Vibration control of floating offshore wind turbines using optimally designed negative stiffness absorbers

Vibration mitigation of the IEA 15-MW Floating Offshore Wind Turbine (FOWT) on the UMaine VolturnUS-S semi-submersible platform is investigated using Vibration Control Systems (VCS), substantially attenuating the coupled tower-platform pitch response and reducing structural loads. A novel application of negative stiffness (NS) KDamper-based absorbers to FOWT is proposed, comprising the KDamper and its extensions, benchmarked against the Tuned Mass Damper. A reduced-order finite element model of the coupled tower-platform system accounts for platform pitch kinematics and second-order geometric effects, validated against OpenFAST through free-decay simulations and under wind-wave loading. The KDamper VCS are formulated via a unified NS network parameterisation, dimensioned through component sizing, and optimised via constrained Harmony Search to determine absorber parameters. VCS performance is assessed, under open-loop loads extracted from fully-coupled OpenFAST simulations, across three combined wind-wave load cases of increasing severity. The EKD and EKDI achieve reductions of up to 42% in tower-top displacement and 37% in platform pitch energy-integral metrics, relative to the baseline, requiring one-tenth the TMD added mass through NS inertia amplification.