Gassing Up
Replacing inert gas in LNG/LPG cargo tanks with cargo vapour before cool-down, eliminating CO2 and water that would freeze at cryogenic temperatures.
In practice
Required when changing cargo grade or returning from drydock. Done with cargo vapour displacing inert gas; the gas-tight watch monitors O2 and HC concentration through sample points. Errors here are the leading source of LNG / LPG cargo claims.
Regulatory detail & full definition
Sequence: tanks first inerted with nitrogen (CO2 < 1%, dew point < -45 deg C), then warm cargo vapour is introduced top-down, displacing inert gas through the bottom. Done at a terminal with dedicated infrastructure, or ship-to-ship from a 'mother' vessel. Skipping risks CO2 freezing at 1 atm in liquid LNG and blocking spray nozzles.
Authoritative source: IGC Code ↗
Related
Source: SIGTTO LNG Operations Manual; IGC Code Ch. 11
High confidence
Editorial confidence: how we grade.