OREANDA-NEWS. October 30, 2012. BP has capped and plugged an abandoned piece of subsea equipment known as a cofferdam that is thought to be the source of a recently spotted surface sheen near the Mississippi Canyon 252 lease block in the Gulf of Mexico.

The cofferdam is an 86-ton, steel container that was lowered over a leaking drill pipe during the Deepwater Horizon response in 2010 in an attempt to capture the oil and funnel it to the surface. It now sits on the sea floor roughly 1500 feet away from the Macondo well.

With US Coast Guard approval, BP on Tuesday deployed Remotely Operated Vehicles (ROVs) and successfully installed a 750-lb cap over an opening on the cofferdam known as the stovepipe and secured it in place with clamps. ROVs also successfully inserted plugs into four small connection ports on the top and sides of the cofferdam. The entire operation lasted approximately 26 hours.

Initial visual inspections of the cap and plugs have observed no oil droplets emanating from any of the openings. BP will now monitor the sheen by satellite for several days to confirm that the cap and plugs are secure.

Last week, a separate ROV survey confirmed once again that the Macondo well and its two associated relief wells were not leaking. It marked the third time since the Macondo well was permanently sealed in September 2010 that well integrity was confirmed by visual inspection.

The latest survey came after BP and Transocean received a Notice of Federal Interest from the US Coast Guard to determine potential sources of the recent sheen. It focused on the wells, wreckage and debris related to the 2010 Deepwater Horizon accident.

BP reported the sheen on Sept. 16 to the National Response Center. The Coast Guard has since determined it is not feasible to recover the sheen and that the sheen does not pose a risk to the shoreline.

The ROV survey last week identified small, intermittent drops of oil coming from the cofferdam’s stovepipe and one of the connection ports, which BP concluded was the probable source of the sheen. A mixture of oil and slushy methane hydrates was trapped inside the cofferdam during the response.

The operation to cap and plug the cofferdam was performed in the presence of the FOSC, as well as representatives from the Interior Department’s Bureau of Safety and Environmental Enforcement (BSEE) and Bureau of Ocean Energy Management (BOEM) and representatives of the Gulf states.