Spreading and Transport Of Oil Slicks on the Open Ocean

Abstract

Abstract An analysis is made of the spreading and transport of oil slicks on the open ocean. Included in the analysis are the spreading of oil spills through the mechanisms of gravity and surface tension, the effect of viscous retardation, and transport due to combined effects of waves, winds, and currents. The role of turbulent diffusion as a transport mechanism is assessed. Analytical results for fixed and moving oil leaks are derived. A method is derived for the tracing of the path of a slick under the influence of transport mechanisms which vary both in direction and magnitude over a period of time. With this method, the configuration of the Chevron slick (March 1970) is estimated using the available data on wind and current transport, and general agreement with on-site observations is obtained. 1. Introduction It is a common experience that crude oil and other petroleum products when spilled on the ocean will rapidly spread over the surface and in time contaminate large areas. The actual process is one of considerable complexity, and analytical models which have been recently developed to describe the spreading of oil slicks are applicable only under very special conditions. The purpose of this paper is to review the analytical work which is presently available and to suggest how this work can be extended and applied to make predictions of oil slick spreading and transport under realistic conditions. We distinguish here between the spreading of oil due to its internal driving forces and the transport due to environmental mechanisms of winds, waves, currents, and water turbulence. Spreading in this sense is primarily a function of certain properties of the oil, such as the density, viscosity, and surface tension relative to water. Transport, on the other hand, depends less on the properties of the contaminant, and can to a rough approximation be estimated if we assume that the slick behaves as if it were surface water. Of course if the source itself is moving, this must be taken into account along with other transport mechanisms. In the open ocean when the natural level of turbulence in the surface layer is small, we associate spreading with a change in slick area, while transport is associated with a change in the position of the slick (or more precisely, its centroid). The range of surface contamination is the result of a complex and dynamic interaction between the spreading and transport mechanisms. Ocean turbulence and also atmospheric turbulence of a certain intensity and scale can be presumed to act primarily to change (increase) the area of the slick in contrast with the other external driving forces. It appears, however, that calculations of the effect of surface turbulence are open to question at the present time. In what follows, we will review the existing knowledge in this field, with emphasis on more recent contributions. Analytical approximations for spreading and transport will be presented. The problem of superposition of spreading and transport modes will be considered, and the interaction between the various mechanisms will be discussed.