CFD investigation on scour beneath different configurations of piggyback pipelines under steady current flow

Scour beneath piggyback pipelines laid on a sandy seabed is numerically investigated by solving two-dimensional (2D) Unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The fluid flow and the sediment transport are resolved using a two-phase flow Eulerian-Eulerian numerical solver SedFoam based on the open-source code OpenFOAM. A parametric study is performed for different Shields parameters defined as theta = Uf2/(s-1)gd50 of 0.068, 0.18 and 0.33 where Uf denotes the shear friction velocity on the seabed, the specific density of the sediment grains is s = rho s/rho f where rho s and rho f are the density of the solid and fluid phases, respectively, g is the gravitational acceleration and d50 is the median grain diameter. Scour processes beneath piggyback pipelines under steady current flow condition are studied for different gap ratios of G/D = 0, 0.15, 0.25 and 0.35 between the main cylinder and the additional ones with a diameter ratio of d/D = 0.3 between them. Mesh convergence studies based on the scour depth and the sediment profile are performed to obtain an appropriate grid resolution. A validation study is performed by comparing the scour depth of the single cylinder cases with the published experimental results obtained by Mao (1986). The effects of different theta and geometry configurations on the scour depth and the sediment profile are obtained and discussed.