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Abbreviation: BOG
Vapour generated naturally as cryogenic cargo (LNG, LPG, ethylene, ammonia) absorbs heat through tank insulation and boils.
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.
Authoritative source: IACS Unified Requirements ↗
Source: IGC Code Ch. 9 (use of cargo as fuel); IACS UR M77 (low-flashpoint fuels)
Editorial confidence: how we grade.