An experimental study on higher order waves and hydrodynamic loading on vertical surface piercing cylinders

Abstract

Estimation of second order wave loading on offshore structures has attracted recent attention in research. This thesis presents the results of an experimental study carried out to quantify second order wave forces on a vertical cylinder in a wave tank. The initial part of this thesis is directed towards formulating and analyzing the wave field present in the laboratory. These results formed part of the input for further studies on wave forces. Due to the physical limitations of the wave tank, it was found that several waves coexist with the progressive wave of interest in the tank. An analysis procedure was developed using a Fast Fourier Transform technique and a least squares curve fitting method to separate the wave of interest from the side effects and identify its principal parameters. Several side effects were quantified in the process. Wave forces on a vertical cylinder to second order were theoretically formulated from a literature survey. Analysis of the measured wave forces in the wave tank involved using a fast fourier transform to identify the first and second order force components. Towards the end of the study, it was found that the proposed formulations for the total wave field in the tank and the wave forces on a cylinder were adequate. There exist many phenomena in a wave tank whose effects on measurements requires further research. It is emphasized that systematic experimental studies of higher order waves form an important part of offshore research development.