Fatigue Testing and Analysis of Full Scale Girth Welded Tubulars

Heavy duty girth welded tubulars are used for many critical structural members offshore, e.g. free spanning pipelines, risers and tethers. The fatigue properties of those girth welds are often the design limiting factor. Today’s fatigue design rules for girth welded tubulars are mainly based on testing of segment specimens machined from a girth weld. Such specimens do strictly speaking not simulate all the features of a complete girth weld in a tubular. Lately some data have become available from rotating bending of full scale tubulars. However data from axial loading of full scale tubulars are rare. In this study full scale (OD 24”, 609.6 mm, thickness 0.812”, 20.6 mm) girth welded tubulars were fatigue tested in axial tension. Each tubular specimen contained three girth welds, equalling almost 6 m of weldment for each test. Both as-welded, TIG-dressed and ground weldments were included in the test program. The fracture surfaces were inspected for determination of the initiation point and its relation to welding flaws. The test results are compared to common fatigue design curves. The significance of embedded weld flaws in ground welds and the relationship to S-N curves for ground welds is discussed.