Owing to pollutants emitted from ships, global warming and environmental pollution are becoming serious problems in the field of world maritime transport. Existing ship fuels include heavy fuel oil (HFO), which is heavy oil, and marine diesel oil (MDO) and marine gas oil (MGO), which are a mixture of heavy oil and diesel; environmental pollutants are discharged from these fuels. Pollutants in the main emission gases include sulfur oxides (SO x), nitrogenoxides (NO x), and carbondioxide (CO 2). The International Maritime Organization (IMO) introduced regulations to limit emissions from ships. According to the SO x regulation, the sulfur content in fuels used in an emission control area (ECA) is limited to 0.5% as of 2020, and according to the NO x regulation, it should satisfy the "Tier II" level in general waters and "Tier III" in the ECA as of 2016. In addition, regulations on CO 2 emissions have been gradually applied by introducing the energy efficiency design index (EEDI). The EEDI refers to the amount of CO 2 (gCO 2 /t•mile) generated by a new ship that carries one ton of cargo for one mile. The EEDI has been proposed by the IMO's Marine Environmental Protection Committee (MEPC) and requires a 10% reduction in CO 2 emissions at each phase as of 2008 (Ivica and Ante, 2015). In the MEPC 72nd conference in May 2018, it was proposed to advance to Phase II (20% CO 2 reduction) in 2020 and to Phase III (30% CO 2 reduction) in 2025. Furthermore, it was proposed to advance to Phase IV (40% CO 2 reduction) in 2030 instead of 2040 (IMO, 2018). Therefore, liquefied natural gas (LNG), which is an eco-friendly fuel, is known to be a promising alternative fuel suitable for reducing the emissions of SO x , NO x , and CO 2 , which are pollutants emitted from ships. According to Alternative Fuels Insight (AFI) provided by DNV-GL, there are 170 LNG-operated ships and 147 LNG-propelled ships as of October 2019, and 55 more ships will be built by 2020 (DNV-GL, 2019). Starting in September 2020, the five largest ports in Korea will be designated as ECAs. Accordingly, domestic coastal ships are also built as LNG-propelled ships, and tugboats, which account for a large proportion, will have to use the LNG fuel. Furthermore, because tugboats push and pull large ships, these boats require high power for the total weight than other coastal ships (Kifune and Nishio, 2015). This indicates that tugboats require a high-power engine and a large LNG fuel tank. As the hull is limited in space, a fuel tank suitable to the shape of a tugboat and having a high volume efficiency is required. The pressurized fuel tank does not affect the gas fuel supply, and it can withstand the maximum design pressure even if the internal pressure of the tank increases due to boil-off gas (BOG). Therefore,