An economic evaluation of operating expenditures for LNG fuel gas supply systems onboard ocean-going ships considering availability

Abstract

This paper proposes a methodology for economic evaluations of liquefied natural gas fuel gas supply (LNG–FGS) systems. To evaluate the economics of various LNG–FGS systems, expected operating expenditures during long-term voyages were compared, including the loss from venting boil-off gas (BOG) to prevent overpressurisation of the fuel tank, penalties for venting BOG, expenses for consuming marine diesel oil when LNG–FGS systems are unavailable, and expenses for electricity consumption. A secondary objective was to compare the economics of various LNG–FGS systems for large-sized ships. An analysis demonstrated that the comparative operational expenditures vary considerably within various design options. Three LNG–FGS systems were compared. As a case study, an Aframax tanker (a highly popular crude carrier) was chosen, and the actual voyage scenario between Ulsan and Antwerp was used for calculating expenses. The results indicated that group FP-P (fully pressurised-type storage tank with cryogenic pump) provides a profit of approximately 7 MUS$ per 25 yrs compared to group UP-P (unpressurised-type storage tank with cryogenic pump); conversely, group FP-P incurs a loss of approximately 5.4 MUS$ per 25 yrs compared to group FP-N (fully pressurised-type storage tank with vapour-generating unit). Nevertheless, group FP-N is valuable for small-sized tanks because the vapour-generating unit requires high energy consumption during an extended initial time to raise pressure. In conclusion, group FP-P is the most economical system if it can compensate for approximately 5.4 MUS$ in capital expenditure and maintenance costs. The proposed methodology reflects the availability of necessary equipment and the results of process dynamic simulation in an actual voyage scenario.