Slow motion responses of compliant offshore structures

An efficient methodology is developed to predict slow-drift responses of slender compliant offshore structures due to ocean waves. It uses a hybrid wave model for computing the incident wave kinematics and the Morison equation for wave loads. An iterative-incremental Newmark-Beta approach is employed to simulate the structure responses in the time domain. The hybrid wave model considers the nonlinear interactions between the wave components in incident waves and hence is able to predict wave kinematics accurately up to the second order of wave steepness. It is observed that when the hybrid wave model is used for computing wave kinematics, the predicted slow-drift responses of a JIP (Joint Industry Project) spar are in excellent agreement with the measurements of the model tests. However, the corresponding predicted slow-drift responses using Wheeler stretching and linear extrapolation methods for wave kinematics do not agree with the measurements.