Verification of Class B S-N Curve for Fatigue Design of Forged Steel Connectors

The fatigue design of forged steel connectors in steel catenary risers (SCRs) is commonly based on the BS 7608 Class B S-N curve, which was derived from longitudinal butt weld data. However, there is very little direct support for this. Therefore, its validity was checked by performing a fatigue test programme involving strip specimens extracted from actual forged J-lay connectors. The effects of stress ratio and surface roughness on fatigue strength were investigated. On the basis of these test results, together with available published data obtained from specimens or structural components that failed in plain steel, the Class B S-N curve was verified. The method was particularly conservative for low stress ratios in the high-cycle regime and a procedure based on the use of the Goodman diagram was devised for correcting the Class B curve for mean stress. Surface roughness in the range investigated, between 3.2 and 8.1μm in Ra, had little effect on the fatigue performance of the forged steel.