Brussels, Belgium ? They’re derided as “environmental timebombs” and “floating garbage dumps.” Yet more than half the world’s 10,000 oil tankers are the old-style, single-hulled variety despite outcries after every disastrous spill, from the 1989 Exxon Valdez in pristine Alaska to this week’s sinking of the Prestige off the verdant coast of Spain.

A U.N. treaty banning single-hulled tankers entered into force this year ” but the phase-in period stretches to 2015.

Until then, European Union officials say their efforts to impose stricter inspections are being subverted by shipowners who steer clear of EU harbors or avoid dropping anchor when they refuel or pick up supplies. Yet oil they spill can wash ashore anyway ” as the cleanup crews scooping sludge from Spanish beaches Wednesday can attest.

“These vessels now avoid European ports because they know it’s risky for them,” said Gilles Gantelet, spokesman for EU Transport Commissioner Loyola de Palacio.

Authorities in Rotterdam, Netherlands, which has the world’s busiest port, say they have the same impression.

“It’s quite obvious that older vessels, those with very bad maintenance, do not enter the port of Rotterdam because the risks are too big for them,” said Minco van Heezen of the Rotterdam Port Authority.

Those that don’t pass muster can be fined or even “chained up for a long time,” he said.

The number of dockings ” and repair work ” done by Dutch shipbuilders has declined over the past few years, said Ruud Schouten of the Netherlands Shipbuilding Industry Assn.

“It’s the same in other west European countries,” he said. “Regulations are tougher here than some other parts of the world, (so) if you have a vessel which is not up to the standards, then it’s better to go elsewhere.”

As business declines in western Europe, work is migrating to low-wage countries in eastern Europe, Asia or on the Arabian peninsula, he said.

The Prestige was loaded in St. Petersburg, Russia, and Latvia and was en route to Singapore when it ruptured in stormy weather last week. It split in two and sank Tuesday, about 150 miles from Spain.

According to the American Bureau of Shipping, which validates a ship’s seaworthiness, the tanker’s last annual inspection was done in May in Dubai, United Arab Emirates. Its last detailed inspection ” done in dry dock in China ” was in May 2001, said Stewart Wade, vice president of the Houston-based agency.

“At the time of this incident, the Prestige was fully in compliance with all of our requirements,” he said.

The International Maritime Organization, a U.N. agency whose motto is “safer shipping, cleaner oceans,” has no information about “ships avoiding particular ports,” said spokesman Lee Adamson in London.

“That’s not to say it’s not happening,” he added.

But he said inspections are carried out under the auspices of the country whose flag the ship is flying. “The owner wouldn’t just choose,” he said. “The flag state authority would have to give its approval.”

The Prestige was owned by a Liberian company but registered in the Bahamas, a so-called “flag of convenience” known as a tax haven. Adamson said his agency has no data on the safety records of ships registered there.