the RAM System Model

 Sufficiency 

The model studied here is complex with respect to any analytical calculation but fairly simple when modelling using a simulation modelling platform. The simulation modelling tool used to build and calculate this model is SPAR^TM (courtesy of Clockwork Solutions L^td).

In the problem studied here there are twenty three systems deployed in three different fields. All twenty three systems have identical structure and for each of which the operational condition is represented by the Reliability Block Diagram (RBD) shown in figure 1, where the generic name for a component is a Line Replaceable Unit (LRU)

Figure 1 Reliability Block Diagram (RDB) of the Operational System Configuration

Every system comprises eleven LRUs selected from six different types. The distinction among different types is a matter of the design of the model concept. Every type may have unique properties such as failure data, repair data, etc.; on the other hand, there can be different types of LRUs with similar inherent properties and other attributes such as identical cost or, there can be two LRUs of the same type which may have different inherent properties (e.g. two engines operating at different loads). However, the attribute according to which a type will unify a set of different LRUs will be whether or not all members of that type share the same spare parts. Hence, the number of types in a problem will be the different number of spare parts types. In this problem, eleven LRUs in a system "consume" six different types of spare parts.

The system composition along with the failure data, repair/replacement data, repair at level B depot and shipment times are given in table 1. Note that only mean values are shown in the table, however, essentially each process should have a descriptive distribution as well. In the current model all the failure distributions are Exponential where the mean appears as the Mean Time To Failures (MTTF - sometimes appears as MTBF for Mean Time Between Failures), all Replacement distributions are Normal where the mean appears as Mean Time To Repair (MTTR) with relatively small standard deviations and all Repairs at the depot and the shipment times are assumed Constants.

Table 1 Failure repair and recycling data of the proposed model.

Each field contains a local storage. Additionally, there is a repair Depot located at Field 1, but which serves equally all fields. The Depot has also its own storage. A diagram of the logistic cycle is shown in figure 2:

Figure 2 Diagram of the logistic cycle of the proposed model..

This logistic cycle is a typical⁽²⁾ supply chain structure for almost any industrial (civil or military) enterprise. Although enterprises may have different operating environments or management with their own special rules, still the general logistic structure can be described as such and any exceptions are implied through data or additional logical rules. In this model, upon failure, the failed LRU is shipped to level B for a repair. At the same time two demands arise: one demand is for a spare part of the same type from the local storage (the field storage). The other demand is made by the field storage, for a spare of that type, from the level B storage; this will balance the cycle since it is expected that the repaired LRU at the depot will return into the depot's storage. There are two possibilities for each demand:

There is a spare available (instantly), in which case it responds to the demand and the demand is erased. With regard to the demand of the field from its local storage, a replacement/installation process will start whereas in the case of the field's storage, a spare will be sent off from the depot's storage to the storage of the demanding field.

There aren't any spares available, in which case the demand will hold until a spare arrives at the corresponding storage.
Upon arrival of the failed LRU at the depot, a demand for a repair team arises as well and the same logic is then applied.

 Hybrid