Monday, March 28, 2011

DNV Macondo BOP Final Report

DNV Macondo BOP report - Drill pipe at an awkward angle
DNV has released its final report into what went wrong with the Blow Out Preventer above the Macondo well - it was drill pipe not being cut properly, due to being at an awkward angle when the rams tried to cut it.

At the time of the accident, there was a drill pipe tool joint between the upper annular ram and the upper variable bore ram. When both of these rams were closed around the drill pipe, forces from the flow of fluids pushed the tool joint into the upper annular ram.

This meant that the when the blind shear ram was closed, it did not close the drillpipe smoothly, but pushed the pipe at an awkward angle, which meant it did not seal.

Note - it was not a problem of a tool joint being positioned between the blind shear rams at the time they were activated and the rams not being able to cut them (as many people thought it might be) - but the tool joint being in a position such that the rams could not cleanly cut the drill pipe.

An additional contributing factor was the fact that the upper annular ram, which closes around the drillpipe but does not squash the drillpipe, was closed at the time, because of the negative pressure tests which were carried out.

So the drillpipe did not have freedom to move - this also means that the rams were trying to close the drill pipe in a scenario which might have not been previously tested.

The liquids flowing through the well made it buckle between the upper annular and upper variable bore rams, which also led it to squash in an awkward way.

So as the blind shear rams closed, part of the drill pipe cross section ended up being trapped between the ram block faces, so the blocks did not fully close.

The evidence suggests that the blind shear rams were activated on the morning of April 22nd (the date the rig sank) - at this date the hydraulic plunger to the autoshear valve was cut, DNV says - although there is no way to be sure exactly when it closed, it could have been activated earlier by the deadman / automatic mode failure system.

When the drill pipe was sheared on April 29 with the casing shear rams, the flow just found a different route, going through open drill pipe at the casing shear rams, and up the wellbore to the blind shear rams.

DNV recommends that the industry makes further studies what effect flow through the drill pipe tubing and blow out preventer components can have on the ability for the BOP to close, with possible buckling of the drill pipe.

It also recommends that the industry should study the effects of tubulars being fixed or constrained in the blow out preventer as the rams close.

DNV recommends that the industry should also look at potential effects of certain activities (for example conducting negative pressure tests) can have on the ability for a BOP to operate in an emergency.