Symmetric oscillatory flow induces asymmetric lift on a bottom-mounted horizontal cylinder

Peak hydrodynamic loads and on-bottom stability of subsea pipelines and cables laid on seabed are major design considerations in engineering practice. The analogous problem of a bottom-mounted cylinder subjected to wave-induced sinusoidal oscillatory flow has been investigated in different experimental and numerical studies, which coincidently reported an interesting and puzzling phenomenon where symmetric sinusoidal oscillatory flow is shown to induce asymmetric lift on the cylinder between the two halves of an oscillation period (T). The present study unveils the physical mechanism for this lift asymmetry through new analysis based on both flume experiment and numerical simulation. By decomposing the pressure into a reference pressure for which the cylinder is removed, and a dynamic pressure due to the presence of the cylinder body, we found that the degree of lift asymmetry (ΔCL) is proportional to both the difference in reference pressure between two time instants T/2 apart and the width of seal/contact commonly used to connect the cylinder and the plane boundary. Based on the physical understanding of the lift asymmetry, a method is proposed to predict ΔCL and to convert the time history of the lift coefficient (CL) obtained from an experimental/numerical study to that induced by actual subsea conditions, which enables quantification of the peak lift and on-bottom stability of the pipeline/cable.