Fatigue calculation at hot spot in cope hole welded details using finite element analysis

Finite Element Analysis (FEA) is widely used to perform fatigue calculations for geometric singularities at welded components. The analysis methodologies are described in design codes and recommended practices such as DNV, IIW and Eurocode. The focus in the present study is the application of hot spot stress methodology on a weld detail located at the cope hole in a pile sleeve connection of a jacket substructure. Finite element analysis is used to calculate the geometric stress where the influence factor (INF) technique has been implemented to calculate the hot spot stress at the weld location. The INF methodology is used as the preferred approach compared to the traditional nominal stress method due to its ability to capture the stress response in complex welded details. Generally, a mid-surface shell model excluding the weld is used to model the welded components in FE analysis and a stress extrapolation method is applied to calculate the hot spot stress at the fatigue critical location. Here a full solid model of the cope hole detail including the weld geometry has been used for fatigue calculation as benchmark to calibrate the weld modeling techniques using shell elements for the analyses. The results confirmed that the weld geometry and stiffness has a significant influence on the hot spot stress calculation at the considered cope hole. Thus, the weld geometry and stiffness must be included into the finite element model for an accurate fatigue damage calculation of such details. The calibrated results showed that the mid surface shell model can still be used if an appropriate weld stiffness is included in the finite element model.