Deepwater Horizon

The Vessel

Deepwater Horizon was a semi-submersible deepwater drilling rig owned and operated by Transocean Ltd., the world's largest offshore drilling contractor. She was built at Hyundai Heavy Industries in Ulsan, South Korea, delivered in 2001, and operated under a nine-year charter to BP for ultra-deepwater exploratory drilling in the Gulf of Mexico. Her design was a dynamic-positioning, fifth-generation semi-submersible: two parallel pontoons supporting six columns that in turn supported the main deck structure, 121 metres long and 78 metres wide, with a drilling derrick rising 80 metres above the deck.

She was one of the most capable drilling rigs in the world. Her operational specifications allowed drilling in water depths of 2,400 metres to a drilled-well depth of 9,100 metres, the outer envelope of ultra-deepwater oil exploration in 2010. She was classed by Det Norske Veritas and flagged under the Marshall Islands, a conventional ownership structure for offshore rigs that placed her nominally outside American regulatory jurisdiction while she operated in the American exclusive economic zone.

In February 2010 she was assigned to BP's Macondo Prospect, a 50 per cent BP-owned exploratory lease in Mississippi Canyon Block 252, 66 kilometres off the Louisiana coast. The Macondo well had been drilled from 6 February 2010 and had encountered difficulties throughout: gas kicks, loss of drilling fluid, issues with the blowout preventer, pressure discrepancies that suggested hydrocarbons might be migrating into the cement being placed to seal the well. The drilling had already fallen 43 days behind schedule and cost BP approximately $1.5 million per day of operation.

The Voyage

By 20 April 2010, the Macondo well had been drilled to a total measured depth of 5,600 metres below the rig deck. The well was to be temporarily abandoned by cementing: production-casing cement was to be emplaced at the bottom of the well, followed by an inert spacer, followed by saltwater displacement of the drilling mud in the riser to prepare the rig for departure. The sequence was standard. The execution was flawed.

A series of decisions taken between BP engineers in Houston and Transocean drilling personnel on the rig, between 09:00 and 17:00 on 20 April 2010, has been reconstructed in detail by the subsequent investigation. The negative pressure test, a standard check for well integrity, was conducted twice with ambiguous results and interpreted on the rig as acceptable despite the pressure anomaly. The decision not to run a cement bond log, the physical inspection that would have detected the inadequate cement job, was made by BP management as a cost-saving measure: the CBL would have added approximately 12 hours to the rig schedule. The positive displacement of drilling mud with seawater began at 17:05.

At 21:45 on 20 April 2010, gas began to enter the riser from the Macondo well. Drilling mud and seawater began to blow up the riser and onto the rig deck. The blowout preventer, an 80-ton hydraulic stack on the seafloor whose function was to close off the well in exactly this scenario, failed to do so. Its blind shear ram, the specific component designed to cut through the drill pipe and seal the well, either did not activate or did not successfully cut. The gas reached the rig deck at 21:49.

The Disaster

The gas ignited at 21:49:30 on 20 April 2010. The first explosion destroyed the drill floor, the mud-gas separator, and the forward helideck; the second explosion at approximately 21:51 spread the fire through the accommodation block. Eleven crew members, all working in or near the drill floor or mud-processing spaces, died in the first ninety seconds. The surviving 115 crew were evacuated by lifeboat and by rescue vessels over the following two hours; all of them reached the surface ship Damon B. Bankston by early morning on 21 April.

The rig burned for 36 hours. The drilling mud and diesel aboard fed a fire that the Louisiana Coast Guard and U.S. Navy firefighting support could not extinguish. She sank at 10:22 on 22 April 2010, coming to rest on the seabed 450 metres northwest of the wellhead, broken into three large sections. The riser pipe connecting her former position to the seafloor bent and separated; the Macondo well began releasing oil and gas to the Gulf of Mexico at a measured rate of 62,000 barrels per day.

The well continued to flow for 87 days. Every attempt to close it short of a relief well failed, from the initial "top kill" drilling-mud injection through the "junk shot" polymer attempt. On 15 July 2010 a newly-built capping stack was successfully mated to the wellhead and sealed the flow; the final cement kill of the well from two relief wells was completed on 19 September 2010.

Approximately 4.9 million barrels of crude oil had been released into the Gulf of Mexico between 20 April and 15 July 2010, the largest marine oil spill in human history. The spill contaminated 2,100 kilometres of Gulf coastline from Louisiana to Florida, killed birds, fish, sea turtles, and marine mammals in numbers that have never been fully counted, and required the deployment of 1.8 million gallons of dispersant chemicals whose own ecological effects are still being studied.

The Legacy

The National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, established by President Obama in May 2010 and chaired by Bob Graham and William Reilly, issued its final report in January 2011. The Commission's conclusions were that the disaster was the product of systemic industry-wide regulatory failure, corporate cost-cutting prioritisation over safety, and inadequate federal oversight of offshore drilling. The report's specific findings against BP centred on the cement-integrity decisions; against Transocean on the blowout preventer maintenance; against the Minerals Management Service (the federal regulator, since renamed the Bureau of Safety and Environmental Enforcement) on inadequate inspection and industry capture.

Eleven people died. Among them: Jason Anderson (35), Aaron Dale Burkeen (37), Donald Clark (49), Stephen Ray Curtis (39), Gordon Jones (28), Roy Wyatt Kemp (27), Karl Kleppinger (38), Keith Blair Manuel (56), Dewey Revette (48), Shane Roshto (22), Adam Weise (24). Nine were Transocean employees, two were M-I Swaco contractors. Their families received a combined settlement of approximately $150 million from BP and Transocean in civil settlements over 2010-2013.

BP pleaded guilty to fourteen federal criminal charges in November 2012, including eleven counts of seaman's manslaughter. The corporate fine of $4.525 billion was the largest corporate criminal settlement in American history to that date. BP's total liability for the Deepwater Horizon disaster, across criminal, civil, and natural-resource-damage claims, reached approximately $65 billion by 2018. No BP executive was convicted of a felony. Donald Vidrine and Robert Kaluza, two BP "company men" on the rig at the time of the blowout, were charged with manslaughter; charges were dropped in 2015 and 2016 respectively.

The Bureau of Safety and Environmental Enforcement, the reconstituted regulator, implemented revised well-design standards, mandatory third-party inspection of cement jobs, and redesigned blowout-preventer testing protocols in the Well Control Rule of 2016. The Trump administration partially rolled back the rule in 2019; the Biden administration restored the stricter version in 2021.

The Deepwater Horizon is the largest industrial accident in the offshore oil industry's history and the largest marine oil spill ever recorded. The 11 dead are commemorated at the Deepwater Horizon Memorial Park in Jackson County, Mississippi, inaugurated in 2011, and on a plaque at the Transocean corporate headquarters in Vernier, Switzerland. The rig itself lies on the Gulf of Mexico floor at 1,525 metres, broken in three, still partly visible on side-scan sonar surveys of the Macondo debris field.