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.

Read more