Plant-wide maintenance decisions are strongly influenced by constraints such as annual maintenance budget limits, availability metrics, environmental indices, etc. In 1994, the American Society of Mechanical Engineers (ASME), published a research report (CRTD-Vol. 20-3, 1994) describing a fully-quantitative methodology for the economic optimisation of maintenance activities in multi-component systems. This approach was aimed at optimising inspection and maintenance schedules for fossil fuelled power plant components. The approach requires multi-disciplinary skills in decision theory, probabilistic damage modelling and engineering financial analysis.
In 2001, TWI extended the ASME methodology by developing a decision model applicable to pipelines subject to internal and external corrosion. This work was undertaken as part of TWI's internal core research program (CRP) and will soon be published as CRP Report 13179. Central to this methodology is a decision model which is formulated by combining the probability of failure and the consequential failure cost. The decision model is then optimised by varying the scheduled year of the maintenance cost, while maximising the net present value (NPV) of the maintenance action by considering the cost of the action against the probable cost of failure if no action is taken.
The probable cost of the failure is determined as the product of the failure probability and the direct failure consequence costs. The use of NPV in the decision model allows for the inclusion of inflation rates, interest rate factors, tax rate factors and the depreciation of capital expenses in the optimisation process. The methodology can be applied to individual equipment, as well as several items of equipment within a multi-component system, e.g. a sub-sea pipeline network.
High level of detail
This fully-quantitative methodology for the economic optimisation of maintenance activities is best applied to equipment in the 'ageing stage' of their life cycle. It allows for the inclusion of safety and environmental, as well as economic, constraints on the optimisation problem. The primary disadvantage, as with all quantitative risk analysis methods, is that the optimisation computation requires a very high level of detail in the input information. In addition, the analyses are complex and will require the use reliability analysis software. For these reasons, plant managers would normally only employ this methodology with assistance from an external specialist. However, TWI has shown, through its CRP activities, that substantial financial benefits can be realised through the use of multi-component economic maintenance optimisation.
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