Field verification of linear and nonlinear hybrid wave models for offshore tower response prediction

Accuracy of the prediction of the dynamic response of deepwater fixed offshore platforms to irregular sea waves depends very much on the theory used to determine wave kinematics. A common industry practice consists of using linear wave theory, which assumes infinitesimal wave steepness, in conjunction with empirical wave stretching techniques to provide a more realistic representation of near-surface water kinematics. The current velocity field is then added to the wave-induced fluid velocity field and the wave-and-current forces acting on the structure are computed via Morison's equation. The first objective of this study is to compare the predicted responses of Cognac, a deepwater fixed platform, obtained from several popular empirical wave models with the response Cognac predicted based on the hybrid wave model. The latter is a recently developed higher-order, and therefore more accurate, wave model which satisfies, up to the second-order in wave steepness, the local mass conservation and the linear free surface boundary conditions at the instantaneous wave surface. The second objective of this study is to correlate the various analytical response predictions with the measured response of Cognac. Availability of a set of oceanographic and structural vibration data for Cognac provides a unique opportunity to evaluate the prediction ability of traditional analytical models used in designing such structures. The results of this study indicate that (i) the use of the hybrid wave model provides predicted platform response time histories which overall are in better agreement with the measured response than the predictions based on the various stretched linear wave models; and (ii) the Wheeler stretching technique produces platform response time histories which overall are more accurate than those obtained by using the other stretching schemes considered here.