Sensitivity testing for improved efficiency of a statistical oil-spill risk analysis model

The US Minerals Management Service performs oil-spill risk analyses using, in part, a statistical model of hypothetical oil-spill trajectories. The “Oil-spill Risk Analysis” (OSRA) model is driven by analyzed sea surface winds and model-generated ocean surface currents. The OSRA model calculates thousands of oil-spill trajectories over extended areas of US waters and tabulates the frequencies with which the simulated oil spills contact the geographic boundaries of designated natural resources within a specified number of days after the simulated spill events are initiated. The limits of computer mass storage and speed impose constraints on the number of oil-spill trajectory simulations and the time step of the numerical integration generating the spill trajectories. Also, the model winds and ocean currents have limited spatial and temporal resolution, producing sensible limits on the spatial density of the simulated oil spills and on the numerical integration time step. This investigation attempted to determine the smallest practical spatial interval between simulated oil spill release sites and the shortest integration time step beyond which the frequencies of oil-spill contact essentially do not change. In addition, we investigated the number of hypothetical oil spills initiated per day from a single location and two methods of numerical integration to generate the spill trajectories—simple forward time stepping and the fourth order Runge–Kutta method. Applying the OSRA model to the Gulf of Mexico using 9 years of winds and ocean current data, we found a smallest distance between simulated spills of 6 nautical miles and a shortest integration time step of 1 h using the fourth order Runge–Kutta integration method. A 20-min time step would be needed with simple forward time stepping. Initiating hypothetical spills eight times per day produced essentially the same contact probabilities as initiating spills once per day. These intervals were practical for OSRA model runs over the US portion of the Gulf of Mexico using commonly available desktop personal computers.