A recent investigation by the National Research Council (NRC) of the National Academy of Sciences concluded that in the area of dispersant effectiveness, the two most important factors that need to be addressed and fully characterized in terms of efficacy are energy dissipation rate and particle size distribution. A 16 m wave tank facility was designed and constructed in 2004 and extended to 32 m in 2006 to expand the range of controlled sea state conditions. It was constructed at the Bedford Institute of Oceanography (BIO) in collaboration between Fisheries and Oceans Canada (DFO) and the U.S. Environmental Protection Agency (EPA) to specifically address these factors in controlled oil dispersion studies. Oil dispersion is the breakup of oil slicks at sea into small droplets. Natural oil dispersion occurs for oils that are inherently dispersible, but the process can be accelerated using dispersants. Dispersants are chemicals containing surfactants that reduce the surface tension between oil and water, resulting in the formation of oil droplets (oil-in-water emulsions). The dispersion of an oil slick is also greatly enhanced in the presence of waves. Waves provide mixing energy, which breaks the surface oil film and propels oil droplets into the water column. Thus, in the context of oil spill response operations, dispersion is a physical-chemical process, whose effectiveness depends on the chemical properties of both the dispersant and the oil and the mixing energy generated by the physical action of waves.
This project directly addresses the NRC recommendations, namely, measuring dispersant effectiveness at different sea state conditions (turbulence) and measuring particle size distribution and relating it to mass balance. The wave tank at BIO is able to produce breaking waves at precise locations in the tank reproducibly and is fully equipped to enable measurements of dispersed oil in the water column. A laser in-situ scattering and transmissometry (LISST-100X) particle size analyzer will be used to measure particle size distribution. In support of operational use of dispersants, the application of a multiple simultaneous scattering and fluorescence sensor technology will be developed under this program to provide real-time field data on the concentration and 3-dimensional profile of dispersed and non-dispersed oils at sea.
The goal is to generate various types of sea states, including breaking waves, and to quantify the corresponding energy dissipation rate at each sea state condition and then measure the resulting oil dispersion at those conditions. The tank is equipped with a flap-type wave maker that generates waves with periods varying from about 0.5 to 1.5 seconds. On the opposite end of the tank, a series of inclined screens is in place to absorb wave energy and minimize reflection. Currents can be generated using two sets of vertical manifolds perforated in the long sides of the tank and downstream of the wave maker flap. Such a system was used in previous research to simulate tidal currents in a wave tank (Boufadel et al., 1998; Boufadel, 2000). By closing or opening the valve of each hole, a wide range of velocity configurations can be considered, the simplest being a uniform distribution with depth.
Dr. Simon Courtenay
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