updated 5/26/2010 7:14:24 PM ET 2010-05-26T23:14:24

What happened aboard the Deepwater Horizon that led to the blast that killed 11 workers and led to the massive Gulf oil disaster? Below's what is known so far.

April 10: The blowout preventer (BOP), equipment atop the wellhead designed to seal the well in an emergency, is “fully tested” by employees of Transocean Ltd., the drilling operator, under supervision of BP employees.

April 17: After reaching an oil reservoir, drilling is halted at 18,360 feet below sea level. The drill bit is retracted and the crew of the Deepwater Horizon begins making preparations to plug and temporarily abandon the well, which is to be brought into production by BP at a later date.

April 19: On April 19, employees of Halliburton, the cement contractor on the Deepwater Horizon, pump cement into the casing and to the bottom of the well. The cement passes through a one-way valve, then rises back up into the annulus, the narrow gap between the steel casing and the rock walls of the drill hole. The cement used in such an operation is “the consistency of a gray Cream of Wheat,” according to Robert MacKenzie, managing director of energy and natural resources at FBR Capital Markets and a former oil industry cementing engineer. “It’s like taking a straw and blowing into a cup of water. The bubbles — or the cement in this case — go down the middle and come up the outside.” The cement is intended to hold the drill pipe in place, protect it and prevent gas from leaking up the outside of the well.

After the cement is pumped, a rubber wiper plug — similar to the rubber element in a syringe — is inserted in the casing and pushed down to the lower valve by heavy drilling liquid pumped in behind it.

April 20, 12:30 a.m.: “Positive tests” — in which pressure inside the casing is increased as high as 6,500 pounds per square inch to test the wellhead seal assembly — are conducted through 7:30 a.m. and produce satisfactory results, according to BP documents later provided to Congress.

April 20, 10:30 a.m.: Crew members again test the blowout preventer through noon, including testing the “blind shear rams” designed to cut and seal the drill pipe. The device also passes this test, the documents show. (During this period, a crew from the testing firm Schlumberger that had been on the rig for two days preparing to conduct an acoustic test of the cement — known as a “cement bond log” — is placed on a regularly scheduled helicopter flight to the mainland without ever having been asked by BP to perform the test, according to the New Orleans Times-Picayune, which quoted a spokesman for Schlumberger. The omission of that test — described by Halliburton official Tim Probert during congressional hearings on May 11 as “the only test that can really determine the actual effectiveness of the bond between the cement sheets, the formation and the casing itself” — is likely to be a focus of the investigation.

April 20, 5 p.m.:  About 16 ½ hours after the cement was pumped, a “negative” pressure test — in which pressure inside the casing is lowered to see if gas is leaking into the casing — is conducted. BP officials later acknowledge that the test produced an “anomalous” result — a reading of 1,400 pounds per square inch in the drill pipe, but zero pressure in the “kill” and “choke” lines attached to the blowout preventer.

(There is disagreement among BP officials about what happened next, according to Rep. Henry Waxman, D-Calif., and chairman of the House Energy and Commerce Committee. He said at a hearing on May 12 that James Dupree, BP senior vice president for the Gulf of Mexico, told staff members that the well didn’t pass the test and that a follow-up test proved unsatisfactory as well. But BP lawyers told the committee that further pressure tests were taken and that company officials determined that the additional results justified ending the test and proceeding with well operations.)

April 20, 8 p.m.:  The crew aboard the Deepwater Horizon begins displacing drilling mud in the casing pipe with seawater prior to placing cement plugs in the well and temporarily abandoning it.

April 20, 9:08 p.m.: The crew discontinues pumping operations to displace the drilling mud with seawater. But real-time pressure readings from a “standpipe” – piping on the oil rig that is connected to high-pressure pumps and the drill pipe – indicate pressure of 1,000 pounds per square inch within, rather than the zero reading that would have been expected.

April 20, 9:46 p.m.: The real-time data show “several abnormal-appearing entries” between 9:46 p.m. and 9:48 p.m., indicating that pressure within the standpipe has reached nearly 3,500 pounds per square inch. 

April 20, 9:49 p.m.: Transmission of real-time data ceases. Methane gas and possibly other hydrocarbons “kick” up the well toward the Deepwater Horizon, pushing a mixture of drilling mud and seawater before them.

Crew members aboard the Deepwater Horizon unsuccessfully attempt to activate the blowout preventer on the seabed to cut off the escaping gas and oil. BP officials later tell congressional investigators that the complex 450-ton piece of “failsafe” equipment only partially restricted the torrent. Crewmembers also try to activate the “emergency disconnect,” which shuts the BOP and disconnects the rig and riser from the well bore. It too fails.

After pushing out the oil and drilling mud, the escaping heavier-than-air gas cascades out of the top of the rig, about 240 feet above the deck of the Deepwater Horizon, and spills down into the operations area. Moments later, it ignites and causes an explosion, killing 11 crew members and forcing 115 others to flee for their lives. A supply vessel working alongside the rig reports that in addition to oil and water, cement-like debris also rains down on its deck, suggesting that cement from the bottom of the well also was hurled upward by the escaping gas.

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