Study On Fatigue Life Prediction Of Subsea Wellhead And Conductor In Deepwater Drilling

With the development of deepwater and ultra deepwater in China,the safety of deepwater oil and gas drilling and production engineering has received more attention.As an important part of deepwater oil and gas exploitation system,subsea wellheads and conduits bear marine environment load and production load for a long time,which are prone to fatigue failure and cause serious accidents.Aiming at the stress and fatigue problems of subsea wellhead and conduit system in deep water,the interaction test of string and seabed soft soil is carried out.Combined with theoretical model and finite element analysis,the mechanical properties of subsea wellhead and conduit under different working conditions are studied,and the fatigue damage and life prediction are of great significance.Through the analysis of the composition and stress of the subsea wellhead and the conduit,and on the basis of soil dynamics,the dynamic model of the interaction between the subsea wellhead and the casing string below and the front bottom was established considering the effect of the lateral dynamic load on the wellhead and the string.The analysis method of the horizontal load pile and several common p-y curve methods were introduced.According to the similarity theory and stiffness equivalent scheme,the offshore casingsoft soil interaction platform was established,and the interaction test between offshore shallow soil layer and casing column was carried out.The effects of transverse static load amplitude,transverse dynamic load amplitude,action period,cycle number and vertical load on the mechanical properties of casing column such as bending moment and transverse displacement were analyzed.The curve fitting of bending moment was carried out by using six polynomial interpolations,and the distribution of soil resistance around the model pipe column was obtained.Combined with the finite element analysis of the column model,the feasibility of the physical test scheme was verified.The mechanical properties of underwater wellhead and conduit system were studied,and the finite element analysis model of underwater wellhead and conduit system was established.According to the four working conditions of drilling,completion,production and workover,the stress distribution and lateral displacement of underwater wellhead under each working condition were analyzed,and the influence law of underwater wellhead and conduit strength was studied from the aspects of soil properties,wellhead top load and conduit mud discharge height.Based on the strength analysis results of subsea wellhead and conduit system,the annual fatigue damage of subsea wellhead and conduit system under various working conditions was evaluated according to the fatigue accumulation theory of subsea wellhead,combined with the wave-current fatigue load and the stress-life curve of wellhead material.According to the time-length ratio of each working condition,the safe production life of subsea wellhead and conduit system is predicted.Finally,the effects of different factors on catheter fatigue damage are analyzed,and corresponding technical measures are proposed.