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Page 1: Multiphase flows sensor response database

Flow Measurement and Instrumentation 14 (2003) 219–223www.elsevier.com/locate/flowmeasinst

Multiphase flows sensor response database

Hoi Yeung∗, Abba IbrahimCranfield University, Cranfield, Bedford MK43 0AL, UK

Abstract

The requirement for reliable and yet cost effective multiphase meters has recently become increasingly more important. Multiphasemeters are especially needed in cases when several operators share the use of production and transportation facilities in the exploi-tation of marginal oil and gas reserves.

The employment of a sensor array comprising a combination of conventional instrumentation is seen as a cost effective andreliable solution to multiphase metering due to their simplicity, robustness and reliability.

As part of the UK National Flow Programme, Cranfield University, working in conjunction with the National Engineering Labora-tory (NEL), has collected the response from a range of sensors when subjected to three phase (oil/water/gas) flow conditions. Theultimate objective of the project is to enable the industry to develop novel signal processing techniques so that the flow rates ofthe three individual phases can be determined from simple, robust sensors. This database is available to industry and academia forthe evaluation and development of signal analysis and measurement strategies from the Flow Programme via Cranfield University.

This paper presents the detail of the sensor spool piece, experimental arrangement and tests that were carried out. The sensorspool piece comprising of two capacitance meters, two conductance meters, a gamma densitometer, an absolute pressure transducer,two differential pressure transducers and a thermocouple, was designed and assembled by Cranfield University. After the initialcommissioning trials, the spool piece was transferred to NEL. Data was collected over a wide range of flow conditions with twosalinity levels of 50 and 100 g/l MgSO4. Tests were also carried out at two different locations in the flow loop so that the usercan explore possible installation effects on the data. A total of 531 data test series are in the database. 2003 Elsevier Ltd. All rights reserved.

Keywords: Multiphase; Signal processing; Sensor response

1. Introduction

In the drive to reduce costs in the exploitation of mar-ginal North Sea oil and gas reserves especially in remoteenvironments, i.e. where subsea tiebacks are employed,it is now necessary for operating companies to shareresources and minimise the use of bulky equipment off-shore. This increases the use of shared production andtransportation facilities and reduces the use of fluid sep-aration equipment. As a consequence, in order to controlproduction to allocate produced fluids to a particularsource, the flow has to be measured in its mixed ormultiphase state. This means that multiphase metershave become vital for the effective exploitation of mar-ginal oil and gas reserves.

∗ Corresponding author.E-mail address: [email protected] (H. Yeung).

0955-5986/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0955-5986(03)00028-1

Realising the need for reliable and accurate multi-phase meters, the oil industry and government bodies,over the last decade, have supported the developmentand demonstration of multiphase metering technology.As a result, multiphase meters are increasingly deployed.However, the cost of multiphase meters remains rela-tively high. The employment of a sensor array compris-ing of a combination of conventional instrumentation hasbeen seen as a cost effective and potentially reliable sol-ution to multiphase metering due to their simplicity,robustness and reliability. As a result of a survey projectin the 1996–1999 Flow Programme, National Measure-ment System Policy Directorate, The Department ofTrade and Industry, UK, it was recommended that asensor database for multiphase flow should be created.This is because the cost of collection of sensor responseto multiphase flows under representative conditions isexpensive and beyond the means of research institutionsand small innovative companies. If such a database wereavailable, it would encourage the development of innov-

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220 H. Yeung, A. Ibrahim / Flow Measurement and Instrumentation 14 (2003) 219–223

ative signal analysis techniques which is the key to thedevelopment of cost effective multiphase meters.

In the 1999–2002 Flow Programme, Cranfield Univer-sity has assembled such a sensor spool piece. The sensorresponse was collected over a wide range of oil, waterand gas flowrates in the Multiphase Flow Facility ofNEL. This paper summarises the rational of the selectionof the sensors and test conditions. Typical response sig-nals are also included.

2. Sensor spool piece

2.1. Selection criteria

There is a wide range of sensors (both commercialand laboratory) that are used to study multiphase flows.They can be intrusive or non-intrusive. For betterreliability and easier maintenance in industrial con-ditions, non-intrusive sensors are more desirable as theyare less prone to erosion, corrosion or excessive pressuredrop problems. Some of the non-intrusive sensors tomeasure multiphase flows are:

i. absolute pressureii. temperatureiii. differential pressureiv. conductancev. capacitancevi. gammavii. X-rayviii. microwaveix. ultrasoundx. infrared

The absolute pressure and temperature sensors areprimarily used to determine the average conditions at themeasurement point for the estimation of fluid propertiessuch as density, viscosity and surface tension rather thanthe characteristics of multiphase flows.

In the selection of sensors used in the sensor array,the following criteria were considered:

i. known behaviour in oil/water/gas flowsii. frequency (or dynamic response)iii. complexity of sensor output processingiv. commercial availabilityv. costvi. non-intrusive designvii. reproducibilityviii. ruggedness/complexity

We acknowledge that there is a limit to how manysensors could be tested. Thus, in the selection, prefer-ence was given to sensor technology that is well under-stood, developed and, preferably, commercially avail-

able. The latter will give confidence for the eventualdeployment of the technology. It is important that theoutput of sensors does not require very complex dataprocessing, as this would limit the integration with othersensors. Multiphase meters are often located in harshenvironments and the sensor has to be rugged and robust.

Potential sensors were then rated A, B or C againstthese criteria, with A representing the best and C beingthe worst as shown in Table 1.

2.2. The sensor array

Using the consideration mentioned above, a combi-nation of sensors was eventually selected. All the sensorsand equipment are commercially available. The sensorsare generic in nature and thus their response should besimilar to sensors of other makes. This means that thetechniques developed by the user of the database shouldbe applicable to other similar sensors and thus theirefforts will not be restrained and restricted to that of thespool piece.

The multiphase sensor spool comprises of sensors inthe following order:

Sensor Distance fromdensitometer(mm)

1 Gamma ray densitometer 02 Capacitance sensor 1 5903 Conductance sensor 1 10604 Capacitance sensor 2 15005 Conductance sensor 2 19606 Absolute pressure transducer 23307 Differential pressure 2640

transducer 18 Differential pressure 2740

transducer 29 Thermocouple 2840

Table 1Selection of sensors

Sensor Criteria

i ii iii iv v vi vii viii

Absolute pressure A A A A A A A ADifferential pressure A A A A A A A AConductance A A A B B A A AImpedance A A A B B A A AGamma A B A A B A A BX-ray A B B B C A A BUltrasonic B B B B B A B BMicrowave B B B B B A B BInfrared/optical B C C B B A B B

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221H. Yeung, A. Ibrahim / Flow Measurement and Instrumentation 14 (2003) 219–223

Fig. 1. Sensor spool piece as in NEL.

The overall length of the spool piece is about 3.43 mlong and weighs about 400 kg as shown in Fig. 1.

The temperature and absolute pressure transducers areused to determine the properties of the mixture. Thegamma ray densitometer provides a measurement of themixture density. The capacitance and conductance sen-sors are needed to cope with oil continuous and watercontinuous flows. It is envisaged that when the capaci-tance, conductance and gamma densitometer sensors areconsidered together, they should be able to give the rela-tive composition of the three phases. The differentialpressure sensors measure the differential pressure acrossthe top and the bottom of the pipe (the pipe is mountedhorizontally), the measurement is an indication of theliquid holdups. All these sensors are responsive to flowregime changes. The use of two capacitance sensors, twoconductance sensors and two differential pressure trans-ducers allows cross correlation between the sensor pairsto determine phase velocity.

3. Experimental arrangement

The multiphase test facility is based around a 3-phaseseparator, which contains the working bulk fluids. Theoil and water are recirculated around the test facilityusing two variable speed pumps. Nitrogen is used as thegas phase. Oil and water used are stored in tanks underthe separator and in tanks kept outside the building. Theoil used for this test is a mixture of Forties and Berylcrude oil—D80, topped to remove light ends andincrease flashpoint to about 75 °C, with kerosene addedto restore original viscosity (approx. 33° API gravity).

Heat exchangers allow the temperature of the oil andwater to be maintained within ±1 °C over the range ofapproximately 10–40 °C.

The spool piece was mounted at two locations. In thefirst location, it was furthest away from the mixing sec-tion so that the flow pattern was developed at the spoolpiece. The second location was much nearer to the mix-ing section where the flow pattern was not fully

developed. A 100 mm (4 in.) Perspex visualisation sec-tion was installed immediately upstream of the sensorspool piece so the operator can give a subjective classi-fication of the flow pattern.

The flowrates of nitrogen, water and oil were variedso that the spool piece was subjected to a wide range ofwater cut and gas void fractions (GVF).

Fig. 2 shows the full test matrix plotted on a flowregime map. This is a flow regime map typical of flowregimes generated by the flow loop. Tests were carriedout with water salinity of 50 and 100 mg/l of MgSO4.A total of 531 test conditions were recorded during thetest programme.

Traditionally, the ‘dynamics’ of a sensor signal isoften filtered out. With multiphase flows, however, thesmall signals that were considered as noise could beimportant. For example, the signal from a differentialpressure transducer could contain information of the sur-face waves in stratified flows. The signals from the sen-sors were sampled by a dedicated computer at a rate of250 Hz.

4. Sensor database

4.1. Typical sensor signals

Typical gamma densitometer, capacitance sensor,conductance sensor and differential pressure transducer

Fig. 2. Flow regime map showing the matrix of data collected.

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Fig. 3. Gamma count versus time.

signals are shown in Figs. 3, 4, 5 and 6, respectively.The conditions for the test are as follows:

Water cut 45%Gas volume fraction 70%Superficial gas velocity (Usg) 1.86 m/sSuperficial liquid velocity (Usl) 0.74 m/sObserved flow regime

SlugWater salinity 100 g/l MgSO4

Temperature 42OCPressure 5.17 bar

Fig. 3 shows the output from the gamma densitomer.Because of the statistical nature of gamma radiation, inmost applications gamma densitometers output an aver-age reading (typically 8–10 Hz) over a sampling period.This low data frequency is unsuitable for this project.Instead, the raw count rate was sampled every 1 ms (1kHz) so that the time varying information could beobtained. The output from the capacitance and conduc-tance sensors are shown in Figs. 4 and 5. As the flowunder the test condition was oil continuous, the conduc-tance sensor response is essentially constant. This is incontrast to the capacitance sensor with the sensor output

Fig. 4. Output from capacitance sensors.

Fig. 5. Output from conductance sensors.

Fig. 6. Output from differential pressure transducers.

fluctuation showing the presence of liquid slugs. Thereis a time lag between the two signals. The signals arevery similar though the dissipation of the waves at theslug tail is clearly shown. The differential pressuresacross the top and bottom of the pipe are shown in Fig.6. Again the signals are very similar. There is a surgein pressure pulses as the slug passes.

4.2. Sensor database

The sensor signals for the 531 tests were recorded onfive CD-ROMs, a total of 2.7 GB of data. The data fileswere in ASCII format so that they could be easilyretrieved for further analysis. The air, water and oil flowrates, temperature and absolute pressure at the spoolpiece are also included in each of the data files. A tem-plate for visualisation of the signals is also provided.Full instructions and technical information about the sen-sors are also included on the CDs.

These CDs are now available to industry and acade-mia who are interested in developing advanced signalanalysis techniques and to develop cheap and robustmultiphase meters. The data CDs can be obtained fromCranfield University, [email protected]

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5. Conclusion

In responding to the needs of the industry, a set ofsensor response under a range of three phase (oil, water

and gas) conditions have been collected. This data isnow available to industry and academic researchers whoare interested in developing advanced signal analysistechniques for multiphase metering.