TIP 4 - Use of Dispersants to Treat Oil Spills

Mechanism of dispersion and dispersant composition

Following a spill of oil onto the sea surface, some of the oil in the resultant slick may disperse naturally into the water column. The extent to which this occurs depends on the type of oil spilt and the mixing energy of the sea. Oils with a lower viscosity are more amenable to natural dispersion than those with a higher viscosity. In general, crude oils will disperse to a greater extent than fuel oils.

Natural dispersion takes place when the mixing energy, provided by the waves and wind, is sufficient to overcome surface tension at the oil/water interface and break the oil slick into droplets of variable sizes. The larger oil droplets will rapidly resurface and coalesce to reform an oil slick. The smaller droplets will remain suspended in the water column, due to wave motion and turbulence, and will be further diluted by subsurface currents.

The process of natural dispersion takes place in moderately rough seas, with breaking waves and winds above 5 m/s (10 knots). For example, severe storm conditions in Shetland, UK, during the grounding of the tanker BRAER in 1993, caused the majority of the 85,000 tonne cargo of Gulfaks crude oil, a very low viscosity oil, to be dispersed naturally, with minimal shoreline impact.

Dispersants are designed to enhance natural dispersion by reducing the surface tension at the oil/water interface, making it easier for wave motion to create many more small oil droplets (Figures 1b and 1c). Dispersants are a blend of surfactants (surface active agents) in a solvent. The solvent has two functions: to act as a ‘thinner’, reducing the viscosity of the surfactant, so that it can be sprayed, and to promote the penetration of the surfactant into the oil slick.

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