Wind Turbine Design: Evaluation of Dynamic Loads on Large Offshore Wind Turbines
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A design method for combined aerodynamic and structural (aeroelastic) design of large wind turbine blades has been developed, with the purpose of facilitating conceptual design, parametric studies, optimisation, or cost analysis of offshore wind turbines using more advanced aero-servo-hydro-elastic analyses. The aerodynamic design is based on blade element momentum theory, which is the most common approach for engineering analysis of wind turbines, due to a combination of speed and accuracy. A parametric blade model is developed which allows the blade geometry, in terms of airfoil thickness and chord distributions, to be described with only a few parameters. The chord and twist distributions are then determined with respect to the airfoil characteristics and the design tip speed ratio to yield optimal glide number and induction factors. The focus is on a realistic and manufacturable design with near-optimum properties and a smooth aerodynamic shell spanning from a cylindrical shape at the blade root to thin airfoils close to the tip. The structural design is based on a parametric internal structural definition constrained by the shell geometry. The structure is divide in six material zones consisting of leading and trailing edge, fore and aft shells, main spar, and shear webs, and initial material layup of each zone is defined from a parametric description developed in an earlier large-scale blade design study. The final structural design is found through an iterative process, by determining the beam properties using laminate theory of slender, thin-walled beams, and investigating the material strains and blade deflections of a set of quasi-static design load cases. The design load cases were selected from the design standards after a thorough discussion and evaluation of the most severe load cases, based on aero-servo-hydro-elastic wind turbine simulations. The blade designs are found to have realistic properties in terms of blade mass and stiffness distributions and natural frequencies. This is argued for by comparing with available wind turbine blade data from commercial wind turbines and reference wind turbine design studies. The structural integrity has been evaluated for a range of different blade designs with different design parameters, rotor diameters, and blade materials. Both ultimate loads and fatigue loads have been investigated using realistic material properties and appropriate safety factors. Aeroelastic stability issues have been investigated with a focus on classical flutter for long and slender blades. The design method has been implemented as a design tool in MATLAB. A graphical user interface facilitates an efficient and intuitive design process, making it suitable for both academic and industrial use.