The cap installed on the top of the well is basically a second blowout preventer.
Then there was the blowout preventer, a huge stack of valves on the sea floor.
It is not clear if there was any attempt to activate the blowout preventer before the explosion.
Would a different type of culture have motivated BP to check that blowout preventer on the Deepwater Horizon rig?
Systems that were planned to be redundant, like the five-story blowout preventer, turned out to be susceptible to complete collapse.
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If both barrels fail there is the possibility of putting a new blowout preventer on top of the old one.
Officials had planned to detach a device called a blowout preventer from the well and replace it with a new one.
The remote-controlled submarines are being used by BP in an attempt to activate the well's blowout preventer to choke the flow of oil.
To do this, BP has been replumbing and rewiring the blowout preventer, paying particular attention to its choke pipe and the kill pipe.
Various sizes of caisson will be built, including one 15 metres (50 feet) or so in diameter, large enough to fit over a whole blowout preventer.
As early as five hours before the explosion drillers noticed unexpected loss of fluid in the riser pipe, indicating leaks in a part of the blowout preventer.
Other companies involved included Transocean, the owner of the rig and responsible for the safety valve known as the blowout preventer, and Halliburton, who provided cementing services.
One is to deal with this blowout preventer stack and to take -- to use every capability to try to get it -- the rams to close.
Subsequently the company rushed the Deepwater Horizon well into production, ignoring danger signals especially related to the blowout preventer, which Tony Hayward now acknowledges must be totally redesigned.
Mr Wells says that once the control pod has been reinstalled on the blowout preventer and the manifold is attached to the Q4000, the top kill could start.
The Interior Department granted the company permission to begin "certain limited preparatory activities" -- including work related to the installation of a blowout preventer -- on August 30.
Bradford County Emergency Management Agency Deputy Director Skip Roupp was quoted as saying that there was a crack in the top part of the well, below the blowout preventer.
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Washington (CNN) -- The public may have seen the last video images of the failed blowout preventer from the BP oil well disaster, as authorities expressed possible security concerns.
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With that leak sealed, the plan is to inject heavyweight "kill mud" into the lower part of the blowout preventer, followed by cement that will effectively entomb the well.
At that depth, a key piece of equipment is the blowout preventer, which weighs up to 300 tons and caps the well, holding back ultrapressurized oil deep in the Earth.
"You have here an unprecedented event never before have you seen a blowout at such depth and never before has a blowout preventer failed in this way, " BP spokesman Andrew Gowers said.
Removal of the device will need to be done carefully, because the blowout preventer may hold valuable forensic evidence as to why it failed on April 20, triggering the massive explosion.
That starts off with a "junk shot, " in which material such as rubber tires and golf balls is pumped into the shut-off valve, or blowout preventer, that sits atop the well.
The blowout preventer's control pod, which was also originally connected to the lost rig, has been taken up to the Q4000, where it has been tested and attached to new cables.
The trial will determine the causes of the spill, and assign responsibility to the parties involved, including BP, Halliburton, Transocean, and Cameron, which manufactured the blowout preventer meant to stop oil leaks.
The primary culprit behind the explosion appears to be the blowout preventer, a two-story-tall stack of valves on the seafloor that was built by Cameron in 2001 and designed to guillotine shut an out-of-control well.
He said in such a situation, normal procedure would be for the contractor's crew to secure the well by closing off the huge valve sitting on the seabed known as a blowout preventer.
The risks of a spill are lower because the target is primarily gas, and the shallow water depth (approx. 100 ft) allows the blowout preventer to be located on the rig at the surface.
On May 26th BP, the company responsible for the well that Deepwater Horizon had drilled, started to pump heavy drilling mud into the well's blowout preventer, a set of valves on the sea floor.
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The goal of the hearings is to examine the design and performance of the drilling rig's blowout preventer, a mammoth set of valves that is the last line of defense against an oil spill.
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