Drift of an articulated cylinder in regular waves

Abstract

Potential flow theory is used to investigate the wave induced harmonic response and the mean drift of an articulated column in regular waves. The mean drift horizontal force is evaluated by means of the Stokes expansion to second order in wave steepness. Analyses based on both near field and far field formulations are shown to give identical expressions, provided that the second-order forces at the intersection between column and seabed are included in the near field approach. The latter have not been considered in previous studies concerned with drift forces on floating bodies. It is shown that the drift forces on a column, although of second order, can excite pich responses of first order: this is because articulated columns are designed to have a low natural frequency in the tilt mode, relative to wave frequencies. Comparison of the theoretical results with experimental data, from a model tested in regular waves, suggests reasonable agreement for the drift forces over a range of frequencies and two wave heights.