The present chapter describes a numerical model complex for hindcasts of waves, currents and water levels in the central and southern parts of the North Sea. The model complex is based upon two generalized numerical modelling systems, SYSTEM 20 for waves and SYSTEM 21 for currents and water levels. The theoretical background for the two numerical modelling systems is described, and the validity of the North Sea models is demonstrated through comparisons with measured data. Finally, the use of the model complex in a reassessment of environmental design data for the Ekofisk field is described.
Accurate predictions of extreme values of winds, waves, currents and water levels are essential basic data required for the design of marine structures. The safety of the structures, as well as the possibility of developing an economic design, relies above all on the reliability and accuracy of the underlying design data.
The growth in offshore activities through the 1970s and early 1980s has led to the development of increasingly advanced tools for the accurate assessment of environmental design data. The hindcast techniques, which are used for computing the wave, current and water-level conditions from known (measured and analysed) historical meteorological conditions are today considered an integral part of the accepted procedures for design data assessment. The advantage of using hindcast data covering the most severe events during a period of many years, compared to using measured data from a relatively short period of time, is well known and well documented by now (see, e.g., Wang and Mehaute, 1983). In addition, the hindcast techniques provide a spatial resolution which cannot in general be achieved by measuring.
The two most important modelling systems applied by DHI in hindcast projects are SYSTEM 20 for hindcasts of waves and SYSTEM 21 for hindcasts of currents and water levels. Both systems were developed by SHI in the 1970s and both systems have been applied in numerous hindcast studies all over the world during the past decade. A brief description of each of the two systems is provided in this section.
The DHI wind wave modelling system, SYSTEMS 20, is a second-generation discrete spectral type modelling system which described the wave field by the direction-frequency wave energy spectrum. This means that the area to be modeled is represented by discrete points in a grid, each point representing an area of ¿x ¿¿y km2, where ¿x and ¿y are the mesh widths in the x-and y-directions respectively. The water depth at each point is specified ads the average depth of the area represented by that point. Figure 1 shows the model grid of the SYSTEMS 20 North Sea model. Two areas with embedded fine gird models are also shown.
SYSTEM 20, being a discrete spectral model, furthermore implies that the wave conditions for any given location (grid point in the model) are described in terms of the distribution of wave energy in a number of energy.