The piece of equipment at the center of the investigation into the Deepwater Horizon explosion and spill was being retrieved from a mile below the sea, BP announced Friday.
The blowout preventer "was successfully detached" from the Macondo well, spokesman Daren Beaudo said.
"There is no oil coming from it," he said of the well.
Late Friday, the government said another blowout preventer had successfully been placed on the blown-out well.
"During the period of time between the removal of the damaged Blow Out Preventer (BOP) and installation of the replacement BOP, there was no observable release of hydrocarbons from the well head, said Retired Coast Guard Adm. Thad Allen, the government's point man on the oil spill response.
"This is an important milestone as we move toward completing the relief well and permanently killing the Macondo 252 well," Allen said.
Earlier Friday one of several live feeds of the procedure from underwater robots showed a cloudy brown substance coming from the well. Beaudo said the "trailing brownish material" was residual drilling mud pumped into the well in earlier operations.
BP said the 50-foot, 300-ton device was detached from the wellhead at 1:20 p.m CDT.
Earlier in the day, a vessel had latched onto the equipment to raise it from a mile beneath the sea.
Undersea video showed the device suspended in the water. A crane on the Helix Q4000 was being used for the task.
Equipment key to investigation
The blowout preventer is considered a key piece of spill evidence.
A 12-person government evidence team is waiting to take possession of the blowout preventer when it reaches the surface. Investigators will examine it and hope to gain insight into why the device failed to prevent the spill.
Investigators know the explosion was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before igniting.
But they don't know exactly how or why the gas escaped. And they don't know why the blowout preventer didn't seal the well pipe at the sea bottom after the eruption, as it was supposed to. While the device didn't close — or may have closed partially — hearings have produced no clear picture of why it didn't plug the well.
Lawyers will be watching closely, as hundreds of lawsuits have been filed over the oil spill. Future liabilities faced by a number of corporations could be riding on what the analysis of the blowout preventer shows.
On Thursday, engineers removed a temporary cap as a prelude to raising the massive piece of equipment that failed to prevent the worst offshore oil spill in U.S. history.
The replacement blowout preventer will deal with any pressure caused when a relief well BP has been drilling intersects the blown-out well.
Once that intersection occurs sometime after Labor Day, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.
The April 20 rig explosion killed 11 workers and led to 206 million gallons of oil spewing from BP's well. BP was leasing the rig from owner Transocean Ltd.
Plug put to test
With the blowout preventer removed, a lot was riding on the stability of a plug created when mud and cement were pumped down into the well from the top. Essentially, the pressure exerted downward served to counter the pressure coming up.
But Rice University engineering professor George Hirasaki said there was still uncertainty about whether the cement settled everywhere it needed to in order to keep oil and gas from finding its way up.
"Just because it didn't flow when they tested it doesn't mean the cement displaced all of the oil and gas," Hirasaki said.
That's why many people have felt that finishing a relief well and pumping mud and cement in through the bottom would be the ultimate solution to the crisis, said Hirasaki, who was involved in the oil containment effort in the Bay Marchand field off Louisiana after a rig burned in the early 1970s.
Another potential risk: What happens if the crane attached to the blowout preventer accidentally drops the 50-foot, 300-ton device onto the wellhead?
By itself, that might not cause more oil to spew, as long as the plug held, but it would make it difficult to continue the operation, Hirasaki said.
"It would crush everything," he said. "It would be hard to place another blowout preventer on top of it. Right now the wellhead condition is in good condition. But if you dropped it, everything could be opened up."
Allen on Wednesday told reporters during a visit to BP's U.S. offices in Houston that engineers believe the crane will be able to handle the weight of the blowout preventer and some fragile pipe that is believed to be lodged inside.
But if the crane were to swing like a pendulum, that could cause problems, which is why officials waited for rough seas at the site to calm down before continuing with removal of the blowout preventer.
Allen said there is no "significant risk" of more oil leaking into the environment. But he said that after the cap and blowout preventer are removed, "The goal there will be to secure the annulus as quick as we can."
The annulus is an area between the inner piping and the outer casing.