Energy and economic analysis using boil-off gas onboard liquefied natural gas tanker vessel

Abstract

Liquefied natural gas (LNG) tanker vessel is operated to carry natural gas at cargo temperature of around -163°C. The containment system is designed to allow heat to escape into the cargo area. Around 0.15% of boil-off gas (BOG) is typically present in the cargo volume per day. Thus, the cargoes from the loading terminal are not fully delivered to the client due to the BOG phenomenon. This thesis looks into the utilization of BOG in the boiler. Modem marine boilers installed onboard the LNG carriers are capable of burning 100% BOG or heavy fuel oil (HFO). It can also use both BOG and HFO in a dual fuel burning operation. A Malaysia International Shipping Corporation (MISC) Berhad vessel which has operated for five voyages and has the same shared route of loading and discharge port was identified as a sample. The data main source was from the vessel's engine log book. The investigation is conducted by profiling the steam system onboard. On further analysis, the estimated fuel costs and operating characteristics of the facility were determined, focusing on evaluating the fuel to steam conversion efficiency of the boiler. It was found that during the laden voyage, more energy saving of BOG was obtained as compared to HFO in the range of 72.9% to 97.6% whilst in the ballast voyage, less BOG was obtained in the range of7% to 75.9%. The percentage energy saved varies whereby the percentage is high when vessel encountered shorter routes or purposely more BOG been utilized through forced boil-off method. The plant efficiency is about the same regardless of the type of fuel burnt in the boiler, whether it is burning combined BOG and HFO, 100% BOG or 100% HFO. The only difference is the amount of fuel burnt in the boiler. Hence, it is the owner best interest to decide which fuel to use that provides lesser fuel consumption and less cost.