Effect of Floater Flexibility on Global Dynamic Responses of a 15-Mw Semi-Submersible Floating Wind Turbine

Abstract

Structural flexibility of the floater plays a significant role in accurate predictions of the global dynamic responses of ultra-large floating wind turbines (FWTs). The conventional global dynamic analysis of FWTs often considers the floater as one rigid body, which cannot capture the correct resonant responses of the floater (especially the column that supports the tower). In this paper, a new approach where the floater is divided into multiple rigid bodies connected by flexible beams is developed for advanced modelling of flexible floaters in combination with coupled time-domain simulations. The hydrostatic and hydrodynamic loads on each body are obtained under the assumption of one-body hydrodynamics of the whole floater. These external loads, together with gravitational loads and inertial loads, are implemented in the beam-based finite element model of the floater to carry out a time-domain analysis. The proposed approach is applied to a 15-MW semi-submersible FWT by comparing the numerical results under the rigid (one-body for the floater) and flexible (multiple-bodies for the floater) floater assumptions for the constant wind, regular wave, irregular wave and turbulent wind conditions. The effects of the floater flexibility in the responses of the tower, floater and mooring lines are investigated for these conditions.

Keywords: Semi-submersible floating wind turbine, floater flexibility, dynamic response, hydrodynamic model, structural model