The internal investigation BP included in their report and made public last week about how the disaster occurred indicates that the crew almost prevented the explosion and did succeed in operating the blowout preventer.
A summary of what happened according to BP is based on eyewitness reports of those workers who survived. The 11 workers who died were the most directly involved in the operation and so without their input BP put together a time line to explain what they think happened.
Other companies and government agencies are still doing their own investigations and may reach different conclusions to what BP has.
At 9:41pm on the night of April 20th the crew realized a blow out was about to happen when drilling mud blew out of the top of the well pipe. They then operated the valves on the blow out preventer to cut off the gas flow from coming up the pipe. The valve on the ocean floor did close but failed to seal the pipe completely.
By this time the fast rising gases in the pipe had already travelled above the preventer valves and were racing towards the drill rig through the mile long pipe from the ocean floor.
By 9:47pm the crew had managed to get the preventer to close off the pipe completely and stop the gas. Eye witnesses said that the mud stopped coming out of the pipe at that point and they thought the crew was bringing the problem under control once they had now managed to limit the gas volume to just what was in the pipe above the preventer. Alarms then began sounding on the platform warning of a gas build up.
There are two ways for well gas reaching the surface to be handled. The normal way is to direct the well discharge into an on board containment system which separates the gas from the other sludge in the pipe and gets rid of the gas. There are stiff federal fines for letting oil escape into the ocean and so this is the default system that is used. At the time the crew probably didn’t have enough information to decide to change protocol.
The other system just vents the gas and sludge overboard into the water. When the gas reached the on board system there was too much of it for the system to handle and it became overloaded. It’s not known if the crew tried to divert the gas overboard through the other system once they seen the problem.
The report says that it is likely at this point that the gas surrounding the platform got sucked into the engine room and a spark there caused an explosion at 9:49 which killed all power.
With no power the crew had no way of having the blow out preventer do its final task of shearing off the pipe on the ocean floor and sealing off the well. Instead, after the explosion the pipe eventually broke off between the blow out preventer and the platform and the valve which had the pipe sealed off worked its way open again allowing the oil to spill into the gulf.