Planned Maintenance System (PMS)
PMS allowed ship owners or operators to carry out routine maintenance (predominantly by the ship’s crew) at predetermined intervals. The obvious reason for introducing PMS was due to the prohibitive costs associated with breakdown and emergency repairs. Especially profound were failures that were likely to result in total loss, claims from injuries or excessive downtime. The real challenge with a breakdown was its unpredictability, i.e., when it will occur and where it will occur. Today a policy of breakdown maintenance is usually not instituted unless the ship itself or its equipment is at the last leg of its life or when a company is scheduled to close or cease operations.
Insurance companies that carried out research into planned maintenance showed a significant decrease in breakdowns and damage to ships. Increased reliability and on-board safety were seen as consequential. This is what led to the IACS publishing requirements for PMS on board. Compliance with the ISM code (chapter 5, section 10) meant that the PMS database should include all shipboard vital equipment, and all equipment should have a clearly defined maintenance plan (constructed according to the manufacturer's recommendations, Class requirements and good seamanship practice). However, PMS in many ways was found wanting which lead to condition based management and maintenance of machinery.
Condition Based Maintenance System (CBM)
Condition based diagnosis of equipment meant that either it was likely to fail or not fail. CBM entails determining the equipment's health and acting:
When detecting circumstances that shorten machinery lifespan
Before it develops into a major failure
Only when maintenance is actually necessary
Condition-based maintenance permitted the maintenance personnel to undertake only what was necessary, thereby minimizing spare parts cost, time spent on maintenance and downtime. Despite its usefulness, there are several challenges to the use of CBM. It requires improved instrumentation of the equipment. Often the cost of sufficient instruments can be quite large, especially on equipment that is already installed. Therefore, it was important for the company to decide the importance of the investment before adding instrumentation to all equipment. Often visual inspections are considered to form an underlying component of condition monitoring, however this is only valid if the inspection results can be measured or critiqued against a documented set of guidelines. For these inspections to be considered as condition monitoring, the results and the conditions at the time of observation must be collated to allow for comparative analysis against the previous and future measurements.
In the absence of previous inspections, when the evaluation of the condition was performed in isolation it was then only Condition Assessment. The assessment was largely left to the observer’s competence and experience which resulted in errors in assessment of apparent condition. There is currently a trend to standardize the training and experience requirements for technical personnel involved in Condition Monitoring of machinery. This trend is reflected in the ISO18436 international standard. Although CBM achieves improved reliability and reduced maintenance costs, there seems to be general consensus that it has failed to improve consistent optimum and peak performance.
Has this been your experience? What can we do to mitigate this? Are there other options?
Let me know your thoughts.