考虑温度影响的水下井口疲劳损伤计算方法

Calculation method of subsea wellhead fatigue damage considering thermal effect

水下井口系统与隔水管系统共同构成连接平台与地下油藏的通道。钻井、完井、修井作业过程中泥浆循环导致井筒温度发生变化,并且平台运动、波浪等产生的循环动载荷通过隔水管作用于水下井口,造成水下井口疲劳损伤。首先建立深水井筒温度场计算模型,将温度场计算得到的井筒温度分布施加于水下井口精细有限元模型,采用局部等效方法得到水下井口的等效梁模型,把等效梁模型作为子模型代入隔水管-井口耦合模型中进行水下井口动态响应和疲劳损伤计算,研究泥浆循环作业过程中井筒温度对水下井口疲劳热点处疲劳损伤的影响,并研究不同水泥环返高对水下井口疲劳损伤的影响。结果表明,对于水泥环返高无缺陷的水下井口,50h内不考虑温度影响时最大疲劳损伤发生在导管接头处,其值为1.00×10^(-2);考虑温度影响时最大疲劳损伤发生在套管接头处,其值为3.59×10^(-2)。当水泥环返高缺陷分别为-2 m和-5 m时,与水泥环返高无缺陷相比,套管(接头和焊缝)的疲劳损伤减小,而导管(接头和焊缝)的疲劳损伤增加,最大损伤均发生在套管接头处,其值分别为3.55×10^(-2)和3.48×10^(-2)。

The subsea wellhead and drilling riser system form a channel connecting the floating platform and underground reservoir resources.Mud circulation during the drilling,completion and workover operations results in the variation of wellbore temperature.The subsea wellhead is subjected to circulation dynamic loading caused by platform movement and waves and currents loads through the risers,leading to fatigue damage in it.Firstly,the calculation model of deep-water wellbore temperature field is established to obtain the wellbore temperature which is applied to the fine finite element model of the subsea wellhead.Secondly,the equivalent beam model of subsea wellhead is achieved by using local equivalent method,and is then substituted into the riser-wellhead coupling model for dynamic response analysis and fatigue damage calculation as a sub-model.Finally,the influence of wellbore temperature on the fatigue damage of subsea wellhead at fatigue hotspot during mud circulation operation process is researched,and also the influence of different cement level on fatigue damage of subsea wellhead is investigated.The results show that,for the subsea wellhead without cement defect,the maximal fatigue damage will be 1.00×10-2 in50 h when the temperature effect is not taken into account,which occurs at the conductor connector of the subsea wellhead.While the maximal fatigue damage will be 3.59×10-2 in the same time duration,which occurs at casing connector when temperature effect is considered.When the defect of cement top is-2 m and-5 m,respectively,compared with cement defect,the fatigue damage at the casing(connector and weld)will decrease,while that of the conductor(connector and weld)will increase.The maximal damage of the subsea wellhead will be 3.55×10-2 and 3.48×10-2,which both occur at the casing connector.