Fatigue testing strategies for the X65 steel catenary riser with small-scale specimens considering the effect of welding residual stress

To accurately evaluate the fatigue performance of a full-scale deep-water steel catenary riser (SCR) using small-size specimens, six fatigue testing strategies were explored, considering the effect of welding residual stress. Through a comparison with the full-scale resonant bending fatigue testing results, the most equivalent strategy using small-scale specimens was identified. The results indicate that the test strategies applying constant stress underestimated the fatigue life, compared with that of a full-scale specimen; meanwhile, fatigue life values obtained from the low-stress ratio testing strategy were higher, particularly in the low-stress range region. Comparatively, the fatigue lives obtained for the 100 mm-wide specimens without cutting were higher in the high-stress range region. The variable-applied mean stress strategy using the 25 mm-wide welded joint specimen was the most suitable for equivalence with the full-scale fatigue testing, with only a 9.7% difference in the fatigue life testing results. The difference between the applied mean stress and the actual transverse welding residual stress under various fatigue testing strategies is the key factor affecting the equivalence of the fatigue testing results.