Catenary riser

For a catenary riser, the lower section of the riser rests on or near the seabed in the touch-down zone (TDZ). Accurate modelling of the seabed interaction in this region is important, as the cyclic lift-off and lay-down of the riser during vessel motion drives fatigue damage at the touch-down point. The hang-off area at the top of the riser is equally critical, as it is subject to high dynamic loading from vessel motions. Both the TDZ and the hang-off area are key locations for assessment of extreme loads and fatigue.

In this example the catenary riser is connected to an FPSO at a water depth of 300 m. The riser runs from a fixed anchor point at the seabed to a hang-off point on the vessel, forming a free-hanging catenary shape. The total nominal riser length is 680 m, discretised into five segments using two thin-walled pipe cross-sections with different diameters. The riser carries an internal fluid with a density of 200 kg/m³.

The motion at hang-off is prescribed via a Response Amplitude Operator (RAO) applied to the FPSO support vessel. The vertical hang-off angle is fixed using a boundary change load step at the end of the static analysis sequence, which allows the riser to reach its correct catenary configuration before the angle constraint is imposed.

The static analysis follows a defined load sequence applying gravity, vessel displacement, current, seafloor friction, and finally the boundary change in sequence. Seafloor contact is modelled with a spring formulation, capturing normal, axial, and lateral stiffness and friction along the entire riser length.

A local axis system is set up at the hang-off point to make the riser-to-vessel angle easy to extract. It is built from a node fixed to the vessel at the same position as the hang-off node, with three 1 m lines running to slave supernodes offset one metre along the global x-, y- and z-axis. Being fixed to the vessel, it follows the same RAO as the FPSO, so the relative angles are read directly in the vessel’s moving frame of reference.

The model uses scriptable variables and conditions to support parametric studies. Three conditions are defined. The simple condition co_WaCu defines a single environmental load case with Hs = 5.0 m, Tp = 10.0 s, wave direction 90°, and current velocities of 0.2 m/s (surface) and 0.1 m/s (depth). The condition set cs_check_line_length sweeps the riser length variable dl over three values (0, 50 and 100 m) and the element count variable nel over (0, 10 and 20), keeping the hang-off angle and top tension unchanged while varying the anchor position. The condition co_rot_free_hangoff applies a model variation that removes the boundary change load step from the static sequence, leaving all rotations at the hang-off free — this is used to assess the effect of the rotational constraint on the riser response.