Diesel Engine Conversion for LNG Fuel – Worth Doing?

By George Backwell at May 19, 2012 09:23
Filed Under: General

A  25,000 dwt product tanker, Bit Viking became the first vessel in the world to be equipped with a fully mechanical propulsion system powered by LNG as a primary fuel when the plant was converted by Wärtsilä last year. Now, looking at the operating period retrospectively, owners Tarbit can say whether they believe the conversion was worthwhile; but first the background.

The Ship & the Conversion Project

A purpose built tankship for coastal deliveries of petroleum products, on time charter to Statoil since delivery, Bit Viking is employed in a trade from Mongstad refinery to oil terminals along the Norwegian coast; a route that gives ready access to LNG bunkers.

Designed by Wärtsilä Ship Design, built by Edwards Shipbuilding in Shanghai and launched in 2007, Bit Viking was equipped with twin-screw propulsion with each propeller powered by Wärtsilä 6L46 engines running on heavy fuel oil (HFO).

'Bit Viking'with On-deck LNG Bunker Tanks: Image credit Tarbit

Owners Tarbit initiated the engine conversion project in cooperation with Statoil primarily to reduce noxious exhaust gas emissions, and for that worthy aim obtained a 75% contribution of the costs from Norway’s ‘Næringslivets Hovedorganisasjon (the Confederation of Norwegian Business and Industry). With the  conversion to LNG fuel the operators would also gain qualification for reduced government NOx emission payments tax.

Modification of the Existing Engines

Modification of the exist­ing Wärtsilä L46 engines to the 50DF configuration included:

    •    Honing out the cylinder bores from a diameter of 46 cm to 50 cm,
    •    Replacing the cylinder heads
    •    New cylinder liners and anti-polishing rings
    •    New upper sections to the engine’s connecting rods
    •    Fitting dual-needle injection valves
    •    New engine turbochargers for DF operation
    •    Camshaft components for DF Miller-valve timing and
    •    A new UNIC engine control system

Added components included a gas rail pipe and gas admission valves, a pilot fuel system, pilot fuel oil filter, common-rail piping and pilot fuel oil pumps and exhaust gas waste gates.

Two LNG bunker tanks with a capacity of 500m3 each were installed on deck.

Was it Worth the Trouble?

Reportedly the main aim of the conversion has been achieved with sulphur oxides (SOx) emissions completely eliminated, and reductions in nitrogen oxides (NOx) emissions by 95 per cent, particulate matter emissions by 99 per cent and carbon dioxide (CO2) emissions by 25 per cent.

It is not clear whether the operator's fuel bill for LNG bunkers has been less than it would have been for MGO prior to engine conversion, but after six months of LNG operation, Tarbit do say that the maintenance costs have been equivalent to running conventional engines on heavy fuel oil.

Anders Hermansson, Tarbit's Technical Manager told Wärtsilä recently there  had been a reduction in fuel consumption, adding: “During six months of more or less trouble free trade we have gained a cleaner engine room. We have closed down all the fuel separators, we don’t have any smoke from the funnel and the crew are quite happy.”

The apparent success of the project was the result of cooperation between many stakeholders including Statoil, Germanischer Lloyd, the flag state, Tarbit and Wärtsilä.

 

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