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Introduction: The purpose of any oil spill response is to minimise the damage that could be caused by the spill. Dispersants are one of the limited number of practical responses that are available to respond to oil spills at sea.When oil is spilled at sea, a small proportion will be naturally dispersed by the mixing action caused by waves. This process can be slow and proceed to only a limited extent for most situations. Dispersants are used to accelerate the removal of oil from the surface of the sea by greatly enhancing the rate of natural dispersion of oil and thus prevent it from coming ashore. Dispersed oil will also be more rapidly biodegraded by naturally occurring microorganisms. The rationale for dispersant use is that dispersed oil is likely to have less overall environmental impact than oil that persists on the surface of the sea, drifts and eventually contaminates the shoreline. The development of modern dispersants began after the Torrey Canyon oil spill in 1967. Many lessons have been learned since that spill, and consequently the modern dispersants and application techniques in use today have become an effective way of responding to an oil spill. For example, the dispersant response to the Sea Empress spill in 1996 demonstrated that dispersants can be very effective and prevent a much greater amount of environmental damage from being caused (6). This chapter describes the chemistry and physics of dispersants, planning and decision-making considerations, and finally their practical application and operational use in oil spill response.

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Small-scale laboratory methods were used to simulate the weathering processes that occur when crude oil is spilled at sea. Changes caused by evaporation and water-in-oil (w/o) emulsification were studied separately. W/o emulsions were assessed for chemical dispersibility using the Institut Français du Petrole (IFP) and Mackay-Nadeau-Steel-man (MNS) methods. Larger scale experiments were performed in a meso-scale flume. Crude oil was weathered for three days and then sprayed with dispersant. The results show that emulsion breaking is an important part of the mechanism of chemical dispersion. IFP, MNS, and Warren Spring Laboratory (WSL) tests, conducted on w/o emulsions recovered from the flume, produced much lower levels of dispersion than did treatment in the flume. The standard test procedures do not permit emulsion breaking to proceed to the extent observed in the flume. A sea trial also was conducted. Preliminary evaluation of the results shows that dispersant application partially broke the w/o emulsion that had rapidly formed. Dispersion proceeded at a slow rate but the treated slick was removed from the surface more rapidly than the control slick. The degree of dispersion was difficult to quantify by visual observation due to the weather conditions. A combination of remote sensing, surface sampling, and subsurface fluorometry provided a more reliable estimate.

Oil viscosity has been perceived as a major factor affecting the dispersibility of oil. Very high viscosity oils—20,000 centistokes (cs) or more—can readily be observed as resisting the breakup of the oil into dispersed droplets. However, there are instances where a relatively viscous oil will disperse much more readily than another oil of similar viscosity. An extensive study has been conducted at ExxonMobil Research facilities in New Jersey to define the molecular makeup of 14 viscous heavy fuel oil products and determine the property of the viscous oils, besides viscosity, that influences dispersibility. Dispersibility was measured by a standard laboratory dispersant test using a COREXIT dispersant selected from the U.S. Environmental Protection Agency (EPA) National Contingency Plan (NCP) Product Schedule. Initially, IATROSCAN (TLC) and gas chromatography data failed to show any correlation between chemical properties, such as sulfur, aromatics, paraffins, resins, vanadium, nickel content, etc., and dispersibility. However, the analysis did identify a statistically significant relationship between a parameter based on normal paraffin content and dispersibility, which helps explain anomalies such as low viscosity oils that do not disperse. These results are expected to aid in guiding oil spill response for viscous oils.