Subsea pipelines placed at or near the seabed are exposed to waves and underwater currents. The unstable seabed, irregular terrain, erosion of sediments or installation procedures may lead to free span in sections along the length of pipelines. This problem can be explained in simplified terms as a cylinder near an impermeable wall subjected to boundary layer based shear flow. It is common practice to use fixed cylinder hydrodynamic coefficients (drag, lift, and interia) to calculate the pipeline stability with the assumption that the pipeline is either trenched or lying on the seabed. However, due to the gap between the pipe and the seabed, the force coefficients have a strong dependence on the relative gap.

In this paper, a moving wall is considered rather than a fixed wall to avoid the confusing interaction of wall boundary layer and thus focus completely on wall proximity effects to evaluate the lift and drag forces. In particular, the wall-induced lift force is due to two competing mechanisms in subsea pipelines. First, the vorticity generated at the surface of the cylinder cross-section advects and diffuses downstream. The presence of a nearby wall breaks the axisymmetry of the wake vorticity distribution. The resulting induced velocity also breaks the symmetry and results in an effective lift force that tends to move the pipeline away from the wall. Second, from inviscid theory, one can argue that the flow relative to the cylinder will accelerate faster in the gap between the cylinder and the wall. The resulting low pressure in the gap will induce a lift force directed toward the wall. In the first part, we focus on the wall-induced lift force in the finite Reynolds number regime using a Spectral Element method to perform direct numerical simulations (DNS) with high accuracy. The effects of gap ratio will be investigated in detail to characterize the oscillating forces on the cylinder. In the second part, this paper will focus on studying how seabed can affect two tandem cylinder arrangements as a function of spacing and the interaction of wall-induced forces with the wake dynamics between the two cylinders.

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