NEW YORK (Reuters) - The Obama administration may succeed in pushing through its offshore drilling ban, despite fierce resistance from the oil industry, since a piece of machinery in short supply has left oil companies and the environment glaringly vulnerable to another oil spill.

The offshore skimming devices -- seagoing vessels that suck up spilled crude -- are the first line of defense in the contingency plans that big oil companies are required to submit when they drill in the deep waters of the U.S. Gulf of Mexico.

But the vast majority of skimming capacity listed in “worst case scenario” plans to combat major Gulf spills is already deployed to clean up BP’s leak, according to copies of the plans made public by Congress and lists of vessels active in the cleanup that were obtained by Reuters.

With few skimmers in reserve, any new spill could be harder to fight, including one caused by a hurricane during the Atlantic storm season that forecasters say could be one of the most intense on record.

That may give U.S. Interior Secretary Ken Salazar the justification he needs to quickly issue a new deepwater drilling ban after a district court struck a first one down.

“We are working hard to issue a new moratorium in the coming days,” Interior Department spokeswoman Kendra Barkoff said, without offering further details.

BP, Exxon Mobil Corp. and Shell Oil Co, among the biggest operators in the Gulf, would rely largely on the same armada of skimmers, according to contingency plans that were released by lawmakers investigating the BP blow-out.

Many of those vessels are among the 58 largest skimmers already cleaning up the biggest Gulf spill ever, one that has forced the closure of more than 80,000 square miles of fishing area and put the future of U.S. offshore drilling in doubt.

For instance, 84 percent of the skimming capacity Shell lists in its Eastern Gulf “worst case scenario” spill contingency plan is engaged in the BP effort, according to an equipment manifest given to Reuters. Two big spill response firms told Reuters that over 90 percent of their resources are already at work on BP’s spill.

The bulk of skimmers listed in oil companies’ contingency plans are controlled by a single spill response firm, Virginia-based Marine Spill Response Corp. (MSRC), formed and funded collectively by big oil companies after Alaska’s Valdez spill in 1989 and run by a former BP executive.

With BP’s blow-out still gushing up to 60,000 barrels per day, the Gulf clean-up effort may drag on for months or years, even if BP can plug its blown-out well in August as planned.

Following the Valdez spill, offshore skimming vessels remained in action for more than a year.

“If you don’t have the equipment to respond to a spill, you can’t be allowed to drill,” said Dan Lawn, a former oil safety inspector for Alaska’s Department of Environmental Conservation.

“The contingency plans should be revoked because they are worthless right now.”

The U.S. Coast Guard, charged with overseeing offshore spill response, did not respond to requests for comment.

U.S. District Judge Martin Feldman of New Orleans last month struck down the Obama administration’s first moratorium issued in May -- which halted drilling in waters deeper than 500 feet for six months. Feldman ruled it “arbitrary and capricious.”

While any ban is controversial since U.S. Gulf oil projects account for a third of the country’s oil production and thousands of jobs, Salazar pledged to press ahead regardless.

Grilled on Wednesday by a Congressman who said a ban would bring irreparable harm to Louisiana’s offshore industry, Salazar said: “The greater irreparable harm would be if there was another blowout, when there is not the oil response capability to even deal with the current Horizon event.”

FEW SKIMMERS IN RESERVE

More than 7,000 U.S. Gulf-based spill response vessels -- including skimming units -- and around 50,000 people are involved in the Horizon cleanup, the largest and most complex spill response ever. The vessels have recovered more than 28 million gallons of oil-water mixture so far.

The deployment of skimmers at BP’s spill has expanded more than fivefold since early June, and 550 skimmers were at work as of Friday, according to a release from spill responders. They expected 750 skimmers in action by August.

In preparation for offshore disasters, scores of skimming vessels are usually kept at staging areas in the Gulf Coast, but few are idle now.

BP’s Gulf contingency plans call for racing skimmers to a Gulf spill in as little as 6.5 hours.

A contingency plan for Shell, the No. 2 Gulf producer, shows it could race 24 skimmers with capacity to suck up 162,752 of oil per day to a potential blow-out. At least 16 of the skimmers, and all of the largest ones, are engaged. Those that may still be available could collect less than 25,818 barrels, the vessel lists showed.

Shell declined comment on its Gulf contingency plans.

A person close to the company said Shell’s contingency plan for the Gulf envisages an unlikely scenario with multiple spills. There is a “considerable amount” of safety equipment available still available, the person said, and Shell would still rely on a Gulf-based fleet of skimmers.

In addition, Shell could quickly import more boom, dispersant and other safety gear from Europe, the person said.

To be sure, skimmers are no silver bullet. They often collect less than 20 percent of oil that reaches the sea surface, experts say. But used with barges, tugs, absorbent booms and dispersants, skimmers play a major part in keeping oil from fouling beaches, especially if they are deployed fast.

Some support is already arriving from abroad.

One Taiwanese vessel that arrived this week, the so-called A Whale, is a converted supertanker with capacity to process up to 500,000 barrels per day of oil and water mix. It may gain Coast Guard approval to operate this weekend.

But the bulk of the world’s offshore skimmers are on standby for spill responses elsewhere. Only 15 foreign response vessels were at work on the Gulf spill last week.

And even the A Whale is unlikely to free up other U.S. skimmers from their ongoing work, since an aggressive spill response requires up to hundreds of agile skimmers to cover the rapidly-expanding area of oil slicks, experts say.

BP’s spill is no longer a single slick but a “massive collection of smaller patches of oil,” response commanders wrote on Friday.

STORM RISK

Hurricanes bring more risk for oil companies in the Gulf, often requiring the deployment of skimmers after they pass.

In 2005, Katrina ravaged the region, laying waste to several drilling platforms and causing spills of at least 6.5 million gallons, more than half of the Valdez spill volume.

After the Valdez disaster, MSRC deployed seven of its largest, ‘responder-class’ skimmers during the 2005 storm season.

Today, 12 of the firm’s 15 responder-class vessels are dedicated to BP’s spill, MSRC spokeswoman Judith Roos said. The firm also operates dozens of smaller skimmers.

“Should another event occur, the Coast Guard has the authority to determine where to direct our resources,” Roos said.

Shell, BP and Exxon Mobil’s contingency plans all call for relying heavily on MSRC skimmers.

Employees at National Response Corp. and Ampol, the two other response firms listed in all three companies’ Gulf response plans, told Reuters they have deployed more than 90 percent of the equipment they had available in the Gulf.