Behavior of shade-weathered and photo-oxidized crude oil with and without application of dispersant under breaking wave conditions

Chemical dispersants are a critical tool in crude oil spill response, facilitating the breakup of surface oil into fine droplets that disperse into the water column, thereby reducing shoreline contamination. However, their effectiveness on photo-oxidized oil, which undergoes chemical transformation due to sunlight exposure, remains a subject of concern. This study evaluates the performance of Corexit 9500 on photo-oxidized Hibernia crude oil through a large-scale breaking wave experiment conducted at Ohmsett. Crude oil samples were applied to the water surface and subjected to breaking waves with and without the application of dispersant, while droplet size distribution (DSD) and concentration behaviors were recorded. For comparison, tests were also conducted using shade-weathered oil. In the absence of dispersant, the volume median droplet diameter (d₅₀) for various experiments ranged between 300 and 730 μm at different locations, and tended to increase with the degree of weathering. In the presence of dispersant, the d₅₀ was less than 10 μm, and showed only a slight increase with the degree of weathering. Simulation results were also predicted qualitatively based on the energy dissipation rate of the wave breaker. Statistical models were fitted to the resulting DSD by volume. In the absence of dispersant, the DSD was initially Gaussian-like or Weibull-like, and gradually evolved into Weibull or Lognormal distributions as larger droplets resurfaced. In the dispersant-treated cases, the Weibull and Lognormal distributions more accurately represented the data, with the Weibull distribution being slightly better.