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
(CO
2
) and water (H
2
O)
oil slick
surfactant
attracts
water
attracts
oil
oil
surfactant
microorganism
water
IPIECA-IOGP Oil Spill Response Joint Industry Project
2
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:
l
only effective, low toxicity products are approved; and
l
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:
l
the factors that determine the effectiveness of
dispersant; and
l
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.
1
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).
Salinity
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
3
At-sea monitoring of surface dispersant effectiveness
Factors
determining the
effectiveness of dispersant
1
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.