Various methods of simulating wave kinematics on the structural members of 100-year responses

Abstract

The main force acting on an offshore structure is usually due to wind-generated random waves. According to the Morison equation, the wave force on a cylindrical member of an offshore structure depends on wave kinematics at the centre of the element. It is therefore essential to accurately estimate the magnitude of wave-induced water particle kinematics at all points in a random wave field. Linear random wave theory (LRWT) is the most-frequently used theory to simulate water particle kinematics at different nodes of an offshore structure. Several empirical techniques have been suggested to provide a more realistic representation of the near-surface wave kinematics. The empirical techniques popular in the offshore industry include Wheeler stretching and vertical stretching. Most recently, two new effective methods (effective node elevation and the effective water depth) have been recently introduced. The problem is that these modified methods differ from one another in their predictions. Hence, the aim of this study is to investigate the effects of predicting the 100-year responses from various methods of simulating wave kinematics accounting for the current effect. In this paper, four versions of the wave kinematics procedure have been tested by comparing the short-term probability distributions of extreme responses. For all current cases, the highest vertical ratios for zero, positive and negative current cases are 1.414, 1.175 and 1.831, respectively. It is observed that even for positive-current cases, the difference between Wheeler and vertical stretching predictions is quite high and cannot be neglected. Thus, further investigation is necessary to resolve this problem and the outcomes in providing useful design information for the oil and gas industry.