Slug Catcher Two-Phase Flow Modeling and Numerical Simulations

Abstract In Oil & Gas applications, the use of pipelines that carry both liquid and gas phases of fuels extracted from on-shore and off-shore subsea wells is constantly growing. In these conditions, flow regimes such as slug flow and plug flow can appear. If these flow regimes are established, damage at the downstream devices, such as the treatment systems located at the end of the pipeline, can occur. It is therefore necessary to separate the phases before they are treated. The purpose of a slug catcher is to separate the phases at the exit of the pipeline and to send them separately to the respective treatment systems. In this paper, a series of fluid dynamic simulations using a CFD methodology to predict the separation performances of a typical finger-type slug catcher geometry are carried out. A typical geometry which resembles real installations is considered. Different computational models were tested to find the solution that would give more accurate results with the minimum computational effort. For this purpose, comparisons between static models (computationally less expensive) and transient models (more accurate) were carried out. The influence of different models of turbulence and the influence of the computational grid on the final results were also evaluated. Guidelines for the correct implementations of these kinds of simulations are reported and the impacts of modeling assumptions on the expected results are discussed. The main technical contributions of the paper are:evaluating the operation of a slug catcher in different flow conditions;testing the validation of the slug catchers by means of numerical simulations;verifying the design choices in order to optimize the geometry of the slug catcher in relation to the conditions of use. Introduction Multi-phase pipelines are very common in oil & gas applications. This kind of pipeline can carry both a liquid and a gaseous phase. The simultaneous presence of the two phases leads to the formation of biphasic flow as stratified flow, bubble flow or slug flow. These phenomena have been studied by many researchers both for the complexity of the problem and the consequences that the formation of these phenomena can cause on the device installed downstream of the pipeline. In particular the slug flow is a potentially very dangerous phenomenon that can seriously damage the device downstream of the pipeline. This kind of problem occurs both in nuclear industry and in oil and gas ones. In the 90s Paglianti et al [1] analyzed several flow variables to clarify the intrinsic features of the flow regimes. With diffusional analysis, employed to analyze time series from a capacitance probe, three different flow behaviors in the slug flow region are identified. The plug flow occurs at low Froude numbers, at intermediate Froude numbers the elongated bubble flow occurs while ate the higher Froude numbers the slug flow occurs. Frank [2] has investigated the formation and the propagation of slug flow in a horizontal circular pipe with a series of a numerical simulation by using the commercial CFD code ANSYS CFX. With these simulations the inlet and boundary conditions that lead to the formation of the flow regime were investigated. Vallée et al [3] have studied two horizontal channels with rectangular cross-sections and the hot-leg of the German Konvoi-reactor. The three geometries were investigated by using both experimental data and computational fluid dynamics simulation. From these studies, the authors obtained a model to predict the formation of the slug flow in the nuclear reactors cooling lines. Bartosiewicz et al [4] have simulated the slug flow behavior by using several CFD codes in order to match the experimental data. Azzopardi and Smith [5] have investigated the two phase phenomena that occurs in a T junction studying the effect of the downstream geometry and the condition of the incoming gas and liquid flow.