Containment structure to straddle riser; collect leaking oil in gulf

A subsea containment structure was lowered 5,000 ft to the seabed, and oil spill response crews worked May 7 to precisely place the structure over a leaking riser from the collapsed Deepwater Horizon semisubmersible in the Gulf of Mexico.

Paula Dittrick
OGJ Senior Staff Writer

HOUSTON, May 7 -- A subsea containment structure was lowered 5,000 ft to the seabed, and oil spill response crews worked May 7 to precisely place the structure over a leaking riser from the collapsed Deepwater Horizon semisubmersible in the Gulf of Mexico.

A crane operator worked May 7 to position the metal containment structure so that it straddles the riser, which dug a trench 8 ft below the seabed when Transocean Ltd.’s semi sank on Apr. 22 after a fatal Apr. 20 fire and explosion (OGJ, May 3, 2010, p. 31).

The semi drilled the Macondo well on Mississippi Canyon Block 252 for BP PLC and its partners, Anadarko Petroleum Corp. and Mitsui Oil Exploration Co. Ltd. BP is the operator. The well struck oil and associated gas with the oil being 35° gravity. Cause of the accident remains under investigation.

A spreading oil leak in the gulf off Louisiana is being fed by a leak from the wellhead and a leak from the collapsed Deepwater Horizon riser. Most of the oil is coming from the riser. An estimated 5,000 b/d of oil is leaking from the two leaks.

The 98-ton metal containment structure is expected to settle into the mud over the riser, helping form a seal around one oil leak. After being placed, the containment structure will collect oil, gas, and water that will be pumped to Transocean’s Discoverer Enterprise drillship for processing and storage, said Doug Suttles, chief operating officer of BP Exploration & Production Inc.

Speaking during a May 7 news conference in Robert, La., Suttles said the biggest technical hurdle could come from the formation of gas hydrates because of cold temperatures in 5,000 ft of water.

Within the containment structure’s riser going to the drillship, BP will use a pipe within a pipe and pump warm water from the surface into the outer pipe to prevent formation of gas hydrates, Suttles said.

He estimates May 10 as the soonest date that the containment structure could become operational.

Wellhead leak
BP and industry experts have stopped trying to activate the shear rams on an existing blowout preventer stack (BOP) at the wellhead, Suttles said May 7.

‘We’ve exhausted those options,” he said. “We are continuing to look for whatever technology we can find,” to stop the flow of oil.

A group of 20 experts from across the world is studying the possibility of using “a top kill” method, which would involve pumping first rubber cuttings and then heavy fluid through the existing BOP to seal the well. It’s a technique that has been used on land and shallow water but never in deep water, Suttles said.

“It has risks,” he said. “We don’t want to do anything to make it (the oil spill) worse.”

Another option being studied for sealing the wellhead is to put a BOP on top of the existing BOP stack. “This also is very complex,” Suttles said.

Meanwhile, a relief well in the process of being drilled had reached 8,700 ft of its 18,000 ft TD as of May 7, Suttles said. It’s estimated the relief well could take 3 months to complete, and a second relief well also might be drilled.

Contact Paula Dittrick at paulad@ogjonline.com.

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