Numerical Simulation Of The Flow Line Of SCR And Seafloor Interaction

With the increasing demand of global oil and gas, a new kind of steel tube structure form-steel catenary riser (SCR) is being widely used in a variety of deepwater development projects. Present domestic studies on the application of shallow riser are already very mature, but the research on deep-sea riser is still in the initial stage, and widely used pipelines in foreign deep-sea oil and gas development are catenary risers with bending stiffness and torsional rigidity.The riser of this structure, which is different from the top tension riser(TTR), is with remarkable geometrical nonlinearity and a large deformation problem.The marine environment of SCR is more complex, and the stiffness of the seabed soil has important influence on SCR fatigue life. The study found that most deep soil is soft clay, which has very strong nonlinear characteristics, and especially in large cycle load the plastic property and hysteretic behavior is very significant. In addition, under the influence of the floating body slow drift movement and extreme conditions, the flow line of SCR will be dragged out of the seabed and keep rising, and now the soil produces viscous force against pipeline rising motion, which seriously affect the ultimate stress of the pipe. Therefore the study on riser/seafloor interaction in the touch down zone has very important significance on the application of SCR.In this paper, steel catenary risers are for the study and finite element software ANSYS is for the numerical simulation of riser-seafloor interaction. Due to the structure characteristics and linear elastic material of the sag bend of SCR, the sag bend model is established based on the catenary equation in ANSYS; Flow line is tiled on the seabed surface. Through analyzing the soil model from the riser-seafloor vertical interaction of foreign test data, the skeleton curve and reload curve (soil suction and no soil suction) of seabed soil is approximately simulated by using soil equivalent stiffness. Finally, riser-seafloor interaction model is established according to the different soil equivalent stiffness, and the influence on riser-seafloor interaction of the slow drift motion and heave motion of the upper floating body is studied, especially on the buried depth and bending moment of the pipe in the touch down zone. The results show that the seabed pipeline has a large deformation under the action of slow drift as the initial penetration in the soil, and touch location and maximum buried depth position also change obviously; but the touch location changes little in heave motion. While reloading the soil and using equivalent stiffness to simulate the soil, the test suggests that the change of the touch location and buried depth containing soil suction is smaller than those containing no soil suction; soil suction has little influence on top counter force and bending moment. Bending moment has an extreme value under the action of heave motion or slow drift motion in three kinds of equivalent stiffness model of soil. Modal analysis shows that a different top constraint has little influence on modal frequency.