The usual method of calculating design hydrodynamic forces on a template-type structure involves the prediction of water particle velocities and accelerations from a design wave height and period. The forces on elements are then calculated from that description of the wave kinematics. Much theoretical effort has been devoted to the vital step of developing wave theories with which to predict the kinematics from the surface description of the waves. However, until recently, instrumentation permitting verification of the theories under realistic conditions has not existed. Measurements made with a wave staff and electromagnetic current meters in tropical storm Delia in the Gulf of Mexico in 1973 were apparently the first measurements of wave kinematics under storm conditions. Comparisons of these measured velocities with those predicted by unidirectional theories commonly used in design practice showed considerable scatter and a bias toward overprediction. However, velocity statistics and spectra predicted using directional spectral concepts and linear theory compared satisfactorily with the data. The comparisons support the conclusion that it is more important for a wave theory to include directional spreading effects than to account properly for the nonlinear free surface boundary conditions.
The calculation of hydrodynamic forces on a fixed structure during a storm requires knowledge not only of wave heights, but also of the subsurface flow due to the waves. The usual design approach is to use-a unidirectional regular wave theory to calculate the velocity and acceleration (kinematics) of the water below the waves and then use a force theory to calculate the forces on structural members due to that flow. The accuracy of the wave theory is thus very important to the overall accuracy of the force predictions.
Due to instrumentation problems and logistic difficulties, measurements of wave kinematics in the ocean have been rather rare. Bowden and White (1966) used the electromagnetic flowmeter developed by the Nationa1 Institute of Oceanography in the intertidal zone at the mouth of the Mersey River. Later, Simpson (1969) used the same instrument to study waves arriving at the end of a Blackpool pier. Measurements somewhat further offshore were made at the Naval Undersea Center oceanographic research tower by Thornton and Krapohl (1974) using Engineering Physics Company flow meters. Although the 19-meter water depth for this experiment was comparable to depths of interest to platform designers, the waves present consisted of a long low Pacific swell. The measurements made by the Ocean Current Measuring Program (OCMP) during tropical storm Delia (Forristall et al., 1977) seem to be the first particle velocity measurements made under storm conditions.
The wave theories presently used can be tested by taking the recorded wave height and period and using the theory to predict the horizontal velocity of the water at the depths of the current meters. Comparisons of the predicted and observed velocities show a fair amount of scatter, but also a clear and persistent bias toward overprediction.
