In an engineering library a few years ago I came across an older book called Frozen Fire that detailed some of the serious accidents that occurred in the early days on LNG storage and discussed the potential hazards.
According to this book, an LNG vapor cloud does not rise but clings to the ground (which makes sense since it is cold). If a breach of one of these very large tanks occurred, the vapor cloud could spread over an enormous area.
The Wikipedia page on LNG has a link to a Sandia Labs study on LNG leak hazards. It says that if there is not a nearby ignition source, an LNG vapor cloud from a large spill can extend a mile or more.
When 25000 cubic meters of LNG starts burning, you don't want to be in the vicinity.
It is extremely difficult for Methane vapor to disperse to an only 15% concentration and remain cold enough to be heavier than air. Methane's specific gravity is on 0.55.
If it has just vaporized in a dense, cold cloud, then it is almost entirely too rich to ignite. To mix with the warmer air to only a 15% concentration, it has picked up most the heat from the ground and air and is rising and dispersing.
LNG has been transported worldwide for 4 decades. Safety systems have improved a lot in that time while the volume being handled has greatly increased.