Platform differences may contribute as well. SUBMEPP's results are derived from a sample of submarine components and MSP's results are derived from a sample of surface ship components. Corrective maintenance accomplished during a submarine overhaul is not captured by 3-M OARS. Not until the boat is delivered to the fleet is corrective maintenance reported to 3-M OARS.
For the purposes of this paper it should be stated in mathematical terms how SUBMEPP categorized components as characteristic E or C. None of the components within this group had a regression line with a perfect slope of zero. Some had a negative slope and some had a slight but positive slope. For a component to be deemed characteristic C, the slope had to exceed 0.003X, and it had to have a coefficient of determination of at least 0.1 (10% of the variation is explainable by age). The slope of 0.003X was judged to be too slight to qualify as a component experiencing wear out. It would take 33 years for the failure rate to increase from 10% per year to 20% per year.
Maintenance Plan Changes
The majority of components analyzed by SUBMEPP did not demonstrate an age and reliability relationship and consequently, many existing time directed component overhauls have been deleted from class maintenance plans. These deletions have allowed the Navy a substantial cost avoidance for submarine depot availabilities. The term avoidance is used here because one can not project beyond the age span of study to predict future probabilities of failure. Components may or may not experience failure rate increases and that will be a future determination when maintenance strategies for these components are revisited. SUBMEPP's review of components does shed light on the effectiveness of many overhaul periodicity extensions made in the early 1980's however. The majority of components that fit non-wear out characteristics D, E and F once had overhaul periodicities half as long.
The RCM approach is to extend or eliminate overhaul periodicities in the absence of an age and reliability relationship. The decision whether to extend the periodicity or delete the action entirely often depends on the consequences of failure. Extensions are more appropriate for components with safety related failures for which no effective condition monitoring techniques have been devised. Deletions are more appropriate for non-safety related components. Maintenance plan strategies should not be based entirely on failure rates viewed at the equipment level. Individual failure modes should be viewed in isolation as well to determine if an age and reliability relationship exists. If so, a surgical maintenance approach may be appropriate where only a piece part or subassembly is replaced.
The portion of components analyzed by SUBMEPP, that did demonstrate an age and reliability relationship, was further analyzed to determine if a time directed maintenance task was appropriate. For non-severe failures, where there are no additional costs attributed to failure beyond material and labor to repair the component, a fix-when-fail strategy may still be more cost effective. Labor and overhead cost differences must be taken into account. And if there are mission or collateral damage costs associated with failure, condition monitoring can sometimes be substituted for a time directed task. Condition monitoring must detect potential failure conditions and allow a known and sufficient time period for adequate correction. A more surgical maintenance strategy may be appropriate as well. Pareto's rule that 80% of the problems are generally caused by 20% of the actors, has been validated by RCM analyses.
Maintenance professionals should concentrate on the few "bad actors" which degrade reliability. Also, if one took the physical component health approach, they must calculate the population that has survived to the age where a time directed task is desired. The action may be saving only a small portion of the population.
When pursuing the system health approach, one must recognize that steady increases in unreliability are generally non-sustainable. At some point, the majority of items that are going to fail have failed and the influence of corrective maintenance improves reliability. Therefore one must compare the area under the curve both before and after the time of the desired maintenance action to determine the payback. Sometimes there is a dramatic failure rate increase followed shortly thereafter by a swift decrease. In such a case, there may not be area enough under the curve to warrant the investment of a time directed task (see figure 7).
Figure 7. Reliability Effect Possible with 5 Year Overhaul
Oftentimes newly manufactured components and newly restored components are thought of as apples and apples when they should be considered apples and oranges. SUBMEPP analysis has shown that reliability after overhaul is not always equivalent to the reliability of a newly manufactured component. To compare the two, the analyst would choose not to restart the lifecycle clock of a component when it had a planned renewal. This will cause the application to graph reliability beyond the renewal age. If the component's planned overhaul was effective in improving the health of the component, one would see a sudden reliability improvement as exhibited in figure 8. If no change were observed, one would conclude that although replacement with a new component would affect reliability, overhaul of the existing component would not. SUBMEPP recently analyzed an air dehydrator. Age and Reliability graphs for the dehydrator showed that the component did experience increased failure rates as it aged and corrective maintenance was not improving the situation. Because of backup capabilities, the failure modes in question did not have significant consequences, however consideration was given to restoring the unit from a cost containment perspective. The system engineer took the additional step of comparing the effectiveness of past overhauls to the reliability exhibited by a new component and found that little improvement would be gained from an overhaul.
Figure 8. Various Reliability Effects with 5 Year Overhaul
Soon after the development of the feedback data analysis application, a SUBMEPP combat systems engineer analyzed Trident class torpedo tubes. Torpedo tubes are comprised of barrels, breech and muzzle doors, latches, linkages, slide valves, rotary actuators, power cylinders, safety interlocks, indicators and numerous other sub-assemblies. The class maintenance plan for the torpedo tubes included a time based maintenance action to replace hydraulic power cylinders every 160 months. Each torpedo tube has five cylinders. Functional failures for these components are mission critical as they render a tube inoperable, or degrade performance to an unacceptable level. Even though there are multiple torpedo tubes, a full complement of operational torpedo tubes is deemed necessary for readiness. Two of the power cylinders operate the torpedo tube slide valve. Over half of the observed discrepant conditions associated to inoperability of the slide valve were attributed to the hydraulic power cylinders and only one of those discrepancies was judged to be a functional failure. The predominant mode of failure was external leakage of hydraulic fluid and as previously stated, these were judged to be non-functional failures. They were potential functional failures if left untreated. Figure 9 displays the Age and Reliability curve for the slide valve power cylinders. The failure pattern is random with no correlation with time. In fact, the regression line has a slightly negative slope of 0.0003X. There is no evidence indicating that the valves should be replaced at 160 months. Moreover, the engineer found that existing condition monitoring tasks were applicable in monitoring and maintaining system health. Periodic pressure and cycle time tests are able to detect degradation before performance is compromised, and allow sufficient time for repair or replacement of cylinders. Age and reliability findings for the remaining power cylinders were similar. The engineer deleted the requirement to replace torpedo tube power cylinders at 160 months and this lifecycle cost avoidance for Trident class submarines was determined to be $2.3 million. If the current reliability trend holds consistent over the submarine lifecycle, that avoidance will be actual savings.
Figure 9. Age and Reliability Graph for Torpedo Tube Slide Valve Power Cylinders
The vast majority of steady state random failures exhibited by the sample of submarine components analyzed by SUBMEPP support the 1961 finding that "reliability and overhaul time control are not necessarily directly associated topics". At the conclusion of an RCM analysis it has been uncommon at SUBMEPP to prescribe time directed component renewals. Prescription of condition monitoring tasks has been the prevalent strategy to maintain safe operation and required asset functionality. The once held belief that time is the best guide in scheduling major equipment overhauls should no longer be ascribed to. More appropriately, maintenance professionals should continue the evolution of devising conditioning monitoring parameters to better assess component health. Advents in technology are making this easier with each passing day. Whenever possible, the component should communicate to the maintainer when maintenance is appropriate or necessary. However, the findings of this paper should not dissuade one from analyzing the effects of time on component reliability. Age and reliability correlations do exist for many components and as components in various systems, platforms and facilities experience ages never before observed or studied, past results are subject to change. Moreover, maintenance engineers must always be on guard to prevent safety-related failures, which haven't occurred, but could occur at older lifecycle ages. Material condition assessment is appropriate for these instances. Maintenance plans must periodically be revisited to assess past decisions and to devise new strategies based on current best practices and new technologies. In the end, there is no substitute for an in-depth, thorough and comprehensive review of maintenance feedback data.
The author acknowledges the contributions of Kenneth E. Gerber, co-developer of SUBMEPP's Feedback Data Analysis System.
American Management Systems, Inc., "Age Reliability Analysis Prototype Study", N00024-92-C-4160, November, 1993.
Michal, J. "Reliability Modeling and Estimation Using U.S. Navy 3M Maintenance Data, Naval Postgraduate School Monterey, California, September, 1995.
Nowlan, F. and H. Heap, "Reliability Centered Maintenance", MDA 903-75-C-0349, December, 1978.
Tim Allen is a Reliability Analyst Leader at Submarine Maintenance Engineering, Planning and Procurement (SUBMEPP), a Naval Sea Systems Command field activity located in Portsmouth, NH. He has worked at SUBMEPP for the past sixteen years. As a member of the Reliability Centered Maintenance group, he co-developed SUBMEPP's Feedback Data Analysis System. Tim trains system engineers in the principles and methodologies of RCM and works collaboratively with them to engineer maintenance plans. He was previously an engineer for submarine atmospheric and seawater systems. Tim received a Bachelor of Science in Mechanical Engineering Technology at the University of Maine in 1986. In 1997, Tim received a Master of Business Administration degree at New Hampshire College.