The owner of the drilling rig that exploded in the Gulf of Mexico last year released an internal investigation report Wednesday that largely blames oil giant BP for the disaster.

Transocean said in the report that the explosion on the Deepwater Horizon on April 20, 2010, and resulting oil spill resulted because of a succession of well design, construction, and temporary abandonment decisions that compromised the integrity of the well and compounded the risk of its failing.

Among Transocean's findings:

  • BP did not properly communicate to the drill crew the lack of testing on the cement or the uncertainty surrounding other critical tests on measures used to temporarily seal the well.
  • BP's use of a technically complex nitrogen foam cement program for sealing the well was of minimal quantity, left little margin for error, and was not tested adequately.
  • Cement contractor Halliburton and BP did not adequately test the cement slurry used to seal the well.
  • BP also failed to assess the risk of the temporary abandonment procedure used at Macondo. BP generated at least five different temporary abandonment plans between April 12, 2010, and April 20, 2010. After this series of last-minute alterations, BP proceeded with a temporary abandonment plan that created risk and did not have the required government approval.

The Swiss firm said many of those decisions were made by well owner BP in the two weeks before the incident.

Transocean said, based on its evidence, BP failed to properly assess, manage and communicate risk.

The report was a contrast to BP's own internal report into the disaster, which blamed a cascade of failures by multiple companies.

Government investigations also have spread around the blame.

The various findings will be debated as numerous lawsuits make their way through court. The companies involved in the disaster have sued each other seeking to recoup their losses or expected losses from the disaster.

BP officials did not immediately respond to an email seeking comment.

Eleven rig workers were killed and the government estimates some 780 million litres of oil spewed from BP's Macondo well 1.6 kilometres beneath the sea before the well was capped three months later.

It was the worst offshore oil spill in U.S. history, staining hundreds of kilometres of shoreline, hurting fisherman and businesses and prompting new rules for deepwater drilling.

BP has already spent or committed tens of billions of dollars to clean up the mess and compensate victims.

With files from The Associated Press