Dual-Fuel-Electric Propulsion Machinery Concept on LNG Carriers
Keywords:LNG carriers, Boil-off gas, Dual-fuel engine, Electric propulsion
Human efforts to devise optimum propulsion for their vessels are as old as the vessels themselves. Today these efforts are even more determined as modern shipping requires propulsion systems that are increasingly reliable, available, cost-efficient and able to meet high ecological criteria. The heat transfer towards liquefied gas stored in tanks results in boil-off during cargo handling or voyage. The rate of the evaporated gas amounts to 0.13% per day during the voyage of a fully loaded ship. Steam turbines have been a dominant form of propulsion on liquefied natural gas - LNG carriers for over forty years. Until recently, the possibility of using boil-off gas as fuel for boilers has been the reason for installing steam plants as the only means of propulsion of LNG carriers. However, it has been proved that these plants are not sufficiently efficient due to adverse impacts on both emissions and the vessel’s operating expenses. It has also been found out that dual-fuel-electric propulsion is the most effective alternative to steam. Shipping companies select electric propulsion primarily because it provides excellent manoeuvrability and increased availability, allows reduction of the machinery space and better arrangement of shipping capacity and, naturally, because of lower fuel costs. This paper discusses the newest technologies and the operation principle of the low-pressure four-stroke dual-fuel diesel engine, specificallythe 12V50DF and 9L50DF types produced by Wärtsilä company, and the concept of the dual-fuel-electric propulsion for the new generation of LNG carriers.