Bridging the Gap Between Dispersion Efficiency at Various Scales

Dispersion effectiveness (DE) is a means of measuring the ability of dispersants to enhance the tendency for natural dispersion of oil into the water column. Understanding the DE of spilled oil in marine environments is a necessary aspect of predicting the transportation and fate of spilled materials. In the absence of studying crude oil spill in marine environments, scaled models, at the wave tank and laboratory scales, have been employed to predict DE. To better understand the conditions at each scale, researchers have characterized the hydrodynamics in the laboratory scale vessels (example: baffled flasks) and wave tanks (example: Ohmsett); however, the scalability between systems is a challenge due to the complexity of the apparatus and the disparity in systems. As a result, laboratory scale DE measurements could not be directly correlated to DE measurements at test tank or ocean scales. Identifying the necessary parameters, such as energy dissipation rate and dilution, was needed to model dispersion effectiveness across scales. The goal of this project was to understand the physics and develop a model correlating the formation and transport of oil droplets across scales, from laboratory, to wave tank, and finally ocean scale. Data from previous laboratory and Ohmsett tank experiments were used as a basis for the model development. This modeling effort was completed through a contract with the New Jersey Institute of Technology, with a value of $363,389.
To model DE across scales, Computational Fluid Dynamics (CFD) was used to identify and test the necessary parameters and compare modeled results to previous experiments. Dispersion effectiveness measurements performed in previous studies were used to develop and test the modeling approach. Baffled flask tests were performed with fresh and weathered ANS and Corexit 9500 at dispersant-to-oil ratios of 1:20 and 1:50, along with no dispersant controls. Following completion of the baffled flask test protocol, the droplet size distribution of dispersed oil was measured using a LISST-100x. The VDROP model was used to simulate the conditions in the baffled flasks matching the observed droplets size distribution. Wave tank experiments were performed at Ohmsett using weathered Hibernia. Large-eddy simulation models were used to represent the conditions of plunging breakers and identified the water surface elevation, velocity fields, and vorticity distributions in the Ohmsett tanks. These modeled results were coupled with the VDROP model to predict the droplet size distribution. Modeling the hydrodynamic conditions at both scales was required to understand how to scale data between experimental setups.
Modeling results using VDROP identified the critical variables needed to scale DE. Required inputs to model droplet size distribution, and therefore dispersant effectiveness, are the interfacial tension (based on the dispersant-to-oil ration), oil viscosity, and energy dissipation rates based on ocean conditions. This model allows for more accurate predictions of the droplet size distribution of a target oil in specific sea states. Responders will be able to more accurately predict crude oil transport and dissipation into the water column with and without dispersant applications and ultimately predict the fate of spilled crude oil.