Tuesday, 25 January 2011

BP’s Subsea Oil Recovery System

Since the explosion and collapse of the Deepwater Horizon oil rig several weeks ago, engineers have been scrambling to come up with a way to staunch the flow, at least temporarily, until a relief well can be drilled. One of the most promising ideas seemed to be the Subsea Oil Recovery System, a tremendous dome intended to be placed over the well head to cap the largest oil geyser in history, 5,000 feet beneath the sea floor in the Gulf of Mexico. The dome was lowered but the operation had to be halted because of ice crystals that formed, blocking the funnel that was supposed to pump the oil up and out of the dome.

The design of the Subsea Oil Recovery System isn’t a new one. BP built the system in Louisiana, and the design was based on similar constructions that were used during the aftermath of Hurricane Katrina. At that time, containment systems were built to protect wellheads, but they were used in shallow water. However, the conditions for this current scenario are quite different because the floor of the sea a mile deep are quite muddy. So engineers had to add mud flaps to the base of the dome, with the intention of sealing off the leak more thoroughly.

The Subsea Oil Recovery System consists of a 40-foot-tall concrete box, designed to be lowered over the leaking well. A funnel coming from the box pumps the oil up to the Deepwater Enterprise, a tanker on the surface of the water, which collects the oil, stores it, and ships it to shore. The oil would then be shipped to a terminal on shore for storage. This process would continue until a more permanent solution could be developed. BP said that the Deepwater Enterprise was capable of storing up to 139,000 barrels of oil, and could process up to 15,000 barrels of oil per day. With this system, BP was hoping that it could collect up to 85% of the oil currently leaking from the sea floor, which would dramatically lessen the growing threat to the coastline.

This approach to temporarily capping a leaking well hasn’t been used since the 1970s, when the oil company called ARCO attempted to do something similar by lowering a rudimentary type of funnel over an oil leak that was occurring naturally in the Santa Barbara Channel. The efforts of ARCO (which was later bought by BP) were successful, and led to the use of similar strategies in shallow water. But the Subsea Oil Recovery System was by far the largest such structure, and the challenges were far greater because of the extreme depth of the water. A BP spokesman said that the company wasn’t sure whether or not the equipment would work at such depths, but they had done extensive modeling and engineering, and they thought it would give them the best chance to corral the oil.

It took about two weeks to build the dome and it took another three days to carry it 50 miles out into the gulf and slowly lower it. But once it was lowered in place, icy hydrates, a sludgy mixture of water and gas, clogged the funnel structure at the top of the box, blocking any chance for the oil to be pumped out. Engineers knew there was a possibility of hydrates forming, but they didn’t know that they would form so quickly and in such large quantities as to plug up an opening 12 inches in diameter.

The containment box was lifted and moved about 600 feet away from the well head while BP reworks its strategy and tries to come up with alternate solutions. One idea under consideration is to build a smaller dome for containment, which might be less vulnerable to the buildup of crystals. This approach would take another two or three days to deploy. Another approach being bandied about involves shooting concrete and mud into the blowout preventer on the well, which is a device designed to shut down the flow of oil during a leak. This technique might help in plugging up the well, but it would take two or three weeks and is not a guaranteed solution. Also, BP is considering trying the Subsea Oil Recovery System again, if they can find a way to keep the crystals from accumulating and blocking the funnel.

In the meantime, small balls of tarry sludge have begun washing ashore onto beaches, and white sea birds are sporting brown oil stains on their feathers. And Gulf Coast residents watch, wait, and pray.