Treatment of Low Stresses in Fatigue Design of Girth Welds Under Spectrum Loading

Although the use of Miner’s rule for calculating the cumulative damage due to spectrum loading is well established, there is conflicting evidence as to how stresses below the constant amplitude fatigue limit (CAFL) should be assessed. They are known to be damaging under spectrum loading, but the extent of that damage is not clear. Since such stress levels potentially contribute the majority of the fatigue damage from wave motion and vortex-induced vibration in risers, this issue is highly significant both economically and with respect to safety. Therefore, fatigue tests were performed on strip specimens cut from girth-welded steel pipes under constant and variable amplitude loading to investigate this issue. The test results were assessed on the basis of Miner’s rule used in conjunction with various methods of extrapolating the S-N curve beyond the CAFL to establish the most suitable for correctly representing the damaging effect of stress ranges below it. A fracture mechanics fatigue crack growth analysis was also performed to calculate the fatigue lives expected under the test loading spectrum.