Buckled pipe prevented BOP from protecting Deepwater Horizon rig
24 Mar 2011
Washington – The blowout preventer (BOP) stack on the BP Deepwater Horizon rig failed because a piece of drill pipe had become trapped in the shutoff mechanism when oil started flowing from Macondo well, a US-commissioned report by Det Norske Veritas has found.
The position of the drill pipe between the shear and bore rams led to it buckling within the wellbore, said the Norwegian company’s report into the failure of the huge valve set that should have halted the disaster in the Gulf of Mexico. This, it concluded, meant that the BOP could not properly seal off the well.
The BOP stack, built by Cameron, consisted of:
• A Lower Marine Riser Package (LMRP) containing two annular preventers and two control pods
• The lower section of the BOP Stack contains five sets of rams. These rams are referred to as the blind shear rams (BSR), the casing shear rams (CSR), upper variable bore rams (VBR), middle VBRs and Llower VBRs. The LMRP sits on top of the lower section of the BOP.
• Two electronic control pods fitted to the LMRP, to receive signals from the control panels that are located on the rig itself, and then activate various solenoids in turn functioning various hydraulic circuits and mechanical components on the BOP stack.
At the time of the accident, the rig was drilling an exploratory well at a water depth of approximately 5,000 feet in the Macondo prospect. The well is located in Mississippi Canyon Block 252 in the Gulf of Mexico.
Prior to the loss of well control on the evening of 20 April, 2010, the upper annular was closed as part of a series of two negative tests. About 30 minutes after the second test, fluids from the well began spilling onto the rig floor.
At 21:47h the standpipe manifold pressure rapidly increased from 1200 psig to 5730 psig. The first explosion was noted as having occurred at 21:49h. At 21:56h an emergency disconnect sequence (EDS) was activated from the bridge – the final recorded well control attempt from the surface before the rig was abandoned at 22:28h.
A drill pipe tool joint was located between the upper annular and the upper VBRs. With both of these units closed on the drill pipe, forces from the flow of the well pushed the tool joint into the Upper Annular element. This created a fixed point arresting further upward movement of the drill pipe.
The drill pipe was then fixed but able to pivot at the upper annular, and horizontally constrained but able to move vertically at the upper VBRs. Forces from the flow of the well induced a buckling condition on the portion of drill pipe between the upper annular and upper VBRs.
The drill pipe deflected until it contacted the wellbore just above the BSRs as the well began flowing. The piece of the drill pipe located between the shearing blade surfaces of the BSRs was off centre and held in this position by buckling forces.
As the BSRs were closed, the drill pipe was positioned such that the outside corner of the upper BSR blade contacted the drill pipe slightly off centre of the drill pipe cross section.
Part of the pipe cross section was outside of the intended BSR shearing surfaces and would not have sheared as intended. As the BSRs closed, a portion of the drill pipe cross section became trapped between the ram block faces, preventing the blocks from fully closing and sealing.
Since the deflection of the drill pipe occurred from the moment the well began flowing, trapping of the drill pipe would have occurred regardless of which means initiated the closure of the BSRs. In the partially closed position, flow continued through the drill pipe trapped between the ram block faces and subsequently through the gaps between the ram blocks.
When the drill pipe was sheared on 29 April, 2010, using the CSRs, the well flow pattern changed to a new exit point. At this point, the flow expanded through the open drill pipe at the CSRs and up the entire wellbore to the BSRs and through the gaps along the entire length of the block faces and around the side packers.
