This is a parametric study on how blade and tower loads for a prototypical downwind offshore wind turbine are affected as the tower geometry and blade properties are changed. Downwind turbines have the potential to reduce the cost of energy, as blades can be more flexible and lighter, but the tower shadow induces additional structural vibrations. In order to reduce the latter, a fairing around the tower has been introduced. The length of the fairing is varied, adjusting the rotor overhang accordingly. Additionally, the blade weight and stiffness are adjusted. The blade and tower fatigue loads are, thereby, significantly decreased. In the first case, a maximum reduction of 8% and 28% (for the blade root bending and tower bottom moment, respectively) was achieved, compared to a downwind version of the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine on a monopile tower. Using softer and lighter blades resulted in loads even lower than for the conventional upwind rotor of the NREL turbine, up to 5% and 13% less for the blade and tower fatigue loads, respectively. The increased overhang increased the mean tower bending moments, suggesting that an optimal downwind turbine needs to be designed with a compromise between these fatigue and ultimate loads. The power production stayed approximately the same as that of a conventional wind turbine or was slightly higher. V C 2013 AIP Publishing LLC.
Offshore wind turbines are becoming more common due to the scarcity of suitable land sites. By going offshore, maintenance costs become one of the driving expenses. Hence more reliable components should be implemented on offshore wind turbines. The down-wind rotor configuration does not require as powerful yaw drive as the upwind rotor configuration to align with the wind direction. Thus the yaw system can be simpler with fewer components that can fail and require maintenance. This paper presents numeric simulation studies of how the tower shadow impacts the blades when they pass through the wake behind the tower. The work concentrates on bottom-fixed offshore wind turbines designed for the specifications of the NREL offshore 5-MW baseline wind turbine. The blade response has been compared for a full truss tower and a conventional tubular tower to show how the different tower shadows influence the blades. The blades on the more transparent truss tower experience less root flapwise moment fluctuations due to the weaker tower shadow. The simulations were performed by means of GH Bladed, version 3.82.
Abstract. Experimental laboratory testing of vortex-induced structural oscillations in flowing water is an expensive and time-consuming procedure, and the testing of high Reynolds number flow regimes is complicated due to the requirement of either a large-scale or high-speed facility. In most cases, Reynolds number scaling effects are unavoidable, and these uncertainties have to be accounted for, usually by means of empirical rules-of-thumb. Instead of performing traditional hydrodynamic measurements, wind tunnel testing in an appropriately designed experimental setup may provide an alternative and much simpler and cheaper framework for estimating the structural behavior under water current and wave loading. Furthermore, the fluid velocities that can be obtained in a wind tunnel are substantially higher than in a water testing facility, thus decreasing the uncertainty from scaling effects. In a series of measurements, wind tunnel testing has been used to investigate the static response characteristics of a circular and a rectangular section model. Motivated by the wish to estimate the vortex-induced in-line vibration characteristics of a neutrally buoyant submerged marine structure, additional measurements on extremely lightweight, helium-filled circular section models were conducted in a dynamic setup. During the experiment campaign, the mass of the model was varied in order to investigate how the mass ratio influences the vibration amplitude. The results show good agreement with both aerodynamic and hydrodynamic experimental results documented in the literature.
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