随着近年来深水作业水深的增加，水下井口的疲劳损伤问题变得非常突出。采用第6代钻井平台比采用第3代平台的井口和导管疲劳损伤水平增加10-20倍。由于海洋浅部软土对井口和导管起支撑作用，所以土反力对井口和导管的疲劳损伤和疲劳寿命影响很大。但是，目前国内外对耦合动力土反力影响井口和导管疲劳损伤的机理和规律研究不够深入，也缺乏针对井口多轴疲劳的相关理论研究。本项目拟建立考虑隔水管、防喷器组、水下井口、导管和浅部软土耦合作用下的水下井口和导管系统多轴疲劳损伤模型，重点研究考虑“循环衰退”效应下动力土反力作用的井口和导管多轴疲劳损伤理论和方法；同时针对不确定性条件下的井口和导管疲劳寿命预测模型和时变疲劳可靠度分析方法进行研究。以便揭示耦合土反力作用的水下井口和导管的多轴疲劳损伤机理、完善复杂动载作用下的水下井口和导管疲劳设计方法，为我国深水油气钻采作业安全提供一定的理论支撑和技术保障。

Subsea wellhead fatigue damage is not a critical issue in the process of shallow drilling, but it played a prominent part during deepwater drilling operations. The fatigue damage of wellhead and conductor of the 6th generation drilling platform is more than 10-20 times of the 3rd generation. The shallow seabed soft soil plays the role of supporting the casing string. As a result, the influence of soil reaction force on the fatigue life of the wellhead and conductor systems is very large. However, the study for the fatigue damage mechanism of the wellhead and conductor systems, especially for the multi-axial fatigue in deepwater operations are far from enough. Base on the study of wellhead stability and conductor bearing capacity under deepwater drilling operation, according to the basic theories of soil dynamic and multi-axial fatigue mechanic, it is necessary to establishes the models of subsea wellhead and conductor systems, and obtain a modified dynamic model which description shallow seabed soil reaction force based on conductor-soil model test and finite element analysis. Then propose a random multi-axial fatigue damage model for the wellhead considering drilling platforms, riser, BOP stack, wellhead, conductor and soil reaction’s coupling effect. Meanwhile, it is necessary to analyze the time-dependent reliability of deepwater wellhead fatigue under the uncertainty soil condition and other factors based on the probability theory. In order to reveal the multi-axial fatigue damage mechanism of subsea wellhead and conductor systems under coupling soil reaction, improve the fatigue design method of subsea wellhead. The results will provide theoretical basis and technical support for deepwater oil and gas drilling operations in China.