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
–
Page 25
Figure 19: The British testing equipment for assessing the toxicity
9.3
Biodegradability tests
Dispersants and dispersant/oil mixtures are often tested for biodegradability. There is no
consensus on a standard method for testing biodegradability of dispersants and various
adapted standard tests on organic material are in use (e.g. the method used in France
–
Standard NF 90 346).
9.4
Other tests
Standard analytical methods are used for testing other properties (density, viscosity, etc.) if so
required by the competent authorities.
10.
DOSAGES OF DISPERSANTS AND APPLICATION RATES
The amount of dispersants which needs to be applied to a certain quantity of oil, in order to
achieve a desired level of dispersion, depends on the oil type, its weathering degree, its
thickness, the environmental conditions (e.g. waves), and the dispersant itself.
In certain cases as during the “Sea Empress” incident in 1995, the oil is easily di
spersible and
therefore a low dosage (oil/dispersant ratio) may be sufficient, whilst in other incident less
favorable (low dispersibility of the oil), it may be suitable to increase the dosage.
Practically, it is advisable to refer to the dose recommended by the manufacturer (often 5% for
“concentrates”), dosage which can be adjusted during the operations on the basis of certain
average figures.
In general terms conventional dispersants or 2
nd
generation (hydrocarbon based
dispersant) are usually applied in doses of approximately 30 - 50 % of estimated oil quantity
for low viscosity oil (up to 1000 cSt) and 100% for oils in the viscosity range of 1000 - 2000 cSt.
Figures for concentrate dispersants or 3
rd
generation, are in the range of 5 % for oils of up
to 5 000 cSt, and 5 - 10% for treatment of oil between 5 000 and 10 000 cSt. Treatment of oils
with viscosities of more than 10 000 cSt is considered ineffective. For fresh light oils easily
dispersible viscosity less than 500 cSt a dosage lower than 5% may be sufficient.
Considering the application rate versus the oil thickness required application rates can be
calculated on the basis of generally accepted rules for the assessment of oil thickness (dark
patches of oil are assumed to be approximately 0.1 mm thick and areas covered by a thin oil
sheen are estimated to be between 0.001 and 0.01 mm).
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
–
Page 26
Regardless of the spraying device used, application rate is determined by the discharge rate of
dispersant pump, speed of the vessel or aircraft and the width of the area covered by the spray
(swath). The relation between these variables is the following:
Application rate = Discharge rate / Speed x Swath
Consequently, given the constant swath of the available spraying equipment, the required
application rate for each particular slick area can be achieved by:
a) either selecting the appropriate discharge rate of the dispersant pump;
b) or selecting the appropriate speed of the vessel or aircraft.
Very often, an average treatment rate of 100 litres of concentrate dispersant per hectare,
corresponding to oil thickness of 0.1 mm and a dose of 1:10 is used in approximate
calculations for the use of dispersants.
Figure 20: Ship mounted modern application spraying equipment (source SINTEF)
11
SYSTEMS FOR THE APPLICATION OF DISPERSANTS
Selection of the dispersants‟ application technique basically depends on:
the type of dispersant available;
the type of spraying device available;
although the size and location of the spill must also be taken into consideration.
Several dispersant spraying systems exist and they can be grouped in accordance with the
carrier for which they were designed:
aircraft mounted spraying systems;
boat mounted spraying systems;
portable units for individual use.
11.1
Aircraft mounted spraying systems
As a result of advantages offered by the aerial spraying of dispersants (good control and
assessment of results, rapid response, high treatment rates, optimum use of the product,
regardless of the sea state), a number of spraying systems have been developed for use with
both fixed and rotating wing aircraft (helicopters). Existing units are either of a type which can