Optimal Inspection and Maintenance Strategies for Structural Systems

The aim of this thesis is to give an overview of conventional and optimal reliability-based inspection and maintenance strategies and to examine for specific structures how the cost can be reduced and/or the safety can be improved by using optimal reliability-based inspection strategies. For structures with several almost similar components it is suggested that individual inspection strategies should be determined for each component or a group of components based on the reliability of the actual component. The benefit of this procedure is assessed in connection with the structures considered. Furthermore, in relation to the calculations performed the intention is to modify an existing program for determination of optimal inspection strategies.

The main purpose of inspection and maintenance of structural systems is to prevent or delay damage or deterioration to protect people, environment, and investments made in the structure. The inspection and maintenance should be performed so that the structural system is operating as much of the time as possible and the cost is kept at a minimum and so that the safety of the structure is satisfactory. Up till now inspection strategies have been based on experience and judgement with a few exceptions within the areas of aircraft structures and offshore structures. For some aircraft structures in the U.S. Airforce, inspections are planned when the average crack is expected to exceed a given level according to Yang, 1980, and in Pedersen et al., 1992, it is described how a reliability-based inspection strategy is used for offshore structures at the Tyra Field. However, methods are being developed to determine strategies that are optimal with regard to the expected costs and that also fulfil requirements for the reliability of the system, i.e. optimal reliability-based inspection strategies. Such strategies make it possible to include the uncertainty in loads and material parameters.