## 1 Introduction  
As climate change and energy crises continue to worsen worldwide, there is an increased eagerness to develop cleaner alternatives to fossil fuels. Wind energy is one of the most promising renewable energy sources. Nowadays, the power and complexity of wind turbines (WTs) are rapidly increasing. Complex electromechanical systems and uncertainties of extreme operating environments of wind turbines lead to serious accidents resulting in huge economic losses and threats to human lives [1; 2; 3]. To eliminate the weaknesses in complex system, redundancy designs are widely used for the improvement of system reliability and availability. In engineering practice, active redundancy, passive redundancy, partly loaded redundancy, and change over redundancy are commonly applied to enhance system reliability [4; 5; 6]. Reliability-redundancy allocation aims to obtain the optimal redundancy level of each subsystem to maximize the system reliability within allowable resources of the entire system [7; 8]. However, most redundant components have been treated as a parallel configuration without considerations of the failure mechanism of redundant systems [5]. Survival signature-based reliability analysis method has been applied to systems with multiple types of components, and excellent results are also achieved [9; 10; 11]. Many researchers have previously studied reliability importance analysis (Marginal reliability importance (MRI) and Joint reliability importance (JRI)) and reliability sensitivity analysis for complex systems. [PERSON] [12], and [PERSON] [13] introduced the JRI to explore how two components in a system interact in contributing to the system's reliability. The JRI was extended from two components to multi-components [14]. [PERSON] et al. [15] used the survival signature to perform imprecise system reliability analysis and component importance analysis. [PERSON] and [PERSON] [16] studied the reliability of a one-shot system containing multi-functional components. [PERSON] et al. [17; 18] performed the reliability sensitivity analysis for coherent systems using survival signature and system reliability optimization considering swapping existing components. However, the effects of component swapping on system reliability and component importance have not been extensively studied in the existing literature. In reality, components swapping is of great importance in improving the reliability of redundant systems, i.e., offshore wind turbines, which have high-reliability requirements. Due to the scale and complexity of the redundancies in complex systems, it is a challenge for us to express the change of the structure function of redundant systems upon swapping, which causes the fact that the system reliability with component swapping can not be realistically evaluated. Furthermore, the quantitative measure of MRI and JRI of redundant components can help allocate resources (design, inspection and maintenance) reasonably. However, the listed literature did not explore the influences of component swapping on system reliability, MRI and JRI of critical components. Moreover, the uncertainties of failure rates of components were not studied. Component swapping developed from survival signature is an effective method to enhance system reliability by swapping between failed components and working components of the same types in a redundant system, which can significantly improve the resilience of systems to failures [19]. Actually, this kind of maintenance strategy is already used to prevent the shutdown of wind turbines and avoid huge losses in reality. However, the influences of components swapping on system reliability have not yet been explored due to the change of system structure upon swapping. Moreover, the MRI and JRI with considerations of component swapping and fuzzy failure rates can not be conducted as well.