This paper presents the result of a series of experiments of forces on a fixed vertical truncated column due to Stokes 5thLorder - like waves in wave tank. An effort was made to generate the waves as close as possible to the theoretical Stokes 5th-order waves. The horizontal and vertical velocities were measured under a typical wave to find the difference between the theory and experiment. The measured forces (Fx, Fz) increase almost linearly with the wave steepness and with wave period at the given steepness. The horizontal and vertical force TF (Fx/H/2 and Fz/11/2) due to a 2.0 s period wave at H/L =0.049, amount to 1.35 and 1.05 times the corresponding LTF. The theoretical LTF underestimates the measured horizontal force LTF, while overestimates the measured vertical force LTF.
Hogben and Standing (1975) compared linear and Stokes 5th-order theory predictions of the inertia force on a bottom-mounted column using Morison load model and concluded that the difference between the linear and nonlinear forces for typical north sea wave conditions was not large. The difference was attributed to the wave deformation due to the surface-piercing structure, which cannot be accounted for by the linear theory. The above nonlinear problem may formally be investigated by extending the diffraction theory to 5th order approximation on the basis of the Stokes expansion procedure. The 2nd-order approximation was done by Lighthill (1979), Molin (1979) and others. The 3rd-order diffraction forces were studied by Faltinsen et al. (1995) and Malenica and Molin (1995). There arc two other potential alternatives, both in timedomain simulation approach. One of them is numerical wave tank (NWT) and the other one is the universal linear system model (ULSM) for prediction of the particles horizontal velocity field including the crest region.
