Graduation Year


Document Type




Degree Granting Department

Industrial Engineering

Major Professor

O. Geoffrey Okogbaa, Ph.D.

Committee Member

Tapas. K. Das, Ph.D.

Committee Member

A.N.V. Rao, Ph.D.


Failure-repair process, Non-steady state, Renewal theory, Simulation, Instantaneous availability, Sensitivity analysis


Most modern systems are equipped with very complex, expensive, and high technology components whose maintenance costs have become an increasingly large portion of the total operating cost of these systems. Thus, the efficacy of the maintenance policy for these and related systems has become a major concern to both manufacturing and design engineers. Different kinds of maintenance strategies have been proposed to solve the problem. While some of these have proven effective, there is yet no definitive approach that has been found that support the maintainability requirements of transient systems or systems that exhibit transient behavior. Transient behavior is the notion of non-steady state operation, which is the characteristic of system operation during its useful life. For designing convenience most of the maintenance strategies have assumed negligible maintenance or repair time which is not practical.

In this research an opportunistic maintenance (OM) approach is implemented on a multiunit system that exhibits transient behavior. Under OM policy, if a maintenance event has been scheduled for certain components and in the process of implementing the scheduled maintenance of these targeted components, the maintenance of other components whose maintenance times are in close proximity is also implemented at the same time. As a result, the time and cost of marshalling and staging maintenance resources are reduced. As part of the system effectiveness measure, the instantaneous system availability based on the transient nature of the system, is estimated using the renewal theory approach. An advantage of modeling system failure process as a renewal process is that the system failure causes and the underlying probability structure associated the distributions are tracked and identified.

Using simulation, and assuming Weibull distribution failure times and lognormal distribution repairs or maintenance times, a cost model is developed that minimizes the overall maintenance cost of the system. This cost framework is then used to evaluate total maintenance costs incurred while implementing OM and PM policies. The optimal replacement times for the components of the system for the PM policy are obtained using analytical formulation. The results of the simulation model show that the OM policy is more economically viable as compared to the PM policy. A sensitivity analysis is performed to explore the robustness of the system parameters. The results of the sensitivity analyses show that the total system maintenance cost is lowest at optimal maintenance intervals for individual components. Furthermore, another measure of system effectiveness, the instantaneous availability of the system is estimated and compared with the system maintenance costs for various maintenance intervals. It is observed that to attain high availability the maintenance interval of the components should be as low as possible which increases the maintenance cost. From a design perspective, it is important to compare availability with cost because different organizations typically assign different levels of significance to cost versus availability.