1409 Reliability, Risk and Safety – Ale, Papazoglou & Zio (eds) © 2010 Taylor & Francis Group, London, ISBN 978-0-415-60427-7 Optimal inspection strategy for rubble-mound breakwaters with time-dependent reliability analysis D.V. Nguyen, P.H.A.J.M. van Gelder, H.J. Verhagen & J.K. Vrijling Delft University of Technology, Delft, The Netherlands ABSTRACT: This paper presents a comprehensive maintenance strategy with optimal inspection planning and its application to rubble mound breakwaters. In order to fulfil designated functions, a break- water has to be inspected during its lifetime and has to be repaired if necessary. Actions of inspecting and repairing form a maintenance strategy. In the paper rational maintenance decision-making approaches are discussed. The proposed maintenance strategy combines time-dependent and condition-dependent maintenance with event-dependent inspection. Optimal inspection planning is obtained by cost optimiza- tion with a safety constraint. In practical cases, it is possible to achieve an optimal inspection plan when relevant parameters of strength and load are existing and available. Then a maintenance strategy applied to rubble mound breakwater is developed based on the reli- ability analysis of breakwater systems. The optimi- sation problem is formulated with cost component quantification. Optimal inspection plan is found when a minimum total expected cost of maintenance is achieved with a minimum reliability constraint. The paper is structured as follows. A brief intro- duction on reliability analysis of rubble mound breakwaters is presented in Section 2. Discussion on maintenance strategies is given in Section 3. In Section 4, the optimisation problem is formulated. Finally, conclusions are presented in Section 5. 2 RELIABILITY ANALYSIS OF BREAKWATER SYSTEMS 2.1 Introduction to reliability analysis of rubble-mound breakwaters Several concepts in the context of reliability analysis should be mentioned including system, failure and fault tree. A structure can be modelled by a system including subsystems and compo- nents. A failure is defined as a condition in which a structure or a structural component loses its designated functionalities. Interaction (or correla- tion) between failure modes and the contribution of each one to the system failure can be defined in a fault tree. A fault tree gives a logical succession of all events that leads to one unwanted top event. The probability of failure of the whole system, equal to the probability that the top event occurs, can be calculated based on probabilities of failure of components and their correlations. 1 INTRODUCTION Structural systems in general and breakwater systems in particular are subject to deterioration in time due to various factors such as the cumulative loading, the environmental effects or the ageing of materials. Selecting a rational maintenance strategy, which is formed by a combination of inspecting and repairing actions, is of main concern for engi- neering operation management. Over the past decade, optimal maintenance strategies following probabilistic and reliability approaches has been developed widely. Faber (2002) and Straub & Faber (2005) introduced and discussed the concepts and applications of reliability-based and risk-based inspection planning for structural system (RBI). In the field of coastal and hydraulic engineering, several researches on the maintenance model have also been presented. van Noortwijk & van Gelder (1996) proposed an optimal mainte- nance decision model for berm breakwaters that dealt with breakwaters’ breaching processes. Vrijling (2003) developed a framework for the maintenance of water defense systems which focused on distin- guishing different maintenance strategies. Recently a thorough discussion on the monitoring, inspect- ing and repairing procedures of flood defence sys- tems was presented by Buijs (2007). This paper proposes a comprehensive mainte- nance strategy for breakwater systems that includes the time, load and state dependencies. With this strategy the optimal inspection planning is derived. In this paper the functional system, functional failures, failure modes and reliability analysis of a breakwater system are presented. Rational maintenance optimisation approaches are discussed.