Fatigue analysis of the vortex-induced vibration in a drilling riser-wellhead system in ultra deepwater

The fatigue induced by vortex in a riser occurs frequently during ultra deepwater drilling operations,while the wellhead system fatigue caused by vortex-induced vibration(VIV) in a riser may be more severe.The present paper established an integral finite element model for the riser-wellhead system by using the VIV fatigue analysis algorithm and calculation flow chart of the ultra deepwater riser-wellhead system.A modal analysis was performed to extract the vibrating mode,slope and curvature of all nodes in multi-stage modality.A detailed VIV fatigue analysis was carried out by applying SHEAR7 to identify key fatigue places of the riser-wellhead system,assess the VIV fatigue life of the system,and investigate impacts of the top tension,blowout preventer(BOP),conductor dimension and wellhead mud-line height on the VIV fatigue performance of the wellhead system.A practical analysis on the riser-wellhead system of an ultra deepwater well in the South China Sea indicated that multi-stage modal vibration occurred more readily in the ultra deepwater riser-wellhead system,the maximum fatigue damage was located on the conductor,and the fatigue life of the system could meet operation requirements.An appropriate increase of top tension,a small BOP,a conductor with large bending rigidity,and a reduction of the wellhead mud-line height would effectively improve the VIV fatigue performance of the wellhead system.