For the main shaft of wind turbine of certain type, shaft fracture occurs at the variable section of the shaft during early stage of operation. In order to validate the failure analysis, finite element analysis of the main shaft was performed. The analysis results demonstrate that there is a severe stress concentration that leads to the formation of initial cracks at variable inner diameters of the main shaft. Also, the stress in the variable part mainly resulted in the pressure of the bulging joining sleeve applied on the main shaft via the gearbox planet carrier shaft. According to the above analysis, local structure improvements were carried out through increasing the action area between the bulging joining sleeve and the main shaft. The finite element simulation results show that the stress concentration in the variable section of the improved shaft decreases significantly. The improvement of shaft strength decreases the possibility of crack formation and its growth, thus enhancing the reliability of the main shaft. This analysis process and the results of this study can provide a reference in shaft fracture analysis and also technical support for improvement in the design of wind turbine main shafts.

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In a riser-drill string coupling system, the drill string extends from platform to downhole, and its exterior tube is divided by mud line into two parts: riser for upside and borehole for downside. Due to such a pipe-in-pipe structure, an improved dynamic model is proposed to take the multipoint interactions between the inner and outer pipes into consideration. The dynamic responses of this system are analyzed by Computer Aided Engineering (CAE) techniques; specifically, it is numerically simulated in Abaqus; then, both the parametric sensitivity analysis and the main effect analysis are carried out in Isight to determine the optimization parameters and the optimization strategy. Moreover, six-sigma algorithm in Isight is applied to simultaneously drive the neighborhood cultivation genetic algorithm (NCGA) to conduct multiobjective optimization and drive the Monte Carlo method to analyze the stability of the obtained optimal solution. Based on the above investigations, a software package is developed via the secondary developments of both Abaqus and Isight. By this way, the optimization design of the riser-drill string coupling system based on dynamic analysis can be conducted effectively and efficiently.

High-strength bolts are widely used in wind turbines and play a significant role in their operation. In this paper, in order to study the ultimate strength of high-strength bolts in the connection between the hub and main shaft in a 2.5 MW wind turbine with pretension effects, two kinds of finite element models are presented: a solid bolt model and a simplified bolt model. Theoretical calculation was used to compare these models with the results of finite element methods. The ultimate strength analysis results showed that the simplified bolt model was the most efficient and useful in terms of computational time and memory usage. Based on this study, the simplified bolt model was used in the fatigue calculation considering multiple random fatigue loads, and the fatigue life of high-strength bolts was determined by combining the S-N curve of the bolt, based on the Palmgren-Miner Linear damage accumulation hypothesis.

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