This is was the riser looks like that sits between a drilling rig on the surface and the well on the ocean floor.
Transocean officials said Wednesday it appeared the accident was caused by a blowout, the uncontrolled flow of hydrocarbons up the rig’s riser, followed by an ignition of the hydrocarbons. The company said it didn’t know for sure, but that was their best lead so far.
There’s equipment that’s supposed to prevent just such accidents, called (not surprisingly) blowout preventers, or BOPs. But it does not appear that has worked, particularly since officials are unable to shut off the flow of oil to the fire.
A blowout can occur at many stages of the drilling and production process, but in this case it appears the crews had finished drilling a while ago and were putting metal casing down the well hole, and were putting cement around the casing to keep it in place.
Analysts with FBR Capital Markets seem confident in their data collection which spells out two possible occurences:
We do not know for certain, but according to our sources, the rig had recently set and cemented a tapered 7"x 9 5/8" tapered casing and were somewhere in the process of displacing the riser with seawater and subsequently setting a surface plug when the well blew out. According to one anecdote, the rig was probably less than 24 hours from leaving location when the blowout occurred.