Insights from detailed numerical investigation of 15 MW offshore semi-submersible wind turbine using aero-hydro-servo-elastic code

Abstract

Offshore wind turbines are getting larger in terms of dimensions to maximize power generation. Large wind turbine offshore structures can be installed in deep water locations, where they have the advantage to generate higher power due to high wind speeds. To achieve this, wind turbine manufacturers are introducing larger and scaled-up designs to meet the global energy demands. However, these models suffer from large structural loads and deformations when subjected to varying environmental loadings. Therefore, it is necessary to fully comprehend the multibody coupled dynamics of such large offshore structures before proceeding towards their installation in open oceans. This paper carries out an extensive investigation on the ultimate load analysis and fully coupled dynamic response of the 15 MW reference turbine mounted over a UMaine VolturnUS-S semi-submersible floating platform. We used OpenFAST to carry out the stability and full-system linearization studies followed by a detailed time-domain analysis for the power production, parked and fault conditions. We began with a brief background theory on linearization and eigenanalysis for identifying the natural frequencies followed by discussion of the Campbell diagram showing the full-system natural frequencies. It was observed that the multi-blade coordinate code does not correctly pick the natural frequencies. Additional free decay tests have been carried out to verify the full-system natural frequencies. The variation of mean, minimum and maximum values of various characteristic loads and platform motions with respect to different environmental conditions are also presented. We also discussed the change in dynamic response due to the yaw misalignment, wind–wave misalignment, and second-order wave forces. The parked and fault conditions were found to be the critical design load cases (DLCs) for the design of IEA 15 MW wind turbine mounted on Volturn US-S semi-submersible. Furthermore, this paper gives an insight of the important DLCs to be considered while computing the design loads for similar large wind turbine structures.