Results are presented on the factors relating to the dissipation of oil spills at sea, including evaporation, emulsion formation, spreading, and natural dispersion into the water column. For Ekofisk oil, 20% evaporates in about 7.5 hours and, while emulsion formation is as rapid as for Kuwait crude, the resulting viscosity is low and insufficient to allow interference with the natural spreading and dispersion rates. Spreading has two components. One is controlled by surface tension-viscous drag forces and the other is wind-induced. Together they contribute to the two dimensional dissipation of the oil so that subsequent oil concentrations in the sea are of necessity, low. These concentrations were measured for naturally dispersing and chemically dispersed slicks. The chemically-dispersed slicks were of two kinds. One was previously weathered for three hours, the other was of controlled thickness and was dispersed immediately upon being laid. Resulting concentrations of oil in the sea are low and of short duration compared with those required to give observable effects in laboratory toxicity studies. No significant deleterious effects were found to result from the dispersion of oil slicks at sea using low toxicity dispersant chemicals; also it was noted that, in any case, substantial quantities of oil can be expected to enter the sea before oil recovery operations can be mounted.

This paper examines the costs of various cleanup techniques, drawing mainly on information from recent oil spills from ships. Analyses of the costs for dispersant application and for the use of offshore mechanical recovery equipment are compared in relation to their effectiveness and scope for reducing shoreline cleanup costs and damage from oil pollution. Spill size, oil type, coastline character, and the extent of pollution all contribute to the observed differences in shoreline cleanup and disposal costs. The influence of these factors is evaluated and criteria are proposed for selecting the most appropriate cleanup response to suit the circumstancees of a particular oil spill. On occasions, a response at sea is both necessary and worthwhile, but for most spills shoreline cleanup, including protection of sensitive resources, is likely to be the most cost-effective option.

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Laboratory tests can demonstrate the effectiveness of dispersants relatively easily but it is far more difficult to assess effectiveness in field conditions. In many oil spills, timely application of dispersants is the best approach. It is therefore necessary to study their use in field trials and actual incidents to see what lessons can be learned about the amounts used relative to the amount of oil spilled, types of oil on which dispersants are relatively effective, methods of application, the period after release into the sea during which dispersants remain effective, and the influence of sea conditions and temperatures. This paper discusses these questions, reviewing published data on the performance of dispersants infield trials and in actual oil spills in which staff of the International Tanker Owners Pollution Federation Ltd. have been involved in recent years. Recommendations are made regarding further work in the laboratory and field that appears necessary to determine the conditions under which dispersants are likely to be most effective.

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