Not impossible, but it would be very difficult for an enemy submarine to move within 40 miles of New Orleans and not be detected by the US Navy. I’d say its much more likely that the cause is some kind of mistake made in operating the equipment on the rig. Keep in mind that there’s almost always some natural gas that moves up the well pipe with oil and if that gas is not handled properly and flared off, it could cause an explosion. All that diesel fuel has to be handled properly too or it can cause an explosion and fire.
If they were either drilling or producing, I would lean a lot more towards the MOAB- Mother of all Blowouts. But they were installing and grouting in casing. As soon as you stop drilling, doesn’t the risk of a blowout drop? I only found one case of a blowout happening other than during production or drilling. The Ensco 51, Gulf of Mexico, 2001 - no fatalities, and the rig survived: http://home.versatel.nl/the_sims/rig/ensco51.htm
I’m an ex-volunteer firefighter, and I work in the transportation industry. Regular #2 diesel has a flash point of 125 F. Marine Diesel (#6/#2 blend) is minimum 150 F. Yeah, it’s combustible (NFPA hazard class 2, “ignites if heated”) but nothing like gasoline, a class 3 flammable liquid with a flash point of -45 F (yep, 45 below zero!)
I will concede that a chunk of drilling equipment from a blowout could have pierced a fuel bunker.