基于监测数据的深水水下井口循环弯矩计算方法

水下井口的疲劳完整性是海洋油气田长期安全开采的前提。工业界往往采用贴应变片直接测量水下井口应变来计算弯矩和疲劳损伤，但水下井口应变片粘贴困难且不能长时间连续工作。采用对水下井口监测方法，基于隔水管—防喷器组—水下井口的运动和力学特性，考虑防喷器组惯性力矩建立系统耦合动力学方程，最终形成基于监测数据的水下井口循环弯矩计算方法。以南海某深水水下井口为例，建立隔水管—防喷器组—水下井口系统有限元模型并进行动态分析，提取隔水管底部张力、转角、防喷器组加速度及转角等参数，代入所建立的系统耦合动力学模型，得到水下井口弯矩与有限元计算结果吻合良好。研究表明只需通过在线监测获得所需的输入数据，无需监测水下井口应变即可获取水下井口循环弯矩。建立的系统耦合动力学模型可为水下井口疲劳完整性评估提供理论依据。

Monitored data-based calculation approach for cyclic bending moment of subsea wellhead in deepwater

The fatigue integrity of subsea wellhead is a critical prerequisite for the long-time safe production of offshore oil and gas industry. Routinely, the bending moment and fatigue damage were calculated by direct strain measurement results of subsea wellhead using strain gauge in industrial sector. Whereas, it was difficult to paste the strain gauge for subsea wellhead and to monitor continuously for a long time. Using monitoring method for subsea wellhead, based on the mechanical characteristics and motion of riser-LMRP/BOPs-subsea wellhead system, the coupling dynamics equation was built to form the monitoring data based calculation method of cyclic bending moment for subsea wellhead taking the moment of inertia into consideration. Taking a deepwater wellhead system in the South China Sea as a case study, the finite element model of riser-LMRP/BOPs-subsea wellhead system was established to carry out dynamic analysis. The calculation parameters of lower riser tension and rotation angle, LMRP/BOPs acceleration and rotation angle were extracted and then substituted into the established coupling dynamics model to calculate the cyclic bending moment of subsea wellhead, and the calculated results agree well with that of the finite element simulation. The results show that the cyclic bending moment of subsea wellhead could be obtained by online monitored input data without monitoring strain of subsea wellhead. The established coupling dynamics model in this paper could provide a theoretical basis for fatigue assessment of subsea wellhead.