Omae 2003-37238 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 SN curves for non-tubular joints. Conclusions made based on the study provide more background on the S-N curve application in fatigue assessment. INTRODUCTION The S-N curve method is the most widely used procedure to establish the fatigue strength (life) of structural details in both offshore and ship engineering practices. The application of the S-N curve method is governed by a variety of engineering standards that typically share some features. These common features include: • The format and implicit confidence bounds of the ‘design S-N curves’ (which are established from experimental data from specimens that are considered representative of the structural detail being assessed); • The categorization of generic structural details into particular S-N curve ‘classifications’ (which are based on the geometry and loading of the detail; and also occasionally• specifics of the welding procedure, or • the extensiveness of the non-destructive examination (NDE) to be provided during structural fabrication, • or both); • Adjustments to the design S-N curves to account for other considerations such as structural element thickness and the relative corrosiveness of the environment that the detail will experience. While sharing these general features, the specifics of each of these features in the various reference standards can be quite different. This can result in disparities of fatigue life predictions. ABS TECHNICAL PAPERS 2003 142 Sensitivity of Fatigue Assessment to the Use of Different Reference S-N Curves Internationally, relevant sources of specifications governing the application of the S-N curve approach to offshore structures are the U.K. Welding Institute (TWI) and the International Institute of Welding (IIW). The former are well known to the offshore industry as they were published in various editions by the U.K. regulatory bodies, the Department of Energy (DEn) and the Health and Safety Executive (HSE). In the USA and in other countries where the prevalent offshore structural engineering standards have relied on the API’s recommended practice-RP 2A, the API and by reference, the American Welding Society (AWS) S-N curves are used to establish fatigue strength. ABS has studied the various S-N curves in use for offshore structures and has developed its “Guidance on the Fatigue Assessment of Offshore Structures”, [1]. The Guidance provides proposals that reflect the features of the international standards. This has resulted in the development of new sets of S-N curves that are referred to in the Guidance as the ABS S-N curves. But, the Guidance also recognizes the application of API and AWS S-N curves for major areas of fixed (buoyant and non-buoyant) platform structures that are sited on the U.S. Outer Continental Shelf. This paper compares the ABS and AWS S-N curves for non-tubular joints in offshore structures and presents the results of fatigue assessments that have been performed using these data. This exercise will illustrate the background and application of these standards, and it will also demonstrate the sensitivity of the fatigue life predictions to the reference standards employed.