BP Group Chief Executive Tony Hayward listens to an operation update from the U.S. Coast Guard

• The cement and shoe track barriers -- and in particular the cement slurry that was used -- at the bottom of the Macondo well failed to contain hydrocarbons within the reservoir, as they were designed to do, and allowed gas and liquids to flow up the production casing.
• The results of the negative pressure test were incorrectly accepted by BP and Transocean, operator of the drilling platform, although well integrity had not been established.
• Over a 40-minute period, the Transocean rig crew failed to recognise and act on the influx of hydrocarbons into the well until the hydrocarbons were in the riser and rapidly flowing to the surface.
• After the well-flow reached the rig it was routed to a mud-gas separator, causing gas to be vented directly on to the rig rather than being diverted overboard.
• The flow of gas into the engine rooms through the ventilation system created a potential for ignition which the rig’s fire and gas system did not prevent.
• Even after explosion and fire had disabled its crew-operated controls, the rig’s blow-out preventer on the sea-bed should have activated automatically to seal the well. But it failed to operate, probably because critical components were not working.

"It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy. Multiple parties, including BP, Halliburton and Transocean, were involved," said Tony Hayward, BP's outgoing chief executive.

“To put it simply, there was a bad cement job and a failure of the shoe track barrier at the bottom of the well, which let hydrocarbons from the reservoir into the production casing. The negative pressure test was accepted when it should not have been, there were failures in well control procedures and in the blow-out preventer; and the rig’s fire and gas system did not prevent ignition," he said.

“Based on the report, it would appear unlikely that the well design contributed to the incident, as the investigation found that the hydrocarbons flowed up the production casing through the bottom of the well,” added Hayward, whom BP recently reassigned to Russia.

The April 20 fire, explosion and sinking of the Deepwater Horizon drilling platform killed 11 workers, and the subsequent oil spill killed unknown numbers of birds, fish and other sea creatures.

Gulf communities from Louisiana to Florida are still cleaning up oil that washed ashore, and scientists say at least one large underwater plume of oil could create further environmental havoc.

BP has established a $20 billion compensation fund to pay claims, and this week gave $10 million to the National Institutes of Health to study the potential health effects of the spill.

"We are determined to learn the lessons for the future and we will be undertaking a broad-scale review to further improve the safety of our operations. We will invest whatever it takes to achieve that. It will be incumbent on everyone at BP to embrace and implement the changes necessary to ensure that a tragedy like this can never happen again," Hayward said.

Chairman of the Board Carl-Henric Svanberg said, “I believe this report will be of significant value in helping the overall understanding of how this tragedy occurred. It is of the utmost importance to the board to ensure that BP learns from this and further enhances the safety of its operations for the future.”