14 Delay Time Modelling

In this chapter we present a modelling tool that was created to model the problems of inspection maintenance and planned maintenance interventions, namely Delay Time Modelling (DTM). This concept provides a modelling framework readily applicable to a wide class of actual industrial maintenance problems of assets in general, and inspection problems in particular. The concept of the delay time was first mentioned by Christer in 1976 in a context of building maintenance, Christer (1976). It was not till 1984, the concept was first applied to an industrial maintenance problem, Christer and Waller (1984). Since then, a series of research papers appeared with regard to the theory and applications of delay time modelling of industrial asset inspection problems, see Christer (1999) for a detailed review. The delay time concept itself is simple which defines the failure process of an asset as a two-stage process. The first stage is the normal operating stage from new to the point that a hidden defect has been identified. The second stage is defined as the failure delay time from the point of defect identification to failure. It is the existence of such a failure delay time which provides the opportunity for preventive maintenance to be carried out to remove or rectify the identified defects before failures. With appropriate modelling of the durations of these two stages, optimal inspection intervals can be identified to optimise a criterion function of interest. The delay time concept is similar in definition to the well known Potential Failure (PF) interval in Reliability Centred Maintenance, Moubray (1997). It is noted however, that two differences between these two definitions mark a fundamental difference in modelling maintenance inspection of assets. First, the delay time is random in Christer's definition while the PF interval is assumed to be constant. Secondly the initial point of a defect identification is very important to the set up of an appropriate inspection interval, but ignored by Moubray. Nevertheless, Moubray did not provide any means of modelling the inspection practice, while