Welds are one of the commonly used joint types and are employed extensively in subsea oil and gas production equipment. Commonly used weld joints in subsea components are fillet, butt, full-penetration, plug, and girth. Fatigue is one of the critical failure modes for welded joints. Welded joints are complex to analyze for fatigue loading due to the microstructure change during the welding process. The welding process also induces residual stress in the heat affected zone (HAZ) surrounding the weld. This, in turn, can adversely affects the fatigue life of the joint.
The S-N fatigue approach is commonly used for weld fatigue analysis due to the simplicity of this method. Industry standards such as DNV, IIW, BS-7608, and ASME BPVC Sec VIII Div. -2 or -3 are typical references for this type of analysis. For subsea specific applications, DNV-RP-C203 and BS-7608 are generally used because these two standards provide S-N curves for welds in “air” as well as in “seawater with cathodic protection”. These two codes also provide S-N curves for various weld geometries ranging from simple fillet welds to complex tubular joints. Some of the weld fatigue analysis techniques used in the subsea industry are the: nominal stress approach, structural hot spot stress approach, effective notch stress approach (ENS), structural stress method (ASME VIII-2, -3) and the Fracture mechanics based fatigue crack propagation (FCG) approach.
This paper presents the fatigue analysis of fillet welds in bore inserts using the ENS method. In the ENS method, a 1mm radius notch is modelled at the weld root or toe, see Figure 1, which yields a finite weld root stress. The stress analysis is carried out using FEA and the stresses on the notch along with the appropriate fatigue curve are used to estimate the weld root fatigue life.