Nonlinear Finite Element Analysis By ANSYS For Steel Catenary Risers

In recent years, with the increasing development of deepwater oil and gas activities, the complicated deepwater environment poses greater challenges for the analysis and design of riser system which connects the submarine pipeline and platform. The research on theory of the shallow-water risers has been relatively mature by now, but it is still at an early stage for deep-sea risers.In this thesis, numerical simulation of the steel catenary riser considered the nonlinearity of geometry for hanging section and contact with soil by the general purpose finite element analysis software ANSYS. For geometric nonlinearity of the hanging section, the riser initial configuration was obtained by iterative calculation and good agreement was observed with the results of theoretical catenary equations. Based on this initial static configuration, the riser response analyses subjected to both the current and wave load were conducted. For nonlinearity of contact with soil, contact element was employed to simulate the interaction between riser and soil. Finite element model of the riser/soil system was set up in consideration of the nonlinear feature of the soil. The comparison of the present results and the existing results was performed and they were in good agreement. The effect of the pipe weight, soil model and friction coefficient on the influence of the riser bending stresses and penetration were also analyzed.Based on the study of nonlinearity of steel catenary riser and incorporating the DP soil constitutive model into the SCR/soil overall finite element model, this thesis carried out riser static and dynamic analyses under both current and wave actions. The results show as follows:under the sole action of buoyancy and gravity, the greater the elastic modulus of the soil, the smaller the deformation of the flowline, but the change in equivalent stress and bending moment is small; soil cohesion has little impact on flowline deformation, bending moment and equivalent stress. Consider the loads of wave and current, there's maximum bending stress at the touch down point while little change in bending stress is seen close to the water surface. Current velocity unchanged, increasing wave height and wave period, amplitude of the displacement near the surface increases with the same response period as wave; but it has relatively small effects on displacements at TDP and flowline though similar response period is observed. Wave height and period being constant, the bending stress and average displacement at TDP both increase with the increase of current velocity. Increasing both current velocity and wave height and period, it is seen that riser average displacement, amplitude of the displacement and bending stress all increase except for a decrease of the average displacement at TDP.