The vortex shedding characteristics for submarine pipelines in a subcritical regime are quantitatively investigated. The swirling strength analysis is conducted on the wake flows around the pipelines. For a single wall-free pipeline, the peak value of the time-averaged nondimensional swirling strength (WD2/U2) is almost constant with Reynolds number (Re = 1.0×104 ~ 4.0×104). For a pipeline close to the bottom, the peak value of WD2/U2 decreases with the decrease of the gap ratio (e/D) when e/D < 0.3, owing to the suppression of vortex shedding. For piggyback pipelines near the bottom, the vortex shedding characteristics are dependent on both the gap ratio (e/D) and the spacing ratio (G/D). Four different wake patterns are found in swirling strength fields around the piggyback pipelines. The vortex-induced vibrations of the piggyback pipelines are further conducted and the relationship between the magnitude of swirling strength and the amplitude of vibration is established. It can be concluded that the nondimensional swirling strength is an important parameter in quantifying the vortex shedding and the suppression of vortex shedding.
Free spans often occur below the submarine pipelines due to the longterm actions of waves and currents or the unevenness of the seabed. Vortex shedding is formed and exerts a periodic transverse force on the spanning pipeline, leading to the vortex-induced vibration (VIV) of the structure. Therefore, the investigation on the mechanism and the suppression of vortex shedding from submarine pipelines has great significance on the design and operation of pipeline engineering. Sometimes the offshore pipelines of different diameters are laid on the seabed as a bundle for technical and economic reasons. The piggyback pipelines, which consist of a main (large) pipe and a piggyback (small), are the most popular configuration of the pipeline bundles. This would be beneficial to the stability of pipelines.
