A Novel Probabilistic Fatigue Assessment Tool and its Application to an Offshore Riser Joint

Abstract

Although the complete stress field of a structure is available after finite element analysis, traditional S-N based fatigue assessment is merely based on the (peak) stress cycles at the critical location of the structure and generally deterministic in its nature. This is true both for fatigue assessment by “hand” and by post-processor. At the design stage, fatigue-crack growth is generally limited to “worst-case” cracks. This paper gives a brief presentation of a novel tool for probabilistic fatigue assessment, LINKpfat, both capable of performing weakest-link fatigue analysis and fatigue-crack growth analysis based on computationally efficient determination of stress-intensity factors by means of weight functions.Both the traditional peak-stress module and the weakest-link module of LINKpfat are applied to the fatigue assessment of an offshore riser joint with moderate stress concentrations for three different load cases. It turns out that the peak-stress analysis may overestimate the load capability of the riser with about 35% by neglecting the size effect. The latter is consistently accounted for by the weakest-link module.