On risk reduction principles in the context of maintenance optimisation modelling

This article addresses the relation between maintenance optimisation modelling and safety risk management. Most models for determining optimal maintenance strategies are based on a classical cost-benefit rationale, whereby all effects studied are transformed into monetary value and different concerns are balanced based on expected value calculations. If the failure of a system could have negative safety effects, there is a potential conflict between such an optimisation approach and the risk management principle saying that risk should be reduced to a level that is as low as reasonably practicable (ALARP); at least this is so if an interpretation of ALARP applies where verifying ALARP includes cost-benefit assessments but also extend beyond these. The ALARP principle is also typically implemented in combination with risk acceptance criteria, which could imply a restriction on the set of permissible maintenance policies and hence lead to suboptimal maintenance scheduling from an optimisation modelling point of view. This article considers the implementation of the ALARP principle, in combination with risk acceptance criteria, in the context of maintenance optimisation, addressing two different interpretations of the ALARP principle. A numerical example is included to show how one of these would affect the determination of an optimal policy. Two key points discussed are that risk reduction can be achieved in other ways than by adjusting the decision parameter of a given maintenance optimisation model, and that according to one of the interpretations the philosophy and practice of the ALARP principle extends beyond mechanical cost-benefit optimisation.