03 Lifetime extending maintenance model (LEM)
Lifetime-Extending Maintenance model
This page presents the basic principles of the Lifetime-Extending Maintenance (LEM) model, which has been developed by the Civil Engineering Division of the Netherlands Ministry of Transport, Public Works, and Water Management (Rijkswaterstaat) in co-operation with HKV Consultants. The LEM model is a program for Life-Cycle Costing (LCC) with which cost-optimal maintenance decisions can be determined under uncertainty.
Lifetime-Extending Maintenance (LEM)
A new model of lifetime-extending maintenance has been developed with 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. Lifetime-extending maintenance is defined to be maintenance of one component (e.g. a coating protecting steel) to extend the lifetime of another component (e.g. the steel).
Optimisation of LEM and replacement
The lifetime-extending 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 (the Net Present Value) 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 (e.g. the loss of steel thickness due to corrosion).
Application to LEM of coating on steel
The LEM Model has been successfully applied in a case study to optimise the maintenance of a coating protecting steel. An example of a cost-optimal frequency of lifetime-extending maintenance is shown in Figure 1. The expected lifetime of the steel can be extended by grit blasting (with 0.1 mm loss of steel thickness), as well as placing a new coating. The Net Present Value of the costs of lifetime extension (removing old coating and some steel, as well as placing a new coating) and replacement of the steel are shown in Figure 2.