Vapour generated naturally as cryogenic cargo (LNG, LPG, ethylene, ammonia) absorbs heat through tank insulation and boils.
In practice
On LNG carriers BOG is monitored continuously by the GVU and balanced against engine demand and reliquefaction capacity. The duty engineer adjusts the boiler / gas-burning unit setpoint as cargo level and ambient temperature change. Excess BOG that cannot be burned or reliquefied is vented as a last resort, usually only after a protracted port stay.
Regulatory detail & full definition
Boil-off gas results from heat ingress through cargo containment systems, however well insulated. Modern LNG carriers use it as fuel for steam turbines, dual-fuel diesel engines, or gas turbines via the GVU (Gas Valve Unit). Small carriers use a reliquefaction plant to return BOG to the tank as liquid. Boil-off rate (BOR) is a key carrier specification — typical 0.10–0.15% of cargo per day for membrane LNG carriers, lower for moss-type.