Methane Slip During Cargo Operations on LNG Carriers and LNG-Fueled Vessels

Abstract

Abstract In this paper was presented the problems of methane leakages during cargo operations on LNG carriers. Also the leakages are possible on LNG fueled vessels. Due to green-house effect from methane on the atmosphere it should be done some measures to avoid it. Building the cargo tanks with very high capacity, utilization of better thermal insulations limits the quantity of boil-off (BOG). It is used as a fuel in marine power plant, only the overage should be liquefied again. The leakages attend all cargo operations which methane goes directly to the atmosphere through pressure-vacuum valves and gas freeing installation to mast riser or by the ventilation system from cargo pump or compressor room. To minimize the slip on LNG carriers the re-liquefaction systems are installed. They are based on cooling systems which boil-off gas (mainly methane) is liquefied at ambient pressure in temperature about −161.5°C by pre-cooled nitrogen gas at temperature about −180°C. Compressed nitrogen to a pressure about 25MPa through multistage compressors with intercooling systems is expanded step by step (in intercoolers) to pressure about ambient reaches the temperature about −180°C. The re-liquefaction system needs delivering a lot of electric energy. The total level of methane leakages from mining to the last consumer may be different and sometimes very high. The leakage level starts as minimal 1% and may be raised up to 10%. It was indicated the undertaken actions and next possibilities of methane slip limitations.