Analysis of Simulataneous Wave Force and Water Particle Velocity Measurements

Abstract

ABSTRACT A constant drag coefficient of 0.61 was calculated over a Reynolds number range of 2×105 to 8×105 from simultaneous measurements of-wave-induced velocities and forces. The average inertia coefficient was 1. 20. The constant drag coefficient found for a Reynolds number as low as 2×105 allows a simulation of design wave force with a smaller scale space frame structure. The drag (inertia) coefficients were determined from drag (inertia11y) dominant portions of individual wave cycles. The measured wave force power spectrum agrees well with that calculated from measured velocities with average drag and inertia coefficients. INTRODUCTION The objective of this study was to establish the feasibility of measuring wave forces on a small-scale structure that could be scaled up to typical design loads on a conventional jacket-type offshore platform. It is essential to show that a supercritical flow condition (typified by constant drag coefficient) occurs over the entire structure to make such scaling possible. It is not possible, however, with existing wave-force data to show at what Reynolds number this supercritica1 condition begins, because the water-particle velocity must be inferred from the surface wave elevation. By measuring both the wave force and the particle velocity, one can remove the uncertainty due to wave theory and possible steady ocean currents affecting the calculated drag coefficient. When unambiguous data that can be scaled up to typical design loads are obtained from a smaller scale test structure, they can be used to calibrate present-day design wave-force calculations [4]. Currently used design force calculations for a space-frame structure are based on wave-force measurements on a single pile, typically the wave-force data collected by Chevron during Hurricane Carla [2,3,9]. Design waves are assumed to be smooth and long crested (two-dimensional). Actual waves, however, are neither. The spatial distribution of particle velocities calculated for idealized design waves can be different from those for real, random waves. For the same average velocities, the drag forces on structural members are subject to random variation due to turbulence. The net result of these effects can be randomly varying force vectors over the space of a structure. Because of the changing directions of force vectors, not all of the wave forces are additive. The actual structural load in real waves may be different from when the forces are assumed in the same direction over the entire structural space, as in present design wave-force calculations. Scaling Wave Forces In order to simulate one physical system with another of different size, the two systems are required to be geometrically similar. In addition to geometrical similarity, certain dimensionless physical parameters must be preserved so that the experimental results of one system can be applied to another. Such parameters can be found by dimensional analysis of the governing equations for the physical system under consideration.