Assessment of chemical dispersant effectiveness in a wave tank under regular non-breaking and breaking wave conditions

Current chemical dispersant effectiveness tests for product selection are commonly performed with bench-scale testing apparatus. However, for the assessment of oil dispersant effectiveness under real sea state conditions, test protocols are required to have hydrodynamic conditions closer to the natural environment, including transport and dilution effects. To achieve this goal, Fisheries and Oceans Canada and the US Environmental Protection Agency (EPA) designed and constructed a wave tank system to study chemical dispersant effectiveness under controlled mixing energy conditions (regular non-breaking, spilling breaking, and plunging breaking waves). Quantification of oil dispersant effectiveness was based on observed changes in dispersed oil concentrations and oil-droplet size distribution. The study results quantitatively demonstrated that total dispersed oil concentration and breakup kinetics of oil droplets in the water column were strongly dependent on the presence of chemical dispersants and the influence of breaking waves. These data on the effectiveness of dispersants as a function of sea state will have significant implications in the drafting of future operational guidelines for dispersant use at sea.     

Effects of temperature and wave conditions on chemical dispersion efficacy of heavy fuel oil in an experimental flow-through wave tank

The effectiveness of chemical dispersants (Corexit 9500 and SPC 1000) on heavy fuel oil (IFO180 as test oil) has been evaluated under different wave conditions in a flow-through wave tank. The dispersant effectiveness was determined by measuring oil concentrations and droplet size distributions. An analysis of covariance (ANCOVA) model indicated that wave type and temperature significantly (p < 0.05) affected the dynamic dispersant effectiveness (DDE). At higher temperatures (16 °C), the test IFO180 was effectively dispersed under breaking waves with a DDE of 90% and 50% for Corexit 9500 and SPC 1000, respectively. The dispersion was ineffective under breaking waves at lower temperature (10 °C), and under regular wave conditions at all temperatures (10-17 °C), with DDE < 15%. Effective chemical dispersion was associated with formation of smaller droplets (with volumetric mean diameters or VMD ? 200 μm), whereas ineffective dispersion produced large oil droplets (with VMD ? 400 μm).     

Oil viscosity limitation on dispersibility of crude oil under simulated at-sea conditions in a large wave tank

This study determined the limiting oil viscosity for chemical dispersion of oil spills under simulated sea conditions in the large outdoor wave tank at the US National Oil Spill Response Test Facility in New Jersey. Dispersant effectiveness tests were completed using crude oils with viscosities ranging from 67 to 40,100 cP at test temperature. Tests produced an effectiveness-viscosity curve with three phases when oil was treated with Corexit 9500 at a dispersant-to-oil ratio of 1:20. The oil viscosity that limited chemical dispersion under simulated at-sea conditions was in the range of 18,690 cP to 33,400 cP. Visual observations and measurements of oil concentrations and droplet size distributions in the water under treated and control slicks correlated well with direct measurements of effectiveness. The dispersant effectiveness versus oil viscosity relationship under simulated at sea conditions at Ohmsett was most similar to those from similar tests made using the Institut Francais du Pétrole and Exxon Dispersant Effectiveness (EXDET) test methods.     

Effects of chemical dispersants and mineral fines on crude oil dispersion in a wave tank under breaking waves

The interaction of chemical dispersants and suspended sediments with crude oil influences the fate and transport of oil spills in coastal waters. A wave tank study was conducted to investigate the effects of chemical dispersants and mineral fines on the dispersion of oil and the formation of oil-mineral-aggregates (OMAs) in natural seawater. Results of ultraviolet spectrofluorometry and gas chromatography flame ionized detection analysis indicated that dispersants and mineral fines, alone and in combination, enhanced the dispersion of oil into the water column. Measurements taken with a laser in situ scattering and transmissometer (LISST-100X) showed that the presence of mineral fines increased the total concentration of the suspended particles from 4 to 10microl l(-1), whereas the presence of dispersants decreased the particle size (mass mean diameter) of OMAs from 50 to 10microm. Observation with an epifluorescence microscope indicated that the presence of dispersants, mineral fines, or both in combination significantly increased the number of particles dispersed into the water.     

Application of entropy analysis of in situ droplet-size spectra in evaluation of oil chemical dispersion efficacy

In situ droplet-size distributions were measured using a laser in situ scattering and transmissiometry (LISST-100X) particle size analyzer during the evaluation of natural and chemical dispersion efficiency of crude oils under different wave and current conditions. An entropy grouping of the in situ dispersed oil droplet-size spectra has classified the multi-modal droplet-size distributions into different groups based on similar droplet-size spectra characteristics within groups and distinction between groups. A generalized linear logistic regression model was fitted to analyze the effects of a number of factors and their interactions on the grouping of oil droplet-size spectra. The grouped results corresponded to the oil dispersion efficiency at different levels. This new method for droplet-size distribution data analysis can have significant implication in field evaluation of natural and chemical dispersion efficiency of oil.     

Evolution of droplets in subsea oil and gas blowouts: Development and validation of the numerical model VDROP-J

The droplet size distribution of dispersed phase (oil and/or gas) in submerged buoyant jets was addressed in this work using a numerical model, VDROP-J. A brief literature review on jets and plumes allows the development of average equations for the change of jet velocity, dilution, and mixing energy as function of distance from the orifice. The model VDROP-J was then calibrated to jets emanating from orifices ranging in diameter, D, from 0.5 mm to 0.12 m, and in cross-section average jet velocity at the orifice ranging from 1.5 m/s to 27 m/s. The d₅₀/D obtained from the model (where d₅₀ is the volume median diameter of droplets) correlated very well with data, with an R² = 0.99. Finally, the VDROP-J model was used to predict the droplet size distribution from Deepwater Horizon blowouts. The droplet size distribution from the blowout is of great importance to the fate and transport of the spilled oil in marine environment.     

Development of a unified oil droplet size distribution model with application to surface breaking waves and subsea blowout releases considering dispersant effects

An oil droplet size model was developed for a variety of turbulent conditions based on non-dimensional analysis of disruptive and restorative forces, which is applicable to oil droplet formation under both surface breaking-wave and subsurface-blowout conditions, with or without dispersant application. This new model was calibrated and successfully validated with droplet size data obtained from controlled laboratory studies of dispersant-treated and non-treated oil in subsea dispersant tank tests and field surveys, including the Deep Spill experimental release and the Deepwater Horizon blowout oil spill. This model is an advancement over prior models, as it explicitly addresses the effects of the dispersed phase viscosity, resulting from dispersant application and constrains the maximum stable droplet size based on Rayleigh-Taylor instability that is invoked for a release from a large aperture.