Gulfspill Version 2.0: a software package for oil spills in the Arabian Gulf

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Abstract

A software package is described for the prediction of the movement and fate of oil spills in the Arabian Gulf. The package is designed to run on a PC operating under Windows and has user-friendly graphical interfaces for model input and for examination of the output. The package contains several optional programs: a simple trajectory model, as well as models for large-scale spills that encompass a significant area of the water body and for smaller spills that remain localized in an arbitrarily small sub-region. The models are based on a three-dimensional hydrodynamical model of the Arabian Gulf that has been tested and calibrated to a high degree of accuracy. The fate of the oil encompasses the processes of evaporation, emulsification, dispersion by wave action into the water column, beaching on the coasts (and possible re-entry into the water body) and mechanical spreading. Examples are shown to illustrate typical input and output screens and to demonstrate the accuracy of the model.

Introduction

For many years the KFUPM Research Institute has had operational numerical models for determining the movement and fate of oil spills in the Arabian Gulf (Lehr et al., 1982, Belen et al., 1983, Lardner et al., 1988a, Lardner et al., 1988b, Al-Rabeh et al., 1989, Al-Rabeh and Gunay, 1991, Al-Rabeh et al., 1992, Al-Rabeh, 1993). Initially these were designed as research tools for use by the Institute's staff in contracted projects. Later they were made easier to use by the addition of graphical interfaces for both input and output and about two years ago Version 1.0 of the model OILPOL was made generally available, upgraded a year later to Version 1.1 (Al-Rabeh et al., 1995). Subsequent work has led to numerous improvements in the numerical model and in the user interface, and Version 2.0 of OILPOL has now been released. At the same time, three other related models, GULFTRAK, OILPOL-S and QUIKSLIK, have been developed and now all four models have been released as a single package under the name GULFSPILL. This new package is, we believe, the most accurate, comprehensive and user-friendly package of oil pollution models of the Arabian Gulf that is available and its description will be the goal of this paper.

OILPOL is a large-area model with the water velocities computed using a three-dimensional hydrodynamical model with a 10 km grid covering the whole Gulf. It is suitable for prediction of the behaviour of large oil spills for which the slick spreads over a wide area, the 1991 Al-Ahmadi spill being an extreme case in point. It runs efficiently, the hydrodynamics taking less than 1 s of CPU time on a Pentium 133 for each hour of simulation, while the oil fate and transport takes a further 1 to 5 s depending on the size of the spill.

OILPOL-S is designed to provide more detailed and accurate computation for a spill that is restricted to a limited sub-region of the Gulf. The hydrodynamics in the sub-region is computed using a finer grid size, to be chosen by the user, with the boundary conditions on the outer boundary of the given sub-region being computed by an algorithm similar to that used in OILPOL except that a 5 km grid is used to cover the whole Gulf. The cost of this extra detail is considerably longer CPU times for the hydrodynamical part of the computation, the minimum for a small sub-region being about 10 s for each hour of simulation on a Pentium 133 while for a sub-region grid of dimension 100×100 and mesh-size 500 m this increases to around 50 s.

Most oil spill simulation models in use today either have very crude models of the hydrodynamics of the relevant water body or ignore them altogether, simply taking the rule that the oil moves with a certain fraction of the wind speed at a certain angle to the wind direction. Such models have two advantages: they can rapidly be applied to any sea region and they do not require as much computer time to run as a model that computes the full hydrodynamics. While models developed specifically for a given region can always be expected to be more accurate for spills in that region than such general purpose models, it is unavoidable that they do require longer CPU time. For this reason we have included in the package a program called QUIKSLIK that is the same as OILPOL except that the hydrodynamics is replaced by a simple wind velocity rule. The user who needs a quick and rough answer regarding the movement of a spill can opt to use this module.

The fourth module in the package, called GULFTRAK, is a trajectory model that can be used to compute the track of any observed floating object in the Gulf, for example an observed patch of oil. It contains no fate algorithm but simply predicts the future position of the oil. The hydrodynamics is computed by a three-dimensional model similar to that used in OILPOL. This model also has a hindcast facility, that is the track can be traced backwards in time in order to locate possible sources of the spill.

In Section 2 we shall briefly summarize the theoretical basis of the models and shall provide some examples showing comparisons of model output with measured data. We shall also discuss the modifications and improvements in Version 2.0. In Section 3 we shall describe the graphical interfaces in the GULFSPILL package.

Section snippets

Hydrodynamics

The three basic questions of practical importance concerning a given oil spill are where will it move to, when will it get there, and what will its state be when it arrives. Obtaining a reasonable answer to the third question requires the use of reliable fate algorithms, which we shall discuss later. The first two questions are generally the more critical as far as combatting the spill is concerned, and predicting their answers accurately depends on having a good model of the hydrodynamics of

The graphical interfaces

The GULFSPILL package comprises the four programs, OILPOL, OILPOL-S, QUIKSLIK and GULFTRAK. Each of these is designed as three modules: an input interface, a run module and an output interface. The run module has been coded in FORTRAN while the two interfaces were developed in Microsoft Visual Basic. In this section we shall give a brief description of the main features of these two graphical interfaces. We shall describe the interfaces for OILPOL-S and GULFTRAK; the interfaces for OILPOL and

Discussion

The GULFSPILL package contains programs that allow the prediction of the transport and fate of spilt oil in the Arabian Gulf, either large spills affecting a substantial part of this water body or small, localised spills. One component of the package allows either forecasts or hindcasts of the motion of any floating object (such as a patch of oil).

The package is founded upon a three-dimensional model of the hydrodynamics of this water body that has been extensively tested and calibrated. It is

Acknowledgements

The authors would like to acknowledge the support provided by the Research Institute of the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.

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