ESTIMATING THE RESPONSE GAP FOR TWO OPERATING AREAS IN PRINCE WILLIAM SOUND, ALASKA

Abstract

ABSTRACT Technological advancements in oil spill response systems have contributed to more proficient oil spill response operations. Yet, there are still times when oil is being shipped but environmental conditions, such as wind, waves, temperature, and visibility, preclude effective spill response operations. The Response Gap is this window between the point of maximum mechanical response capacity and the weather-based limits of oil transportation. To quantify the Response Gap for two operating areas in Prince William Sound (PWS), Alaska, historical datasets of the environmental factors known to affect the open-water mechanical response system were assembled. Each dataset contained observations related to four environmental factors: wind, sea state, temperature, and visibility. These datasets were used in a “hind-cast” to evaluate how often environmental conditions exceed the response operating limits. Response operating limits were determined based on a thorough review of the published literature, existing contingency plans, regulatory standards, and after-action reports, with the objective of establishing realistic limits for the existing open-water response system. Response limits were then coded using the colors red (response not possible), yellow (response possible but impaired), and green (response possible) for a particular environmental factor during each operational period. A Response Gap Index (RGI) was calculated to incorporate the interactions between environmental factors. Once the RGI was computed for each observational period, the dataset was summarized to produce an estimate of the amount of time that the Response Gap existed. The met-ocean climatology is characterized using histograms and joint-probability distribution plots, with the RGI superimposed. At Hinchinbrook Entrance, sea state exceeded the operating limits 19.2% of the time and wind exceeded the limits 2.9% of the time. When the environmental factors were considered together, the response limitations were exceeded 37.7% of the time. Not surprisingly, the response limits were exceeded more often in winter (65.4% of the time) than in summer (15.6% of the time). Results for Central PWS indicated that the response limitations were exceeded only 12.6% of the time. The paper discusses ways to improve the present subjective quantification of response limits, particularly through additional field trials and modeling of mechanical recovery systems.