Research On Cross-Sectional Mechanical Model And Fatigue Life Analysis Of Deepwater Flexible Risers

Unbonded Flexible risers are the lifeline of offshore oil and gas exploitation.With the development of offshore oil and gas exploration to deep water,complex and volatile environment in deep water brings more challenges to the application of the flexible risers.As a key part of the riser system,mechanical analysis and design of the flexible risers is particularly important.Based on summarizing the current situation of research and analyzing the development trend of the flexible risers,in this paper,the research works include:the mechanical properties of the flexible pipes under the axisymmetric loads and bending load;the effect of ovalization on the bending properties of the flexible pipes;the fatigue life of the flexible pipes and a set of methods for fatigue analysis of flexible pipes is established.The major research contents are summarized as follows:1)Study on the cross-sectional mechanical model of the flexible pipes under the axisymmetric loadsBased on the structural characteristics of the flexible pipes,the helical element and cylindrical element are taken as the main research object.The relationship between the displacement and the strain of the helical element under axisymmetric loads is established,and the influence of the radial strain,curvature and torsion of the helical element on the structural stiffness of the helical layer is further introduced.The contact constrained conditions between the flexible pipe layers are introduced into the whole stiffness matrix of the flexible pipes by using the penalty function method,then a theoretical model has been developed for analyzing the structural response of the flexible pipes under the axisymmetric loads.The experimental results are compared with the results from the theoretical model,which validates the effectiveness of the proposed theoretical model.2)Numerical modelling on structural response of the flexible pipes under the axisymmetric loadsA reasonable numerical model of the flexible pipes under axisymmetric loads is established with the framework of the computer code ABAQUS.The tensile armors are simulated by beam elements.The contact between the flexible pipes layers is fully considered.The structural response is calculated under the axisymmetric loads.The calculated results are compared with the theoretical model,which validates the effectiveness of the proposed numerical model.Then the proposed theoretical model is further adopted to analyze the structural response of the flexible pipes under the axisymmetric loads,and the influencing factors are obtained on axial and radial stiffness of the flexible pipes.3)Theoretical modelling on structural response of the flexible pipes under the bending loadBased on the bending theory of Kebadze,the critical curvature,axial force and bending stiffness of the helical strip are deduced according to the axial equilibrium relationship of the helical strip under the bending deformation.A numerical model of the helical strip with the framework of the computer code BFLEX is developed.The structural response is calculated under the bending load.The influencing mechanisms of the axial and transverse slip on bending properties of the flexible pipes are revealed.The calculated results are compared with the theoretical model,which validates the effectiveness of the proposed theoretical model.Then the proposed theoretical model is further adopted to analyze the influences of external pressure,fricition coefficient and helical angle on the bending properties of the flexible pipes.4)Influence of cross-sectional ovalization on the bending properties of the flexible pipesBased on the theoretical model of the flexible pipes under bending,a new theoretical model considering the ovalization is presented for the flexible pipes under the bending deformation.The axial force,critical curvature and bending stiffness of the helical strip have been derived in the new model.The calculated results are compared with the bending theoretical model,which validates the effectiveness of the new proposed model.Then this model is further adopted to study the effect of cross-sectional ovalization on the bending properties of the flexible pipes under bending deformation.5)Fatigue life analysis of the tensile armor layer of the flexible pipesA comprehensive numerical model of the flexible pipes for the global analysis with the framework of the computer code OrcaFlex is developed to investigate the dynamic response of the flexible pipes in time domain,including stochastic waves,ocean currents,platform motions and the bending stiffener between the flexible pipe and platform.The obtained time-history response curves from the global model are applied to the local model proposed in this paper.Based on the stress time-history curves of the tensile armor layers,S-N curve and Miner's rule,the fatigue life of the tensile armor layers is calculated,the most possible region causing the stress fatigue failure is observed,and the influence of ovalization,mean stress and friction coefficient on the fatigue life of the flexible pipes is studied as well.