COMPARISON OF ELECTRIC POWER AND PROPULSION PLANTS FOR LNG CARRIERS WITH DIFFERENT PROPULSION SYSTEMS

摘要

Dual-Fuel Electric Propulsion has become the preferred solutions for LNG carrier newbuildings in the range from 140 000 cbm to 200 000 cbm. The experience from the first vessel delivered is very good with respect to the performance of the electric propulsion system. In the last two years only a few were ordered with traditional steam turbine propulsion system, and two-stroke propulsion with reliquefaction is so far only selected for LNG carriers with more than 210 000 cbm capacity. A fourth alternative with Gas Turbines has also been considered but yet not ordered for LNGC. All these different propulsion systems require a HV power plant, either to supply only the cargo handling plant and/or reliquefaction plant or combined with electric propulsion. For steam turbine propulsion vessels the utilization of this power plant is relative low as the cargo pumps are only operated at the discharging terminal. The two stroke propulsion system requires an even bigger power plant because of the reliquefaction compressors of 5-6 MW. The utilization of this power plant is slightly higher than for steam turbine, but also for this alternative the HV power system is only fully utilized at the discharging terminal. Further the starting methods are discussed for the reliquefaction compressors, but the use of frequency converter is the only solution to ensure a stable and reliable power plant. Dual-fuel Electric Propulsion LNG Carriers vessels are ordered in France, and last year also in Korea and Japan. These vessels combine the high voltage power system for both cargo handling and propulsion; hence the total installed power is less than for any other type of propulsion. Frequency converters are also natural part of electric propulsion systems, and the power systems for these two alternative propulsion systems are similar, also with respect to design and engineering practice. In an electric power system of this size and complexity, special consideration must be made to essential characteristics as short circuit levels, harmonic distortion, transient conditions, starting of large consumers etc. Recorded performance of the electric propulsion system shows the possibilities to operate the propulsion motors in constant power, which gives stable operation of the power plant operating in gas mode, even at rough sea conditions. Also the crash stop performance is superior to other propulsion systems because of the ability to stop and reverse the propeller in less than 30 seconds.