Fatigue Strength Analysis for the Condition Assessment Program

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This paper describes the technical state of fatigue strength analysis of longitudinal stiffeners in the NK-CAP (Condition Assessment Program). In the NK-CAP fatigue strength analysis, spectral fatigue analysis is done on all connections where longitudinals pass through ordinary web transverses and transverse bulkheads, based on all stress components caused by external wave pressure, internal tank pressure, hull girder bending and relative deformation of primary members. Epoch-making technology that takes into account the effects of different tank loading conditions on the fatigue strength of ship structures has been employed in the analysis program. The effect of mean stress due not only to welding residual stress but also to static stress in still water, which varies depending on tank loading conditions, is evaluated based on the hot-spot stress approach. This method is the same as that specified in the Guidelines for Fatigue Strength Assessment of Tanker Structures that were published in November 2002 by ClassNK. In the course of application to some fifty tankers, the technical background and applicability of the NK-CAP fatigue analysis program have been validated by a good correlation between the analysis results and the acquired damage experience.
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