Study of Flow Around a Circular Cylinder — Influence of Neighborhood Effects and Seabed Proximity

Abstract

Flexible risers, widely used in offshore engineering, are long multilayer tubes designed to carry fluid — such as oil and natural gas — from the seabed to the sea platforms. In this scenario, the risers have to be able to withstand efforts ranging from their own weight, water column pressure, to dynamic loads resulting from sea currents. This study aims to evaluate the influence of sea currents on flexible risers. As for this interaction, the fluid passage was evaluated in a uniform flow around the crosssection of the tube in a two-dimensional analysis. The seasonality and variation of sea currents over time make it necessary to study different flow regimes. Therefore, a variation of Reynolds number and type of flow is applied, thus obtaining results for the laminar regime. In the flow around an obstacle is interesting the study of the vortex shedding. This phenomenon generates pressure variation on the surface structure, causing dynamic efforts that can induce oscillations, increase drag force and cause structural failure if the frequency of vortex shedding approaching one of the natural frequencies in a structure. Therefore, the drag and lift coefficients and Strouhal for Reynolds numbers in the laminar regime are so important. They are obtained through modeling using computational fluid dynamics software, ANSYS(R) Fluent, and then compared with the results of other models in the literature. Under the laminar regime, a study of the influence of the proximity of one riser on the other was carried out. Also, the influence of the vertical position of the riser (depth) was investigated. Thus, the force coefficients are obtained as a function of the distance from one riser to the other and the ratio of the asymmetry of the domain respectively. It can be observed that for low distances between the cylinders has a region of speed considered null, appearing as if they were not two bodies, but only one. And as the cylinders move away, that region narrows, until the second cylinder has the velocity profile similar to the first one. It could be observed that when the cylinder was in the seabed, the values of coefficients varied significantly, as did the Strouhal number.