32 - Wärtsilä 1-2004

The Ship Power Supplier

Until now, steam turbines have dominatedas propulsion machinery for LNG carriers,with their apparent reliability and the easewith which they can burn the boil-off gasfrom the ship’s cargo tanks while at sea.

The low fuel efficiency of steam turbines,however, has already encouraged almost allother shipping segments to switch todiesel-powered ships. As a result of bothincreased fuel efficiency and increased cargocapacity, an LNG carrier with dual-fuelelectric machinery will deliver more naturalgas to the offloading terminal even when gasis used as fuel throughout the voyage.

Dual-fuel electric propulsionThe order last autumn for a 153,000 m3

LNG carrier by Gaz de France at Chantiersde l’Atlantique proves the point.

The ship, due for delivery in 2005, willbe propelled by dual-fuel engines andelectric propulsion. The heart of the systemis four dual-fuel engines – three 12-cylinderand one 6-cylinder Wärtsilä 50DF – givinga combined output of 39.9 MW.

The ship will be the largest LNG carrierin service. It will be employed to carryliquefied natural gas (LNG) from Norwayor Egypt, but is also designed for thealternative of trading on the spot market.

This is the second of two similar ordersto Wärtsilä. In 2002, four 6-cylinder 50DFengines, with an aggregate output of

22.8 MW, were ordered for Gaz de FranceEnergy’s 75,000 m3 LNG carrier, alsobeing built at Chantiers de l’Atlantique inSainte Nazaire.

Once delivered later this year, Gaz deFrance Energy will be the first LNGcarrier in service featuring this newpropulsion system. The membrane-typevessel will be employed to carry LNGfrom Algeria to France. The service speedis 16 knots, which can be achieved withthree of the four generating sets. Like itsnewer sister ship, the vessel is alsodesigned for spot market trading, such asvoyages to the USA.

Optimized propulsion from Wärtsilä“Wärtsilä has done extensive research tofind a more attractive propulsion solutionfor LNG carriers,” said Mikael Mäkinen,Group Vice President, Marine Division,when the second order was placed.

“We have studied the specificrequirements of the LNG trade to determinethe desired characteristics of the optimumpropulsion plant, and we have nowdeveloped a number of attractive solutions.”

Dual-fuel engines have so far beensuccessfully applied in eight onshore powerplants and a number of marineinstallations, including two FPSOs and twooffshore support vessels. LNG carrieroperators will be watching with interest

when the first ship with Wärtsilä 50DFengines takes to the sea later this year.

Major benefitsThe basic propulsion solution for theseLNG carriers uses dual-fuel engines forelectric power generation. The electricpower is supplied to an electric propulsionsystem, fairly similar to the diesel-electricpropulsion systems on modern cruise ships.The solution offers LNG carriers a numberof significant advantages.

Greater earning powerWhilst making maximum use of the gasfuel (boil-off from the cargo of liquefiednatural gas) to develop useful power,Wärtsilä 50DF engines have a much lowerfuel consumption overall and thus loweroperating costs than the conventional steamturbine plant.

Wärtsilä’s calculations show that theearnings of a 145,000 m3 dual-fuel electricLNG carrier sailing between the MiddleEast and Japan will be several millions ofdollars per year higher than the annualearnings of a steam turbine powered ship ofthe same size.

Fuel flexibilityThe Wärtsilä 50DF, derived from thesuccessful Wärtsilä 46 diesel engine, is atrue dual-fuel engine, using low-pressure

Dual-fuel diesel

Recent orders for Wärtsilä 50DF dual-fuel engines for two LNG carriers, eightengines in all, are no coincidence. This engine type promises big benefits for LNGoperators including larger cargo capacity, lower fuel consumption, higher flexibilityin operation, and lower emissions. Gaz de France is the first to take the plunge.But surely not the last.

The 153,000 m3 LNG carrier byGaz de France will be equipped withthree 12-cylinder and one 6-cylinderWärtsilä 50DF engines.

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natural gas as primary fuel. The engine canbe run alternatively in gas mode or liquidfuel mode.

It is also fully capable of switching overfrom gas to liquid fuel (marine diesel oil)automatically should the gas supply beinterrupted, while continuing to deliver fullpower.

When running on natural gas, ignition istriggered using a very small quantity ofliquid fuel. Marine diesel oil can be used asa secondary fuel.

Lower emissionsThe Wärtsilä 50DF engines also have muchlower stack emissions than a steam plant.The NOX emissions of the Wärtsilä 50DFengines are about one-tenth those of theequivalent diesel engines.

The combination of the engines’ low fuelconsumption and their maximum use ofnatural gas means the Wärtsilä 50DFengines also have low CO2 emission levels.

Higher transport capacityBeing more compact than steam turbines,diesel-electric propulsion enables muchimproved flexibility in the machinerylayout. This enables increased cargo capacityfor a given displacement, or alternativelysmaller ship dimensions for a given cargocapacity.

Further developments of the dual-fuelelectric LNG carrier could include locatingthe prime movers on the main deck. Onlythe electric propulsion motors, sea waterintakes with the necessary pumps, and thebilge and ballast systems, would still need tobe arranged on the tank top. Added to spacesavings, this would simplify thearrangements for ventilation, fire insulationand extinguishing and escape ways.

Furthermore, dual-fuel generators can bedelivered as compact, containerized, fullyfunctional power modules. They can betested before being lifted onboard, thusenabling savings in installation andcommissioning cost and time. �

engines for LNG carriersby Glenn Mattas, Sales, Wärtsilä Finland Oyand Barend Thijssen, Solutions, Wärtsilä Corporation

The Wärtsilä 50DFDeveloped from Wärtsilä’s verysuccessful type 46 diesel engines, theWärtsilä 50DF engines have cylinderdimensions of 500 mm bore by 580mm piston stroke. Available inconfigurations with six, eight andnine cylinders in line, and 12, 16 and18 cylinders Vee-form, the Wärtsilä50DF engines develop 950 kW percylinder MCR at 500 or 514 rev/minfor 50Hz and 60Hz electricitygeneration respectively.

Gas fuel is supplied at a lowpressure (less than five bar) to theengines. In gas mode, the Wärtsilä50DF engines operate according tothe lean-burn Otto process. Gas isadmitted into the air inlet channels ofthe individual cylinders during the

intake stroke to give a lean, premixedair-gas mixture in the enginecombustion chambers. Reliableignition is obtained by injecting asmall quantity of diesel oil directlyinto the combustion chambers aspilot fuel which ignites bycompression ignition as in aconventional diesel engine.

The Wärtsilä 50DF engines use a‘micro-pilot’ injection with less thanone per cent of the fuel energy beingrequired as liquid fuel at nominal load.Electronic control closely regulates the‘micro-pilot’ injection system andair-gas ratio to keep each cylinder at itscorrect operating point between theknock and misfiring limits. �