For a steel catenary riser system, the touch-down point is often a concern as it can influence the structural response of the riser and its performance. Although, riser configurations can be designed to withstand wave loading, internal pressure, external pressure, and thermal loads; the design of risers taking into account the interaction between the riser and the soil, with cycles of contact and separation, still remains challenging. More specifically, when the riser contacts the seabed surface, it penetrates into the soil material. This penetration can cause suction-type behavior when the riser moves away from the surface. Further, it causes permanent deformation in the soil so the riser has to traverse additional distance prior to re-contacting the seabed surface. For formulating a robust riser design using the finite element method, accurate modeling of this nonlinear riser-soil interaction is needed.
In the majority of situations, risers are modeled with beam elements. This is an efficient approach considering that a riser-pipeline system can be several thousand meters in length and using shell or brick elements for modeling such a system becomes computationally expensive. In addition, the seabed surface is often modeled as a rigid surface. The riser-soil interaction behavior is then incorporated into the finite element analysis either by defining springs at the contacting nodes or by modifying the normal contact stiffness for the interaction. However, these methods do not entirely capture the nonlinear riser-soil interaction behavior.
In this paper, we discuss the modeling of the nonlinear riser-soil interaction through the use of a user subroutine that runs in conjunction with a finite element analysis using Abaqus. This subroutine is intended to provide a special contact interaction property definition between the riser and the seabed. This contact property definition focuses primarily on the normal behavior although frictional resistance is also considered. The results of the simulation and the insights gained are included.