Comparison of two maintenance policies for a multi-unit system considering production and demand rates

In this paper, two new condition-based maintenance policies are developed for a multi-unit system. This study integrates maintenance, production and demand into a single model. The system consists of N identical, independent units, each subject to gradual deterioration. Production rate of each unit varies in different working states and demand rate of the system is assumed to be constant. Cost of sending maintenance crew to perform maintenance is high, so simultaneous replacement of several units can be cost effective. Units are inspected at equidistant time epochs, and if production rate of the system at an inspection time is below a critical level, maintenance is initiated. First maintenance policy prescribes corrective replacement of failed units, and second maintenance policy prescribes corrective replacement of failed units as well as preventive maintenance of unhealthy units.The expected long run cost rate model is developed for a general multi-unit system to determine the optimal level. The maintenance model and the maintenance policies presented in this paper are new research contributions to the area of modeling and maintenance control of multi-unit systems. Our objective is to determine the optimal parameters for both maintenance policies minimizing the long run expected average cost per unit time. The developed optimization algorithms provide also the optimal inspection intervals for each policy. The problem is formulated and solved in the semi-Markov decision process (SMDP) framework. Numerical examples are carried out to demonstrate the proposed optimization model and to compare the two maintenance policies. Sensitivity analysis is conducted to observe the effect of number of turbines, distribution of replacement time, and several cost components on the optimal level and on the long run expected average cost rate.