Study On Nonlinear Cross-sectional Mechanical Response Of Unbonded Flexible Risers And Failure Characteristics Of Helical Layers

In the exploitation of marine oil and gas,unbonded flexible risers are relatively efficient transmission tools for oil and gas in deep water.With the growing demand of oil and gas and the development of ocean engineering technology,the scale of oil and gas exploitation in deep water is gradually expanding.The unbonded flexible risers are multi-layer composite structures with complex forms.The study on cross-sectional mechanical responses and typical failure modes of unbonded flexible risers is crucial to ensure the safe production and transportation of marine oil and gas.South China Sea is one of the four largest offshore oil and gas resource zones in the world.It has abundant oil and gas reserves and the prospect of exploitation is broad.Therefore,there is an urgent need to improve the technology about the exploitation of offshore oil and gas.Industrial and academia have not fully grasped the mechanical properties of unbonded flexible risers in deep water yet.To some extent,it is significant to carry out research on the nonlinear cross-sectional mechanical response and typical failure modes.In this paper,the non-linear cross-sectional responses of the unbonded flexible risers were studied from the material nonlinearity,geometric nonlinearity,hysteresis responses under cyclic loads and coupling loads.Based on the equivalent stiffness theory,the crushing study of carcass and pressure armor was carried out.Meanwhile,the numerical calculation was used to study the lateral buckling failure of helical strips in tensile armor layers.Firstly,the unbonded flexible risers and typical failure modes of different layers were described.The structures and functions of carcass,pressure armor,tensile armor layer and sheath were respectively described.The research status of cross-sectional mechanical responses,the crushing of the carcass and pressure armor,and the lateral buckling failure of tensile armor layer was summarized at home and abroad.Secondly,based on the summary of theoretical methods and numerical methods for the mechanical response of unbonded flexible risers,combining with the law of conservation of energy and the Ramberg-Osgood elastic-plastic model,the influence of material nonlinearity on the cross-sectional mechanical properties was studied.The influence of geometrical nonlinearity was also analyzed using a numerical model.The results showed that material nonlinearity of tensile armor had the greatest influence on the nonlinear responses of cross-sections.The friction between layers had an impact on the hysteresis responses of unbonded flexible risers under cyclic loads,such as tension,compression,torque and pressure.The mechanical response of unbonded flexible riser was predicted by theoretical methods and numerical methods.Meanwhile,the research on the typical failure modes of different layers in unbonded flexible risers is essential to the safety of operation.For the crushing study of carcass and pressure armor,based on summarizing the existing equivalent thickness methods,the equivalent stiffness method was theoretically modified and geometric coefficient was added to the existing formula.The linear relationship between the geometric coefficient and the reciprocal of the radius was derived by derivation.Based on the above derivation,a theoretical-numerical equivalent thickness method—modified equivalent stiffness method—was developed.The geometric coefficients of carcass and pressure armor were calculated by numerical calculation,which verified the theoretical derivation process.The above theoretical-numerical equivalent thickness method was verified by radial compression experiments and numerical calculations.Compared with other existing methods,the results of carcass and pressure armor showed that the equivalent thickness calculation method proposed in this paper greatly improved the calculation accuracy.This paper discussed the influences of superposition,lay angle and friction coefficients on the stability of the geometric coefficient.Similar to the carcass and pressure armor layer,the tensile armor layer is also heical layer and formed by a plurity of heical steel strips,which maily bear aixal loads.In the present paper,the numerical simulation of the lateral buckling of the tensile armor was carried out.The numerical model was simplified to improve the computational efficiency.The simplified model was verified by comparison with the existing experimental results.Combining with the theory of stability of helical strips,the lateral buckling process of the tensile armor under wet environment had been analyzed in depth.