Fatigue design is based on the SN-curve (Miner-Palmgren) approach and a semi-probabilistic design method. To properly account for the effect of inspection and the inherent uncertainty, a probabilistic fracture mechanics approach need to be adopted. Two of the more important variables included in the fracture mechanics model are the initial crack depth, a0, and the Probability of Detection (POD) values for the inspection methods EC/MPI. This paper presents the results obtained for a0 and POD in studies performed by Aker Offshore Partner during the last four years. More than 4000 NDT (Non Destructive Testing) inspection results for jacklet structures in the North Sea, in the period 1972 to 1995 have been classified, evaluated and collected in a database. The collected inspection history has been used to get new values for a0 and POD. The two values are assumed to be exponentially distributed. This study indicates a mean value (distribution parameter) of the initial crack depth and the POD püarameter to be 0.38 mm and 1.95 mm, respectively. The distribution parameter of 0.38 mm refers to one crack occurrence in the hotspot area in each joint, which is relevant for fatigue analysis. This value is equivalent to a distribution parameter of 0.56 mm when one crack is assumed to occur in each joint. The initial crack depth distribution is based on an observed mean fabrication crack depth of 0.94 mm with a frequency of one crack per three joints. By back-calculation of the reported fatigue cracks, the distribution of their initial crack size was established to be exponentially distributed with a mean value of 1.0 mm, corresponding very well to the reported fabrication defects. Thus, fatigue cracks can be considered to grow from the fabrication defects present from installation. The inspection history provides data for assessing the effect of grinding of the weld profile. A method of predicting the number and size of un-detected fatigue cracks are presented. The upper limit of the residual crack size may indicate how deep the grinding shall be performed when a crack is no longer detected.


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    Titel :

    In-service observations of cracks in North Sea jackets - a study on initial crack depth and POD values


    Weitere Titelangaben:

    Betriebliche Beobachtung von Rissen in Plattformanlagen in der Nordseee. Eine Untersuchung der Initialwerte von Risstiefe und Wahrscheinlichkeit der Rissentdeckung


    Beteiligte:
    Moan, T. (Autor:in) / Vardal, O.T. (Autor:in) / Hellevig, N.C. (Autor:in) / Skjoldli, K. (Autor:in)


    Erscheinungsdatum :

    1997


    Format / Umfang :

    9 Seiten, 4 Bilder, 7 Tabellen, 5 Quellen



    Medientyp :

    Aufsatz (Konferenz)


    Format :

    Print


    Sprache :

    Englisch






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