Experimental Study on Interference Effect of Three Risers Arranged in Tandem with Variable Spacing Ratios

Taking the three-riser group arranged in tandem as the research subject, an experimental study was carried out on the risers arranged in tandem. The purpose is to explore the sensitivity of the dynamic response of each riser to spacing ratio and reveal the physical mechanism of riser groups under the interference effect. The spacing ratios of the adjacent risers are 4.0, 5.0, 6.0, and 8.0. At the spacing between the risers of 4.0 D , the strong feedback effect increases the cross-flow (CF) displacement amplitude of the upstream riser. The shielding effect is the key factor affecting the interference effect on the midstream and downstream risers. At low reduced velocities, the shielding area initially appears, the displacement amplitude of the midstream and downstream risers varies greatly, the vibration of the two risers is still dominated by the first-order mode, and the transition between adjacent vibration modes is restrained. The multi-frequency superposition phenomenon is very significant at high reduced velocities. The most sensitive interference spacing under the test conditions is obtained. Due to the separation of the incoming flow and the double shielding effect of the upstream and midstream risers, the regular vortex-induced vibration in the wake area of the downstream riser is broken, and the vibration in the two directions is weakened. In general, the interference effect is more significant for the CF vibration of the three-riser groups than the in-line (IL) vibration.