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 fixed/free (fixed at the top and free at the bottom). The top end of the riser was fixed to the forced oscillator to give the effects of horizontal vessel motion. The model test was performed at the Ocean Engineering Basin of the Korea Research Institute of Ships and Ocean Engineering (KRISO). In this study, the displacements along the riser length were measured using underwater camera systems. Various oscillation conditions were considered in order to investigate the effects of period and amplitude. The oscillation period was determined considering 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, statistical value, and snapshot. The cross-flow (CF) responses along the riser length in the experiment were determined by the KeuleganCarpenter number at each point of the riser. A comparison result of the IL and CF responses shows that they were excited by different dominant frequencies from the top-end motion and vortex shedding, respectively. While one top-end oscillation frequency appeared in the IL, multi-peak frequencies were investigated at all positions along the riser length in the CF, which is induced by traveling waves.