Wave Tank Studies on Dispersant Effectiveness as a Function of Energy Dissipation Rate and Particle Size Distribution

The use of chemical dispersants can be an effective means to combat oil spills at sea. There has been renewed interest in the use of chemical dispersants due to escalated oil spill incidents, logistical constraints of traditional spill response options, and the development of new generation, low-toxicity, high efficiency dispersant formulations for potential use on oils covering a greater viscosity range. For the assessment of chemical dispersant effectiveness under realistic sea states, test protocols are required to produce hydrodynamic conditions close to the mixing, transport and dilution effects found in the natural environment. To meet this requirement, a wave tank has been designed and constructed at the Bedford Institute of Oceanography (BIO) to evaluate chemical dispersant effectiveness under different wave conditions with energy levels ranging from regular non-breaking waves to plunging breakers. The hydrodynamics of these wave conditions were characterized using an autocorrelation function method applied to in-situ velocity measurements. Quantification of oil dispersant effectiveness was based on observed changes in dispersed oil concentrations and oil-droplet size distribution using a laser in-situ scattering and transmissometry particle size analyzer. Evaluation of chemical dispersant effectiveness in a batch mode established quantitative relationships between dispersant effectiveness and energy dissipation rate under a variety of simulated wave conditions. The results indicated that 53% to 90% of the test crude oils have been dispersed in the presence of chemical dispersants and only 10% to 20% were dispersed under control conditions in the absence of chemical dispersant. The characterization of the in-situ dispersed oil droplet size distributions indicated that the physical dispersion generated monomodal lognormal oil droplet size distributions of larger median diameters, whereas chemical dispersion produced bior tri-modal lognormal oil droplet size distributions of smaller median diameters over a wider range. The wave tank in flow-through mode simulating waveand current-driven hydrodynamic conditions revealed that nearly 8 % to19 % of the test crude oils were dispersed and diluted under regular wave and breaking wave conditions, respectively, in the absence of dispersants. In the presence of dispersants, about 21% to 36% of the crude oils were dispersed and diluted under regular waves, and 42% to 62% under breaking waves. Consistently, physical dispersion under regular waves produced large oil droplets, whereas chemical dispersion under breaking waves created small droplets. These data on the effectiveness of dispersants as a function of sea state are significant contributions to the development of improved predictive models on dispersant effectiveness and better operational guidelines for dispersant use. Results of our experiential studies using the wave tank system have advanced our mechanistic understanding of dispersant effectiveness under ambient field conditions. The research has shown that use of dispersants in deep water environments under moderately energetic wave conditions is a promising countermeasure technology for driving floating oil into the water column. Chemical dispersant Corexit 9500 is effective in all low, moderate and high energy test conditions, whereas SPC 1000 can also be very useful in moderate to higher energy conditions. Effective dispersion of oil to achieve small droplet formation is dependent on wave energy and the presence of a chemical dispersant. If either of these two factors is missing, dispersion can only take place in very high energy sea states. Use of chemical dispersants drastically reduces the time required for thorough dispersion of oil into small oil droplets in the water column to take place in comparison to natural dispersion. When the window of opportunity for oil spill response is narrow (i.e. rapid remediation is critical due to proximity to environmentally sensitive areas), a quick and effective response such as what can be achieved using dispersants may be deemed necessary even under high energy conditions because of their ability to through accelerate the process and efficacy of oil dispersion into the water column and to facilitate oil biodegradation.