The Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE) and U.S. Coast Guard concluded that BP, Transocean and Halliburton all share blame in the April 20, 2010 Macondo oil spill in the U.S. Gulf of Mexico through a combination of well design, approach to well control and missed steps.
The final investigative report, conducted by a joint investigative team led by BOEMRE and the Coast Guard, found that the Macondo blowout was the result of a series of decisions that increased risk and a number of actions that failed to fully consider or mitigate those risks.
"While it is not possible to discern which precise combination of these decisions and actions set the blowout in motion, it is clear that increased vigilance and awareness by BP, Transocean and Halliburton personnel at critical junctures during operations at the Macondo well would have reduced the likelihood of the blowout occurring," BOEMRE said in the report.
Operator BP was ultimately responsible for conducting operations at Macondo that would ensure the safety and protection of personnel, equipment, natural resources and the environment. Transocean was responsible for conducting safe operations and protecting personnel on board the rig, and Halliburton, as a BP contractor, was responsible for conducting the cement job, and its subsidiary Sperry Sun had certain responsibilities monitoring the well.
The panel conducting the investigation found that BP well designers set the casing in a location that created additional risks of hydrocarbon influx. "Even knowing this, BP did not set additional cement or mechanical barriers in the well."
BP also made two additional decisions that further increased risks: the decision to have the Deepwater Horizon crew install a lock-down sleeve as part of the temporary abandonment procedure, and BP's decision to use a lost circulation material as spacer, which risked clogging lines used for well integrity tests, the panel noted.
Personnel from both BP and drilling contractor Transocean failed to conduct an accurate negative test to assess the integrity of the production casing cement job, the report said. The crew performed temporary abandonment procedures while unaware of the failed cement job below. "Unfortunately, the rig crew then limited its kick detection abilities by deciding to bypass the Sperry Sun flow meter when displacing fluid from the well overboard."
The failure of the rig crew to stop work on the Deepwater Horizon after encountering multiple hazards and warnings also was a contributing cause of the Macondo blowout. "The Deepwater Horizon rig crew missed signs of a kick and thus was delayed in reacting to the well control situation," the panel found.
"Once the flow reached the rig floor, the crew closed the upper annular and upper variable bore ram and diverted the flow to the mud gas separator. The mud gas separator could not handle the volume of the blowout and explosions followed. Forensic analysis suggests the explosions had damaged the rig's multiplex cable and hydraulic lines, which were rendered inoperable by the blowout preventer stack's blind shear rams, by the time a crew member on the bridge had activated the emergency disconnect system.
The blowout's force, and possibly the force from drill pipe in the riser, buckled the drill pipe, putting it in a position where it could not be completely sheared by the blind shear ram blades. As a result, the blind shear ram, when activated on April 20 or 22, could not shear the drill pipe and seal the wellbore.
The panel made a number of recommendations, such as implementing regulations that require the negative pressure testing of wells where the wellbore will be exposed to negative pressure conditions, such as when the BOP and riser are disconnected from the wellhead during permanent or temporary abandonment procedures. "Had the Deepwater Horizon crew interpreted the negative test properly, the blowout may have been averted," the report said.
At least two barriers, including one mechanical and one cement barrier, should be required for wells undergoing temporary abandonment procedures. "Having a cement plug and an additional mechanical barrier would added an increased safety factor," the panel found. "While the Macondo well did have dual float valves, the Panel does not believe that float valves should be considered a mechanical barrier."
Other recommendations include the incorporation into the Code of Federal Regulation parts of the American Petroleum Institute's Recommended Practice Parts 1 and 2, that would require a minimum hole diameter of 3.0 inches greater than the casing outer diameter; rathole mud density greater than cement; and mud conditioning volume greater than one annular volume. The definition of safe drilling margins should also be expanded to encompass pore pressure, fracture gradient and mud weight.
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