Nonlinear Hydrostatic and Hydrodynamic Effects of Semi-Submersible Floating Offshore Wind Turbines

Floating offshore wind turbines (FOWTs) are subject to nonlinear hydrostatic and hydrodynamic loads due to their intricate platform geometry. However, most fully coupled simulation tools for FOWTs rely on linearized hydrostatics and frequency-domain potential flow models transformed into the time domain, which assume a stationary waterline and wetted surface. To overcome these limitations, this study develops an innovative coupled simulation tool named FAST2WASIM (F2W), which incorporates nonlinear hydrostatic and hydrodynamic effects while preserving computational efficiency for engineering applications. This paper first describes the overall framework of the F2W methodology, outlines its underlying hydrodynamic theory, and presents the numerical model of the OC4 DeepCwind semi-submersible FOWT. Response predictions under a range of test conditions are then compared between FAST and F2W, demonstrating the validity of the proposed tool and revealing the inadequacies of conventional linear methods in predicting the hydrodynamic behavior of semi-submersible FOWTs. Finally, a novel structural analysis workflow for semi-submersible FOWTs based on F2W is introduced, and its differences from the traditional approach are examined in terms of computational time and structural stress outputs. This work offers an efficient and high-fidelity approach for simulating nonlinear hydrodynamics of semi-submersible FOWTs and provides valuable insights for practical engineering design.