* BP says sheen likely emanating from bent riser pipe on sea floor
* Transocean says not responsible for oil originating below surface
* U.S. lawmaker calls for inspection by undersea robots
By Chris Baltimore
HOUSTON, Oct 11 (Reuters) - An oil sheen spotted on the Gulf of Mexico near the site of the sunken Deepwater Horizon rig matches samples from BP Plc's ill-fated Macondo well, the U.S. Coast Guard said.
BP reported a sheen on Sept. 16 in block 252 of the Mississippi Canyon, about 50 miles (80 km) off the Louisiana coast. Test samples indicate that "the sheen correlates to oil that originated from BP's Macondo Well", the Coast Guard said in a statement late on Wednesday.
Swiss-based Transocean Ltd owned the Deepwater Horizon drilling rig and London-based BP was the operator of the Macondo well, which ruptured on April 20, 2010, killing 11 workers and unleashing the worst U.S. offshore oil spill.
The latest reported sheen is the first in recent months, although there have been several near the site over the past two years.
According to the Coast Guard, the sheen likely comes from wreckage on the sea floor, not the well itself.
"The exact source of the sheen is uncertain at this time, but could be residual oil associated with wreckage and/or debris left on the seabed from the Deepwater Horizon incident in 2010," the Coast Guard said. "The sheen is not feasible to recover and does not pose a risk to the shoreline."
The Coast Guard notified BP and Transocean that "either party or both may be held accountable for any cost associated with further assessments or operations related to this sheen."
BP said it will continue to work with the Coast Guard to identify likely sources. It said its analysis shows that the sheen's likely source is a bent riser pipe that once was connected the rig to the well head.
A BP spokesman said an undersea inspection by BP in September 2011 confirmed that the Macondo well was sealed, and "we have seen no evidence from this latest sheen that leads us to believe otherwise."
A Transocean spokesman pointed to a February 2012 ruling from U.S. Judge Carl Barbier that Transocean is not responsible under the Oil Pollution Act for undersea oil discharges from the well.
"We will rely on the lab analysis as to the origin of the oil, and defer to the recent ruling of the federal court on the question of responsibility," Transocean spokesman Lou Colasuonno said.
A U.S. lawmaker said the U.S. government should require BP to utilize remotely operated undersea vehicles to survey the ocean floor around the site.
"BP must do everything in its power to ensure this well does not rupture or leak, and they should be held responsible if it does," said U.S. Representative Edward Markey, a Massachusetts Democrat. "BP still has billions to pay to the people of the Gulf and the U.S. government, but the Gulf region also deserves the peace of mind that this well is dead once and for all."
Two and a half years ago, the well spewed 4.9 million barrels of oil into the Gulf of Mexico for 87 straight days, unleashing a torrent of oil that fouled the shorelines of four Gulf Coast states and eclipsed the 1989 Exxon Valdez spill in Alaska in severity.
The well was capped with cement on Sept. 19, 2010, which U.S. officials said had "killed" the leaking well for good.