Proposed Method for Dealing with Boil-off Gas on board LNG Carriers during Loaded PassageDownload PDF
In tankers carrying liquefied natural gas a portion of the cargo, generally between 0.1 and 0.25 percent per day evaporates as a result of imperfect insulation. This boil-off gas has normally been used to fire the ship’s boilers. In view of rising price of natural gas, it will be more economical to re-liquefy boil-off gas and returns it to the LNG cargo tanks. It is therefore desirable to provide means for solving the boil-off gas problem. This paper provides an overview of current transporting methods of natural gas by using different ways like pipelines, liquefied natural gas, compressed natural gas and pressurized liquefied natural gas. Also, it describes the boil-off gas problem in LNG carriers and the different methods of solving this problem which include venting BOG or burning it in the gas combustion unit or by using BOG as fuel. Moreover, it states the different re-liquefaction processes in order to return BOG to LNG cargo tanks. Finally, the proposed method is to handle this boil-off gas in two PLNG containers; each container has a capacity of 3612 cubic meters for capacity of 266000 cubic meters LNG carrier in order to save BOG quantity.
Keywords: LNG, PLNG, CNG, re-liquefaction, BOG