BfR-Wissenschaft 
55 
Figure 9.1 (adapt, from Merlin, 2015) 
Oil dispersible or not? (Figure 9.2) 
The effectiveness of chemical dispersion depends on the nature of the oil; the viscosity at 
ambient temperature constitutes one of the most important factors. 
Chemical dispersion is usually possible for viscosities under 5,000 cSt (with some excep 
tions, for example, in the case of hydrocarbons high in paraffin). When viscosity increases 
beyond 5,000 cSt, the chances of success decrease quickly. More than often, dispersion is 
considered not suitable for viscosities ranging from 10,000 cSt and beyond. 
Besides its intrinsic physic-chemical characteristics, oil viscosity increases with time spent at- 
sea, due to the (met-oceanic driven) weathering process. As a result, its dispersibility de 
creases with time, and oil is generally dispersible for a limited duration referred to as the 
“window of opportunity for chemical dispersion”: 
• To estimate the viscosity of an oil, once spilled at-sea, and the window of oppor 
tunity for dispersion, various data-processing models can be used to calculate the 
weathering of a given oil (according to both its composition and the met-oceanic 
conditions); 
• For an oil of high viscosity, the greater the mixing energy (wave action, sea state), 
the higher the potential for an efficient action of chemical dispersants; 
• Oil Pour Point 4 is also a characteristic to be considered, knowing that when ambi 
ent temperature is a few degrees less than the pour point value, oil ceases to be 
fluid and becomes undispersible. 
It should be noted that, whenever particular oils at risk of being spilled have been identified in 
a given NOSCP (e.g. oil frequently transported in -or in the close vicinity- of national waters, 
import/storage harbor activities, etc.), it is recommended to carry out dedicated studies to 
assess the “window of opportunity for dispersion” for those oils. This can be done through 
modelling tools, as well as through laboratory estimations of oil dispersibility vs. weather 
ing/viscosity. Results may be integrated in tables showing the expected oil viscosity, and the 
4 Pour point: the temperature under which an oil does no longer flow, according to specific laboratory conditions 
(ASTM -D97/ IP 15)