船舶主机说明书L70MCC

7 VibrationAspects

Thevibrationcharacteristicsofthetwo-strokelowspeeddieselenginescanforpracticalpurposesbe,splitupintofourcategories,andiftheadequatecountermeasuresareconsideredfromtheearlyprojectstage,theinfluenceoftheexcitation

Thenaturalfrequencyofthehulldependsonthehull’srigidityanddistributionofmasses,whereasthevibrationlevelatresonancedependsmainlyonthemagnitudeoftheexternalmomentandtheen-gine’spositioninrelationtothevibrationnodesof

sourcescanbeminimisedorfullycompensated.

Ingeneral,themarinedieselenginemayinfluencethehullwiththefollowing:

ExternalunbalancedmomentsThesecanbeclassifiedasunbalanced1stand2ndorderex-ternalmoments,whichneedtobeconsideredonlyforcertaincylindernumbers

Guideforcemoments

Axialvibrationsintheshaftsystem

Torsionalvibrationsintheshaftsystem.

TheexternalunbalancedmomentsandguideforcemomentsareillustratedinFig.7.01.

Inthefollowing,abriefdescriptionisgivenoftheiroriginandofthepropercountermeasuresneededtorenderthemharmless.

Externalunbalancedmoments

Theinertiaforcesoriginatingfromtheunbalancedrotatingandreciprocatingmassesoftheenginecreateunbalancedexternalmomentsalthoughtheexternalforcesarezero.

Ofthesemoments,the1storder(onecycleperrevo-

theship.

A–

B–

C–

D–

1st2nd

1st

C C

A

B

D

CombustionpressureGuideforce

StayboltforceMainbearingforce

ordermomentvertical1cycle/revordermomentVertical2cycle/rev

ordermoment,horizontal1cycle/rev.

lution)andthe2ndorder(twocyclesperrevolution)needtobeconsideredforengineswithalownumberofcylinders.On7-cylinderengines,alsothe4thorderexternalmomentmayhavetobeexamined.Theiner-tiaforcesonengineswithmorethan6cylinderstend,moreorless,toneutralisethemselves.

Countermeasureshavetobetakenifhullresonanceoccursintheoperatingspeedrange,andifthevi-brationlevelleadstohigheraccelerationsand/orvelocitiesthantheguidancevaluesgivenbyinter-nationalstandardsorrecommendations(forin-stancerelatedtospecialagreementbetweenship-ownerandshipyard).

Guideforcemoment,

HtransverseZcycles/rev.Zis1or2timesnumberofcylinder

Guideforcemoment,

XtransverseZcycles/rev.Z=1,2...12

1780682-8.0

Fig.7.01:Externalunbalancedmomentsandguideforcemoments

1stordermomentson4-cylinderengines

1stordermomentsactinbothverticalandhorizon-taldirection.Forourtwo-strokeengineswithstan-dardbalancingtheseareofthesamemagnitudes.

Forengineswithfivecylindersormore,the1stordermomentisrarelyofanysignificancetotheship.Itcan,however,beofadisturbingmagnitudeinfour-cylinderengines.

Resonancewitha1stordermomentmayoccurforhullvibrationswith2and/or3nodes,seeFig.7.02.Thisresonancecanbecalculatedwithreasonableaccuracy,andthecalculationwillshowwhetheracompensatorisnecessaryornotonfour-cylinderengines.

Aresonancewiththeverticalmomentforthe2nodehullvibrationcanoftenbecritical,whereasthereso-nancewiththehorizontalmomentoccursatahigherspeedthanthenominalbecauseofthehighernatu-ralfrequencyofhorizontalhullvibrations.

Adjustablecounterweights

Aft

Fixedcounterweights

counterweights

Adjustable

Fore

Asstandard,four-cylinderenginesarefittedwithadjustablecounterweights,asillustratedinFig.

7.03.Thesecanreducetheverticalmomenttoanin-significantvalue(although,increasingcorrespond-inglythehorizontalmoment),sothisresonanceiseasilydealtwith.Asolutionwithzerohorizontalmo-mentisalsoavailable.

Fig.7.03:Adjustablecounterweights:431151

Fixedcounterweights

1781678-7.0

1780684-1.0

Fig.7.02:Statisticsoftankersandbulkcarrierswith4cylinderMCengines

1780676-9.0 1780692-4.0

Fig.7.04:1stordermomentcompensator Fig.7.05:Statisticsofverticalhullvibrationsintankers

andbulkcarriers

Inrarecases,wherethe1stordermomentwillcauseresonancewithboththeverticalandthehori-zontalhullvibrationmodeinthenormalspeedrangeoftheengine,a1stordercompensator,asshowninFig.7.04,canbeintroduced(asanoption:431156),inthechaintightenerwheel,reducingthe1stordermomenttoaharmlessvalue.Thecompen-satorcomprisestwocounter-rotatingmassesrun-ningatthesamespeedasthecrankshaft.

Witha1stordermomentcompensatorfittedaft,thehorizontalmomentwilldecreasetobetween0and30%ofthevaluestatedinthelasttableofthischap-ter,dependingonthepositionofthenode.The1storderverticalmomentwilldecreasetoabout30%ofthevaluestatedinthetable.

Sinceresonancewithboththeverticalandthehori-zontalhullvibrationmodeisrare,thestandarden-gineisnotpreparedforthefittingofsuchcompen-sators.

2ndordermomentson4,5and6-cylinderengines

The2ndordermomentactsonlyintheverticaldi-rection.Precautionsneedonlytobeconsideredforfour,fiveandsixcylinderengines.

Resonancewiththe2ndordermomentmayoccurathullvibrationswithmorethanthreenodes.Con-trarytothecalculationofnaturalfrequencywith2and3nodes,thecalculationofthe4and5nodenat-uralfrequenciesforthehullisarathercomprehen-siveprocedureand,despiteadvancedcalculationmethods,isoftennotveryaccurate.Consequently,onlyaratheruncertainbasisfordecisionsisavail-ablerelatingtothenaturalfrequencyaswellasthepositionofthenodesinrelationtothemainengine

A2ndordermomentcompensatorcomprisestwocounter-rotatingmassesrunningattwicetheen-ginespeed.2ndordermomentcompensatorsarenotincludedinthebasicextentofdelivery.

SeveralsolutionsareshowninFig.7.06forcom-pensationoreliminationofthe2ndordermoment.Themostcostefficientsolutionmustbefoundineachcase,e.g.:

Nocompensators,ifconsideredunnecessaryonthebasisofnaturalfrequency,nodalpointandsizeofthe2ndordermoment

Acompensatormountedontheaftendoftheenginedrivenbythemainchaindrive,option:431203

Acompensatormountedonthefrontend,drivenfromthecrankshaftthroughaseparatechaindrive,option:431213

Compensatorsonbothaftandforeendcom-pletelyeliminatingtheexternal2ndordermo-ment,options:431203and431213

Brieflyspeaking,compensatorspositionedonanodeornearitareinefficient.Ifitisnecessary,solu-tionno.4shouldbeconsidered.

Adecisionregardingthevibrationaspectsandthepossibleuseofcompensatorsmustbereachedatthecontractstagepreferablybasedondatafromsisterships.Ifnosistershipshavebeenbuilt,werecommendtomakecalculationstodeterminewhichoftheabovesolutionsshouldbechosen.

Ifnocompensatorsarechosen,theenginecanbedeliveredpreparedforretro-fittingofcompensatorsontheforeend,seeoption:431212.Thedecisiontopreparetheenginemustalsobemadeatthecon-tractstage.Measurementstakenduringseatrialorinservicewithfullyloadedshipcanshowwhetherthereisaneedforcompensators.

Ifnocalculationsareavailableatthecontractstageweadviseorderingtheenginewitha2ndordermo-mentcompensatorontheaftend,option:431203,andtomakepreparationsforthefittingofacom-pensatoronthefrontend,option:431212.

Ifitisdecidedneithertousecompensatorsnorpre-parethemainengineforretro-fitting,thefollowingsolutioncanbeused:

Anelectricallydrivencompensator,option:431601,synchronisedtothecorrectphaserelativetotheexternalforceormomentcanneutralisetheex-citation.Thistypeofcompensatorneedsanextraseatingfitted,preferablyinthesteeringgearroomwheredeflectionsarelargest,andthecompensatorwillhavethegreatesteffect.

Theelectricallydrivencompensatorwillnotgiverisetodistortingstressesinthehull,butitismoreex-pensivethantheengine-mountedcompensatorsaslistedabove.Morethan70electricallydrivencom-pensatorsareinservicewithgoodresults.

MomentcompensatorAftend,option:431203

MomentcompensatorForeend,option:431213

Centrelinecrankshaft

CompensatingmomentF2CxLnodeoutbalancesM2V

M2V

M2V

M2V

NodeAFT

F2C

Lnode

MomentfromcompensatorM2CoutbalancesM2V

node

4

3

node

1780680-4.0

Fig.7.06:Optional2ndordermomentcompensator

Fig.7.07:1stand2ndordermomentcompensator

PowerRelatedUnbalance(PRU)

Toevaluateifthereisariskthat1stand2ndorderexternalmomentswillexcitedisturbinghullvibra-tions,theconceptPowerRelatedUnbalancecanbeusedasaguidance,seefig.7.07.

1784923-6.0

PRUNm/kW Needforcompensator

from0to60 notrelevant

from60to120 unlikely

from120to220 likely

above220 mostlikely

PRU

Externalmoment

Enginepower

Nm/kW

Inthetableattheendofthischapter,theexternalmoments(M1)arestatedatthespeed(n1)andMCR

WiththePRU-value,statingtheexternalmomentrelativetotheenginepower,itispossibletogiveanestimateoftheriskofhullvibrationsforaspecificengine.Basedonserviceexperiencefromagreater

numberoflargeshipswithenginesofdifferenttypes

ratinginpointL1ofthelayoutdiagram.Forotherspeeds(nA),thecorrespondingexternalmoments(MA)arecalculatedbymeansoftheformula:

n2

MMxA kNm

A 1 n

andcylindernumbers,thePRU-valueshavebeenclassifiedinfourgroupsasfollows:

1

(Thetoleranceonthecalculatedvaluesis2.5%).

1780681-6.2

Fig.7.08a:H-typeguideforcemoments Fig.7.08b:X-typeguideformoments

GuideForceMoments

Theso-calledguideforcemomentsarecausedbythetransversereactionforcesactingonthecrossheadsduetotheconnectingrod/crankshaftmechanism.Thesemomentsmayexciteenginevi-brations,movingtheenginetopathwartshipsandcausingarocking(excitedbyH-moment)ortwisting(excitedbyX-moment)movementoftheengineasillustratedinFigs.7.08aand7.08b.

TheguideforcemomentscorrespondingtotheMCRrating(L1)arestatedinthelasttable.

Topbracing

Theguideforcemomentsareharmlesstotheen-ginebutmayexciterelativelargevibrationsifareso-nanceoccurintheengine/shipstructuresystem.

Asadetailedcalculationofthesystemisnormallynotavailable,MANB&WDieselrecommendthatatopbracingisinstalledbetweentheengine'supperplatformbracketsandthecasingsideforthefirstvesselinaseries.Forfurtherinformationpleaseseesection5‘Topbracing’.

Themechanicaltopbracing,option:483112com-prisesstiffconnections(links)withfrictionplatesandalternativelyahydraulictopbracing,option:483122toallowadjustmenttotheloadingcondi-tionsoftheship.Withbothtypesoftopbracingtheabove-mentionednaturalfrequencywillin-creasetoalevelwhereresonancewilloccurabovethenormalenginespeed.Detailsofthetopbrac-ingsareshowninsection5.

DefinitionofGuideForceMoments

Duringtheyearsithasbeendiscussedhowtodefinetheguideforcemoments.Especiallynowthatcom-pleteFEM-modelsaremadetopredicthull/enginein-teraction,theproperdefinitionofthesemomentshasbecomeincreasinglyimportant.

H-typeGuideForceMoment(MH)

Eachcylinderunitproducesaforcecoupleconsist-ingof:

1: Aforceatcrankshaftlevel.

2:Anotherforceatcrossheadguidelevel.Thepo-sitionoftheforcechangesoveronerevolution,astheguideshoereciprocatesontheguide.

AsthedeflectionshapefortheH-typeisequalforeachcylindertheNthorderH-typeguideforcemo-mentforanN-cylinderenginewithregularfiringor-deris:

N•MH(onecylinder).

Formodellingpurposethesizeoftheforcesintheforcecoupleis:

Force=MH/L kN

whereListhedistancebetweencrankshaftlevelandthemiddlepositionofthecrossheadguide(i.e.thelengthoftheconnectingrod).

Astheinteractionbetweenengineandhullisattheengineseatingandthetopbracingpositions,thisforcecouplemayalternativelybeappliedinthosepositionswithaverticaldistanceof(LZ).Thentheforcecanbecalculatedas:

ForceZ=MH/LZkN

Anyotherverticaldistancemaybeapplied,soastoaccommodatetheactualhull(FEM)model.

Theforcecouplemaybedistributedatanynumberofpointsinthelongitudinaldirection.Areasonablewayofdividingthecoupleisbythenumberoftop

bracingandthenapplyingtheforcesinthosepoints.

ForceZ,onepoint=ForceZ,total/Ntopbracing,totalkN

X-typeGuideForceMoment(MX)

TheX-typeguideforcemomentiscalculatedbasedonthesameforcecoupleasdescribedabove.How-everasthedeflectionshapeistwistingtheengineeachcylinderunitdoesnotcontributewithanequalamount.Thecentreunitsdonotcontributeverymuchwhereastheunitsateachendcontributesmuch.

Aso-called‘Bi-moment’canbecalculated(Fig.7.08):’Bi-moment’=S[force-couple(cyl.X)•distX]

inkNm2

TheX-typeguideforcemomentisthendefinedas:MX=‘Bi-Moment’/L kNm

Formodellingpurposethesizeofthefour(4)forces

(seeFig.7.05)canbecalculated:

Force=MX/LX kNwhere:

LX:ishorizontallengthbetween’forcepoints’(Fig.7.05)

SimilartothesituationfortheH-typeguideforcemoment,theforcesmaybeappliedinpositionssuitablefortheFEMmodelofthehull.ThustheforcesmaybereferredtoanotherverticallevelLZabovecrankshaftcentreline.Theseforcescanbecalculatedasfollows:

ForceZ,onepoint=Mx•L kN

Lz•Lx

Forcalculatingtheforcesthelengthoftheconnectiingrodistobeused:L=2660mm

AxialVibrations

Whenthecrankthrowisloadedbythegaspressurethroughtheconnectingrodmechanism,thearmsofthecrankthrowdeflectintheaxialdirectionofthecrankshaft,excitingaxialvibrations.Throughthethrustbearing,thesystemisconnectedtotheship`shull.

Generally,onlyzero-nodeaxialvibrationsareofin-terest.Thustheeffectoftheadditionalbendingstressesinthecrankshaftandpossiblevibrationsoftheship`sstructureduetothereactionforceinthethrustbearingaretobeconsidered.

Anaxialdamperisfittedasstandard:431111toallMCenginesminimisingtheeffectsoftheaxialvibra-tions.

Thefiveandsix-cylinderenginesareequippedwithanaxialvibrationmonitor(431117).

TorsionalVibrations

Thereciprocatingandrotatingmassesoftheengineincludingthecrankshaft,thethrustshaft,theinter-mediateshaft(s),thepropellershaftandthepropel-lerareforcalculationpurposesconsideredasasystemofrotatingmasses(inertias)interconnectedbytorsionalsprings.Thegaspressureoftheengineactsthroughtheconnectingrodmechanismwithavaryingtorqueoneachcrankthrow,excitingtor-sionalvibrationinthesystemwithdifferentfrequen-cies.

Ingeneral,onlytorsionalvibrationswithoneandtwonodesneedtobeconsidered.Themaincriticalorder,causingthelargestextrastressesintheshaftline,isnormallythevibrationwithorderequaltothenumberofcylinders,i.e.,fivecyclesperrevolutiononafivecylinderengine.Thisresonanceisposi-tionedattheenginespeedcorrespondingtothenaturaltorsionalfrequencydividedbythenumberofcylinders.

Thetorsionalvibrationconditionsmay,forcertaininstallationsrequireatorsionalvibrationdamper,option:431105.

Basedonourstatistics,thisneedmayariseforthefollowingtypesofinstallation:

Plantswithcontrollablepitchpropeller

Plantswithunusualshaftinglayoutandforspecialowner/yardrequirements

Plantswith8-cylinderengines.

Theso-calledQPT(QuickPassageofabarredspeedrangeTechnique),option:431108,isanal-ternativetoatorsionalvibrationdamper,onaplantequippedwithacontrollablepitchpropeller.TheQPTcouldbeimplementedinthegovernorinordertolimitthevibratorystressesduringthepassageofthebarredspeedrange.

TheapplicationoftheQPThastobedecidedbytheenginemakerandMANB&WDieselA/Sbasedonfi-naltorsionalvibrationcalculations.

Four,fiveandsix-cylinderengines,requirespecialattention.Onaccountoftheheavyexcitation,thenaturalfrequencyofthesystemwithone-nodevi-brationshouldbesituatedawayfromthenormalop-eratingspeedrange,toavoiditseffect.Thiscanbeachievedbychangingthemassesand/orthestiff-nessofthesystemsoastogiveamuchhigher,ormuchlower,naturalfrequency,calledundercriticalorovercriticalrunning,respectively.

Owingtotheverylargevarietyofpossibleshaftingarrangementsthatmaybeusedincombinationwithaspecificengine,onlydetailedtorsionalvibrationcalculationsofthespecificplantcandeterminewhetherornotatorsionalvibrationdamperisnecessary.

Undercriticalrunning

Thenaturalfrequencyoftheone-nodevibrationissoadjustedthatresonancewiththemaincriticalor-deroccursabout35-45%abovetheenginespeedatspecifiedMCR.

Suchundercriticalconditionscanberealisedbychoosingarigidshaftsystem,leadingtoarelativelyhighnaturalfrequency.

Thecharacteristicsofanundercriticalsystemarenormally:

Relativelyshortshaftingsystem

Probablynotuningwheel

Turningwheelwithrelativelylowinertia

Largediametersofshafting,enablingtheuseofshaftingmaterialwithamoderateultimatetensilestrength,butrequiringcarefulshaftalignment,(duetorelativelyhighbendingstiffness)

Withoutbarredspeedrange,option:407016.Whenrunningundercritical,significantvaryingtorqueatMCRconditionsofabout100-150%ofthemeantorqueistobeexpected.

Thistorque(propellertorsionalamplitude)inducesasignificantvaryingpropellerthrustwhich,underad-verseconditions,mightexciteannoyinglongitudinalvibrationsonengine/doublebottomand/ordeckhouse.

Theyardshouldbeawareofthisandensurethatthecompleteaftbodystructureoftheship,includingthedoublebottomintheengineroom,isdesignedtobeabletocopewiththedescribedphenomena.

Overcriticalrunning

Thenaturalfrequencyoftheone-nodevibrationissoadjustedthatresonancewiththemaincriticalor-deroccursabout30-70%belowtheenginespeedatspecifiedMCR.Suchovercriticalconditionscanberealisedbychoosinganelasticshaftsystem,leadingtoarelativelylownaturalfrequency.

Thecharacteristicsofovercriticalconditionsare:

Tuningwheelmaybenecessaryoncrankshaftforeend

Turningwheelwithrelativelyhighinertia

Shaftswithrelativelysmalldiameters,requiringshaftingmaterialwitharelativelyhighultimatetensilestrength