OPERA (oil pollution: environment risk assessment) was a joint research project dealing with the development of an ensemble of software packages related to oil pollution. The basic software developed consists of 3 components:
a conventional vertically integrated hydrodynamic model able to compute tide and wind induced circulation in shallow seas;
a model able to predict the movement, spreading and aging of oil slicks at the surface of the sea;
a highly accurate advection diffusion model capable of describing the behaviour of the dissolved fraction.

The governing equations of the hydrodynamic model are conventional vertically integrated shallow water wave equations.
In the surface model, the oil released at sea is represented by one or several slicks, of which the movement, spreading and ageing are computed. The velocity of the centre of mass of patch is a linear function of the wind speed and of the water current. The spreading is based on the assumption that the shape of the slick remains circular as time passes. The ageing processes taken into account are: evaporation, emulsification, dispersion, dissolution and direct sea air exchanges.
A numerical scheme has been developed to compute the solution of the vertically integrated equation expressing the conservation of mass of a dissolved substance. The time stepping procedure is based on the implicit alternate direction approach. The spatial derivatives are approximated by means of compact fourth order differences using the Adam method.

During the coding of the various computer programs, particular attention has been paid to using only one standard programming language, FORTRAN 77. The models have been implemented on several computers ranking from a supercomputer (a Convex C230) to a portable computer (a Zenith turbo 386). The models are not site specific and they can be used for a large variety of computational domains. A pointer system has been defined and is used to identify the characteristics (coastlines, open boundaries, etc) of the area of interest. Two coordinate systems, a Cartesian coordinate system and a spherical coordinate system, can be handled by the models.

A 3-dimensional oil spill model, PARCEL, has also been developed, based on the particle approach, where the oil volume spilled at sea is divided into a large number of particles which are tracked individually. During the course of an integration, the particles can be at the sea surface or in the water column and the horizontal and vertical movements of the oil droplets are computed.