(171b) Optimization Methodology of Bog Rate for the LNG Carriers

A new propulsion system, such as Dual Fuel Diesel Engine (DFDE), has replaced the conventional steam propulsion system. Even though Main Engine Gas Injection (ME-GI) has not been commercially proven yet, it has been accepted and stacked 35 orders as of February 2015. Another prototype dual fuel diesel engine, XDF, is introduced to the industry, which requires low pressure fuel gas. Those engine types have higher fuel efficiency than conventional propulsion systems, which technically requires less fuel consumption than the one of conventional. The conventional BOG rate may exceed the fuel consumption and requires reliquefying it in order to maintain cargo tank pressure. Fuel gas demands associated power requirements are depending on various factors, load condition (laden or ballast), voyage speed, LNGC propulsion efficiency, etc. If the normal BOG is insufficient, forced LNG will be vaporized and fed to the engines. Extra BOG needs to be combusted in order to maintain the cargo tank pressure. The optimum BOG rate has been long term industry concern.

This paper discusses differences of two new propulsion systems in view of fuel gas supply system and demand. The optimization process of the BOG ratio for both propulsion systems is also discussed with case study transporting about 2 MTPA of LNG from Gulf of Mexico to JKM. This study can be directly applied to other optimization of BOG ratio for an LNG carrier dedicated piston voyages transporting a certain volume of LNG.