Study On Steel Catenary Riser Numerical Model And Fatigue Life In Touchdown Zone

At present,steel catenary riser is an important form of riser in offshore oil and gas development.In the process of use,the place near the contact site of the riser is affected by the combined action of environmental load and complex excitation such as the heave movement of the upper floating body.And the fatigue failure is easy to occur.Therefore,the fatigue assessment of steel catenary riser is the focus of related engineering design.The scientific riser-soil interaction model will make the evaluation more accurate.Through the comparison of the two forms of models,this paper studies the static and dynamic response of the steel catenary riser and evaluates the influence of relevant parameters on the fatigue life of the contact site.In this paper,the seabed soil is regarded as a whole,and the overall static analysis of the elastic-plastic plane soil model is carried out,and the accuracy of its static equilibrium state is verified by catenary solution.Three characteristic points,maximum buried depth point,maximum stress point and suspension point,are found by equivalent stress,bending moment and groove depth.On this basis,it is found that the maximum equivalent stress of the whole pipe will increase with the increase of soil stiffness.The maximum groove depth will decrease with the increase of soil stiffness.And the rate will both become slower and slower.Through dynamic analysis,it is found that fatigue failure is most likely to occur near the maximum buried depth.In this paper,the seabed soil is also defined as a series of nonlinear springs,and the static analysis of the model is compared with the static analysis results of the elasticplastic plane soil model,which verifies the rationality of the P-y curve model.The establishment of nonlinear spring soil calculation method will be more in line with the real situation.The effects of groove depth and suction coefficient on the contact deformation,equivalent stress and bending moment of steel catenary riser are analyzed.Based on the dynamic analysis of the nonlinear spring soil model,it is found that the increase of the vertical motion amplitude of the floating platform,the groove depth and the suction coefficient will lead to the decrease of the fatigue life of the steel catenary riser.Through the calculation of an example,the reliability and applicability of the proposed method are verified.The relationship curves of the relevant parameters are different,among which the change of the vertical motion of the floating platform has the most obvious influence on the fatigue life.