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

Abstract

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 θ=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=ρs/ρf where ρs and ρf 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 θ and geometry configurations on the scour depth and the sediment profile are obtained and discussed.