A tanker voyage is usually described as a delivery. An LNG voyage is better understood as a cycle, because what the ship does on the way back determines what it can do when it arrives.
The two legs
The laden voyage carries cargo from the export terminal to the import terminal. This is the leg that earns.
The ballast voyage returns the ship, tanks discharged, carrying seawater ballast for stability and a small quantity of LNG called the heel. It earns nothing, costs fuel and time, and is roughly half the round trip.
That asymmetry is why freight is quoted as a daily rate rather than a price per tonne delivered. The charterer pays for the ship’s time, laden or not, and a route’s economics depend on the round trip rather than the distance one way.
Heel: the cargo that is not delivered
The tanks must stay cold. If they warm up, the next loading cannot begin until they are chilled again, and chilling a warm ship is slow.
So a few per cent of the cargo is deliberately retained after discharge. Its boil-off does the cooling, exactly as it does on the laden leg: liquid evaporates, takes latent heat with it, and what remains stays at temperature. On arrival at the load port the tanks are already cold and loading can start within hours.
Heel is a genuine cost. It is cargo that was produced, paid for and not sold, and it is consumed to keep the ship ready. So the quantity is calculated rather than guessed: enough for the voyage length, the expected weather and the ship’s own boil-off rate, plus margin, and no more. On a very long ballast leg a ship may arrive with almost none left.
Cooldown, and what happens when it goes wrong
Arriving cold means a short cooldown: LNG sprayed into the tanks through spray headers, bringing the last of the structure down gradually. Gradually matters — thermal shock is a real risk, and the operation is deliberately slow.
Arriving warm is a different problem entirely. A ship coming out of dry dock has tanks full of air at ambient temperature, and the sequence is long: inerting to remove oxygen, gassing up to replace the inert gas with methane vapour so no carbon dioxide remains to freeze, then cooldown. It takes days rather than hours and consumes a meaningful quantity of LNG.
This is why a ship’s schedule is not simply a matter of sea time, and why unplanned delays that let the tanks warm are expensive out of proportion to their length.
Where the time actually goes
Sea time is the largest block but far from the whole cycle. Loading a conventional cargo takes on the order of twelve hours at typical rates, plus connecting, cooling the arms and completing the custody transfer measurement. Discharge is comparable. Waiting for a berth can add days, and canal transits add their own scheduling.
A round trip that looks like four weeks of sailing is commonly five to nine weeks door to door.
Why the cycle drives freight rates
Because a ship’s earning capacity is round trips per year, not miles sailed, anything that lengthens the cycle removes capacity from the market as surely as scrapping a vessel would.
A canal closure that forces a longer route does it. Congestion at a loading terminal does it. So does a change in trade pattern: cargoes moving from a short route to a long one absorb more ships for the same volume, and rates rise even though no ship has left the fleet.
That is the mechanism behind the violent swings in charter rates. The fleet is small, it cannot grow quickly because the yards that can build these ships are few, and voyage length is the variable that decides how much of it is effectively available.
Reading it in the data
This site holds no vessel positions and no voyage records, and nothing here should be read as tracking. What it does hold is the fixed geography the cycle runs over: which terminals export, which import, and which chokepoints sit between them on the usual routes. That routing is an editorial model of typical laden voyages, not observation.