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XIII. BOP Stack Conclusions

The Panel concluded that on April 20, 2010, the Deepwater Horizon crew
closed the upper annular on the BOP stack at 9:43 p.m. as part of its initial efforts
to control the well. At 9:47 p.m., the crew closed the upper variable bore rams as
part of their continuing efforts to control the well. When the well blew out, there
was a drill pipe tool joint located between the closed upper annular and the
closed upper variable bore rams.

A. Cause of the BOP Stack Failure

The Panel concluded that there are two possible scenarios for when the
BSR was activated. First, the BSR could have been activated by the AMF while
the pipe was placed in compression by the upper annular preventer immediately
following loss of communication with the rig. Second, the BSR could have been
activated by the autoshear on April 22, while the pipe was placed in compression
by the weight of the drill pipe and upward forces of the well. Under either
scenario, the BSR failed to shear the drill pipe and seal the wellbore because the
pipe was placed in compression outside of the cutting surface of the BSR blades.
The Panel concluded that failure of the BOP to shear the drill pipe and seal the
wellbore was caused by the physical location of the drill pipe near the inside
wall of the wellbore, which was outside the BSR cutting surface during
activation on April 20 or April 22.

B. BOP Stack Failure Contributing Causes

The BSR could not fully close and seal because a portion of the drill pipe
became trapped between the BSR block faces. The elastic buckling of the drill
pipe forced the drill pipe to the side of the wellbore and outside of the BSR
cutting surface, and was a contributing cause of the BOP failure.

C. BOP Stack Failure Possible Contributing Causes

Approximately 28 feet of drill pipe between the upper annular and the
upper variable bore ram elastically buckled within the wellbore, causing that
portion of the drill pipe to be off‐center. Either of the following scenarios or
some combination thereof, led to elastic buckling of this pipe in this location:

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