TLP-Supported NREL 5MW Floating Offshore Wind Turbine Tower Vibration Reduction Under Aligned and Misaligned Wind-Wave Excitations

The paper presents numerical study of the TLP-supported NREL 5MW wind turbine model under aligned and misaligned extreme wind-wave excitations. The analyses are conducted using OpenFAST dedicated software as well as specially developed simulation framework based on Comsol Multiphysics FEM model that was imported to MATLAB/Simulink environment using eigenvalue decomposition method. The Comsol/MATLAB/Simulink simulation model was equipped with 20 ton passive tuned vibration absorber (TVA) similarly to a standard OpenFAST’s solution, or with a controlled 10 ton TVA, utilising magnetorheological (MR) damper, operating simultaneously with the force actuator (forming hybrid H-MR-TVA), or independently – forming semiactive MR-TVA. The implementation of semiactive/hybrid 10 ton TVA tuned to tower 1st bending mode for the TLP-supported NREL 5MW wind turbine is a promising alternative to 20 ton passive TVA. Both, MR-TVA and H-MR-TVA, provide more favourable tower vibration mitigation results for aligned wind-wave conditions: the reduction of maximum tower deflection reaches 11.2% with respect to 20 ton passive TVA, and 14.9% with regard to structure without the TVA. The reduction of root-mean-square tower deflection is up to 4.2% / 2.9%, respectively, for along-the-waves critical direction. TVA stroke reduction reaches up to 18.6% (regarding 20 ton passive TVA). For misaligned excitations, controlled 10 ton TVA solutions excel in tower extreme deflection reduction as well as TVA stroke minimisation. The maximum tower deflections are reduced by up to 4.3% / 4.8% with regard to 20 ton passive TVA for 45° / 90° misalignments (respectively). This is accompanied with up to 22.2% / 34.4% stroke reduction (45° / 90° misalignment respectively). On the contrary, 20 ton passive TVA is slightly better in lowering the root-mean-square tower deflections: 0.1% / 2.3% for 45° / 90° misalignments. It is also worth emphasizing that maximum stroke of the selected H-MR-TVA variant does not exceed 2.427 m across all the regarded worst-case-scenario wind and wave excitations, which opens a field for more flexible 10 ton TVA location options, including tower sections that are favourable vibration mitigation solutions’ locations. The implementation of 10 ton passive TVA tuned to tower 1st bending mode is not recommended – the uncontrolled TVA of low mass ratio is ineffective for the regarded demanding excitation conditions. The results of the current research may be used for the design of the full-scale controlled TVA system serving the TLP-supported floating offshore NREL-5MW-class wind turbine structure.