Optimisation of wind turbine foundations

Muzofa, Tinotendaishe Daniel (2017-12)

Thesis (MEng)--Stellenbosch university, 2017.

Thesis

ENGLISH ABSTRACT: This study seeks to optimize the foundations of tall wind turbine support structures in South Africa. It is postulated that the concrete quantity can be reduced in gravity based foundations by incorporating the backfill material into the foundation design. The study was based on the development of finite element models consisting of gravity based foundation embedded in a founding material and connected to a wind turbine tower. Three main analyses were computed namely; Static non-linear push over analysis, Cyclic load analysis and Structural eigenvalue analysis. The purpose of the study was to investigate (a) soil-structure interaction in order to assess the infinite vs. finite soil stiffness effect on the natural frequency of the structure, (b) the stress distribution in the foundation in order to replace under-stressed parts with backfill material yet maintaining the same structural stiffness and vibrational behaviour that complies with the soft-stiff structural vibration frequency domain, (c) the feasibility of using high density concrete towards foundation size reduction, as well as water usage reduction. Finally, a structural performance and cost comparison is drawn between a conventional and alternative foundation systems. A conceptual design guideline for the foundation system is also created for both geotechnical and structural design of the foundation. The study concludes that soil-structure interaction does influence the natural frequency of the wind turbine tower. Stiff soils result in higher natural frequency and less stiff soils result in a lower natural frequency. Secondly the critical sections that require structural concrete in a gravity based foundation are the section directly below the tower base and also the base of the foundation. The rest of the concrete is primarily for rotational stiffness as a counterweight. Lastly, volumetric aggregate replacement is a feasible solution towards increase in the density of concrete and consequently reducing the concrete foundation size. The feasibility being based on the mechanical behaviour of the concrete material. The study recommends that finite element analysis be used in order to develop an optimum wind turbine foundation design.

AFRIKAANSE OPSOMMING: Hierdie studie poog om die fondamente van die ondersteunende strukture van hoë windturbines in Suid-Afrika te optimeer. Daar word gepostuleer dat die hoeveelheid beton in swaartekrag gebaseerde fondasies verminder kan word deur die opvulmateriaal in die fondasie-ontwerp in te sluit. Die studie ontwikkeel eindige element modelle wat bestaan uit ’n swaartekrag gebaseerde fondasie, ingebed in die ondergrond, en verbind met ’n windturbine toring. Drie hoofontledings is uitgevoer, naamlik; Statiese nie-lineêre omkantelingsanalise, gedrag onder sikliese belasting en strukturele eiewaarde-analise. Die doel van die studie was om (a) grond-struktuur interaksie te ondersoek ten einde die oneindige teenoor. eindige grondstyfheidseffek op die natuurlike frekwensie van die struktuur te assesseer, (b) om uit die spanningsverdeling in die fondasie dele met lae spanning met terugvulmateriaal te vervang, maar steeds dieselfde strukturele styfheid en vibrasiegedrag behou wat voldoen aan die sag-stywe strukturele vibrasie frekwensie domein, (c) die haalbaarheid van die gebruik van hoë-digtheid beton ten einde volume-vermindering, sowel as waterverbruik vermindering te bewerkstellig. Laastens word ’n strukturele prestasie en kostevergelyking getref tussen ’n konvensionele en alternatiewe fondamentstelsel. ’n Konseptuele ontwerpsriglyn vir die fondamentstelsel word ook geskep vir beide die geotegniese en strukturele ontwerp van die fondasie. Die studie lei tot die gevolgtrekking dat grond-struktuur-interaksie die natuurlike frekwensie van die windturbine toring beïnvloed. Stywe grond veroorsaak hoër natuurlike frekwensie en minder stywe gronde lei tot ’n laer natuurlike frekwensie. Tweedens, die kritiese deel wat strukturele beton benodig in ’n swaartekrag gebaseerde fondament, is die gedeelte direk onder die toringbasis en ook die basis van die stigting. Die res van die beton is hoofsaaklik vir rotasiestyfheid as ’n teengewig. Laastens is volumetriese vermindering ’n haalbare oplossing deur gebruik van hoë-digtheid beton wat aggregaat van hoë digtheid bevat. Die haalbaarheid is gebaseer op die meganiese gedrag van die betonmateriaal. Die studie beveel aan dat eindige element analise gebruik word om ’n optimale ontwerp van windturbines te ontwikkel. Sleutelwoorde: Wind, Turbine, Grondslae, Grondstruktuurinteraksie, Optimering

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102862
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