环空带压对深水水下井口疲劳损伤的影响规律

海洋深水油气井测试过程中，高温产液上返时会加热周围套管及多层套管环空内的液体，引起液体在密闭井筒环空中膨胀，产生环空带压。环空带压的存在会改变水下井口疲劳热点处的应力状态，进而对水下井口疲劳损伤产生不利影响，制约了深水油气井长期安全高效运行。为了给深水油气井的长期安全运行提供更加科学的指导，考虑环空液体物性参数、井筒环空液体热膨胀和环空体积变化的耦合影响，建立了水下井筒环空带压计算模型，采用迭代法对环空带压进行了求解，将获得的环空带压施加到水下井口有限元模型上，然后以高压井口头与表层套管的焊缝为研究对象，研究了环空带压条件下水下井口疲劳热点处的应力状态；在此基础上，分析了环空带压、水泥浆返高和高压井口头出泥高度对水下井口疲劳损伤的影响规律。研究结果表明：①环空带压的存在会加剧水下井口的疲劳损伤，压力越高，疲劳损伤越严重；②表层套管外水泥浆返高与泥线的距离越大，水下井口的疲劳损伤越小；③高压井口头出泥高度越大，水下井口疲劳损伤越大。结论认为，有效地控制水下井口的环空带压与合理地设计井身结构，有助于减少水下井口的疲劳损伤。

Effect of annular pressure on the fatigue damage of deepwater subsea wellheads

In the testing process of offshore deepwater oil and gas wells, the produced high temperature (HP) fluid can heat the surrounding casing and the liquid in the annulus of multi-layer casing during its upward flowing, which will lead to liquid expansion in the sealed wellbore and induce sustained casing pressure. The existence of sustained casing pressure can change the stress state at the subsea wellhead fatigue hotspot which can bring adverse influence on the subsea wellhead fatigue damage and consequently restricts the long-term safe and efficient operation of deepwater oil and gas wells. In order to provide scientific guidance for the long-term safe operation of deepwater oil and gas wells, this paper established a model for calculating the sustained casing pressure of a subsea well by considering the physical parameters of annulus fluid and the coupling influence of thermal expansion of annulus liquid and annulus volume change. The sustained casing pressure was solved by using the iteration method, and the calculated sustained casing pressure was introduced into the finite element model of subsea wellhead. Then, the stress state at the fatigue hotspot of the subsea wellhead under the condition of sustained casing pressure was studied by taking the weld between surface casing and HP wellhead housing as the research object. Finally, the influence laws of sustained casing pressure, top of cement and HP wellhead housing height above the mud line on the subsea wellhead fatigue damage were investigated. And the following research results were obtained. First, the existence of sustained casing pressure can aggravate the subsea wellhead fatigue damage. The higher the sustained casing pressure is, the more serious the fatigue damage is. Second, the farther the top of cement outside the surface casing is from the mud line, the less the subsea wellhead fatigue damage is. Third, the subsea wellhead fatigue damage increases with the increase of the HP wellhead housing height above the mud line. In conclusion, the subsea wellhead fatigue damage can be mitigated by controlling the sustained casing pressure of the subsea wellhead effectively and designing the casing program reasonably.