Page 39: of Maritime Reporter Magazine (April 2006)

April 2006 39 fuel options currently being proposed and adopted use electric power genera- tion to drive electric propulsion systems such as conventional propellers driven by electric motors.

Diesel Electric Propulsion

Diesel electric propulsion systems are considered attractive because they com- mand higher power density than steam turbines and, as a result, provide more flexibility in terms of machinery arrangements, allowing designers to obtain more overall cargo capacity. The high efficiency of dual fuel, combined with its low fuel consumption, further gives owners and operators the opportu- nity to reduce their operating costs and increase their earnings. Electronically controlled dual-fuel engines also give off less carbon dioxide emissions than steam turbine plant, making them a more environmentally friendly choice.

Finally, the ability of dual-fuel engines to operate on gas or on liquid fuel pro- vides increased operational flexibility to support the varying demands of the ship's operating schedule. The use of methane in diesel engines means that the ships can now be propelled by diesel electric systems. These systems usually consist of a number of generators form- ing one common supply, both for propulsion and general ship use. The main advantages of diesel electric propulsion are the low noise and vibra- tion levels compared to slow-speed diesels, improved fuel consumption compared to steam turbines and flexibil- ity with respect to the engine room lay- out. The disadvantages are the low power-to-weight ratio and inherent inef- ficiency compared to a diesel engine propulsion system.

The Reliquefaction Alternative

Reliquefaction is a method of convert- ing boil-off gas back into LNG and returning it to the cargo tank. In this way, the volume of LNG loaded at the loading port is the same as the volume discharged at the receiving terminal.

When fitting a reliquefaction plant, a minimum of two of each of the major items of equipment are to be provided to allow the boil-off from stored cargo to be reliquefied without the need to 'flare' cargo with any one major item of equip- ment out of action. However, the requirement to provide a complete standby unit may be waived provided there is the ability for boil-off gas to be 'flared', burnt in a thermal oxidizer, in the event of a breakdown.

Alternatives to LNG

Although the growth of LNG shipping currently appears unbounded, it is possi- ble that in the future, alternative meth- ods of transport and gas conversion may become both necessary and economical- ly viable.

Gas by Wire

One option being considered by the industry is the 'gas by wire' solution.

Gas by wire uses locally generated DC current transmitted over long distances and converted ashore for AC use.

Developments in capability and cost reduction have resulted from develop- ments in semiconductors and insulating materials which, in turn, have con- tributed to the possibility of energy lev- els equivalent to 10 billion cubic meters of gas being economically transported over distances of 3,000 km and beyond.

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