Abstract
Monopile towers used for offshore wind turbines are sensitive to Vortex-Induced Vibration (VIV). Here, their structural response to VIV was experimentally investigated with models of a partially and a fully immerged offshore monopile. The partially and fully immersed model cylinders had aspect ratios of 18.75 and 28.13, respectively. They were subject to reduced velocities of up to 9.78 and 5.45, respectively, corresponding to Reynolds numbers of up to 77120 and 63680, respectively. Normalised transverse motion amplitudes of up to 1.19 were measured. “Strouhal-like” numbers down to 0.13 were obtained, and they decreased with increasing flow velocity and corresponding motion amplitude. The 3D Digital Image Correlation (DIC) processing method was used to measure response without influencing model properties and the flow field. To assess the suitability of DIC for VIV investigations, results obtained from the conventional technique using triaxial accelerometers were compared. The influence of different processing methods, pattern designs and their assemblies, the repeatability of the model tests, and the influence of ventilation were investigated also. Additionally, forces and torques were measured directly. Results from these three measurement techniques were compared and discussed.
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