Why it happens
No insulation is perfect. Heat leaks into a cargo tank from the sea, the air and the surrounding structure. LNG sits at its boiling point, so any heat that gets in does not warm it much. It boils a little of it instead.
That is the useful part. The evaporating fraction carries away its latent heat, and the liquid left behind stays at minus 162 degrees. A cargo tank is not a refrigerator; it is a giant version of a wet cloth cooling by evaporation. The cost is that the cargo slowly shrinks.
Boil-off rate is the measure, expressed as a percentage of cargo volume per day. A modern purpose-built carrier is typically quoted at around 0.1 per cent per day and older tonnage closer to 0.15. On a three-week voyage, 0.1 per cent a day is roughly two per cent of the cargo, which at any realistic gas price is a lot of money.
Real rates are not constant. Warm seawater raises them. Partly filled tanks raise them, because there is more surface area per unit of liquid and more sloshing. Rough weather raises them, because motion stirs the liquid and breaks up the stratification that would otherwise suppress evaporation.
Three things you can do with it
Burn it. The oldest and still the most common answer. A steam turbine will burn boil-off happily, which is why steamships survived so long in this trade. Modern two-stroke dual-fuel engines do the same far more efficiently: ME-GI, ME-GA and X-DF designs all take gas directly, so the vapour that would have been lost propels the ship. On a laden voyage a modern ship’s natural boil-off is roughly in the region of what its engines want to consume, which is a neat coincidence of physics and design.
Reliquefy it. A refrigeration plant aboard re-chills the vapour and returns it to the tanks. This is the choice when the cargo is worth more than the fuel saved, when the engine cannot absorb all the vapour, or when the ship must sit still for long periods. Two approaches are common. Full reliquefaction plants use a nitrogen or mixed-refrigerant cycle to condense the gas outright; Wärtsilä is the main supplier and has the largest installed base. Subcoolers such as Air Liquide’s Turbo-Brayton take a different route, chilling the liquid rather than condensing all the vapour, and are compact enough to retrofit.
Vent it. Releasing methane to atmosphere is a safety measure of last resort, not an operating practice. Methane is a potent greenhouse gas and the cargo is valuable, so venting is avoided by design.
Why some ships reliquefy and others do not
The decision follows from the engine and the trade.
A ship with a high-pressure two-stroke engine burns gas efficiently, so more of the boil-off has commercial value as fuel, and the case for reliquefaction is about handling surplus vapour rather than all of it. Some Q-class vessels were built with full reliquefaction precisely because they were designed with conventional diesel propulsion and needed to preserve the whole cargo.
A ship that spends time waiting, floating storage, or trading on short voyages where the engine is not running hard, accumulates vapour it cannot burn. Reliquefaction earns its keep there.
Our vessel pages record whatever the tracker says about boil-off handling, and the propulsion type, so it is usually possible to see which strategy a given ship uses. Where the field is empty we say “Not reported” rather than inferring it from the engine.
Heel, cool-down and the empty leg
After discharge a ship does not run its tanks dry. It keeps a heel, a small quantity of LNG, deliberately retained. On the ballast voyage back, that heel boils and keeps the tanks cold.
The reason is that warming a cargo tank and cooling it again is slow, wasteful and stressful for the structure. A full cool-down takes many hours and consumes a large volume of LNG sprayed into the tanks to bring the metal down gradually. Keeping the tanks cold with a heel avoids that entirely, and on a modern ship the heel also fuels the return voyage.
This is why a “empty” LNG carrier is never quite empty, and why a ship taken out of service and warmed up cannot simply be pressed back into a trade at short notice.
Why it matters beyond the ship
Boil-off is not only a shipping question. Storage tanks at terminals boil off too, and terminal boil-off compressors handle it in much the same way, returning gas to the send-out stream or reliquefying it.
It also shapes the methane emissions picture for LNG as a fuel. The chain’s climate footprint depends heavily on how much methane escapes rather than being burned or recovered, and boil-off handling is one of the places where that is decided. A fleet that burns or reliquefies its vapour looks very different from one that vents.