Dynamic response of a top-tensioned riser under vessel motion

Model tests of a top-tensioned riser model were carried out as a part of a joint industry project, with the purpose of verifying the calculations of the riser analysis program RIFLEX. Sinusoidal motion in one direction was imposed at the top end of the riser model to simulate vessel motion. The tests were carried out in still water. Bending strain and acceleration were measured in both in-line (IL) and cross-flow (CF) directions along the riser model, so that the global response could be obtained through post-processing of the measured signals. Numerical simulations were performed and the results were compared with results from the model tests. This paper discusses interesting aspects of this comparison as well as the general dynamic behaviour of the top tensioned riser.
It was found that the dynamic responses of a top tensioned riser with vessel motion can consist of not only the in-line responses due to vessel motion at the riser top end, but also cross-flow vortex-induced vibrations (VIV) under conditions when Keulegan–Carpenter number is relatively small. Cross-flow VIV response is estimated using the VIVANA software and compared to the measured response. The main conclusion is however that the riser analysis program RIFLEX can predict the global dynamic responses sufficiently well.