A comparison of an oil plume without dispersant (left or a) and one with dispersant at a 1:100 dispersant to oil ratio (right or b). The insets show typical droplet sizes in magnified sections corresponding to the white squares. Image credit David Murphy.
Scientists conducted laboratory experiments with a simulated oil plume to assess how chemical dispersants affect a crude oil jet as it transitions into a plume under crossflow conditions. The researchers found that counter-rotating vortices within the plume strongly interacted with oil droplets. Millimeter-scale droplets that escaped these vortices either defined or broke through the upper boundary of the plume. As dispersant concentration increased, the number of smaller droplets (micrometer) also increased. Compared to the larger droplets, these smaller droplets rose more slowly, and this slower rise rate inhibited the droplets or stopped them completely from breaking through the upper boundary of the plume. Vertical vortices within and beneath the plume entrained small droplets, and since higher dispersant concentrations increased the number of smaller droplets, the amount of oil trapped beneath the plume increased. The researchers published their findings in Journal of Geophysical Research: Crude oil jets in crossflow: Effects of dispersant concentration on plume behavior.
Subsea oil well blowouts, such as the Ixtoc I and Deepwater Horizon, can discharge large amounts of oil droplets that rise to the sea surface, form slicks, and harm environmentally or economically important coastlines and impact public health. Because the magnitude and type of damage largely depends on oil fate, understanding how chemical dispersants affect oil fate is important to inform decisions about dispersant use. The authors in this study addressed this need by simulating an underwater oil plume in a towing tank, injecting oil and dispersant-oil mixtures (1:100 and 1:25 dispersant-to-oil ratios, DOR), and examining oil droplet dynamics and transport. Ratios reflect varying subsea dispersant injection levels that might be present in an oil jet/plume blowout (Brandvik et al., 2013 and Johansen et al., 2013).
Using high-speed imaging, the team documented the plume structure. Using particle image velocimetry, they then observed the dominant flow patterns and quantified velocity distributions of the plumes. Finally, the team measured droplet size distributions in the evolving plume using holography.
The turbulent jet broke the crude oil jet into droplets, and the droplet size decreased as dispersant concentrations increased. The bottom boundaries of the crude oil and 1:100 DOR plumes did not significantly differ, but both formed at a higher elevation than the 1:25 DOR plume. The upper boundaries of the plumes differed substantially, with plume height increasing as less or no dispersant was present and droplet size was larger.
Plume turbulence did not affect the vertical trajectory of droplets in the crude oil plume, and its upper boundary diffused as large droplets (3-5 millimeter) escaped. Plume turbulence had a small effect on the vertical trajectory of droplets in the 1:100 DOR plume, and only a few droplets (~2 millimeter) escaped its upper boundary. Plume turbulence dominated the trajectories of droplets in the 1:25 DOR plume, and no droplets (micrometer) escaped its upper boundary.
The evolving strength of the counter-rotating vortices and size distribution of droplets generated when the oil jet breaks up help define the oil plume structure in a crossflow. However, the study did not account for how droplet interactions affect the structure and trajectory of the counter-rotating vortices nor did it explain flow phenomena during early jet to plume transition.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Dispersion Research on Oil: Physics and Plankton Studies II (DROPPS II) consortium.
The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.
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