Effect of Nonlinear Wave Kinematics on Dynamic Response of Spars

The difference between various approximate methods to compute the wave kinematics and forces acting on a Spar platform up to the instantaneous free water surface is investigated. Three types of procedures are considered; i.e., extrapolation, stretching (both simple extensions of linear wave theory) and the hybrid wave model (which involves decomposition of the incident wave into first- and second-order components). Of particular interest for the dynamic response of a Spar are the nonlinear (second- and higher-order) low-frequency forces. The effects of the different procedures are compared analytically and numerically for the inertia forces using Morison et al.’s equation as reported in 1950, but the conclusions can be extended to diffraction theory formulations.