In traditional offshore pipeline design, the on-bottom stability of submarine pipelines is governed by the Morrison’s equations. According to this set of equations, offshore pipelines are designed to satisfy two stability conditions: the submerged weight of the pipe has to be greater than the lift force, and the horizontal frictional force should exceed the combined drag and inertia forces.
It is common practice to used fixed hydrodynamic coefficients (drag, lift and inertia) to calculate the pipe stability, based on the assumption that the pipeline is either trenched or in contact with the seabed. However, due to uneven seabed topology and/or scouring, a gap may exist between the pipe and the seafloor. In such a case, the force coefficients not only depend on the relative gap between the pipe and the seabed. Moreover, in unsteady oscillatory flow (induced by waves), the time-dependent laminar or turbulent characteristics of the boundary layer become important.
In this paper, a computational fluid dynamics (CFD) model is presented to study on-bottom stability of offshore pipelines in close proximity to the seabed. Numerical simulations of fluid flow are performed to evaluate the lift, drag and inertia forces exerted on a subsea pipeline when subjected to both steady current (tides) and oscillatory flow (waves). The evolution of the hydrodynamic coefficients with the relative gap between the pipe and the seafloor is investigated, and the effect of boundary proximity on the stability is revealed.