BALTIC SEAENVIRONMENT PROCEEDINGS No. 59 - Helcom

sea furface, and persistent oils, which in contrast dissipate more slowly
and usually require a clean-up response. Non-persistent oils include gasolene,
naptha, kerosene and diesel whereas most crude oils and heavy refined
products have varying degrees of persistence depending on their
physical properties and the size of the spill.
Crude oils of different origins have a wide range of physical and chemical
properties, whereas refined products have well-defined properties irrespective
of the crude oil from which they are derived.
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Evaporation
The rate and extent of evaporation is determined primarily be the volatility
of the oil. The greater the proportion of components with low
boiling points the greater the evaporation. The initial spreading rate
of the oil affects evaporation since the larger the surface area, the
faster the light components will evaporate. Rough seas, high wind
speeds and warm temperatures will also increase the rate of evaporation.
In broad terms, those oil components with a boiling point below
ZOO'.C will evaporate within a period of 24 hours in temperate conditions.
When exetremely volatile oils are spilled in confined areas,
there may be a risk of fire and explosion.
Dispersion
Waves and turbulence at the sea surface act on the slick to produce
droplets with a range of sizes. The rate of dispersion is largely dependent
upon the nature of the oil and the sea state, proceeding most
quickly in the presence of breaking waves. Slick thickness, which is
related to the amount spilled and the degree of spreading, is an important
factor in the rate of dispersion since smaller droplets are
produced from thin films. Oils which remain fluid and can spread unhindered
by other weathering processes may disperse completely in moderate
sea conditions within a few days, see the Braer-incident in the Shetlands.
Emulsification
Many oils exhibit a tendency to absorb water to form water-in-oilemulsions
increasing the volume of pollutant by a factor of between
three and four. These emulsions are often extremely viscous and, as a
result, the other processes which would cause the oil to dissipate are
retarded. This is the main reason for the persistence of light and medium
crude oils on the sea surface. In moderate to rough sea conditions,
most oils rapidly form emulsions, the stability of which is dependent
on the concentration of asphaltenes. Oils with asphaltene contents
greater then 0,5 % tend to form stable emulsions, often referred
to as "chocolate mousse" whilst those containg less are likely to
disperse.
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