Analysis of a two-phase inspection model with competing risks

Condition monitoring, as it is usually refered to, is a maintenance strategy where decisions are taken depending on regularly measured equipment states or conditions. In literature and in practice, condition monitoring is often advocated as an efficient tool for cost-effective maintenance. It reduces uncertainty with respect to actual states of equipment, and may thus avoid unnecessary repair or replacement, compared to techniques such as time based preventive maintenance. However, in many cases it requires capital investment for monitoring equipment. In this paper we analyse a basic model for the economic evaluation and optimisation of condition monitoring techniques. The model is based on the assumption that for a certain type of failure mode, equipment passes through an intermediate state between 'as-good-as-new' and 'failed'. This so called 'bad' state can be detected by the inspection technique, allowing preventive maintenance to be planned before equipment failure. Other 'invisible' failure modes may be involved, and these are modelled as a competing risk to evaluate their influence. Apart from providing optimality results for inspection frequencies, this paper especially focuses on sensitivity analyses. Hereby we identify the most important model parameters, showing that the full model can be simplified without seriously affecting optimal decision taking