A new approach for intermediate-scale open-sea experimental activities on offshore structures. Application to spar buoys for wind energy exploitation via a 1:30 scale activity

The thesis deals with the introduction of new techniques, both analytical and experimental, for the dynamic identification of rigid body motions of floating structures. The central hypothesis of the thesis consists in the proposal of realizing intermediate-scale open-sea experimental activities on offshore structures, as alternative or complement to the traditional small-scale ones, realized in indoor laboratories. The main advantages of the proposed activities, as widely documented in the thesis, are the cost reduction and the possibility of increase the model size and the duration of the experiments. However, due to the non-controlled marine environment, traditional identification methods, based on the measurement of the dynamic response of the structure (output) to external loads (input) with known law, cannot be directly applied. Consequently, to validate and support the new approach proposed, these methods have been adapted to the open-sea case and a pilot experimental activity on a 1:30 scale model of a spar floating support for offshore wind turbines (OC3- Hywind) has been realized at sea in the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy). The wind turbine has been represented in fully-parked rotor conditions, through a lumped mass at the top of the tower. The realization of the experimental activity has allowed the Author to propose practical solutions to many challenges arising when dealing with open-sea experiments on floating structures. Among the others it is relevant to mention the reduction of the scale effects, the design of the mooring system on inclined seabed, the choice of measurement sensors and techniques, and so on. The results of the experimental activity have been processed and discussed, obtaining, as expected, the characterization of the dynamic behavior of the model structure, in terms of rigid body motions. Moreover, a numerical model of the structure has been implemented in Ansys AQWA, and has been calibrated using the ex perimental data collected. The comparison between this model and the literature results, along with the interpretation of the experimental results, confirms that intermediate-scale open-sea experimental activities may provide more reliable information than the small-scale indoor ones, concerning the identification of some dynamic properties of the structure, e.g. the damping coefficients in the case study considered in this thesis. However, since wave energy content is restricted to certain limited frequency ranges, traditional input-output methods, thought adapted to the open-sea case, cannot guarantee a complete dynamic identification of the model structure in the frequency domain. Consequently, to fill this gap, a new approach has been proposed, introducing output-only identification methods, typical of civil engineering. Among the others, a classical frequency domain method, namely Extended Frequency Domain Decomposition (EFDD) Method, has been selected, and it has been revisited to meet the requirements of the new field of application proposed. In particular, one crucial assumption of the method, i.e. that of white noise input load, has been weakened, being typical wave spectra narrow-banded. EFDD method has been applied to the response time histories of a numerical model of the floating spar structure considered in this study, again built in Ansys AQWA, confirming its efficiency and thus proving the feasibility of complete dynamic identification of rigid body motions of floating structures via intermediate-scale open-sea experimental activities.