Quantitative risk analysis for gas leak and dispersion in cargo compressor room of 174K ME-GI LNG vessels

Abstract

Recently, many high-pressure gas-fueled liquefied natural gas (LNG) vessels combined with high-pressure fuel gas compressors or high-pressure pumps/vaporizers have been built because LNG fuel is considered environmentally friendly and satisfies the International Maritime Organization (IMO) requirements. However, there are many reasons to examine its safety requirements, such as gas leakage, explosion, and fire. Because LNG carriers under the IGC code have sailed for long time, there are many adequate safety regulations available. However, safety regulations for LNG-fueled vessels are still insufficient because LNG-fueled vessels with the IGF code do not have enough reference. Most safety regulations applied for the IGF code are in accordance with the safety requirements of IGC code to cover the insufficient safety regulations in IGF code and to prevent risk in LNG-fueled vessels. In particular, the gas-detection system applied in the machinery room by IGF/IGC code only defines the number of gas detectors. There are no rules for their locations, so the gas detectors are installed in agreement among the ship owners, shipyard, and classification societies. The minimum number of detectors in a machinery room (cargo compressor room) by IGF/IGC code is three, but there are no rules for the detecting points. Therefore, ship owners do not heavily rely on the detecting system defined by IGF/IGC code. Therefore, this study considers gas dispersion in the cargo compressor room of an LNG carrier equipped with high-pressure cargo-handling equipment, bringing up reasonable methods of safety regulation and the number and locations of gas detectors specified in the IGF/IGC code. To perform LNG gas-dispersion simulation in the cargo compressor room, the geometry of the cargo compressor room and the arrangement of the equipment and piping are designed with the same 3-dimensional size as the actual structures in the vessel. Scenarios for a gas leak were examined for high pressure of 305 bar. Pinhole sizes for high pressure are 4.5 mm, 5.0 mm, and 5.6 mm. The results show that the cargo compressor room of 174K ME-GI LNG vessels has no serious risk areas regarding the flammable gas concentration, as it is verified that the ventilation assessment was safe for a 5.6-mm pinhole for a high-pressure leak and gas-rupture condition. However, based on the computation fluid dynamics (CFD) simulation, it is verified that the actual gas-detection sensors in the cargo compressor room should be moved to other optimum points and their quantity should be increased. The CFD results of this study will be useful for risk-based design and analysis, and optimum gas-detection points can be applied.