Optimal structural design for offshore wind energy system

• Main Author: Chew, Kok Hon
• Other Authors: Ng Yin Kwee
• Format: Theses and Dissertations
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Civil engineering::Structures and design; DRNTU::Engineering::Mechanical engineering::Alternative, renewable energy sources; DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics; DRNTU::Science::Mathematics::Applied mathematics::Optimization
• Online Access: http://hdl.handle.net/10356/69616
• Institution: Nanyang Technological University

Offshore wind industry has been gaining much momentum over the last decade due to increasing energy demand and global appeal to create a balanced energy mix. Current wind energy research and development has been focusing on driving down the cost of energy. Support structures of offshore wind turbines are site-specifically designed and can be optimized for cost reduction. However, the task is computationally expensive due to the highly-constrained, non-convex and non-linear nature of the design problem. A good depth of detail in the problem formulation can give useful insights in the practical design process, but may also compromise the efficiency. Therefore, the main objective of this thesis is to investigate an efficient, effective and practical automated structural optimization framework for designing the support structures of offshore wind turbines, where efficiency relates to the convergence speed to attain the optimal design; effectiveness measures the degree of success to converge to the correct optimal solution reliably and practicality represents the quality of a design problem being mathematically formulated to fit to the industrial requirements. The thesis begins with the development of an in-house modeling and structural analysis module for offshore wind turbine system, which is based on a decoupled aero-servo loading acting onto a linear hydro-elastic model. Code comparison and verification were carried out against the fully-coupled non-linear simulations in commercial software FEDEM Windpower. Under the normal operating wind-wave conditions, the partially decoupled approach was able to give accurate response analysis and fatigue assessment, as the bottom-fixed offshore wind structures behave linearly. However, when subjected to severe wind and wave loads, non-linearity within aerodynamic and hydrodynamic drag forces become prominent, and the partially decoupled approach tended to generate conservative results in the extreme load analysis. Secondly, the thesis studied analytical analysis in the dynamic sensitivity computation to eliminate the inaccuracies related to finite difference approximations. Analytical formulae were developed to calculate gradients for various design assessment criteria prescribed by the industry, including ultimate limit state (ULS) checks and fatigue limit state (FLS) checks in both time- and frequency-domains. Comparison studies against finite difference schemes for simulated stress data show that the analytical gradients could avoid numerical artifacts that typically occurred in the analysis of extreme load constraints for beams that experienced alternating tension and compression modes. Moreover, the fatigue damage gradients were very sensitive to response sensitivities, where factors like step sizes, time steps and locations of assessment could influence the accuracies of finite difference approximations greatly. Thirdly, an integrated analysis and optimization framework was developed for the design optimization of offshore wind turbine support structures. Case studies were performed on the UpWind monopile, OC4 jacket and UpWind jacket. Results show that the framework was reliable and consistent in delivering superior efficiency and accuracy in the optimization study, as compared with the conventional optimization method that is based on finite difference gradients. The global optimum could be achieved in the design optimization process, where the large number of design constraints implemented can possibly lead the optimizer to find the global optimum. Among the list of design constraints implemented, the buckling and fatigue load constraints had significant influence over the design, where each component is oriented to maximize the utilization against the prescribed limit state functions. The framework has enabled the design process of the entire wind turbine support structure system, including the pile penetration depth to be carried out simultaneously in an integrated manner, hence could possibly lead to a more optimized design solution.