Derivation of spreading parameters of oil at sea for oil spill modeling

Oil spill models are commonly used to simulate the large-scale (tens to hundreds of kilometers) transport of oil spills in the oceans. The values of the spreading parameters of these models are obtained empirically by fitting to observed slicks, thus they do not account explicitly for the effects of waves. In addition, there is little success in using these values to predict the spread at smaller scales (tens of meters to a few kilometers). This works attempts to better understand the physics of oil movement in the ocean by focusing on the small-scale mechanisms. The investigation also leads to evaluation of small-scale spreading parameters. The Random Walk Method is used in a Monte Carlo simulation framework to track the transport of oil due to the effects of waves, buoyancy, and turbulent diffusion. The small-scale spreading parameters are then calculated using the Method of Moments. Our results indicated that the approach for using a spreading coefficient becomes after a time equal to about 30 wave periods. This corresponds to a travel of the centroid of about two wave lengths. At larger scales, the longitudinal spreading coefficient increased with distance from the initial location and the lateral spreading coefficient became equal to the turbulent diffusion coefficient. The vertical spreading coefficient reached a value that is smaller than half of the turbulent diffusion coefficient, which is due to the presence of the upper boundary (the free surface) and buoyancy.