An internal Transocean investigation into the explosion and loss of the Deepwater Horizon found that the tragedy was the result of ‘compromised' well integrity due to a series of well design, construction and temporary abandonment decisions made by operator BP, Transocean said 22 June.
The Transocean investigation, launched shortly after the 20 April 2010 blowout at the Macondo well in the deepwater Gulf of Mexico's Mississippi Canyon block 252, blamed the disaster on ‘a succession of interrelated' decisions by BP that increased the risk of a blowout.
‘The decisions, many made by the operator, BP, in the two weeks leading up to the incident, were driven by BP's knowledge that the geological window for safe drilling was becoming increasingly narrow,' Transocean said.
‘Specifically, BP was concerned that downhole pressure – whether exerted by heavy drilling mud used to maintain well control or by pumping cement to seal the well – would exceed the fracture gradient and result in fluid losses to the formation, thus costing money and jeopardizing future production of oil.'
Transocean accused BP of failing to properly assess, manage and communicate risks to Macondo contractors. The company said drill crews were not properly informed about problems with tests measuring the integrity of the cement slurry used to plug the well and that BP ran through a ‘series of last-minute alterations' to the temporary abandonment plan before settling on a method that did not have the required approval of federal regulators.
Despite lost circulation events and kicks while drilling the Macondo well in 5000ft water depths, Transocean said BP proceeded with its original plan to cement a long-string casing rather than alter the casing design to reduce the risk of a well control incident.
‘The resulting cement program was of minimal quantity, left little margin for error, and was not tested adequately before or after the cementing operation. Further, the integrity of the cement may have been compromised by contamination, instability and an inadequate number of devices used to center the casing in the wellbore,' Transocean said.
BP and Halliburton, which provided cementing services, ‘did not adequately test the cement slurry program, despite the inherent complexity, difficulties and risks associated with the design and implementation of the program and some test data showing that the cement would not be stable', Transocean said.
The blowout and fire aboard the Deepwater Horizon rig killed 11 workers and poured an estimated 4.9 million barrels of oil into the Gulf of Mexico.
Transocean said it was not clear why the drill crew did not detect a pressure anomaly until shortly before the explosion, but that the crew undertook appropriate well control actions once the influx of hydrocarbons had been identified. The well's blowout preventer had been properly maintained, Transocean said, but was overcome by ‘extreme dynamic flow' that caused the drill pipe to bow inside the BOP, preventing the shear rams from completely closing – a conclusion also reached by a recent Det Norske Veritas forensic examination of the device.
‘The loss of evidence with the rig and the unavailability of certain witnesses limited the investigation and analysis in some areas,' Transocean said. ‘The team used its cumulative years of experience but did not speculate in the absence of evidence. The report of the team does not represent the legal position of Transocean, nor does it attempt to assign legal responsibility or fault.'
The results of an internal investigation by BP released last September sought to spread blame more widely, attributing the disaster to ‘a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces.'
A federal joint investigation team is set to release a final report on the Macondo disaster by 27 July. OE