Numerical modeling of flow and hydrodynamic forces around a piggyback pipeline near the seabed

Steady boundary layer flows around a two-cylinder configuration near a plane wall is investigated numerically. The Reynolds-averaged Navier-Stokes equations are solved using an upwind finite-element method with a k-ω turbulence model closure. The numerical model is validated against independent experimental data for flow past a single cylinder near a plane wall. The two cylinders investigated in the present study are structured in the so-called piggyback configuration, in which the small cylinder is placed directly above the large cylinder. The diameter ratio of the small cylinder (d) to the large cylinder (D) is set at a constant value of 0.2. Different values of the gap (e) between the large cylinder and the plane wall and the spacing (G) between the two cylinders are investigated. The effects of the gap ratio e/D and the spacing ratio G/D on the flow around and the hydrodynamic forces on the cylinders are investigated. Four vortex shedding modes are found around the two-cylinder system. It is found that the vortex shedding mode is dependent on the gap and spacing ratios defined in this study. The variations of the hydrodynamic forces with the gap and spacing ratios are also quantified.