State-of-the-art design methods for wind turbine towers

Abstract

The costs of wind energy have to be reduced in order to be competitive with conventional methods to generate electricity. The tower costs contribute significantly to the costs of a wind turbine. This thesis aimed to suggest improvements in wind turbine tower design in comparison with industry standards nowadays in order to reduce the costs of wind turbine towers. It has been investigated if modeling the mass, geometry and stiffness of the tower flanges into aeroelastic codes affects the simulation results. The effect of geometry and stiffness is not contributing as much as modeling the mass. Modeling flanges as point masses is sufficient to represent the flanges in the aeroelastic code. A constraint damping layer between the flange connections is proposed. Such a layer can be used to increase the damping of the tower. In this way, the fatigue loads on the tower can be reduced. An improved flange design optimization method is suggested. A cost performance function is created, reducing the flange costs of more than 2.5% in comparison with the optimization method used nowadays within Siemens Wind Power. Standardization of flange connections in wind turbine towers is also considered. This is beneficial for the costs of handling equipment and tower internals, as project specific design and certification of these components can be omitted. Other advantages of flange standardization are risk mitigation and supply chain benefits. Standardization of the flanges leads to a costs increase up to 7%. The tower sections can be designed less conservative if sector based fatigue loads will be considered. In combination with sector based SN-curves and stress concentration factors the tower can be directed in such a way that the fatigue loads are less severe. Up to 6% tower mass reduction can be realized for fatigue driven tower designs. The Effective Equivalent Stress Hypothesis and the Gough-Pollard algorithm, multiaxial fatigue life prediction models proposed by literature, are implemented to investigate the influence of combined loading on tower welds. According to the above multiaxial fatigue models, load safety factors between 1.16 and 1.40 are required when evaluating fatigue life in tower welds with the conventional uniaxial method. In this way the additional damage due to combined out-of-phase loading is incorporated in the fatigue life prediction. However, usage of correct multiaxial fatigue models instead of safety factors increases the accuracy of fatigue life predictions.