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[ Probe Deepens on New Oil Linked to BP Site ]

BP and its rig contractor, Transocean, will deploy robotic submarine cameras to the wreckage of the ill-fated Deepwater Horizon at the floor of the Gulf of Mexico on Sunday, as they, under U.S. Coast Guard supervision, try to pinpoint the source of an oil sheen that appeared at the site last month, the government said last night.

The Coast Guard earlier this week said its own laboratory testing showed oil from the slick matched oil from BP’s 2010 spill. This latest development indicates the government is not satisfied with BP’s explanation that the oil likely came from a leak in the bent riser pipe that once connected the Deepwater Horizon drilling rig to the Macondo well.  Officials have noted that the pipe lies on the bottom of the sea floor with both ends open.

Now, the Coast Guard says, BP and Transocean have agreed to make satellite observations and to mobilize “ROVs,” or remotely operated vehicles, to examine the original Macondo well area, including the wreckage, debris, and the riser on the sea floor.

This is not the first time that authorities have investigated reports of oil near the Macondo site. At least one leak in the immediate aftermath of the spill was traced to another well. And exactly one year ago, responding to other reports of oil on the Gulf surface near Macondo, the Coast Guard released robot submarine video of both the riser and the well (see photo above) to show that no leaks from the debris were visible. But this is the first time since the capping of the BP well that new oil has been chemically fingerprinted to match the 4.9 million barrels that gushed into the Gulf in the spring and summer of 2010.

To gain some insights on what might be happening at the site of the worst oil spill in U.S. history, I emailed Robert Bea, professor emeritus in engineering at the University of California, Berkeley, a former oil industry executive who has spent years studying catastrophic engineering failure. He has closely followed the Deepwater Horizon disaster from the beginning.

“I am skeptical that the current leakage is coming from the riser,” he said by email. “The riser has been collapsed for a long time… exposed to severe currents and hurricanes… lots of movements to encourage oil to escape if it could.”

Instead, he suggests another possible explanation:  “The leakage could be coming from the fissures—fractures to the sea floor that connect to the ‘sealed’ Macondo well…. or that connect to fractures in the Macondo reservoir.”

This would not necessarily be cause for alarm, Bea said, noting that there are numerous natural oil seeps in the sea floor. Indeed, more than 1,300 barrels of oil a day seep naturally into the Gulf of Mexico, according to a 2002 report by the U.S. National Research Council of the National Academy of Sciences.

But any seeping of fresh oil from the Macondo reservoir could greatly complicate the negotiations now underway to settle BP’s liability for the 2010 disaster.

The Coast Guard’s original statement, while noting the source was unknown, did say the oil “could be residual oil associated with wreckage and/or debris left on the seabed,” and some of the original news  coverage of these latest developments said the oil tested showed signs of drilling mud, which would bolster that conclusion.

But I called the U.S. Coast Guard Eighth District in Louisiana yesterday to check on this point. Spokesman  Ryan Tippets said that the Coast Guard’s lab tests did not detect presence of drilling mud in the oil sample.

Bea said the only way to pinpoint the source is to do the kind of survey that now appears to be planned: “The source/s of the current oil ‘leakage’ can be determined after a thorough ROV camera survey is done of the immediate and surrounding areas,” he wrote by email, adding that such an endeavor is “not quick, easy, or free.”

For a deeper look at deepwater drilling, the science of oil on water, and the geography of the debris that now lies on the sea floor around BP’s Macondo well, see our National Geographic cover story, “Is Another Deepwater Disaster Inevitable?,” and our follow-up stories (“Why The Gulf Oil Spill is Not Going Away,” “Gulf Spill Pictures: Ten New Studies Show Impact on Coast,”and “Gulf Spill Anniversary News and Pictures.”) (Related: Quiz: How Much Do You Know About the Gulf Oil Spill?)

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