In an internal accident investigation report, BP said it and Transocean,the owner of the Deepwater Horizon rig drilling the Macondo well, failed to notice that the cement used within the well had failed to contain the oil and gas.
But BP said drilling contractor Transocean had missed further danger signs and may have reduced the time to respond to the accident by diverting escaping hydrocarbons to an onboard mud gas separator rather than overboard.
BP also faults oilfield service giant Halliburton for its cement, the contents of which may have prevented it fully sealing the well.
"It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy," said the company's outgoing chief executive Tony Hayward, who has announced his resignation as a result of the accident.
"Multiple parties, including BP, Halliburton and Transocean, were involved," he said.
Hayward said BP's design of the well, which has come under scrutiny since the incident happened on April 20, was not likely to have contributed to the blowout.
"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," he said.
"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," Hayward said.
NEGLIGENCE
The 193-page report on the incident, which caused the largest oil spill in US history, was compiled by a team of BP investigators led by Mark Bly,BP's head of safety and operations.
BP's report on the incident precedes probes by the Justice Department and other US agencies, which are expected to center on whether BP or its rig partners were grossly negligent in their role in the disaster.
Some US politicians have suggested that cost-cutting moves by BP may have played a role in the accident, something BP denies.
BP hopes to avoid the key legal measure of gross negligence, a ruling which would likely make it liable for some $20 billion of fines over the US Clean Water Act.
Indeed, BP has denied gross negligence on its part, saying that the accident was a complex event that involved various companies' decisions.
Transocean, the owner of the rig that was hired by BP to drill Macondo,has denied wrongdoing, claiming that its blowout preventer had been tested and was not faulty.
Halliburton has also said its cement work wasn't to blame for what for the well blowout.
In its report, BP said its testing had identified faults with the composition of the cement slurry used by Halliburton.
The tests showed that it was not possible for the slurry to stabilize if it had a nitrogen content of more than 50%, while the slurry used in the Macondo well had a nitrogen content of 55-60%, making it "likely unstable," BP said.
"Improved engineering rigor, cement testing and communication of risk by Halliburton could have identified the low probability of the cement to achieve zonal isolation," it said.
RIG WORKERS
Transocean may have contributed to the scale of the disaster due to a failure by its rig workers to notice and act on hydrocarbons flows into the well bore, BP said.
"The results of the negative pressure test were incorrectly accepted by BP and Transocean," BP conceded.
But later, "Over a 40-minute period, the Transocean rig crew failed to recognize and act on the influx of hydrocarbons into the well until the hydrocarbons were in the riser and rapidly flowing to the surface," BP said.
BP said, had rig workers decided to divert fluids exiting the well overboard rather than the Deepwater Horizon's mud gas separator system, there may have been more time to respond, reducing the consequences of the accident.
"Transocean's shut-in protocols did not fully address how to respond in high flow emergency situations after well control has been lost. Well control actions taken prior to the explosion suggest the rig crew was not sufficiently prepared to manage an escalating well control situation," BP said.
BLOWOUT PREVENTER FAULT
Once the gas reached the rig and exploded, the blowout preventer on the seabed should have activated automatically and sealed the well, but this did not happen.
BP said this was "probably because critical components were not working," also preventing the BOP being triggered manually.
BP said an examination of the BOP control pods following the accident revealed that there was a fault in a critical solenoid valve in the yellow control pod and that the blue control pod batteries had insufficient charge; faults which "likely" existed at the time of the accident.
BP said also said the investigation team found indications of potential weaknesses in the testing regime and maintenance management system for the BOP.
"The condition of critical components in the yellow and blue pods and the use of a non-OEM part, which were discovered after the pods were recovered, suggest the lack of a robust Transocean maintenance management system for Deepwater Horizon BOP," BP said.
China Chemical Weekly: http://news.chemnet.com/en/detail-1411716.html
