Simplified Modelling for Risk Assessment of Hydrocarbon Spills in Port Area

Abstract

Several authors, on the basis of mathematical models of different complexity, have faced the study of dispersion and transformations following an open sea oil spill. The different phenomena involved in the release evolution were quantified: convective transport by sea currents; wave action; turbulent diffusion; buoyant action on dispersed particles; superficial spreading of the oil due to different density between sea water and hydrocarbon; and evaporation and emulsification rates. In dealing with a release in the port area, besides the environmental consequences, the most severe hazards are linked to fires and explosions due to the formation of a flammable cloud by evaporated hydrocarbons and air and subsequent ignition. If a cloud of sufficient size is formed and ignition occurs instantly, a large fire, jet flame or fireball may occur, but significant blast-pressure damage is unlikely. Instead, the blast effect produced by vapour cloud explosion (i.e. cloud formation and ignition delayed typically by some minutes) can vary greatly depending on the flame propagation velocity, confinement and turbulence and can result in extensive damage. This paper considers such events in confined areas, focusing on individual intervention time and types more suitable for containing the risk on an acceptable level. Considering that intervention time and dispersion time are comparable, the basic variables allowed for in developing the mathematical model were oil spill spreading rate, hydrocarbon evaporation rate, cloud transport and dispersion into the atmosphere. The simplified model allows identifying risk areas and intervention times in confined regions and ports of relative simple geometry. The same model can also be applied to more complex geometry, typical of port areas, obtaining conservative results for a preliminary risk evaluation. The results of the model, even if conservative, have not yet been tested with laboratory or full-scale data.