A bivariate optimal replacement policy for a cold standby repairable system with repair priority

In this article, an optimal replacement policy for a cold standby repairable system consisting of two dissimilar components with repair priority is studied. Assume that both Components 1 and 2, after repair, are not as good as new, and the main component (Component 1) has repair priority. Both the sequence of working times and that of the components' repair times are generated by geometric processes. We consider a bivariate replacement policy (T , N) in which the system is replaced when either cumulative working time of Component 1 reaches T , or the number of failures of Component 1 reaches N, whichever occurs first. The problem is to determine the optimal replacement policy (T , N)* such that the long run average loss per unit time (or simply the average loss rate) of the system is minimized. An explicit expression of this rate is derived, and then optimal policy (T , N)* can be numerically determined through a two-dimensional-search procedure. A numerical example is given to illustrate the model's applicability and procedure, and to illustrate some properties of the optimal solution. We also show that if replacements are made solely on the basis of the number of failures N, or solely on the basis of the cumulative working time T , the former class of policies performs better than the latter, albeit only under some mild conditions. © 2009 Wiley Periodicals, Inc.