Vapour generated naturally as cryogenic cargo (LNG, LPG, ethylene, ammonia) absorbs heat through tank insulation and boils.
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.