「防漏」多一層 美海域禁駛單殼油輪

As of January 1, 2015 single-hull oil tankers are no longer allowed in U.S. waters.

The Oil Pollution Act of 1990, passed after the disastrous 1989 Exxon Valdez oil spill in Prince William Sound, Alaska, required that all new tankers and tank-barges be built with double hulls to prevent similar oil spills.

For the first 25 years after the law was passed, single-hull tankers were still allowed to operate, but now those tankers are at the end of their operational lives and can no longer carry oil as cargo in U.S. waters.

The National Oceanic and Atmospheric Administration, NOAA, says the double-hull requirement was phased in gradually “because of the difficulty of converting existing single-hull tankers to double hulls, and retiring the single-hull tankers more rapidly would have been a major disruption to world shipping.”

The January 1, 2015 deadline did not bring about a major change because most of the tankers calling on U.S. ports have had double hulls for years.

In some collisions involving these double-hull tankers, not a drop of oil was spilled.

Twenty years after the Exxon Valdez oil spill, for instance, the Norwegian tanker SKS Satilla collided with a submerged oil rig in the Gulf of Mexico. The collision tore a huge hole in the tanker’s side, but none of the 41 million gallons of crude oil on board was spilled.

But single-hull tankers have spilled oil in the years since the Oil Pollution Act became law.

In 2004, the Cyprus-registered tanker Athos I struck a submerged anchor in the Delaware River while preparing to dock at a refinery in Paulsboro, New Jersey. The spill of 265,000 gallons of heavy crude oil fouled the river near Philadelphia, Pennsylvania.

For vessels with a single hull, one plate of steel is all that separates the oil on board from the ocean. If the hull is punctured from a collision or grounding, an oil spill is likely to occur.

A ship with a double hull has two plates of steel with empty space between them, essentially a hull within a hull. The second hull creates a buffer zone between the ocean and the oily cargo.

Although they are likely to prevent oil spills, the double hull requirements apply only to tankers and tank barges. Container ships, freighters, cruise ships, and other types of vessels are still built with single hulls. While these ships carry a lot less oil than a tanker, a large non-tank vessel can still carry a lot of fuel oil, and some have caused some big spills.

For instance, when the single-hull cargo ship Cosco Busan, operated by Fleet Management Ltd of Hong Kong, struck the San Francisco-Oakland Bay Bridge in November 2007 it spilled 53,569 gallons of bunker fuel oil into the San Francisco Bay.

The spill fouled beaches from the Marin Headlands to San Mateo County, killed nearly 7,000 birds and wiped out as much as 29 percent of the spawning herring that winter.

Double hulls don’t prevent all oil spills from tankers, NOAA acknowledges, but the design has been credited with reducing the amount of oil spilled, especially during low-speed groundings and collisions.

In 1992, two years after the Oil Pollution Act, the International Convention for the Prevention of Pollution from Ships, called the MARPOL Convention, was amended to require all newly built tankers have double hulls.