Nondimensionalized semi-empirical equation to predict secondary load cycles on vertical cylinders of different diameters

Abstract

Theoretical solutions including the Morison equation have been widely used to estimate the wave forces exerted on offshore structures of vertical cylindrical configuration. However, none of those methods can accurately predict the wave force of the secondary load cycle induced by a steep wave acting on the cylinder. In this study, regular waves of different wave steepness are generated using an experimental wave flume with a piston-type wavemaker, and the forces exerted on the cylinders of three different diameters are investigated. Using the data-fitting method, the difference between the experimentally measured wave force and that calculated by the Morison equation is empirically formulated, and the effects of key non-dimensional parameters on the force difference function are analyzed. The results show that the amplitude and phase of the force difference function are mainly governed by the incident wave condition including the relative depth, relative amplitude, and relative elevation. On the other hand, the effect of the relative radius is negligible when the diameterto-wavelength ratio satisfies the small-scale structure condition. The proposed equation is valid for predicting the non-breaking regular wave forces exerted on a vertical cylinder when the secondary load cycle occurs and can be utilized in wave-structure interaction problems.