System performance analysis of complex systems with failure dependence

Abstract

Many technical systems are becoming more and more complex, consisting of multiple components and prone to failure dependences. Within such systems with failure dependences, the failure of a component or some components may accelerate the degradation of other components. Such failure dependences can significantly reduce system reliability and lead to catastrophic consequence if not well considered and effectively mitigated. In addition, within the complex multi-component systems, the failure dependences are commonly not expected to be single but multiple and heterogenous, which further complicates the investigation of system operation and maintenance management.

The subsea system is typical example of complex system with failure dependence. The subsea system is part of the offshore oil and gas industry that performs various tasks and operates in the seabed or underwater environment. Given the long-term exposure to hostile environmental conditions encompassing high pressure, low temperature, salinity, and corrosion, et. al, ensuring the normal operation of the subsea system via effective maintenances becomes very important. Additionally, due to the complex functions, the subsea system rarely operates independently by a single device. In many cases, the subsea system may inevitably suffer the coupling effect of natural degradation and failure dependences, resulting in more severe consequences. Therefore, to guarantee the system performance to ensure its long-term stable operation in extreme environments, it is crucial to deal with these complex failure dependence situations of the subsea system.

Currently, the effect of failure dependence has not been well studied neither in the reliability analysis, maintenance management of the complex system nor in its sustainable relationship with the surroundings. As a result, it is desirable to conduct a thorough investigation on the impacts of failure dependence in complex multi-component systems. Our research identifies the subsea complex system as an ideal case example for such investigation.

This Ph.D. thesis aims to propose comprehensive methodologies to conduct reliability analysis, maintenance management, and sustainability evaluation for the complex systems considering the failure dependence. The aim is refined into the following four specific research objectives that are addressed in one conference paper and four journal articles:

Elucidate the definitions of terminologies related to failure dependence and clarify delimitations for various types of failure dependence. Based on that, mechanisms of component degradation and cascading process are better categorized and understood. The study could improve the recognition and comprehension of failure dependence during system design and operation phases. Develop a system reliability analysis model for complex systems with multi-state components considering overloads. The cascading process is examined with its stop scenarios and influencing factors. It is expected to present insights to optimize the design and maintenance of complex loading dependent systems with overloads. Establish a general maintenance model for complex systems subject to failure dependences. Impacts of heterogeneous failure dependences on component degradation within the subsea system are explored by this model, with the aim of optimizing maintenance strategies to improve the system availability. Propose an integrated framework to conduct sustainability evaluation for complex systems subject to failure dependence. This framework is capable of thoroughly examining the coupling effect of component degradation, failure dependence and maintenance management on the sustainability. It concerns the sustainability evaluation of complex subsea systems from environmental, social, and economic perspectives. Thus, it provides guidelines for long-term and sustainable optimization of maintenance strategies.

From an academic standpoint, this thesis proposes approaches and models to assess the effects of failure dependences. The suggested approaches and models reveal the degradation patterns of components subjected to failure dependence and the development mechanisms of cascade processes. From the practical viewpoint, this thesis serves as a reminder to designers, operators, and safety personnels regarding the significance of acknowledging failure dependences in complex multi-component systems. Furthermore, it offers implications to minimize the failure dependence during the system design stage or implement effective measures to mitigate such dependences during system operation and maintenance stage.

To conclude, this thesis provides a comprehensive overview of failure dependence issues in complex multi-component systems, as well as contributes to the reliability analysis, maintenance management and sustainability evaluation of the subsea systems subjected to failure dependences.