BfR-Wissenschaft
15
Natural dispersion of the oil
Natural dispersion is the phenomenon by which spilled oil forms oil droplets, with a wide
range of sizes, suspended in the water column through the action of waves. The intensity of
this phenomenon is directly linked to the level of mixing energy provided by waves and wind.
It is widely agreed that larger oil droplets will resurface and coalesce to regenerate an oil
slick, and will do so more quickly than smaller droplets. Only very small oil droplets can become
permanently entrained in the water column and can drift according to the subsurface
currents. Nevertheless, natural dispersion of oil in the sea water column is not a predominant
process and it will not be the main behaviour of the oil slick in the environment (Le Floch et
al., 2002).
In addition, the rate of natural dispersion is greatly reduced by changes in the characteristics
of the oil caused by weathering processes (evaporation, dissolution, emulsification and photo-oxidation).
These processes will reduce the natural dispersion rate by increasing the viscosity
of the oil.
Chemical dispersion of the oil
By adding dispersants, the natural dispersion of the oil in the water column is increased. In
fact, the surfactant contained in the dispersant formulae will allow the breaking waves to
convert a huge amount of oil from the slick into very small oil droplets (IPIECA, 2001). In addition,
the distribution size of oil droplets is very homogeneous and, usually, droplet diameters
range between 10 and 50 pm.
The low buoyancy of these small droplets (oil + surfactant) will not allow them to resurface
mainly due to the intrinsic mixing energy of the sea. In addition, dilution of these droplets in
the whole water column will be induced by the subsurface currents and, depending on their
intensity, the oil concentration can drop rapidly.
Observations made in the field during the Sea Empress accident illustrate perfectly this
trend. During this accident close to the coast of Wales in 1996, major dispersant spraying
operations were carried out (over 440 tonnes were sprayed) and oil concentrations were
monitored in the upper water column (Table 4.1).
Table 4.1: Oil concentration in the upper water column after dispersant spraying operations during the
Sea Empress incident (Lewis et al., 2006)
Time after dispersant application
Oil concentration in the upper water
column (ppm)
Just after treatment
10
2 days after treatment
1
1 week after treatment
0.5
1 month after treatment
0.2
3 months after treatment
Background level
Evolution of commercial dispersant formulations
Surfactants are substances that are widely used in the detergent industry and are well known
for their efficacy on oil and grease. They were used in large quantities during the Torrey
Canyon accident (1967). Approximately 10,000 tonnes of detergents were directly used for
beach clean-up and 14,000 tonnes of solvents were released into coastal waters in Cornwall,
UK. However, these detergents contained surfactants dissolved in oil fractions composed of
aromatic hydrocarbons with low boiling points. Those aromatics are well known for their car-
