Concept for protection against overpressure caused by BOG generated during ship-to-ship LNG bunkering

Abstract

This study investigated the overpressure occurring during the ship-to-ship bunkering of LNG (Liquefied natural gas) to LNG-fuelled large ships and suggested a proper measure to handle LNG and boil-off gas (BOG) against overpressure. Two primary causes of BOG were identified: one from the contact of LNG with relatively hot equipment and piping segments, and the other from the onboard fuel tanks filled with high-pressure BOG and LNG. The latter was found to dominate the former, implying that the protection measure should address the BOG from the fuel tank before the bunkering operation. Comparing the BOG-handling alternatives of venting into the atmosphere, reliquefaction, and absorption into LNG in a pressure vessel, the last was considered the most appropriate in view of cargo loss, risk, operation time, and equipment cost. Process dynamic simulations performed for several operational cases demonstrated that the BOG-generation rate and the consequent pressure buildup in the absorbing vessel were primarily determined by the size, pressure, and heat capacity of the fuel tank as well as the amount of the remaining LNG. Because the filling limit allowed for the thermal expansion of the cryogenic fuel, the feasible bunkering operation range was primarily governed by the pressure and the liquid level of the absorbing vessel. The peak pressure of the operational cases indicated that the maximum operating pressure for the BOG absorbing vessel should be approximately 4.5 bara in the considered operational cases.