Joint optimization of condition-based and age-based replacement policy and inventory policy for a two-unit series system

This paper proposes a joint optimization problem of the condition-based and age-based replacement policy and spare parts inventory policy for a series system with two non-identical units. Unit 1 is subject to soft failure and condition-based maintenance actions are adopted. Unit 2 is subject to hard failure and maintenance actions are performed based its usage. Besides the replacement policies, the (0,1) inventory policy is used to order the spare part of unit 1 and the (s,S) inventory policy is employed for unit 2. Unlike the previous studies where the maintenance policy and inventory policy are optimized separately, the replacement and order activities are simultaneously determined at decision epochs in this paper. Moreover, the system state transition probabilities, the expected maintenance costs and expected sojourn times would be derived by utilizing the semi-Markov decision process. The optimal preventive replacement thresholds of both units and the order level of unit 2 are simultaneously determined to minimize the system average maintenance cost per unit time. An illustrative example of a two-unit wind turbine system is used to demonstrate the priority of the proposed joint optimization policy.