raptures: resolving the tugboat energy...

Amsterdam, The NetherlandsOrganised by the ABR Company Ltd

DayPaper No.

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iNTRODUCTiONTugboat operators are looking for ways to make their vesselsmoreefficientandfriendliertotheenvironment,toreducecosts,meetregulatoryrequirements,catertomarketdemandsforcleaneroperations,orsimplytoimprove‘green’credentials.Efficiencyimprovementsthatreducefuelconsumptiongenerallyreduceoperatingcostsandemissionsatthesametime,adoublebonus.Theconnectionbetweenfuelconsumptionandgreenhousegasesisclear–CO2emissionsaredirectlyproportionaltofuelconsumption.Butwhataboutotheremissions,suchasunburnedhydrocarbons,NOx,andparticulates?Fordieselprimemoversthesedependnotonlyonfuelconsumed,butalsoonhowtheequipmentisoperatedandtheemissionstechnologybuiltintotheengine,oraddedasoff-engineequipment.Asalwaystherecanbetrade-offstoconsiderifnewequipmentrequiressignificantcapitalinvestment.

Ontugs,thepowerdirectedtopropulsionisgenerallyresponsibleforthebulkoffuelcostsandemissions.Withtheexceptionofarelativelysmallnumberofdieselelectrictugs,beginningwithLuna(1930)andexamplessuchastheUSNavy’sYTBlargeyardtugs(1940s),thebulkoftugstodaycontinuetobepoweredbydieselmainenginescoupledtopropellersor,inthecaseofmore modern tugs, azimuthing propulsors (Z-drives), or VoithSchneiderPropellers(VSPdrives).Electricpowerforhotelneedsandauxiliaryequipmentistypicallysupplied by one or more small generator sets.

These systems have generally worked well and, untilveryrecently,therehasbeenlittleincentivetochange.Butthefuel-pricerollercoasterrideandheightenedconsciousnessoftheenvironmentalimpactofgreenhousegasesandotheremissionshavebroughtaboutare-examinationofthestatus

quo,andanewinterestinalternativepropulsionequipmentconfigurationsthatoffersomepotentialforimprovementsinfuelconsumptionandemissions–andpreferably both.

Designersandoperatorsoflargeanchor-handlingtugs and other offshore support vessels have been quickertoembracedieselelectricpowersystemsasanalternativetodieselmechanical.Thisisnotonlyforthepurportedfuelsavings,butalsofortheflexibilitythatacommonpowergenerationsystemoffersinvarying the power distribution between propulsion, dynamicpositioning,pumps,anddeckequipmentloadsasoperationschange.However,forsmallertugsthatdonothavethesamevarietyofequipmentortheneedforextendeddynamicpositioning,anyimmediateadvantagesarenotasobvious.Furthermore,thetimehascometolookbeyonddieselelectrictowhatisachievablewithnovelintegrationsofothercommerciallyavailabletechnologiesortheapplicationsofmorerecent‘alternativepower’technologies,suchasalternatefuelsorbatteryhybrids.Alongwithalternateequipmentchoices,operatorsmayalsoachievecostsavingsbysimplychanginghowexistingequipmentisoperated.

To make the best use of the opportunities alternative tug propulsions may offer, Robert Allan Ltd wanted to beequippedtoefficientlyandquantifiablyanswerthefollowingbasicquestionswhendesigninganewtug:

• Whatarethemostpracticalpoweringoptionsavailablefortheapplication?

• Howdotheseoptionscompareintermsoffueleconomyandemissionsfortheintendedoperations?

• Whatisthecostpremiumorsavings?

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RAptures: Resolving the Tugboat Energy Equation

Vince den Hertog,PEng,Ken Harford,PEng,Robin Stapleton,EIT,Robert Allan Ltd, Canada

SYNOPSiS Withdemandsonalltypesofvesselstoreduceemissionsandfuelconsumption,itisoftendifficulttoseparatethefactsfromthesalespitches.Facedwiththefrequentquestionof“Howbesttogogreen”(andalsosavesomemoney),RobertAllanLtdsetabouttocreateatooltoanalyseawiderangeofoptionsfortypicalharbourtugpropulsionsystems.RAptures (RobertAllanLtd:PoweringTugsforRealEnergySavings)isastraightforwardanalyticaltoolwhichcanbeusedforbothahigh-levelsystemcomparisonattheveryearlystagesofanewdesign,andalsoforthelatterdesignstagecomparisonofveryspecificmachineryselectionoptions.Withbasictugpowerandanoperatingprofileasinput,adirectcomparisonofdiversepropulsionsystemoptionsispossible,withoutputsoffuelconsumption,emissionsandinstalledcosts.

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• Ifoperationschange,whatwillbetheimpactonemissionsandfuelconsumption?

To that end, the RAptures programme was developedasatooltomakeeffectivecomparisons,evaluate the trade-offs, and generally zero in onthepoweringconfigurationthatbestfitstherequirementsandgoalsoftheclient.Appreciatingthatwidevariationsinvesselsize,requiredperformance,anddutyprofilemeanthereisno“one-size-fits-all”solution,theprogrammewasdesignedattheoutsettomodeldiverseapplicationsandconfigurations.

This paper gives an overview of the RAptures programmeanddiscussesresultsfromahypotheticalcase-studyforatypicalRobertAllanLtd28mRAmparts 2800tugtoillustratetheprogramme’scapabilitiesandpotential.

TERMS OF REFERENCERAptureswasdevelopedwiththesespecificobjectivesinmind:

• Toproviderealisticestimatesoffueleconomyand emissions that are not based on guesses or supplier promises, but on reliable fuel consumptionandemissionsdatathatspecificallytakesintoaccounttheperformancerequirementsfor the vessel and the day-to-day operational profile;

• To model how the operating points of the machineryarelinkedtotheactivitiesofthevessel, whether it is idling, free-running, operating atBP,andwhatotherpower-consumingequipment,suchaswinchesorpumps,isbeingoperatedatthesametime;

• Torecognisethat,fortugs,bothBPandhullresistancearecriticalaspectsthatdominatethepowerdemand,andprovideameanstopredictBPandfree-runningresistance;

• Toconsiderhowfuelconsumptionandemissionsofeachinstalledprimemovervarieswithoperatingpoint,basedondatafromcertifiedtestresultsorothersources;

• Toallowoperatingprofilestobedefinedthatarerealisticrepresentationsofanoperator’santicipatedday-to-dayusageofthevessel;

Toaccuratelycalculatefuelconsumptionandemissionstotalsforseveralcandidatepoweringconfigurationsoperatingunderthesameoperationalprofilesimultaneously,sothatlike-for-likecomparisonscanbemade;

• Topresentcostcomparisonsthattakeintoaccountequipmentcapitalcosts,andcoststhatvarywiththeoperator’sdutyprofile,includingfuel,equipmentmaintenanceandoverhaul;

• Makeiteasytoquicklyevaluate‘what-if’scenarios.

THE RAptures TOOL The RAptures tool is a software program developed asaspreadsheetformaximumflexibilityandtransparency.Theprogramisorganisedintoseveraltypesofdistinctmodules(Figure 1):propulsion,dutyandpower,poweringconfiguration,andequipment.Thisarchitecturemakesitstraightforwardtoestablishnewconfigurationsandadjustparameters.Ingeneral,eachmodulerequiresinputsregardingdifferentaspectsofthevessel,equipmentorutilisation.Dataforaspecificvesseloritsequipmentdetailscanbeentereddirectly,orstockdatafromapre-enteredlibrarycanbereferenced.

Propulsion moduleAnessentialelementforpredictingfuelconsumptionand emissions at different operation points is a model of how the ‘demand’ for power by the propellersvarieswithrequiredstaticthrust,orfree-running speed. Not only is power important, but alsothecorrespondingrev/minsinceequipmentefficienciesandprimemoverfuelconsumptionandemissions are governed by both.

Iftherelationshipsbetweenthrust,free-runningspeed,shaftpowerandshaftrev/minarealreadyknown, either from model tests or sea trials, this datacanbeentereddirectlyintothedutyandpowermodule(seebelow).Ifunknown,thepropulsionmodulecanbeusedinstead.Themodulecombinesasimplepolynomial-basedpredictionofpropellerandnozzleperformancewithapredictionofvesselresistancederivedfromanexpansionofmodeltestor trials data from similar vessels. These parameters establishthenecessarystaticthrustandself-propulsionrelationships.Withthesoftwarecurrentlyorientedtowardtugs,thepropulsionpredictionsinthemodulearesetupspecificallyformodellingZ-drives with Kaplan-style propellers in 19A nozzles. Othernozzleorpropellermodelscanbebuiltin.

Duty and power moduleWithin the duty and power module, the user first defines a set of different possible ‘operating conditions’whichconstituteallthestatesofsignificantinterestinwhichthetugcanoperate.Avarietyofdifferentconditionscanbedefined.Thesearetypicallyidle,differentfree-runningtransitspeeds,anddifferentstaticthrustlevels.The‘operatingprofile’isthendefinedbyallocatingthepercentageoftimespentateachconditionwithinonestandardoperatingperiod(typicallya24-hourday.)Foreachoperatingcondition,themoduleusesthefree-runningandstaticthrustmodelparametersprovidedbythepropulsionmoduletocalculatepropulsionpowerdemands.Otherloads,includinghotelanddeckmachineryelectricalpower,arealsoentered.Thesepowers,andthedurationsallocatedin the operating profile, are then used by the poweringconfigurationmodule.

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Powering configuration modulesThepoweringconfigurationmodulesformtheheartof the software, taking input from the user and other modulestodeterminewhatequipmentoperatesinanygivenconditionandreturningparametersrelatedtofuelconsumption,emissionsandcostsrelatedtothatcondition.Thereisonemoduleforeachpoweringconfigurationofinterest.Allareallfedthesamedatafrom the duty and power module, but return different resultssocomparisonscanbemade.Itisthesameasrunningallconfigurationsthroughthesamevirtualtrials.Themainpurposeofeachpoweringconfigurationmoduleistotakethepowerdemandassociatedwitheachoperatingconditionandtranslateitintocorrespondingoperatingpoints(powerandrev/min)fortheprimemovers,takingtheintermediateefficiencies,losses,andgearreductionsintoaccount.Withthisinformation,thefuelconsumptionandemissionsaredeterminedateachoperationconditionthroughtheequipmentmodules(seebelow)andsummeduponthebasis of the running times.

Intheexampleconsideredinthispaper,RAptures assumesfixedpitchZ-drivesareusedforallpoweringconfigurations,thedifferencesbeingonlyintheequipmentconfigurationsupplyingpowertothem.Mechanicallossesingearsandbearingsaremodelledasfixedpercentagesofpowerwithinthepoweringconfigurationmodule.Thisisinaccordancewithtypicalengineeringpractice,althoughtheprogramisamenabletoaccountingforspeed-dependentorotherlinearornon-lineareffectsifneeded.

Costsarealsocalculatedwithineachpoweringconfigurationmodule.Tomakerealisticandstandardisedcomparisonsbetweenpoweringconfigurations,life-cyclecostsareworkedoutforequipmentpurchase,

maintenanceandfuelfor20years.ANetPresentCost(NPC)iscalculatedforeachconfiguration,whichrepresentshowmuchmoneywouldhavetobeinvestedin the market today at a given rate of return (interest rate) tocreatethecashflowtopayfor20yearsofoperation.Interestandpredictedescalation(inflation)ratesareadjustabletosuittheclient’saccountingpracticesandeconomicprojections.Inthecalculationofequipment-relatedcosts,RApturestakesintoaccounthowthelife-cyclecostsareaffectedbyspecificvesseloperatingprofiles.Feweroperatinghoursonequipmentgenerallytranslatesintolowerannualmaintenancecostsandmoreyears between overhauls.

Capturingthiscost-savingeffectisimportant,particularlywherepoweringconfigurationsallowmachinerytobeshutdownwhenthepowerisnot needed. Operating in this manner, however, couldresultinmorewearandtearontheenginesandstartersduetomorefrequentstartingandstopping, thermal stresses, and wear from start-uplubeoildeficiencies.Generally,theseaspectswouldtypicallyhaveonlyasmalleffect,butshouldbesubjectivelyevaluatedininstanceswherethereareonlysmalldifferencesbetweentwocontendingarrangements.TherearecaseswhereRAptures needs tobesupplementedwithacombinationofoperatingexperience/preferencesfromowners,andintuitiveinput from designers.

Inthecasestudypresentedbelow,RAptures considersthoseitemsrelatedtothesupplyofpowertothemainpropulsionshaftingsuchasdieselengines,marinegears,generators,electricmotors,drivesandancillaries.CostsfortheshaftingitselfandtheZ-drivesarenotincluded,theseitemsbeingassumedmoreorlessequalforallconfigurations.

Figure 1: RAptures Architecture.

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Equipment modulesAseparateequipmentmoduleisestablishedforeachspecificmakeandmodelofdieselengine,genset,electricmotororotherequipmentitemsthatareusedwithinthepoweringconfigurations.Dataonfuelconsumption,emissions,andcost(usuallyprovidedbytheequipmentsupplier),isentereddirectlyintoitsmodule.Thisdataisthenmappedwithfittedpolynomialcurvessothatpredictionscanbemadeonthebasisofany given operating point and passed to the powering configuration.Wherespecificdataisnotavailablefromequipmentmanufacturers,genericequipmentmoduleswithtypicalcharacteristicsarecreated.

Inthecasestudypresentedbelow,threeequipmenttypesaredefined:mainengines,VFD-drivenpropulsionmotorsandfixedspeedgeneratorsets.Formainengines,specificfuelconsumptionaswellasspecificemissions(NOx,unburnedhydrocarbonsandparticulate)ing/kWhismappedaccordingtoenginerev/minandoutputpower.Forfixedspeedgensets,dataismappedwithrespecttogensetoutputpoweronly.WiththeVFD-drivenpropulsionmotors,driveandmotorefficiencyismappedformotorspeedandtorque.

CASE STUDY The Robert Allan Ltd RAmparts 2800 (Figure 2) isarelativelyrecent28mdesignwithinthehighlysuccessfulRAmparts series of tugs. To date, all ofthesetugshavebeenpoweredbyconventionaldieselmechanicalpoweringarrangements.Inlightofthepopularityofthistypeoftug,itislogicaltowonderwhetherfutureversionscouldbenefitfromaswitchtodieselelectricorotheralternativepoweringconfigurations.Furthermore,iftherearebenefits,howsensitivearethesetothespecificdutyinwhichthetugwillbeengagedonadailybasis?Toanswerthesekindsofquestions,RAptures is

applied to a notional version of the RAmparts 2800, withtheobjectiveofcomparingdieselmechanicaltothreealternativecandidate-poweringconfigurations.Giventhedependencyoftheoutcomeonthespecific

characteristicsofthecandidateequipmentandassumedtugoperatingprofile,theintentionisnottomake a generalisation on the best arrangement for tugs in general, but rather to show how RApturescanprovideresultstargetedtoaspecificcase.Thesametypeofanalysiscanbeappliedtoanyothervesselsconsideringotherequipmentoroperatingprofiles.

Powering configuration optionsInthecasestudy,fourpoweringconfigurationsareexaminedfortheRAmparts 2800:

• DieselMechanical(DM);• SeriesDieselMechanical/Electrical(SDME);• DieselElectric–RunningStandby(DERS);• DieselElectric–ColdStandby(DECS).

Theoutcomefortwodifferentoperatingprofilesarecompared:

• Harbourduty–Inthisdutythetugisprimarilyengagedinshipberthing/unberthinginaharbourenvironment, where transits are short. Between ship handlingoperations,thevesselspendsasignificantamount of time loitering.

• Shipassistduty–Inadditiontoshipberthing/unberthing,thetugistaskedwithaccompanyingships entering and leaving the harbour over a greaterdistanceforlightescortoperations.Overall,comparedwiththeharbourduty,thetugspendsmore time at higher transit speeds and less time at operationsinvolvinghighstaticthrust.

AllpoweringconfigurationsassumetwinfixedpitchZ-drives.Hotelloadsare80kWandmaximumpeakdeckmachineryloadsare80kW.Alldeckmachineryisassumedtobeelectricalorelectric/hydraulic.Specificequipmentdataisobtainedfromavarietyofspecificmanufacturerspublisheddatasheets.Allenginesarehigh-speeddieselsfromacommonmanufacturerandareEPATierIIrated.Figure 3 (at end of paper) shows thespecificfuelconsumptionoftheprimemoversasafunctionofthepercentratedpower.

TheDieselMechanicalconfiguration(Figure 4, at end ofpaper)hastwo2,000bkWmainenginesmechanicallycoupledtotheZ-drives.Hotelanddeckmachineryloadsaresuppliedbytwo175kWship’sservicegenerator sets arranged for independent operation.

TheSeriesDieselMechanical/Electricalconfiguration(Figure 5, atendofpaper)isanexampleofanalternative arrangement, but one that makes use of fairlyconventionaltechnology.Itincludesthesametwo2,000bkWmainenginesastheDMconfiguration,exceptthatanelectricmotorisnowinstalledbetweeneachofthemainenginesandZ-drivessothatthemotor is in series with the main engines. Power to the Z-drivesisprovidedbyeithertheelectricmotororthemain engines, but not to both at the same time. During Figure 2: RAmparts 2800 built by Cheoy Lee (2008).

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loiterorlowspeedtransit,theelectricmotorsdrivetheZ-drivesfromgensetpower;themainenginesaredeclutchedandnotrunning.Whenhigherpowerisrequired,themainenginesarestartedupandclutched-in,andtheelectricmotorsarefree-wheeled.Mainenginepowerispassedthroughtheelectricmotorssothemotorshaftsaresizedaccordingly.

This arrangement is similar to the one used in serieshybridsystems,egFoss’sGreen Assist hybrid tug,exceptthatthemotorsarenotalsogenerators,anddonotcontributetorquetothepropulsionshafting when the main engines are operating. TheelectricmotorsareAC-typecontrolledwithVariableFrequencyDrives(VFDs)ratedfor250kW.Electricpowerforthemotorsandforthevessel’sotherelectricloadsissuppliedbyeitheroneortwogensets, one of 560kWe, the other of 175kWe.

BothDieselElectricconfigurations(Figure 6, at endofpaper)havetwo2,000kWelectricpropulsionmotorsdirectlydrivingtheZ-drives.Electricalpowerisprovidedbyacombinationofthefollowinggensetsconfiguredtoallowstagingandfullloadsharing:2x2,200kWe;1x560kWe;1x175kW.ThetwoDEconfigurationsdifferonlyinhowthegeneratorsareoperated.

IntheDieselElectric–RunningStandby(DERS)configuration,whenBPoperationsareanticipated,the two main generators are started up and paralleled (runningstandby)sothatmaximumpowerisavailableandthesystemcanrespondalmostimmediatelytoanydemandfluctuationscommandedfromthebridge.However,duringlowerspeedtransitsnotinvolvingbollard operations, one or more main gensets are shutdownandthesmallership’sservicegenerator(s)provides power.

IntheDieselElectric–ColdStandby(DECS)configuration,itisassumedthatanygeneratoroperating surplus to the immediate power demand canbeshutdownimmediately,evenduringbollardoperations, and is only started up when needed (coldstandby).WhilethiswouldcertainlyreduceresponsivenesssubstantiallyincomparisontoDERS,especiallyduringbollardoperations,andisofquestionablepracticality,itisincludedforcomparison.

Tug operating profilesTheoperatingprofiles(Figure 7, at end of paper) for both the harbour and ship assist duties are based on a 12-hour daily operation, 360 days per year. On harbour duty,thetugspendsasignificantamountoftimeidlingandtransitingatlowspeeds,mixedwithshortperiodsof high BP operations. On ship assist duty, more timeisspentathigherspeedtransittorendezvous/assist ships, and less time on BP operations. These scenariosareconsideredreasonablyrepresentativeoftypicaldutyforthepurposeofthiscasestudy.

RESULTSResults for the harbour tug are presented in Figures 8-11(atendofpaper).Thenineoperatingconditionsthatcompriseeachoperatingprofile(duty)areshownalongthex-axis.Thewidertranslucentbarsindicatethepercentageoftimeallocatedforeach.ThenarrowersolidbarsarefueloremissionsquantitiesforeachPoweringConfiguration.Totalsaregivenonthe right.

Fuel consumption/CO2 emissions SinceCO2 emissions vary in proportion to fuel consumption,theseaspectsareintrinsicallylinkedincomparingtheconfigurations.Undertheharbourduty,thetwodieselelectricoptionsemergeastheleastfavourable. While there are some savings at lower powers, where the minimally loaded main engines of theDieselMechanicalarrangementsufferfromlowspecificfuelconsumptions,thechartillustratesthatmostofthefuelisstillconsumedduringstaticthrustoperations.Thisisthecasedespitethefactthatmuchmoretimeisactuallyspentloiteringormovingslowly.Atthehigherpoweroperatingpoints,thedieselelectricconfigurationssufferfromelectricalsystemlosses.Overallfuelconsumptionfordieselelectricishigherbyapproximately16percent.

Ofthetwodieselelectricoptions,theRunningStandby(DERS)systemcomesoutworstsinceallgeneratorsarerunningcontinuouslywheneverBPoperationsareexpected.ThisisincontrastwiththeColdStandby(DECS)configurationwheregensetsareonlystartedwhenneededandshutdownagainwhennot.Inreality,operatinggensetsthiswaywouldbeimpracticalformosttypicalharbouroperations,aspowerdemandvariesrapidly where diesel gensets need time to start up and parallel.Neverthelessthecomparisongivesanindicationofthepotentialsavingsinfuel,andunderscoreshowbatteriescanhaveabeneficialroletoplayinprovidingameanstorespondtoshort-termpowerfluctuationwithouttheneedtostartupagenseteachtime.

WiththeSDMEconfiguration,thesituationisbetter.SlightlylessfuelisconsumedthanwiththeDieselMechanicalconfiguration.Thisisasonewouldhope,sincetheSDMEarrangementisintendedtoavoidthepitfallsoftheothers:inthecaseofDieselMechanical,mainenginesoperatingunloadedatlowpoweroperatingpoints;forthetwoDieselElectricconfigurations,comparativelyhighelectricallosses.However,thedifferenceisfairlysmall,only3percentlessfuelthanDieselMechanicaland,onitsownperhaps,notincentiveenoughtomovetothisconfigurationwithouttheprospectofadditionaladvantagesofreducedemissionsorlowercosts,aspectswhicharediscussedbelow.(Figure 8).

NOx, Hydrocarbon (HC) and particulate emissionsWithdieselprimemovers,NOx,HCandparticulateemissionsvarywithequipmentdesign,fuel

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management, off-engine measures, rate of power variations, and a host of other operational and environmental parameters. Data is not always easy to comeby,thesourceoftenbeingthesupplier,wherepossiblydatawascollectedtoproveconformancewithregulatoryrequirements.Forthiscasestudy,theemissionsdatawasbasedontypicalpublishedvaluesforthespecificprimemoversusedinthecasestudy.Asthedataisforsteadystateloadconditions,itdoesnotcapturetheeffectsofrapidtransientloadfluctuations.Nevertheless,comparingemissionsonaquasi-steadystatebasisisareasonableapproach.

Acommontrendisseenintheemissionscharts(Figures 9-11, atendofpaper):whereprimemovermachineryisforcedtooperateawayfromitsdesignoperatingpoint,specificemissions(emissionperbrakekW)tendtobecomeworse.Whileimprovementsinemissionstechnologymayreducethissensitivity,itischaracteristicoftoday’smarinemachinery.Forthisreason,atlowpowersthebenchmarkDieselMechanicalconfigurationhastheworstemissionsofallthepoweringconfigurationsduetothemainenginesrunningsofarbelowtheirratedpower.Forthelow-andmid-powersofidleandtransitoperations,dieselelectricconfigurationsfarebetterthanDieselMechanical,asoptimalloadingpointsforsmallergeneratorsarepossible.However,atthehigh-powerBPconditions,dieselelectriclosesitsadvantagesincetheinherentelectro-mechanicalconversionlossesreduceefficiencycomparedwithDieselMechanical.

TheabilityoftheSDMEconfigurationtooperateelectricallyatlowpower,optimallyloadingthegensets,butswitchingovertodirectDieselMechanicalforhigherspeedtransitandBPconditionsgivesittheedgeovertheconventionalDieselMechanicalconfiguration.Asthechartsclearlyindicate,theadvantageissubstantialasaresultofthesignificantpenaltyDieselMechanicalsuffersunderlow-powerloiteringconditions.ThenetresultisthatemissionsfromtheSDMEconfigurationarethelowest overall.

COSTSBarchartsofthecosts(Figure 12, at end of paper) comprisethreecomponents:fuel,equipmentmaintenance,andequipmentpurchase.NetPresentCostsinthisexamplearecalculatedonaninterestrateof5percent,anannualescalationof2percent,atafuelpriceof$0.60/litre.

Itisimmediatelyapparentthatthetwodieselelectricoptionsarethemostexpensiveowingtotherelativelyhighcapitalcostofthefourgensets,plusthecostsforelectricpropulsionmotors,switchgearandVFDs.Thehigherfuelconsumptionisalsoafactor,buttoalesserextent.Asthebreakdowninthechartshows,itisprimarilythepurchasecostofequipmentthatpushesthetotalcostbeyondtheotherarrangements.Maintenancecostsareless,particularlyfortheColdStandbyversion,butnotsufficientlytobringthetotalcostbelowtheDieselMechanicalandSDMEconfigurations.

ThecomparisonofDieselMechanicaltoSDMEisinterestingsincetheSDMEconfigurationcomesoutaslessexpensiveoverallthanDieselMechanical.Thesmalldifferenceinfuelconsumptionreducesfuelcostsbyapproximately3percentbelowDieselMechanical,buttherealsignificanceliesintheequipmentpurchaseandmaintenancecosts.AlthoughtheequipmentpurchasecostisgreaterfortheSDMEconfiguration,aswouldbeexpectedgivenrelativelyhighcostfortheelectricmotors,drivesandgenerators,overtimethisisoffsetbyconsiderablylowerlife-cyclemaintenancecosts.WiththeSDMEarrangement,themainenginesaccumulatefewerrunninghoursandthisisanadvantagesincetheyaremoreexpensivetomaintainand overhaul than the gensets.

ForanoperatorconsideringtheSDMEoption,thefactthatthelowermaintenancecostmorethancompensatesfortheadditionalequipmentpurchasecostisakeyconclusionthatillustrateshowallcostelementsneedtobeconsideredcollectively.Regrettably,makingaccuratepredictionsofperhourmaintenancecosts,timebeforeoverhaul,andtheactualcostofoverhaulcanbeadifficultproposition.Datafromsupplierscanbenotoriouslyvariablegiventhemanyfactorsthatcomeintoplay,includinghowrigorouslytheequipmentisoperated,themaintenanceskillleveloftheoperator,andenvironmentalandlogisticalaspectsthatcanconstrainservicingintervals.Forthiscasestudy,datafromvariousequipmentsupplierswasblendedtoarriveatvaluesconsideredtobereasonablyrepresentativeofequipmentofthetypeunderconsideration.Ofcourse,thebestsourceofdataisfromtherecordsoftheoperatoritselfforsimilarequipmentandoperationalconditions,ifavailable.

Despitetheinherentuncertaintyinmaintenancecostprojections,thisdoesnotdetractfromthekeypointthatthecasestudyresultssuggestthattheSDMEconfigurationcouldhaveasignificantcostadvantageover20yearsbysavingmaintenanceonthemainengines,atleastforthesubjectvesselunderthegivendutyprofile.

COMPARiSON TO SHiP ASSiST DUTY With RAptures,oncetheconfigurationoptionsaredefined,itisamatterofafewkeystrokestoseehowchangingtheoperationalprofileinfluencesfuelconsumption,emissionsandcost.Toillustratethis, results for the same vessel, with the same four poweringconfigurationoptions,arepresentedfortheshipassistdutyasdefinedabove.

Lookingatfuelconsumptionfirst(Figure 13, at end ofpaper),itisimmediatelyclearthat,withmoreoftheworking day spent in transit and less time loitering, fuel consumptionissignificantlyhigherthanwiththeharbourtug duty. Most of the fuel is used for the high-speed transit at 12.5 knots, not during bollard pull, as is the casewiththeharbourduty.

Itisworthnotingthatatthe12.5knottransit,thereiseffectivelylittledifferenceinhowtheDieselMechanical

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andSDMEconfigurationsoperate:bothareusingthemainenginesonly,directlycoupledtothepropulsionshafting.ThesingledifferenceissomelossoftheshaftpowerpassingthroughtheinactiveelectricmotorintheSDMEconfiguration.ThesamesituationexistsbetweenthetwoDieselElectricconfigurations:thesamenumberofgeneratorsisrunningineachcase.FortheRunningStandbyversion,thestandbygeneratorsarerunningonlywhenbollardoperationsareanticipated,notfortransit.Thereforetheapproximately12percentgreaterfuelconsumptionoftheDieselElectricconfigurationisattributablemainlytotheloweroverallefficiencyarisingfromelectricalconversionlosses.

Thedominanceofthis12.5knottransitoperatingpointalsocarriesoverintoemissions(Figures 14-16, atendofpaper).Thegeneraltrend,whencomparedto the same results for harbour duty, is that the Diesel ElectricconfigurationscomparelessfavourablytotheDieselMechanicalandSDMEconfigurations.Atthatpowerlevel,theelectro-mechanicalconversionlossesoutweightheefficiencypenaltyinrunningthemainengines at a lower load level.

IncomparingtheDieselMechanicalandSDMEconfigurations,withtheshipassistdutythereisanarrowingofthedifferencesinallareas,includingcosts(Figure 17, atendofpaper).Sincemoretimeand fuel is spent in transit in the ship assist duty, andlesstimeloitering,theSDMElosessomeoftheadvantagesithadinrunningmoreefficientlyandproducinglessemissionsatlowpower.However,overallitremainsanattractiveoption.

SUMMARYForthecasestudyfortheRAmparts 2800 tug there islittlebenefittothetwodieselelectricconfigurations(DERS,DECS)overtheothertwoconfigurationsintermsofcostoremissionsforeitheroperationalprofile.ComparedwiththeconventionalDieselMechanicalconfiguration,SDMEappearstobeaveryattractiveoption,particularlyfortheharbourdutydefined.

CONCLUSiONSThe RAptures program is proving to be a versatile toolforassessingandcomparingthefuel,emissionsandcostperformanceofanyalternativepoweringconfigurationsthatprogressivetugoperatorsmaywanttoconsiderforreducingcostsandemissions.Thespecificresultsfromtheprogramcanbeusedtoverifyoperationalbenefitclaims,andcanpreventinvestmentintechnologiesthatmayprovedisappointing.ItsdevelopmentisdrivenbyRobertAllanLtd’scommitmenttothedesignofcleaner,moreefficientvesselsandrecognitionthat,inweighingthetrade-offsbetweenpoweringconfigurationoptions,thewidelyvariableoperationalprofilesoftugboatspreventanyonesolutionfrom being a universally best one.

Asillustratedbythecasestudy,thistypeofanalysisisessentialtotheprocessofproperlyweighingthepotentialimprovementsagainstthecostsofalternativepoweringconfigurations.WithRAptures as a tool, Robert Allan Ltd isreadytoworkwithclientstoachieverealenergyandemissions savings.

Figure 3: Equipment specific fuel consumption.

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Figure 4: Diesel Mechanical (DM) Configuration.

Figure 5: Series Diesel Mechanical/Electrical (SDME) Configuration.

Figure 6: Diesel Electric Configurations (DERS, DECS).

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Figure 7: Operational Profiles Duty Cycles.

Figure 8: Fuel consumption – harbour duty.

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Figure 9: NOx emissions – harbour duty.

Figure 10: Hydrocarbon emissions – harbour duty.

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Figure 11. Particulate emissions – harbour duty.

Figure 12: Costs – harbour duty.

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Figure 13: Fuel consumption – ship assist duty.

Figure 14: NOx emissions – Ship Assist Duty.

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Figure 15: Hydrocarbon emissions – Ship Assist Duty.

Figure 17: Costs – Ship Assist Duty.

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Figure 16: Particulate emissions – Ship Assist Duty.