150 lbs of TNT is a 2.2 Richter, as is the pressure energy release. If there were two combined, that would be a 2.4 - 2.5 Richter event.
I definitely think it was a thermal cut, but the fold of the pipe wall is inconsistent. I've never seen any internal pressure explosion resulting in that shape. It's just very weird. Of course we can't be sure if the clean cut was the initiating event, or the result of a post event investigation removing the damaged section. In any case, I can't imaging how that fold got there, especially the way the rest of the pipe appears to have remained relatively circular. That looks like a huge crimping tool had a go at it, but I'm not suggesting it.
There are residual tension stresses in the pipeline from maintaining the catenary "S" shape, not allowing too much sag in the lower curve that would cause a local buckle, as the pipe is pulled off the barge. It's considerable, but absolutely not enough to cause more than maybe a half foot of snapback if cut. Cutting more often causes the two ends to displace laterally, far more than axially.
The surrounding water should have kept any pipe fragments rather close by, as even aerodynamically shaped bullets don't travel far under water. Another reason to suspect pipe removal, as no evidence of any fragments or 25m of bent speghetti pipe has been seen yet.
At 80m, it's a mixed gas dive for anything longer than 5 min, or a very long time decompressing.
Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."