Abstract Subjected to complicated and cyclical dynamic loads caused by waves, currents and vessel motions and transferred by drilling risers, the subsea wellhead (SW) is prone to fatigue failure, which could lead to the loss of well integrity and even catastrophic accidents. This paper presented a novel semi-decoupled model based local stress-strain approach to predict the fatigue damage of SW. Using the proposed method, a local fine finite element model of SW was built to analyze local load-strain curves of fatigue hot-spots and the equivalent model of the SW. Further, the equivalent model was put into the global model to extract load-time curve by dynamic analysis. After the strain-time curve was determined by the calculated load-strain and load-time curves, the strain range could be calculated by the stabilized hysteresis curve and rainflow counting. Subsequently, the fatigue damage was calculated by the modified strain-life curve and Palmgren-Miner's rule. The results of a case study indicated that the fatigue damage of high pressure wellhead (HW) weld was higher than that of low pressure wellhead (LW) weld, as the LW has the larger outside diameter and wall thickness causing the larger bending stiffness. Compared with the conventional S-N method of fatigue analysis, the calculation results of the proposed semi-decoupled model based local stress-strain approach would be more efficient and reasonable, and the related conclusions could provide reference for life extension of SW in deepwater oil and gas development.

中文翻译：

---

基于半解耦模型的海底井口系统疲劳损伤局部应力应变预测方法

摘要 海底井口（SW）承受由波浪、水流和船舶运动引起并由钻井隔水管传递的复杂周期性动态载荷，容易发生疲劳失效，从而导致井的完整性丧失，甚至发生灾难性事故。本文提出了一种新的基于半解耦模型的局部应力应变方法来预测 SW 的疲劳损伤。利用所提出的方法，建立了SW局部精细有限元模型，分析了疲劳热点局部载荷-应变曲线和SW的等效模型。进一步将等效模型放入全局模型中，通过动态分析提取加载时间曲线。通过计算的载荷-应变和载荷-时间曲线确定应变-时间曲线后，应变范围可以通过稳定滞后曲线和雨流计数来计算。随后，通过修正应变-寿命曲线和 Palmgren-Miner 法则计算疲劳损伤。案例研究结果表明，高压井口 (HW) 焊缝的疲劳损伤高于低压井口 (LW) 焊缝，因为 LW 具有更大的外径和壁厚，从而导致更大的弯曲刚度。与传统的SN疲劳分析方法相比，本文提出的基于局部应力应变法的半解耦模型计算结果更加高效合理，相关结论可为深水油气SW寿命延长提供参考。发展。疲劳损伤由修正的应变-寿命曲线和 Palmgren-Miner 规则计算。案例研究结果表明，高压井口 (HW) 焊缝的疲劳损伤高于低压井口 (LW) 焊缝，因为 LW 具有更大的外径和壁厚，从而导致更大的弯曲刚度。与传统的SN疲劳分析方法相比，本文提出的基于局部应力应变法的半解耦模型计算结果更加高效合理，相关结论可为深水油气SW寿命延长提供参考。发展。疲劳损伤由修正的应变-寿命曲线和 Palmgren-Miner 规则计算。案例研究结果表明，高压井口 (HW) 焊缝的疲劳损伤高于低压井口 (LW) 焊缝，因为 LW 具有更大的外径和壁厚，从而导致更大的弯曲刚度。与传统的SN疲劳分析方法相比，本文提出的基于局部应力应变法的半解耦模型计算结果更加高效合理，相关结论可为深水油气SW寿命延长提供参考。发展。因为 LW 具有更大的外径和壁厚，导致更大的弯曲刚度。与传统的SN疲劳分析方法相比，本文提出的基于局部应力应变法的半解耦模型计算结果更加高效合理，相关结论可为深水油气SW寿命延长提供参考。发展。因为 LW 具有更大的外径和壁厚，导致更大的弯曲刚度。与传统的SN疲劳分析方法相比，本文提出的基于局部应力应变法的半解耦模型计算结果更加高效合理，相关结论可为深水油气SW寿命延长提供参考。发展。