Topics on the design of tubular steel wind turbine towers
The wind turbine tower is mainly a simple cantiliver beam. However, its section forms a thin-walled cylindrical shell and therefore, several issues arise during the analysis such as the local buckling of the shell structure or the stress concentrations ar
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Introduction The wind turbine tower is mainly a simple cantiliver beam. However, its section forms a thin-walled cylindrical shell and therefore, several issues arise during the analysis such as the local buckling of the shell structure or the stress concentrations around the door opening which must be thoroughly examined. The design is governed by the extreme wind loading; earthquake loading should also be taken into account when designing the turbine tower on seismic hazardous areas.
1. Evaluating the tower shell thicknesses 1.1 General aspects The evaluation of the shell thicknesses of the tower is performed by the plastic limit state design [LS1] and buckling limit state desingn [LS3] as they are described in EC-3-1-6. The wind tower is a cantilever beam; in this sense, the simplest way is to perform a hand calculation, considering the tower clamped to its base. A computer analysis using a linear model can also be done. In this case the various courses of the tower are represented as linear elements. The tower can be considered either as clamped to the base, or including the foundation using a linear beam grid. By the previous type of approach the internal forces at every point of the tower are calculated. Special cases as the local buckling of the shell or the stress state at specific points, like flange positions or door opening must be examined by applying special calculation models. However, as nowadays the designer has advanced computational tools in his hands, it is now becoming easy to model the whole tower with all the details included using an integrated Finite Element model. Constructing such a model, the designer should evaluate directly the stress state at any point of the tower (including flange connections, door opening etc.)
C. C. Baniotopoulos et al. (eds.), Environmental Wind Engineering and Design of Wind Energy Structures © CISM, Udine 2011
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In the paragraphs below, an analysis of the tower using the aforementioned methods is attempted. The prototype tower examined has a height of 76.15 m. The shell diameter at the base is 4.30 m and the diameter at the tower top is 3.0 m. Shell thicknesses vary from 30 mm at the bottom to 12 mm ato the top. The tower is divided into three parts connected together by bolted flanges.
1.2 Tower Loading 1.2.1 Vertical loads [G] In a structural model, the self-weight of the shell is usually estimated directly by the FE software, as a function of the geometry and the unit mass of the steel elements. The contribution of the platforms and the ancillary equipment (ladders, cable racks etc.) to the total weight of the tower could be neglected. The weight of the nacelle, including the blades and the rotor, is provided by the manufacturer. In the example at hand, it is assumed to be equal to: Gr=1067,00kN, having the center of gravity shifted horizontally +0,725m from the axis of the tower and vertically +0.50 to +1.00 m above the upper flange level (+76.15 m). 1.2.2 Extreme Wind loads [W] The loads over the tower stem are calculated,
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