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AND CONCLUSIONS 1. INTRODUCTIONThe Submarine Line Terminal muipment (SLTE) is part of the land-based equipment of a submarine network. It is considered to be a repairable system and is housed in the Terminal Station (TS). This study is concerned with the methodology for the calculation of the unavailability of the SLTE. The traditionally-accepted method of calculating the unavailability of the SLTE is to assume an unlimited number of spares in the TS. Because of the rapidly increasing capacity of submarine networks, it is becoming more important to quantify accurately the unavailability of the equipment, taking into account the exact limited number of spares provided.Furthemore, the spares replacement policy has a bearing on unavailability. For example, a Central Store (CS) may be provided in order to reduce the effective Mean Time To Repair (MTTR).The terminal equipment considered comprises m independent sub-systems in series, each of which comprises nl, i=l.m identical units following the same distribution, taken to be exponential. The problem is then reduced to calculating the unavailability of a repairable series system composed of n' independent, identically distributed units, taking into account the different replacement options. The first replacement option takes into account the spares in the TS only. The second replacement option includes both the spares in the TS and in the CS.The Markov method is used to model these systems and to calculate their unavailability, the results are supported using Montecarlo simulations. Simplified formulae are derived to approximate the results. Numerical examples are presented to show the advantage of introducing a central store and to validate the approximations. ' As only one subsystem is studied at a time, the subscript is dropped. Several authors have addressed the problem of how to estimate the required number of spares in different stores and how to control these stocks. For example, Lawrence et at (Ref. 1) studied a stock control policy against cost for the National Coal Board followed by Lampkin et at (Ref. 2). who presented in his paper a method of calculating optimum re-order levels for consumable (non-repairable) systems. Much research has been carried out on the optimisation of spares required to achieve a specified availability. Most approaches use the Poisson distribution to estimate the required number of spares (Ref. 3, 4). In practice, however, it may not be possible to provide the number of spares predicted due to financial or other constraints and the inverse problem must be solved. The model presented in Ref. 5 combines the time to repair if an item is spared and the t i m e to repair if no spares exist, but one of the assumptions of the model is that the failed item cannot be repaired during the mission t i m e . Indeed this paper (Ref. 5)aims at presenting the operational availability of a system whose mission t i m e is a maximum of 180 days. Since the design life of a submarine system is 25 years, this model is not applicable here. The model prese...

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