A robust estimation of the response of floating wind turbines through piecewise linearization

Abstract

Abstract Floating wind turbines (FWT) enable access to substantial wind resources in deep waters. They are hence anticipated to contribute significantly to the carbon-neutral target. Popular simulation tools for this relatively new offshore technology adopt the linear potential flow theory borrowed from the offshore oil and gas industry to evaluate the hydrodynamic forces, which are calculated around the equilibrium position of the platform. However, the compliance of the floating platform can potentially lead to large motions under combined wind and wave actions. To address this issue, the present work proposes a new piecewise linearization approach that can capture the nonlinearity by re-linearizing the wave-platform interaction system at instantaneous platform positions (operating points). A state-basis transformation algorithm is developed to ensure that the consistent physical basis is used across all operating points when calculating the fluid radiation force using the state-space representation. This new approach is implemented in a FWT Simulink model, and an open-source boundary element method code, Nemoh, is used to calculate the hydrodynamic force for the linearized wave-platform system at each operating point. Free vibration tests of a 5-MW ITIBarge FWT are examined to demonstrate the effectiveness of the piecewise linearization approach. The results obtained by this new approach are compared to the common practice of linearizing around the equilibrium, and the new approach is found to be able to conduct a fast and robust evaluation of the nonlinear hydrodynamics for FWTs.