Wave Impacts on the Column of a Gravity Based Structure

Abstract

An accurate prediction of the wave loading on the columns of any offshore structure is an integral part of the design / re-assessment process. Earlier studies have investigated the nonlinear loads on single columns covering a wide range of diameters (expressed as a ratio of the incident wave length) and addressing a variety of incident wave steepness. As a result, it is generally accepted that wave-structure interaction effects can be important even outside the diffraction regime and may be one possible cause of structural dynamic response. The importance of wave-structure and wave-wave interaction effects will again be highlighted in the present study; the emphasis of the work being the occurrence of wave impacts on the columns of a gravity-based structure in random seas involving very large, steep waves. This paper presents the results of an experimental study of a gravity based structure in a severe wave climate. The structure comprises an arrangement of storage caissons located on the sea bed in relatively shallow water, with four large diameter surface-piercing columns. The primary purpose of the present paper is to provide a comparison between the incident waves and the measured loads. This shows that robust predictions of the slamming load cannot be made without taking into account the geometry of the structure, including that of the underlying caissons. It is also shown that the largest loads frequently do not correspond to the tallest or steepest incident waves. This emphasizes the importance of correctly incorporating the nonlinear wave-wave interactions and hence the need to undertake long random wave tests in offshore engineering design.