As Texas Cajun wrote, this kind of methane bubble is common, and usually controlled.
The methane escaped into the atmosphere ( a reliable oxygen source ) formed an explosive mixture and found probably one of several ignition sources. The gas cloud exploded, then the leaking gas continued to burn.
What I don’t get is why the rig sank. The fire was well above the ‘water line.’
I was thinking the weight of the firefighting water had somwthing to do with it. That’s an issue with boat fires, I know.
hours and hours of heat....buckling steel plates.....no computer or human control over the ballast system....no pumps for dewatering.... IMO, most of the water from those firenozzles on those boats just ran off, HEAT is what sank that rig...