A model test on the response characteristics of a free hanging riser

Abstract

A series of model tests were performed to investigate the response characteristics of a free hanging riser under forced oscillation conditions. The boundary conditions were considered to be fix-free (fix at top and free at bottom). The top end of the riser was fixed to the forced oscillator to give the effects of the horizontal vessel motion. The model test was performed at Ocean Engineering Basin of KRISO. In this study, the displacements along riser length were measured using the underwater camera system. The various oscillation conditions were considered to investigate the effects of period and amplitude. The oscillation period was related to the eigenvalues in the In-line(IL) response. The IL responses of the experiment were compared with the time simulation results of OrcaFlex. The responses of two results had a good agreement on time series, the statistical value, and snapshot. The Cross-flow(CF) responses along the riser length on the experiment were found to be determined by Keulegan-Carpenter number at each point of the riser. From the comparison of two responses, the responses in IL and CF were shown to be excited by different dominant frequencies from the top motion and vortex shedding, respectively. While one top-end oscillation frequency in IL appeared, multi-peak frequencies at all positions along the riser length in CF were investigated, which is induced by the structural propagation. ？ 2019 by the International Society of Offshore and Polar Engineers (ISOPE).