Page 43: of Maritime Reporter Magazine (September 2001)

route of using a steam turbine in which boil-off is consumed. "The reliquefaction of evaporated gas will increase the amount of LNG deliv- ered to the customer, and the benefit from the sale of this reliquefied LNG will far exceed any difference in operat- ing costs between a diesel engine with reliquefaction plant and steam turbine," observed the company.

MAN B&W considers that its

S70MC-C and L70MC-C low-speed designs are especially well-suited to a twin diesel engine layout where the requirement is for a redundant installa- tion, while the very wide, 900 mm-bore family would cover a single-engine specification. The single diesel engine solution could encompass back-up propulsion in the shape of retractable azimuth thrusters or the adoption of an electric motor acting on the shaftline.

Wartsila Corporation implemented work two years ago on the adaptation of its 32DF dual-fuel engine in response to prospects of demand for LNG as an alternative fuel in the offshore, ferry and coastal shipping markets. Development of the marine DF range, soon to be extended with a gas engine version of its powerful 46-series medium-speed diesel, has also been influenced by the need for a high-performance source of power using boil-off gas for LNG trans- port as well as production and storage.

Derived from the Wartsila 46, a six- cylinder 50DF engine was run on the laboratory testbed last year, and an 18- cylinder variant started an extended test program at the group's Italian factory in

Trieste during May 2001. The first com- mercial pilot installation is expected to be in operation towards the end of this year, giving the concept its debut in the land-based power generation sector.

It is estimated that running costs for a large LNG carrier using the 50DF in a twin-engine plant could be some 20-30 percent less than with the conventional steam turbine solution. It is also claimed that newbuilding installation would be easier, offering potential savings in ship- yard man-hours.

Wartsila's DF gas engine development makes it possible to utilize the boil-off gas efficiently, and dispenses with the need for a boiler. Although direct-drive is one option, Wartsila also advocates electric propulsion using the low-pres- sure dual-fuel DF gas engines as the prime movers in main gensets. High thermal efficiency, operating flexibility and safety are the main arguments in the comparison. The redundancy conferred by such an installation is as relevant to the pressing requirement for service dependability in the LNG supply chain as it is to safety.

DF engines can be run on gas or on

September, 2001 light fuel oil and heavy fuel oil. Instead of a spark plug for ignition, as normally used in lean-burn gas engines, the lean air-gas mixture is ignited by a small amount of diesel fuel injected into the combustion chamber. The relatively minute amount of diesel fuel injected, typically less than 1 percent at nominal load, has considerable merit from an environmental as well as economic standpoint. It is reckoned that the NOx emissions of a DF engine are about one- tenth of those of a standard diesel engine.

The 50DF employs a wider bore and lower mean effective pressure than the

Wartsila 46 engine, but retains the same stroke and crankshaft speed. The DF type has an output of 950-kW per cylin- der, compared to the maximum rating of about 1,150-kW/cyl attained with the latest D version of the 46 diesel. Since it will be made available as both an in-line and vee-form engine, the 50DF covers

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