Naval vessels are complex platforms that combine highly advanced telecom, radar, defensive and offensive technologies, as well as high power propulsion systems. The goal of a vessel is to provide high mission reliability i.e. a high probability of mission success.
Naval missions have unique characteristics:
- Long mission times
- Low or no access to spare parts other than COB (Carry On Board)
- Harsh conditions
Several system design methods exist for achieving high mission reliability:
- High reliability of each equipment
- Hot redundancy
- Spare parts
High equipment reliability is always preferred because it reduces the need for redundant / spare part units. However, most equipment is procured from 3rd parties and it is not always possible to obtain equipment with the desired reliability.
During initial system design a process of reliability allocation should be conducted to identify equipment that requires redundancy / spare parts. Reliability allocation allows you to design effective systems with the right number of redundancies, and to provide realistic MTBF requirements to OEMs, such that the system is expected to comply with the mission reliability requirement.
During detailed design, an accurate reliability analysis should be conducted to verify compliance with the required mission reliability.
In many industries spare parts analysis is conducted late in the design process. This cannot be the case when designing naval systems because COB spare parts are limited by weight and cost, and have a significant effect on the mission reliability. Therefore, redundancies and spare parts must be considered from an early design stage.
Example: Communication System
Consider the case of a communication system with the following requirements:
- 2 transceivers must be operational
- 10 end units must be operational
- Mission reliability requirement is 98% for a mission duration of 60 days
Reliability allocation with no redundancies and no spare parts provides an MTBF requirement for the transceivers and end units of 855,310 hours.
Figure 1: Reliability allocation of system with no spares, Reliability Block Diagram (RBD) screenshot from BQR software
The MTBF requirement is too strict for both the transceivers and end units. Therefore, spare parts have to be added. Spare parts can be modeled in RBD software using stand-by models. By adding 2 spare end units and 1 spare transceiver the MTBF requirement is reduced to 28,352 hours:
Figure 2: Reliability allocation of system with spares, screenshot from BQR RBD software
This MTBF requirement is much more realistic. Next an MTBF of 40,000 was provided by the OEM for the transceivers, this value can be fixed and then the end units minimal MTBF requirement reduces slightly to 26,841 hours:
Finally, MTBF of 30,000 was provided for the end units from the OEM. Then the mission reliability is calculated to be 98.46%:
Figure 3: Analytic Mission Reliability calculation, screenshot from BQR RBD software
This simple example shows how reliability allocation and calculation can be used to optimize the system design and ensure compliance with the performance requirements.