BALTIC SEAENVIRONMENT PROCEEDINGS No. 59 - Helcom

1.3
2.
d) Spreading
Spreading ist one of the most significant processes during the early
stages of a spill. The main driving force behind the initial spreading
of the oil is its weight. A large instantaneous spill will therefore
spread more rapidly than a slow discharge. This gravity assisted spreading
is quickly replaced by surface tension effects. During these early
stages, the oil spreads as a coherent slick and the rate is influenced
by the viscosity of the oil. High viscosity oils spread slowly and
those spilled at temperatures below their pour point hardly spread at
all. After a few hours the slick begins to break up and form narrow
bands or "windrows" parallel to the wind direction.
Observation of Oil at Sea
Aerial reconnaissance is essential for an effective response to oil spills,
both to facilitate the location of oil at sea and to improve the control of
clean-up operations. It is necessary to locate the oil in order that timely
measures may be taken to protect coastlines threatened by pollution.
The aircraft chosen for aerial observation must feature good all round visibility
and carry suitable navigational aids. Over nearshore waters the flexibility
of helicopters may provide an advantage, for instance, in surveying
an intricate coastline with cliffs, coves and islands. However over the open
sea the requirements for rapid changes in flying speed, direction and altitude
are less acute and instead the speed and range of fixed-wing aircraft
are generally desirable. Details see chapter 5.
It can be assumed, that nearly 90 % of the total outflow quantity of oil in
case of a tanker accident cover only 10 % of the entirely contaminated seasurface.
These slick concentrations must be selected with proper remote sensing
equipment, at least consisting of a side-looking-airborne radar for
wide range detection and an infra-red-line-scanner for the slick analysis.
In case of a spontaneous outflow e.g. with 10.000 m3 of a North Sea Crudeoil
a coverage of 25 - 50 skm can be assumed after 24 hours spreading time.
Pollutions of this dimension need the aerial assistance in order to fulfil
the a.m. assumption with a clean-up coverage capacity of at least 10 % of
the polluted surface. For predictions of the oil spill movement it has been
found empirically that floating oil will move downwind at about 3 % of the
wind speed. Additional movements of oil can be superimposed in proportion
to the presence of surface currents.
Annex 1 shows a table containing a Guide to the Relation between the
Appearance, Thickness and Volume of Floating Oil.
Strategy
Highest priority must always be given to prevention of any kind of pollution
or threat of pollution. Lightering operations can eliminate or minimise the
risk of an outflow. If - in case of a collision or grounding - ruptured
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