Fatigue Life Analysis Of Surface Cracks On Welded Joints Of Deepwater Risers

With the continuous reduction of oil and gas reserves in the land and offshore and increasing demand for petroleum resources,the exploitation of oil and gas has developed from the shallow sea to the deep sea,and gradually advanced to the ultra-deep sea.As a key component connecting the offshore platform and the subsea wellhead,the deep-water riser serves to isolate oil wells and seawater,transport oil and gas,and support various pipelines.In the deep sea,the riser is in a harsh environment and may be subjected to one or more loads,such as uniform flows,waves,and platform movements.The uniform flow is one of the most important loads.When the uniform flow through the riser,Vortex-induced vibrations are caused by alternating vortexes on both sides.At the same time,the combined action of the movement of the offshore platform and the flowing fluid in the riser will result in more complex coupling movements in the deep-water riser.These phenomena will shorten the fatigue life of the deep-water riser.In addition,deep water risers are mostly welded from high-strength steel.During the welding process,the surface cracks are easily generated at the welded joints,and will gradually expand under the action of vortex-induced vibration.In severe cases,the fracture of the deep-water riser may be invalid.At the same time,the riser welded joint will also produce a local welding residual stress field during the welding process,which will further affect the fatigue life of the riser during service.Most of the existing studies on vortex-induced vibration of deep-water riser only consider the influence of single factor,but rarely consider the effects of multiple factors such as internal and external flow and top tension.In addition,the fatigue life prediction of the riser is also lacking.Research on the fatigue extended life of welded joints.Therefore,this paper has carried out research on the above issues,the main work is as follows:(1)A vortex-induced vibration prediction model for deep-water risers was established.Based on the small deformation theory of beam element and Euler-Bernoulli equation,considering the factors such as internal and external fluid and top tension,the vortex-induced vibration model of deep-water riser is established and iteratively solved in the time domain.The numerical results of the model are compared with the experimental results of Song et al.and the numerical simulation results of Xu et al.,which proves the effectiveness of the vortex-induced vibration model in predicting the vortex-induced vibration of deep-water riser.(2)The fatigue crack propagation life prediction model of the welded joint of deep-water riser under vortex-induced vibration is established.Firstly,the rainflow counting method is used to simplify the dynamic response of the deep-water riser into several full-scale full-cycle stresses,which are then converted into the equivalent stress rms.On this basis,based on the fatigue crack propagation theory,the Newman-Raju formula is adopted.The Paris formula and the Forman formula establish a fatigue propagation life prediction model for the surface crack of welded joints in deep water risers.The numerical results of the model are compared with the experimental measurements of Park,Huang and Iranpour.It is shown that the welded joint of deep-water riser shows that the crack fatigue extended life model is effective for predicting the fatigue life under vortex induced vibration.(3)The influence mechanism of main factors on the fatigue life of surface cracks of welded joints in deep water risers is analyzed.Based on the prediction method of surface crack fatigue life of welded joints in deep water riser,the influence mechanism of key factors such as top tension of deep-water riser,flow fluid outside the pipe,flowing fluid in the pipe and welding residual stress on the fatigue life of welded joints was discussed.The results show that due to the influence of internal flow,the deep-water riser mainly has two extreme values of bending stress,which are close to the top end and the lower end respectively.With the increase of the fluid velocity in the pipe,the bending stress of the welded joint near the bottom end is greatly increased.As a result,the fatigue life of the welded joint near the bottom end is greatly reduced;the outflow flow rate has a greater influence on the welded joint than the inflow flow rate.With the small increase of the outflow,the fatigue life of the welded joint is greatly reduced;as the top tension increases,the bending stress decreases,and at the same time,the axial tensile stress increases correspondingly;the occurrence of the welding residual stress field,It causes a large tensile stress in the crack depth direction,accelerates the expansion of fatigue cracks at the welded joint,and reduces the fatigue life of the welded joint.In addition,as the crack expands,the residual tensile stress becomes residual compressive stress,which slows down.The expansion of the fatigue crack of the welded joint increases the fatigue life of the welded joint.