Horizon blowout. It appears that from what I hear there was gas under the annular, a big rubber and steel spincter at the top of the BOP. It seals off between the pipe in the hole and the casing. That space is called the annulus. They had the annular shut for some time and as the hole was cemented they had just tested BOPs were probably confident there would be nothing under the annular.
Whenever any of the blowout preventers, including an annular are opened it is good practice to open up a choke line which gives access under the preventers to monitor any pressure build up.
Hydroatatic pressure at 5000' is about 2500psi with salt water.
At 10,000' near the bottom of the well it is almost 5000psi. Assuming gas had migrated from the bottom of the well, with the annular closed and no expansion possible you would expect there would be some increase in pressure that could be seen at the BOP if they looked.
The matter was made much worse by the fact that the heavier drilling fluid had been displaced with sea water, leaving less hydrostatic head to resist the gas expansion.
When the annular was opened, I assume blindly, all hell broke loose. The gas bubble pushed the water out of the well with a 250' plume, so you have to think it would not take long to travel the 5000' seabed to surface.Usually it starts slow but increases rapidly.
There is amother spincter like apparatus just under the rig floor. Called the diverter, it can close around the pipe, sealing off the annulus at surface. At the same time it opens valves to direct gas flow overboard.
The diverter takes about 30sec to function closed, depending on what the operating pressure is set at.
If the diverter is set too low to close fully, then in that situation it would be too late. If the driller called someone for instructions it would probably be too late.
All that is hypothetical as I dont know for sure the chain of events, only info from a hand who claims to have been on the rig.
What I do know for sure is that the Zapata Lexington blew out in the same area back in 1985 after they opened their annular with a gas bubble under it. They had just killed a kick and didnt know that there was some gas trapped in the BOP between a ram and an annular. They were confident all was good. Four hands killed. There was a big conference and one of the things as I recall that came out of it was to always check for pressure under preventers before opening. There was only a few barrels of gas at about 2000' and the expansion was enough, even from that depth to destroy the rig floor and derrick when the fire started. One big mistake they made was to divert into the mud pits to avoid pollution with their oil based mud.
