Disaster 07 · Engineering disasters
The pressure test that was read the hopeful way
Crew on an oil rig explained away a failed safety test, and hours later the well blew out, killing 11 people and causing the largest marine oil spill in US history.
- BP, Transocean and Halliburton
- Gulf of Mexico, United States
- 2010
- 3 min read
On this page
Before you readWhat would you do?
You test a new supplier's product before a big order. Two of ten samples fail. The sales rep says the failed ones were probably damaged in shipping. You are under pressure to place the order today. What should you do?
Pick an answer now. The file tells you whether you were right.
01The file in one minute
1A failed test
Pressure readings showed the well was not sealed. The crew accepted another explanation and carried on.
2Blowout and fire
Hours later the well blew out and the rig caught fire. Eleven workers died.
387 days of oil
The seabed safety valve failed. A court found about 3.19 million barrels reached the Gulf.
4The lesson
Decide before a test what counts as failure, and test any easy explanation for a bad result.
02Timeline
20 April 2010
The crew calls a worrying pressure test a success. That evening the well blows out.
22 April 2010
The Deepwater Horizon rig sinks.
15 July 2010
The well is capped after 87 days of flowing oil.
January 2011
The national commission reports a chain of risky decisions under time pressure.
03What happened
In April 2010 the drilling rig Deepwater Horizon, owned by Transocean and working for BP, was finishing an exploration well called Macondo, about 1.5 kilometres under the sea in the Gulf of Mexico. The last steps were to seal the well with cement and test that the seal held.
On 20 April the crew ran a negative pressure test, which checks whether oil and gas can leak into the well. The readings showed pressure building where it should not. The team accepted an explanation that the readings were caused by something else, called the test a success and carried on.
That evening, oil and gas surged up the well and onto the rig, then ignited. Eleven workers died. The blowout preventer, a large valve stack on the seabed meant to seal the well in an emergency, failed to close it. The rig sank two days later.
Oil flowed for 87 days until the well was capped on 15 July. Estimates of the spill vary. A US court later found about 3.19 million barrels had entered the Gulf, and government scientists had estimated more.
04Why it happened
The national commission that investigated found no single cause. It pointed to a chain of decisions under time and cost pressure, many of which saved time and added risk. The negative pressure test was the clearest warning, and the crew explained it away.
Once people expect a step to pass, an odd result tends to be explained away instead of investigated. The commission also found that the companies involved did not share information well, so nobody had the whole picture.
The idea behind this caseLesson: Check what AI tells you →
05Lessons for decisions
- Decide before a test what result counts as a failure, so you cannot reinterpret it afterwards.
- When a check fails and someone offers an easy explanation, test the explanation before moving on.
- Keep a list of choices that saved time but added risk, and look at them together, not one by one.
- Make sure the different groups working on a job share warning signs with each other.
07 — Sources
- Deep Water: The Gulf Oil Disaster and the Future of Offshore Drilling · National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling · 2011-01
- Macondo Blowout and Explosion · U.S. Chemical Safety and Hazard Investigation Board · 2016
- Deepwater Horizon oil spill · Wikipedia · 2026