Study Improves Knowledge about Dispersants’ Net Effect on Oil Fate

Visualization of oil droplet and water mixing processes.  Under a breaking wave, oil is entrained and broken into various-sized droplets and distributed. While the oil droplets are suspended, the wind helps move the slick. Droplets resurface upwind from their original location, the distance depending on the suspension time.  A droplet’s rise speed is determined by its diameter, and its time in the water column is determined by its mixing depth and droplet diameter. (Image provided by Marieke Zeinstra)

(Click to enlarge) Visualization of oil droplet and water mixing processes. Under a breaking wave, oil is entrained and broken into various-sized droplets and distributed. While the oil droplets are suspended, the wind helps move the slick. Droplets resurface upwind from their original location, the distance depending on the suspension time. A droplet’s rise speed is determined by its diameter, and its time in the water column is determined by its mixing depth and droplet diameter. (Image provided by Marieke Zeinstra)

Scientists analyzed literature on surface oil dispersion processes to improve how (natural and chemical) dispersion could be quantified in oil spill prediction models.

They suggest that chemical dispersion is an amplification of natural dispersion and that oil properties and wind-wave mixing are key model parameters for calculating net dispersion effectiveness (oil fate change in slick size, location, and water column). They published their findings in the September 2015 edition of Marine Pollution Bulletin: The NET effect of dispersants—a critical review of testing and modelling of surface oil dispersion.

Models that simulate chemical dispersion require an input for dispersant effectiveness, which is not yet quantified.  First author Marieke Zeinstra said existing studies, using scales from small laboratory tests to larger wave tank tests and sea trials, focused on different dispersion process characteristics, making analyzes challenging. She explained their team’s approach, “Dispersion of spilled oil at sea is not a binary or yes/no thing; it is a transient process. We defined separate steps, and searched the literature for clues on what determines the outcomes for this process. Following this approach, the collected information could be combined into a more elaborate conceptual model that made a lot more sense.”

Study author Marieke Zeinstra-Helfrich. (Photo provided by Zeinstra)

(Click to enlarge) Study author Marieke Zeinstra-Helfrich. (Photo provided by Zeinstra)

High oil viscosity hinders dispersion by helping oil resist entrainment, counteracting its breakup into small droplets, and preventing dispersants from reaching the oil-water interface. However, high viscosity oil drops that do get into the water column rise to the surface a bit more slowly. High wind speed increases the energy and frequency of breaking waves and thereby enhances dispersion by increasing oil entrainment and facilitating its breakup into droplets and mixing into the water column. On the other hand, high winds make targeting chemical dispersants more difficult. Low-to-no wind speeds reduce dispersants’ effectiveness as there are no breaking waves to submerge oil. Conditions falling between those extremes are conducive to chemical dispersant use, allowing it to reduce oil droplet size and coalescence.

Zeinstra said this study helped identify available information and areas that need further investigation for better dispersion modelling. The team suggested future research should more precisely quantify oil’s entrainment rate, droplet size distribution, mass transfer coefficients for surfactant to the oil layer, and dispersant’s effects on physical oil properties. “The goal is for future responders to be able to predict how much oil will disperse naturally and how much more will disperse when dispersants are applied, leading to an estimate of net effect in cost-effective oil spill response decision making.”

The study’s authors are Marieke Zeinstra-Helfrich, Wierd Koops, and Albertinka J. Murk.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Center for Integrated Modeling and Analysis of Gulf Ecosystems (C-IMAGE). Other funding sources included the Wageningen UR IPOP TripleP@Sea Innovation Program (KB-14-007).

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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