Impacts of Mooring-Lines Hysteresis on Dynamic Response of Spar Floating Wind Turbine

Abstract

Floating wind turbines often experience larger-amplitude motions caused by wind and ocean wave loads, while mooring-lines, such as catenary and taut mooring-lines, make the structure configurations along with an analysis of the global response more complicated compared to a fixed support foundation. Moreover, the restoring performance of dynamic mooring-lines exhibits a significant hysteresis behavior, and this hysteresis behavior may have profound impacts on the structural response of floating wind turbines under environmental loads. In this study, using the coupled finite element method, a dynamic simulation model is developed to study the motion responses of a spar floating wind turbine under consideration of mooring-lines hysteresis. In order to consider large-amplitude motion and nonlinear behaviors of catenary mooring-lines, a FEM (finite element method) model is developed based on a combination of 3D nonlinear beam elements and the super-element approach, and the interaction between mooring-lines and seabed is also included. Using our FEM numerical simulations, firstly, the restoring performance of mooring-lines and its hysteresis behavior are studied. Then, the motion responses, e.g., the displacements of the spar float undergoing various wave loads, are examined. The numerical results show that: the restoring stiffness of mooring-lines exhibits significant hysteresis behavior, and the restoring force is directionally dependent. Due to the hysteresis of restoring performance, for a case of regular wave conditions, little change of the spar surge in a steady-state is seen; however, for a case of extreme wave loads, the motion response gets about 14.4% smaller, compared with the quasi-static cases.