Nonlinear dynamic response analysis of marine risers under non-uniform combined unsteady flows

Abstract

Considering the top-end platform surge induced by periodic wave, the encountered flow field of marine risers under the actual marine environment is approximate to a combined sheared and sheared-oscillatory flow, which is non-uniform in both time and spatial domains. A published time domain force-decomposition model is proposed to predict the riser's nonlinear dynamic response under non-uniform combined unsteady flows. A 90 m flexible riser is chosen as the simulated object, and a large amount of the combined flow cases with different oscillation periods and combined ratios are designed. With the fundamental study on a steady sheared flow case, the structural dynamic response characteristics under non-uniform combined flows are investigated systematically. The overall dynamic response presents travel wave pattern, and the high-frequency response is easier to be excited for the riser's top and bottom segments. Some time-sharing response characteristics like amplitude modulation, frequency transition and mode jumps are captured, and they are affected by the flow field parameters significantly. Under the long oscillation period cases, the intermittent dynamic response follows the accordant periodicity with the current velocity. While under the short oscillation period cases, the unstable structural vibration occurs persistently without clear regularity. The combined ratio also influences the RMS displacement, excited frequency bandwidth and travel wave propagation velocity evidently. Associated with VIV mechanism analysis, the individual response characteristics under different flow field parameter combinations are explained reasonably.