High fatigue reliability is one of the most important design criteria for welded offshore steel structures. Due to repeated wave loading fatigue cracks may initiate and grow in welded joints that are important for the integrity of these structures.

The present paper presents the methodology and the practical calculations for risk based inspection planning for fatigue cracks in welded details. Due to the uncertainty in the variables involved in the problem the planning has to be carried out by stochastic modeling and risk based assessments. Scatter in potential crack growth has to be analyzed by applied probabilistic facture mechanics and the uncertainty in the performance of the actual inspection technique has to be determined. With given risk acceptance criteria the practical outcome of the analyses is recommended inspection techniques and associated planned inspection time intervals. The classical theory is outlined and the latest recommendations from a Joint Industry Project recently completed by Det Norske Veritas in Norway are discussed. Discussion on how to model the fatigue process correctly is emphasized, particularly the role of time to crack initiation versus the subsequent crack propagation phase. Proper modeling of these two phases is crucial to get the potential crack path correct and thus obtain a reliable estimate of the probability of crack detection. For the crack propagation phase the selection of geometry functions is addressed.

A practical case study for life extension of an offshore oil loading system is finally presented. A structural significant item in the system is the steel gooseneck connection for the sub-sea flexible loading hoses. The case is an interesting one in the way that two similar systems were analyzed independently, one analysis based on the tradition S-N approach and one where the decisions were reliability based by applying the Risk Based Inspection approach.

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