Loading an LNG carrier that is already cold takes a few hours. Loading one that is warm takes days, and the sequence in between is one of the more exacting routines in commercial shipping.
Why a warm tank cannot simply be filled
Two independent reasons, and either alone would be enough.
Air. A tank that has been open contains air. Methane and air form a flammable mixture between roughly 5 and 15 per cent, and filling a tank of air with methane means passing through that range on the way. Nobody does this deliberately.
Thermal shock. The membrane is a thin metal skin bonded to insulation and supported by the hull. Pouring liquid at minus 162 degrees onto warm steel cools the surface far faster than the structure behind it, and the differential sets up stresses the containment was not designed to absorb in a single event.
So the tank has to be made both inert and cold, in that order, gradually.
The full sequence
Drying. Air is dried to remove moisture that would freeze. Water ice in the wrong place blocks valves and fouls instruments.
Inerting. An inert gas generator burns fuel to produce a low-oxygen flue gas, which displaces the air until oxygen falls below a few per cent. The atmosphere is now non-flammable, and methane can be introduced safely.
Gassing up. The inert gas is itself a problem: it contains carbon dioxide, and carbon dioxide freezes solid at around minus 78 degrees, well above cargo temperature. Left in place it would sublime out as the tank chills and block whatever it reached. So the inert gas is displaced in turn by methane vapour, usually produced by vaporising a small quantity of LNG, until the carbon dioxide content is down to a fraction of a per cent.
Cooldown. LNG is sprayed into the tank through spray headers in the dome. It evaporates on contact, the vapour carries heat away, and the structure cools from the top down. Temperature sensors at multiple heights track progress, and the rate is deliberately limited — this is the step that risks thermal shock, and rushing it saves hours against a repair that costs weeks.
Doing it the short way
A ship that has just discharged and retained a heel skips almost all of it. The tanks never warmed, the atmosphere is still methane, and only a modest cooldown is needed to bring the structure from the temperature it drifted to back down to loading condition.
That is the whole purpose of heel, and it is why the quantity is calculated for the specific voyage rather than chosen once. Arrive with enough and loading starts within hours. Arrive without and the ship is out of service for a day or more while it repeats the long sequence — burning cargo, missing its berth window, and earning nothing.
When ships go warm deliberately
Dry-docking requires it. So does a change of cargo grade in some circumstances, a repair inside the tanks, or a long lay-up where maintaining the cold is not worth the loss.
Warming up is the sequence in reverse: the tanks are heated, the methane displaced by inert gas, and the inert gas eventually by air, at which point the space can be entered. Each stage exists to avoid passing through a flammable atmosphere, and each is slow for the same reason cooldown is.
Why it shows up in the commercial terms
A ship’s condition on arrival is not a technical detail; it is a contractual one. Charter parties specify the tank condition a vessel must present at the load port, who supplies the LNG consumed in cooling, and what happens if the ship arrives warm and delays the terminal.
It is also why the voyage cycle rather than the voyage is the right unit for thinking about an LNG carrier. The state the ship arrives in is determined by what it did on the leg before, and a schedule that ignores that produces plans the vessel cannot keep.