Numerical Study on the Hydrodynamic Force on Submarine Pipeline Considering the Influence of Local Scour Under Unidirectional Flow

The effect of local scour on the hydrodynamic force upon a submarine pipeline under unidirectional flow has been numerically investigated. The flow field around the pipeline is obtained using the Navier–Stokes equations with the SST k-ω turbulence model, and the sediment transport model, considering suspended load and bed load, is accounted for. Firstly, the influences of the Reynolds number (1 × 104 ≤ Re ≤ 1 × 105) and Shields number (1.2 ≤ θ/θcr ≤ 2.5) on the scour below the pipeline are analyzed; then, the effect of local scour on the hydrodynamic force upon the pipeline is examined by comparing with the condition that the pipeline is put on the flat seabed. It is found that the presence of local scour leads to a significant effect on the hydrodynamic force acting on the pipeline. Additionally, the Reynolds number affects the hydrodynamic force significantly, while the Shields number has a relatively low effect. The reduction coefficient (λ) is adopted to quantify the influence of the local scour around the pipeline on the hydrodynamic force. According to the reduction coefficient, the presence of local scour increases the drag coefficient by about 10% when the Reynolds number is 1 × 104, while it decreases the drag coefficient significantly when the Reynolds number is larger than 2 × 104, and the reduction coefficient trends towards a constant value with the increase in the Reynolds number.