Time-domain floater stress analysis for a floating wind turbine

There are increasing focuses on developing cost-effective floating wind turbines, for which efficient stress analysis methods are needed for floater structural design. Most of the today's studies focus on global analysis methods in which the floater is assumed as a rigid body or multiple rigid bodies and the stress distributions in the floater cannot be directly obtained. As part of the COWI Fonden funded EMULF project, a summary about the methodology, the numerical modeling procedure and the verification for stress response analysis of a semi-submersible floater for a 15MW wind turbine is presented. This analysis procedure includes the regeneration of the hydrodynamic pressure loads on the external wet surface of the floater due to wave diffraction, radiation and hydrostatic pressure change, and the application of these pressure loads, together with the time-varying gravity due motions, the inertial loads and the forces/moments at the boundaries (i.e. tower bottom and mooring line fairleads) of the floater to obtain the deformation and the stresses of the floater in the time domain. The analysis procedure is implemented in a developed MATLAB code and the DNV software package. The importance of the different hydrodynamic pressure components was discussed considering representative sea states. A verification of the obtained stress time series and statistics using this method against the regeneration from a linear frequency-domain approach was made considering irregular wave actions only, and a very good agreement was obtained. The developed methodology can provide an efficient solution for structural design analysis of floating wind turbines.