Utilization of Subjective Information to the Problem of Structural Maintenance

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In-service inspection on ship structure plays an important role for maintaining safety against fatigue damage. However, when we want to perform reasonable inspection, it is necessary to clarify the present reliability level of structural members as well as the desirable level.From this point of view, fatigue reliability analysis under repeated inspections is carried out for the six structural members of bulk carrier. Crack initiation and propagation properties of the members, probability of crack detection by the visual inspection, target failure probabilities of the members and etc., which are essential information in the reliability analysis, are collected by the questionnaire asked to the navel engineers.Then fatigue reliability analysis and sensitivity analysis are carried out using the questionnaire results. Markov Chain Model is employed for this purpose, in which crack length exceeding 200mm or 500mm was defined as the failure event. Through the calculations, effect of repeated inspections at the present condition on fatigue reliability, and the inspection intervals necessary to achieve the target failure probability are investigated for the six members. The major conclusions are drawn as follows :(1) Fatigue characteristics and POD curves by visual inspection were obtained for the six locations of bulk carrier by the questionnaire.(2) If ISI of two years interval is applied, al least 60 to 70% of initiated cracks are detected by the ISI during 20 years.(3) ISI of two years interval can reduce the failure probability Pf (200) under no inspection to 1/3 and the Pf (500) to 1/5, respectively.(4) Inspection interval of 6 months is necessary for the achievement of target Pf (200) and 1.5 years for target Pf (500).(5) For the design requirements such as the improvement of reliability, reduction of weight and simplification of inspection, the solutions to respond to the requirements were extracted.