Abstract
Semi-submersible platforms are widely adopted as support structures for floating wind turbines (FWT). Morison-like, quadratic drag terms are typically used for modelling viscous loads on the platform columns and pontoons, with drag coefficients obtained from model tests. One of the challenges when adopting this modelling approach is to achieve a simultaneous representation of damping effects and viscous excitation. In this paper, a systematic procedure is presented to calibrate the drag coefficients of a semi-submersible platform supporting a 12 MW FWT, based on model test results. First, the pontoon coefficients are calibrated based on the heave and pitch motions. Then, it is assumed that viscous excitation in surge is mainly associated with the upper part of the columns, which are subjected to more significant action of the waves. Thus, the coefficients at this portion of the columns are tuned to retrieve the low-frequency motion components not captured by potential theory alone, while the coefficients at the bottom of the columns are adjusted for achieving the necessary level of damping. The final coefficients are chosen based on comparison between the model test results and multiple combinations of the drag coefficients associated with different components of the platform.
