BP plc last Thursday said it had confirmed the integrity of the deepwater Macondo well in the Gulf of Mexico, as well as its associated relief wells. The producer said it conducted “an extensive subsea survey” to identify potential sources of a surface sheen near Mississippi Canyon Block 252, which is near the blowout site.

“The latest survey marks the third time since the Macondo well was permanently sealed in September 2010 that it has been visually inspected at the sea floor and confirmed not to be leaking,” BP said. “The nearly three-day-long inspection was also successful in identifying the cofferdam, a piece of containment equipment used during the Deepwater Horizon response, as the probable source of the surface sheen.”

BP conducted the survey with Coast Guard oversight, jointly with Transocean Ltd., owner of the Deepwater Horizon rig, which was destroyed when the well blew in April 2010. The survey also was performed in the presence of the federal on-scene coordinator for the oil spill, as well as representatives from the Interior Department’s Bureau of Safety and Environmental Enforcement, Bureau of Ocean Energy Management and state on-scene coordinators from Louisiana, Mississippi and Florida.

The Macondo well was capped on July 15, 2010 and permanently sealed with cement on Sept. 19, 2010, BP said. “The well later was plugged and abandoned with the approval and oversight of the U.S. government. Well integrity was confirmed during and after the abandonment process.” In August 2011, “BP once again confirmed the integrity of the Macondo well and the two associated relief wells” after a separate surface sheen was reported near the well site. BP reported the new sheen on Sept. 16 to the National Response Center.

An 86-ton steel cofferdam was lowered over a leaking drill pipe at the Macondo well site in May 2010 in an attempt to capture the oil and funnel it to the surface, BP said. A mixture of oil and slushy methane hydrates was trapped inside the cofferdam during the response. Last Wednesday a video inspection by a remote operated vehicle “observed small, intermittent drops of oil coming from an opening at the top and another on one side of the cofferdam. Samples of the droplets have been collected from the opening at the top, known as the stovepipe, and will be analyzed to confirm a match with the sheen. Droplets were also observed coming out of a small connection port on one side of the cofferdam.”

The Coast Guard determined that it is not feasible to recover the sheen and that it does not pose a risk to the shoreline, said BP. The survey confirmed that no oil is leaking from the Macondo well, nor from the relief well that ultimately intercepted and permanently sealed it. A backup relief well that never intercepted Macondo also was surveyed and confirmed to have integrity, said BP. “In addition, the entire length of the 4,500-foot riser was visually inspected, with separate passes made for the top and two sides, and no oil was seen leaking.”

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