Finite Element Analysis of Compact Reinforced Concrete substructure to a bucket foundation and offshore windturbine
Translated title
Finite Element Analyse af højstyrkebetonovergangsstykke mellem et bøttefundament og en havbaseret vindturbine
Authors
Dorph, Minik ; Sipavicius, Aurimas
Term
4. term
Publication year
2010
Submitted on
2010-06-10
Pages
139
Abstract
Dette projekt tager udgangspunkt i, at overgangsstykker mellem bøttefundamenter og tårne i havvindmøller typisk udføres i stål og derved er sårbare over for korrosion og udmattning i svejsede forbindelser. Formålet er at undersøge, om et skalformet overgangsstykke i materialet Compact Reinforced Composite kan udgøre et levedygtigt alternativ. Metodisk er der opbygget og valideret en finit element-model ved at sammenligne kritiske foldningslaster for kendte skalkonstruktioner (kuppel, cylinder og bøttefundament) med analytiske løsninger; der blev opnået god overensstemmelse. Selve overgangsstykket, der har en “flaskehals”-geometri, er analyseret som en aksisymmetrisk halvmodel med jord, mens tårnet er udeladt for at fokusere på den strukturelle respons. Materialeadfærd er beskrevet med en ikke-lineær Concrete Damaged Plasticity-model, og laster omfatter hydraulisk tryk fra lineær bølgeteori (med antaget drag-fordeling kontrolleret ved bølgetop) samt vindlast. I en indledende elastisk undersøgelse af fem geometrier viste hydraulisk tryk begrænset betydning for snitkræfter og ustabilitet, mens vindlasten var dimensionerende. To forslag med hhv. ét og to armeringslag blev efterfølgende vurderet. En lineær foldningsanalyse indikerede ingen foldningsrisiko for de foreslåede løsninger, og materialebrud blev dermed dimensionsgivende. Geometriske imperfektioner baseret på lineære foldningsformer blev inddraget i en ikke-lineær analyse, som viste, at overgangsstykket ikke var følsomt over for imperfektioner. Samlet peger resultaterne på, at et overgangsstykke i Compact Reinforced Composite er en realistisk mulighed for bøttefundamenter til havvindmøller, under de anførte forudsætninger og lastantagelser.
This thesis addresses the vulnerability of steel transition pieces between bucket foundations and wind turbine towers to fatigue at welded joints and corrosion, and investigates whether a shell-type transition substructure made from Compact Reinforced Composite could be a viable alternative. A finite element analysis (FEA) framework was built and verified by comparing critical buckling loads of benchmark shell structures (a dome, a cylinder, and a bucket foundation) with analytical solutions, showing good agreement. The substructure, shaped like a bottle neck, was modeled as an axisymmetric half-model including soil, while the tower was omitted to focus on structural response. Concrete behavior was represented using a nonlinear Concrete Damaged Plasticity model, and loading included hydraulic pressure from linear wave theory (with an assumed drag pressure distribution checked at wave crest) and wind. An initial elastic study of five geometries indicated that hydraulic pressure had limited influence on section forces and instability, whereas wind was the governing load. Two reinforced concepts with one and two rebar layers were then proposed. Linearized prebuckling analysis showed no buckling risk for the proposed designs, making material failure the governing design criterion. Geometric imperfections based on prebuckling modes were included in nonlinear analyses, which indicated low sensitivity to imperfections. Overall, the results suggest that a Compact Reinforced Composite substructure is a feasible transition piece solution for bucket-founded offshore wind turbines within the stated assumptions and loading approach.
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