Probabilistic Design of Wind Turbine Structures: Design Studies and Sensitivities to Model Parameters

Abstract

Several societies have envisaged renewables as sources of energy for ecological and geo-strategical considerations. Amongst others, wind energy has gained considerable interest in the past decades due to its high potential to fulfil the aspirations of the societies that opted for it. However, harnessing offshore wind energy poses challenges such as cost of energy reduction, handling of very large structures, randomness pertaining to the metocean environment, and need for better understanding of the mechanical behavior of the structures. Three means are employed in this thesis for cost reduction: decrease of conservatism level, improvement of design procedures, and development of innovative structural systems that suit well for large wind turbines. The increasing size of the structure introduces new problems that were not present for small structures. These problems include: (i) the preparation of models with sufficient adequacy in replacement of models whose validity ranges were restricted for small size structure; (ii) the upscaling of supplementary structures like mass dampers whose volume or mass become prohibitive; (iii) the satisfaction of fatigue lifetime requirements for jacket substructures. In addition to being aggressive, conditions for offshore environments and the associated models are highly uncertain. Appropriate statistical methodologies should be used in order to design robust structures, which are structures whose engineering performance is not significantly affected by reasonably small changes of the environmental conditions. Recent inspections of some installed wind turbines on monopiles have unveiled serious damage to the grouted joints. The subsequent investigations revealed a misunderstanding of phenomena related to the mechanical behaviour of the grouted joints. Explanations have been proposed by previous studies and the present thesis investigates one of the derived solutions. This study addresses these challenges sorted in three research areas: (i) Area 1: reduction of conservatism; (ii) Area 2: Lifetime improvement; and (iii) Area 3: Innovative systems. These research areas are differentially implemented through tasks on various wind turbine structures (shaft, jacket, semi-floater, monopile, and grouted joint). In particular the following research questions are answered: How are extreme and fatigue loads on a given structure influenced by the design of other structures on the same wind turbine? How can loads be prepared in order to be exchanged between the designers / manufacturers of different wind turbine components with better accuracy and lower conservatism level? • How can fatigue lifetime of large substructures at deep waters be extended? What techniques are suitable? To which extent do they act on the structures? What are their efficiencies? • How can innovative structures be developed/adapted which allow installations in deeper waters while maintaining low fatigue load levels and being economically competitive to floating structures? Why is the innovative concept efficient? • How do the design parameters individually impact the design of monopiles and their engineering performance? What could be the effect of the interaction of these parameters? • How do the design parameters influence the long term survival of the grouted joint under normal conditions? Given the computational cost of the finite element simulation, how can the grouted joint be assessed under extreme loading taken into account the uncertainties in the variables? Respectively, the principal contributions and findings of the present work are: Format for load exchange. A preparative method of loads to be exchanged between the different stakeholders of a complex design process is introduced. The development of this method is buttressed on the stress calculation algorithm. For the extreme loads, the promoted method is in line with the standards’ recommendations relative to determination of the extreme loads as the highest of the peak averages of each mean wind speed. Comparison between this method and the conventional load format seen in literature shows that additional structure capacity is revealed by the proposed method and that material saving is possible. Improvement of fatigue lifetime. Three methods for fatigue lifetime improvement have been developed for jacket substructures. The first focuses on the joint design methodology. Clear guideline rules have been established to help designers to reduce stress concentration factors at joints. The second intends to reduce the vibration of braces based on the application of magneto-rheological dampers. Modelling methods and effectiveness are presented together with installation steps. The third employs an aero-elastically tailored rotor to alleviate fatigue loads on the support structure. Whereas the rotor optimization process was not done within the present study, its effect on the substructure is shown in this work. Semi-floater: An innovative substructure. The semi-floater concept has been introduced by previous studies. In this work, the detailed design of the universal joint has been proposed together with the installation process of the substructure. A design process of such substructure type has been presented along side with an algorithm to design mooring line at the preliminary phase. Monopile: Influence of model parameters. The individual influences of some key model parameters (damping, construction errors, soil properties) and their interactions have been quantified in a comparative manner. It has been established for example that the soil-structure interaction can interact with the construction errors to amplify the fatigue demand at some hotspots of the monopile. Grouted joint: Mechanical behavior and determinant parameters. A probabilistic design approach based on a detailed finite element model has been developed. In order to reduce the computationally expensive analysis of the joint, a method based on load criteria is proposed and the adapted probabilistic analysis process is explained. The influences of the steel wall thickness, of the conical angle, and of the grout length have been respectively identified. Recommendations are given to improve the design process.