Fatigue performance of riser quality girth welds: Analysis of TWI and DNV's databases

Abstract

In the fatigue design guidelines which focus on welded joints, when the applied loading is of variable amplitude (variable amplitude loading, VAL), the slope of the S‐N curves at endurances above the slope transition point (STP) changes from m to m + 2 in DNV‐RP‐C203 and BS7608. However, the position of the STP is different in BS7608 and DNV RP C203. This results in different calculated fatigue lives and the difference in calculated fatigue life can be significant. In order to provide better guidance to industry on the approach to use for fatigue analysis of girth welds in pipelines and risers specifically, TWI and DNV carried out a project where the ultimate aim was to provide guidance on a common S‐N curve suitable for calculating the fatigue life of girth welds with focus on the slope change area of the S‐N curves. This was attempted by re‐analyzing TWI's and DNV's databases of girth weld fatigue test results and comparing the outcomes. Although it was not possible to agree on a single STP from the analysis carried out, regression analysis of high endurance results from the databases suggest that an STP between the two current values 1E7 and 5E7 may be plausible. This is supported by fracture mechanics analysis, which suggests that the position of the STP depends on the fatigue Class, and the magnitude of the applied stress ranges.