Risers and sections of pipeline in free-span can be subjected to cyclic loading from flow-induced vibrations and other fluctuating forces leading to fatigue loading of the structure. Single-sided welds are of particular interest, however current design codes take a pessemistic view of their fatigue performance. Improvements in the quality of the finished weld produced by modern pipeline welding technology, and indeed advances made in knowledge about fatigue of such welds, mean that revision of the design guidance is now warranted. This paper reviews the fundamental basis of the existing S-N curves used for North Sea applications and considers their applicability in the light of research on fatigue of girth welds. Based on the review, proposels are made for revised fatigue design rules for girth welds in pipelines. The S-N curves and teratments noted in this report present the best available design guidance for fatigue of pipline girth welds at the present time. The most significant relaxation in these new guidelines is the removel of the very severe treatment for unbacked single-sided girth welds, currently ClassF2/HSE Class 1.52 combined with an additional stress magnification factor for even moderate misalignment (within fabrication tolerances for example). Other relaxations are in the thickness effect penalty, and in the calculation of the stress magnification factor for axial misalignment in girth welds. These new guidelines represent an improvement over existing treatments and, as such, they formed the basis of the new NORSOK rules. There exists scope for further improvement at the time when confidential results become available, and in the event that further work is undertaken.
Guidance for fatigue design and assessment of pipeline girth welds
2003
19 Seiten
Aufsatz (Konferenz)
Englisch
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