Abstract
Local scour around a submarine piggyback pipeline in combined waves and current is investigated experimentally. Based on the experimental results, the scour evolution and scour morphology are firstly analyzed. Then, a comparison with the equilibrium scour depth Seq between the present experimental data and predicted results is conducted. After that, the correlation between the dimensionless scour timescale T* and the maximum Shields parameter θcw is investigated, and a formula is obtained to describe the variation trend between T* and θcw for different gap ratios G/D. Furthermore, the parametric study is carried out to study the effects of Reynolds number Red and θcw on Seq, respectively. The results indicate that the Seq below the piggyback pipeline increases when the gap ratio G/D increases from 0 to 0.1, and it gradually decreases when G/D > 0.1. For a given KC, the Seq increases with the increase of the ratio of velocities Ucw. In addition, when Ucw is fixed, a higher KC results in a greater Seq. The T* is closely related to θcw and G/D. The higher Red and θcw both tend to result in the greater scour depth below a piggyback pipeline in combined waves and current.
Highlights
Zhao and Cheng [22] established numerical models to simulate the scour process around a piggyback pipeline in a steady current, and the results indicated that the gap ratio G/D
For G/D = 0, the small pipeline acts like a rigid spoiler attached on the top of the large pipeline, which significantly increases the block area and turbulent velocity around the pipeline and causes more soil particles to be mobilized, and leading to more serious scour
For G/D = 0, the small pipeline acts like a rigid spoiler attached on the top of the large pipeline, which significantly increases the block area and turbulent for the piggyback pipeline, especially for a small gap ratio, such as G/D = 0 and 0.1
Summary
Submarine pipelines are vital for transporting oil, natural gas, wastewater, and hydrocarbons in ocean engineering. Gao [24] conducted an investigation experimentally on local scour around a piggyback pipeline considering the vortex-induced vibration in a steady current Through this test, the effects of gap ratio, pipeline diameter, and installation angle on scour depth were analyzed. Zhao et al [25] adopted the laboratory experiments and numerical models to study the effects of gap ratio, inflow Reynolds number, and pipe diameter on the scour depth in a steady current. Yang et al [27] examined and discussed the effects of a small pipeline on scour topography in the vicinity of a piggyback pipeline under a steady current by numerical simulation, indicating that the small pipeline installed at the upstream edge of the large pipeline leads to more serious scour. In the present study, a series of laboratory experiments are carried out to investigate the scour morphology, scour depth, and scour timescale around a piggyback pipeline in combined waves and current. The local scour below the piggyback pipeline is remarked upon and discussed comprehensively
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