Previously, BP and the government aimed to remove segments of drill pipe from the BOP before attempting to replace it. But some of drill pipe pieces have settled into the BOP, Allen said during a press conference in Washington accessible by telephone.
The time line for the next steps entail making preparations Friday, Saturday and Sunday to remove the BOP and then taking out the BOP between Monday and Tuesday, according to Allen. "If for some reason it does not come free, we will cut the pipe at the wellhead," he said of the original BOP.
"In sum, we are prepared to place the new BOP sometime next Tuesday or Wednesday followed by testing," Allen said. With that time line, the target date for having a relief well intercept Macondo would be September 7 or 8.
At least a couple of factors could alter the schedule: weather and any challenges with opening rams on the BOP, Allen said.
The Macondo disaster began when the BP Macondo exploratory well blew out on April 20, causing explosions and the sinking of the Transocean Deepwater Horizon rig, the deaths of 11 workers and the worst marine oil spill in US history.
No oil has flowed from Macondo since July 15, when BP successfully installed a sealing cap or capping stack atop the malfunctioning BOP.
By then, however, a government panel has estimated that 4.9 million barrels of oil escaped from the well, including 800,000 barrels captured by BP with a variety of collection devices deployed during the effort to close it down.
THREE SCENARIOS FOR BOP REMOVAL ON THE DRAWING BOARD
Allen and BP believe they effectively plugged Macondo on August 5 when engineers pumped 500 barrels of cement into the primary well casing.
But they still consider interception by a relief well at the reservoir 13,000 feet below the sea floor as the only permanent solution.
And, they have decided to replace the original BOP with a working model as an extra precaution prior to interception by the relief well poised about 100 feet from its target.
Allen said Friday he has not heard anyone at BP suggest they would not be able to remove the BOP.
But they had been most concerned about a 3,500-foot length of drilling pipe broken during the blowout and extending below the BOP into the well casing.
With the fishing expedition deemed unsuccessful and the prospect for extraction of that pipe eliminated, Allen said BP will follow three different scenarios for replacing the BOP.
After preparing the BOP for removal over the weekend, he said BP will begin the process on Monday by first removing the sealing cap that is no longer needed due to the August 5 cementing on the well.
But they will place it on the sea floor near the wellhead in case they need it again.
Then, he said, BP will use the Helix Q4000 multipurpose vessel to unlatch the BOP from the sea floor 4,993 feet below the surface.
"If it pulls free, we will cut whatever pipe is beneath it," Allen said.
But, he said, if it sticks, they will deploy what he dubbed "the gentle tug" using 80,000 pounds of pressure to pull it up and cut the pipes holding it down.
In the event the BOP continues to resist, he said the engineers will open the rams on the BOP to release the pipe, lift it off and sever any pipes that continue to hold it.
Because the BOP and the long pipe may have value as evidence in investigations of the event, Allen said a custodial team is on the site ready to preserve the chain of custody on items retrieved.
China Chemical Weekly: http://news.chemnet.com/img/articles/140/1409957_0.pdf
