Wärtsilä LNG-fuelled engines for offshore vesselsNews // November 2, 2001
The vessels have diesel-electric propulsion. Each vessel will have four Wärtsilä 6L32DF dual-fuel engines, each of 2020 kW output at 720 rev/min driving the main generating sets. Two electrically driven azimuthing thrusters will provide propulsion, and the diesel-electric plant will also supply all shipboard power requirements. The vessels, measuring 94.9 m long by 20.4 m beam, will have a maximum speed of 17.2 knots.
The two vessels have been ordered by the Norwegian owners Eidesvik AS and Simon Møkster Shipping AS, and their design was developed together with the ship consultant Vik-Sandvik AS. When delivered in 2003, the vessels will be long-term chartered to Statoil and employed delivering supplies to oil and gas platforms in the North Sea.
Natural gas is an unusual fuel in the marine world, and these are the first vessels to use it in the offshore supply vessel industry. The fuel has environmental benefits; it burns very cleanly and NOx emissions can be much reduced. This is important in Norway, which undertook to reduce by 2010 national NOx emissions by about a third from the 1999 level. In the case of these supply vessels, the savings in NOx emissions, estimated at 390 tonnes a year, will be taken as a credit to offset emissions at Statoil's land-based facilities.
These vessels give a unique opportunity for Wärtsilä's 32DF dual-fuel engines. They run simultaneously on natural gas and diesel oil, and can be switched over from gas to liquid fuel automatically should the gas supply be interrupted while continuing todeliver full power. The environmental benefits extend beyond NOx emissions, which are about one-tenth those of the standard diesel version. The combination of the 32DF engine's low fuel consumption and its maximum use of natural gas means the 32DF also has low CO2 emissions.
The Wärtsilä 32DF engine was introduced in 2000 to marine applications to meet the requirements of a new safety class for installations with a gas pressure of less than 10 bar in a single-pipe arrangement. It thus provides an alternative to the Wärtsilä32GD gas-fuelled engine, which has been highly successful in the offshore market.
Whereas the gas-diesel engine (as in the Wärtsilä 32GD type) injects high-pressure gas fuel into the engine cylinders, the 32DF type employs gaseous fuels at low pressures. In gas mode, the 32DF engine operates according to the lean-burn Otto process. Gas is admitted into the air inlet channels to individual cylinders during the intake stroke to give a lean, premixed air-gas mixture in the engine combustion chamber.Reliable ignition is obtained by injecting a small quantity of diesel oil directly into the combustion chamber as pilot fuel. The 32DF engines use a "micro-pilot" injection with less than one per cent of the fuel energy requirement at nominal load.
An important contribution to satisfactory running of the Wärtsilä 32DF engine comes from the application of full electronic control. It uses an electronic control system based on the Wärtsilä WECS 8000 system. When running on gas with a premixed air-gasmixture and micro-pilot injection, the combustion must be closely controlled to prevent knocking and misfiring.
The WECS 8000 control system provides control of the air-gas ratio, and the quantity and timing of the pilot fuel injection to keep every cylinder at the correct operating point between the knock and misfiring limits. The pilot fuel system is a common-rail system with one engine-mounted high-pressure pump supplying diesel oil to the injection valves at a constant pressure of 900 bar. The gas fuel is supplied to the engine at a pressure of less than four bar.