Gas-liquid Two-phase Flow in Pipelines

Abstract

ABSTRACT A combined theoretical and experimental investigation to study the behaviour and characteristics of gas-liquid two-phase flow in horizontal pipelines is described in this paper. A simple theoretical model was developed to provide predictions of liquid hold-up in equilibrium stratified two-phase flow regines and transition boundaries into slug flow. In order to calculate the liquid bold-up, the pressure drop equations were considered. The pressure drops in the two phases together with the shear stresses at the walls and interface were solved numerically. The criterion employed for predicting the onset of slug formation is wave instabiltiy which occurs when the wave velocity is imaginary. The experimental study wes conducted over a wide range of flow conditions to investigate pressure loss, liquid hold-up, flow pattern transitions and slug/bubble characteristics. The results presented in the paper are only flow pattern transitions and slug/bubble characteristics. The experiments were conducted on 0.2 and 0.4m diameter, 400m long loops at BHRA for a range of superficial air and water velocities up to VSG=24m/s and VSL=2m/s. The agreement between the experimental observations and the predictions of the transition into slug flow is considered excellent. Based on the average experimental data generated from these loops and from a separate study conducted on different fluids and pipelines, various empirical correlations for calculating the slug/bubble characteristics have been developed. Comparisons were also made between the measurements and current design methods, thus stating their range/limitations of applicability. 1. INTRODUCTIONS The search for more economic recovery methods, particularly for marginal offshore oilfields, sometimes in deep water, has resulted in decisions to transport mixtures of gas and liquid (two-phase flow) simultaneously in large diameter pipelines over relatively long distances. Optimization of the design and successful operation of two-phase pipeline systems requires a substantial knowledge of the behaviour and characteristics of such flows. Various studies have shown that at present no single theory or correlation can satisfactorily predict the characteristics of two-phase gas-liquid flow in pipes over a wide range of conditions. Gas-liquid flow is a more complex phenomenon than single-phase flow primarily because the distribution of the two phases is normally unknown and difficult to specify quantitatively. Unfortunately, the present design methods used within the industry are primitive compared to their single-phase counterparts. Some of these methods are based on empirical correlations derived from small scale laboratory systems with small diameter pipes and using simple test fluids such as air and water at low pressure. Extrapolation of these correlations to large diameter, high pressure oil and gas pipelines usually results in unacceptable errors. Another approach to this problem has been the development of very simple steady-state models that cannot reliably predict the wide variety of possible multiphase flow phenomena. A typical example of these phenomena is slug flow whereby the liquid phase aggregates into a slug that completely fills the cross-section of the pipe and is propelled along it at high velocity by the gas phase. The presence of these slugs can often be troublesome in practical applications (giving rise to sudden pressure pulses, causing vibration of pipes, ..etc), and the prediction of both the onset and characteristics of slug flow is of considerable industrial importance.