• Dick writes about Earth and planetary science for Science magazine.

Deepwater Horizon Was Not Pushing the Envelope When It Caught Fire

30 April 2010 5:00 pm

The technological and ecological disaster unfolding on the Gulf Coast may end up being one for the record books, but the tragic failure to contain the deep-seated pressures 7 kilometers beneath the Gulf of Mexico was not likely the result of overreaching. In wells 65 kilometers off the coast of Louisiana, "the depths and pressures are rather routine" for the oil industry, says petroleum engineer Kenneth Gray of the University of Texas, Austin (UT Austin). "That doesn't mean they're easy." It does mean that even in routine operations, there is not much room for error.

The $350 million Deepwater Horizon was not operating near its design limits on 20 April when uncontrolled oil and gas shot up its drill pipe and ignited, eventually sinking the vessel. It was designed to drill through as many as 9144 meters of rock in as much as 2438 meters of water. Last year, it had drilled the world's deepest well at 10,680 meters of rock and water. But at the time of the blowout, the well extended only 5486 meters beneath the sea floor in 1500 meters of water.

The oil industry has developed technologies to hold in the 125-megapascal (18,000-pound-per-square-inch) pressures encountered at even such lesser depths. One is the weight of heavy drilling "mud" circulated down the drill pipe during drilling.

When in 1979 drillers lost mud circulation on the "IXTOC I" platform off the coast of Mexico, the resulting blowout spewed several times more oil each day than in the current disaster. That went on for 10 months, producing the second largest spill in history.

Deepwater Horizon 's containment failure may have come in the cement injected between the well wall and its steel liner, experts say. According to press reports, workers had just finished that "cement job." A failure could have also come in the temporary cement plug set in the well. Problems with cement—a batch that fails to properly set up, for example—were the cause of 18 of 39 blowouts in the Gulf of Mexico over a 14-year period, according to a 2007 report from the U.S. Minerals Management Service, the agency that supervises offshore drilling.

Whatever the cause of the rush of oil and gas up the well, the disaster also required a failure of the blowout preventer (BOP) sitting on the sea floor over the well. BOPs can fail to close off a well in any number of ways. In the case of the 1977 Ekofisk Bravo blowout in the North Sea, workers reinstalled the BOP upside down after maintenance. All efforts to close the Deepwater Horizon BOP using remotely operated submersibles have so far failed. That suggests to petroleum engineer Paul Bommer of UT Austin that "maybe the BOP got the signals [to shut] and nothing happened."

Ironically, the ultimate solution may be more drilling into pressurized rock. Precisely directed drilling from one or more other platforms can intersect the problem well kilometers below the sea floor despite the target being only 20 centimeters wide. The pinnacle of the driller's art and technology may yet come to the rescue.

For more on the gulf oil spill, see our full coverage.

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