Sensitivity of Fatigue Assessment to the Use of Different Reference S-N Curves

The S-N curve based fatigue assessment approach is the most widely used one in both ship and offshore industry, in contrast with a fracture mechanics approach. The S-N approach, implemented by either simplified or spectral method, has to apply S-N curves to calculate fatigue strength. The S-N curve, which represents the number of cycles (N) of a constant amplitude stress range (S) that will cause a fatigue failure, is normally developed based on experimental data. Which S-N curve should be applied to a particular detail depends very much on the geometry of the detail, welding information as well as loading condition. There are various S-N curves published by different institutions, e.g., S-N curves published by UK HSE, IIW, AWS etc. The newly developed ABS “Guidance on Fatigue Assessment of Offshore Structures”, [1], proposes the ABS S-N curves, in which two categories of joints, tubular and non-tubular, are included, and both size and environment effects are taken into account. However, the application in Gulf of Mexico is also influenced by API recommendations. In API RP 2A, AWS S-N curves are referred, which in US practice is accepted for fixed (buoyant and non-buoyant) platform deck structures. The objective of this paper is to address the difference between different S-N curves and to present the detailed results of fatigue assessment by using different S-N curves for non-tubular joints. Conclusions made based on the study provide more background on the S-N curve application in fatigue assessment.