Transocean concluded the blowout preventer on the rig was adequately maintained but was never able to completely seal the well which spewed crude oil into the Gulf of Mexico from April 20 until July 15, 2010. The well was declared sealed Sept. 19.
The accident, which killed 11 rig workers, was a "result of a succession of interrelated well design, construction and temporary abandonment decisions" made mostly by BP, the report obtained by the San Antonio Express-News said.
Transocean, which owned the South Korean-built Deepwater Horizon rig and leased it to BP, blamed Halliburton for not adequately testing a nitrogen foam cement mix used on the final seal of the well, said BP's well design led to the risky cement job, and said neither risk was communicated to the rig's crew by BP.
The report said BP took risks because officials were worried the pressure of heavy drilling mud from the rig would lead to fractures of the deepwater Macondo exploratory formation possibly affecting future oil production.