02 May 2010

Containment Dome in 6-8 Days; Blowout Preventer Defective?

BP PLC chairman Lamar McKay told ABC's "This Week" that he can't say when the well a mile beneath the sea might be plugged. But he said he believes a 74-ton metal and concrete box — which a company spokesman said was 40 feet tall, 24 feet wide and 14 feet deep — could be placed over the well on the ocean floor in six to eight days. _ Source
Design Evolution of Subsea BOP (PDF)
Officials from both BP and the US government are stating that the blowout preventer (BOP) at the Deep Horizon site was defective (scroll). All efforts to close the BOP valves have been ineffective -- and it will be from 6 - 8 days before the containment dome can be placed over the seafloor leak of high pressure hydrocarbons.
Casings, multiple pipes one inside another, are inserted into the well to extract drilling mud. A mechanism of valves called a “blowout preventer” sits on top of the well on the sea floor to prevent oil from gushing out. The 450,000-pound blowout preventer can shut the opening, or close tightly around a drill or pipe inserted into the well.

As drilling proceeds, mud flows straight up through the blowout preventer to a 21-inch-diameter long pipe called a “riser” and up to the floating rig, BP Exploration and Production officials said.
Much of the work on oil platforms and rigs has to do with inserting and extracting equipment in and out of these nesting pipes and operating the blowout preventer to ensure there are no leaks, Robertson said.

It’s delicate work, he said, because petroleum is often with mixed natural gas underground and is highly pressurized. If gas leaks into the mechanism, it will expand as it rises to the surface, and can accelerate into an explosion, Robertson said. Once the gas blows, it takes only a spark for fire to erupt.

...Robertson said there are multiple safety mechanisms in place to prevent accidents such as this, including ones to detect increasing gas pressure and protocols to prevent fire aboard the rig. “There would have been a dozen barriers that had to fail in order for this accident to happen,” he said.
_LATimes
While the cause of the leak remains unclear, speculation centres on two areas: the "cementing" process to secure the well walls – work that contractor Halliburton insists was completed 20 hours before the accident – and the "blowout protector". The equipment, supplied by US firm Cameron International, "is the ultimate failsafe mechanism", Hayward said. "And for whatever reason, it failed to operate." _Guardian
Differences Between Undersea BOPs and Dry Land BOPs
"There is a major difference in land and jackup BOPs compared to those working offshore, explains Erving Schneider, Director of Marketing, Cameron. "Jackup BOPs stay dry on deck. However, deepwater BOPS have to work on the seafloor. To do that, you need a riser and electronics to operate it."

While the basic concept of the BOP structure is the same, subsea BOPs have an added locking mechanism, which allows the crew to close it hydraulically. Land and jackup BOPs have a manual lock, Schneider explained.

Deepwater subsea control systems remotely power seabed BOP valves and monitor the chokes and gauges in the system. Surface transmissions flow through hydraulic lines or electric umbilicals to the Central Control Manifold. This connects to separate manifolds on each well and transmits the signal to and from the blowout preventers, hydraulic connectors, and valves of the BOP.

Hydraulic power fluid, which flows to the BOP stack components, comes from a surface pump or actuator unit through an umbilical, a separate hose, or a rigid conduit connected with the riser joints. Of course, these subsea controls are duplicated to ensure redundancy. And due to the remote location of subsurface systems, they are called on to operate reliably for longer working periods than surface systems.

The subsea control system must react quickly to the surface commands. The minimum requirement for BOPs to close is 45 seconds or less. Annular BOPs must close within 60 seconds. _Rigzone

Deep Horizon crew members had tried to close the BOP, and remotely operated undersea vehicles have made multiple attempts to close the BOP -- all unsuccessful.
BP chief operating officer Doug Suttles said interviews with Transocean workers on the rig revealed crewmembers tried to activate the BOP from the rig's bridge before the fire forced them to evacuate, but the BOP did not close off the well.

Suttles also revealed that BP remotely-operated vehicles (ROVs) had hit "subsea access points" that should close the BOP, but that they also failed to trigger the mechanism to shut.

"We don't know why the BOP failed to stop the flow," he said. "Ultimately we will recover the BOP, get it to the surface and find out."

"I'm sure Transocean, who actually owned blowout preventer, will be interested to find out why it didn't work," Suttles said.

An MMS official estimated that the SWAT teams would have all the Gulf's offshore drilling units inspected in the next seven days. _UpstreamOnline

It is likely that the undersea leak will continue over the next week -- until the containment dome can be readied and lowered into place. There is also a possibility that the pressure driving the leak will be diminished or partially impeded by various dynamic geological processes along the length of the undersea hydrocarbon flow. To be blunt: No one knows exactly how much oil is leaking or will leak out over the next week to several weeks from now. Engineers who are directly involved are no doubt hoping for the best, but preparing for the worst.

A look at the evolution of undersea BOP design PDF Excerpt:
Today’s deepwater BOPs can be required to remain subsea for extended periods of time ranging from 45 to 90 days for a single well, to more than a year in cases where drilling and completions on multiple wells are required. In all cases, however, when the BOP is called on to function in an emergency situation, it is the main barrier protecting human life, capital equipment and the environment.
...Therefore, it must function without fail.

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