Hydrodynamic studies of a 15 MW semi-submersible FOWT to assess the suitability of the inclusion of a damper system

Floating Offshore Wind Turbines (FOWT) can exploit the high energy density found in the offshore environment, with turbines now reaching up to 15 MW in size. At the same time, however, the energetic environment and the massive size of the device present significant challenges in the motion stabilization and mooring system. To overcome these challenges, a tuned mass damper (TMD) has been considered for integration in the FOWT for peak motion reduction. This paper investigates the baseline responses including motion, dynamic response, and tensile loading of the mooring line for a 15MW FOWT on a semi-submersible platform without TMD to identify the damageable motion and the impacts of the TMD on the motion response under wave-wind environmental loadings. The comprehensive analysis is conducted in a package for the dynamic analysis of offshore marine systems, named as Orcaflex. The dynamic and motion characteristics of the 15MW FOWT are analysed and compared under different environmental parameters. The wave and wind parameters are quantified by the 20-years statistical data of the Celtic Sea including both operational and extreme conditions (with a 50-year return period). Subsequently, the key parameters of TMD are investigated by configuring different combinations of mass, damping coefficients and stiffnesses. The preliminary results of the study show that the TMD system can successfully mitigate extreme motion characteristics, however this is strongly dependent on damping properties. Unsuitable TMD designs may increase the motion responses of FOWT and the tensile loading on the mooring line. Therefore, the TMD properties have to be adjusted based onsite environmental conditions). With this consideration, an active TMD with changeable damping properties will be conducted in future research.