Current-induced scour beneath initially elevated subsea pipelines.

Highlights • New equation for predicting the maximum seabed shear stress beneath the pipe. • The likelihood of scour occurring increases with current velocity but decreases with pipe diameter. • New equation for predicting the equilibrium scour depth beneath the pipe. • Including the upstream seabed shear stress improved the correlation to experimental data. • The influence of the Reynolds number is small. Abstract When a subsea pipeline is laid on an uneven seabed, certain sections may have an initial elevation with respect to the far-field seabed, e o , and thus potentially affecting the on-bottom stability of the pipeline. This paper focuses on quantifying the effects of the upstream dimensionless seabed shear stress, θ ∞ , and Reynolds number, Re , on: (1) the maximum dimensionless seabed shear stress beneath the pipe, θ max , to be compared to the critical shear stress in order to determine whether scour would occur and progress towards an equilibrium state; and, (2) the dimensionless equilibrium scour depth beneath the pipe, S eq / D. Using a 2-D Reynolds averaged Navier-Stokes (RANS) approach along with the k-ω Shear Stress Transport (SST) turbulence model, a parametric study involving 243 computational fluid dynamics (CFD) simulations was conducted. The simulation results were used to develop a closed-form equation for the prediction of θ max. Subsequently, experimental measurements of S eq / D have been compiled from published literature, to develop a new closed-form equation for the prediction of S eq / D with a high correlation to the experimental data. In summary, we present two closed-form equations for the prediction of θ max and S eq / D for pipelines with an initial e o / D , which are applicable for both clear-water and live-bed conditions. The effects of θ ∞ and Re have been included, albeit Re having a small influence as compared to the other parameters. [ABSTRACT FROM AUTHOR]