The Mixing of Oil Spills Into the Sea by Breaking Waves
Authors: Naess A. 
Published: December 2007

An important pollution-control problem on the high seas is the mixing of an oil spill into the sea caused by breaking waves. This paper presents a theoretical model for determining the amount of oil mixed into the sea by breaking waves. The model is based primarily on probabilistic methods, with experimental observations and data as the foundation. Numerical examples are given.

Introduction

Due to the strong increase in activity on the Norwegian continental shelf over the past few years and the ensuing danger of a major oil spill accident, national research into the different aspects of removal and containment of oil spills under adverse weather conditions has experienced a concurrent rapid growth.The work presented in this paper is part of a research program being carried out at The Ship and ocean Laboratory (SOL) in Trondheim to gain sufficient knowledge to give proper design criteria for oceangoing oil containment barriers and removal equipment.A problem intimately connected with this is the assessment of the oil distribution with depth under given weather conditions. The primary cause for this vertical distribution of oil is to be found in the presence of breaking waves, which lead to a mixing presence of breaking waves, which lead to a mixing of the oil into the water. This, of course, significantly affects the ability of a mechanical oil boom and/or skimmer to remove oil spills from the sea, and it may well prove to represent the ultimate barrier to such efforts, as they all require the oil to be concentrated near or on the water surface. The importance of this problem has made it highly desirable to try to find problem has made it highly desirable to try to find methods of estimating the vertical distribution of oil in the sea as a function of environmental conditions.At the time we started to study the mixing of oil into the water column, very little work had been done or reported in the literature on this problem. However, we should mention the work by S. Leibovich. He seems to have been the first to attempt to calculate the vertical dispersion of an oil slick into the water column, and his paper served as an inspiration for this work even though we disagreed with his line of attack. In working with this problem we soon realized that if any progress was to be made there were quite a few gaps in the existing knowledge that needed immediate attention. In an attempt to fill in some of the most conspicuous ones, a series of experiments on the effect of breaking waves on an oil slick were performed in one of the wave tanks at SOL.Of particular concern were the effective mixing depths induced by breaking waves passing the oil slick and an estimate of the size distribution of oil droplets formed by the breakup of the oil slick due to the strong mixing created by the passing breaker. Both these factors will influence significantly the expected amount of oil at a certain depth below the surface. During these experiments it was observed that the time constant of the mixing process induced by a breaking wave generally was much smaller than the time periods to be expected between the passing of breaking waves.The mixing mechanism we have described is not the only one present.

JPT

P. 1113