Serious safety lapses by oil rig owner and operator Transocean contributed to the massive blowout and spill at a BP well in the Gulf of Mexico, the U.S. Coast Guard said in a report on the 2010 disaster.

A Coast Guard investigation "revealed numerous systems deficiencies, and acts and omissions by Transocean and its Deepwater Horizon crew, that had an adverse impact on the ability to prevent or limit the magnitude of the disaster," the agency said in a statement on last April's blowout at the BP well.

The 288-page report found Transocean had piled up "numerous deficiencies in the area of safety" in the years leading up to the April 20 explosion and spill that killed 11 people and fouled a wide section of the Gulf.

A team of investigators from the Bureau of Ocean Energy Management, Regulation and Enforcement and the Coast Guard found poor maintenance of electrical equipment, bypassing of automatic shutdown systems, and lack of training of personnel for emergencies, the report said.

"These deficiencies indicate that Transocean's failure to have an effective safety management system and instill a culture that emphasizes and ensures safety contributed to this disaster," the Coast Guard statement said.

Transocean had numerous international safety code violations, repeated maintenance deficiencies including neglected safety inspections, and a record of safety incidents that were not properly investigated, the report said.

The rig operator also failed to ensure that its onboard managers and crew had sufficient safety training and knowledge and were unaware of procedures for shutting down the rig in a way that could have averted an explosion, it said.

The initial report will be included as part of the final Coast Guard investigative report, which is expected to be released by July 27, the statement said.

A barrage of court claims pitting BP against its partners in the Gulf of Mexico oil spill could lay the groundwork for billions of dollars in settlements to spread the costs of the disaster.