An increasing number of ship types now sail on Liquefied Natural Gas (LNG). The characteristics of LNG are very different to conventional bunker fuel oils including the phenomenon of ‘Boil of Gas’ (BOG) which occurs as the liquefied gas gradually evaporates in the fuel tanks of ships. The resulting build-up of pressure in ship fuel tanks is a safety hazard so ships are designed to manage the tank pressure, which is usually by burning the BOG in engines.
Meanwhile, environmental regulations increasingly require ships to connect to Onshore Power Supply (OPS) whilst at the quayside. OPS avoids the need for the ship to generate its own power from marine fuels, thereby reducing emissions at berth. However, since OPS replaces the need for onboard engines to run, LNG-fuelled ships need to manage the pressure from BOG in another way.
In this study, the typical energy demand of containerships at berth is assessed, and the types of LNG tanks and onboard systems available for BOG consumption or other forms of pressure management are inventorised. Using multiple scenarios, the study estimates the amount of BOG generated during stays at berth, the corresponding energy content, and the potential for onboard utilisation thereof – with and without OPS use. These estimates help assess whether the generated BOG exceeds the ship’s actual energy needs at berth, which could lead to energy wastage or, in the absence of appropriate BOG management, a build-up of tank pressure that could trigger emergency venting of methane to the atmosphere, which is a potent greenhouse gas.
Due to the possibility to use BOG in dual-fuel boilers at berth during an OPS port call, the study assesses the risk of emergency venting to be relatively low – although the installation of a gas-fired or dual-fuel boiler could not be confirmed for all inventorised ships. The analysis also shows that the rate at which BOG would need to be consumed by the boiler typically exceeds the ship’s actual steam demand, resulting in the BOG being combusted without use. This implies energy wastage and associated avoidable emissions should no additional BOG management measures be implemented.
This study was conducted at request of NABU (the German Nature And Biodiversity Conservation Union).