Riser clashing

When two risers are suspended close together from the same floating vessel, differences in their properties combined with the vessel’s motions can drive them toward one another and toward adjacent structures such as pontoons or columns. Contact between risers, or between a riser and the hull, can cause coating damage, local wear and fatigue, so the clearance between neighbouring lines is an important design check. This example is built to study such clashing and to evaluate the collision risk in the post processor.

In this example two flexible risers are suspended from a vessel at a water depth of 100 m. Each riser runs from a fixed anchor point at the seabed to a hang-off point on the vessel, forming a free-hanging shape with a steep wave profile. The total nominal riser length is 160 m, discretised into three segments: a short bare section at the anchor, a mid buoyancy section, and a long bare section up to the hang-off. Each riser carries an internal fluid. The two risers are geometrically identical but are assigned different drag coefficients — so that they respond differently in the same environment and move toward each other during the simulation.

A short horizontal line fixed to the vessel represents a pontoon, acting as the adjacent structure the risers can clash with. The risers hang off the vessel a small distance apart, and the pontoon is positioned nearby so that all three lines come into close proximity as the vessel moves.

The motion at hang-off is prescribed via a Response Amplitude Operator (RAO) applied to the support vessel, giving the wave-frequency response of both riser hang-offs and both pontoon ends. In addition, the slow drift of the vessel is introduced by reading the low-frequency vessel motion from a file, so both wave-frequency and low-frequency contributions to the relative riser motion are captured.

The static analysis follows a defined load sequence, applying gravity and geometry, vessel displacement, current, and finally an initial-stress step in sequence to establish the equilibrium configuration before the dynamic simulation. The dynamic analysis is an irregular time-domain simulation of 150 s driven by the wave environment and the low-frequency motion file, using a Newmark-β integration scheme with a start-up ramp and updated proportional damping. Displacements are stored for all nodes of both risers and the pontoon so that the distances between the lines can be reconstructed afterwards.

The collision risk is assessed from the plots in the post processor, which report the distance between the lines. Note that the distance is calculated between line centres, so the contact radius of each line must be taken into account when evaluating the results.