During the last 30 years a methodology for planning in-service inspection of fatigue cracks based on probabilistic methods has been developed. Due to the nature of the fatigue phenomena it is well known that minor changes in basic assumptions can have significant influence on the predicted crack growth lives. Calculated fatigue lives are sensitive to input parameters using standard design analysis procedures. Calculated probabilities of fatigue failure using probabilistic methods are even more sensitive to the analysis methodology and to input parameters to the analyses. Fracture mechanics analysis is required for prediction of crack sizes during service life in order to account for probability of detection after an inspection event. Analysis based on fracture mechanics needs to be calibrated to that of fatigue test data or S-N data. Thus, use of these methods for planning inspection requires considerable education and engineering skill. Therefore the industry has asked for guidelines that can be used to derive reliable inspection results using these methods. DNV has during the last years performed a joint industry project on use of probabilistic methods for planning in-service inspection for fatigue cracks in offshore structures. The recommendations from this project are now being included in a DNV Recommended Practice. The main background for this document is presented in this paper.

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