabilityofintactfloatingstructuresbutinanyfmalassessmentitisalsonecessarytopayattentiontononlinearstabilityparametersandtotaketheoverturninganddynamiceffectofwindintoaccountDamagestabilityisimportantforallfloatingstructuresandusuallyrequiresconsiderationsoflargeanglesandnonlineareffects.Sections4.4to4.11discussmanydetailedaspectsoflinearandnon-linearstabilityanalysis.Designaspectsarehighlighted,foreachtypeofstructurementionedabove,inSection4.12.Sections4.13to4.14discussthecalculationofhydrostaticstiffhesswhichisanimportantinputtohydrodynamicmotionanalysisandsometimestostructuralanalysis.Damagestabilitymustbeaddressed,notonlyfromananalyticalpointofviewbutalsotosatisfyrulerequirementswhichreflectthemanypracticallessonsgainedfromoperationalexperience.Therearealsorequirementsforincliningteststodetenninetheactualpositionofthecentreofgravity.ThevariousrulerequirementsandtheirinfluenceondesignisdiscussedinSections4.15to4.20.Section4.21reviewslimitationsandpossiblefuturedevelopments.4.2IntroductionAfloatingvesselmustbedesignedtobeasafeworkingplatfonnwhileundertheinfluenceofthefullrangeofenvironmentalconditionswhichitmaybeexpectedtoencounterduringitslife.'Stability'isameasureofthevessel'scapacitytoresistoverturningmomentsandwithstandtheenvironmentalforcesofwind,currentandwaves.Tnaddition,itmustpossesssufficientreservestabilitythatitsoperationisnotoverlyrestrictedandliftingofweightsbycranesorpumpingoffluidscantakeplaceasrequired.Inpractice,awiderangeofenvironmentalconditionswillbeencountered.Nonnaloperationscanbecontinuedsafelyinmostconditionsbuttheremaybeother,moreextremecaseswherenonnaloperationswillceaseandthevesselwilladoptasurvivalmode.Inthiscondition,risersmaybedisconnectedandthedraftreducedtoenhancethestabilitycharacteristicsortoincreasefreeboardtoavoidwaveslamandpreventwateringress.Somevessels,includingTLPsaredesignedtooperatethroughsevereweatherwithnochangeofstatus.Anothereffectwhichneedstobeexaminedduringdesignisthefloodingofoneormorecompartmentsduetodamage.Thiswillaffectthedraft,trimandheelofavesseland,consequently,changesthevariousmeasuresofstability.Hancox(1996)givesagoodreviewofthevariousoperationalconditionsandrelatedissuesforfloatingoffshorestructures.CriteriaarelaiddownbyCerti:tyingAuthoritiesconcerningacceptableintactanddamagedstability(,Rules').ThesetypicallytaketheformofprescriptionsaboutthepropertiesoftheGZcurve(discussedinthefollowingsection)anditsrelationshiptotheenvironmentalforce,likeoverturningmoment,whichmightbeencountered.Therulesalsoinfluencethedimensionsofcompartmentsandthelocationsofbulkheadsandhorizontalflats.Forstaticstability,weightandbuoyancyarethetwogoverningparametersandtheyareusuallyexpressedintomes(t).Fortheanalysisofdynamicstability,theequationsofmotionhavetobeconsideredandaconsistent4-2Floatingstructures:aguidefordesignandanalysissetofunitsisrequired.Forthisreason,theequationswillbesetupwithweightsspecifiedinNorkNandmassinkgort.Tonne(t)maybeusedtorepresentaforce,butfordynamicanaiysisthiswouldrequiremasstobemeasuredinunitsof9.81tonne.Finally,theanalysisoffloatingstructuresrequiresacloseliaisonbetweenthenavalarchitectandthestructuralengineer.Thenavalarchitectconsidersweights,buoyancyandmotionresponsetoensurestabilityandsurvivalwhilethestructuralengineerconsidersforces,pressuresandstrengthtoensurethatdetailedstrengthrequirementsaresatisfied.Thischapterprovidesbasicinputtotheworkofbothdisciplines.Thelightweightanddeadweightofafloatingunitaredefmedas:lightweightdeadweighttheweightofthefloatingunit,fullyequippedbutexcludinganypersonnel,consumables,fuel,cargo,etc.theweightintheoperationalmodeincludingpersonnel,consumables,fuel,cargo,etc,butexcludingthelightweightitselfTheRulesdefinefourbasictypesofvesselandcriteriavaryaccordingly.'Columnstabilised'unitsincludesemi­submersibles,'surface'unitsarepoweredship-shapedvesselsand'bargetype'unitsareunpoweredfloating,surfacevessels.Jack-upsareclassedas'self-elevating'units.Sincetheseareonly'floating'structuresduringvoyagesbetweendifferentsites,theyarenotconsideredexplicitlyinthischapter.4.2.1DownfloodingAfloatingbodymaybesusceptibletodownfloodingifopeningsbecomesubmergedunderthecombinedactionofheel,trimandwaves.Bothcausesareaddressedinthestabilityassessment.Animportantstabilityparameteristhedownfloodingangle.Thisangleistheleastangleofheelatwhichopeningsinthehull,superstructureordeckhouses,whichcannotbeclosedweathertight,immerseandfloodingstartstooccur.Notethatdoors,hatches,ventilators,windows,etc.whichareprovidedwithclosingappliancesthatcanbesecuredwatertightarenotconsideredasdownfloodingpoints(seealsoSection4.6).4.2.2NomenclatureInthischaptertherewillbemuchdiscussionoftheunderwatershape