Study On Dynamic Response Of Steel Catenary Riser Considering Pipe-soil Contact Effect

With the gradual development of deep sea oil and gas resource,the application field of marine risers also move from shallow water to deep water.The steel catenary riser(SCR)systems are widely used in deep-water and ultra-deep-water oil and gas exploitation because of its unique advantages,such as low cost and strong adaptability to working environment.In recent years,it has become the preferred riser systems for deep-sea offshore platforms.The working condition and stress of SCR are very complicated.There will be a continuous interaction between the pipeline and seabed under the combined action of hydrodynamic loads and floating body movements.This continuous contact effect can cause the liquefaction and plastic deformation of seabed soil.The trench will be formed under the pipe-soil cyclic interaction.Besides,with the increase of water depth,the influence of the sea current becomes larger.The riser's drag force will increase and the vortex induced vibration(VIV)will also happen.Both pipe-soil interaction and VIV can cause severe fatigue damage to the riser,which will lead to the adverse effects on the safety and reliability of the whole offshore platform.A detailed discussion on this issue was performed in this paper.The numerical analysis method was used to simulate the SCR overall dynamic response and explore the pipe-soil contact effect as well as the law of VIV.The sensitivity analyses on the main factors which affected the fatigue life were carried out and the fatigue life in hotspot area was calculated precisely.The main content of this paper can be listed in the following aspects.The complex contact between offshore pipeline and seabed created by laying,installing and operation processes will generate the external load of pipeline,which lead to the pipeline strength problem.The nonlinear finite element software ABAQUS was used to carry out the numerical simulation of the interaction between pipeline and seabed.Based on the ALE method as well as the modified hard contact method,the main parameters of pipeline penetration and rising process were studied.The results of numerical simulation were in good agreement with the results of NGI large scale tests,which showed a well simulation of generation and release process of both soil resistance and soil suction.The results verified the feasibility of numerical method.The results of parameter analysis showed that the soil friction angle and cohesion had a significant impact on the pipe-soil interaction.The overall dynamic response model including offshore platform,SCR and seabed was established,which based on the nonlinear hyperbolic soil stiffness hypothesis proposed by Randolph and Quiggin.The finite element model was used to discuss the effect of vessel offsets,soil shear strength,soil suction and nonlinear seabed parameters.The most important thing in this research was to check the trench shape change and investigate its effect on pipe-soil interaction.On this basis,the fatigue life prediction of the SCR was carried out and the influence of main parameters was analyzed.The Iwan-Blevins wake oscillator model was used to simulate the VIV phenomenon under the different flow field.The effect on VIV induced pipesoil interaction was studied and the riser's fatigue life was calculated.The results showed that the ocean flow profile had a significant impact on VIV performance as well as its induced pipe-soil interaction.In conclusion,this paper discussed the overall dynamic response and analyzed the fatigue damage of deep-sea pipeline,which provided references for the further study on pipe-soil cyclic interaction and deep-sea riser's VIV phenomenon.What is more,the study offered a theoretical basis to the offshore platform fatigue life and safety prediction in practical engineering.