Experimental Studies on Vortex-Induced Vibration of a Piggyback Pipeline

Offshore pipelines of different diameters are often seen in piggyback arrangements in close proximity. Under the effects of external flows, the pipelines may experience vibration. Reliable prediction of the vibration amplitudes is important for the design and operation of these structures. In the present study, the effect of the position angle (alpha) and gap ratio (G/D) of a piggyback pipeline on the amplitude of 1DOF vortex-induced vibration (VIV) was investigated experimentally in a wind tunnel. The diameter ratio d/D of the two cylinders was 0.5. Five position angles, namely, alpha = 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees, and six gap ratios at each angle, G/D = 0, 0.1, 0.2, 0.3, 0.4, 0.5, were tested. It was found that both alpha and G/D affected the amplitude of vibrations significantly. For all gap ratios, the amplitude of vibrations increased from alpha = 0 degrees to alpha = 90 degrees and then decreased to a minimum value around alpha = 135 degrees. The maximum amplitude occurred around alpha = 90 degrees when G/D = 0, and the minimum occurred around alpha = 135 degrees, when G/D = 0.2-0.3. At other position angles, the vibration amplitude was less sensitive to G/D, especially when the latter was between 0.1 and 0.4. These results verified those obtained using numerical methods and are invaluable to engineers when designing offshore piggyback pipelines.