[7] At the time of the explosion, the Deepwater Horizon was on Mississippi Canyon Block 252, referred to as the Macondo Prospect, in the United States sector of the Gulf of Mexico, about 41 miles (66 km) off the Louisiana coast.

[15] However a Wall Street Journal analysis "painted a more equivocal picture" with Transocean rigs being disproportionately responsible for safety related incidents in the Gulf and industry surveys reporting concerns over falling quality and performance.

The rig won an award from the MMS for its 2008 safety record, and on the day of the disaster, BP and Transocean managers were on board to celebrate seven years without a lost-time accident.

[15]In February 2009, BP filed a 52-page exploration and environmental impact plan for the Macondo well with the Minerals Management Service (MMS), an arm of the United States Department of the Interior that oversees offshore drilling.

[16] In the event an accident did take place, the plan stated that due to the well being 48 miles (77 km) from shore and the response capabilities that would be implemented, no significant adverse impacts would be expected.

[16] The Department of the Interior exempted BP's Gulf of Mexico drilling operation from a detailed environmental impact study after concluding that a massive oil spill was unlikely.

[25] The rig's mechanic stated that the well had been experiencing problems for months, and that the drill repeatedly kicked due to resistance from high gas pressure.

[27] A confidential survey commissioned by Transocean weeks before the explosion stated that workers were concerned about safety practices and feared reprisals if they reported mistakes or other problems.

[28] The survey raised concerns "about poor equipment reliability, which they believed was a result of drilling priorities taking precedence over maintenance."

[34] Patrick O'Bryan, BP Vice President of drilling, was on the platform two hours prior to the explosion[35] to celebrate seven years without a "lost-time incident" with the rig's crew.

[34] A House Energy and Commerce Committee statement in June 2010 noted that in a number of cases leading up to the explosion, BP appears to have chosen riskier procedures to save time or money, sometimes against the advice of its staff or contractors.

"[41] After the explosion, Adrian Rose stated that abnormal pressure had accumulated inside the marine riser and as it came up it "expanded rapidly and ignited.

"[14] According to BP's internal investigation, a bubble of methane gas escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding.

[48][49] Lifeboats took 94 workers to the Tidewater-owned supply boat Damon Bankston, with no major injuries, four were transported to another vessel, and 17 were evacuated by helicopter to trauma centers in Mobile, Alabama and Marrero, Louisiana.

The workers were transported to a hotel in Kenner, Louisiana, where they were provided with food, medical attention, and rooms with showers, and asked to fill out incident response forms.

[59] On the morning of April 22, Coast Guard Petty Officer Ashley Butler stated that "oil was leaking from the rig at the rate of about 8,000 barrels (340,000 US gallons; 1,300,000 litres) of crude per day.

[45] Butler warned of a leak of up to 700,000 US gallons (17,000 bbl) of diesel fuel, and BP Vice President David Rainey termed the incident as being a potential "major spill.

[46] The report says BP employees and those at Transocean incorrectly interpreted a pressure test, and both companies neglected signs such as a riser pipe losing fluid.

[69] On November 8, 2010, the inquiry by the Oil Spill Commission revealed its findings that BP had not sacrificed safety in attempts to make money, but that some decisions had increased risks on the rig.

[71] One of the decisions that met with tough questions was that BP refuted the findings of advanced modelling software that had ascertained over three times as many centralizers were needed on the rig.

[72] A slide briefly appeared on the Oil Spill Commission's website and republished by The New York Times enumerating eight "risky" and "unnecessary" steps that BP was deemed to have taken.

[76] In May 2011, MOEX Offshore, which owned a 10% stake in the well through a subsidiary and which in turn itself was majority-owned by Mitsui & Co., agreed to pay US$1.08 billion to settle BP claims against it over the accident.

With MOEX agreeing to share in the financial burden—though "the agreement isn't an admission of liability by either party"—the possibility of such a ruling by the Department of Justice seemed to some to be perhaps lessened.

[80] In December 2011, Cameron International agreed to pay a $250 million settlement to BP PLC to settle all claims related to the Deepwater Horizon with neither party admitting responsibility.

[81] In September 2014, Halliburton agreed to settle a large percentage of legal claims against it over the Deepwater spill by paying $1.1 billion into a trust by way of three installments over two years.

[82] On September 4, 2014, U.S. District Judge Carl Barbier ruled BP was guilty of gross negligence and willful misconduct under the Clean Water Act (CWA).

[83] In July 2015 BP reached an $18.7bn settlement with the US government, the states of Alabama, Florida, Louisiana, Mississippi and Texas, as well as 400 local authorities.

[84] In January 2018 a detailed estimate of the "Ultimate Costs of the Oil Spill", published in the Journal of Corporate Accounting and Finance, amounted to US$145.93 billion.