GASTECH 2OO2

PROGRAMME

Janne Kosomaa, Project Manager, Product and Application Development, Wartsila

Janne Kosomaa (29) graduated from Helsinki University of Technology (HUT) as M.Sc. in Mechanical Engineering in 1999. He started his professional career as Project Engineer at Wärtsilä U.K. in Aberdeen in 1998. His main duties were technical support for operation and maintenance of offshore gas engine power plant. In August 2000 he moved on as R&D Engineer to Product and Application Development department in Wärtsilä Corporation Marine Division in Finland, Turku. In June 2002 he was promoted Project Manager. Marine application development for gas engines is among his main duties.

August 6th 2002

GASTECH 2002Qatar, 13th - 16th October

The DF-Electric LNG Carrier ConceptJanne Kosomaa, M.Sc.Wärtsilä Corporation

Marine Division

August 6th 2002

Kosomaa 1

ABSTRACTLNG carrier propulsion has traditionally beenbased on a steam turbine power plant. The mainreason for this has been the ability to utilisecargo boil-off gas as fuel. This propulsionsystem is also considered to be very reliable.However, the economical and environmentaldisadvantages of steam turbine propulsion aredriving operators to seek alternative means forLNG carrier propulsion.

Handling of the boil-off gas sets thetechnological limits for alternative LNG carrierpropulsion system. Recent technologicaladvances have finally brought some solutionsthat enable to displace the steam turbinepropulsion. One technology is based onreliquefaction of boil-off gas, the other on usingit as a fuel.

In addition to operating economy andenvironmental issues, also the very specialrequirements of LNG carrier operation have tobe considered carefully when selecting thepropulsion power plant. Safety and redundancyare the most important features required fromthe alternative power plant. Electric propulsionbased on Wärtsilä dual fuel engines is shown tobe a strong alternative for LNG carrierpropulsion system (figure 1).

INTRODUCTIONThe increasing demand to supply gaseous fuelfor the needs of the more environmentallyfocussed energy production is also affecting thevolume of marine transport of gas. A clear needfor more Liquefied natural gas (LNG) carriers isevident: 10 – 12 ships are predicted to be builtannually over the next few years compared tothe 6 – 8 ships built every year over the pastdecade. Furthermore, the need to replaceexisting capacity will also amplify this need – infact nearly all of the gas carriers built since thebeginning of the trade in the 1960`s are stilloperational.

The traditional steam turbine propulsion hasserved LNG carriers well for over three decadesnow. However, recent developments intechnology finally enable also alternativepropulsion systems to be considered also for thisship type. Ever-increasing consciousness aboutcost and environment are putting a lot ofpressure to move away from steam turbine into anew era in LNG carrier propulsion.

For the start this paper will handle shortly themain characteristics of LNG carriers and LNGshipping business to give some idea about thereasons behind the propulsion concepts thathave been used and are being proposed for this

FIGURE 1. Artistic impression of membrane-type DF-electric LNG carrier

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ship type. In this paper some alternativepropulsion concepts will be overviewed. Thefocus of this paper is to introduce the Dual Fuel(DF) engine technology together with the DF-electric LNG carrier concept in more detail.

CHARACTERISTICS OF LNGCARRIERS

The LNG trade has traditionally been based onlong term shipping contracts and dedicatedfleets of ships sailing on the fixed routes andschedules between the rather limited number ofLNG terminals of the world. The LNG supplychain does not have much buffer capacity and itis very important that the cargo is delivered ontime. Heavy penalties are imposed on delays.

Steam turbine propulsion dominates in thevessels currently operating in the global LNGcarrier fleets. Original reasons for this has beenavailability of high power output combined withthe possibility to use low-grade fuels.Maintenance of the turbines is relatively lowcost and infrequent and the systems areconsidered proven and reliable.

The key issue, however, is the possibility toutilise the boil-off gas from the cargo tanks.This boil-off gas must be disposed of somehowand the simplest way is to burn it in a boiler.Actually, this has previously been the onlyfeasible method of disposing the boil-off gasand getting some use of it at the same time. Thisfeature has prevented any other propulsionsystem entering LNG carrier market and madesteam turbine a natural choice for LNG carriers.

The available amount of natural boil-off-gasdepends on the ship design specification andoperating conditions. A natural boil-off rate of0,15% per day is typically considered as adesign point. However, values as low as 0,10%have been reported in operation onboard modernvessels. On the other hand the boil-off rate canalso be higher in unfavourable conditions.During ballast voyage the amount of availableboil-off gas varies between 10-50% compared tothe laden voyage depending on how many tankswill contain heel LNG.

As well as the quantity, also the gas compositionvaries during the voyage. The natural boil-offgas is mainly a composition of methane andnitrogen. The Nitrogen content can be high atthe beginning of the laden journey anddegreases during the trip. This results in varyingheating value of the boil-off gas. On ballastvoyage major part of the boil-off gas isgenerated from the tank cooling spray. In thisprocess also the heavier hydrocarbons of theLNG evaporate. This results in higher energycontent of the gas, but lower the methanenumber at the same time. The samephenomenon applies to forced boil-off gas.

As stated above, both the quantity and heatingvalue of the boil-off gas will vary considerablyduring a return voyage. Alternative LNG carrierpropulsion system has to include machinery thateither utilises boil-off gas as fuel or reliquefiesit. Therefore the above properties of boil-off gashas to be kept in mind when configuring themachinery systems.

Safety is of utmost importance for LNGshipping business, and LNG carriers have anexcellent safety record. Reliability of steamturbine propulsion has helped to achieve thistogether with strict terminal rules and robustship design. For example, the number of tugsrequired to enter and leave a terminal is definedin the terminal rules. Also propulsion power hasto be available all the time during harbouroperations. The design life of LNG carrier isvery long, up to 40 years. All these featureshave to be taken into account also on thepropulsion plant design.

WINDS OF CHANGEDue to the increasing demand and supply ofliquefied natural gas the number of short-termcontracts and even spot cargoes has increasedand will continue to do so in the future.Currently some LNG carriers have even beenordered without any contract or route securedfor the ship, which is previously unheard in theLNG business. From the shipping point of viewthis means that the operators are bound to lookfor ships with more operational flexibility and

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efficiency in response to varying contractualsituations. Primarily this calls for a flexible andefficient propulsion plant able to accommodatedifferent ship speeds and alternative operatingprofiles.

The main drawback of the traditional boiler andsteam turbine system is the inefficiency, andhence high fuel consumption, of the propulsionplant (figure 2). The lack of alternative usagefor the boil-off gas has lead into thinking thatthe boil-off gas would be free. Alternativemethods to utilise boil-off gas have forced tochange this thinking. Furthermore, in modernLNG carriers the amount of natural boil-off gasis decreasing due to advances in tank insulationtechnology and design. Hence the energy in thenatural boil-off gas is far from sufficient toproduce the propulsion power needed for therelatively high operating speeds. Therefore,forced boil-off gas or heavy fuel oil is needed totop up the fuel demand of the boilers, which isyet another argument encouraging shippingcompanies to look for a propulsion plant withhigher efficiency.

High fuel consumption of a steam turbinepropulsion plant, in addition to being costly,leads directly to high CO2 emissions (figure 3).Carbon dioxide emissions of ships are believedto get evermore focus in the future. On the otherhand, NOx emissions of traditional LNG carrierare very low due to the characteristics ofcombustion process in the boiler. SOx emissionsof steam turbine propulsion are alsoconsiderable because of the usage of heavy fuelto top up the energy requirement. Typically on aladen voyage around 50% of the energyrequirement comes from heavy fuel, and up to80% respectively during ballast voyage.Environmental aspects are getting moreimportant all the time, and it is easy to predictthat legislation on emissions will not get anyeasier during the 40 year lifetime of LNGcarrier.

Among other arguments that are often heardcriticising steam turbine propulsion areincreasing lack of competent steam operators,poor manoeuvring characteristics, and limitedpropulsion redundancy.

FIGURE 2. Efficiency comparison between DF-electric and steam turbine propulsion

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ALTERNATIVE PROPULSIONCONCEPTS

GeneralThe diesel engine has for decades dominated allother sectors of the merchant shipping exceptLNG carriers. The accumulated experience ofthousands of propulsion plant installations basedon diesel machinery has helped to ensure thesuccessful development of this technology.Meanwhile steam plant development hasvirtually stood still as there has been practicallyno market for marine applications since the1973 oil crisis. Also, the recent development ofdual fuel (diesel and gas) operated engines,derivative from heavy fuel diesel engines, havemade it possible to use the boil-off gasefficiently, and therefore propulsion based ondiesel or dual fuel engines is a strong option formodern LNG carriers today.

Gas turbines have also been proposed for LNGcarrier propulsion using mechanical or electricpropulsion. Gas turbines are light and virtuallyvibration free and have some dual fuel

capabilities (using MGO as backup fuel).However, gas turbines have been available for along time also for LNG carrier applications, aswell as for other marine applications, but theyhave not been considered feasible. The mainreasons being relatively high-pressure gasrequired and inefficiency of the power plant.The inefficiency can be to some extentcompensated with combined cycle system,where waste heat is recovered. However, thiswill, together with the required auxiliarygenerators, make the installation much morecomplex, heavier, space demanding andexpensive.

When specifying propulsion machinery optionsfor LNG carriers it is essential to consider thedifferences in operating profiles, fleetconfigurations and shipping routes. The basiccase today is a 138000 m3 vessel with anoperating speed of around 19,5 knots and thecorresponding power required at propeller ofapproximately 26 MW. However, futureoperating profiles of LNG carriers will requiremore flexibility from the power plant. Already

SOx [ton/year]

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FIGURE 3. Emission comparison between steam turbine and DF-electric propulsion.

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there are ships being build that would normallyoperate about 15 knots speed, but have to becapable of doing 19 knots on spot cargo trade.Then it is very important that the power plantwill be efficient also at part load operation. Themaximum required electrical power for cargopumping and other consumers is roughly 6 MW.At the lowest the electrical load can be less than1 MW.

As stated above, the energy in the boil-off gaswill vary considerably during a return voyage ofLNG carrier. When converting the energycontent available in the in the boil-off gas of a"full size" LNG carrier into mechanical power atthe flywheel using a modern dual fuel gasengine figures ranging from the 12 MW of theworst case up to 25 MW at the best situation canbe calculated for the laden voyage. In ballastconditions the figures are typically half of theabove, but can be even less. This means thateven at the best case the natural boil-off wouldnot be enough to cope with the energyconsumption and either forced boil-off gas orsupplementary liquid fuel is needed to make upthe shortfall. The selection of supplementaryfuel depends on the result of a feasibility studytaking into account not only the operatingprofile of the ship but also the trends and effectsof the unstable oil price and further moreavailability of liquid fuel of correct grade at thevicinity of the LNG terminals.

One option to be considered is not to use theboil-off gas as fuel for production of propulsivepower at all. Instead, this gas is reliquefied andreturned back to the cargo tanks to be carried tothe final destination. In such an option thepropulsion would be based on heavy fuelburning diesel engines just as in almost anyother modern large cargo vessel in operationtoday.

Steam turbine propulsion has been accused ofrelatively large machinery room space demand.The machinery space could in theory be madeconsiderably shorter by using almost any of theproposed alternative propulsion concepts.However, many existing LNG terminals set alimit for maximum draught and displacement of

LNG carrier. Therefore the benefit of a shortermachinery space usually cannot be fully utilisedfor increased cargo capacity with the same shiplength. Also, the structural limitations of cargocontainment systems may prevent fully utilisingthe benefit of a shorter machinery space foradditional cargo capacity. In this sense themembrane-type cargo containment system hasan advantage over the spherical-type system.

The following paragraphs describe someoptional propulsion configurations for LNGcarrier, which have been considered in order tosee what are the merits and drawbacks of thevarious concepts.

Propulsion Based on Low-Speed DieselEngines

A single screw single two-stroke main enginewould be the simplest solution for shippropulsion. However, this arrangement wouldnot provide any redundancy in case of failures.The extremely high demands for safety andavailability mean that market is not likely toaccept this solution for LNG carriers.Redundancy can be achieved with twin enginearrangement or by fitting an auxiliarypropulsion drive (APD).

Electric auxiliary propulsion drive on a single-screw single-two-stroke installation wouldrequire a complex and expensive gear and clutcharrangement. In order to utilise the APD also asa shaft generator, the system should include atwo-speed gearbox and a controllable pitchpropeller. Also adequate auxiliary generatorcapacity has to be installed to cover the powerrequirement of APD in addition to the usualelectric consumers.

Twin-screw twin-two-stroke mechanicalpropulsion could well compete with otheralternative propulsion solutions in fuelconsumption and machinery investment cost,offering also redundancy and manoeuvrability.Two directly coupled slow-speed engines coulddo the job, but they would have to utilise CP-propellers and possibly also gearboxes.

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As stated previously, in order to utilise astandard diesel engine running with heavy fuel,a boil-off gas reliquefaction plant would have tobe installed onboard. The reliquefaction plantshave been tested in marine conditions but thetechnology is not yet considered mature. Thereliquefaction plant is quite expensive, morethan five million USD, and consumesconsiderable amount of energy (about 3,5 MW).Therefore, in addition to the technical risk theutilisation of reliquefaction plant is verysensitive to LNG price compared to heavy fuelprice.

Furthermore, many of the operators arenowadays very conscious about theenvironmental aspects. These vessels aretransporting "the cleanest fuel" for the powergeneration market, and it would seem to benatural to use the same fuel onboard the vesselas well. Without additional equipment, NOxemissions of two-stroke engine are relativelyhigh. High SOx emissions are also inevitablewhen heavy fuel is burned.

There are also gas diesel low speed enginedesigns readily available for the market. Thiskind of engine requires a large reciprocatingcompressor plant to provide high-pressure gasinjection. Such two-stroke diesel engines havebeen tested, but further development and designwould be necessary to make it commerciallyavailable. However, the biggest obstacles are theparasitic energy consumption of the high-pressure fuel gas compressor (approximately 6%of the power output of the engines), the complexcontrol system required, and the industryreluctance to accommodate high-pressure gassystems onboard.

Among other concerns that have been raisedabout utilising two-stroke engines in LNG-carriers is the vibrations induced by the rigidlymounted power plant. This may increase the riskof fatigue damage for the cargo tanks during thelong lifespan of the ship.

Mechanical Propulsion Based onMedium-Speed Engines

Propulsion with medium-speed diesel engineswould be naturally multi-engine installationoffering inherently some redundancy. Theinstallation can be based on single- or twin-screw arrangement.

A single-screw twin-four-stroke diesel engineoption requires a moderate gear, couplings, anda CP-propeller. Continuous low load operationon heavy fuel is not desirable, and thereforePTO and shaft generator would probably not befeasible due to low load in port for the runningof large main engine (electrical cargo pumps).Therefore, adequate auxiliary generator capacityhas to be installed to cover the powerrequirement of electric consumers.

A twin-screw diesel-mechanic medium-speedsolution could have two (single-in-single-outgearboxes) or four engines (twin-in-single-outgearboxes). In case of four-engine arrangement,where engine size becomes naturally smaller,one main engine could drive a primary generatorsuitable for cargo operation, which could lead toreduced installed auxiliary generator capacity.

Again, the utilisation of standard heavy fueldiesel engines requires installing areliquefaction plant associated with thepreviously mentioned drawbacks.

There are also medium-speed gas diesel enginesavailable for propulsion purposes. Thetechnology is available and widely used formany years in marine (offshore) installations.High-pressure gas would have to be used as fuelin such a mechanical drive application andsimilarly to the two-stroke gas diesel engine,this has not been considered feasible for LNGcarrier.

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Electric Propulsion Based on Medium-Speed Engines

Electric propulsion offers by far the mostflexible alternatives for the machineryarrangement. It is also easy to build redundancyin the system with divided engine rooms andauxiliary systems. Medium-speed generator setscan be either heavy fuel burning diesel enginesor low-pressure dual fuel engines. Propulsionsystem based on heavy fuel engines willnaturally require reliquefaction plant to takecare of the boil-off gas. Dual fuel engines canuse either gas or MDO as fuel. With electricpropulsion, there is no need for separateauxiliary generator sets, so actually the totalinstalled power can be reduced. With electricalcargo pumps one generator set should be able tohandle the power demand for cargo operations.

Single-screw electric propulsion with FP-propeller may be selected if appropriateredundancy is built into the electric drivesystem. More than one electric motor can beused for one shaftline either in tandemarrangement (slow-speed electric motors) orthrough a gearbox (high-speed electric motors).The electric motors can also be double woundfor additional redundancy.

Twin-screw propulsion can be configured eitherwith podded drives or FP-propellers driven byelectric motors. Another possibility would be toutilise a single-screw FP-propellercomplemented by a podded drive replacing therudder. Using the “contra rotating propellerprinciple” considerable propulsive efficiencygain is possible. The main propeller would caterfor approximately half of the requiredpropulsive power. The other half would beprovided by the podded drive.

Also electric propulsion system based oncombination of heavy fuel generator sets andgas burning generator sets has been proposed.This is based on the desire not to install anyreliquefaction plant and utilise natural boil-offgas only as fuel (and no forced boil-off gas) andtop up the remaining energy requirement withheavy fuel. In order to optimise the power plant

in varying boil-off gas energy flows requires acomplex control system. Furthermore, the totalinstalled power would be high. In order todispose the all the boil-off gas on laden voyage,about 29MW worth of gas burning electricpropulsion equipment must be installed. On theother hand to cover the top up energyrequirement on ballast voyage, up to 27 MWheavy fuel burning diesel power has to beinstalled. Total installed power would beconsiderable, perhaps even 65%, higher thanpropulsion plant including only heavy fuel oronly dual fuel engines.

Combined Mechanical and ElectricPropulsion

Combined mechanical and electric propulsionhas been proposed based on the desire not toinstall any reliquefaction plant and utilisenatural boil-off gas only as fuel (and no forcedboil-off gas) and top up the remaining energyrequirement with heavy fuel. Then one or moreheavy fuel operated engines would be driving amechanical propulsion line, and additionalgenerator sets burning boil-off gas wouldprovide electric power for a booster drive. Thebooster drive could be a POD or an electricmotor connected to a gearbox. There aretechnically several possibilities to realise thisconfiguration. However, similarly to the APDsystem, this configuration is very expensive andcomplicated. Even more important obstacle forthis arrangement is the high cost of excessiveinstalled power similarly to the case that waspresented in the previous paragraph.

THE DF-ELECTRIC LNGCARRIER CONCEPT

GeneralThe approach Wärtsilä considers most feasiblefor LNG carrier is based on the electricpropulsion where the prime movers in the powerplant are four-stroke low-pressure dual fuelengines. The main arguments in the comparisonare high thermal efficiency and safety, as well asflexible and efficient use of the installedmachinery. The selection of either single screw

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ship or twin screw has to be based on theoperating profile and redundancy requirementsspecific for each project.

The number of engines and the power output ofeach unit are determined by the shaft powerneeded and also by the degree of redundancyrequested. Generally speaking on a typical138000 m3 ship with the need for approx. 34MW total engine output the power plant wouldconsist of four dual fuel engine generating setsof the Wärtsilä 50DF type engines. The MCRoutput of these engines is 950 kW/cylinder andthe thermal efficiency as high as 48%!

The recommended plant consisting of four 9-cylinder units (4 x W9L50DF) would produce34,2 MW, and some redundancy for thesituation where one of the engines would be outof service. On the other hand it would alsoprovide welcome flexibility for the differentoperating modes such as manoeuvring, waitingfor port access, loading and unloading.

Recent studies suggest that the most beneficialsolution to top-up the need for additional energyis to use forced boil-off instead of diesel fueloil. Therefore, main fuel for engines is gas, andmarine diesel oil will be used as pilot and back-up fuel only. This solution in combination withthe dual fuel engine electric propulsion iseconomically very attractive both in installationcost and operation.

As the DF engine is operated on low-pressuregas, between four and five bar at the engineinlet, the fuel gas compressor package isessentially similar (only multi-stage instead ofsingle-stage) to the one already in use in thecurrent fleet equipped with steam boiler andturbine propulsion. The main difference is thatthe total efficiency of the dual fuel-electricpropulsion is well above 40% compared to thereported less than 30% of the steam plant. Thedifference is even higher on part load.

Moreover, flexible preventive maintenance atsea and during port calls is possible, which hasnot been the case with the steam plant or withlarge single 2-stroke alternatives. Electric

propulsion technology is available today and hasbeen proven in various marine applications forseveral decades. Wärtsilä dual fuel engines haveaccumulated considerable number of operatinghours in land- based installations and are maturefor marine installations. This has been alreadyrecognised by recent orders booked for the32DF-type generating sets for offshore andmarine installations. Perhaps even moreimportant recognition is the Wärtsilä 50DForder for the first LNG carrier to be utilisingDF-electric propulsion. In other wordseverything is ready for taking the next steptowards the modern, efficient LNG carrierpropulsion.

The Wärtsilä Dual Fuel (DF) EngineThe Wärtsilä DF-engine is a medium-speed,four-stroke, turbo-charged, trunk piston, non-reversible engine, which can be run alternativelyin gas mode or diesel mode. In gas mode it runsas a lean burn engine utilising the Otto-principle. The ignition is initiated by injecting asmall amount of diesel oil (pilot fuel), giving ahigh-energy ignition source for the main fuel gascharge in the cylinder (figure 5). In diesel modethe DF-engine works just like any diesel engine,utilising traditional jerk pump fuel injectionsystem (figure 4). Transfers between the twooperating modes happen without interruption inpower supply.

With a lean fuel mixture it is possible to achievegood engine characteristics regarding efficiencyand emissions. However, at higher loads theuseful operating window between detonationand knocking is very narrow. To stay within theoperating window and have optimalperformance for all cylinders regarding safety,efficiency and emissions in all conditions,Wärtsilä DF-engine features a system to controlthe combustion process individually for eachcylinder. This makes it possible to obtainoptimal operating performance at conditionswhere gas quality, ambient temperature etc.varies. In order to achieve the required accuratecombustion control, the gas admission and pilotfuel injection is electronically controlledindividually for each cylinder. The control is

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automatically handled by Wärtsilä EngineControl System (WECS).

LNG boil-off gas is very good fuel for theWärtsilä DF-engine. As stated previously, theonly considerable variation in the natural boil-off gas composition is the Nitrogen content.Nitrogen as such is not harmful for the engine –the air that we and also the engines normallybreathe contains more than 78% Nitrogen.

However, as it is an inert gas and will notcontribute to the combustion the energy content(heating value) of the BOG is lower than that ofpure methane. The nitrogen content in thevapour phase of the LNG can be as high as 30%in volume especially at the beginning of theloaded trip. This is not a problem for theWärtsilä DF-engine as the engines can beoperated at their nominal output without de-rating with such a gas mixture.

IGNITION BY PILOTDIESEL FUEL

COMPRESSION OFAIR & GAS

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FIGURE 5. Dual fuel engine operating principle in gas mode.

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FIGURE 4. Dual fuel engine operating principle in diesel mode.

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The DF-engine is designed to operate in gasmode at the same safety level as in diesel mode.The safety is based on early detection ofproblems that can lead to a hazard, followed byimmediate actions to bring the situation to a safestate. Depending on type of problem detected, itcan initiate alarm, transfer to diesel mode,shutdown of the engine, or emergency shutdownof the engine.

The DF-Electric LNG Carrier DataWärtsilä's concept ship is constructed as a dualfuel electric driven single screw LNG carrierwith four cargo tanks. The hull has transomstern, single skeg aft body and a bulbous bow.The propulsion machinery and accommodationspaces are arranged in the stern part. The CargoMachinery Room shall be arranged apart fromthe accommodation space on the upper deck.Two cargo tank system variants can be applied;membrane-type and spherical-type (figure6).Main dimensions of the variants can be seen intable 1.

The main machinery consists of four dual fuelengines, each driving an AC-generator. Thepropellers are driven by two AC-propulsionmotors through couplings and one commonreduction gear.

TABLE 1. Main Dimensions.Main particulars: Spherical Membrane

Length over all 290 m 290 m

Length btw.perpendiculars 275 m 275 m

Breadth 48 m 43 m

Depth to upperdeck

27 m 27 m

Draught, design 11 m 11 m

Draught, scantling 12 m 12 m

Air draught 59 m 56 m

To enhance the redundancy of the propulsionplant the main engine rooms and casings aredivided with fire resistant bulkhead. Mainengine rooms are under diminished air pressureand these rooms have own compressed air duct.Combustion air to the each main engine isprovided using dedicated duct.

The four main dual fuel engines are of Wärtsilä9L50DF type The engines are equipped withdirect diesel pilot fuel injection, low pressuregas system and dry sump.

FIGURE 6. Artistic impressions of spherical and membrane type DF-electric LNG carriers.

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Wärtsilä 9L50DF marine medium-speed dualfuel engines:

Number 4Engine designation 9L50DFNumber of cylinders 9Engine speed nominal 500 rpmMaximum continuous rating 8550 kWTotal installed power 34,2 MW

The service speed of the ship shall be about 19,5knots at the design draught of 11.0 m and with15% sea margin which corresponds to 27 MWshaft power and about 90% DF-engine power(figure 8). The power for accommodation andmachinery auxiliary consumers is about 1 MW.

Propulsion machinery:

Propeller 1 x FPPPropulsion power 27 MWBow Thrusters 2 x 1000 kW

Emergency engine:

Power Output 1 x 500 kW

Reduction gear:

One common type gear with twin input andsingle output.

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FIGURE 7. Machinery arrangement of DF-electric LNG carrier

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Shaft line:

Shaft line consist of shaft seals, shaft bearings,intermediate shaft and propeller shaft.

Propeller:

Five bladed fixed pitch propeller. The propellerand propeller shaft are coupled through conicalkeyless friction joint.

Main generators and power transmission:

Four main generators are feeding ship’s electricnetwork and via variable speed drive system twoAC-propulsion motors.

Nominal power of AC-propulsion motors:

2 x 13,5 MW

The main propulsion machinery arrangementcan be seen in figures 7 and 9.

DF Gensets

El-motors & Gearbox

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FIGURE 9. The DF-electric LNG carrier main propulsion machinery.

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FIGURE 7. Propulsion power requirement.