A Study on the Optimization of BOG Handling for LNG-Fueled Ship under Various Bunkering Scenarios

Abstract

Liquefied natural gas (LNG) as a marine fuel is considered as a realistic and feasible solution that complies with the stringent emissions regulation issued by International Maritime Organization (IMO). For LNG-fueled ships, the bunkering process of LNG and heavy fuel oil are completely different since the cryogenic liquid transfer generates a considerable amount of boil-off gas (BOG). In this study, the commercial software, Aspen HYSYS V10, for process design is used to investigate and analyze the optimization in the dynamic simulation on the BOG handling between the cargo tank of a bunking ship and bunker tank of a receiving ship (LNG fueled ship) under various bunkering scenarios. With respect to the modeling of the study, for the standard ship-to-ship (STS) and truck-to-ship (TTS) LNG bunkering methods, the diameter of the bunkering lines are set as 8 inch and 3 inch while that of the BOG return pipelines are set as 4 inch and 2 inch to satisfy the pressure of the receiving ship and BOG generation, respectively. The capacities of the cargo tank and fuel tank for bunkering and receiving ships are set as 4,538 m3 (70 m3) and 700 m3 (70 m3) for the STS and TTS LNG bunkering methods, respectively. The results indicated that the BOG amount with different LNG bunkering scenarios is variable. The BOG flow rate varies proportionally with the temperature difference, methane number and diameter of BOG/LNG pipe in case of temperature, methane number (MN) and pipe diameter disturbance and inversely with respect to the bunkering time limit after 20 min. in case of different bunkering time limits. Additionally, for the optimal BOG handling (STS bunkering method), it is necessary to control the bunkering time within 120 min. since additional BOG is generated when the capacity of the pump exceeds 100,000 kg/h. Meanwhile, when the diameter of the BOG line (DB) divide the diameter of the LNG line (DL) DB/DL = 0.5 is considered the best value both in the STS and TTS LNG bunkering methods, thus the tank pressure difference between bunkering and receiving ship may be reduced. It is believed that the results of the research could provide feasible assistance for STS and TTS LNG bunkering for the ports, and could give a specific guideline for the amount of the BOG generation and the standardized diameter of pipeline ratio.