Jip 4 At-sea monitoring of surface dispersant effectiveness

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1. Dispersants can be applied by

aircraft, as well as by vessels or

by subsea injection

2. Dispersants reduce interfacial tension between

oil and water so that oil slicks can break apart.

3. Dispersants are blends of surfactants and solvents. Surfactant molecules

are comprised of two parts: an oleophilic (oil-seeking) part and a

hydrophilic (water-seeking) part. These molecules attract water on one

end and oil on the other end.

4. Wave motion breaks up the oil naturally; surfactants enhance this

process. Tiny droplets remain neutrally suspended in the water column

and are more readily available for biodegradation by organisms.

5. Microorganisms convert ingested oil into mostly carbon dioxide



) and water (H



oil slick










IPIECA-IOGP Oil Spill Response Joint Industry Project


Figure 2  How dispersant products work

the water through turbulence, currents and natural

dispersion. The majority of the oil in these droplets

will be rapidly biodegraded by naturally occurring

hydrocarbon-degrading organisms, largely due to the

increased surface area of the oil droplets. The

ultimate fate of most of the oil is to be biologically

converted to carbon dioxide and water.

Dispersants can be applied rapidly over large areas and

in relatively rough weather conditions where mechanical

recovery and other response options are not feasible. In

addition, dispersant use also generates significantly less

waste in comparison to other response techniques.

Successful dispersant application requires effective

logistical planning and resources. Airborne (aircraft or

helicopters) or vessel mounted systems are the accepted

platforms for applying dispersant to floating oil.

It is appropriate that dispersants are regulated to

ensure that:


only effective, low toxicity products are approved; and


there is an authorization process to identify the sea

areas where approved products may be used.

If the surface application of dispersant is chosen as a

response technique, it is important to ensure that

procedures are established to confirm its effectiveness

throughout the response effort. This may influence

operational decisions such as increasing dispersant

dosage or ceasing dispersant operations.

The purpose of this document is to provide guidance on

estimating the effectiveness of dispersant application in

the field by describing:


the factors that determine the effectiveness of

dispersant; and


how the effectiveness of dispersant application in the

field may be monitored, evaluated and verified.

The key factors that determine the effectiveness of a dispersant are described below. Further

details are provided in the IPIECA-IOGP Good Practice Guide on the surface application of

dispersants (IPIECA-IOGP, 2015a)

Composition of the dispersant product

The blend and proportion of surfactants will influence the product’s effectiveness. Most

dispersants consist of a blend of two or three non-ionic surfactants, and sometimes include an

anionic surfactant. Most modern surfactants used in dispersants are also widely used in

household products, e.g. soaps, shampoo, detergents, etc., and are actually less toxic than many

of these products.


Sea state

Rapid dispersion of dispersant-treated oil begins at a wind speed of approximately 7 knots

(3.6 m/s—equivalent to a light to gentle breeze) with wave heights of 0.2 to 0.3 metres.

However, dispersants can be sprayed onto floating oil in flat, calm conditions, and dispersion will

begin as the sea state increases. Gale-force winds with speeds greater than 35 knots (18 m/s)

and wave heights of 5 metres are generally the upper limits for spraying dispersant from aircraft,

although dispersants have been applied from aircraft in winds greater than 50 knots (25.7 m/s).

Waves are also the driving energy behind the formation of water-in-oil emulsion, which increases

an oil’s viscosity and can thereby reduce dispersant effectiveness (see Oil type and its physical

properties, below).


Most commercially available dispersants have been formulated to be most effective in seawater

with a salt content (salinity) of 30 to 35 psu (practical salinity units). The effectiveness of these

dispersants will be decreased in brackish waters (salinity of 5 to 10 psu) and can be very low in

fresh water.

Oil type and its physical properties

Viscosity and pour point are the properties of primary importance for dispersant applicability. The

viscosity of spilled oil generally increases with time as it ‘weathers’ through evaporation and

emulsification, influencing the effectiveness of dispersants. As the viscosity of a floating oil

increases with time the probable effectiveness of dispersants will decline due to the reduced


At-sea monitoring of surface dispersant effectiveness

Factors determining the

effectiveness of dispersant 


Environment Canada Study: Fingas et al. (1995): The effectiveness testing of oil spill-treating agents. In: The Use of

Chemicals in Oil Spill Response. ASTM International.

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