Numerical Analysis of Dynamics of Unbonded Flexible Riser Carrying Slug Flows

Unbonded flexible risers are widely used in deepwater oil and gas production. The cross-section of the flexible riser is composed of multi-layer structures, and the riser is under the effects of the deepwater environment. Accurate prediction of the mechanical behavior of flexible risers is important to ensure safe operation. The dynamics of the unbonded flexible riser involves the axisymmetric model, bending model and riser-seabed interaction model. In this paper, the contact force between each layer is obtained by solving the axis-symmetrical model, and the nonlinear bending model is established. The global dynamic model of the flexible riser is established, and the nonlinear spring model is used to simulate the riser-seabed interaction. The results show that before reaching the critical curvature, the bending moment increases linearly with the curvature, and the bending stiffness is mainly provided by the helical layer. When the curvature exceeds the critical curvature, the bending moment increases nonlinearly with the curvature, and the bending stiffness is mainly provided by the polymer layer. With the increase of tension and internal pressure, the contact pressure between each layer increases, and the critical curvature increases. In the global dynamic analysis, the maximum axial force is at the hang-off point. The bending moment in the touchdown zone (TDZ) reaches a maximum value, and this position changes under the platform motion. The slug flow induces an additional excitation force due to the change of mass.