ABSTRACT

A methodology for crack propagation analysis which takes into account the change in sea area and season has been proposed. A technique for configuring ‘storms’ without trial and error is developed in order to configure ‘storms’ for numerous sea areas and seasons in a short period of time. The fatigue crack propagation life of a weld of a bottom longitudinal of an AFRAMAX TANKER is examined using the developed techniques. In this study, the validity of the developed techniques is demonstrated that the calculated propagation life gives acceptable estimation with the measured one. The effect of the changes in sea area and season on the fatigue life is discussed.

INTRODUCTION

At the present rules of major classification societies, fatigue assessment is one of mandatory assessments. Fatigue load throughout ship life affects the fatigue crack propagation behavior. In the conventional studies, the time history of wave induced load was modeled as timeindividual random waveforms. It is known that the fatigue crack propagation rate changes with the load sequence, but this effect cannot be taken into account in this approach. Tomita et al (1995) proposed ‘storm model’ in order to take into account the load sequence effect. This model simulates wave induced stress histories at rough sea conditions, and it is configured by crescendo-decrescendo amplitude loading blocks. Tomita's "storm model" was derived from the long term distribution of individual wave height. This model cannot take into account the relation between significant wave height and mean wave period, and it is not consistent with theory of ship motion analysis. Kawabe et al (2004) proposed a modified "storm model". Kawabe's "storm model" was derived from wave scatter diagrams (e.g., GWS data), and it overcomes the drawbacks of Tomita's model.

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