Multi-Phase Flow Metering in Offshore Oil and Gas Transportation Pipelines: Trends and Perspectives

Abstract

Multi-phase flow meters are of huge importance to the offshore oil and gas industry. Unreliable measurements can lead to many disadvantages and even wrong decision-making. It is especially important for mature reservoirs as the gas volume fraction and water cut is increasing during the lifetime of a well. Hence, it is essential to accurately monitor the multi-phase flow of oil, water and gas inside the transportation pipelines. The objective of this review paper is to present the current trends and technologies within multi-phase flow measurements and to introduce the most promising methods based on parameters such as accuracy, footprint, safety, maintenance and calibration. Typical meters, such as tomography, gamma densitometry and virtual flow meters are described and compared based on their performance with respect to multi-phase flow measurements. Both experimental prototypes and commercial solutions are presented and evaluated. For a non-intrusive, non-invasive and inexpensive meter solution, this review paper predicts a progress for virtual flow meters in the near future. The application of multi-phase flows meters are expected to further expand in the future as fields are maturing, thus, efficient utilization of existing fields are in focus, to decide if a field is still financially profitable.

Full text

sensors
Review
Multi-Phase Flow Metering in Offshore Oil and Gas
Transportation Pipelines: Trends and Perspectives
Lærke Skov Hansen †,‡, Simon Pedersen *,†,‡ and Petar Durdevic †,‡
Department of Energy Technology, Aalborg University, 6700 Esbjerg Campus, Denmark; lsh@et.aau.dk (L.S.H.);
pdl@et.aau.dk (P.D.)
*Correspondence: spe@et.aau.dk; Tel.: +45-99403376
† Current address: Aalborg University, Esbjerg Campus, Niels Bohrs Vej 8, 6700 Esbjerg, Denmark.
‡ These authors contributed equally to this work.
Received: 22 March 2019; Accepted: 6 May 2019; Published: 11 May 2019


Abstract:
Multi-phase flow meters are of huge importance to the offshore oil and gas industry.
Unreliable measurements can lead to many disadvantages and even wrong decision-making. It is
especially important for mature reservoirs as the gas volume fraction and water cut is increasing
during the lifetime of a well. Hence, it is essential to accurately monitor the multi-phase flow
of oil, water and gas inside the transportation pipelines. The objective of this review paper is
to present the current trends and technologies within multi-phase flow measurements and to
introduce the most promising methods based on parameters such as accuracy, footprint, safety,
maintenance and calibration. Typical meters, such as tomography, gamma densitometry and virtual
flow meters are described and compared based on their performance with respect to multi-phase
flow measurements. Both experimental prototypes and commercial solutions are presented and
evaluated. For a non-intrusive, non-invasive and inexpensive meter solution, this review paper
predicts a progress for virtual flow meters in the near future. The application of multi-phase flows
meters are expected to further expand in the future as fields are maturing, thus, efficient utilization of
existing fields are in focus, to decide if a field is still financially profitable.
Keywords: multi-phase flow; offshore; oil and gas; flow metering; instrumentation
1. Introduction
One major problem in the offshore oil and gas industry is monitoring of multi-phase flow
consisting of oil, water and gas in pipelines [
1
,
2
]. Due to difficulties regarding subsurface
instrumentation the multi-phase flow contributes to a huge problem at offshore installations [
3
,
4
].
In Figure 1a typical offshore installation of a well-pipeline-riser system is illustrated. As it can be seen
from the figure, most of the process is placed subsurface, which enhances the problem. For vertical
wells as in Figure 1the system consists of three main sections being the vertical pipe from the reservoir
to the seabed, the horizontal subsea pipeline, and the vertical riser-pipeline from the seabed to the
separation platform [
5
]. Since subsea instrumentation is extremely expensive and cumbersome,
monitoring of the multi-phase flow is often reduced to the top of the vertical riser-pipeline and
following pipelines, which is located above sea.
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Sensors 2019,19, 2184 2 of 26
Figure 1. Subsea manifold and transportation pipelines to separation platform.
Poor measurements of the multi-phase flow can lead to big uncertainties regarding important data,
which due to small measurement errors and round-offs can end up with a huge error margin in the
end of the oil recovery process [
6
]. The problem is well-described and investigated, but the potential
errors in the measurements are often not being accounted for during the oil recovery process, and a lot
of models and empirical algorithms can hereby be questioned upon their accuracy. Poor accuracy of
multi-phase flow measurements can have a huge effect on:
•Model prediction, history matching and future of reservoir [7–12].
•Control of flow patterns [13,14].
•Separation [15].
•Chemical injection [16].
•Emulsion layer [16].
•Corrosion-rate [17].
Although the focus of this review paper is multi-phase flow in the upstream transportation
pipelines, the flow measurement is also of importance to other parts of the oil recovery process.
Tthe produced water (PW) treatment is affected by accurate and reliable measurements of the
flow. PW treatment is a product from the separation process, which occurs after Figure 1on the
separation platform. It has been documented that control of flow patterns (e.g., slugs) can reduce
the separation efficiency of the separator, and hereby optimize the PW treatment process [
18
–
20
].
Slugs can be controlled by a feedback system feed with flow measurements from e.g., a multi-phase
flow meter (MPFM).
The Danish Environmental Protection Agency has specified regulations on the PW with respect
to dispersed oil discharged into the ocean [
21
–
23
]. All Danish platforms in the North Sea are
by law required to discharge less than 222 ton of oil per year in total [
21
,
24
]. The regulations
are stated by The Danish Environmental Protection Agency based on requirements from OSPAR
Commission, which protects the marine environment and biodiversity in the North-East Atlantic
Ocean [
25
]. The amount of dispersed oil in PW can carefully be monitored and hereby reduced
by the implementation of e.g., MPFMs, as they can optimize the separation process and chemical
injection. In the permissions provided by The Danish Environmental Protection Agency given from
2019 to 2023 it can be seen from Figure 2that the amount of PW and discharged dispersed oil is
increasing with exception of some years due to shut down of big platforms (reconstruction of Tyra
field in 2019–2022 [
26
,
27
]). The increase of PW is a result of maturing fields with increasing water
cut. This is supporting the importance of accurate measurements of the multi-phase flow and, hereby,
the necessity of MPFMs. The increase in PW can also be a huge problem, if the legal requirements are
not adjusted to this change, as it will be difficult to respect the given law. The data from Figure 2are

Sensors 2019,19, 2184 3 of 26
based on reports from The Danish Environmental Protection Agency regarding discharge permissions
of dispersed oil in PW for the Danish North Sea fields operated by Total E&P Danmark A/S, Hess
Danmark ApS and INEOS Oil & Gas [21–23].
Figure 2.
Produced water and discharged dispersed oil from the Danish platforms in the North Sea.
The platforms include: Dan, Gorm, Halfdan, Tyra, Syd Arne and Siri. The fields are operated by Total
E&P Danmark A/S, Hess Danmark ApS and INEOS Oil & Gas. The blue graph illustrates the total
amount of produced water from the fields. The red graph illustrates the discharged dispersed oil in
the PW.
This article will cover monitoring of multi-phase flow in the main production upstream in the
1st stage separation process. It will evaluate the respective methods based on how they can be
implemented and applied to benefit the entire oil recovery process. First, an introduction to the
problem will be described. Sections 1.1–1.3 will introduce the most crucial issues with unreliable
and inaccurate multi-phase flow measurements. Section 2will explain the conventional method
for measuring multi-phase flow, where Section 3will describe the existing technologies and present
some non-commercial prototypes. After an introduction to current technologies, some industrial and
commercial MPFMs will be presented and compared in Section 4. Lastly, a discussion and conclusion
will be made in Sections 5and 6, evaluating and predicting the future of MPFMs based on the results
obtained in this study.
1.1. Model Prediction, History Matching and Future of Reservoir
With an accurate model of the reservoir, it is possible to approximate the future behavior of the
field, to perform computer simulations and to manage the reservoir. The model is periodically updated
based on observed reservoir behavior, which is based on e.g., multi-phase flow measurements of
the wells. The approach is called history matching and has been described in [
7
–
12
], where possible
optimization options are also introduced. History matching is an approach where the current reservoir
model is fitted to reproduce the past behavior, so that the oil recovery is at its maximum over the
lifetime of the field. This can e.g., be done by enhanced oil recovery process [
28
–
31
]. To see the
behavior of the model and the oil production, decline curve analysis is employed. Decline curve
analysis is applied for production forecasting and reserves estimation [
32
]. The decline curve analysis
is introduced in [
33
–
35
], and based on the decline of the curve, enhanced oil recovery methods such
as water injection are decided and implemented. The data for the decline curve analysis is based
on performance history and observed production over time. These datasets, which e.g., consist of
the phase fraction and velocity of the multi-phase flow, need to be accurate to avoid errors in the

Sensors 2019,19, 2184 4 of 26
model. Hence, the water decision of injection and injection volumes is dependent on the accuracy of
the multi-phase flow measurements of the wells.
1.2. Flow Regimes
Many studies investigate the flow pattern inside the transportation pipelines [36–40]. Especially
three-phase flow of water, oil and gas can be a huge challenge in the oil and gas industry [
41
].
There are several factors that can affect the liquid–gas flow pattern, which has been stated in [
42
] and
is listed below:
•Phase properties, fractions and velocities.
•Operating pressure and temperature.
•Diameter, shape, inclination and roughness of the transportation pipe.
•Presence of e.g., valves, T-junctions and bends.
•Pipe direction: vertical, horizontal or incline/decline.
•
Type of the flow: whether the flow is in steady-state, pseudo steady state or unsteady (transient).
There are several types of flow patterns such as bubble, churn, annular, disperse and slug
flow [
43
–
46
]. The latter can be difficult to monitor and prevent, and requires an accurate feedback
system with reliable measurements, in order to prevent slugging from occurring inside the pipes.
The input for the feedback system can e.g., be multi-phase flow or pressure measurements. It is proven
that flow measurements are a better control variable compared to e.g., pressure, as long as the flow
measurements are accurate. The effectiveness of a cascade controller, which is i.a. based on flow
measurements, is presented in [
47
]. This is supporting the importance of reliable and accurate flow
measurements provided by e.g., MPFMs.
1.3. Separation and Chemical Injection
In the gravity separator the three phases will start separating right after entering the gravity
separator. Small impurities from the well and corrosion inhibitors added to the flow can lead to the
presence of foam between the oil and gas mixture [
16
,
48
]. In addition to the foam between the oil and
gas phases there will also occur an emulsion layer between the oil and water phases. The thickness
of the emulsion layer depends on i.a. the residence time inside the gravity separator [
48
]. Both the
foam and emulsion layer has a negative impact of the separation process and hereby the total oil
recovery [
16
]. Chemicals are added to prevent the foam and thick emulsion layer inside the separator.
The amount of chemicals added depends on the composition of the fluid, and accurate measurements
of the multi-phase flow are hereby essential. The chemical injection is a very expensive process
and unnecessary injection will only lead to poor oil quality and unnecessary expenses [
49
]. Hence,
the need for measurement of all three phases prior separation is necessary. Another way to prevent a
thick emulsion layer between the oil and water is electrostatic coalescence, which has been described
in [
50
–
52
]. The stream of the multi-phase flow inside a pipeline can also be directly connected to
corrosion [
53
]. In the presence of slug flow conditions as explained in Section 1.2, the multi-phase flow
has the impact of increasing the protective surface scales/films inside the pipeline. This can eventually
increase the corrosion rate since the slug flow will lead to higher fluctuations of the surface shear
stress [
54
]. Each problems can be prevented by obtaining correct measurements of the multi-phase
flow, which will be an input in a feed-back system. Reliable and accurate equipment is essential to
monitor the multi-phase flow and hereby to improve the overall oil recovery.
2. Conventional Flow Measurement Technology
This section will provide the reader with an introduction to the conventional technology to
measure and monitor the multi-phase flow. The produced multi-phase flow of a reservoir can
vary depending on the location and lifetime of the well [
55
]. Previously the multi-phase flow has

Sensors 2019,19, 2184 5 of 26
been measured using a test gravity separator, which measured each single phase flow from the
output [42,56,57]
. Commonly used meters for single-phase flow measurements can e.g., be a venturi
meter, turbine meters or coriolis flow meters [
1
,
58
]. The conventional flow technology is illustrated
in Figure 3. If the fraction or velocity of each phases is to be obtained, the multi-phase flow can be
transported to a test platform, where the test separator is located.
Figure 3.
Oil production system with test separator and 1st stage separator. The flow inside the pipe is
denoted as either M for multi-phase flow or S for single phase flow. After the test separator each phase
flow is ideally measured by a single-phase flow meter (FM).
Inside the test separator the multi-phase flow is separated into three different single-phase flows.
After each outlet of respectively oil, gas and water a single-phase flow meter is installed labeled as
FM on Figure 3. The conventional method with the test separator is reliable and accurate but not
suitable for real-time monitoring of the multi-phase flow, as the process is simply too slow [
30
]. It takes
time for the three phases to divide inside the separator, which means that the single-phase output
will not provide instantaneous and real-time measurements [
59
]. Since the measurements are not in
real-time it can not be used in a feedback system to prevent e.g., slugging or overdosage of chemical
injection. Other disadvantages regarding the conventional technology is that the test separator has
a huge footprint and contributes to extra load on the given platform [
42
]. Also the measurements
are not performed in-line [
59
]. Due to these disadvantages regarding the conventional test separator,
new technologies and instruments have been investigated and implemented for commercial use.
The upcoming sections will cover the new technologies to measure the multi-phase flow.
3. Multi-Phase Flow Metering
Generally when measuring the multi-phase flow the mass and volumetric flow rates of water,
oil and gas need to be obtained. As described in [
6
] Equation (1) is valid for the volumetric flow rate
Q
.
Q=A(αvg+βvw+χvo)(1)

Sensors 2019,19, 2184 6 of 26
A
is the cross-section area of the pipe.
α
,
β
and
χ
are the gas void fraction, water fraction and oil
fraction respectively.
vg
,
vw
and
vo
is the instantaneous velocity of gas, water and oil. The sum of the
fraction of the three phases should equal one, which means that only two of the three phase fractions
need to be measured. Equation (1) can then be simplified as Equation (2):
Q=A(αvg+βvw+ [1−(α+β)]vo)(2)
To calculate the mass flow rate
M
of the multi-phase flow, the density of each phase needs to be
obtained. The mass flow rate is calculated in Equation (3):
M=A(αvgρg+βvwρw+ [1−(α+β)]voρo)(3)
where
ρg
,
ρw
and
ρo
is the density of the gas, water and oil [
6
]. To monitor the volumetric and/or
mass flow rate of the multi-phase flow, different technologies have been invented. Over a lifetime of
the well each of the parameter can vary. Hence, it is important for the MPFM to measure both density,
velocity and phase fraction of the flow.
Some of the most common methods will be introduced in the next sections. For a technology to
be sufficient for multi-phase flow measurement, it should be non-intrusive, flow regime independent,
accurate, reliable and able to measure the entire fraction range of each of the phases [
55
]. The upcoming
technologies will be discussed on their ability to monitor the multi-phase flow. Section 3.1.1 examines
electrical capacitance tomography, Section 3.1.2 examines electrical resistance tomography, Section 3.1.3
examines electromagnetic tomography, Section 3.1.4 examines microwave tomography, Section 3.1.5
examines electrical impedance tomography and lastly Section 3.1.6 examines optical tomography.
Section 3.2 will introduce a different technology called gamma densitometry. Section 3.3 will introduce
a technology called virtual flow meters (VFM), which consists of i.a. differential pressure transmitters
such as an orifice plate or venturi meter.
3.1. Tomography
Tomography is an imaging process technology, which is widely used when measuring the
multi-phase flow in the offshore industry [
55
,
60
–
70
]. The advantages of tomography is that the sensors
are placed at the periphery of the pipe and are hereby not a causing any obstruction to the flow [
71
].
Equation (4) is applied to determine the volume flow of a phase [56]:
Qx=a
N
∑
i=1
fi(x)vi(4)
fi
is the volume fraction of phase
x
and
vi
is the flow velocity in the
i
-th element.
N
is the
elements that the cross-section of the pipe is divided into with an equal area
a
. The tomography
technology is illustrated in Figure 4, where two series of images are taken simultaneously. Each series
are representing a cross-section of the pipe, which are cross-correlated to obtain the velocity profile
vi
.
The volume fraction distribution of the phase
fi(x)
can be obtained directly by the images provided
from the tomographic sensors [56].

Sensors 2019,19, 2184 7 of 26
Figure 4. Multi-phase flow measurement using tomography imaging process.
3.1.1. Electrical Capacitance Tomography (ECT)
Electric capacitance tomography has been used for many years and the technology has been
widely investigated [
72
–
78
]. In the electrical capacitance tomography a multi-electrode sensor obtains
capacitance measurements. The electrodes are located peripherally around the pipe causing no
interruption with the flow. For an ECT sensor the capacitance is changing when the di-electric
material distribution is changing [
56
,
79
]. The technology for an electrical capacitance tomography
sensor is illustrated in Figure 5. A typical ECT sensor consist of between 8, 12 and 16 measurement
electrodes [56,80].
Figure 5.
ECT sensor with 8 electrodes around the pipe. One electrode is excited and the capacitance
is measured.
One issue regarding ECT is the imaging reconstruction algorithm, which induces an inverse
problem. Different approximation methods have been used to solve the inverse problem and linear
back projection (LBP) is commonly used. Reference [
81
] has developed a new reconstruction algorithm,
which they claim is able to image two- and three-phase flows. The algorithm is based on an analog

Sensors 2019,19, 2184 8 of 26
neural network multi-criteria optimization image reconstruction technique and shows both accurate,
consistent and robust results. The algorithm works for transient multi-phase phenomena in gas-liquid
and gas-liquid-solid flows. Some non-commercial ECT techniques have been proposed, where [
82
]
presents a void fraction measurement system for two-phase flow. The measurement error of the
system is less than 5%, and the method is suitable for the void fraction measurement of bubble flow,
stratified flow, wavy flow, slug flow, and annular flow. Another promising non-commercial technique
is presented in [
83
]. The prototype is a multi-phase flow meter for oil-continuous flows. The technique
is an improved AC-based ECT system and it shows less than 3% absolute error for oil-water flows
with a water liquid ratio (WLR) < 35%.
3.1.2. Electrical Resistance Tomography (ERT)
ERT is contrary to ECT applied when the continuous phase is conducting [
56
]. It can be a challenge
that the continuous phase needs to be conducting, since the phases can vary e.g., during a slug cycle.
During a slug cycle the gas volume fraction (GVF) can vary from 0–100% and the continuous phase
is hereby not guaranteed to be conducting at all times. ERT has also been widely described and
investigated [
84
–
88
]. The electrodes of an ERT sensor are located in direct contact with the flow inside
the pipe.
The ERT technology is only suitable for measurements in vertical pipes, since the electrodes are in
direct contact with the flow. If the electrodes are frequently exposed to the gas phase (non-conducting
phase), as will happen in horizontal pipes under stratified, wave, slug or plug flow, the electrodes might
lose their continuous electrical contact with the measured flow [
89
]. In [
89
] a method is presented,
so ERT can be implemented for two-phase flow in horizontal pipes. The method is called Liquid Level
Detection and takes account for the electrodes that are exposed to gas (air) and hereby monitors the
position of the water surface. M. Wang [
90
] has also invented a method for ERT sensors to address
the challenges of electrodes with no contact to the conductive fluid. A disadvantage regarding ERT
is that the technology is primarily suitable when the continuous phase is conducting. Therefore,
when the flow is water continuous, ERT should be applied. This is due to the electrical conductivity
of water compared to oil, which will appear as an insulator [
91
]. Due to this it can be beneficial to
combine the ERT and ECT technology, to obtain a sensor technique that can obtain capacitive and
resistive properties simultaneously [
92
]. The design of a multi-modal tomography system based on
ERT and ECT has been described in [
93
–
96
]. In [
97
] a dual-modal sensor is presented, which is able
to measure the multi-phase distribution in a flow. For a gas-oil-water concentration consisting of
50% oil/water (30% water and 20% oil), the sensor is able to reconstruct the images such that the
concentration is calculated to 49.69% oil/water (30.17% water and 18.53% oil). The ECT mode is used
when WLR is less than 40% (oil-continuous flows) and ERT is used when WLR is higher than 40%
(water-continuous flows).
3.1.3. Electromagnetic Tomography (EMT)
Electromagnetic waves use the permittivity of a fluid to determine the fraction of each phase
in a multi-phase flow [
98
]. The sensor consists of excitation coils, which produce a magnetic field.
The sensors are not in direct contact with the flow [
56
]. Water has a higher permittivity than oil and
gas and the sensor is therefore, more sensitive to water flows. The permittivity is represented with the
symbol epsilon. Water has a permittivity at
εr(water)
= 81, oil at
εr(oil)
= 2.2–2.5 and gas at
εr(gas)
=1[
98
].
The EMT sensor is not the most convenient technology, when it comes to monitoring of multi-phase
flow in the offshore industry. Since the measurements are based on the electrical conductivity and the
magnetic permeability of the medium, it will require a high excitation frequency to increase the signal
from the sensor [
99
]. In a recent research [
100
] a combination of magnetic induction tomography (MIT)
and electromagnetic velocity tomography (EVT) showed promising results in measuring the velocity
of the continuous phase (water) in a two-phase flow consisting of oil and water. MIT and EVT are both
types of electromagnetic tomography techniques. The prototype shows great accuracy for single-phase

Sensors 2019,19, 2184 9 of 26
water flow with a relative error of only 0.012%, but lacks in accuracy with a ratio of 65.80% water in
water-in-liquid multi-phase flow. Here the relative error is 12%, which is a huge error-margin, if the
meter should be implemented for multi-phase flow measurements.
3.1.4. Microwave Tomography (MWT)
One way to obtain electromagnetic waves is by using microwave tomography [
98
,
101
,
102
].
A microwave tomographic sensor consists of both receiving and transmitting antennas [
103
]. By using
an electromagnetic field the electromagnetic waves will be transmitted at different angles and hereby
create an image of the flow inside the pipe, while comparing with an uniform permittivity background
at the receiving part [
77
,
103
]. The hardware of a microwave sensor consists of a source that generates
the microwave signals, a detection part that detects and measures the microwaves, a routing part
that converts the signals into multi-views (images) of the flow and lastly microwave transmitting and
receiving antennas [
77
]. The microwave tomography technology is not widely used for multi-phase
flow measurements due to the image reconstruction algorithm, as this is too slow for real-time imaging
of the dynamic behavior of the multi-phase flow [
103
]. An experimental MWT system is presented
in [
104
], where an 8-port sensor is designed for oil-gas-water flows. The image construction is not yet
accurate enough to be implemented for industrial and commercial application, as the image quality
and hereby meter accuracy decreases with increasing frequency. Further investigation is essential to
present an accurate MWT meter for multi-phase flow measurements at offshore installations.
3.1.5. Electrical Impedance Tomography (EIT)
Electrical impedance has been described in [
105
,
106
]. This technology is often used in the
pharmaceutical industry to test respiratory and lung function [
107
]. The electrodes are located
periphery around the pipe and have electrical contact with the flow inside the pipe but do not
cause any obstruction to the flow [
80
]. A current is injected through the cross-section of the pipe
and the corresponding electrode voltage is measured. To calculate the fraction of each phase an
algorithm is used. The input to the algorithm is the applied current pattern and the electrode voltages,
which will then reconstruct an image based on the electrical conductivity and permittivity of the
flow [
108
]. EIT is not widely used in the oil and gas industry, but [
105
] shows promising results with
a recently developed measurement system that can produce real time 3D images. These are to be
used together with an algorithm to monitor the multi-phase flow. The system consists of 80 surface
electrodes and is capable of producing 15 frames per second in 3D. Another EIT sensor described
in [
109
] presents a multi-mode prototype, with a combination of capacitive and resistive/conductive
mode. By experiments the relative errors between measured and calculated values are below 1.64%
for the capacitance mode and less than 2.68% for the conductive mode. The sensor requires further
development before application but indicates promising results for measurements of multi-phase flow.
3.1.6. Optical Tomography
Optical tomography uses illumination such as absorption, diffraction and reflection of light as a
method to measure the multi-phase flow within a cross-section of a pipe [
60
,
110
]. The main component
of an optic sensor is a light source and a camera to sense the reflected light. Since the sensor is measuring
the transparency of the flow inside the pipe with respect to absorbed and reflected light, the sensor
needs a transparent window inside the pipe in order to be able to detect the light [
111
]. Another
disadvantages regarding optical tomography is that in multi-component flow such as multi-phase
flows, a bubble of e.g., air/gas can cause misleading measurements. This is due to the curving and
reflecting surface of the bubble, which can cause an intense beam of light to be reflected within the
flow and hereby confuse the camera of the optic sensor [
80
]. Optical tomography technology has
been described in [
112
], where the sensor is investigated for both single and two-phase pipe flows.
The sensor shows accurate measurements but are limited to flow situations with up to 15% gas fraction.

Sensors 2019,19, 2184 10 of 26
The lack of ability to measure the gas fraction in the entire range makes the optical tomography not
suitable for monitoring of multi-phase flow in the oil and gas industry.
3.2. Gamma Densitometry
Besides tomography another convenient technology is gamma densitometry. Gamma densitometry
uses a radioactive source to obtain measurements of the multi-phase flow. Gamma is preferred
because of its ability to measure spacial distribution based on the atomic number and density of a
material [
111
,
113
,
114
]. When gamma-rays are radiated from the source to the detector, the ray will
attenuate depending on the absorption of radiation of the flow within the pipe [
115
]. Depending on
the detected gamma quanta at the detector, the sensor is able to measure even small changes in the
density differences of the fluid, and can hereby obtain accurate measurements of the fraction of each
phases [
116
]. To measure both the phase volume fraction and velocities, and not only the average fluid
density, the gamma densitometer is often installed together with an equipment such as e.g., a venturi
meter or an orifice plate. In [
117
] researchers presents a single clamp-on gamma densitometer unit,
which is able to determine both phase volume fractions and velocities to predict the individual phase
flow rates of vertically upward multi-phase flows. The method yield promising improvements on
the accuracy, but still needs more investigation as the densitometer is flow dependent. Gamma
densitometry has also been tested and evaluated in [
118
–
120
]. A disadvantage with gamma-rays is the
salinity content in the water. Saltwater has a higher attenuation coefficient than freshwater, which will
cause errors in the measurements if the salinity content changes [
98
]. To avoid these errors additional
equipment is needed. A single-beam gamma densitometer with an accuracy of 0.97% (phase fraction
measurements) is presented in [121]. The applied gamma source is Am-241, with radiation energy of
59.5 keV. The accuracy of the desitometer is increased by increasing the measuring time and the location
of the radioactive source with respect to the pipe. With increased measuring time and the radioactive
source at the center of the pipe, an accuracy of the phase fraction measurements on 0.53% can be
achieved. Another method for detecting the flow regime and void fraction by the use of a gamma
source is presented in [
122
,
123
]. The method is based on dual modality densitometry using artificial
neural network (ANN) and presents error less than 1% between estimated and simulated values.
3.3. Differential Pressure Meters
Many multi-phase flow meters use differential pressure (DP) transmitters to measure the
difference in the pressure in two given points inside the pipe. The most common differential pressure
transmitters used in the offshore industry are an orifice plate or a venturi meter, due to their accuracy,
in-line measurements and small footprint. Using Bernoulli’s principle it is known that increasing
the velocity of the fluid will cause a decrease in the pressure. By obtaining the differential pressure,
the flow rate of the fluid can be calculated [
124
,
125
]. Both the orifice plate and the venturi meter
creates a disturbance to the flow which enables the DP transmitters to measure the pressure difference
between two points. Together with a software tool consisting of empirical algorithms, some DP
meters can provide multi-phase flow measurement with the same accuracy as tomography based
meters or gamma densitometers. These meters are called virtual flow meters (VFM) and with the
simple instrumental equipment, these meters can contribute to a cheaper solution for the offshore
industry [
126
]. The meter consist only of an orifice plate or a venturi meter and already available
measurements at offshore installations such as temperature and pressure transmitters. The main
limitation of the meter is that the fluid composition must be constant. To avoid this problem void
fraction sensors and gamma densitometers are combined with the VFM measurements.
3.3.1. Orifice Plate
Orifice plates can be used to measure the flow velocity of a fluid within a pipe [
127
–
129
].
As explained in [
130
] an orifice plate works by applying a thin plate with a small opening inside the
pipe. The orifice plate is illustrated in Figure 6.

Sensors 2019,19, 2184 11 of 26
Figure 6.
Principle of an orifice plate. Interruption of the flow inside a pipe due to an orifice plate.
DP transmitters are measuring the pressure difference at a point before and after the orifice plate and
the velocity of the fluid is hereby obtained by Bernoulli’s equation.
The point where the flow is experiencing the maximum of convergence is called vena contracta.
Vena contracta is occurring just after the orifice plate as illustrated on Figure 6with the diameter noted
as
dvc
. The differential pressure transmitters measures the pressure in the regular pipe diameter (
dpipe
)
and at vena contracta and hereby calculates the pressure difference and by Bernoulli’s equation obtains
the velocity of the fluid.
For vertical orifice plates the volumetric flow rate in terms of the pressure difference (
∆P
) is
calculated as Equation (5):
Q=CdAoriYs2(∆P+ρg∆z)
ρ(1−β4
d)l(5)
βd
is the ratio between the diameter of the pipe and the diameter of the orifice,
z
is the change in
elevation and
Cd
is the discharge coefficient.
Aori
is the area of the orifice plate,
ρ
is the density of
the fluid and
g
is gravity. The discharge coefficient for an orifice plate (thin sharp edged) is around
0.61 [131]. Yis the expansion coefficient, which is defined as Equation (6):
Y=Cd,c
Cd,i
(6)
Y
depends on the discharge coefficient for compressible (
Cd,c
) and incompressible (
Cd,i
) flows [
132
].
For incompressible fluids
Y
= 1 and for compressible fluids the expansion coefficient will be defined
by the discharge coefficients as defined in Equation (6). The orifice is often used in a combination with
another instrument to measure and calculate the mass flow rate of the multi-phase flow. An orifice
plate meter is presented in [
126
]. The measurements of the meter are compared with simultaneously
measured data from a test separator and shows 3.52% measurement error with respect to the standard
volume flow rates of oil, water and gas.
3.3.2. Venturi Meter
A lot of studies has shown the effectiveness of measuring the multi-phase flow by using a venturi
meter [
125
,
133
–
137
]. The principle of a venturi meter is common to the principle of an orifice plate

Sensors 2019,19, 2184 12 of 26
and illustrated in Figure 7. A venturi meter is said to have the lowest pressure loss compared to other
differential pressure transmitters [65].
Figure 7.
Principle of a venturi meter. The DP transmitters are located before the pipe is converging
(d1) and when the pipe is most converged (d2).
To obtain the volumetric flow rate using a venturi meter, the same equation as the orifice plate
is used. The only difference in Equation (5) is that the discharge coefficient is higher for a venturi
meter compared to the orifice plate. For a venturi meter the discharged coefficient (
Cd
) is between
0.984–0.995 [
131
]. The venturi meter is widely used with respect to multi-phase flow in the offshore
industry. In [
65
] a solution has been presented for a two-phase flow meter consisting of an ECT sensor
and a venturi meter. The ECT sensor is measuring the void fraction information’s while the venturi
meter is obtaining the velocity of the two-phase flow. The venturi meter has also been investigated
together with an ERT sensor. This is described in [
137
], where the method is presented. The ERT
sensor measures the real-time flow pattern, while the void fraction and mass quality is calculated and
determined by the presented model. The mass flowrates are calculated based on the mass quality and
the differential pressure across the venturi meter. For bubble and slug flow the root mean square error
of the total mass flowrate is less than 0.03 and 0.06 respectively. The relative error is less than 5%.
3.4. Wet Gas
When the flow inside the pipe is gas dominant and the water and oil fraction together is less
than 5%, wet gas conditions are valid [
6
,
138
]. A wet gas flow can be defined by Lockhart-Martinelli
parameter, which is a dimensionless number ranging from 0 to 0.3. Zero is representing a completely
dry gas [
139
]. As described in [
140
] the Lockhart-Martinelli parameter can be defined as Equation (7).
XLM =sS uper f i cial Liqui dIn ertia
Su per f ici alG asI nerti a =ml
mgrρg
ρl
(7)
where
ml
is the mass flow rate of the liquid and
mg
is the mass flow rate of the gas.
ρg
and
ρl
is the
density of gas and liquid respectively. To predict the wet gas flow pattern the Lockhart-Martinelli
parameter combined with the gas and liquid densimetric Froude number is used [
140
]. The gas and
liquid densimetric Froude number is defined as Equations (8) and (9) [140]:
Frg=
Usg
pgD sρg
ρl−ρg(8)
Frl=
Usl
pgD rρl
ρl−ρg(9)

Sensors 2019,19, 2184 13 of 26
D
is the internal diameter of the pipe,
g
is the gravitational constant,
ρg
and
ρl
is the densities of
gas and liquid, and
Usg
and
Usl
is the superficial gas an liquid velocities calculated by Equations (10)
and (11).
Usg =mg
ρgA(10)
Usl =ml
ρlA(11)
mg
and
ml
is the mass flow of gas and liquid. Wet gas flow meters can consist of i.e., an orifice plate,
which has been described in [140] or a venturi meter [138].
3.5. Summary of Current Technologies
Sections 3.1–3.4 has outlined some of the current technologies for multi-phase flow measurements.
Based on the investigation and experimental results especially ERT, EIT and gamma densitometry
have shown promising and reliable results based on the accuracy of the presented prototypes. All of
the presented prototypes are listed in Table 1for a better overview. Though, the prototypes has
been widely tested, further investigations should be done before application of the prototypes for
industrial use in e.g., the offshore industry. The prototypes with the poorest accuracy is the VFMs
by [
65
,
126
,
137
]. The ECT meter from [
82
] shows a measurement error on 5%, which is higher than
some of the other presented prototypes. The most accurate prototypes are the two MPFMs with a
radioactive source. Reference [
121
] shows less than 0.53% measurement error, while [
122
,
123
] shows
less than 1% mean absolute error.
4. Comparison of Industrial MPFMs
In this section some industrial MPFMs will be presented. They will be listed based on i.a. their
technology, advantages and disadvantages and will be illustrated in Table 2. Please note that the
tabular only represents a small amount of the existing commercial MPFMs on the market. It gives a
small insight to the industry and which technologies the industry has implemented. It should also be
noted that the values given in the table is based on data sheets provided by the manufactures and that
it has not been possible to verify the data.
From the table it can be seen that the presented MPFMs with a radioactive source tends to have a
better accuracy compared to the MPFMs with no radioactive source. The MPFMs provided by Emerson
(Roxar) 2600 M and 2600 MG contains no radioactive source and has a poor accuracy compared to the
other MPFMs. Also the MPFM provided by Khrone Oil & Gas contains no radioactive source and has
a poor accuracy especially for the gas rate. Based on the data sheets the most accurate MPFMs from
the table are the meters provided by Schlumberger, Weatherford and Pietro Fiorentini (Flowatch HS).
The three meters all contain a radioactive source and is claimed to operate in the entire range (0–100%
WLR, 0–100% GVF ).

Sensors 2019,19, 2184 14 of 26
Table 1. List of current non-commercial prototypes to monitor multi-phase flow.
Non-Commercial Prototypes
Source Technology Accuracy Advantages Disadvantages
[82] ECT 5% Measurement error. New improved image reconstruction
algorithm. Non-radioactive. Mostly based on static experimental data.
[83] AC-based ECT 3% Absolute error. Non-radioactive. Only suitable for oil-continuous flows.
[97]Dual-modal sensor:
ECT & ERT
Not informed. Measurements follow the
expected trend.
ECT mode when continuous phase is
oil (WLR < 40%), ERT mode when continuous
phase is water (WLR > 40%). Non-radioactive.
Needs more investigation upon
dynamic evaluation.
[100] MIT & EVT Single-phase: 0.012% relative error.
Multi-phase: 12% relative error.
Robust, low-cost and non-radioactive solution.
Only suitable for water-continuous flows.
Needs further improvements w.r.t. accuracy.
[104] MWT
Not informed. Measurements and simulated
values show the same trend.
Designed is intended for the process industry
and oil-gas-water flow imaging.
Needs improvements of image quality, when
frequency is increased.
[105] EIT Not informed. Measurements show the
expected trend. Designed for industrial application. Needs further development before
application (e.g., new measurement system).
[109] EIT Capacitance mode: 1.64%.
Conductive mode: 2.68%
Fast and robust image restoration algorithm.
Simple hardware design. Only preliminary tests.
[112] Optical tomography.
Gas inclusion: 0.21% void fraction error.
Evans Blue solution: 2.17% void
fraction error.
Fast data acquisition. Limitations of larger GVF than 15%.
[121]Gamma
densitometry. 0.53% measurement error. Non-intrusive and reliable. Not tested with gas injections. Contains
radioactive source.
[122,123]Dual modality
densitometry. 1% Mean absolute error. Non-intrusive and able to identify
flow regimes. Contains radioactive source.
[126] VFM (Orifice plate) 3.52% measurement error. No radioactive source. Limitation: fluid composition must remain
constant during the measuring period.
[65]VFM
(venturi meter + ECT)
Not informed, but performs good
flowrate measurements. High quality images from the ECT sensor. Over- and underestimated measurements
based on employed model (5 different).
[137]VFM
(venturi meter + ERT)
5% relative error (bubble and slug flow).
10% relative error (annular and
stratified flow).
Improved measurement performance.
No radioactive source. Flow regime dependent.

Sensors 2019,19, 2184 15 of 26
Table 2. List of industrial multi-phase flow meters.
Industrial Multi-phase Flow Meters
Manufacturer Emerson (Roxar)
[141–143]
Halliburton [
144
]
Schlumberger
[145,146]
Weatherford
[147–149]
Pietro Fiorentini
[150–155]ABB [156]
KROHNE Oil &
Gas [157]
Footprint Small Small Small Small Small Small Small
Radioactive source Optional None X X Optional None None
Range
2600 M/MG: 0–100% WLR
0–85% GVF
2600 MV/MVG: 0–100% WLR
0–100% GVF
0–100% WLR
0–100% GVF
0–100% WLR
0–100% GVF
0–100% WLR
0–100% GVF
Flowatch 3I/HS:
0–100% WLR
0–97% GVF
Wetgas meter:
0–100% WLR
0–100% GVF
0–100% WLR
0–100% GVF
0–100% WLR
0–98% GVF
Technology EIT VFM Gamma
densitometry
VFM (venturi,
gamma,
sonar array).
Venturi, gamma
densitometry, EIT. VFM (Orifice plate) Magnetic
resonance (MR)
Accuracy
2600 M/MG:
Liquid rate:±8–10% relative.
Gas rate:±8–10% relative.
Water cut:±3–5% absolute.
2600 MV/MVG:
Liquid rate:±3–5% relative.
Gas rate:±6–8% relative.
Water cut:±2–4% absolute.
±5%
Liquid rate:±5%.
Gas rate:±5%.
Water cut:±2%.
Flowatch HS:
Liquid rate:
±3% relative.
Gas rate:
±5% relative.
Water cut:
±2% absolute.
Gas: (GVF > 99%):
2% of reading.
Gas: (90% < GVF > 100%):
3% of reading.
Gas: (80% < GVF > 90%):
5% of reading.
Gas: (GVF > 80%):
10% of reading.
Liquid: (90% < GVF >
100%): 5% of reading.
Liquid rate:
3–5% MV.
Gas rate:
8–10% MV.
Water cut:
3–5% MV.
Repeatability <2% ±1% ±2%
Advantages Non-radioactive solutions and
add-on equipment available.
No radiation.
Simple and
cheap solution.
Measurements
in full range.
No flow
calibration plus
add-on equipment.
Measurements in
full range.
Add-on options
available.
Measurements in
full range.
Non-radioactive
solutions and
add-on
equipment available.
No radiation. Simple and
cheap solution.
Measurements in
full range.
No radiation.
Disadvantages Not full GVF depending
on model.
Radioactive
source.
Radioactive source.
Not full GVF
depending
on model.
Much calibration
before start-up. Not full GVF.

Sensors 2019,19, 2184 16 of 26
5. Discussion of Advantages and Disadvantages Regarding MPFMs
The next sections will cover a discussion of the instrumental perspective of MPFMs based on the
cost, maintenance, footprint, radioactive source and calibration. This will contribute to the conclusion
and the future predictions for the MPFMs.
5.1. Cost
The price for a MPFM can vary between 100,000–500,000 US dollars, depending on the
requirements for the meter [
42
]. The prices will vary depending on whether the MPFM is installed
on- or offshore or if the location is topside or subsea. The operational cost for a MPFM is around
10,000–40,000 US dollars per year. This is a huge saving compared to the conventional test separator,
which has an operational cost around 350,000 US dollars per year [
42
]. It is clear that VFMs are
much cheaper compared to other MPFMs, as they only consist of measurements based on simple
conventional field instruments and empirical algorithms.
5.2. Maintenance
Whether the MPFM consists of pressure and temperature sensors, gamma-ray source etc. can have
an influence of the maintenance of the instrument. Maintenance is always a cumbersome procedure
when it is performed offshore. If maintenance is performed on an unmanned platform, the procedure
can be challenging and expensive due to shipping of the right personnel to the platform. Another
contributing factor to the expenses of maintenance is whether the equipment i located sub sea or above
sea level. If the equipment is located sub sea the maintenance is assigned specially educated divers
and consultants, which is a cumbersome and expensive procedure [158,159].
Pressure and temperature sensors are installed so that they can easily be adjusted or replaced by
new sensors, which makes maintenance more doable. As illustrated in Figure 8, a sensor is measuring
either the pressure or temperature on a fluid flowing inside a pipe. The valve can easily be shut down
without any interruption of the fluid, and the sensor can be replaced without a complete shut down
of the oil production. If the MPFM receives the mass flow and density distribution from a coriolis,
the maintenance and replacement can cause more difficulties [
160
,
161
]. Even though the coriolis is
suppose to have lower maintenance due to no moving parts, it can still cause problems. The coriolis
is installed inline of the pipe as illustrated in Figure 9, and maintenance such as replacement of the
coriolis will require a complete shutdown of the production.
Figure 8.
A PT sensor placed on a pipe. The PT sensor is replaced without causing any affection on the
oil production due to the location of the valve.
Figure 9.
A coriolis meter placed inline of the pipe. Replacement of the meter will cause a shut down
in the oil production in the given location.

Sensors 2019,19, 2184 17 of 26
Maintenance of a gamma source is not required often. The radiation of the source will decay over
time with respect to the half-life time of the radioactive source. If a gamma source is to be maintained
or replaced, the procedure is expensive as specialized and certificated personnel is shipped to the
concerned platform.
5.3. Footprint
The footprint of the equipment is essential at offshore installations. A compact platform makes
huge constructions and equipment impossible and the footprint of a MPFM should be as small as
possible. Therefore a solution with an inline MPFM is preferred instead of the huge test separator.
5.4. Radioactive Source
Radioactive materials requires careful supervision during execution, operation and disposal.
Handling radioactive materials requires permission and experts to meet the given law [
162
,
163
].
This means that a MPFM consisting of a radioactive source requires special educated employees, when
handling the meter. This is contributing to a higher OPEX and CAPEX due to external employees
shipped to the offshore platform for implementing, operating and disposal of the radioactive material.
5.5. Calibration
For the MPFM to operate, it needs calibration in the form of input data from time to time.
Especially VFMs will need much calibration before start-up to estimate and produce the empirical
equations for the software. Contrary to this other MPFM will only need calibration when the accuracy
of the data is drifting over time. PT-sensors have a long-term stability and are calibrated with respect to
the electric signal from the sensor. Over the time a PT-sensor will drift from the initial zero point offset.
By calibration, the zero point offset can be re-adjusted increasing the accuracy. Pressure calibration is
done by venting the sensor with ambient air and hereby trimming the offset, so that the zero point is
again matching [
164
]. Gamma-ray source requires special educated personnel and strictly permissions
and is therefore, a more comprehensive technology to calibrate. The calibration of a gamma source
depends on the half-life time of the source [
121
]. For a given period stated by the manufacturer,
the radioactive source must be calibrated to take account for the loss of intensity of the source. If the
half-life time is short, the calibration needs to be performed more frequently compared to a radioactive
source with a longer half-life time. The calibration is done by measuring the count-rates of the radiation
with respect to each single phase [118,165–167].
6. Conclusion and Predictions for MPFMs
This review article has presented and discussed the newest trends in the offshore oil and gas
industry with respect to multi-phase flow measurements. As stated in this article, the accuracy
and reliability of multi-phase flow measurements are essential for allocated production data and
model prediction. The value of MPFMs can be of great importance over the entire lifetime of a field,
but especially as mature fields turn into brown fields over the production life of the field. Hence, it is
essential to monitor production and stage of each well as this can estimate the lifetime and future
development of a field. In the latter years of the production life of a well, the focus of production
is increasing rather than the exploration of the well. For mature wells, it is extremely important to
accurately measure the multi-phase flow, as the water cut will increase and the reservoir pressure
will decrease.
Tomography and gamma-ray densitometry have been widely investigated, and commercial
meters have been developed. The technologies have been further developed, and some new and
promising solutions and prototypes have been tested. Some of the prototypes show measurement
errors of less than 0.53%. Commercial MPFMs where illustrated and discussed as well. The most
accurate MPFM is a gamma densitometer based on experiments of the prototypes and data sheets of
the commercial products. Even though gamma densitometers have the greatest overall accuracy and

Sensors 2019,19, 2184 18 of 26
are able to measure the flow independent of the composition of the phases, the radioactive source is
often a considerable limitation. Maintenance and calibration of a radioactive source requires special
demands and safety to respect the law. A MPFM solution without a radioactive source is therefore,
preferred. Sensor fusion-based DP meters with a software tool (VFM) have also shown progress in
recent years. This method is to be preferred in most cases, as this is non-intrusive, non-radioactive
and cheap compared to other MPFM technologies. Although the accuracy of VFMs is not as great as
gamma densitometers, the solution should be greatly considered due to less maintenance requirements
and price.
The number of MPFMs and investigations of accurate and intelligent technologies of multi-phase
flow measurement are expected to continuously increase and expand in the coming years because
of maturing fields and the focus upon continuous oil production. The overall issue is to design a
commercial solution, which can accurately measure the entire GVF and provide accurate and real-time
measurements. This should be without compromising safety and the footprint on the respective
platform. Other essential qualities for future MPFMs are low maintenance, availability and easy
operation. A possible solution could therefore, be further investigation of VFMs, as this has the
potential to fulfill the qualities for accurate and reliable multi-phase flow measurements. Eventually
VFMs could be combined with e.g., tomography technologies in sensor fusion to obtain even more
accurate and reliable meters.
Author Contributions:
Manuscript preparation, L.S.H.; Reviewing and editing, L.S.H and S.P and P.D.;
Supervision, S.P and P.D.
Funding:
This research was funded by Danish Hydrocarbon Research and Technology Centre (DHRTC) and
Aalborg University (AAU) joint project—Virtual Flow (AAU Pr-no: 886037).
Acknowledgments:
The authors would like to thank DHRTC for the financial support for the project and Aalborg
University for supporting the publication charges of this review article.
Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
aCross-section of pipe divided with equal area
ACross-section area of the pipe
Aori Area of orifice plate
ANN Artificial neural network
CCapacitance
CdDischarge coefficient
Cd,cDischarge coefficient for compressible flows
Cd,iDischarge coefficient for incompressible flows
CAPEX Capital expenditures
dpip e Diameter of pipe
dvc Diameter of pipe at Vena Contracta
DInternal diameter of the pipe
DP Differential pressure
ECT Electrical capacitance tomography
EIT Electrical impedance tomography
EMT Electromagnetic tomography
ERT Electrical resistance tomography
EVT Electromagnetic velocity tomography
fi(x)Volume fraction of phase x
FM Flow meter
FrgGas densimetric Froude number
FrlLiquid densimetric Froude number
gGravitational force
GVF Gas volume fraction

Sensors 2019,19, 2184 19 of 26
LBP Linear back projection
mgMass flow rate of the gas
mlMass flow rate of the liquid
MMass flow rate
MIT Magnetic induction tomography
MR Magnetic resonance
MPFM Multi-phase Flow Meter
MWT Microwave tomography
NNumber of elements
NIR Near infra red
OPEX Operating expenses
P Pressure
PET Positron emission tomography
PEPT Positron emission particle tracking
PT Pressure and temperature
PW Produced water
QVolumetric flow rate
QxVolumetric flow rate of phase x
Usg Superficial gas velocity
Usl Superficial liquid velocity
vgInstantaneous velocity of gas
viFlow velocity in the i-th element
voInstantaneous velocity of oil
vwInstantaneous velocity of water
VPotential difference
VFM Virtual Flow Meter
VMS Virtual Metering System
WLR Water liquid ratio
XLM Lockhart-Martinelli wet gas parameter
YExpansion coefficient
zChange in elevation
αGas void fraction
βWater fraction
βdRatio between diameter of pipe and diameter of orifice
χOil fraction
ΓElectrode surface
εPermeability distribution
εr(gas)Permittivity of gas
εr(oil)Permittivity of oil
εr(water )Permittivity of water
φElectrical potential distribution
ρgDensity of gas
ρlDensity of liquid
ρoDensity of oil
ρwDensity of water
σConductivity distribution
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