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Spring 2011 Vol. 11 Number 1

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Dispersant-clotted oil and fresh crude 9 miles from Deepwater Horizon, ©Christopher Berkey/epa/Corbis

Drilling for Answers

What Went Wrong During the Deepwater Horizon Accident?

When the Deepwater Horizon oil rig exploded in the Gulf of Mexico a year ago, killing 11 people, it marked the start of what turned out to be the largest accidental oil spill in history. A mile below the ocean's surface, BP's exploratory Macondo well had blown, triggering the rig's explosion. By the time the well was capped three months later, an estimated 5 million barrels of oil had gushed into the Gulf. The disaster provided a terrible demonstration of the risks and complications posed by deepwater drilling operations.

In the weeks following the explosion, the U.S. Department of the Interior asked a committee of the National Academy of Engineering and National Research Council to examine the probable causes of the accident and identify measures that could prevent such a catastrophe in the future. The committee issued its preliminary findings in November.

It may not be possible to definitively establish exactly which mechanisms caused the blowout and explosion, says the interim report, given the deaths of potential witnesses, the loss of the oil rig and important records, and the difficulty in obtaining reliable forensic information from the Macondo well. However, the committee says that it has been able to develop a good understanding of a number of key factors and decisions that may have contributed to the disaster.

Of particular concern was the apparent lack of a suitable approach for managing the risks, uncertainties, and dangers associated with deepwater drilling, the report says. Skimming and support vessels and drilling rigs surrounding Deepwater Horizon site, U.S. Coast Guard photo by Lt. Cmdr. Paul RooneyIt cites many questionable decisions about a number of technical and operational breakdowns that occurred prior to the accident. "It's also important to note that these flawed decisions were not identified or corrected by BP, its service contractors, or by the oversight process employed by the U.S. Minerals Management Service and other regulatory agencies," said Donald C. Winter, chair of the committee, former secretary of the Navy, and professor of engineering practice at the University of Michigan.

The events suggest insufficient checks and balances for considering well safety and for making critical decisions affecting the schedule for "temporarily abandoning" the Macondo exploratory well -- in other words, sealing it for later use. The report points out various dubious decisions, such as continuing abandonment operations at the site despite tests that indicated the cement put in place to temporarily seal the well was not an effective barrier to prevent gases from entering it. In addition, several clear failures in monitoring the well appear to have been partly responsible for its blowout.

The report also notes that a previous loss of hydrocarbon circulation in the Macondo well in the weeks before the accident presented missed opportunities to take actions that would mitigate future risks. For its final report, due out this summer, the committee will examine ways to establish practices and standards to foster a culture of safety and methods to ensure that schedule and cost decisions do not compromise safety. The committee will assess the extent to which there are gaps, redundancies, and uncertainties in responsibilities of agencies and professional societies that oversee deepwater drilling operations, and it will consider the merits of an independent technical review to provide operation checks and balances by enforcing standards and reviewing deviations.

The committee will also evaluate forensic evidence from the Macondo well's blowout preventer, which had just been recovered at the time the interim report was issued. --  Molly Galvin

Interim Report on Causes of the Deepwater Horizon Oil Rig Blowout and Ways to Prevent Such Events. Committee for the Analysis of Causes of the Deepwater Horizon Explosion, Fire, and Oil Spill to Identify Measures to Prevent Similar Accidents in the Future, Transportation Research Board and Division on Earth and Life Studies (2010, 28 pp.; available only online from the National Academies Press).

The chair of the committee is Donald C. Winter, former secretary of the Navy, and professor of engineering practice, University of Michigan, Ann Arbor. The study is funded by the U.S. Department of the Interior.

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Copyright 2011 by the National Academy of Sciences