Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). To avoid this problem, empirical techniques such as Wheeler and vertical stretching methods are frequently used to provide a more realistic representation of the wave kinematics in the near surface zone. Previous investigation shows that these two different methods of simulating water particle kinematics on the probability distribution of extreme responses could be significant leading to uncertainty as to which method should be used. Modified version of LRWT; effective node elevation and effective water depth methods are introduced which would significantly reduce the computational effort for evaluation of water particle kinematics in the near surface zone. While the offshore industry recognizes that different methods of simulating water particle kinematics lead to different responses, no systematic investigation has been conducted to investigate the effect of this on the probability distribution of the extreme responses. Thus, in this paper, the more efficient time simulation (ETS) method has been used to compare the magnitude of the 100-year responses derived from different methods. First, these methods and their differences are reviewed. Then, the ETS method will be used to calculate the 100-year responses from different methods are compared and the design implications will be commented on.


Offshore structures are used worldwide in a variety of water depths and environments and for a variety of functions. However, their primary use is for extracting oil and gas from the bottom of the sea. These structures must be designed to withstand a variety of loads such as the gravitational load, earthquake load together with wind and wave forces during their intended service life. However, the dominant load is generally due to wind-generated random waves. Morison's equation (Morison et al, 1950) is frequently used to calculate wave hydrodynamic loads on the cylindrical members of an offshore structure from wave-induced water particle kinematics. It can therefore be concluded that accurate estimation of wave-induced water particle kinematics is a key step for accurate prediction of wave loads on the structure.

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