Fatigue Analysis and Design Optimisation of Offshore Wind Turbine Support Structures
Translated title
Udmattelsesanalyse og Designoptimering af Støttestruktur til Havvindmøller
Author
Laustsen, Jonas Mathias
Term
4. term
Education
Publication year
2014
Abstract
Offshore vindmøllers støttestrukturer skal være lette og samtidig modstå mange års cykliske påvirkninger. Dette speciale udvikler og afprøver en beregningsmæssig metode til at minimere massen af en støttestruktur under en udmattelsesbetingelse med realistiske, ikke-proportionelle laster. En 3D Bernoulli–Euler bjælke-baseret finite element model med tyndvægget cirkulært tværsnit er opstillet, hvor spændinger evalueres i udvalgte punkter. Modellen er verificeret mod en ANSYS reference for globale flytninger, tværsnitsspændinger og egenfrekvenser med god overensstemmelse, dog med forventede forskelle i tværskæringsspændinger pga. bjælkeantagelser. Laster stammer fra en generisk 5 MW vindmølle og behandles som ikke-proportionelle via en modificeret Wang–Brown rainflow-tælling kombineret med Findleys critical plane udmattelsesmodel og Palmgren–Miners lineære delskade. Optimeringen udføres som sekventiel lineær programmering med adaptive flyttegrænser, hvor objektfunktionen er den totale masse og bibetingelsen er en skadesgrænse i alle evalueringspunkter. De nødvendige gradienter bestemmes ved en analytisk følsomhedsanalyse baseret på direkte differentiering og verificeres mod centrale differenser. Implementeringen demonstreres på en fast indspændt bjælke under ikke-proportionel last, hvilket understøtter metodens anvendelighed. Uddraget indeholder ikke kvantitative optimeringsresultater, men viser et sammenhængende og verificeret grundlag for masseoptimering af offshore støttestrukturer med udmattelse som begrænsning.
Offshore wind turbine support structures must be lightweight yet endure many years of cyclic loading. This thesis develops and tests a computational framework to minimize structural mass subject to a fatigue constraint under realistic, nonproportional load histories. A 3D Bernoulli–Euler beam finite element model with a thin-walled circular cross-section is formulated, with stresses evaluated at selected points. The model is verified against an ANSYS reference for global displacements, cross-sectional stresses, and natural frequencies with good agreement, acknowledging expected differences in shear stress treatment due to beam theory assumptions. Loads from a generic 5 MW turbine are handled as nonproportional using a modified Wang–Brown rainflow counting method coupled with the Findley critical plane fatigue model and Palmgren–Miner linear damage. Optimization is performed via sequential linear programming with adaptive move limits, minimizing total mass while constraining damage at all evaluation points. Required gradients are obtained through an analytical design sensitivity analysis based on direct differentiation and checked against central finite differences. The implementation is demonstrated on a cantilever beam under nonproportional loading, supporting the feasibility of the approach. While the excerpt does not report quantitative optimization outcomes, it presents a coherent and validated basis for fatigue-constrained mass optimization of offshore support structures.
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