Today, BP released a report on its own internal investigation into the Deepwater oil spill, which continued uncontrolled for months following what the report calls "a complex and interlinked series" of error and failures. The report is one of a number in progress, performed by both the companies involved and various governmental agencies, and it's not intended to be complete—there's an entire section devoted to information the investigators would have liked to have access to, but didn't. Nevertheless, it provides a detailed chronology of what went wrong, and why none of the safety equipment and procedures that were in place were effective.

The events that triggered the Deepwater failure started with the preparations for the departure of the ship that drilled the well, a process termed "abandonment." At this point, the well extended into what the report terms the "primary reservoir sands" from which hydrocarbons would be harvested. Since these are under pressure relative to the top of the well, the intent was to seal it using concrete. It was at this point that things started to go wrong.

The concrete was injected as a "foam slurry," with the gaseous portion of the foam provided by nitrogen. The small area at the tip of the well that would be filled with concrete ensured that only a small volume of cement would be used, which the report notes increases the possibility that a small amount of contaminants would have an overly large impact. Human error played a role here, as well; there were incomplete lab tests of the concrete, and no fluid loss additives were put in place.

In discussing the fluid loss in the cement mixture, the report refers to "Halliburton slurry," which highlights another problem: several companies were working together to seal off the well for abandonment, and each had to rely on the others to do things right. The recommendations portion of the report has an entire section entitled "Contractor and Service Provider Oversight and Assurance," which indicates that the company needs to take steps to establish minimum standards for its contractors in everything from process to safety procedures.

The report concludes that, because of the problems with the cement, the foam destabilized, enabling the nitrogen to break out of the mixture, and an improper seal resulted. The nitrogen then leaked into heavier cement meant to provide a more robust seal, destabilizing that. This enabled hydrocarbons to enter a collar around the drill pipe and gain access to the rest of the well, although the precise route through the hardware isn't clear.

Problems of this sort should have been caught by further testing, and the report indicates that they could have been. The well (correctly) passed a positive pressure test, which indicated that it was stable to excess pressure pumped in from the surface. The rig workers next performed a negative pressure check, to test if the concrete seal would block the flow of hydrocarbons once the pressure supplied by the rig was released. These tests indicated a problem—some of the liquid involved in the testing bled off, and anomalous pressure readings were seen—but were "incorrectly accepted as successful," in part because there were no standards in place for what constituted a negative test.

Fail-safes that failed

With the concrete supposedly in place, the crew began replacing the drilling mud used to maintain pressure with seawater. During this process, the pressure in some of the equipment began to increase, but the flow from the well was masked by the process of removing the drilling mud. By 8pm that evening, hydrocarbons were already entering the drill pipe, with accompanying pressure increases, but nobody appears to have noticed. (The presentation that accompanied the report's release shows real-time graphs of pressure changes, and highlights the significant events.)

When the pumps that were clearing out the drilling mud shut down at about 9:30pm, the explosion was only a half-hour away. Discussions of what to do started as the pressure rose and, within 10 minutes, workers had attempted to bleed the pressure and close the drill pipe. By this point, hydrocarbons were already rising through the pipes, and drilling mud was overflowing onto the rig floor. An automated system intended to shut the blowout preventer occurred only two minutes before the explosion.

The report includes a three-dimensional reconstruction of the flow of gas throughout the drilling rig. Its pressure was simply higher than any of the diverters were designed to handle, so methane spewed out of multiple locations on the rig. Some of these, the report notes, were well away from areas designated as being at high risk of fire. One of these was the intakes for the engine room; the rig's Fire and Gas System failed to shut these down automatically. Either the engines or an electrical system in an area that wasn't rated a fire risk set off the explosion that ultimately destroyed the rig.

That returns us to the automated system that was attempting to shut the blowout preventer. Even though it kicked in, once the explosion happened at the surface, it lost power. Shortly afterwards, the rig began to drift, which caused structural damage to the connections.

Redundant hardware on the sea floor was designed to operate independently. Unfortunately, it also failed; one of the devices had a fault in a critical piece of hardware, while the second had insufficient charge in its batteries to function.

All told, the report identifies eight key findings about the events that led to the disaster, and they span the spectrum from hardware failures to poor practices. With that many things having gone wrong, there are extensive recommendations about how BP can improve their practices and those of their subcontractors for future work. The company is promising to do these things, but these sorts of sweeping changes will take time, and may not be easy to implement with hardware that's already in the field.

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