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Multiphase flowmeters (MPFMs) available today are mostly based on a combination of differential pressure measurement, provided by at least one flow constriction, such as a Venturi device, with phase holdup measurement based on nuclear techniques, electromagnetic techniques, or both. This paper investigates the issues of combining the ultrasonic Doppler velocity measurement with the differential pressure measurement for multiphase flow metering. The intention is to measure at least one “useful” velocity of the flow, such as the bulk liquid velocity or the homogeneous mixture velocity, which can then be combined with a measured differential pressure across a “constriction” device, such as a Venturi, to derive parameters needed to calculate liquid and gas flow rates. An ultrasonic Doppler velocity sensor has been tested in combination with a Venturi under multiphase flow conditions. The results show that, due to the effect of gas bubbles in the liquid phase, the interrogation depth of the ultrasonic wave is often limited to a shallow liquid region near the pipe wall. As a result, the ultrasonic Doppler sensor cannot reliably measure the bulk liquid velocity, or the homogeneous velocity. It is generally difficult to interpret the measured Doppler velocity and to derive the flow rates of a multiphase flow without employing an elaborate flow model. This work was part of a project cofunded by the UK Technology Strategy Board 1 , with industry partners The University of Manchester, Schlumberger Gould Research, and TUV-NEL. Introduction Many multiphase flow meters utilize differential pressure measurement combined with a mixture density or gas holdup measurement [1, 2]. A typical example is a dual-energy gamma-ray system that measures the density and the water-liquidratio (WLR) of multiphase mixture at the throat of a Venturi that provides the differential pressure measurement [3]. The measured density of the mixture (hence gas/liquid holdup) and WLR are then combined with the differential pressure across the Venturi to derive the flow rates of the three individual phases: oil, gas, and water. Other approaches used in MPFMs include combining differential pressure measurement with velocity measurement to derive the liquid and gas flow rate [1]. A recent MPFM [4] utilizes differential pressures across two flow constrictions and an ultrasonic Doppler velocity measurement to obtain the flow rates of gas, oil and water. It was claimed that the ultrasonic Doppler sensor could measure the velocity of a homogeneous multiphase flow, even though no flow homogenization was used in the MPFM. As remarked in [2], the direct velocity measurement technique widely used in MPFMs today is based on cross-correlation, which tends to measure the velocity of large and distinctive dispersed-phase flow structures, such as gas slugs. As a result, the velocity measurement based on cross correlation is not necessarily useful because it is difficult to find consistent correlations between a slug velocity and the “useful” velocities, such as the velocity of homogeneous mixture, which is defined as the total volumetric flow rate of all the phases divided by the cross-section of the pipe bore, or the velocity of bulk liquid phase. 1 The Technology Strategy Board is a business-led governmental body that creates economic growth by promoting the UK’s role as a global leader in innovation. Sponsored by the Department for Business, Innovation, and Skills (BIS), it brings together business, research, and the public sector, accelerating the development of innovative products and services to meet market needs, tackle major societal challenges, and help build the future economy.

Issues of a Combination of Ultrasonic Doppler Velocity Measurement with a Venturi for Multiphase Flow Metering