Abstract

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 (Uw), and the period of the incoming oscillatory flow (Tw). Reynolds number is 1 × 104 based on Uw and D. A seabed roughness ratio ks/D (ks 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–ω 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, ks/D, and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail.

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