Almost everything about how LNG is built, shipped and regulated follows from about six numbers. They are worth knowing before anything else, because once you have them most of the industry’s apparent peculiarities stop being peculiar.
It is cold, not compressed
LNG is held at roughly minus 162 degrees Celsius at close to atmospheric pressure. It stays liquid because of the temperature alone.
This is the single most common misunderstanding, and it matters because the intuition people import from LPG is exactly wrong. Propane and butane liquefy under modest pressure at ordinary temperatures, which is why a barbecue cylinder is a thick steel pressure vessel at ambient temperature. Methane cannot be liquefied that way at all: above its critical temperature of about minus 82.6 degrees Celsius, no amount of pressure will condense it. You have to take the heat out.
So an LNG tank is not a pressure vessel. It is a very good vacuum flask, and the engineering problem is insulation rather than containment strength.
It is about 600 times smaller than the gas
One cubic metre of LNG makes roughly 600 cubic metres of natural gas at standard conditions. That ratio is the entire commercial case for the chain: it is what turns a cargo of gas from an absurdity into a shipload.
It is also the number behind the safety analysis. A spill does not stay a spill; it becomes six hundred times its volume in vapour, and the modelling of how that vapour spreads is what sets exclusion distances around terminals.
It is lighter than water and floats
At 430 to 470 kilograms per cubic metre, LNG is a little under half as dense as water, and a cubic metre weighs a bit less than half a tonne. Composition drives the variation: a cargo rich in ethane and propane is heavier and carries more energy per cubic metre than one that is nearly pure methane.
Two consequences follow. LNG spilled on water floats and boils vigorously, drawing heat from the water beneath it, and it can do so fast enough to produce a physical explosion with no combustion at all — a rapid phase transition, which damages structures nearby but releases nothing like the energy of a chemical explosion. And because ships are volume-limited rather than weight-limited, an LNG carrier is a large hull carrying a relatively light cargo, which is why the vessels look so much bigger than their deadweight suggests.
The cargo is its own refrigerant
Nothing on a ship or in a tank actively chills LNG in normal operation. Heat leaks in through the insulation, a small fraction of the liquid evaporates, and the latent heat that evaporation takes away is what keeps the remainder cold. The boil-off is not a defect to be eliminated; it is the cooling system.
That is why a carrier’s specification quotes a boil-off rate rather than a refrigeration capacity, and why the interesting engineering question is what to do with the vapour rather than how to prevent it. Boil-off and reliquefaction covers the options.
It also means the liquid changes as it sits. Methane boils off preferentially, so what remains gets slowly heavier and higher in heating value. That is ageing, and on a long voyage it is measurable.
It burns only within a narrow band
Liquid LNG does not burn. What burns is methane vapour mixed with air, and only between roughly 5 and 15 per cent by volume. Below 5 per cent there is not enough fuel; above 15 per cent there is not enough oxygen. Methane’s autoignition temperature, around 537 degrees Celsius, is high compared with most liquid fuels, so it does not ignite readily on hot surfaces.
The practical effect is that an LNG hazard is a vapour hazard with a defined geometry. A release produces a cold cloud that is flammable within a particular envelope, and everything from emergency shutdown design to terminal siting is built around predicting where that envelope reaches.
Ordinary steel will not do
At minus 162 degrees, carbon steel becomes brittle: it loses the ability to deform before it breaks, so a defect that would be harmless at ambient temperature propagates as a crack. The industry’s answer is a short list of materials that stay tough in the cold — nine per cent nickel steel and increasingly high-manganese steel for tanks, aluminium alloys for exchangers and Moss spheres, stainless steel and Invar for membranes.
This single material fact is why cryogenic construction is expensive, why the supplier list for the cold end of a plant is so short, and why a leak that lets cold liquid reach the hull of a ship is treated as an emergency rather than a maintenance item.
What it is not
LNG is odourless, colourless, non-toxic and non-corrosive. The white plume seen above a spill or a vent is condensed water vapour from the air, not the gas itself, which is invisible.
It is also not odorised. The mercaptan smell added to domestic gas is added downstream, after regasification, because the odorant would freeze in the liquid. A leak of LNG or its cold vapour gives no warning by smell, which is why gas detection at terminals and on ships is instrumented rather than left to people.