Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
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Page 10
5.
FACTORS AFFECTING THE ACTION OF DISPERSANTS
Regardless of the application technique (Chapter 10) and dosage used (Chapter 9),
dispersant action will primarily be determined by:
type of oil to be treated;
contact dispersant/oil;
mixing;
weather conditions.
5.1
Type of oil
Characteristics determining the type of oil which can be chemically dispersed are basically:
a) Viscosity:
Only oils with viscosity at seawater (ambient) temperature of not more than 5 000 cSt (most
fresh crudes, medium fuel oils) are considered to be chemically dispersible by presently
existing products. Chemical dispersion of oils with viscosity between 5 000 and 10 000 cSt
may be uncertain (reduced); chemical dispersion above 10 000 cSt (heavy, weathered and
emulsified crudes, heavy fuels) is very little or non effective.
Even oils with low initial viscosity are likely to reach quickly the limits proposed above (often 24
hours from the moment of spillage) due to the weathering process. The time during which oil
remains dispersible is called “the window of opportunity for dispersion”. It would vary according
to the type of oil and the meteorological and oceanographic conditions (mainly temperature,
agitation/wind).
The more viscous the oil is the more agitation (waves) is required for its chemical dispersion.
b) Pour point:
Oils with a high paraffin (wax) content i.e. with a high pour point can cease to be dispersable if
ambient temperature is significantly lower than their pour point.
c) Oil emulsification:
With the emulsification process, the oil viscosity increases, and dispersant are generally not
effective on water-in-oil emulsions ("chocolate mousse"). However, when the emulsion is very
fresh, (not entirely stabilized) research studies showed that dispersants may be effective. In
such a case, the dispersant application can be undertaken in two stages : a first application to
break the emulsion and therefore to reduce the oil viscosity, followed with a second application
to carry out the dispersion itself.
Figure 7: Weathered oil emulsion which dispersion remains uncertain
Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
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Page 11
5.2
Dispersant/oil contact
In order to achieve a good dispersant/oil contact, a dispersant needs to be sprayed onto the
floating oil in such a way as to reach the surface of oil and not to penetrate through the oil
layer. These goals are achieved by combining appropriate spraying technique (Chapter 10)
and appropriate droplet size. Optimal droplet size is considered to be in the range of 350 and
1000 µm, or approximately 700 µm. Smaller droplets will be carried away by wind and may
never reach the oil, while the bigger ones penetrate through the oil layer and enter directly in
contact with the water without having sufficient time to bind themselves to the oil. Application
spraying system should be chosen to reach such requirements.
5.3
Mixing
Once the dispersant has come in contact with oil and the oleophilic end of its molecule has
been attached to oil, the dispersant/oil mixture needs to be agitated in order to be broken down
in droplets and dispersed in the sea-water mass.
Natural agitation of the sea surface (waves) is required for completing this process (e.g. sea
state 2, Beaufort 3).
In some cases, if the wave energy is insufficient (very calm sea), on limited pollution, the mixing
of dispersant/oil system and water can be supplied locally:
by sailing through the oil slick and stirring it with bow wave and propeller action;
by mixing oil and water with fire hoses.
Figure 8: Ship applying dispersant
(a part of mixing energy is generated by the bow wave)
5.4
Weather conditions
Chemical dispersion of oil is less affected by adverse weather conditions than other spill
response methods (e.g. containment and recovery). In addition, weather conditions do not
directly affect the physicochemical process of dispersion, but rather the application of
dispersants.
Winds may blow the sprayed dispersants away from the target area and consequently cause
significant loss of product. In case of the aerial spraying of dispersants, high winds may also
affect the safety of spraying aircraft.
Waves: Whilst waves provide the required mixing energy to enable the dispersion process (the
more energy is provided, the better is the dispersion); large waves or breaking waves can also
be an obstacle and render spraying operation difficult for boats. Interaction between dispersant
and oil slicks broken by the wave effect can also be reduced since part of the dispersant would
be sprayed directly on the water surface rather than on the oil.