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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 81
Edited by: B.H.V. Topping
Paper 102

Analysis and Design of the Foundations of Wind-Turbo Generators using the Finite Element Method

R.T. Teixeira+ and R.C. Barros*

+Faculty of Science and Technology, University Fernando Pessoa, Porto, Portugal
*Department of Civil Engineering, University of Porto, Porto, Portugal

Full Bibliographic Reference for this paper
R.T. Teixeira, R.C. Barros, "Analysis and Design of the Foundations of Wind-Turbo Generators using the Finite Element Method", in B.H.V. Topping, (Editor), "Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 102, 2005. doi:10.4203/ccp.81.102
Keywords: wind turbines, wind energy conversion systems, finite elements, preliminary design, seismic analysis.

The present work addresses the analysis and design of several reinforced concrete foundations of the 65 m high wind-turbo generator towers, to be constructed at the wind energy farm at 'Serra do Barroso', in the northern district of Vila Real. On these trunk-conic towers are mounted turbines of three 37.5 m blades.

The study is divided in three major phases. The first phase corresponds to the static analysis of the foundation slab, where the structural strength is verified under the actions transmitted by the superstructure, namely the effect of the wind action. The simulation of the static behaviour of the foundation slab is achieved with a formulation of finite elements for thick slabs based upon Reissner-Mindlin plate theory [1]. The second phase corresponds to the study of the dynamic behaviour of the structure of the aeroturbine, through the quantification of the generalized actions transmitted to the foundation by the metallic tower under regulatory seismic actions. The third phase corresponds to the reinforced concrete design and detailing of the foundation slab, and the verification of the safety of the connection between the metallic tower and the foundation slab.

The analysis quantifies the permanent actions as well as the variable actions due to wind (on tower, blades and rotor) for two wind scenarios, associated with 90o and 45o wind incidences. For the quantification of the wind actions on the conic tower, the tower was subdivided into 2m height segments. The characteristic dynamic pressure corresponds to the average between the lower dynamic pressure and the upper dynamic pressure at the segment length height. The total bending moment at the foundation level (base moment) is the cumulative sum of the bending moment contributions induced by the equivalent static wind forces on all the tower segments of the subdivision. Also, the horizontal force at the foundation level (base shear) is the cumulative sum of the segmental contributions to the static forces.

The most unfavourable configuration for wind actions on the tower [2] corresponds to the situation in which one of the blades is coincident with the axis of the tower while the remaining two are positioned symmetrically in relation to the same axis (Figure 102.1). Two different scenarios were considered for orientation of the blades with respect to the direction of wind gust. In the first scenario, the most unfavourable, it is assumed that in the presence of strong winds the rotation device of the blades is not functional in one of the blades [2]. In this scenario the most stressed blade is the vertical blade. The second scenario corresponds to the situation in which the operation of the orientation device (of all the blades) allows that these are positioned parallel to the wind for high wind speeds Although the wind energy generator towers are located at the least seismic risk zone in the country, a seismic verification under standard principles of spectral modal analysis was performed to ascertain thoroughly the anticipated risk. Figure 102.1 collectively contains the layout of the wind turbine for the wind worst scenario configuration, moments mx for the scenario 1 and the wind incidence at 90o and the profile of the first three modes shapes.

Figure 102.1: Wind tower, moment distribution for mx and the first three mode shapes.

Finite element analysis enables the distributions of the pressure at tower footings, settlements under footing slabs, total tower displacements, among others to be obtained. With the stress resultants obtained for the worst load case design combination, the RC foundations were designed according to Eurocode EC2 and to Portuguese standards RSA and REBAP. In view of the geotechnical properties and characteristics at the site, it can be said that the stresses underneath the proposed foundation of the order of 180 kPa are well below the strength of foundation media of around 300 kPa. Moreover, footing up-lift would not occur beyond about 40% of foundation area.

Shames I.H. and Dym C.L., Energy and Finite Element Methods in Structural Mechanics, McGraw-Hill Book Company, New York, 1985.
DNV, "Design of Offshore Wind Turbine Structures", Offshore Standard DNV-OS-J101, Det Norske Veritas, Norway, 2004.

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