Generation and analysis of multi-directional waves

Abstract

Real sea states cannot be represented adequately by a single sine wave. Indeed, wind-generated waves in the ocean have obviously different amplitudes and frequencies, but also come from different directions. Consequently, the distribution of energy with respect to wave direction, which is commonly known as the directional spectrum, is very important in the design of submerged or semi-submersible structures. The use of directional seas in design provides a better representation of the ocean surface and a basis for minimizing the over-design of offshore structures. This study examines the means of generation of such types of environments in the wave basin of the Offshore Technology Research Center. Series of tests, for a wide variety of multi-directional waves, are conducted in order to gather data from wave elevation recorders at different locations in the tank. Drive signals for the pacemaker are computed according to the wave elevation variance table of the sea state to be simulated. Matlab programs, using several spreading functions, are used to prepare the input files. The analysis of the data relies on complex techniques, which will be implemented through the use of either Dr. Borgman's program NUSPEC or the GEDAP analysis software package used at the center. A large part of the results will be presented as contour and three-dimensional graphs, that allows thereafter to compare the different models and also draw some conclusions about the limits of each technique. The final part of the study taluses on freak waves. This interesting type of waves can be modeled by monochromatic waves converging towards the center of the wave basin. Although these waves are quite unusual in the real world, their action on structures at sea can be very destructive. A comparison to the usual type of directional seas is made.