Three-dimensional study on touch down zone of steel catenary riser under cyclic loading in saturated seabed

Abstract

The fatigue mechanics of steel catenary risers is significantly influenced by the cyclic riser-soil coupling dynamic interaction in the touchdown zone. In this study, the penetration resistance and its corresponding soil deformation of a shallowly embedded riser, which withstands a long-term cyclic loading, are simulated using a finite element method. During the numerical analysis in this paper, a dynamic boundary surface model of saturated soil is used instead of the empirical strength degradation curve method, and a riser-soil contact model is considered instead of no-slip tie assumption. A sufficiently large number of riser motion of vertical displacement is simulated using the soil dynamic constitutive model and the riser-soil contact model proposed in this paper. On the basis of the numerical results, an empirical degradation coefficient for the riser penetration resistance is proposed in this paper, which can well evaluate the effect of cyclic number on the resistance. Along the axis direction of the riser in the touchdown zone, an intensive study about the deformation law and failure mode of the seabed soil is analyzed by comparing the equivalent plastic strain at different sections. Finally, some parameters which may affect the riser penetration resistance and soil deformation law of the seabed are discussed in detail. It can be found that varing with the loading cyclic number, the riser penetration resistance is reduced gradually, and the soil deformation surrounding the riser in the touchdown zone presents obvious three-dimensional characteristics, too.