Risk-based inspection

Abstract: Steel structures are subject to deterioration processes such as fatigue crack growth or corrosion. The models describing these processes often contain major uncertainties, which can be reduced through inspections. By providing information on the actual state of the structure, inspections facilitate the purposeful application of repair actions. In doing so, inspections represent an effective risk mitigation measure, for existing structures often the only feasible one. Risk based inspection planning (RBI) provides the means for quantifying the effect of inspections on the risk and thus for identifying cost optimal inspection strategies. By combining the Bayesian decision analysis with structural reliability analysis, RBI uses the available probabilistic models of the deterioration processes and the inspection performances to present a consistent decision basis. Although the principles of RBI were formulated for fatigue deterioration in the early 1990s, its application has in the past been limited to relatively few industrial projects. The complexity of the approach, combined with the required numerical efforts, has hindered its implementation in an efficient software tool and thus its integration into the general asset integrity management procedures of the owners and operators of structures. These drawbacks have motivated the development of generic approaches to RBI. The main idea of the generic approaches is to perform the demanding probability calculations for generic representations of structural details. Based on these “generic inspection plans”, the optimal inspection plans for a particular structure are obtained by means of an interpolation algorithm from simple indicators of the considered deterioration process. Because these indicators are obtained from standard design calculations and specifications, the application of RBI is greatly simplified once the generic inspection plans are calculated. In this work, the generic approaches to RBI are developed together with the tools required for their implementation in an industrial context. This includes a presentation of the general RBI methodology, a review of the probabilistic deterioration models for fatigue and corrosion of steel structures and the description of inspection performance models. Whereas most of these aspects are well established for fatigue subjected structures, new concepts are introduced for the treatment of corrosion deterioration. A framework for the generic modelling is developed and the application is demonstrated on two examples for fatigue and corrosion. Various aspects of the implementation are presented, including the development of a software tool. The generic approaches, due to their computational efficiency, facilitate the integral treatment of structural systems, as opposed to the traditional RBI approaches which focus on individual details. These “system effects” are investigated and it is demonstrated how the inspection efforts can be optimised for entire systems. Additionally a consistent framework is established for the determination of risk acceptance criteria related to inspection planning for structural systems. These system orientated developments ensure that the generic approaches to RBI, which have already demonstrated their efficiency in practical applications, are fully consistent with the objectives of the owners and operators of structures.