The United States Department of Interior issued a report on the results of its forensic testing of the drilling equipment in use at the time of the Deepwater Horizon explosion and subsequent oil spill. The forensic testing on which the report is based focused on the drill pipe contained within the riser and the drill pipe removed from the blowout preventer (BOP) stack (BOP and Lower Marine Riser Package (LMRP)). The following is an excerpt from the report, which was developed by contractor U.S. Det Norske Veritas and will be the subject of public hearings on Apr. 4th at the Holiday Inn Metairie, New Orleans Airport.
What is Considered to Have Happened
Prior to the loss of well control on the evening of April 20, 2010, the Upper Annular (UA) was closed as part of a series of two negative or leak-off tests. Approximately 30 minutes after the conclusion of the second leak-off (negative pressure) test, fluids from the well began spilling onto the rig floor. At 21:47 the standpipe manifold pressure rapidly increased from 1200 PSIG to 5730 PSIG. The first explosion was noted as having occurred at 21:49. At 21:56 the Emergency Disconnect Sequence (EDS) was noted to have been activated from the bridge. This was the final recorded well control attempt from the surface before the rig was abandoned at 22:28.
The Upper VBRs were found in the closed position as-received at the Michoud facility. There was no documented means of ROV intervention to close the Upper VBRs. ROV gamma ray scans on May 10, 2010, confirmed that the ST Lock on the port side Upper VBR was closed. Scans of the starboard side ST Lock on the Upper VBRs were inconclusive. Measurements of the ST Lock positions performed at the Michoud facility confirmed that both ST Locks on the Upper VBRs were closed. Evidence supports that the Upper VBRs were closed prior to the EDS activation at 21:56 on April 20, 2010.
A drill pipe tool joint was located between the Upper Annular and the Upper VBRs. With both the Upper Annular and the Upper VBRs 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 blind shear rams (BSRs). This condition would have most likely occurred from the moment the well began flowing and would have remained until either the end conditions changed (change in Upper Annular or Upper VBR state) or the deflected drill pipe was physically altered (sheared). The portion of the drill pipe located between the shearing blade surfaces of the BSRs was off center 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 center of the drill pipe cross section. A portion 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.
Of the means available to close the BSRs, evidence indicates that the activation of the BSRs occurred when the hydraulic plunger to the Autoshear valve was successfully cut on the morning of April 22, 2010. However, on the evidence available, closing of the BSRs through activation of the AMF/Deadman circuits cannot be ruled out.
In the partially closed position, flow would have 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 April 29, 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.
Primary Cause and Contributing Causes
The failure cause analysis was organized and conducted around a single top event. For the purposes of this investigation, the top event was defined as the failure of the BSRs to close and seal the well. The primary cause of failure was identified as the BSRs failing to fully close and seal due to a portion of drill pipe trapped between the blocks.
Contributing causes to the primary cause included:
• The BSRs were not able to move the entire pipe cross section into the shearing surfaces of the blades.
• Drill pipe in process of shearing was deformed outside the shearing blade surfaces.
• The drill pipe elastically buckled within the wellbore due to forces induced on the
drill pipe during loss of well control.
• The position of the tool joint at or below the closed Upper Annular prevented upward
movement of the drill pipe.
• The Upper VBRs were closed and sealed on the drill pipe.
• The flow of well fluids was uncontrolled from downhole of the Upper VBRs.
Recommendations for Industry
The primary cause of failure was identified as the BSRs failing to close completely and seal the well due to a portion of drill pipe becoming trapped between the ram blocks. The position of the drill pipe between the Upper Annular and the upper VBRs led to buckling and bowing of the drill pipe within the wellbore. Once buckling occurred the BSRs would not have been able to completely close and seal the well. The buckling most likely occurred on loss of well control.
To download the latest “Deepwater Horizon Joint Investigation Report” in its entirety, click here.
