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10.3.2 Oil Layer Thickness
In operational guidelines for chemical dispersion of oil, oil layer thickness is not
considered very relevant for oil fate modelling. It mainly is seen as a parameter
relevant for dispersant dosages: spraying dispersants on a slick that’s too thin
would cause the dispersant to fall through and be lost to the water column (Tamis
et al. 2012). Spraying the thicker areas of the slick is advised; however, for too
high thicknesses, multiple spray passes are advised to reach the effective dosage
(EMSA 2009).
Using a plunging jet setup (Zeinstra-Helfrich et al. 2015a, 2016), the importance of the oil layer thickness in both entrainment and droplet breakup was
revealed: The volume of oil entrained increases proportionally with layer thickness. The availability of oil per unit surface area (oil layer thickness) clearly determines the volume entrained per “mixing event.” With increasing oil layer thickness,
the mean oil droplet size increases, but due to the enhanced entrainment volume,
the absolute amount of oil in small droplets also increases. Still, thin layers have a
higher dispersibility than thick layers due to the larger relative portion of small
droplets produced by an impact. This influence of layer thickness on dispersibility
also is crucial for the modelled behavior of the slick over time. With the same oil
mass, a longer, thin slick is dispersed faster than a short thicker slick (ZeinstraHelfrich et al. 2017). Such rapid removal of the thin slick areas, while thicker parts
remain, has also been observed in situ (Lewis et al. 1998).
These observations confirm that aiming any response at the thick slick portions
(National Research Council of the National Academies 2005) is most effective as
the thin slick part will naturally disperse more easily. Additionally, for mechanical
recovery, removal rates are higher in thick oil.
Based on these outcomes, one can hypothesize that mechanical dispersion on
thicker portions of the slick to enhance spreading could be effective as the resulting
thinner slick will subsequently disperse more easily.
10.3.3 Initial Slick Size
The model study (Zeinstra-Helfrich et al. 2017) only briefly examined the influence
of initial slick length on the dispersion process. It is, however, possible to make
some prognoses based on the different elongation mechanisms.
For favorable conditions (dispersibility factor >0.4), the initial slick length will
not have a large influence on the dispersion process: In these situations, the oil mass
will be rapidly entrained into the water phase. For a larger slick oil will simply move
to the water phase over the larger area.
For less favorable conditions, initial slick length is expected to influence the
outcome. The absolute elongation is minimally affected by initial slick length: A
10 Effects of Oil Properties and Slick Thickness on Dispersant Field Effectiveness…
10.3.2 Oil Layer Thickness
In operational guidelines for chemical dispersion of oil, oil layer thickness is not
considered very relevant for oil fate modelling. It mainly is seen as a parameter
relevant for dispersant dosages: spraying dispersants on a slick that’s too thin
would cause the dispersant to fall through and be lost to the water column (Tamis
et al. 2012). Spraying the thicker areas of the slick is advised; however, for too
high thicknesses, multiple spray passes are advised to reach the effective dosage
(EMSA 2009).
Using a plunging jet setup (Zeinstra-Helfrich et al. 2015a, 2016), the importance of the oil layer thickness in both entrainment and droplet breakup was
revealed: The volume of oil entrained increases proportionally with layer thickness. The availability of oil per unit surface area (oil layer thickness) clearly determines the volume entrained per “mixing event.” With increasing oil layer thickness,
the mean oil droplet size increases, but due to the enhanced entrainment volume,
the absolute amount of oil in small droplets also increases. Still, thin layers have a
higher dispersibility than thick layers due to the larger relative portion of small
droplets produced by an impact. This influence of layer thickness on dispersibility
also is crucial for the modelled behavior of the slick over time. With the same oil
mass, a longer, thin slick is dispersed faster than a short thicker slick (ZeinstraHelfrich et al. 2017). Such rapid removal of the thin slick areas, while thicker parts
remain, has also been observed in situ (Lewis et al. 1998).
These observations confirm that aiming any response at the thick slick portions
(National Research Council of the National Academies 2005) is most effective as
the thin slick part will naturally disperse more easily. Additionally, for mechanical
recovery, removal rates are higher in thick oil.
Based on these outcomes, one can hypothesize that mechanical dispersion on
thicker portions of the slick to enhance spreading could be effective as the resulting
thinner slick will subsequently disperse more easily.
10.3.3 Initial Slick Size
The model study (Zeinstra-Helfrich et al. 2017) only briefly examined the influence
of initial slick length on the dispersion process. It is, however, possible to make
some prognoses based on the different elongation mechanisms.
For favorable conditions (dispersibility factor >0.4), the initial slick length will
not have a large influence on the dispersion process: In these situations, the oil mass
will be rapidly entrained into the water phase. For a larger slick oil will simply move
to the water phase over the larger area.
For less favorable conditions, initial slick length is expected to influence the
outcome. The absolute elongation is minimally affected by initial slick length: A
10 Effects of Oil Properties and Slick Thickness on Dispersant Field Effectiveness…