Fatigue is of significant concern for load bearing offshore structures in the North Sea. The first generation of jacket structures in the Norwegian part of the North Sea in the early 1970'ies was not designed against fatigue failure. The degree of detail and quality of the analysis has been significantly improved since 1995. The present tools for predicting fatigue life or crack growth imply highly scattered estimates. This paper presents a method to utilise the inspection history / in-service observations to measure and improve the accuracy of analysis method. The results presented herein are obtained in studies performed by Aker Offshore Partner during the last four years. More than 4000 NDT (Non Destructive Testing) inspection results in the period 1972 to 1995 have been classified, evaluated and collected in a database. The database also contains the related fatigue life estimates. Cracks were detected in 438 joints, while 181 joints have been evaluated to contain fatigue cracks. The experience with 30 jacket structures and more than 600 platform years in the North Sea clearly demonstrates that the present tools for fatigue crack growth give very uncertain but generally conservative estimates of the fatigue life. For main structural and horizontal framing components approximately 30 per cent of the predicted fatigue cracks is developed. The applied theory also predicts 70 to 80 per cent of the fatigue cracks observed. A first step towards calibration of the prediction tools based on collective experiences has been made. The correlation between in-service observations and theoretical predictions is quantified for different comparison events and different subsets of data. The application area of the theoretical procedures for prediction is identified.


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    Title :

    Predicted versus observed fatigue crack growth. Validation of probabilistic fracture mechanics analysis of fatigue in North Sea Jackets


    Additional title:

    Vergleich zwischen prognostizierter und beobachteter Ermüdungsrissausbreitung. Validierung einer probabilistischen rissmechanischen Untersuchung der Materialermüdung in Plattformanlagen in der Nordsee


    Contributors:
    Vardal, O.T. (author) / Moan, T. (author)


    Publication date :

    1997


    Size :

    10 Seiten, 7 Bilder, 6 Tabellen, 7 Quellen



    Type of media :

    Conference paper


    Type of material :

    Print


    Language :

    English





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