超深水钻井隔水管-井口系统涡激振动疲劳分析

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

超深水钻井作业经常发生隔水管涡激疲劳问题,而由隔水管涡激振动(VIV)引起的井口系统疲劳可能更为严重。根据超深水隔水管-井口系统VIV疲劳分析算法与计算流程,建立隔水管-井口系统整体有限元模型,通过模态分析提取各阶模态下所有节点的振型、斜率和曲率,应用SHEAR7程序进行详细疲劳分析,识别系统关键疲劳部位,评估系统的VIV疲劳寿命,并研究顶张力、防喷器(BOP)、导管结构尺寸及井口出泥高度对井口系统VIV疲劳特性的影响规律。针对南海某超深水井的隔水管-井口系统进行实例分析,结果表明,超深水隔水管-井口系统容易发生多阶模态振动,系统最大疲劳损伤位于导管上,系统疲劳寿命满足作业要求,适当提高顶张力、采用小型化的BOP、增大导管抗弯刚度以及降低井口出泥高度均可有效改善井口系统的VIV疲劳性能。

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.