mud with lighter seawater and checking for flow, is a standard technique for establishing the integrity of the cemented barrier. Multiple negative pressure tests were made, all of which indicated inconclusive and confusing results (BP 2010, 85; Transocean 2011a, I, 29). However, the team mistakenly determined that the negative pressure test had been conducted successfully and proceeded to abandon the well temporarily by displacing drilling mud with seawater, recovering the mud, and discharging overboard the spacer fluid that had been used in previous operations. Various anomalies were noted during this process, starting at roughly 21:00 on April 20. At approximately 21:40, mud was observed flowing onto the rig floor and well control actions were initiated, diverting flow to the mud–gas separator and activating the upper annular and upper pipe rams on the blowout preventer (BOP).

The procedures taken did not reestablish control over the well. Flammable gas alarms on the Deepwater Horizon sounded at approximately 21:47, followed by two explosions at approximately 21:49.

The explosions and resulting fire led to the death of 11 workers and serious injuries to 16 others. The Deepwater Horizon rig sank roughly 36 hours later. Nearly 5 million barrels of oil were released into the Gulf of Mexico (McNutt et al. 2011).


In response to a request from the DOI Secretary, NAE and NRC formed a committee to examine the causes of the Deepwater Horizon–Macondo well blowout, explosion, fire, and oil spill and to identify measures for preventing similar incidents in the future. As part of its task, the committee provided an interim letter report to the DOI Secretary on November 16, 2010. That report presented preliminary findings and observations concerning key factors and decisions that may have contributed to the blowout of the Macondo well, including engineering, testing, and maintenance procedures; operational oversight; regulatory procedures; and personnel training and certification. This final report presents the committee’s overall findings with regard to the causes of the disaster and its recommended approaches for improved safety.


On the basis of its assessment of the evidence collected for this final report, the committee has developed the following findings, observations, and recommendations. The sequence in which they are presented is not intended to

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