Defect Tolerance in Fatigue of Steel Catenary Riser Girth Welds - Two Stages Model

摘要

Steel Catenary Risers (SCR) are known to be subjected to importantcyclic loading often critical for the fatigue resistance of the structure.Normally, specifications for girth welding, in products destined tofatigue applications, express very strict requirements in terms ofmaximum allowable imperfections, with very small allowed embeddedand surface flaws.In previous ISOPE Conference, a paper illustrating the model forsuitable prediction in crack propagation regime was presented, offeringindications for a proper calculation with some reduction in overconservativismwith respect to a standard fatigue life prediction. In thepresent work, a Two Stage Model (TSM) for the fatigue defecttolerance calculations was developed, including the initiation andpropagation stages. The number of cycles to crack initiation is modeledby a local strain approach using the Manson-Coffin equation, whereasthe propagation phase is modeled by fracture mechanics adopting theParis law. Such model is able to take into account the effect of theglobal geometry of the joint and of the local geometry considering weldtoe, weld imperfections, residual stresses and consequent local rising ofmean stress during stress cycling.The model was calibrated on the basis of a wide full scale experimentaltesting program and it was applied to predict fatigue life of X65pipeline girth welded joints. In order to verify the accuracy of thepresented model, the S-N plots constructed from the two-stage modeland full-scale fatigue tests on X65 girth welds with weldingimperfections were compared.