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Gulf oil spill: Drilling technology explained

April 29, 2010 |  5:55 pm


The drilling rig that blew was floating in the deep seas, about 5,000 feet above the sea floor and 40 miles offshore. Such ultra deepwater drilling rigs operate using a series of pipes nested one within another plunging to the sea floor and below, according to Tim Robertson of Nuka Research and Planning Group, an oil production and spill response consultancy based in Alaska.

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.

BP officials said it is too early to know details about how the gulf blast occurred. But they said the rig operators had tapped a new reservoir of oil and were preparing to install a cement plug through the pipes to cap it for future production. It’s possible gas rose up through the riser and exploded.

The riser pipe bent and collapsed, and although the blowout preventer has several mechanisms designed to shut it in an emergency -- including one known as a “dead man’s switch” -- these somehow failed to close.

Oil is now gushing through the blow-out preventer on the sea floor, and through the broken pipes and into the water. It is described as light oil, which evaporates a little more easily than the heavy oil typical of Alaskan wells.

Robertson said both regulations and technology for dealing with oil spills have improved in recent decades. “But the basic physics haven’t changed,” he said. “Once the oil is in water, it’s a losing ball game.”

BP, which owns the oil resource and leases the rig from Transocean, is struggling to stanch the flow, a task made more difficult by extreme depths. The company has dispatched robotic submarines to the sea bottom in an effort to shut off the blowout preventer.

In a separate effort, BP is engineering a giant containment device, like a big funnel, to fit over the entire leak column, from the surface to the sea floor. If the containment device succeeds, it will direct the flow all the way from the blowout preventer to a waiting container ship. The extremely complex technical operation has never been tried before in such deep waters. It could take weeks to complete, BP officials said.

BP is also working to tap into the oil reservoir through another opening, and stop it up, a process that could take three months.

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.

He described a similar accident he survived while working on a fixed rig in Cook Inlet in Alaska in 1987. The gas blew at about 5 p.m., he said. He heard a sound like a jet engine roaring to life. Most of the crew were evacuated. Robertson and a few others stayed to try to stop the leak as gas, mud and sand spurted into the air. At about 11 p.m., they were working below when they heard the roar change: “It went to a different pitch.”

They knew it was a fire. A flying rock had smashed a light, sparking a blaze. They could feel the heat as they escaped in a rescue pod. Fifteen miles away, the light of the exploding platform pierced the night so brightly, “you could read a newspaper,” he said.

The gulf accident may have happened even more quickly. Reading about it, “I re-lived every moment of that night,” Robertson said.

- Jill Leovy

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Photo: This undated photo provided Sunday, April 25 by the US Coast Guard shows the arm of a robot submarine attempting to activate a shutoff device known as a blowout preventer (BOP) to close off the flow of oil at the Deepwater Horizon well head.  Credit: AP Photo/US Coast Guard