Key figures: Tony Hayward (BP CEO), Barack Obama (US President), Thad Allen (National Incident Commander)
Summary
On April 20, 2010, a blowout on the Deepwater Horizon drilling rig, operated by Transocean and leased by BP, triggered a catastrophic explosion in the Gulf of Mexico approximately 40 miles off the Louisiana coast. High-pressure methane gas surged up the marine riser and ignited on the platform, killing 11 workers and injuring 17 others. The rig sank two days later on April 22, rupturing the riser pipe and beginning an uncontrolled discharge from the Macondo well at a depth of approximately 5,000 feet below the sea surface.
The well flowed uncontrolled for 87 days. Multiple intervention attempts — including a “top kill” procedure, the deployment of containment domes, and the drilling of relief wells — were required before a temporary cap was placed on July 15, 2010. The Macondo well was declared permanently sealed on September 19, 2010. By that date, the US federal government estimated 4.9 million barrels (approximately 210 million gallons) of crude oil had been discharged into the Gulf of Mexico, making it the largest accidental marine oil spill in the history of the petroleum industry.
The spill affected approximately 70,000 square miles of ocean surface and caused extensive environmental damage across the Gulf Coast. Federal and state authorities closed large areas to commercial and recreational fishing. Estimated wildlife casualties included 51,000 to 84,000 seabirds, 56,000 to 166,000 juvenile sea turtles, and an estimated 5 trillion newly hatched fish. Deep-sea coral communities and Gulf floor habitats also suffered significant damage.
The Blowout and Immediate Response
The Deepwater Horizon was a semi-submersible mobile drilling unit owned by Transocean and under lease to BP, drilling the Macondo Prospect — exploratory well MC 252 — at a water depth of approximately 5,000 feet (1,524 meters) in the Mississippi Canyon block of the Gulf of Mexico. The well had reached a depth of about 18,000 feet (5,486 meters) below sea level and was in the final cementing phase before a temporary cap was to be placed before the rig moved to its next assignment.
On the evening of April 20, during displacement of drilling mud from the wellbore with seawater — a critical step in the cementing process that reduced the downward pressure column — a negative pressure test was conducted and misinterpreted as satisfactory by the crew. At 9:49 PM, highly pressurized methane gas rose rapidly up the wellbore, expanded as it reached the surface, and ignited in a series of explosions. The rig’s blowout preventer (BOP), a 450-ton stack of rams and valves designed to cut and seal the wellbore in exactly this emergency, failed to function — a failure later attributed to hydraulic system problems and, critically, a missing component in the blind shear rams that were supposed to cut through the drill pipe. At 10:22 PM, a second explosion collapsed the derrick. The fire burned for 36 hours before the Deepwater Horizon sank on April 22.
Of the 126 crew on board, 115 evacuated successfully. Eleven workers — Jason Anderson, Aaron Dale Burkeen, Donald Clark, Stephen Ray Curtis, Gordon Jones, Roy Wyatt Kemp, Karl Kleppinger Jr., Blair Manuel, Dewey Revette, Shane Roshto, and Adam Weise — died in the explosions and were never recovered. Their deaths made the Deepwater Horizon accident the deadliest offshore drilling disaster in United States history.
Containment Attempts and the 87-Day Flow
The rupture of the marine riser when the rig sank created multiple simultaneous leak points at the seafloor. Remotely operated underwater vehicles (ROVs) confirmed oil flowing from the wellbore within 24 hours, but initial estimates of the flow rate — subsequently revealed as grossly understated — led to a delayed grasp of the disaster’s scale. BP initially estimated a flow of 1,000 barrels per day; independent scientists and federal officials ultimately determined the true flow rate was approximately 62,000 barrels per day in the first days after the sinking, declining as reservoir pressure dropped.
BP’s attempted responses to seal the well took four distinct forms over 87 days:
- Cofferdam dome (May 7–9): A 98-ton steel containment dome lowered over the leaking riser failed when methane gas, mixing with seawater at the low temperature and high pressure of the seafloor, formed a crystalline methane hydrate that blocked the dome’s collection funnel.
- Top Kill (May 26–29): BP pumped heavy drilling fluids (“kill mud”) into the blowout preventer at high pressure in an attempt to overwhelm the well’s upward flow. The attempt failed after three days when the kill mud could not overcome the well’s flow rate.
- Lower Marine Riser Package (LMRP) cap (June 3): After cutting the damaged marine riser pipe above the BOP, BP placed a loose-fitting cap on the BOP stack that collected a portion of the flowing oil and gas; a mile-long riser pipe delivered the collection to a surface drillship. This cap reduced — but did not stop — the spill.
- Static kill and cement cap (August 3–5): After drilling two relief wells to intersect the Macondo wellbore, BP performed a “static kill” — pumping drilling mud and then cement through the relief well — that permanently stopped the flow. The Macondo well was declared “effectively dead” on September 19, 2010, 152 days after the initial blowout.
Environmental Damage
The 4.9-million-barrel discharge created a surface oil slick that at its peak covered approximately 70,000 square miles — an area larger than Oklahoma. Subsurface plumes of dispersed oil, at depths of 3,300 to 4,600 feet, were detected by academic research vessels, demonstrating that the oil spread through the water column rather than solely on the surface. BP and its contractors applied approximately 1.84 million gallons of chemical dispersants — primarily Corexit 9500 and 9527 — both at the surface and, in a controversial first-ever application, directly at the wellhead on the seafloor. EPA officials subsequently raised concerns about dispersant toxicity, and the long-term ecological effects of subsurface dispersant application remained a subject of scientific study for years.
The Gulf Coast fishing and tourism industries suffered severe disruption. Federal and state authorities closed 36 percent of Gulf federal waters to commercial and recreational fishing at the height of the spill. The oyster fishery in Louisiana, already stressed by saltwater intrusion, suffered estimated losses of $330 million. The spill occurred during the peak nesting season for five species of Gulf sea turtles; an estimated 56,000–166,000 juvenile Kemp’s ridley sea turtles — the most endangered sea turtle species — were in the spill zone. National shorelines and salt marshes from Texas to Florida were oiled to varying degrees; deep-sea coral communities near the wellsite were found dead within months.
Significance
The Deepwater Horizon disaster prompted the most consequential overhaul of US offshore drilling regulation since the Santa Barbara oil spill of 1969. The Obama administration imposed a temporary moratorium on deepwater drilling and reorganized federal oversight, abolishing the Minerals Management Service and replacing it with the Bureau of Safety and Environmental Enforcement. The disaster demonstrated critical failures in industry self-regulation, blowout prevention technology, and emergency response capacity.
Legally and financially, the consequences for BP were historic. In November 2012, BP pleaded guilty to 11 counts of felony manslaughter, two misdemeanors, and felony obstruction of Congress, agreeing to pay $4.525 billion in criminal fines — the largest criminal resolution in US history at the time. In April 2016, BP reached a $20.8 billion civil settlement with the US government and five Gulf Coast states, the largest environmental damage settlement in US history. Total cleanup and legal costs surpassed $65 billion by 2018.
The spill also galvanized public debate about offshore energy policy, the environmental costs of fossil fuel extraction, and corporate accountability. It remains the defining industrial environmental disaster of the 2010s and a benchmark event in the history of energy regulation.