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NAVY ENGINEERING BULLETIN MARCH 2003 69 failure mode has occurred and the designer must be prepared accordingly. A disciplined, methodical approach to reliability and maintainability is, therefore, essential for naval engineering systems and equipment. What are Reliability, Availability and Maintainability? There would definitely be a case for reliability and maintainability in design but what are these nebulous parameters? How far do we go with our detailed, methodical examination of designs as they evolve? Can reliability and maintainability be measured? Can they be given a number, quantified? Yes, they can. The reliability and maintainability characteristics of a machine or system can be specified for a design in exactly the same way as performance characteristics such as power, pressure or speed. The standard Defence definition of reliability is “The ability of an item to perform a required function under stated conditions for a stated period of time”. It will be gathered that this strict definition of reliability is a statistical probability. For example, a propulsion system might be required to have a 95% probability of remaining in operation without failure for a mission time of, say, three months. Its specified reliability would be 95% for that mission time. Similarly, we can look at the definition of maintainability: "The ability of an item under stated conditions of use to be retained in or restored to a specific condition when maintenance is performed by personnel having specified skill levels under stated conditions, and using prescribed procedures and resources." This is also expressed as a probability. Finally, because some defects can be repaired at sea we need another parameter that can take account of both these characteristics. We call it availability: "A measure of the degree to which an item is in an operable and committable state at the start of a mission when the mission is called for at an unknown (random) time." There are several types of availability. Perhaps the commonest used for marine engineering are intrinsic availability, which is a function of the system or machine design, and operational availability which takes into account logistic delays. Clearly, in the final analyses, operational availability is important to the Commanding Officer of a ship because it represents availability of the ship as a weapon platform. Using statistics to quantify reliability and maintainability, while sometimes unavoidable, can be cumbersome: two parameters, the probability and the mission time (in the case of reliability) have to be stated using this method. For this reason it is usual to assume for reliability, that failures will occur in a random manner. When this assumption is made then the parameter mean time between failure (MTBF) can be used to express the reliability of a system or equipment. As the title suggests, MTBF is calculated by dividing the total equipment running hours accumulated in service by the number of failures that have occurred. It is a characteristic of equipment in a specified design state and may be used to compare the reliability of one system or machine with another. For example, a diesel engine with MTBF of 1000 hours compared to a gas turbine with MTBF of 5000 hours. There are mathematical relationships between MTBF, reliability, missiontime and failure rate such that the reliability of a complete system can be predicted by building from the MTBFs of the component equipment or subassemblies. Reliability prediction may be used to indicate whether a system or equipment is near to the specified target and hence to determine whether the amount of reliability assessment conducted on the design is sufficient. Prediction can also be used to compare the reliabilities of different parts of the design, eg fuel system versus cooling system on a diesel engine. The latter use enables design effort to be directed to where it can do the most good. However a fundamental pre-requisite of reliability prediction is numerical data (MTBF or failure rate) on sub-assemblies, equipment or components, although such data is scarce on mechanical equipment. This underpins the importance of reporting systems for equipment in service although maintainers are, understandably, reluctant to record details such as running hours, details of repair etc, whenever a failure occurs. Reliability, Availability and Maintainability Consideration in Naval Engineering Having established a case for reliability and maintainability to be addressed specifically during design, and identified ways of quantifying these parameters, how do Engineers ensure that they will be included in the design process? In an ideal world Engineers would specify our requirements in numerical terms, just as we do for performance requirements and the manufacturer would design and build the product accordingly. Calculation and test would evidence proof that the requirements had been met. Unfortunately this process is not as easily achieved for reliability and maintainability as it is for performance requirements - particularly for mechanical equipment. Calculation of performance is achieved using well established engineering formulae and is usually quite accurate; testing for performance is, again, fairly straightforward -
70 NAVY ENGINEERING BULLETIN MARCH 2003 there can be no doubt whether a machine has developed a specific power, fuel consumption or pressure. Calculation of reliability, on the other hand, is not accurate in absolute terms. This is partly because of the difficulties of finding accurate numerical data on components; testing to demonstrate reliability (or maintainability) is statistically based and time consuming - while it can be (and sometimes is) done, the time and cost involved sometimes makes it an impractical exercise. Because of the practical difficulties of relying purely on numerically based techniques to demonstrate the achievement of reliability and maintainability, it is usually necessary to resort to additional methods of obtaining reliability assurance. Basically, the process involves having a say in the reliability and maintainability techniques that are to be used by a contractor and generally having some influence or interest in the design as it evolves, notwithstanding the contractors' responsibilities as the designer. There are four fundamentally essential ingredients to a procurement program if reliability and maintainability are to be properly addressed, and these can be described as follows. The Specifications Must Be Right The numerical RAM 1 targets are the starting point for all reliability and maintainability work. If they are not derived from a sound base then the rest of the program may be wasted. The aim is to educate design procurement staff sufficiently to ensure that the targets specified for new equipment are sensible, achievable and demonstrable. RAM requirements may originally be specified for a whole weapon platform (based on operational considerations) then apportioned down to equipment or system level. It is important to ensure that the impact of this apportionment on, say, marine engineering equipment is assessed during the feasibility study to ensure that the RAM target of the equipment is a reasonable figure. It would not be sensible to demand an MTBF of 40 000 hours for a gas turbine, for example, if existing gas turbines had MTBFs of only 3 000 hours; the program of improvement required would be enormous and, even then, may not achieve the result. In some instances, there may be a lack of MTBF information components or equipment in the system being considered. In this case, then, the procurement authority may require a special reporting exercise to be instituted to determine the MTBF of appropriate items in service, or omit quantified RAM targets from the specification altogether. The latter option, while not preferred, may be necessary for minor items not meriting a special reporting exercise, or for large, comparatively reliable, items such as a main thrust block, where the time taken to collect the data would be prohibitively large. If this option were chosen, then reliability assurance would be obtained by placing more emphasis on the qualitative methods available. An essential requirement when specifying quantified RAM targets is to define "failure" unequivocally. This is needed not only for deriving MTBF from service data, but also at the acceptance stage so that there would be no dispute with the supplier. This task is by no means as easy as it sounds; there can little doubt about catastrophic failure, but instances of degraded performance - a diesel capable of only 90% power or a gas turbine with a minor fuel leak, for example - need careful consideration. Particular care is given to relating the RAM requirements to the application of the equipment. There is no point in specifying MTBFs for stand-by equipment such as emergency fire pumps or stand-by generators. If an item is likely to spend most of its life on stand-by then the supplier would be directed to concentrate his RAM effort on probable dormant failures such as stiction, corrosion or brinelling of bearings. Overall, the aim is to advise suppliers of the environment in which their machines will operate, as far as possible. In general, the specification stage is, perhaps, the most important because the procurement authority has to think far ahead. Adequate resources have to be set aside to fund and administer the RAM program, and this can be as much as 10% of the total development cost. If these preparations are not done or the feasibility of the RAM targets is not properly addressed, then retrospective action will be difficult or even impossible and the situation may never be recovered. Choose the Right Techniques A wide range of analyses are available to refine a design from a reliability and maintainability viewpoint but only highly critical items need to have all these analyses applied. Funds are not unlimited and we try to ensure that resources are spent where they will do the most good. A minimum requirement is to learn from past experience. Equipment in new ships is often a repeat of similar items already in service and it is essential to ensure that existing shortcomings are corrected before committing that equipment to, perhaps, another 25 years in the Fleet. The documented history of items proposed for future service is, therefore, examined in detail and a modification program is instituted if necessary.
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