Fatigue of High Quality Girth Welds Available to Purchase

Several studies on fatigue strength of high quality girth welds are summarized and discussed. The fatigue performance of such welds is consistently above the common design classes, as long as key set of influencing parameters are controlled. Fatigue life and crack initiation depend on loading mode, and weld defects, weld geometry, residual stresses, and degree of weld distortions. The welding method, especially for the root, has also proven to be important, e.g. TIG and high quality STT are often superior to Cu-backed roots. In practice the most important factor will be the surface breaking flaws as root LOP, etc. Such flaws are rarely found at the cap side. Thick pipe walls will also reduce the fatigue capacity, e.g. a 45mm wall thickness with 25mm reference, will reduce the with the same amount as above. High fatigue performance requires absence of any weld discontinuity above certain critical sizes which may be a challenge for the accuracy and resolution of NDT systems. In the present study, some important factors that influence the fatigue strength are examined. Based on results and theoretical calculations, the effects of the various crack-like discontinuities are described and compared to current design standards. The geometrical misalignment of the joint (hi/lo) will also influence the fatigue capacity. In bending, the weld cap toe and the weld root are the critical locations. The residual stress distribution in the welded region may, however, alter this. With pipe wall thickness larger than ∼25mm residual stresses can be beneficial to the weld root area. However, due to the scatter in the measurements it is difficult to assess the influence on fatigue life exactly. FE modeling of the welding process is therefore used to supplement this discussion. Improvement methods are available to suppress the critical influence of surface cracks/discontinuities. Grinding of the cap weld toe has in many cases shown significant improvements of girth welds. Also, TIG-dressing, high quality STT, or fillers with high nickel content, have potential for improving the root performance.