Weak cement work
The day before the accident, cement was pumped down a pipe in the well, which squeezed back up the space between the pipe and the rock layers, known as the annulus. That "light, nitrified foam cement slurry" failed to block oil and gas from moving up into the annulus. Cement design, testing, quality assurance and risk assessment was weak.
Once oil and gas got into the annulus, "shoe track barriers" failed to prevent hydrocarbons from entering the pipe. A shoe track is a barrier designed to block hydrocarbons from entering the bottom of the pipe.
Pressure tests misread
A negative pressure test performed on the day of the explosion was misinterpreted as successful by BP well site leaders and the Transocean rig crew. Pressure readings really indicated that hydrocarbons were flowing into the well from the reservoir, but were interpreted as showing that oil and gas were blocked by barriers provided by the cement, shoe track, casing and a casing seal.
Delay in detection
The breach of oil and gas into the well wasn’t detected until they had reached the riser, a pipe that connected the well to the rig.
Failure to regain control
Efforts to bring the well under control failed. Crews first tried to close a blowout preventer at the seabed and a diverter, but oil and gas shot up the riser. Hydrocarbons went to an onboard system that separated drilling mud from natural gas rather than to an overboard diverter line that would have sent them into the sea.
Fast incoming oil and gas overwhelmed the mud gas separator, allowing gas to be vented onto the rig.
Air con propelled the gas
The Horizon's heating, ventilation and air conditioning system likely moved the gas to engine rooms where ignition risk was high.
Blowout preventer failed
The blowout preventer failed to seal the well. The report said the explosions and fire on the rig likely disabled mechanisms that allowed rig crews to seal the well and disconnect the riser. In addition, control pods on the blowout preventer were faulty, and underwater robots failed to manually activate valves in it as oil spewed into the ocean after the disaster.