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In this study, a risk assessment of a Liquefied Petroleum Gas (LPG) marine engine system being developed for installation in the first LPG-powered ship in South Korea was performed and a new standard for a formal safety assessment (FSA) was proposed. Based on the FSA technique, hazards were identified through a failure mode and effect analysis (FMEA) in the first stage. An FMEA workshop was conducted to assess 110 components, and 89 hazards were identified. Of these, 19 failure modes of intolerable level were identified, and risk ranks were divided into four groups. Then, a more objective risk assessment was conducted using fuzzy set theory to compensate for the subjectivity of FMEA. Additionally, a technique for order performance by similarity to ideal solution (TOPSIS) was used to represent the risk rank of individual systems more precisely. By the second stage, risk ranks could be divided into 28 groups through the classification of a total of 89 hazards. Finally, risk control options were presented for high-ranking hazards according to the fuzzy TOPSIS results, and a cost-benefit analysis was performed. Consequently, the gross and net costs of averting a fatality were calculated as US $2.98 million and US $2.93 million, respectively. Through a cost-benefit analysis, the periodic exchange of main critical components was found to be in the range of economic criteria that could be recommended as a safety standard. The risk assessment technique proposed in this study allows a more objective and effective selection of critical hazards that necessitate risk control measures. • An improved risk assessment technique based on formal safety assessment was developed. • Safety policies were proposed for marine vessels using the LPG engine system. • Objective and precise fuzzy TOPSIS FMEA was applied to balance FMEA subjectivity. • Fuzzy TOPSIS FMEA identified and categorized 89 hazards based on risk ranking. • Time required by the FSA procedure to identify critical hazards was reduced. [ABSTRACT FROM AUTHOR]