Storm Wave Kinematics

Abstract

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.

Considerations in the Design Op an Offshore Data Collection Program

“…It is interesting to note that in the field of wave measurement, the instrument types capable of measuring wave direction are limited primarily to wave staffs. It is likely that measurement of wave direction will become more important, particularly in light of some recent results reported by Forristall et al (1978) which indicate that wave direction is an important consideration in wave force calculations. Several manufacturers including Datawell and ENDECO are planning "to manufacture buoys which will monitor both wave direction and vertical displacement.…”