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10.3.1 Main Oil Properties
The viscosity of the oil is considered one of the most important parameters in the
dispersion process. As viscosity is a counteracting force to droplet breakup
(Walstra 2005), high-viscosity oil results in larger oil droplets. The influence of
viscosity as a factor in oil droplet breakup is commonly reported in small- and
large-scale dispersion tests (Zeinstra-Helfrich et al. 2015b) as oil droplet sizes
increase with oil viscosity.
Following Stokes’ law, the oil density is known to increase oil droplet rise speed
(Robbins et al. 1995) and bias dispersibility tests that incorporate a settling step as
a consequence (SL Ross Environmental Research LTD and MAR Incorporated
2011). However, in current oil spill modelling, often a fixed droplet size is assumed
and considered to be stably suspended, regardless of environmental conditions or oil
density (Reed et al. 1999).
In laboratory experiments, oil type did not significantly influence the volume of
oil entrained, with the exception of high-viscosity oil (above 5 Pa.s), of which
60–80% less was entrained than expected based on oil layer thickness (ZeinstraHelfrich et al. 2016). Images of the entrainment process during plunge impact provided evidence demonstrating that the high-viscosity oil layer may prevent droplets
from being sheared off.
A similar entrainment limitation in a plunging jet test was observed by other
researchers (Reed et al. 2009). Reed et al. (2009) reported a twice as high maximum
viscosity but also used a two times greater plunge height. An increasing maximum
viscosity with plunge/wave height is logical as the impact from larger wave heights
is expected to be more successful in separating oil from the floating layer. More
elaborate experiments could provide experimental data to further calculate the
entrainment thresholds for larger wave heights.
Although oil properties clearly influence the process of droplet breakup and
thus droplet size, dispersion model calculations reveal that oil type hardly affects
the overall dispersibility and the oil slick behavior outputs of the model (Fig. 10.2),
partly because weather conditions are a more dominant factor in the model outcome and partly because of the correlation between oil viscosity and density
resulting in opposite effects. Although high-viscosity oil results in a larger mean
droplet size, these high-density droplets rise slower to the water surface than
equally sized low- viscosity (and low-density) oil droplets. The mean droplet rise
velocity of the full droplet size distribution for different oil types therefore only
shows little variation (Fig. 10.3, left panel). In the unrealistic scenario with equal
densities for the chosen three oil types (Fig. 10.3, right panel), mean rise velocity
would be much more affected by oil type. Evidently, the density-viscosity correlation strongly reduces the influence of viscosity on the mean droplet rise velocity.
This means that, when modelling dispersion, both viscosity and density should be
separate inputs, as a disproportion between these qualities can seriously affect the
dispersibility.
10 Effects of Oil Properties and Slick Thickness on Dispersant Field Effectiveness…
10.3.1 Main Oil Properties
The viscosity of the oil is considered one of the most important parameters in the
dispersion process. As viscosity is a counteracting force to droplet breakup
(Walstra 2005), high-viscosity oil results in larger oil droplets. The influence of
viscosity as a factor in oil droplet breakup is commonly reported in small- and
large-scale dispersion tests (Zeinstra-Helfrich et al. 2015b) as oil droplet sizes
increase with oil viscosity.
Following Stokes’ law, the oil density is known to increase oil droplet rise speed
(Robbins et al. 1995) and bias dispersibility tests that incorporate a settling step as
a consequence (SL Ross Environmental Research LTD and MAR Incorporated
2011). However, in current oil spill modelling, often a fixed droplet size is assumed
and considered to be stably suspended, regardless of environmental conditions or oil
density (Reed et al. 1999).
In laboratory experiments, oil type did not significantly influence the volume of
oil entrained, with the exception of high-viscosity oil (above 5 Pa.s), of which
60–80% less was entrained than expected based on oil layer thickness (ZeinstraHelfrich et al. 2016). Images of the entrainment process during plunge impact provided evidence demonstrating that the high-viscosity oil layer may prevent droplets
from being sheared off.
A similar entrainment limitation in a plunging jet test was observed by other
researchers (Reed et al. 2009). Reed et al. (2009) reported a twice as high maximum
viscosity but also used a two times greater plunge height. An increasing maximum
viscosity with plunge/wave height is logical as the impact from larger wave heights
is expected to be more successful in separating oil from the floating layer. More
elaborate experiments could provide experimental data to further calculate the
entrainment thresholds for larger wave heights.
Although oil properties clearly influence the process of droplet breakup and
thus droplet size, dispersion model calculations reveal that oil type hardly affects
the overall dispersibility and the oil slick behavior outputs of the model (Fig. 10.2),
partly because weather conditions are a more dominant factor in the model outcome and partly because of the correlation between oil viscosity and density
resulting in opposite effects. Although high-viscosity oil results in a larger mean
droplet size, these high-density droplets rise slower to the water surface than
equally sized low- viscosity (and low-density) oil droplets. The mean droplet rise
velocity of the full droplet size distribution for different oil types therefore only
shows little variation (Fig. 10.3, left panel). In the unrealistic scenario with equal
densities for the chosen three oil types (Fig. 10.3, right panel), mean rise velocity
would be much more affected by oil type. Evidently, the density-viscosity correlation strongly reduces the influence of viscosity on the mean droplet rise velocity.
This means that, when modelling dispersion, both viscosity and density should be
separate inputs, as a disproportion between these qualities can seriously affect the
dispersibility.
10 Effects of Oil Properties and Slick Thickness on Dispersant Field Effectiveness…