Oil-Spill Risk Analysis for Assessing Environmental Impacts

Abstract

ABSTRACT The Minerals Management Service (MMS), an agency of the U.S. Department of the Interior, maintains a leasing program for commercial oil and gas development on the Outer Continental Shelf in U.S. territorial waters. The MMS performs an oil-spill risk analysis (OSRA) using, in part, a statistical model of hypothetical oil-spill trajectories. The OSRA Model is driven by analyzed sea surface winds and model-generated ocean surface currents. Instead of focusing on individual oil-spill events, the OSRA examines oil-spill risks over long periods of time, ranging from 5 years to decades. The OSRA Model calculates thousands of oil-spill trajectories over extended areas of the U. S. continental shelf 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. A key element of OSRA Model runs is the particle trajectory simulation based on wind velocities and surface ocean currents. It is critical that the trajectory model has a numerical scheme that is stable, accurate, and efficient. Currently, two numerical schemes are incorporated into the model: the Eulerian scheme and the 4th order Runge-Kutta scheme. To test the numerical schemes, a set of analytical solutions to the Navier-Stokes equations is developed. The analytical velocities and particle trajectories are compared with the numerical solutions from the two numerical schemes. The stability and efficiency of the two schemes are discussed. Applying the model to the Gulf of Mexico using 9 years of winds and ocean currents, we find that the OSRA Model with the 4th order Runge-Kutta scheme is much more efficient and able to better represent the circulation patterns and particle movements in the Gulf of Mexico.