Every LNG carrier has to decide what to do with the vapour its cargo produces. There are only four answers, and each of them wastes something.
The four options
Burn it usefully. Boil-off is methane, and a ship that can run on gas can use it as fuel. This is what most of the fleet does, and it is why LNG carriers were burning gas decades before anyone described it as a clean marine fuel.
Turn it back into liquid. Reliquefaction cools the vapour below its boiling point and returns it to the tanks. The cargo arrives intact; the power to do it comes from somewhere.
Burn it uselessly. A gas combustion unit disposes of vapour the engines cannot take — in port, at anchor, at low speed. Nothing is recovered.
Vent it. Permitted in emergencies and otherwise avoided, because methane released to atmosphere is a far worse greenhouse outcome than burning it.
The choice is not made once. A ship makes it continuously, and the equipment fitted determines which options are available.
How reliquefaction works
Almost universally a closed nitrogen cycle. Nitrogen is compressed, cooled, expanded through a turbine so it becomes very cold, and used to chill the boil-off in an exchanger. The condensed LNG is returned to the tanks; the nitrogen goes round again.
Nitrogen is chosen for the same reason it appears in offshore liquefaction: it is inert, so a leak in the refrigerant loop is not a fire. The efficiency penalty against a hydrocarbon refrigerant is accepted for that.
The power demand is substantial — several megawatts on a conventional-sized ship, which is a meaningful fraction of the vessel’s installed generating capacity. Reliquefaction does not save energy; it converts fuel into preserved cargo at a poor exchange rate that happens to be favourable when cargo is expensive.
When it pays
The arithmetic is simpler than it looks. Reliquefying costs fuel and consumes capital; it saves cargo. It pays when the saved cargo is worth more than the fuel burned to save it.
That comparison moves with the LNG price, and it moves a long way. In a high-price market, preserving a cargo is obviously worth burning fuel for. In a low-price market, the cargo is barely worth more than the gas consumed and the equipment sits idle.
It also depends on what the ship’s engines would otherwise do. On a vessel whose propulsion can consume boil-off usefully, the vapour is not wasted by being burned — it displaces fuel oil the ship would have bought. On a vessel whose engines cannot use it, the choice is reliquefy or waste it, and the case is much stronger.
That is exactly why full reliquefaction appeared where it did. The largest Qatari vessels were built with slow-speed diesels that ran on oil and could not take gas, on very long routes carrying very large cargoes. Preserving the boil-off was the only sensible answer, so the ships carried plants big enough to do it.
Subcooling: the partial answer
Modern two-stroke ships created a different problem. ME-GI and X-DF engines do consume gas, efficiently, but at low load or in port they consume less than the tanks produce. The surplus has to go somewhere, and a gas combustion unit is a pure loss.
A subcooler, or partial reliquefaction system, is the compromise. It is smaller and cheaper than a full plant, chills the excess vapour below its boiling point and returns some of it, and reduces cargo loss without pretending to eliminate it. On a modern newbuilding it is close to standard equipment, sized against the engine’s minimum consumption rather than the tanks’ maximum production.
Reading it on a vessel page
Containment, propulsion and boil-off handling are three choices that constrain one another, and knowing two of them usually tells you the third. A ship with slow-speed diesels and no gas capability needs reliquefaction on a long route. A ship with dual-fuel electric machinery generally does not. A modern two-stroke ship probably has a subcooler.
The carrier directory records containment and propulsion as GEM reports them, normalised into families with the original text kept alongside. It does not currently record boil-off handling, and the section is in any case empty until the carrier tracker is loaded.