UPDATE 2-Final BP well plug delayed until September- U.S.
* Final well kill pushed to September
* BP to replace key seabed equipment in interim (Adds BP comment, Transocean letter to BP)
By Kristen Hays
HOUSTON, Aug 19 (Reuters) - BP Plc (BP.L) (BP.N) likely won't put the final plug in its blown-out Gulf of Mexico oil well until September to allow replacement of a critical piece of seabed equipment, the top U.S. oil spill official said on Thursday.
Concern over how to safely proceed after pouring cement in the Macondo well from the top, as well as weather delays, pushed the last step past the U.S. Labor Day holiday on Sept. 6 from mid-August, retired Coast Guard Admiral Thad Allen said at a briefing in Washington.
"As we get to the end, we are very close to putting this well away," Allen said. "I think none of us wants to make a mistake at this point."
Allen authorized BP on Thursday to craft a plan to retrieve the failed blowout preventer atop the Macondo well about a mile (1.6 km) beneath the ocean surface and replace it with another before drilling resumes on the relief well.
Meanwhile, Swiss-based Transocean Ltd (RIG.N), owner of the Deepwater Horizon rig that was under contract for BP when it exploded, has accused BP of withholding evidence about actions that led to the worst offshore oil spill in history.
In a confidential letter to BP executives obtained by Reuters, Transocean said it "appears that BP is withholding evidence in an attempt to prevent any entity other than BP from investigating the cause of the April 20th incident and the resulting spill."
BP said the letter contained "misguided and misleading assertions," and that the company has cooperated with ongoing federal investigations.
The blowout preventer failed when the well ruptured on April 20, causing an explosion aboard the drilling rig that killed 11 workers.
Nearly 5 million barrels of oil spewed into the Gulf, contaminating wetlands, fishing grounds and beaches from Louisiana to the Florida Panhandle.
No oil has leaked into Gulf waters since July 15, when BP sealed shut a provisional cap over the wellhead.
Allen said the blowout preventer is key evidence in investigations of the disaster, including those by the U.S. Department of Justice and a joint probe by the Coast Guard and the U.S. Bureau of Ocean Energy Management.
"We do not want to have damage to that blowout preventer if we can avoid it because it's going to be material evidence of exactly what happened during the event itself," Allen said.
In the meantime, BP is testing pressure in the well and seabed equipment, he said.
Kent Wells, BP's senior vice president of exploration and production, said later the test should confirm that the cement and a seal beneath the blowout preventer will hold in the 24-hour period it will take to remove it and install the replacement.
The relief well is about 50 feet (15 metres) from its target near the bottom of the Macondo well about 13,000 feet (4,000 metres) beneath the seabed.
BP engineers and government scientists were concerned cement injected from the top may have trapped up to 1,000 barrels of oil in the space between the well pipe and the surrounding rock layers.
They want to make sure pumping in mud and cement through the relief well will not increase pressure and force that oil up, where it could leak into the sea or damage the blowout preventer.
Wells said the replacement blowout preventer will be better than the failed one if the relief well intersection indeed increases pressure in the well.
"We just think it's a prudent thing to do going forward," Wells said. (Editing by Doina Chiacu)
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