Model of Lifetime-extending Maintenance

In this paper, a new maintenance model is presented in which both the interval of lifetime extension and the interval of preventive replacement can be optimised. Through lifetime extension, the deterioration can be delayed as such that failure is postponed and the lifetime of a component is extended. Through replacement, the condition of a component can be restored to its original condition. The lifetimeextending maintenance model (LEM model) can be used to optimise maintenance in both the design phase and the use phase. In the design phase, the initial cost of investment can be optimally balanced against the future cost of maintenance (life-cycle costing). In the use phase, the cost of preventive maintenance (lifetime extension and preventive replacement) can be optimally balanced against the cost of corrective maintenance (corrective replacement and failure). The cost-based criterion of the expected discounted costs over an unbounded time-horizon is used to compare different maintenance strategies. The LEM model enables optimal maintenance decisions to be determined on the basis of the uncertainties in the deterioration. It has been successfully applied in a case study to optimise the maintenance of a coating protecting steel. INTRODUCTION For safe and economic design of structures, there are many decision models available. Design optimisation has been a subject of interest for a long time. Although both safety and cost are influenced by time-related deterioration processes and maintenance actions, maintenance is not often considered in the design phase. Designers trying to build a good structure often take measures to avoid damages caused by these processes. Little attention is given to the relation between the investment in these measures, on the one hand, and the risk related to possible damage, on the other hand. Even maintenance managers pay little attention to the economic motivation of the actions taken. Common motives for maintenance are the experience and the personal opinion of the man in charge. Experience can be a good motive when the aim is to avoid failure. A more difficult task is finding an economic optimum for maintenance intervals on the basis of experience only. Therefore, maintenance optimisation is of growing interest and optimisation tools are needed. Such a tool is presented in this paper. It can be applied to perform life-cycle cost analyses for many different structures and many different situations. The development of maintenance management systems based on balancing cost and reliability using life-cycle costing is still in its infancy. Recently, life-cycle costing has been applied to the maintenance of the following structures: bridges (Frangopol, 1998), concrete structures in nuclear plants (Mori & Ellingwood, 1993), and hydraulic structures (Van Noortwijk, 1996).