No single factor caused the Macondo well tragedy in the deepwater Gulf of Mexico (GOM), BP plc said Wednesday. "Rather, a sequence of failures involving a number of different parties led to the explosion and fire which killed 11 people and caused widespread pollution" in the GOM this year.

Based on a four-month investigation led by Mark Bly, BP's head of Safety and Operations, the oil major concluded that decisions made by "multiple companies and work teams" led to the tragedy, which arose from "a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces."

BP was majority owner and the Macondo well operator when the well exploded on April 20. Transocean Ltd. owned the Deepwater Horizon rig, and Halliburton was the primary service operator. The explosion on April 20 killed 11 people and decimated a large portion of the Gulf Coast region commercially.

The report, said BP, is based on information available to the investigating team but it noted that more "relevant information may be forthcoming, for example, when Halliburton's samples of the cement used in the well are released for testing and when the rig's blowout preventer [BOP] is fully examined now that it has been recovered from the seabed. There will also be additional information from the multiple ongoing U.S. government investigations."

Bly's 50-plus member team, drawn from inside BP and externally, concluded that:

Commenting on the report, which was commissioned immediately after the Macondo well explosion, outgoing BP CEO Tony Hayward said, "The investigation report provides critical new information on the causes of this terrible accident. It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy. Multiple parties, including BP, Halliburton and Transocean, were involved.

"To put it simply, there was a bad cement job and a failure of the shoe track barrier at the bottom of the well, which let hydrocarbons from the reservoir into the production casing. The negative pressure test was accepted when it should not have been, there were failures in well control procedures and in the blowout preventer; and the rig's fire and gas system did not prevent ignition."

Based on the report's findings, "it would appear unlikely that the well design contributed to the incident, as the investigation found that the hydrocarbons flowed up the production casing through the bottom of the well," said Hayward.

From the beginning, said BP's incoming CEO Bob Dudley, BP officials said the explosion on the Deepwater Horizon "was a shared responsibility among many entities. This report makes that conclusion even clearer, presenting a detailed analysis of the facts and recommendations for improvement both for BP and the other parties involved. We have accepted all the recommendations and are examining how best to implement them across our drilling operations worldwide...

"We deeply regret this event," said Dudley. "We have sought throughout to step up to our responsibilities. We are determined to learn the lessons for the future and we will be undertaking a broad-scale review to further improve the safety of our operations. We will invest whatever it takes to achieve that. It will be incumbent on everyone at BP to embrace and implement the changes necessary to ensure that a tragedy like this can never happen again."

Based on its findings, BP's investigation team proposed 25 recommendations that it said are designed to prevent future accidents from occurring. The recommendations "are directed at strengthening assurance on BOPs, well control, pressure-testing for well integrity, emergency systems, cement testing, rig audit and verification, and personnel competence."

BP officials said several of the findings could be "considered relevant to the oil industry more generally" and some could be "widely adopted."

The report, available at www.bp.com, drew strong reactions from BP's Macondo well partners.

Transocean said its investigation is ongoing and "will be concluded when all of the evidence is in, including the critical information the company has requested of BP but has yet to receive." The Deepwater Horizon rig operator called BP's report "self-serving" and said it was an attempt "to conceal the critical factor that set the stage for the Macondo incident: BP's fatally flawed well design. In both its design and construction, BP made a series of cost-saving decisions that increased risk -- in some cases, severely."

According to Transocean, decisions made exclusively by BP included:

"As we continue to review BP's internal report...we have noticed a number of substantial omissions and inaccuracies in the document," a Halliburton spokesperson stated. "Halliburton remains confident that all the work it performed with respect to the Macondo well was completed in accordance with BP's specifications for its well construction plan and instructions, and that it is fully indemnified under its contract for any of the allegations contained in the report.

"Deepwater operations are inherently complex and a number of contractors are involved which routinely make recommendations to a single point of contact, the well owner. The well owner is responsible for designing the well program and any testing related to the well. Contractors do not specify well design or make decisions regarding testing procedures as that responsibility lies with the well owner."

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