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FIGURE 9: THE DIRECTED SURVEY APPLIES RCM TO STRUCTURALLY SIGNIFICANT ITEMS ONLY systems tend to fail randomly, but they generally provide us with no warning of impending failure, so that the interval between P and F in Figure 3 is minimal and oncondition maintenance is therefore not feasible. For the electrical/electronic systems in Figure 6, only 6% of failure modes respond to on-condition maintenance, with scheduled restoration and discard applying only to a further 3% of failure modes. Failure Finding on redesign account for a further 7% of failure modes and are therefore still important features of managing the failure of electrical/electronic systems. Overall, there is a tendency for electrical/electronic systems to push us towards the Default actions at the bottom of the RCM Decision Diagram. FIGURE 10: THE STRUCTURES FMEA Figure 6 also shows us that there are still nearly 50% of all failure modes in the mechanical systems for which no form of proactive maintenance is possible, the corresponding figure for the electrical/electronic systems being about 84%. RCM and Structures We now turn our attention to structures which are a form of mechanical system. RCM has been applied to aircraft structures for a number of years, although its application to Naval structures is relatively recent. Structures suffer from three categories of failure modes: Fatigue Damage (FD): Here, applied cyclic stresses resistance to failure. All ships are prone to fatigue failure as they are NAVY ENGINEERING BULLETIN SEPTEMBER 2003 subjected to tensile and compressive forces in heavy seas. Environmental Deterioration (ED): Corrosive and other processes lessen the resistance to failure by reducing cross section, or by producing metallurgical changes at the molecular level. Accidental Damage (AD): Random stress applications reduce resistance to failure by deformations which change load paths and/or accelerate other failure modes. Accidental damage often occurs through docking and removal of systems for maintenance. Complex structural assemblies have a range of functions including longitudinal strength, local strength and containment. Structural failure can affect all of 41 these functions simultaneously. For instance, a corroded structural component may reduce longitudinal and local strength as well as causing a containment breach (flooding/leakage). For this reason there is usually no need to define the functions of structural elements so the structures FMEA can be based on physical components rather than functions, simplifying the analysis process. The number of physical components in a large warship is enormous making the completion of a structural survey of the whole vessel immensely complex and time consuming. Instead, recent experience in the Defence community with the application of RCM to structures has led to the development of a ‘Directed Survey’ on Structurally Significant Items (SSIs). In the model shown in Fig 9, RCM and the Directed Survey would apply to the shaded areas only which include those components prone to FD (red), ED (Green) and AD (blue). For each SSI, the appropriate failure modes and effects are considered within the overall RCM database for the vessel. FD, ED (corrosion and erosion) and AD (random failure) are covered in the 6 Failure Patterns (Figure 1), allowing us to use the RCM decision logic to develop the appropriate survey elements and periodicities. Examples of the RCM software outputs for structures are shown in Figures 10 (SSI FMEA) and
42 NAVY ENGINEERING BULLETIN SEPTEMBER 2003 About the author: Alun Roberts is a Director of The Asset Partnership, based in Sydney. His company is closely allied to the RCM implementation programmes being conducted by the Royal Navy and the US Navy. The Asset Partnership has supported the RNZN, ANZAC and ASASMO in RCM reviews of several systems and is also working closely with the Army in determining the supportability requirements of the new BUSHMASTER Infantry Mobility Vehicle using RCM2. In addition to working in world of Defence, The Asset Partnership works closely with a range of assetintensive businesses including mining, utilities, petrochemicals and manufacturing. FIGURE 11: THE STRUCTURES RCM DECISION WORKSHEET 11 (Structures Decision Worksheet). RCM has been successfully applied to structures on a number of Warship classes in the Royal Navy including Hunt Class MCMV and Type 23 Frigate. The Consolidated RCM Database We have seen how the RCM process applies equally well to mechanical, electrical, electronic systems and to the vessel structure itself. It delivers a management policy for all likely failure modes - some of which we will already have suffered, others of which will not yet have happened – and maximises system reliability and availability. RCM also facilitates very significant reductions in maintenance expenditure. It is less well understood that a comprehensive consolidated RCM database is of immense value for a range of other purposes, including: • Review and optimisation of Useage and Upkeep Cycles (UUC) for a variety of Mission profiles: With little further effort, the impact of extending useage or changing the upkeep cycle can be readily determined. This is because RCM separates truly age-related failures from non agerelated (random) failures and allows us to evaluate the risks of extending upkeep intervals. The holy grail of UUC optimisation is indeed notoriously difficult to achieve by any other route; • Improved forecasting of manning levels required for completion of UUC work packages: The RCM database details proactive, default and corrective actions including the full range of ILS requirements (work content, skills, spares, tools, manuals, drawings etc), allowing work packages to be determined with ease; • Provision of a consistent frame of reference for reporting defects both for individual ships and across Class, particularly if RCM failure codes are embedded in the ship/shore maintenance management system; • Satisfying the needs of regulation, classification and internal audit by demonstrating asset management due diligence. Applying and implementing RCM will require a high level of commitment from the Navy and its contractors, disciplined application of the RCM technology and some time to achieve. The rewards, however, will be enormous. Is the Royal Australian Navy ready for the challenge?
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