Effect of oscillatory boundary layer on hydrodynamic forces on pipelines

Numerical simulations were carried out to investigate hydrodynamic forces on submarine pipelines in oscillatory flows, with a focus on the conditions under which the pipeline diameter D is of a similar order of magnitude to the boundary-layer thickness δ, i.e., δ/D ∼ O(1). Two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with shear stress transport (SST) k-ω turbulence closure were solved using a Petrov-Galerkin finite element method (PG-FEM). The effects of the seabed roughness ks/D and the Keulegan-Carpenter number KC = UmT/D on the hydrodynamic force coefficients were investigated, where ks is the Nikuradse's equivalent roughness, T is the period of oscillatory flow and Um is the amplitude of the oscillatory velocity. The diameter of the submarine pipeline is fixed at D = 0.1 m. The Reynolds number, defined as Re = UmD/υ (where ν is the kinetic fluid viscosity), ranges from 1 × 104 to 4.5 × 104. The numerical results show that the boundary-layer thickness increases with ks. Hydrodynamic force coefficients are significantly affected by δ/D in the range of δ/D ∼ O(1), while δ/D depends on ks/D and KC number. The negligence of velocity reductions in the wave boundary layer leads to overestimations of the submerged weight required for achieving on-bottom stability.