Vortex-induced vibration of a piggyback pipeline half buried in the seabed

Experiments were conducted in a tow tank on the steady current-induced vibration of the smaller flexible cylinder (aspect ratio L/d = 61 and mass ratio m(0) = 1.38) in a piggyback configuration with the main pipe half-buried in the seabed. The effect of the gap-to-diameter ratio (G/d) based on the smaller cylinder diameter is studied for five different configurations from 1.0 to 4.0 and infinity (open flow) at different reduced velocities (Ur) from 2 to 10 based on the 1st modal natural frequency. An underwater optical measurement system was used in the current experiment to acquire both cross-flow (CF) and in-line (IL) vibration with a both spatially and temporally dense format. The results show that the existence of the half-buried larger cylinder will increase the mean drag coefficient for the smaller flexible cylinder and induce a positive mean lift coefficient pushing it away from the larger cylinder. For the smallest gap-to-diameter ratio of G/d = 1.0 when the interaction is the strongest, experiments show that the root mean square (RMS) of the CF vibration will increase due to the appearance of the stronger higher harmonic motion compared to the open flow case. The results of the inversely calculated vortex forces reveal that the 3rd harmonic fluid force in the CF direction is even stronger than the 1st harmonic fluid force for G/d = 1.0. In addition, it is found that there is a strong correlation between the positive lift coefficient in phase with velocity (C-lv) and the counter-clockwise trajectory between the CF and the IL vibrations of the smaller flexible cylinder, even with the existence of the large half-buried main pipeline. (C) 2019 Elsevier Ltd. All rights reserved.