An internal investigation by the owner of the rig that exploded in the Gulf of Mexico last year has largely blamed BP for the disaster.

The Transocean report released on Wednesday said the Deepwater Horizon explosion on 20 April 2010 and resulting oil spill was the result of a succession of well design, construction, and temporary abandonment decisions that compromised the integrity of the well and compounded the risk of its failure. The Swiss firm said many of the decisions were made by well owner BP in the two weeks before the incident.

Transocean said its evidence indicates that BP failed to properly assess, manage and communicate risk.

BP's own internal report on the disaster blamed a cascade of failures by multiple companies. Government investigations also have spread around the blame. The findings by all of the various sides will be argued about for months and perhaps years to come 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.

In addition to owning the well that blew out, London-based BP was leasing the rig that exploded from Transocean. Eleven rig workers were killed and the government estimates some 206 million gallons of oil spewed from BP's Macondo well one mile (1.6km) beneath the sea before the well was capped three months later. It was the worst offshore oil spill in US history, staining hundreds of miles 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.

The Transocean report was the culmination of work by an internal investigation team comprised of experts from various technical fields and other specialists. Transocean said the loss of evidence with the rig and the unavailability of certain witnesses limited its investigation and analysis in some areas.

The findings conclude:

• BP did not properly communicate to the drill crew the lack of testing on the cement or the uncertainty surrounding critical tests and procedures used to confirm the integrity of the barriers intended to inhibit the flow from the well.

• BP adopted a technically complex nitrogen foam cement programme for sealing the well. The resulting cementing job 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.

• Cement contractor Halliburton and BP did not adequately test the cement slurry used to seal the well.