Impact of floater flexibility on tower eigenfrequencies of a spar-type floating offshore wind turbine: measurement-based assessment and model calibration

Abstract

Designing floating wind turbine systems requires integrated load assessments (ILA) using fully coupled hydro-servo-aero-elastic models. In most cases, floater hydrodynamics are represented using potential-flow models for mooring system design and motion estimation, while the floater itself is typically assumed to behave as a rigid body. However, this assumption can significantly affect tower eigenfrequency calculations, especially for large floaters. In this study, we investigate these effects using in situ sensor data from the Zefyros 2.3 MW spar wind turbine. We present a methodology to accurately determine the tower's eigenfrequencies. A rigid-floater model without added mass leads to an average error of 65% for the first tower mode relative to measurements. Including hydrostatic added mass reduces the error to 40%. Further incorporating floater flexibility decreases the error to 4.3%, and accounting for blade flexibility lowers it to just 2.8%. These discrepancies highlight the importance of refining the hydro-servo-aero-elastic model to align with eigenfrequencies derived from finite-element hydro-structural analyses. We present potential model adjustments, assess their impacts, and demonstrate the updated validation process.