The value of one highly touted facet of the Gulf of Mexico oil spill cleanup — the small navy of vessels skimming oil from the surface — has proven all but impossible to measure, which could make it difficult to figure out how much damage BP is liable for when the gusher is finally stopped.
BP and the federal government admit they have no idea how much oil has been collected by hundreds of boats that range from retrofitted fishing vessels to state-of-the art craft designed specifically for the task. The harshest critics say the amount of oil skimmed is as low as 2.9 million gallons of the 87 million to 171 million gallons of crude that have gushed into the Gulf since April 20, but BP and independent scientists alike say there's no real way of knowing.
BP tracks the amount of oil-tainted water skimmers collect — nearly 29 million gallons so far — but not the amount of oil found in that mixture. Part of the problem, BP says, is the variety of vessels and skimming equipment being used.
"We don't have any basis for making an estimate," BP spokesman Bryan Ferguson said. "There are too many different types of vessels and too many other factors."
Other means of collecting oil are easier to measure or estimate. The company has a handle on how much is funneling from the well cap into tankers — some 28.7 million gallons as of Friday. It also releases daily updates on how much oil has been burned from the surface of the water, a calculation it derives from the thickness of the crude, the size of the oil patch and how long it takes to burn.
Of course, even the total spilled is murky. There is a daily federal estimate that hit as many as 171 millions gallons Friday. Put simply, that's enough oil to fill up nearly 113 blimps the size of Goodyear's Spirit of America.
Knowing how much oil has spilled and where it ends up will be a billion-dollar question once the damage from the leak is finally tallied.
Arriving at some final figure will be important for the agencies charged with assessing the toll taken by the oil on the water, beaches and wetlands of the region, according to Larry McKinney, executive director of the Harte Research Institute at Texas A&M University Corpus Christi.
Agencies designated as Natural Resources Trustees will use the calculation of how much oil spilled and where it went in the process of assessing the damage BP is liable for.
"You're talking about billions of dollars," McKinney said. "If they don't get that back from the responsible party, taxpayers are going to have to pay that."
The Sierra Club and other environmental groups doubt the skimmers — which have increased from roughly 100 in the region at the start of June to more than 550 by early July — are worth the resources being devoted to them.
"We're poisoning the Gulf and we need to have as accurate an accounting as possible about how much oil is being released and how much is being cleaned up," said Michael Brune, executive director of the Sierra Club.
Environmentalists who criticize skimming say that under ideal conditions, about 20 percent of that mixture would be oil, but it's generally probably closer to 10 percent.
"In the real world, in good conditions, you get about 10 to 15 percent," said David Guest, a lawyer who runs the Florida chapter of Earthjustice. "Skimmers are a big illusion."
That criticism, though, is just as speculative as claims for skimmers' efficacy, according to marine scientists.
"A 90-10 ratio seems too high on the water side," said Arturo Keller, a biogeochemistry professor at the University of California at Santa Barbara. "At that point, there'd be nothing to skim."
The choppiness of the water, the distance of the oil from the leaking wellhead and even the skill of the boats' crews are all factors that can determine how much crude is being skimmed at any given time. Even the vessels themselves vary widely, from specially designed vessels equipped with on-board oil separation facilities to retrofitted fishing boats towing containment boom.
"It's highly variable," said James O'Donnell, a marine sciences professor at the University of Connecticut's Avery Point campus. "The only way to know might be if each vessel is processing the mixture on board."
Even in the absence of hard numbers about skimmers' value, local officials are so bullish on the boats that supply struggles to meet demand. Leaders in coastal Louisiana parishes have asked BP to pay for having 300 vessels retrofitted as skimmers, and Gov. Bobby Jindal has assigned 20 members of the National Guard to make sure the Coast Guard is deploying as many of the boats as it reports are on the water.
BP is trying to recycle as much of the oil as it can for later use. Oil that can't be processed gets brought to landfills in Louisiana, Mississippi, Alabama and Florida, although the company says it doesn't know how much has been skimmed out of the Gulf.
Neither BP nor the Coast Guard had an estimate on how much the skimmers are costing.
Despite the uncertainty, the demand for skimmers along the coast shows no sign of abating.
"As the spill expands from Louisiana to the entire panhandle of Florida, we are looking at every possible opportunity to bring skimmer capability in there," said retired Coast Guard Adm. Thad Allen, the Obama administration's point person in responding to the spill, at a recent press briefing.
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