BP oil spill report: Which factors were not responsible for the accident?

Sure enough, the presentation moves on to a lengthy assessment of some of the factors that BP believes are not responsible for the accident, which have been cited in Congress as possible causes.

One is the decision to use just six “centralisers” to stabilize the steel casing inside the hole, instead of 21, which modelling had suggested would cut the risk of oil and gas leaks. Only six were used because the BP team working on the well believed – incorrectly – that they had an unsuitable type of centralisers on board the rig. The investigation team concluded that the decision to use only six centralisers “likely did not contribute to the cement’s failure to isolate the main hydrocarbon zones”.

Another is the design of the well, using a “long string” of steel casing right the way down the hole, that was 9 7/8 inches in diameter at the top, and 7 inches at the bottom of the hole, instead of a series of sections separately cemented in place. The report concludes that the use of that long string casing “was an acceptable decision and provided a sound basis of design”.

Central to those arguments is the team’s view that the oil and gas that caused the accident leaked into the well right at the bottom, into what is called the casing shoe, rather than higher up. It does not know how that happened, but argues that almost all the evidence from pressure readings and the flow of oil and gas in the well points to that conclusion. If that is the case, once the oil and gas were in the well, the design of the casing was irrelevant.

Another possible mistake by BP is dismissed: the suggestion that the team working on the well did not circulate drilling fluid, or mud, around the well inside and outside the casing, in what is known as a “bottoms up” procedure. The mud was circulated back to the surface before the well before the cementing started, and showed no sign of leaks of oil or gas, the inquiry team says.

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