Now that OPEC has agreed to raise its crude oil production quotas in hopes of taming high and jittery oil prices, industry experts are growing more concerned about both the capacity and the security of oil tankers, the next link in the supply chain.

The world’s tanker fleet is already stretched thin by robust demand for oil, by looming deadlines for the phase-out of single-hull tankers for safety and environmental reasons, and by lengthening backlogs at the shipyards where new tankers are built. It is far from clear, experts say, whether the existing fleet can handle the new production that Saudi Arabia and others have promised in coming months.

“There is just barely enough shipping capacity at these high production levels,” said Jeffrey Goetz, head of marine projects and consulting at Poten & Partners, a New York-based energy and ocean transport broker.

Charter rates for tankers, which can be even more volatile than oil prices, have been driven up in recent weeks by the tight market. Shipping costs may now add $3 a barrel to the price of oil delivered to the United States from the Middle East, up from about $2 earlier this year, analysts said.

Rates are likely to rise even further if Saudi Arabia steps up its production as much as Saudi officials say it could – by 1 million barrels a day, to about 10 million. Experts say much of that new oil may back up in storage tanks in Saudi Arabia waiting for scarce tanker space.

Tanker security is also a growing issue. Tankers and tanker-loading facilities have already been the targets of attacks by Al Qaeda and other anti-Western groups, threatening to disrupt oil supplies and adding to the upward pressure on prices, maritime insurance premiums and tanker charter rates.

Complicating the picture, new security regulations are scheduled to take effect July 1 for all sizable commercial vessels ships calling at American ports. Among other measures, the new rules require ship operators to make their engine rooms more secure and to demand identification from anyone who comes on board a tanker. Most major tanker owners will probably be in compliance in time, shipping experts say, but some smaller operators may not make the deadline. If enforcement of the new rules leaves some tankers excluded from delivering to the United States, it may aggravate the scarcity.

“At no other time in history have all these factors gone in one way, to make the market this tight,” said Dragos Rauta, technical director of Intertanko, a trade association for tanker owners.

Some experts say that an oil tanker makes an attractive target. “It ticks all the terrorists’ boxes by attracting publicity and raising economic concerns,” said James Wilkes, a director of Gray Page, a London security consultant for the shipping industry.

In October 2002, a small boat filled with explosives came up next to the French tanker Limburg off Yemen and detonated, blowing a hole in the hull. Almost 100,000 barrels of crude oil spilled out and burned. The attack, which killed a member of the Limburg’s crew as well as whoever was on the small boat, was attributed to Al Qaeda.

The attack on the Limburg did not produce the kind of spectacular explosion conjured by popular imagination, and shipping industry experts said it would be extremely difficult for an attacker to get a tanker to blow up that way; even direct missile hits during the Iran-Iraq war generally resulted only in spills and containable fires.

But an attack or accident that crippled even one big oil tanker could have an appreciable effect on the flow of crude, experts say, and the new security regulations do nothing to address a Limburg-style attack. “In the United States, there are 13 million registered pleasure boats alone,” Mr. Rauta of Intertanko said. “Unfortunately, the security legislation has limitations in the practical sense.”

Though there are more than 3,600 tankers in service in the world, about a third of the world’s oil supply is transported by just 435 of them, the towering 2-million-barrel tankers known prosaically in the industry as very large crude carriers, or V.L.C.C.’s. These ships are almost always completely booked, and new ones are slow to reach the market. The shipyards that can build them all have deep backlogs of orders for tankers and other vessels, so a new V.L.C.C. ordered today would probably not be delivered until late 2007 or early 2008 at the soonest.

Tankers of the next largest size, known as Suez Max tankers because they barely fit through the Suez Canal, are also booked close to capacity.

“Going into the fourth quarter, we could see a shortage of tankers” if oil production rises by the forecast two million barrels a day worldwide, said Magnus Fyhr, a Houston-based shipping analyst at Jefferies & Company.

Shippers may be forced to use smaller ships, or keep older single-hull tankers in service longer than planned. After the 1989 Exxon Valdez disaster in Alaska and several major spills in European waters in the 1990’s, both the European Union and the United States adopted rules calling for tankers with single-walled hulls to be phased out in favor of a double-hull wall with about six feet of air space in between, making the oil storage tanks much less likely to be punctured in an accident.

Fleet operators have been actively scrapping older single-hull ships as new double-hulls become available, but even so, about a third of large-scale oil deliveries worldwide this year will still be made by single-hulled vessels, and the looming shortage of tanker capacity may lead operators to extend their service lives.

“The single-hulled tanker is like the fat lady at the dance,” said Frank Gallagher, director of Galbraith’s, a ship brokerage and consulting firm based in London. When plenty of ships are available, most customers prefer to charter a double-hulled ship, Mr. Gallagher said, but when ships are scarce, as they are now, customers are more willing to settle for a single hull.

Provided, that is, that single-hulled tankers are still allowed to dock at the desired ports: some countries and ports have already banned them outright. The International Maritime Organization in London decided in December to move up the deadline schedule for the switchover to double hulls, with nearly all single-hulled tankers to be retired by 2010.

Two recent accidents in the Suez Canal highlighted the reason for the change, Mr. Gallagher said. In both cases, double-hulled vessels ran aground, doing some damage to their outer hulls and delaying delivery of their cargoes but not causing larger problems. Had they been single-hulled tankers, he said, there would have been crude oil spills, and the canal would have been closed to clean up the mess.

“If you go anywhere crowded with single-hulled ships,” Mr. Gallagher said, “they can be dangerous.”