Calculation method of subsea wellhead fatigue damage considering thermal effect

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 in 50 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.