The emergency oil control device on the disastrous Deepwater Horizon oil rig, operated by BP, experienced serious backup system failures, according to a government report.
Some backup control system components "did not perform as intended" on the blowout preventer, designed to stem the flow of oil in the case of abnormal pressure, the report concluded. The news comes after reports emerged that BP could be allowed to start drilling again in the Gulf of Mexico as early as this July.
Forensic testing over many months demonstrated that "not all backup control systems had built in redundancy".
It could not be established whether the automatic 'deadman' switch, designed to trigger the rams, had worked, the report noted.
The report is the final part of the jigsaw of investigations into the Gulf of Mexico oil spill, and it suggests further forensic examination of the BOP could yield important results. It was conducted by specialist consultancy Det Norske Veritas on behalf of the US government.
The principal cause of the failure of the BOP was a bit of piping that had buckled and wedged itself in the valves that were supposed to shut off the oil, the report concluded. But it highlighted some other serious problems involving systems.
But the report also lay blame with a lack of proper industry rules and best practice around systems.
"A review of industry standards indicated they do not stipulate performance requirements for back-up systems (eg closing response times) as they do for primary control systems," it stated. "It is recommended the industry review and revise the requirements for back-up control system performance to be equivalent to the requirements stipulated for primary control systems."
Industry bodies also needed to set clear requirements around testing BOP back-up systems "to assure they will function throughout the entire period of time the unit is required on a well".
They also needed to review the requirements around "evaluating the vulnerability of the back-up control systems of a blowout preventer to assure they are not subject to an event or sequence of events that lead to common mode failure".
BP said in a statement: "In order to get a more comprehensive view of why the BOP failed to work as intended, BP has requested additional testing of the BOP and is presently working with the parties and the court to develop a framework by which that testing will be conducted. We support efforts by regulators and the industry to make BOPs more reliable and effective."
It has insisted in recent months that real time data, and a tight control of staff and technology processes, will be at the heart of turning round its troubled safety record. Cameron, the company that manufactured the BOP on Deepwater Horizion, has not commented.
Transocean, which owned the rig, said the findings "confirm that the BOP was in proper operating condition and functioned as designed". It added: "High-pressure flow from the well created conditions that exceeded the scope of BOP's design parameters."
Edward Markey, a congressman on the House of Representatives' Natural Resources Committee, said the report "calls into question whether oil industry claims about the effectiveness of blow-out preventers are just a bunch of hot air".
Other government reports have questioned BP's decision to ignore the conclusions of modelling software and run with a low number of stability devices. They also stated that abnormal pressure, prior to the explosion, was clearly visible on monitoring devices on the rig. But the devices lacked proper alarms to alert engineers to the problem, reports have concluded.
BP is facing a £13.5 billion US government lawsuit, which in strong language blames it and its partners for failing to use "the best available and safest drilling technology to monitor and evaluate" the Deepwater Horizon drill's conditions and to prevent a spill. BP has said the existence of a lawsuit does not constitute a finding of liability.
There are also reports that the US Justice Department is considering manslaughter charges against it over the deaths of eleven rig workers during the accident.