05 // What actually happened?
The blowout began as a loss of well barriers, not as a sudden BOP malfunction. A critical cement barrier at the bottom of the well had not effectively isolated the hydrocarbon-bearing zone. The negative-pressure test intended to verify that barrier produced unexpected results and was nevertheless accepted as a pass.
Mud displacement removed a major pressure barrier
After the test was accepted, the crew continued temporary-abandonment operations and replaced dense drilling mud with seawater. Drilling mud controls a well by exerting hydrostatic pressure against formation pressure. Removing it reduced bottom-hole pressure while the surface plug had not yet been installed.
The kick developed without timely intervention
CSB concluded that hydrocarbons flowed from the reservoir for almost an hour without human intervention or automated closure. As influx continued, hydrocarbons accelerated upward through the well and the roughly mile-long riser. Once large volumes of gas were above the subsea BOP, even successful shut-in could not remove the hazardous inventory already traveling toward the rig.
The surface response could not safely handle the flow
The diverter was configured to send flow to the mud-gas separator rather than directly overboard. The separator was rapidly overwhelmed, and hydrocarbons discharged from multiple points onto the rig. They found an ignition source, causing explosions and fire.
The BOP story is more complicated than “it did not work”
Pressure evidence indicates that a pipe ram successfully shut in the well for a period. The later emergency blind shear function likely actuated, but CSB concluded that effective compression had buckled the drillpipe off-center so the blind shear ram could not fully close and seal the well. Post-incident examination also revealed latent control-system deficiencies in the redundant emergency pods.