On intermediate-scale open-sea experiments on floating offshore structures: Feasibility and application on a spar support for offshore wind turbines

Abstract

Experimental investigation of floating structures represents the most direct way for achieving their dynamic identification and it is particularly valuable for relatively new concepts, such as floating supports for offshore wind turbines, in order to fully understand their dynamic behaviour. Traditional experimental campaigns on floating structures are carried out at small scale, in indoor laboratories, equipped with wave and wind generation facilities. This article presents the results of a 1:30 open-sea experimental activity on a scale model of the OC3-Hywind spar, in parked rotor conditions, carried out at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy). The aim of the experiment is twofold. Firstly, it aims to assess the feasibility of low-cost, intermediate-scale, open-sea activities on offshore structures, which are proposed to substitute or complement the traditional indoor activities in ocean basins. Secondly, it provides useful experimental data on damping properties of spar support structures for offshore wind turbines, with respect to heave, roll and pitch degrees of freedom. It has been proven that the proposed approach may overcome some limitations of traditional small-scale activities, namely high costs and small scale, and allows to enhance the fidelity of the experimental data currently available in literature for spar floating supports for offshore wind turbines. Keywords 1. Offshore structures 2. Spar 3. Floating wind turbines 4. Physical model 5. Heave damping 6. Roll damping development of reliable dynamic models, able to represent the coupled behaviour of the floating wind turbines [2-3]. While such models are usually implemented by means of numerical codes [4-5], experimental activities play a crucial role for their validation, as well as for the system identification. The experimental activities on floating offshore wind turbines may be classified in two groups, namely small-scale and large-scale ones. Traditional small-scale activities (1:50-1:100) are carried out in controlled environment such as wave tanks and ocean basins, where the desired wind-wave conditions can be reproduced, to measure the dynamic response of the structure and to calibrate opportunely the numerical codes [6-8]. Although the controlled environment allows to achieve very precise and reliable results, these activities have some relevant disadvantages, namely high rental fees of the basins, limited duration of the experiments, and limitations in representing all the relevant physical phenomena at scale level, which may alter significantly the dynamic behaviour of the model with respect to the full-scale structure. On the opposite side, large-scale activities (1:1-1:10) are carried out in open-sea and allow to represent all the relevant features of the offshore wind turbines, including turbine-support interaction, mooring system and grid connection, in relevant operational conditions [9-11]. Clearly, such projects are very expensive and usually represent pilot activities, which are carried out...

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“…Significant benefits brought by the increase of the wind turbine assembly capacity are the improvement of the wind energy utilization efficiency and the reduction of unit power cost. However, the increasing blade size also brings great challenges to designers and experimenters [14][15][16][17][18][19][20]. As one of the main research methods, scale model tests provide the most direct and effective means to evaluate operation performance and nonlinear phenomenon of wind turbines, obtain extreme and specific load distributions, and verify design codes.…”

Review of Recent Offshore Wind Turbine Research and Optimization Methodologies in Their Design

“…Thus, in many cases, a disk of similar drag force was applied to simulate the wind rotor. Another 1:47 Froude scaled model test of the Hywind spar-type FOWT was conducted under various sea states at the Ocean Basin Laboratory at Martinek [28,35]. A 1:50 scale model of the OC3 spar-type FOWT was produced at the State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University [36].…”

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“…The natural wave generation reduces the cost of the activity and allows investigating a wider range of irregular input loads. On the other side, the uncontrolled environment requires new data analysis techniques and design constraint relative to local ultimate limit states, which are useless for scaling purposes but may affect the safety of the structure (Ruzzo et al, 2018).…”