Many previous studies using laboratory test methods have shown that the ability to disperse spilled oils depends on several factors including: spilled oil properties (and how these change with oil weathering), the mixing energy, and the dispersant-to-oil ratio (DOR). There appears to be a ‘limiting oil viscosity’ value that, when exceeded, causes a sharp reduction in the effectiveness of a dispersant. The results obtained in laboratory tests are relative and not absolute, and it has therefore proved very difficult to correlate dispersant effectiveness results from these laboratory tests with dispersant performance at sea. A series of small-scale dispersant tests were conducted at sea in the English Channel in June 2003. Several small test slicks of residual fuel oils of different viscosity grades were laid on the sea and immediately sprayed with different dispersants at different DORs. Observers used a simple ranking system to visually assess the degree of dispersion that occurred when a cresting wave passed through an area of the dispersant-treated oil. Collation of the results showed that there were obvious and consistent differences in the degree of effectiveness observed with different combinations of oil viscosity, dispersant and treatment rate.

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