Numerical investigation of scale effects in modelling scour below offshore pipelines under steady currents

Scale effects must be considered when results of small-scale laboratory tests on scour below offshore pipelines are extrapolated to real-life situations. In the present study, a 2D Finite Element Method (FEM) numerical model for scour simulation was employed to evaluate scale effects in modelling scour below a single pipeline subjected to steady currents. In the model, the Reynolds-Averaged Navier-Stokes (RANS) equations with a k-? turbulence closure are supplemented by a morphological model with both suspended load and bedload sediment transport rates included. Scour simulations were conducted at a number of scales with various Shields parameter values and sediments of two median grain sizes. The numerical results indicate that for the range of parameters investigated in this study, the normalized equilibrium scour depth below the pipe centre decreases in a linear fashion as the pipe diameter increases. Accordingly, an expression delineating scale-effect trends that relate deviations in normalized equilibrium scour depths to those in pipeline diameters is proposed, thereby contributing to an accurate extrapolation of small-scale model test results to prototype scale in modelling scour below pipelines in currents.