Dual-fuel engines cut emissions from supply ships

Two new offshore vessels under const-ruction in Norway will be powered by Wärtsilä dual-fuel engines, capable of running simultaneously on LNG or diesel oil.

Two new offshore vessels under const-ruction in Norway will be powered by Wärtsilä dual-fuel engines, capable of running simultaneously on LNG or diesel oil. The 4,000-ton supply vessels are being built by Kleven Verft for delivery to Eidesvik and Simon Moekster Shipping in 2003. They will then operate in the Norwegian North Sea under a long-term charter to Statoil.

The vessels will be driven by diesel-electric propulsion. Each will have four Wärtsilä 6L32DF engines, with each of these outputting 2.02 mw at 720 rev/min to drive the main generating sets. Two electrically-driven, azimuthing thrusters will provide the propulsion, and the diesel-electric plant will also supply all remaining power needs.

The vessels, both 94.9 m long with a 20.4-m beam, will have a maximum speed of 17.2 knots.

Wärtsilä's 32DF engines were introduced in 2000 in response to new marine safety requirements for installations with a gas pressure of less than 10 bar in a single-pipe arrangement. The engines can switch from gas to liquid fuel automatically should the gas supply be interrupted, continuing to deliver full power.

Natural gas solutions are especially important in Norway, which is working to reduce its nitrous oxide (NOx) emissions by 2010 to one-third of its 1999 levels. The estimated savings in NOx emissions of the two new vessels amounts to 390 t/yr and will be used as credit to offset emissions from Statoil's land-based plants.

Wärtsilä claims its dual-fuel engines emit one-tenth of the NOx generated by standard diesel engines. Furthermore, the 32DF's low fuel consumption, allied to its maximized use of natural gas, leads to lower carbon dioxide emissions.

Unlike gas-diesel engines, which inject high-pressure gas fuel into the engine cylinders, the 32DF exploits gaseous fuels at low pressures. When running on gas, it operates according to the lean-burn Otto process. Gas enters the air inlet channels to individual cylinders during the intake stroke, generating a lean, pre-mixed air-gas mixture in the engine's combustion chamber. Reliable ignition is achieved by injecting a small quantity of diesel oil as pilot fuel into the combustion chamber. According to Wärtsilä, the 32DF uses a "micro-pilot" injection, corresponding to less than 1% of the fuel energy requirement at nominal load.

Combustion must be controlled carefully to avoid knocking or misfiring. Wärtsilä's WECS 8000 electronic control system is used to manage the air-gas ratio, and the quantity and timing of the pilot fuel injection to maintain each cylinder at the correct operating point between the knock and misfiring limits.

The pilot fuel system is a common-rail technique that requires one engine-mounted, high-pressure pump supplying diesel oil to the injection valves at 900 bar constant.

Recently, Wärtsilä also supplied an 18V32DF dual-fuel engine as part of a 5,800-kw power module for the FPSO Berge Hus. This has been upgraded with water handling capability to assume the lead production role on the Ceiba field off Equatorial Guinea. In February, the transfer of operations was completed from the incumbent FPSO, the Sendje Berge.

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