Degraded Blade Stiffness Impacts Floating Wind Turbine Dynamics

IntroductionBlade stiffness degradation can alter the coupled responses of floating wind turbines, but the effects of symmetric and asymmetric degradation may differ. This study investigates their influence on the blade, operational, aerodynamic and platform responses of the NREL 5 MW wind turbine supported by the OC3-Hywind spar platform.MethodsOpenFAST was used to calculate a baseline case and six stiffness-degraded cases under a common turbulent-wind and irregular-wave realization. Equivalent degradation was represented by reducing the sectional stiffness matrices of all three blades or Blade 1 by 10%, 20% and 30%, while retaining the original mass matrices.ResultsSymmetric degradation increased the deformation of all three blades by similar amounts, whereas the deformation change under asymmetric degradation was concentrated in the degraded blade. At a 30% stiffness reduction, the mean blade-tip deformation increased by approximately 33% in the symmetric case and by a comparable amount in the degraded blade of the asymmetric case. The mean operational and aerodynamic quantities and the overall platform time-domain responses remained close to their baseline values. The dominant low-frequency platform-response peaks also remained nearly unchanged. A clear difference appeared near the 1P frequency. Symmetric degradation produced only minor changes, while single-blade stiffness degradation induced pronounced narrow-band amplification in platform sway, roll and yaw. At a 30% single-blade stiffness reduction, the normalized 1P-band response reached up to 8.02 times the baseline value, although its absolute amplitude remained small relative to the overall platform fluctuations.DiscussionSymmetric stiffness degradation mainly affects blade deformation, whereas blade-to-blade stiffness mismatch has a stronger influence on lateral platform responses near the rotor rotational frequency. The 1P-band response is considerably more sensitive to structural asymmetry than the global platform time-domain statistics.