Temporal-spatial mode competition in slug-flow induced vibration of catenary flexible riser in both in plane and out of plane

Abstract

The slug-flow induced vibration (SIV) of a catenary flexible riser with an aspect ratio of 360 is experimentally investigated in this work. Four different flow rates for the internal gas-liquid slug flow are considered in the tests that carried out at a fixed gas-liquid ratio of 3.0 in a small-scale air-water test loop. The temporal-spatial mode competition is discussed in terms of the response amplitudes, frequencies, modal weights, transferred energy between the riser and flow, pressure fluctuations and slug flow characteristics. Experimental results indicate that the modal weights of higher modes increase with the flow rate although the in-plane response is dominated by the fundamental mode, resulting in the intense mode competition. The occurrence of mode transition first emerges at the response trough, presenting the higher second modal weight. The distinct switching between the first and second mode is observed for the pipe section between the response peak and trough, and such mode switching becomes more pronounced as the flow rate grows. The distribution and lengths of liquid slugs passing through the riser affect the mode switching in terms of the distribution and magnitude of fluid forces. The riser storages the energy extracted from the fluid when it approaches to the equilibrium position, while releases the energy as it departs the equilibrium position. The occurrence of out-of-plane response is found when the mixture velocity exceeds 2.0 m/s. The out-of-plane frequencies appear in the in-plane response, and vice versa, manifesting the interaction between the in-plane and out-of-plane responses.