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Chemical dispersion is one of the most cost-effective options to remediate oil spill at open sea. Identifying significant factors that determine in situ droplet size distributions facilitates mechanistic understanding of dispersant effectiveness. In this work, in situ dispersed oil droplet size distributions were characterized during testing of chemical dispersant effectiveness of two dispersants (Corexit 9500 and SPC 1000) on two oils [Medium South American (MESA) and Alaska North Slope (ANS)] under three wave conditions (regular nonbreaking, spilling breaking, and plunging breaking waves) in an experimental wave tank. Results showed that physical dispersion generated monomodal lognormal oil droplet size distributions of larger median diameters, whereas chemical dispersion produced bi- or trimodal lognormal oil droplet size distributions of smaller median diameters over a wider range. Factorial analysis of variance (ANOVA) followed by Tukey's paired comparison statistical data analysis indicated that the volume mean diameters of dispersed oil droplets were reduced by 36 μm (from 122 to 86 μm) by plunging breaking conditions. Volume mean diameters were decreased by 92 μm (from 153 to 61 μm) and 37 μm (from 153 to 116 μm), respectively, by Corexit 9500 and SPC 1000. These results are useful in optimizing operational guidelines for dispersant use, and providing input for modeling transport, fate, and biological effects of dispersed oil. [ABSTRACT FROM AUTHOR]

Numerous laboratory test systems have been developed for the comparison of efficacy between various chemical oil dispersant formulations. However, for the assessment of chemical dispersant effectiveness under realistic sea state, test protocols are required to produce hydrodynamic conditions close to the mixing, transport, and dilution effects found in the natural environment. To this end, we have designed and constructed an experimental wave tank system capable of generating waves of different energy levels, ranging from regular nonbreaking waves to plunging breakers. The hydrodynamics of these wave conditions were characterized using an autocorrelation function method applied to in situ velocity measurements. We report here an investigation of the effectiveness of two chemical dispersants (Corexit 9500 and SPC 1000) on two crude oils (weathered MESA and fresh ANS) under three different wave conditions in the wave tank operated in batch mode. Corexit 9500 dispersed approximately 75% of the weathered MESA and more than 90% of the fresh ANS crude, and SPC 1000 dispersed about 53 and 64%, respectively. Under control conditions (absence of chemical dispersant), only 10 to 20% of the crude oils were dispersed. Quantitative relationships were established between dispersant effectiveness and energy dissipation rate under the different simulated wave conditions. These relationships are essential for the development of accurate predictive models on dispersant effectiveness and operational guidelines for dispersant use. [ABSTRACT FROM AUTHOR]