Prestige drifts in two pieces off the coast of Spain before sinking with most of its cargo. The accident prompted Spain and France to ban single-hull tankers within 200 miles of their coasts.
The 26-year-old, single-hull Prestige split in two and sank, spilling as much as 20,000 metric tons of heavy fuel oil and taking the rest of its 77,000 tons of cargo to the bottom. The catastrophe occurred even though the ship had undergone an extensive examination as part of its five-year special survey in April 2001.
“From our point of view, that this vessel incurred some sort of structural damage 18 months after the survey is a cause of great concern,” said Stewart Wade, a spokesman for the American Bureau of Shipping, the Houston-based classification society that surveyed the vessel.
The disaster also triggered a call by European officials to ban single-hull oil tankers before the 2012 deadline already agreed to by the European Union. The EU’s director general in charge of transport, Francois Lamoureux, said that individual European states should unilaterally ban single-hull oil tankers as a response to the disaster.
Spain and France have already declared that older tankers with single hulls can be banned from their 200-mile exclusive economic zones. On Nov. 30, the Spanish navy escorted a Maltese-flagged tanker out of the 200-mile zone.
Spain also submitted a list of proposals to the annual council meeting of the International Maritime Organization, which began Nov. 25, including the elimination of a transition period for the switch to double-hull tankers. Spain also wants ships that already show deficiencies to be subject to required inspections.
Prestige sank about 135 miles off the coast of northern Spain and Portugal in water 11,800 feet deep.
Before going down, it spilled between 10,000 and 20,000 metric tons of heavy fuel oil, which polluted at least 125 miles of the coast of Galicia, Spain.
As part of the IMO’s civil liability convention, the ship’s owner is liable for damages of up to $80 million. If additional compensation is needed, the International Oil Pollution Compensation Funds, maintained by the oil companies, would pay up to $86 million.
Prestige was a Bahamian-flagged vessel, registered in Liberia, owned and operated by Greek companies and chartered by the Russian-Swiss firm Crown Resources. In its 26-year life, it had two owners. The ship’s last European Port State Control inspection was on Sept. 1, 1999. The inspection revealed two deficiencies involving lifesaving and one general deficiency, but the ship was not detained, according to the ABS.
Prestige’s only previous casualty was a fouled propeller in 1991.
The tanker was on a voyage from Latvia to Singapore when it ran into trouble during a force-10 storm off Cape Finisterre, Spain. On Nov. 13, the vessel appeared to strike an object. There was a shudder, and the ship took on a 25Â° list, according to Wade.
The Dutch salvage company Smit worked to save the vessel and asked the Spanish and Portuguese governments to designate a port of refuge where the ship could be taken to unload the remaining oil. When both governments refused, Smit towed the vessel offshore, where it broke in two and sank.
As a single-hull tanker built in 1976, Prestige would have been scrapped by 2005, according to revisions the IMO made to Marpol (the International Convention for the Prevention of Pollution from Ships) in April 2001. The IMO decided to mandate a staggered phaseout of most single-hull tankers by 2015. The European Union calls for the elimination of single-hull tankers between 2005 and 2012.
In the wake of the Exxon Valdez disaster in 1989, the United States unilaterally banned all single-hull tankers; the ban was phased in starting in 1995 and is tied to the age of the vessel, with all single-hull tankers prohibited by 2010.
Despite the call for the quick banning of all single-hull tankers in Europe, American naval engineers said that the underlying issues are so complicated that such a ban might not be the solution. Certainly double-hulled tankers are safer in the case of groundings, but it’s not clear double-hull rules would have prevented what happened to Prestige.
“Where a ship ultimately breaks in half, the presence of a double hull doesn’t do you much good,” said Robert Levine, manager of new construction for ConocoPhillips marine, which owns oil tankers that serve in the Alaska trade.
It is still unclear what caused the sinking of Prestige. The disaster is being investigated by the Bahamas Maritime Authority, with assistance from the ABS. Crewmembers claimed Prestige struck a submerged object, and then they felt a severe shudder in the ship. The empty No. 2 and 3 starboard wing tanks flooded, and the vessel took on a 25Â° starboard list, according to the ABS.
None of the 27-member crew was injured. Spanish authorities jailed the vessel’s captain, claiming he failed to cooperate with the salvage company.
Prestige developed a 30- to 50-foot crack in the starboard side. “It was some sort of significant damage that took place that resulted in the immediate flooding of those two spaces,” Wade said. A crack that developed over time would not flood the two tanks “in a catastrophic manner such as that,” he said.
“It was very unusual,” Wade continued. “How did this happen? From a structural point of view, this has puzzled us.”
The ABS has asked the International Association of Classification Societies to conduct an audit of all ABS records on Prestige to help determine the cause of the starboard crack. ABS also asked the IMO, the European Commission, the Bahamas Maritime Authority and Intertanko to take part in the audit.
In April 2001, Prestige underwent its five-year special survey in Guangzhou, China, according to the ABS. During the survey, conducted with the help of two ABS surveyors, corrosion was discovered in the No. 3 side-ballast/cargo tanks on both the port and starboard sides.
Repairs were done at the Guangzhou Cosco Shipyard, where 363 tons of steel were replaced. Major repairs were done to the No. 3 side-ballast/cargo tanks (port and starboard), concentrating on the transverse bulkheads and the frames at the upper levels in the tank, according to an ABS release.
During this survey, the side shell plates in side tanks 2 and 3 were particularly examined. No repair was done to the side shell plates because thickness measurements on the plates showed wastage of less than 10 percent, according to the ABS. Wastage in the starboard shell longitudinals was found to be between 10 and 15 percent in the starboard side-ballast/cargo tank No. 3. The repairs were checked and tested by ABS surveyors.
The No. 2 starboard side tank was used only for ballast and had an anti-corrosion coating on it, Wade said. The No. 3 starboard side tank was used for both ballast and cargo, and was not coated.
Although American engineers said they could not speculate on the causes of Prestige’s sinking, they said that older tankers are not necessarily more likely to have structural problems. But detecting corrosion and metal fatigue in older vessels is challenging and complicated.
“Inspecting for corrosion and finding fatigue cracks, and so on, is a very difficult task,” said Bilal Ayyub, an engineering professor at the University of Maryland and a member of the Society of Naval Architects and Marine Engineers’ hull structure committee. “It is a very tedious process. You can have a crack which is several inches in length and wide open, and people might not see it.”
It’s also difficult to detect whether there is corrosion underneath coated tanks, according to naval engineers. And how the repairs are done is critical.
“In some ships, if there is a damaged plate, the decision on whether to repair it or not is highly subjective,” Ayyub said.
It is extremely important where the cut in the plate is made, how the repair is made, how it is welded and where it is attached to the rest of the ship. And sometimes these repairs are made by workers at the shipyard, who may not understand the subtleties of the repair without consulting engineers.
A double-hull tanker is not necessarily more resistant to corrosion. And these ships might be more difficult to survey. “By having a double hull, you end up creating compartments that are difficult to inspect,” Ayyub said.
A leak from the inner shell can occur, allowing oil to become trapped between the two shells. “And this itself can become a hazard,” Ayyub said. The presence of salty water and oil enhance the chances of corrosion.
The larger issue relates to the maintenance of older vessels. “The strength and seaworthiness of the ship is simply a function of how people take care of it,” Levine said. And because there are so many variables that enter into the maintenance of a ship, it is impossible, at this point, to blame the sinking of Prestige entirely on its age.
“We don’t know what happened to her,” said Keith Michel, president of Herbert Engineering Corp., a California firm that specializes in ship design. “We don’t have any particular reason to believe that this was related to the fact that the Prestige was an older vessel. A well-maintained ship can go 30 years.”
Unfortunately, effective maintenance often comes down to economics. “Most of the time, repair and maintenance do not receive the same amount of attention as the initial design,” Ayyub said.
Overall, Levine also believes that the major class societies — the ABS, Det Norske Veritas and Lloyd’s Register of Shipping — are complicating the issues surrounding effective surveys by also selling other products and services.
“The regulatory agencies have put business ahead of being regulatory agencies,” Levine said. “They’re selling engineering services, then approving the same engineering services in a different division. That’s the thing that got Arthur Andersen in trouble. To me, regulatory agencies should be focusing on what they do.”