Boil-Off Gas
Boil-Off Gas (BOG) is the vapour that naturally forms as heat leaks into the extremely cold liquefied cargo of a gas carrier — LNG at about minus 162 degrees Celsius, or LPG — causing a small, continuous fraction of the liquid to evaporate. The cargo tanks are heavily insulated to minimise it, but some boil-off is unavoidable.
The boil-off gas must be managed to keep tank pressure under control. On many LNG carriers it is used as fuel in the engines and boilers, effectively burning the cargo to move the ship; on others, and on LPG carriers, it is reliquefied and returned to the tanks. Managing boil-off well is both a safety and a cargo-quantity, and therefore commercial, matter.
Boil-off is cargo turning to vapour, so how it is handled directly affects both safety (tank pressure) and how much cargo is delivered. Burning it as fuel or reliquefying it is a central design and operational choice on a gas carrier, with real consequences for cargo out-turn and voyage economics.
What happens to boil-off gas on an LNG carrier?
It is typically used as fuel in the ship engines and boilers, or reliquefied and returned to the cargo tanks. Either way it is managed to control tank pressure; controlled venting is a last resort.