Eight catastrophic failures led to the explosion that destroyed the Deepwater Horizon drilling rig in the Gulf of Mexico, killing 11 people and leading to one of the biggest oil leaks in history, according to BP’s long-awaited investigation into the accident.
BP accepts its role in the disaster but also points the finger at two of its contractors.
The accident occurred on 20 April as the team aboard Deepwater Horizon was preparing to temporarily abandon a well it had drilled some 70 kilometres from the US coast.
The day before the accident, the crew had pumped cement to the bottom of the borehole, a standard procedure intended to prevent oil leaking out. On the day of the accident, the team were conducting checks to determine that that the well had been properly sealed.
BP says the accident was caused by the failure of eight different safety systems that were meant to prevent this kind of incident:
The cement at the bottom of the borehole did not create a seal, and oil and gas began to leak through it into the pipe leading to the surface. BP says the cement formulation seems not to have been up to the job.
The bottom of the pipe to the surface was sealed in two ways. It too was filled with cement, and it also contained two mechanical valves designed to stop the flow of oil and gas. All of these failed, allowing oil and gas to travel up the pipe towards the surface.
Pressure test misinterpreted
The crew carried out various pressure tests to determine whether the well was sealed or not. The results of these tests were misinterpreted, so they thought the well was under control.
Leak not spotted soon enough
Whether a well is under control or not, the crew at the surface should be able to detect a flow of oil and gas towards the surface by looking for unexpected increases in pressure in the well. Exactly this kind of increase occurred about 50 minutes before the rig exploded, but it was not interpreted as a leak.
Valve failure no. 2
About 8 minutes before the explosion, a mixture of mud and gas began pouring onto the floor of the rig. The crew immediately attempted to close a valve in a device called the blowout preventer, which sits on the ocean floor over the top of the well borehole. It did not work properly.
The crew had the option of diverting the mud and gas away from the rig, venting it safely through pipes over the side. Instead, the flow was diverted to a device on board the rig designed to separate small amounts of gas from a flow of mud. The so-called mud-gas separator was quickly overwhelmed and flammable gas began to engulf the rig.
No gas alarm
The rig had an onboard gas detection system that should have sounded the alarm and triggered the closure of ventilation fans to prevent the gas reaching potential causes of ignition, such as the rig’s engines. This system failed.
No battery for BOP
The explosion destroyed the control lines the crew were using to attempt to close safety valves in the blowout preventer. However, the blowout preventer has its own safety mechanism in which two separate systems should have shut the valves automatically when it lost contact with the surface. One system seems to have had a flat battery and the other a defective switch. Consequently, the blowout preventer did not close.
“It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy. Multiple parties, including BP, [oilfield services company] Halliburton and [offshore drilling company] Transocean, were involved,” said Tony Hayward, BP’s chief executive.
Originally published in https://www.newscientist.com/article/dn19425-the-eight-failures-that-caused-the-gulf-oil-spill/