What happen:
I don't know how deep the well was but here's a short description of what happen.

They just finish setting 7" production casing-not a liner. Mud weight was 16ppg+ They used light weight cement with nitrogen because of lost circulation problems to set the casing. The cement job went OK. They hung off the casing and tested the seal at 10,000 psi. They set a temporary bridge plulg below 5000' because they were going to come back to complete the well. They displaced the 16 ppg mud in the riser with sea water to get ready to remove it and the BOP's. That's when all hell broke lose. I would think that the pack-off (seal) gave way. The well came in and engulfed the rig with gas. With light winds the engine intakes took it all in and ran away and exploded sending them overboard and setting the engin room on fire. The mud pumps had the same fate. Nine crew members and two mud enginers on the pits were killed. Now get this, they were having a party in the living quarters celebrating 7 years without and accident. Walls were blowned out and several crew member in the quarters sustened broken limps. The safety shut in on the BOP's were activated but they did not work. Most system have a redundancy of two hydraulic and two electric systems. The blind or sheard rams would have controled it. But nothing worked. The riser sheard off some feet above the BOP's and the oil is coming through several sections of the riser where the seals gave way.......Bad,Bad,Bad day for all........This report was giving by my son who is in the industry and was able to get the reports.