Hydrodynamic Forces on a Near-Bottom Pipeline Subject to Wave-Induced Boundary Layer

Hydrodynamic forces on small diameter subsea pipelines and cables placed near seabed are important for their on-bottom stability design. In offshore environments, these pipelines are usually subjected to extreme wave conditions. The present study investigates hydrodynamic forces acting on a pipeline near a flat seabed subjected to a wave-induced boundary layer flow. The Keulegan-Carpenter numbers of the wave-induced boundary layer flow are 20, 140, and 200, defined based on the pipeline diameter (D), the maximum velocity of the undisturbed near-bed orbital velocity (U-w), and the period of the incoming oscillatory flow (T-w). Reynolds number is 1 x 10(4) based on U-w and D. A seabed roughness ratio k(s)/D (k(s) is the Nikuradse equivalent sand roughness) of up to 0.1 and different gap ratios of G/D = 0.05-0.5 between the pipeline and the seabed are considered. Numerical simulations have been carried out based on two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes equations combined with the k-omega shear stress transport turbulence model. A preliminary one-dimensional (1D) simulation is carried out to obtain a fully developed wave-induced boundary layer velocity profile, which is used as inlet flow for the 2D simulations. The numerical model is validated against the experimental data reported by Sumer et al. [1991, "Effect of a Plane Boundary on Oscillatory Flow Around a Circular Cylinder," J. Fluid Mech., 225, pp. 271-300] at KC = 10. Influences of KC, k(s)/D, and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail.