Modelling of the behaviour of hydrocarbons

The behaviour of hydrocarbons ("oil" or "petrol") at sea is very complex and involves numerous physical and chemical processes.

Hydrocarbons are lighter than water. When petrol is discharged on the surface of the sea – voluntarily or accidentally - it will float and spread by the effect of its own weight and the forces of the currents and the wind. These forces will combine and result in movement of the slick. As time passes, the lighter components will evaporate and others will dissolve in the water. Due to the agitation of the waves, little drops will disperse in the water column. Certain types of petrol will form an emulsion of water and petrol that will stay on the surface. The speed of these different physico-chemical phenomena depends on the nature of the petrol, the speed of the wind, the height of the waves and the temperature.

When pollution occurs, it is very useful to know how it will evolve. To help the surveillance teams, prediction models are used of growing complexity, depending on the available information of the pollution. These models use meteorological and hydrodynamic predictions.

Two models are available. The first one uses an idealised representation of an oil slick: a circle with radius, which grows as function of the time and the physico-chemical evolution of the oil (see figure, left). The resulting information is enough to assist on surveillance and pollution combatting operations at sea. Another model allows a more realistic representation of the situation (see figure, right). This is useful when there is risk of impact on the coast and for other models that quantify the effect of the pollution on living and non-living resources.