An experimental investigation into the most prominent sources of uncertainty in wave tank testing of floating offshore wind turbines
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University College Cork
There is an urgent need to replace carbon-based energy sources with renewable energy sources, and floating offshore wind is seen as a critical component in the drive towards energy diversification. Floating offshore wind facilitates accessing a far vaster wind resource that exists in deeper waters, further offshore. Floating offshore wind platforms must undergo wave tank testing in the early stages of development to assess model responses to different wave and wind conditions. Wave tank testing, while highly beneficial, is liable to errors arising throughout the testing campaign. Errors can arise during wave tank setup, testing, and analysis of results. Some of the primary sources of error include errors in the model location within the tank, errors in model parameters like mass, inertia and CoG, and errors due to incorrect replication of mooring forces and aerodynamic forces from the turbine. Scaling wind turbine blade properties can be challenging; this is because aerodynamic forces are scaled using Reynolds scaling, but all hydrodynamic forces are scaled using Froude scaling. For this reason, wind emulation systems are used to replicate the aerodynamic forces from the turbine only. Testing was completed using two very different floating offshore wind concepts. A sensitivity analysis was completed by conducting variations to the wind emulation system used, the model inertia and centre of gravity, and the mooring stiffness of the model. The magnitudes of the variations to the inertia, centre of gravity and mooring stiffness were based on the uncertainty in the values of each of the parameters. Three wind emulation systems of varying complexity were used for this comparison, a simple weighted pulley system, a constant thruster and the software in the loop system developed by CENER. The comparison was conducted to assess the influence of wind emulation systems on the uncertainty of platform response It was found that the effects of each variation conducted were magnified at resonance, and the magnitude of platform response was affected to a greater extent than the period of resonance response. Of all the variations to the model properties conducted, the inertia about the y-axis and location of the centre of gravity along the x-axis affected pitch response to the greatest extent. A 7% change in the inertia about the y-axis coupled with an 8.57% resulted in a 10% change in the period of resonance response for pitch, Tr, and 52% decrease in the magnitude of resonance respsonse for pitch, Tr, mag. Changes in the wind emulation system affected the pitch response most significantly, while the period of resonance response Tr, was mostly unaffected , the magnitude of resonance response Tr, mag, was reduced by nearly 90% when a pulley system was used in lieu of a conventional thruster for a semi-submersible model. Changes in mooring stiffness did not influence the period of resonance response but did affect the magnitude of resonance response, particularly in surge. For a linear horizontal mooring system applied to a semi-submersible model, a 1% decrease in the spring stiffness resulted in a 9% decrease in the magnitude of resonance response for surge, Tr, mag.
Offshore wind , Floating offshore wind , Wave tank testing , Experimental uncertainty , Renewable energy
Lyden, E. 2022. An experimental investigation into the most prominent sources of uncertainty in wave tank testing of floating offshore wind turbines. MSc Thesis, University College Cork.