In the past several decades, maintenance and replacement problems of deteriorating systems have been extensively studied. Thousands of maintenance and replacement models have been created. These models can be categorised in maintenance policies: age replacement policy, random age replacement policy, block replacement policy, periodic preventive maintenance policy, failure limit policy, sequential preventive maintenance policy, repair cost limit policy, repair time limit policy, repair number counting policy, reference time policy, mixed age policy, preparedness maintenance policy, group maintenance policy, opportunistic maintenance policy, etc. Each policy has, depending on the situation, different characteristics, advantages and disadvantages. A maintenance model within the same policy sometimes has different cost structures and/or different maintenance restoration degrees (minimal, imperfect, perfect).
Maintenance aims to improve system availability and reduce failure and downtime. Cost reduction is also necessary. Generally, an optimal system maintenance policy may be the one which either:
- minimizes system maintenance cost rate,
- maximizes the system reliability measures,
- minimizes system maintenance cost rate while the system reliability requirements are satisfied, or maximizes the system reliability measures when the requirements for the system maintenance cost are satisfied.
An optimal maintenance schedule should therefore consider/incorporate various maintenance policies, system configurations, shut-off rules, maintenance restoration degrees, correlated failures and repairs, failure dependence, economic dependence, non-negligible maintenance time, etc