Wave Runup and Forces on Cylinders in Regular and Random Waves

Abstract

A small-scale experimental study was performed to investigate wave runup on rigid full-length and truncated circular cylinders under regular and random sea conditions. Wave elevations were measured around one-half of the cylinder circumference together with the total in-line wave force. The regular wave data were compared with predictions obtained using linear diffraction theory, and the results were consistent with the findings of earlier researchers. A regression analysis was performed on the regular wave data to obtain a relationship between the wave runup and the velocity head. The resulting empirical equation can be used to make preliminary estimates of wave runup on truncated circular cylinders. An approximate formula for predicting the time-dependent wave force on a truncated cylinder is presented. Transfer functions relating the enhanced wave-surface-elevation spectrum and the wave-force spectrum to the incident-wave spectrum are derived. The validity of the wave-force approximation and the adequacy of the linear transfer functions are examined in light of the new experimental data. As in the case of regular waves, the random-wave spectrum model does not adequately predict the root-mean-square (RMS) surface elevation at the front of the cylinder, but does provide a reasonably good estimate of the wave force on full-length and truncated cylinders. Extreme value estimates based on standard statistical procedures are compared with the experimental data and generally found to be conservative.