The promulgation of OPA 90 was in large part a response to public concern over the 1989 Exxon Valdez incident, in which more than 11 million gallons of crude oil were spilled into Alaskan waters. Such incidents involving spillage of more than a million gallons of oil have dominated spill statistics over the past two decades and have focused public attention around the world on the potential hazards of oil spills from large tankers. Compared to earlier five-year periods, there was a decline in the quantity of oil spilled from vessels in U.S. waters³ in the period 1991 to 1995, as well as a reduction in the number of spills of more than 100 gallons. In particular, there were no oil spills of greater than a million gallons from tank vessels in U.S. waters. Between 1991 and 1995, tankers accounted for only about 10 percent of the total oil spilled from vessels in U.S. waters. In contrast, inland and oceangoing barges together accounted for approximately half the total spillage from vessels and were involved in the majority of oil spills in U.S. waters during this period.

The reduction in oil pollution in U.S. waters between 1991 and 1995 cannot be attributed to the requirements of Section 4115, notably the double-hull mandate and the operational and structural requirements aimed at reducing the outflow of oil following incidents that involve single-hull tank vessels. The first compulsory retirements of single-hull vessels did not occur until 1995, and the final rules on operational and structural requirements were not issued until July 1996 and January 1997, respectively. Thus, the timing of actions relating to Section 4115 precludes the possibility that they had a significant impact on oil spills in U.S. waters between 1991 and 1995. Nonetheless, the committee's analytical comparison of double-hull and single-hull designs indicates that properly designed double hulls are potentially more effective than single hulls in preventing and mitigating oil outflow after a vessel casualty. As discussed later, some double-hull vessels (mostly less than 160,000 DWT [deadweight tons]) currently operating—specifically those without longitudinal subdivision through the cargo tanks—will not provide the enhanced environmental protection in all accident scenarios that would be provided by properly designed double hulls.

In the view of the committee, the reduction in oil spillage in U.S. waters between 1991 and 1995 was the result of a number of actions that are in process or emerging, notably: an increased awareness among vessel owners and operators of the financial consequences of oil spills and a resulting increase in attention to policies and procedures aimed at eliminating vessel accidents; actions by port states to ensure the safety of vessels using their ports; increased efforts by ship classification societies to ensure that vessels under their classification meet or exceed existing requirements; improved audit and inspection programs by