Numerical Simulation Of Flow Around Two Pipelines Of Different Diameters Near The Seabed

With the rapid economic development and the impact of global economic integration, human needs and development of marine reach an unprecedented level. The vast ocean is becoming increasingly crowded. During the last one or two decades, there has made great progress in subsea pipelines, such as submarine cables, submarine pipelines and so on. With the development of offshore oil exploration methods and technology, subsea pipeline has been widely used in the exploration of offshore fields, and the research on submarine pipeline has also made great progress. It is dangerous that the submarine pipeline suspended. Domestic and foreign scholars have done a lot of research on the spanning pipeline, The research mainly focuses on the choice of force coefficients of submarine pipeline, the flow field around the submarine pipeline and the vortex-induced vibration of the spanning pipeline. As on the research methodology, it includes physical experiment and numerical simulation.In this paper, Fluent solver of CFD software is used to solve the Navier-Stokes equations of the incompressible viscous fluid, and to make numerical analysis and calculation about the hydrodynamic characteristics of subsea pipeline under the sub-critical Reynolds number. Second-order upwind scheme is used for the discretion of convection term, and semi-implicit SIMPLE algorithm is used to solve the speed and pressure of the control equation. The major work of this paper includes the following aspects:Firstly, in order to validate the reliability of the numerical method and the computational model used in this paper, numerical simulations of flows around a single submarine pipeline at sub-critical Reynolds numbers is carried out. The calculation results are compared with experimental data and numerical result in the literature, which shows rationality of the model established in this paper and can be used in the study of two pipelines of different diameters near the seabed at sub-critical Reynolds numbers.Secondly, two-dimensional incompressible flow around two tandem pipelines of different diameters near the seabed is investigated numerically. The Reynolds-averaged Navier-Stokes equations are solved using finite volume method with a standard k-εturbulence model closure at Re=20000. Two conditions are selected according to the different placement of the small pipeline relative to the large pipeline. Different spacing rations (G/D) raging from 0.4 to 3 between the two pipelines are investigated. The effect of spacing ration on the flow around and the hydrodynamic force coefficients on the pipelines are discussed. It is found that when the small pipeline is placed upstream of the large pipeline, the drag coefficient of large pipeline is smaller than that of the isolated pipeline at the same Reynolds number and there exists a critical spacing ration which transits the vortex shedding from the single-mode to interaction mode. Furthermore, when the small pipeline is placed downstream of the large pipeline, the drag coefficient of small pipeline is negative and the smaller the spacing ration is, the lager the negative drag coefficient is. The vortex shedding mode always presents interaction mode in this condition.Finally, steady boundary layer flows around a piggyback pipeline near the seabed is investigated numerically. The effects of the gap ration e/D and the spacing ration G/D on the flow around and the hydrodynamic forces on the cylinders are discussed. Four vortex shedding modes are found around the piggyback pipeline. It is found that the vortex shedding mode is dependent on the gap and spacing rations. The variations of the hydrodynamic forces with the gap and spacing rations are also quantified.