Performance evaluation of a multi-branch gas\u2013liquid pipe separator using computational fluid dynamics

H Refsnes,M Diaz,M Stanko

doi:10.1007/s13202-019-0708-8

Abstract

The present paper aims to evaluate the performance of a multi-branch gas–liquid pipe separator by means of 3D computational fluid dynamics. This type of separator is attractive for deepwater subsea hydrocarbon fields due to its compactness and reduced weight when compared against traditional gravity vessel separators. The focus of this paper is on studying the internal flow dynamics, the separation efficiency, and the performance with changing and transient operating conditions. Numerical simulations were performed on a numerical prototype of the separator using the inhomogeneous mixture model and assuming that both phases are continuous. Sensitivity analyses were performed on gas volume fraction, outlet pressures, and considering slug flow at the inlet with periods of 2 s and 8 s. The separation efficiency was quantified by calculating the liquid carry-over and gas blowby. For most of the operational conditions studied, separation occurred primarily in pipe branches closer to the inlet while those closer to the outlet exhibited a static liquid level. Reducing the gas outlet pressure caused the height of the liquid in the branches to be reduced. The inlet gas volume fraction did not affect significantly the separation performance, the flow distribution, nor the liquid level inside the separator. Separation efficiencies were not affected significantly with the presence of slugs; however, the liquid level in the branches oscillated significantly. The results and numerical models produced by this study could potentially be used to improve the understanding of this type of separators and improve its efficiency and system-level design.

Highlights

There are challenges when moving processing equipment to subsea, especially in deepwater fields with high hydrostatic pressure
The vertical pipes can be partially filled with liquid, having a spare volume suitable to accommodate for fluctuations in the inlet flows of liquid and gas, e.g., due to slug flow (Sagatun et al 2008)
A small diameter provides short retention time, which makes the separator a compact solution compared to conventional gravity vessels with larger diameters

Summary

Introduction

There are challenges when moving processing equipment to subsea, especially in deepwater fields with high hydrostatic pressure. The following boundary conditions were used for the simulations (locations are indicated in Fig. 2): mixture velocity uniform over the cross-sectional area, gas volume fraction (GVF) at the inlet, and average static pressure at the gas and liquid outlets. Liquid continuous–gas continuous phases and homogeneous mixture: This model employs mixture momentum equations with average properties that depend on the volume fraction.

Results

Conclusion