A third order oscillatory perturbation expansion for sums of interacting long crested Stokes waves is used to represent the waves affecting a vertical truncated circular cylinder. The wave model accounts in part for various nonlinear wave properties such as higher wave crests, shallow wave troughs, the increase of the wavelengths for fixed wave periods, the changes in phase speed for the higher waves and the variation of the velocIty field with depth. Storm-like nonlinear irregular waves are simulated by the Stokes higher-order waves to investigate the extreme particle speed, extreme pressure and extreme horizontal forces. It was found that the maximum particle speeds of the third-order model are smaller than those of the secondorder model for the same maximum wave height. It was also found that the maximum pressure and maximum horizontal force in the third-order model are larger than those in the second-order model.


Competitive and economic design of offshore structures depends on the improved prediction of extreme wave kinematics and extreme wave loads in stormy seas. Forristall (1985) developed a technique for prediction of irregular wave kinematICS using a kinematical boundary condition fit (KBCF). Forristall compared the KBCF to the Delft wave tank measurements of two-dimensional irregular wave kinematics (Bosma and Vugts, 1981) and found excellent agreement. This wave was a linear irregular wave that is obtained by the superposition of Airy waves (St. Denis and Pierson, 1953). Forristall (1986) used the KBCF for comparison with the FULWACK (storm waves) experiment and obtained good agreement. The experiment was designed as a full scale verification of the theory (KBCF), which then had to be extended to three dimensions to include the directional spreading of natural waves. Pierson (1992) developed a Stokes third-order nonlinear interacting irregular wave model which deals with long-crested and deep sea conditions.

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