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BP pins hopes on risky tactic

Wed., May 26, 2010

Ocean depth makes ‘top kill’ effort tricky

As early as today, BP will begin its first attempt to seal the deep ocean well that is spewing oil into the Gulf of Mexico, using a series of high-risk maneuvers that has never been attempted at such low depths.

The so-called top kill effort is increasingly critical for BP, which has come under attack in recent days from Obama administration officials and Gulf Coast states frustrated with the company’s inability to cap the well and stop the worsening environmental disaster.

BP officials were running diagnostics Tuesday on the blowout preventer above the leaking well, a final step before the effort gets under way. BP and federal officials have been prepping for the top kill attempt since the first days after the spill, in large part because they’ve seen the method succeed to seal off gushing wells in the past – most notably, the burning Kuwaiti oil fields from the first Gulf War.

But no one has ever executed the method 5,000 feet below the ocean’s surface. The added complexity of measuring oil flow and working with undersea robots at that extreme depth, along with the difficulty of directing ship traffic on the surface, has slowed scientists’ efforts.

Here’s how it will work:

Heavy mud will be forced into the well to counteract the upward pressure of the leaking oil and gas. Then cement will be poured in after the mud to seal the opening.

If for some reason the mud alone cannot push down the oil, BP officials said they may also try to stop the flow with a “junk shot” filled with golf balls, among other objects.

Success of the venture will depend on loading enough mud and cement into the well to stop the surge of oil and gas – a tricky proposition. Iraj Ershaghi, director of the petroleum engineering department at the University of Southern California, estimated that the upward pressure is likely to be about 9,000 pounds per square inch. At a depth of 5,000 feet, the water pressure bearing down on the leak is about 2,500 pounds per square inch, he said.

That leaves a difference of about 6,500 pounds per square inch of upward pressure at the wellhead, explaining why the oil and gas flowing upward can easily overwhelm the water pressing down on it and why the crude has continued to gush into the ocean.

“This is not a kids’ game, to fight that kind of pressure,” Ershaghi said.

To make up the pressure difference, technicians plan to pump mud into the blowout preventer, a kind of surge protector that sits on top of the wellhead. The device had failed to cut off the flow of oil when the pressure surged too high.

The mud that will be used, drilling mud, is a dense mixture of water and minerals such as bentonite clay. It can be made even denser by adding heavier minerals such as barite and galena.

The heavier the mud, the more it will suppress the flow – but on the flip side, the harder it will be to pump in.

The mud will be pumped from surface vessels with a combined 50,000-horsepower pumping capacity into the internal cavity of the blowout preventer. BP officials said they plan to pump the mud at a rate of up to 40 barrels per minute.

It’s unclear how much mud will be needed to stop the flow of oil, BP spokesman Bryan Ferguson said. It’s possible, he said, that the entire cavity of the blowout preventer will have to be filled.

Once the oil flow has been contained, the hole will be covered with cement to permanently close the well.

Not just junk

 If the top-kill procedure doesn’t work, technicians plan to implement the “junk shot” – shooting in material to keep the mud from escaping.

 Objects in the junk shot – such as rope knots, golf balls and shredded tires – are picked for a reason. Each odd shape serves a different function, and the more varied the shapes of the collected junk, the more effective the clog will be.

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