"Mini-nuke" isn't a very accurate comparison. In the event of a leak, the liquid fuel would need to flow out of the tank and turn into vapor before it could catch fire and burn. As a liquid or highly concentrated vapor it's not flammable. It needs to mix with air first.
The result would be a cloud of gas that burns violently for several minutes, depending upon wind and the speed of the leak. Not the instantaneous, crater-making release of energy that a thermonuclear device produces.
If you suddenly removed the top of a large container of LNG, it wouldn't all instantly vaporise. It would start to boil, rapidly cooling its surroundings due to the latent heat of vaporisation. Large amounts of ice would form on the outside of the container, and pretty soon the boiling rate would drop off substantially as the rate of heat transfer from the now frigid surroundings reduced.
If the vaporising gas was ignited, some of the heat from that combustion would certainly be transmitted to the pool of cold, liquid gas by radiation which would increase the vaporisation rate. The net result would still be a release of energy into the surroundings over probably tens of minutes (as opposed to microseconds in the case of a nuclear bomb).
You can demonstrate this yourself pretty easily using an ordinary 20-lb barbecue tank, a "high pressure" aka 20 psi regulator ($20 on amazon), and a high-pressure burner head (also $20 on amazon). Turn it all the way up - you'll get a great big hot flame at first, but you'll quickly see a shell of ice forming on the outside of the tank, because the process of evaporation drains heat out of the propane very quickly, and it gets cold enough to condense water out of the atmosphere into ice. Within half an hour, that big hot flame will have died down to something very gentle: the tank is still full of propane, but it's now so cold it'll no longer evaporate quickly enough to keep the pressure up. You'll have to turn it off and let it thaw for a few hours before it'll really get going again.
The LHPO runs only on propane, at full bottle pressure (that is, five to ten bars, depending on ambient temperature).
The LHPO may be run from either a bank of propane bottles feeding into a manifold and then proceeding along a main gas pipe to the organ, or from a single propane tank with either liquid or gas feed. If liquid feed, a gasifier must be provided with sufficient peak capacity (see below).
If a single tank, the tank must have at least a three ton filled capacity, to provide enough thermal mass for the required gas delivery rate (see below). The tank connection must be at least 1 1/2 inch inside pipe diameter, to provide for sufficient gas delivery rate.
If a multiple tank installation, at least 28, 33kg capacity bottles are needed. The bottles may not be all close packed into a rectangular array, but must be in one or two rows, with adequate air circulation around the bottle area to provide for heat transfer to the bottles.
If a liquid phase system and a gasifier are to be used, the system must be able to deliver the peak delivery rate (see below) for at least ten minutes continuously."
Well, the double hulls of these ships are inch-thick steel, but I think the dome is less protected. An attack that blew the dome off could open up a large hole and ignite the escaping gas, but it would still take minutes for all of it to boil off, probably freezing as it does.
I guess a big enough rocket could blow the liquid out of the tank, or rupture a large hole in the hull of the ship causing the LNG to dump into the (relatively) warm water and boil off faster. But at some point the real question is: How did the bad guys fire rockets into a secured unloading station?
Inch thick is the same as road plate and means nothing when there's tens of thousands of tons of ship involved. Freight ships are built as light as possible. Steel that you don't have to float leaves room for more cargo weight and increases fuel economy.
The result would be a cloud of gas that burns violently for several minutes, depending upon wind and the speed of the leak. Not the instantaneous, crater-making release of energy that a thermonuclear device produces.