Two hazards get discussed together because both start with a spill, and they are otherwise unrelated. One is a fire. The other is not a fire at all.
Rapid phase transition: an explosion without combustion
LNG is markedly less dense than water and, under the right conditions, will vaporise almost instantaneously on contact with it rather than boiling gradually.
The mechanism is a physical instability at the interface between the two liquids: the temperature difference is so extreme that a stable vapour film cannot form and insulate the LNG the way it normally would, and vaporisation runs away in a fraction of a second. The result is a sharp pressure wave — a physical explosion, driven entirely by the speed of the phase change, with nothing burning.
It requires no ignition source and no flammable mixture, which is precisely what makes it distinct from every other hazard in this list. It can happen even in circumstances where fire is impossible.
The pressure produced can damage nearby lightweight structures and has been observed to eject debris, but it is nowhere near the energy of a chemical explosion of the same fuel mass. It is a genuine hazard for anything close to a marine spill, and a minor one at any real distance.
Pool fires
If a spill finds an ignition source, the result is a pool fire: flame stabilises at the edges of the spreading liquid, fed by vapour rising off it as it boils.
Pool fires burn hot and radiate heat outward at an intensity that depends on the pool’s size and the fuel’s radiative properties. Unlike the rapid phase transition, this hazard scales directly with how much LNG has spilled and for how long, which is why containment design — bund walls, full containment tanks, drainage — exists specifically to limit pool size and duration.
Thermal radiation, not the visible flame, is what actually injures people and damages structures at a distance. Radiation intensity falls off with distance from the pool, and exclusion zones are set at the distance where it drops below a threshold considered safe for people and equipment.
Vapour cloud dispersion, and why it is the third scenario
If a spill does not ignite immediately, the vapour disperses as described in how LNG behaves in an incident: cold and heavy at first, warming and lifting as it entrains air, flammable only within a band of concentration.
If that drifting cloud reaches an ignition source before it disperses below the flammable limit, the result is a flash fire or, in a sufficiently confined and congested space, a vapour cloud explosion. This is a different scenario from a pool fire — the fire occurs away from the spill site, along whatever path the cloud travelled, rather than at the source.
Why safety analysis treats these as three separate scenarios
Each has a different cause, a different consequence, and a different mitigation.
A rapid phase transition is mitigated by keeping LNG away from large bodies of water in an uncontrolled way — hence the design of transfer systems and the placement of containment relative to open water.
A pool fire is mitigated by limiting pool size — bunding, drainage, and the fast shutdown covered by ship–shore interface systems.
A vapour cloud event is mitigated by dispersion distance and by controlling ignition sources across the whole area the cloud could reach, which is a much larger footprint than the spill site itself.
Conflating the three produces a design that manages the wrong distance for the wrong hazard, which is exactly why the codes covered in the IGC Code and SIGTTO treat them as distinct, quantified scenarios rather than one generic “LNG hazard.”
What this means for reading a site
Nothing about a terminal’s layout or a ship’s containment is arbitrary once these three scenarios are understood. Separation distances, bund sizing, gas detection placement and the shape of an exclusion zone are all direct answers to one of these three specific questions, not general caution dressed up as engineering.