Simple flexible riser

This example models a flexible riser in a water depth of 100 m, connected to a semi-submersible (SEMI) vessel at the hang-off point and fixed at the seabed anchor. The riser comprises three segments: a bare lower section, a mid-section with buoyancy elements, and a bare upper section. The buoyancy section increases the net buoyancy locally, giving the riser a lazy-wave or S-shaped configuration in its equilibrium position. This configuration decouples the seabed touch-down region from the vessel motions, reducing dynamic loading and fatigue at the lower end.

The riser is modelled using an axisymmetric cross section, which is the appropriate cross-section type for flexible pipes. Properties — mass, stiffness, and hydrodynamic coefficients — are specified directly rather than derived from pipe wall geometry.

The vessel motions are prescribed through a first-order motion transfer function (RAO) applied to the SEMI support vessel. All degrees of freedom at the hang-off are fixed to the vessel, so the riser top end follows the full six-degree-of-freedom vessel motion.

The model is set up for verification of dynamic response under a numerical wave spectrum. A single condition co_NumSpecCu selects the NumSpecCu wave and current environment for the analysis.

The static analysis follows a load sequence applying gravity and vessel displacement, followed by current. No seafloor contact model is included — the riser anchor is fixed at the seabed and the riser does not rest on the bottom.

The critical areas to assess in this configuration are the hang-off region at the top of the riser, the sag bend where the riser curves downward below the buoyancy section, the hog bend where the riser curves upward above the buoyancy section, and the anchor region at the seabed termination. These locations experience the highest curvatures and dynamic stresses, and are the primary focus for extreme load and fatigue assessment.