Fatigue life prediction method for subsea wellhead welds based on the nonlinear fatigue accumulation model

Abstract

In drilling and completion conditions, subsea wellheads are exposed to enormous alternating bending moment loads from the riser due to complex environmental conditions, causing fatigue damage. The joint weld between the high-pressure (HP) wellhead and the casing is especially vulnerable to fatigue, creating the risk of operational failure and accidents. Although fatigue analysis is vital, the traditional S–N curve method is often not accurate enough. This paper proposed an analytical technique to examine subsea wellhead fatigue by combining subsea wellhead fatigue decoupled analysis with a nonlinear fatigue accumulation theoretical model. This method was used to assess subsea wellhead weld fatigue in a gas field in China's Bohai Sea during drilling and completion. The bending moment load spectrum at the top of the HP wellhead was obtained via global dynamic analysis. The bending moment-stress and bending moment-strain response curves of the welds were extracted using local quasi-static analysis. Finally, the theoretical nonlinear fatigue accumulation model and traditional S–N curve method were used to evaluate subsea wellhead weld fatigue, respectively. The results indicated that the fatigue damage caused by drilling was more significant than completion, while the fatigue damage at the weld between the HP wellhead and the 16″ casing exceeded that of the weld between the low-pressure (LP) housing and the 30″ conductor. Furthermore, the technique proposed in this paper could be used to assess more extensive fatigue damage than when employing the S–N curve method, indicating that the results were more conservative and more suitable for practical engineering applications.