Wave Forces on a Single Surface-Piercing Column: Comparisons Between Theory and Experiment

This paper concerns the nonlinear wave loads acting on a vertical, surface-piercing, column. New laboratory data is presented describing the loads acting on three columns, of varying size, subject to both regular and irregular wave fields; the latter corresponding to realistic JONSWAP spectra. In all cases the flow fields are such that the potential flow forces dominate; with the smallest column diameter lying well outside the linear diffraction regime (D/λ << 0.2, where D is the diameter and λ the wavelength) and the largest just within it. In each case a variety of nonlinearities are considered, together with a range of directional spreads. The measured data, describing the total base shear, and over-turning moment are compared to a second-order diffraction solution and a number of higher order potential force models. The results of these comparisons highlight those flow regimes in which the greatest uncertainty remains. Earlier work, reported by Swan et al. [1], corresponding to the smallest column diameter considered here, showed that the unexpected scattering of high-frequency waves may lead to additional nonlinear loading. The present data shows that the earlier results are in no sense typical of those occurring across the diffraction regime. Indeed, by considering a number of column diameters it is possible to demonstrate that there are a wide range of flow conditions in which the existing potential flow models provide an adequate representation of the applied loading; even in relatively steep waves. Nevertheless, there are also important flow regimes where unexpectedly large nonlinear or high frequency forcing can arise. The purpose of the present paper is to demonstrate both of these cases, to highlight the flow conditions in which each exists, and to comment on the wave conditions leading to the most extreme loading.