A tank of LNG behaves less like a warehouse and more like a slow chemical process. The contents change while they sit, and the discipline of storage operations exists because one of the ways they can change is dangerous.
Ageing
LNG is a mixture, and its components do not boil off equally. Methane, the lightest and most volatile, leaves first.
So the liquid remaining in a tank gets progressively heavier and higher in heating value per unit volume. Over a long storage period the change is measurable, and it matters twice: for custody transfer, because the energy content is what is sold, and for grid specification, because a heavier gas has a higher Wobbe index and may drift outside what the network accepts.
It is also the mechanism behind the more serious problem.
Stratification
Suppose a terminal holds a heel of aged, relatively heavy LNG, and a cargo of lighter LNG arrives. Load the new cargo carelessly and it can settle as a distinct layer rather than mixing.
Two stable layers of different density then sit in the same tank. Each behaves as its own small system: each ages, each boils off, each changes density — the lower layer, under the weight of the upper, boils less freely and warms.
For a while nothing happens. The layers are stable precisely because the denser one is underneath.
Rollover
Eventually the densities converge. The lower layer, warming and losing its density advantage, becomes buoyant relative to the layer above it, and the two invert and mix.
When they mix, the lower liquid — which has been warming under pressure without being able to boil freely — finds itself at the surface and flashes. A large volume of vapour is generated in a short time, potentially exceeding what the tank’s relief system was sized to handle.
This is rollover, and it is the hazard that shapes how LNG storage is operated everywhere.
How it is managed
Not by engineering it out. There is no tank design that prevents stratification; the answer is operational.
Monitor at depth. Density and temperature sensors at multiple heights in every tank, read continuously. Two readings that diverge and stay diverged are the signature of layers forming.
Choose the fill point. Tanks have both top and bottom fill connections. Loading a lighter cargo through the bottom, or a heavier one through the top, forces mixing on the way in rather than allowing settlement.
Know the incoming composition. The decision above requires knowing the density of what is arriving relative to what is already there, which is why cargo composition is analysed rather than assumed.
Recirculate. If the readings suggest layers are developing anyway, in-tank pumps circulate the contents until the profile is uniform again.
None of that is exotic. It is attention, applied continuously, and it is why rollover is rare despite being a well-understood consequence of ordinary operations.
The minimum heel
A terminal cannot run its tanks empty and refill from warm, for the same reason a ship keeps a heel. Warming a tank means an expensive, slow reconditioning before it can take liquid again.
So a working minimum stays in the tank at all times, boiling gently and keeping the structure cold. Inventory management at a terminal is therefore bounded at both ends: enough headroom to take the next cargo, enough liquid to stay cold until it arrives.
Boil-off as a constant
Tanks leak heat and the contents evaporate — a very small fraction of a per cent a day, better than a ship because the tank is stationary and insulation costs no cargo volume.
That vapour has to go somewhere continuously. At a working terminal it goes to the recondenser and back into the liquid stream heading for the grid. At a terminal with no send-out it has to be compressed and exported to the grid as gas, or in the last resort flared. It is never simply vented.
What the tracker holds
None of this is in the data. Global Energy Monitor records terminals, not their operating practice, and this site reports what GEM carries.
It is here because a reader looking at a terminal page and wondering what a storage tank actually does deserves an answer, and because rollover and ageing appear in the glossary and needed somewhere to point.