EVALUATION OF ULTRASONIC TECHNOLOGY FOR MEASUREMENT OF MULTIPHASE FLOW

Transcription

1 EVALUATION OF ULTRASONIC TECHNOLOGY FOR MEASUREMENT OF MULTIPHASE FLOW A Report for National Measurement System Directorate Department of Trade & Industry 151 Buckingham Palace Road London, SW1W 9SS Project No:FEMU04 Date: November 004

2 The work described in this report was carried out under contract to the Department of Trade & Industry ( the Department ) as part of the National Measurement System s Flow Programme. The Department has a free licence to copy, circulate and use the contents of this report within any United Kingdom Government Department, and to issue or copy the contents of the report to a supplier or potential supplier to the United Kingdom Government for a contract for the services of the Crown. For all other use, the prior written consent of TUV NEL Ltd shall be obtained before reproducing all or any part of this report. Applications for permission to publish should be made to: Contracts Manager TUV NEL Ltd Scottish Enterprise Technology Park East Kilbride G75 0QU jduff@nel.uk Tel: +44 (0) TUV NEL Ltd 004

3 NEL East Kilbride Glasgow, G75 0QU, UK Tel: Fax: EVALUATION OF ULTRASONIC TECHNOLOGY FOR MEASUREMENT OF MULTIPHASE FLOW A Report for National Measurement System Directorate Department of Trade & Industry 151 Buckingham Palace Road London, SW1W 9SS Prepared by: Mr C Coull Approved by: Mrs J A Sattary Date: November 004 for Mr M Valente Managing Director Project No: FEMU04 Page 1 of 47

4 CONTENTS Page. SUMMARY INTRODUCTION... 4 BACKGROUND PRINCIPLES OF MEASUREMENT Transit-Time Ultrasonic Flowmeters Pulse Echo Level Measurement CONCEPT METER DESIGN Design Overview Flowmeters Pulse-Echo Interface-Level System Data Analysis EXPERIMENTAL PROGRAMME TEST FACILITY RESULTS Level System Performance Gas Meter Performance Liquid Meter Performance Multiphase Operational Zones Multiphase Results DISCUSSION Factors Affecting Flow Measurement Effect of Low Water Cut Level in Oil Effects of Gas Traducers Developments for a Practical Ultrasonic Multiphase Meter CONCLUSIONS FUTURE WORK ACKNOWLEDGEMENTS REFERENCES Project No: FEMU04 Page of 47

5 SUMMARY An investigation into the feasibility of employing ultrasonic flow measurement techniques to measure multi-phase flow has been performed. A concept meter has been built, employing two commercially available multi-path ultrasonic flowmeters (one designed for single-phase gas, the other for single-phase liquid measurement) and a pulse-echo level measurement system. The meter has been tested in the multi-phase test facility at NEL. Its performance has been analysed in -phase mixtures of oil / nitrogen and water / nitrogen. The meter was tested in mainly stratified flows but also in stratified wavy and wave/slug flow. The multi-phase tests have demonstrated that gas flow measurements can be made to an uncertainty of +/- 5% of reading over a wide range of gas and liquid flowrates. They have also shown that liquid flow measurement is also possible, with a higher uncertainty of +/- 40% of reading under the test conditions. Reasons for the higher uncertainty in liquid flow measurement have been reviewed and consideration has been given to developments in the ultrasonic techniques to improve this measurement uncertainty. Project No: FEMU04 Page 3 of 47

6 1 INTRODUCTION The measurement of multiphase flow has become a vital component in the economic viability of many marginal oil and gas fields, particularly in the UK continental shelf. To reduce capital expenditure, most new fields are now being commingled and tied back to existing platform infrastructures for processing, and established facilities are being de-manned and automated. This has raised a requirement for reliable and lowcost measurement systems to facilitate well testing, production optimisation and commercial allocation of the produced fluids. Multiphase meters are invariably used for this purpose but remain too expensive for the ideal deployment of one unit per well-head, they also frequently use radioactive sources. Ultrasonic flowmeters, on the other hand, offer the possibility of a cheaper, more environmentally acceptable solution, with the added ability to operate at high gas volume fractions (90% +), where the accuracy of multiphase meters deteriorates rapidly. Previous Flow Programme research [1]* has shown the potential benefits of ultrasonic technology in the measurement of multiphase and wet gas flows. This report describes the research project FEMU04 Evaluation of Ultrasonic Technology for Measurement of Multiphase Flow of the National Measurement System three-year Programme for Flow BACKGROUND This project began with a wide-ranging review of ultrasonic technology and its application to multi-phase flow measurement. The survey considered the implications of the proposed ultrasonic meter design and determined the parameters for the experimental work. The proposed design was a natural development of the ultrasonic metering experiments performed for the National Measurement System Programme for Flow [1]. Figure 1 shows the basic meter concept. Gas and liquid flow velocities are measured separately by two ultrasonic flowmeters. Each flowmeter is designed to measure either single-phase gas or liquid flow. An ultrasonic device to measure liquid level in the pipe is also included, from which the relative cross-sectional areas of the liquid and gas can be inferred. With this information it is possible to make measurements of gas and liquid volume flow rates. Gas Ultrasonic Meter Ultrasonic Level Device Liquid Ultrasonic Meter Gas Velocity Gas Area Liquid Area Liquid Velocity Gas Volume Liquid Volume Flow Flow Figure 1 Basic Multiphase Meter Concept Numbers in parentheses [] denote references at the end of this report. Project No: FEMU04 Page 4 of 47

7 The design requires that the flow remains in the stratified regime. This is important for two reasons; firstly, transit-time ultrasonic meters are adversely affected by dualphase flow e.g. bubbles in the liquid or liquid droplets in the gas. The ultrasonic signal is diffracted and scattered at the phase boundaries and the phases have to be well separated, as they are in stratified flow. Secondly, the ultrasonic levelmeasurement system requires a defined boundary between gas and liquid, which is again provided by the stratified regime. Figure shows the possible options for an ultrasonic pulse-echo level measurement system. Ultrasound can be transmitted via the gas phase (A) or through the liquid phase (B); the reflective characteristics of the gas / liquid boundary are acceptable in either direction. Figure Possible level measurement designs The main objective of this project was to evaluate the performance of the proposed design concept in two-phase flow, oil / gas and water / gas, in predominately stratified flow conditions. 3 PRINCIPLES OF MEASUREMENT The ultrasonic multiphase meter proposed includes transit-time ultrasonic meters and a pulse-echo level detection system. This section describes the fundamental principles on how these systems operate. 3.1 Transit-Time Ultrasonic Flowmeters Ultrasonic transit-time flowmeters are based on measurement of the propagation time of acoustic waves in a flowing medium. Generally, the assertion that the apparent velocity along a ray is given by the velocity of sound in the fluid at rest, c f, plus the component of fluid velocity along the ray is applied. To eliminate the velocity of sound from the subsequent derivation, transit times are determined both in the direction of flow and against it. Considering the general ray geometry shown in Figure 3, the upstream and downstream transit times are given by: t ab L = and t ( c v cosθ ) f ba L = ( c + v cos θ) f (1) where v 1 b = vdx () a L Project No: FEMU04 Page 5 of 48

8 d a a l L θ D b b Figure 3 - General Ray Geometry for Transit-Time Velocity Measurement There are four basic methods by which transit-time velocity measurement is performed; direct time differential, phase differential, phase control, and frequency differential. In modern ultrasonic flowmeters the direct time-differential method is most common. Short duration pulses are propagated either sequentially or simultaneously upstream and downstream and the time duration between excitation and reception is determined. The expressions for the upstream and downstream transit times are then solved for the path velocity ( v ) as follows: 1 1 = t t ba ab ( cf + v cosθ) ( cf v cosθ) L (3) t ab t t ab t ba ba v = cosθ (4) L v L t L = = cosθ t t d t t t ab ba ab ba (5) Multiplying v by the cross-sectional area of the flow, A, the volumetric flowrate is obtained. q V = A L d t t t ab ba (6) The transit-time ultrasonic meters used for this project were multiple-path meters (4 parallel paths), these meters calculate the path velocity ( v ) for each path and then calculate the mean velocity using numerical integration techniques. Multiplying this mean velocity by the cross-sectional area returns the measured flowrate. The multiphase meter detailed in this report uses the individual path velocities along with the cross-sectional area determined from the level-measurement device to calculate flow. Project No: FEMU04 Page 6 of 48

9 3. Pulse - Echo Level Measurement In multiphase flows, propagating ultrasonic waves will undergo reflection at the interfacial boundaries between the gas and liquid components. The primary descriptor of this interaction is the characteristic acoustic impedance R = ρ c where ρ is the medium density and c is the propagation velocity of sound (VOS) in the medium. In general practice the VOS in fluids is considered independent of the ultrasonic frequency. The intensity transmission (αt) and reflection (αr) coefficients for normal incidence at a boundary may be obtained from the following expressions ( ) α t = RR R + R ( ) ( ) α r = R R R + R 1 1 (7) (8) where the subscripts indicate the respective media forming the interface. Table.1 below gives approximate acoustic properties for a selection of gaseous, liquid and solid media. Table 1 Approximate Acoustic Properties Medium Steel Perspex Water Air Longitudinal wave velocity, c (m.s -1 ) Density, ρ (kg.m -3 ) Characteristic impedance, R (kg.m -.s -1 ) The significant acoustic impedance mismatch between gaseous and solid media gives rise to interactions that are problematic in relation to many ultrasonic flow measurement techniques. However, the strong reflective nature of the gas/liquid interface can permit the measurement of the interface location. The principle of interface position measurement using ultrasound is based upon measurement of the transit time of the ultrasonic signal as illustrated in Figure 4 and described by the simple equation d = c t 1 (9) Transducer c 1,ρ1 c,ρ d Figure 4 - The Ultrasonic Interface Measurement Technique Project No: FEMU04 Page 7 of 48

10 By using the simple model of stratified flow shown in Figure 5 it is possible to convert the interface level h L into a cross-sectional area for liquid A l and for gas A g. Combining the measurements of gas velocity, liquid velocity and cross-sectional areas it is possible to estimate the volume flows of the gas and liquid by the following simple equation: Q liq = Vliq AL and gas Vgas Ag Q = (10) Gas Oil A L h L Figure 5 Simple Model of Stratified Flow It is by following this basic principle that this project has attempted to measure multiphase flow. More detail of the calculations used in given later in this report. 4 CONCEPT METER DESIGN This section of the report describes the overall measurement principles in the design of the concept multiphase meter. It describes the main components of the concept meter and how they work together to act as a multiphase meter. 4.1 Design Overview The overall design of the meter was fairly simple. The meter was designed to operate ideally in the stratified flow regime, i.e. when gas and liquid are separate with gas running along the top of the pipe and liquid along the bottom. The meter used two 4-path ultrasonic meters, one designed for gas flow measurement and the other for liquid, with horizontal path arrangements; these were supplemented by a pulse-echo interface-level measurement system. These were combined to form the concept multi-phase meter as shown in Figure 6. The meter operated by determining the velocities of the liquid and gas, using the appropriate meter for each measurement. The output flowrate from each meter was not used since this was affected by the presence of a contaminant, i.e. gas in the liquid meter and vice versa for the gas meter. Instead, the individual path velocities were used; for example, in Figure 6 the bottom path of the gas meter fails due to the presence of the liquid, the measurement of the gas velocity was taken from the top three paths. The opposite is true of the liquid measurement, it was taken from the bottom path of the liquid meter. Project No: FEMU04 Page 8 of 48

11 Gas Transducer Standard Transducer Gas Meter Level Sensor Figure 6 Ultrasonic Multiphase Concept Liquid Meter In order to calculate the volume flow of the liquid and gas it is necessary to know not only the phase velocities but also the cross-sectional area of the pipe they occupy. In order to determine this, the height of the interface between the gas and the liquid has to be measured. This was done using a pulse-echo level-measurement system transmitting through the gas and via the liquid phases. Figure 7 shows the concept meter in the multiphase test line. Level Sensor Gas Meter Liquid Meter Figure 7 Concept Meter in Test Line 4. Flowmeters The ultrasonic flowmeters employed in this project were standard commercial meters, designed primarily for single-phase applications. However it should be noted that the gas meter was supplied with transducers specifically designed to operate in wet-gas conditions. This section provides basic details of these meters. Project No: FEMU04 Page 9 of 48

12 4..1 Liquid Ultrasonic Meter Caldon LEFM 40C Model No. LEFM 40C Tube Serial No Conv. Serial No Size 6 The Caldon LEFM40C ultrasonic meter has four parallel paths, all paths are set at 45 degrees to the flow direction in the vertical plane. The path locations are in accordance with the Gaussian integration method and the transducers are a piezoelectric crystal design operating at a nominal frequency of 1 MHz. The meter includes a separately mounted transmitter housing containing the meter s electronic systems. The transmitter performs the excitation and detection of the ultrasonic signal and subsequent data processing and flow measurement calculation. The path velocities are combined using Gaussian-derived weighting factors to determine the volumetric flowrate through the meter. Meter flowrate as well as meter diagnostics, including path velocities, are recorded on a computer using dedicated software via an RS 3 link. 4.. Gas Ultrasonic Meter Sick Maihak FlowSic 600 Model No. FlowSic 600 Serial No Size 6 The FlowSic 600 gas ultrasonic flowmeter has four parallel paths and includes headmounted electronics. The four ultrasonic paths are at 60 degrees to the flow direction and are positioned in a single vertical plane. The paths are spaced at 0.809R and 0.309R for optimised measurement accuracy. The transducers used are designed for wet-gas use. The head-mounted electronics operate the ultrasonic transducers and process the primary measurements to derive the gas volume flow rate. Meter data was recorded on a computer using a dedicated software package via an RS 485 link. 4.3 Pulse-Echo Interface-Level System This section describes the design of the Pulse-Echo Level measurement system and details the bench test results. Design The interface flow cell diagram (Figure 8) shows the design of the levelmeasurement cell. It consists of an 8 NB Perspex spool and two ultrasonic transducers, a gas-coupled transducer and a standard solid / liquid transducer. By using two transducers the system makes two independent measurements of the gas/liquid interface level. Project No: FEMU04 Page 10 of 48

13 Gas Coupled Transducer Gas c g D 8 NB Perspex Section Liquid c l h L c per X Standard Transducer Figure 8 - A Schematic Diagram of the Level Cell The gas-coupled transducer was provided by Imasonic. It was an air-coupled transducer with a single flat element 0 mm in diameter, with an operating frequency of 330kHz. The transducer was in contact with the flow and, to ensure that the pressure across the transducer was equalised, the pipeline pressure was piped to the rear of the active element. The standard transducer had an operating frequency of 1MHz. The standard transducer was secured to the bottom of the flow cell and detected the liquid/gas boundary by transmitting ultrasound through the perspex wall into the liquid phase. The gas-coupled transducer detected the liquid/gas boundary by transmitting from the top of the flow cell directly into the gas. The standard transducer was coupled to the Perspex wall by silicon-based couplant in order to eliminate any air pockets caused by surface roughness. In order to measure the interface position, a data acquisition and control system as shown in Figure 9 was utilised. The system consisted of a PC, a digital oscilloscope, two pulsar/pre-amp units and the two transducers already mentioned. PC GPIB Digital Scope Trig Waveform Pulsar / Pre-Amp Std Ultrasonic Transducer Trig Gas Ultrasonic Transducer Waveform Pulsar / Pre-Amp Figure 9 - A Schematic Diagram of the Ultrasonic System Configuration Project No: FEMU04 Page 11 of 48

14 The pulsar/pre-amp units excited the transducers with a high-voltage pulse that generated an ultrasonic signal, which was transmitted into the pipe. The reflected ultrasonic signals were converted to an electronic signal by the transducer and amplified by the pre-amp. The raw signal was captured by the oscilloscope and recorded and processed by the DAQ software on the PC to determine the liquid-gas interface level. The pulsar repetition rate was controlled by the master pulsar, which sent a triggering signal to both the secondary pulsar and the oscilloscope. The PC controlled the oscilloscope and the data transfer from the scope to the PC. Due to large differences in the time bases required, the waveforms were recorded sequentially to allow the scope s time-base to be adjusted between captures. Time between captures was always less than 00 ms, which was considered insignificant for these tests. The system operated at a sample rate of Hz. A simple threshold-crossing technique was used to determine the arrival of the reflection. Figure 10 shows typical raw ultrasonic signals for the gas and standard transducers and their typical threshold voltages. Amplitude (V) Threshold Voltage (0.V) Transit Time Time (us) Standard Transducer Signal Amplitude (V) Threshold Voltage (0.05V) -0.4 Transit Time Time (us) Gas Transducer Signal Figure 10 Pulse Echo Level System Ultrasonic Signals In cases where the threshold voltage was not reached, the measurement was considered invalid. The non-invasive or non-wetted application of the standard transducer dictated that the transmitted ultrasound encountered the Perspex/liquid interface prior to the liquid/gas interface and the result was a waveform comprised of primary and secondary reflections. A time-window technique was therefore used to ensure that only the gas/liquid reflection was observed by the system. The interface level was calculated using the following simple formulae: For the Gas Transducer h L ( c t ) = D (11) g g Where, h L is the interface level D is the internal diameter c is the speed of sound in the gas and g t g is the transit time of the gas transducer signal from transmission to reception. Project No: FEMU04 Page 1 of 48

15 For the Standard Transducer h L l ( t t ) = c with std per t per X = (1) c per Where c l is the liquid speed of sound c is the perspex speed of sound per t std is the transit time of the standard transducer signal from transmission to reception t per is the transit time in the perspex outward and return and X is the thickness of the perspex wall at the point of transmission. Lengths X and D were carefully measured using a digital micrometer. The sound speeds were determined during the commissioning of the system in the multiphase rig. The Perspex and nitrogen sound speeds were calculated by careful measurement of transit times in completely dry conditions, and the water and oil sounds speeds were calculated during fully immersed conditions. Bench test results Prior to commissioning on the multiphase rig, the system was bench tested using tap water and air, Figure 11. A steel rule with a resolution of 0.5 mm was used as a reference measurement. The bench test showed the interface level measurement to be accurate to within +/- mm over the levels experienced in the multiphase tests (Figure 1). Figure 11 Bench Test of Pulse-Echo system Project No: FEMU04 Page 13 of 48

16 Error (mm) Std Transducer (through Liquid) Gas Transducer Zone of Operation Ref Level (mm) 4.4 Data Analysis Figure 1 Bench Test of Interface Level System (Water / Air) This section describes the data analysis performed on the basic data from the meters and pulse-echo level system. It shows how the basic data was used to obtain multi-phase flow rates. It also defines performance indicators, which are reviewed in Section Calculation of Liquid and Gas Cross-Sectional Areas In order to calculate the volume flow rates of the two phases it is necessary to know both the phase velocity and the cross-sectional area of the pipe that each phase occupies. The cross-sectional areas of the liquid and gas phases were estimated by measurement of the interface level. The Pulse-Echo interface-level system made two independent measurements of level, one through the gas, the other via the liquid phase. For the purposes of this report the interface level was defined as the average of level measurements that achieved the threshold voltage and were verified by the meter path data; this verification is described later in this section. The liquid ( A L ) and gas ( A G ) areas are determined using the simple straight line interface model shown in Figure 5. The areas are calculated using the following equations: A ( ~ 1) + ( ~ 1) 1 ~ ( 1) π cos h L h L h L L = A π 4 G A L A, (13) = A (14) Project No: FEMU04 Page 14 of 48

17 h Where h ~ L L = and A = total pipe area D Under well-stratified flow, the level measurements were observed to be in agreement, generally to within 3-4 mm. However, under certain circumstances, the results obtained varied by a significant degree. In order to determine which of the two level measurements was correct, a level-verification system using information from the two flowmeters was employed. Interface-Level Measurement Verification It was possible to verify the interface-level measurement by monitoring the operational paths on the liquid and gas flowmeters. Figure 13 shows a typical stratified-flow cross-section. In this case the top three gas-meter paths (1,,and 3) operate, while the lowest path (4) fails due to the presence of the liquid. The reverse is true of the liquid meter; only path 4 operates in its case. This information indicates that the interface level must be located between the path 4 of the liquid meter and path 3 of the gas meter. This is considered to be the verified range; any measurements made by the pulse-echo system outside this range are considered to be invalid and therefore are not used in the cross-sectional area calculation. Gas Meter Liquid Meter Level 150 mm Path 1 Path Path 3 Path 4 Figure 13 Example of Level Verification Verified Range 0 mm The verification ranges were set slightly broader than the zones between the paths to ensure that marginal level data was not discarded. Figure 14 shows how the path verification operated in stratified flow with periodic waves. The level measurements clearly show waves occurring approximately 10-1 seconds apart. These can be identified by the gaps in the measurements, showing the front edge of the wave, followed by a high-level peak and a rapid decrease in level after the wave has passed. It can be seen that, as the third wave in the sequence passes, the gas-transducer measurements rise to a very high level, a change that is not matched by the liquid transducer measurement. This effect was observed during the tests and is caused by the wave leaving a droplet of water on the gas-transducer face. The droplet changes the characteristics of the gas transducer making it ring. This ringing causes the pulse-echo system to malfunction and register an exceptionally high level. However, since these levels are outside the verification range, they are rejected. Project No: FEMU04 Page 15 of 48

18 Path Verification Max Interface Level (mm) Accepted Gas Tx Accepted Std Tx Rejected Gas Tx 0 0 Path Verification Min Time (s) Figure 14 Example of Path Verification on Level Measurement 4.4. Calculation of Liquid and Gas Flowrates The liquid and gas velocities were calculated from the indicated velocities of the operating paths in the liquid and gas meters. The restriction of keeping the flow regime in the stratified region meant it was necessary to keep the actual liquid velocity very low, below 0.6 m/s. Gas velocities, however, ranged from 1 to 7 m/s. As a result the flow profiles of the two phases were quite different, as illustrated in the profile diagram in Figure 15. Gas Liquid Figure 15 Illustration of Stratified Flow Profiles Gas and liquid velocity profiles of this type have been observed in actual velocity data collected during the tests. Figure 16 shows path velocities recorded when the liquid level allowed the upper two gas paths and the lower two liquid paths to operate. The reference conditions were 3 l/s oil and 7 l/s gas flow. The path velocities are displayed as percentages of the mean velocity of each phase. It shows the flatter profile of the gas phase. Project No: FEMU04 Page 16 of 48

19 Path 1 Path Gas Mean Velocity.84 m/s Gas Oil Path 3 Path 4 Oil Mean Velocity 0.3 m/s Path Velocity (% of Mean Phase Vel) Figure 16 Path Velocities recorded by Meters The gas flow in these tests was turbulent, hence its velocity profile was fairly flat. It was therefore considered reasonable to estimate the mean gas velocity as the average of the operating path velocities, as the error introduced by flow profile effects would be small compared with other errors in the multiphase measurement. The gas flow rate was calculated as follows: i i Q Gas = x v A g (15) Where i details the operating gas meter paths v i is the path velocity of an operating path x is the number of operating gas meter paths A g is the Gas area as measured by the pulse echo interface level measurement system. The low liquid velocities ensured that the liquid flow regime remained in the laminar or transition regions for most of the testing matrix. Hence the liquid velocity profile was much more peaked than the gas profile. Simply averaging the path velocities in this case would have led to a significant under-reading of the liquid flow. A simple integration method described below was therefore used to calculate the mean liquid velocity. The pipe cross-section is divided into four sectors based on the liquid path locations as shown in Figure 17. The velocity measured by each path was assigned to its pipe sector. The area of the uppermost operational path sector was adjusted to the level measurement made by the pulse-echo interface level measurement system. Project No: FEMU04 Page 17 of 48

20 Path 1 A 1 Path A Path 3 A 3 Path 4 A 4 Figure 17 Liquid Flow Measurement Zones For example, if paths 3 and 4 were operational, the liquid flowrate would be calculated as follows: Q = v A + v A (16) Liq The area of the uppermost sector is adjusted so that the overall area matches that area calculated by the pulse-echo interface-level system. Hence in this case path velocity 3 ( v 3 ) would be multiplied by the adjusted zone 3 area ( A 3 ). A (17) 3 = AL A4 where A L is the liquid area as measured by the pulse-echo interface-level measurement system. In this way the path velocities were weighted to give a more representative estimate of the mean velocity and hence an improved measurement of liquid flowrate Performance Indicators In addition to recording the basic measurements, other diagnostic data was collected. Some of this data has been collated and is used in this report to give a better understanding of how the individual components of the multiphase meter operated. Pulse-Echo Operation Rate (OR P-E ) This parameter describes how successful the pulse-echo system is at detecting an interface level. It is described as No. of Valid Measurements OR P E = *100% (18) Total No. of Measurements A valid measurement requires at least one of the independent measurements (via gas or standard transducer) to have recorded an above-threshold signal that could be verified by meter-path data. In order for the pulse-echo system to be considered operational, the operation rate must be above 30%. Project No: FEMU04 Page 18 of 48

21 Gas-Transducer Pulse-Echo Operation Rate (OR GTx ) Similar to Pulse-Echo Operation Rate, this parameter describes the success rate of the Gas-transducer-based system. Standard-Transducer Pulse-Echo Operation Rate (OR STx ) Again similar to Pulse-Echo Operation Rate, this parameter describes the success rate of the standard-transducer-based system. Number of Paths Gas Meter / Liquid Meter(P G,P L ) This parameter details the maximum number of paths that operated during a test point. Included are paths that operated continuously and those that operated intermittently. Path Reject Level (RL p3/p4 ) This parameter describes the number of unsuccessful measurements on an individual ultrasonic meter path as a percentage of the total number of attempts. Path reject level was used to define path operation / failure. After consideration of the data, reject levels of 50% for the gas meter paths and 80% for the liquid meter paths were selected as the boundary points. These are much higher than would be acceptable under normal single-phase operation of the meters. They were chosen in order to maximise the data available to make multi-phase measurements. 5 EXPERIMENTAL PROGRAMME It was important to test the meter in a stratified-flow regime as this was considered the best conditions for the meter. However, it was also important to determine the limits of its performance and tests in stratified wavy and slug conditions were required. The Flow map produced by Mandhane [], Figure 18, was used to determine a test condition matrix (Table ) that ensured that the meter experienced the range of test conditions required. Project No: FEMU04 Page 19 of 48

22 Figure 18 Multiphase Flow Regime Map by Mandhane [] The experimental programme consisted of two sets of tests, one which tested crude oil with nitrogen and another which tested water with nitrogen. Both tests used the test condition matrix shown in Table. Two test points were performed at each matrix condition and at each test point data were collected for a period of two minutes. Table Test Conditions for Multiphase Tests Liquid Flow (l/s) Superficial Liquid Velocity (m/s) Gas Flow (l/s) Superficial Gas Velocity (m/s) TEST FACILITY The tests reported here were conducted at NEL s multiphase calibration facility. A schematic diagram of the facility, as used for these tests, is shown in Figure 19. The total reserve of oil and water is held in a vessel that acts as a combined storage tank and multiphase separator with water, oil and mixture compartments. The oil and water are drawn from each of the single-phase compartments and are delivered to the flow loop via calibrated reference meters. Also present in the system are sampling loops for the determination of background quantities of oil-in-water and water-in-oil. The gas is injected via gas turbines from an external nitrogen supply. After the oil, water and gas are allowed to co-mingle, the fluids then flow to the test section along a 50 m development length of straight pipe. Project No: FEMU04 Page 0 of 48

23 Separator vessel Water Mixture Oil Pump Densitometer Pump Water-cut it Turbine meters Turbine meters T P Nitrogen supply Turbine meters Test meters Figure 19 - A Schematic Diagram of the Multiphase Facility The oil used in the facility was a crude-kerosene mix with approximate density and viscosity of 846 kg/m 3 and 6.6 cst respectively at the test temperature of 40 C. The water used was a brine solution of density approximately 103 kg/m 3 at the test temperature. 7 RESULTS This section details the performance of the individual components of the concept meter as well showing the overall performance of the meter. 7.1 Level System Performance The pulse-echo system made two independent measurements of interface level, one via a standard transducer transmitting through the liquid phase and another via a gas-coupled transducer. The performance of each measurement and the system as a whole are reviewed. Standard Transducer System Figure 0 shows the operation rates of the standard transducers (OR STx ) in the oil/gas tests. It can be seen from Figure 0 that the operation rate of the standard transducer system is highest at low gas and liquid superficial velocities. High operation rates (i.e. 90% and above) form a triangular zone which extends up to superficial velocities of 0.1 m/s in oil and 3 to 3.5 m/s in gas. This corresponds to the stratified flow region seen in Mandhane s flow map in Figure 18. Outside this region the operation rate drops off steeply. The system operates (i.e. Operation rate >= 30%) up to the maximum tested oil superficial velocity (0.4 m/s) with gas superficial velocities up to 1.35 m/s. At low oil superficial velocities, below 0.1 m/s, it can operate with gas superficial velocities up to 3.8 m/s. Project No: FEMU04 Page 1 of 48

24 Figure 0 Pulse-Echo Operation Rate Standard Transducer Oil / Gas Test A notable exception to the general trend is the high operation level at superficial velocities of 0.17 m/s in oil and 4.4 m/s in gas. This seems to be a false reading caused by an extended Perspex/liquid reflection entering the measurement window and generating a very low liquid-level measurement. This is discussed in Section 8. Normally this false measurement would not be verified by the meter paths, however, due to the failure of the lowest path on the liquid meter, the verified region is extended to the bottom of the pipe and the measurement is accepted. After investigation it is noted that all measurements made by the standard transducer at this flow condition were false and the true operation rate was zero at this flow condition. Identical false readings were also made at the following oil / water superficial velocities, 4.4 / 0.07 m/s and 3.1 / 0.17 m/s. Removing these false readings results in the flow map in Figure 1. Project No: FEMU04 Page of 48

25 Figure 1 Pulse-Echo Operation Rate Standard Transducer Oil / Gas Test Corrected Figure Pulse-Echo Operation Rate Standard Transducer Water / Gas Test Figure shows the operation rate of the standard transducers (OR STx ) in the water/gas tests. It shows that when tested with water the standard transducer operated in a similar way to that seen when it was tested with oil. However, there are some differences to consider. Firstly, the zone of high operation rates (>90%) was significantly reduced, only occurring with water superficial velocities below 0.05 Project No: FEMU04 Page 3 of 48

26 m/s and gas superficial velocities between 1 and 3 m/s. The drop-off seen in the oil /gas tests also occurs in the water / gas test but the gradient is less steep, resulting in the operational area being roughly similar to the oil / gas test. This characteristic may be due to the lower viscosity of the water resulting in a more rippled surface, which will have a tendency to scatter the reflection to a greater degree. As a result the reflection is likely to miss the transducer more frequently. It is also noticeable that the peak seen at 0.17 and 4.4 m/s oil and gas superficial velocities in the oil test was not observed in the these results. A review of the raw signal data confirmed that the extended Perspex / liquid reflection that caused a mis-reading in the oil / gas tests was not present in the water / gas test. This suggests that the background water in the oil may well be responsible for the extended reflection in the oil/gas test (see Section 8.). Gas Coupled Transducer System Figures 3 and 4 show the operation rates of the gas-coupled transducer in oil/gas and water/gas tests respectively. Figure 3 Pulse-Echo Operation Rate Gas Coupled Transducer Oil / Gas Test The gas-coupled transducer system showed similar characteristics to the standard transducer system. The high-operation-rate zone was slightly more restricted than for the standard transducer system in oil, extending to 0.15 m/s oil superficial velocity and.5 m/s gas superficial velocity. The operational zone is, however, significantly greater, extending to the maximum oil velocity tested (0.4 m/s) with a gas superficial velocity up to. m/s. For oil superficial velocities below 0.1 m/s, it can operate with gas superficial velocities up to almost 4 m/s. Project No: FEMU04 Page 4 of 48

27 Figure 4 Pulse-Echo Operation Rate Gas Coupled Transducer Water / Gas Test In the water tests, the gas-coupled transducer system operated at the high operation rate in a small zone up to 0.06 and 1.9 m/s water and gas superficial velocities respectively. Its operational zone extended to the maximum oil velocity tested (0.4 m/s) with a gas superficial velocity up to m/s; and for oil superficial velocities below 0.1 m/s, it operated with gas superficial velocities up to almost 4 m/s. There is an exception at superficial velocities of 1.1 m/s in gas and 0.17 m/s in water, where the operation level falls to 5%. Combined Pulse Echo System Combining the results of the two systems, it is possible to obtain better operational coverage, Figure 5 shows the operation rate of the combined pulse-echo system for the oil / gas test, the false standard-transducer readings shown in Figure 0 have been removed. Project No: FEMU04 Page 5 of 48

28 Figure 5 Pulse-Echo Operation Rate Combined System Oil / Gas Test The combined system benefits from the advantages of both systems, i.e. the greater coverage of the high operation zone at low oil and gas superficial velocities from the standard transducer and the improved operational zone (>30%) at higher oil and gas superficial velocities from the gas transducer. In effect, at low oil superficial velocities (<0.06m/s), the high operation zone (>90%) is maintained up to gas superficial velocities of almost 3.5 m and at higher oil superficial velocities of about 0.15m/s up to gas superficial velocities of 1.5m/s. The operational zone (>30%) extends to gas superficial velocities over 4m/s at low oil superficial velocities (<0.1m/s) and to gas superficial velocities of m/s at maximum oil superficial velocity tested of 0.4m/s. A similar effect is seen in the combined system water / gas test results shown in Figure 6. The combined approach benefits from both the superior operation rate of the standard transducer at low water superficial velocities and from the better operation of the gas transducer at high gas and water superficial velocities. The high operation zone is more restricted than in the oil/gas tests, with a maximum gas superficial velocity of 3 m/s at low water superficial velocities (<0.05 m/s) and up to gas superficial velocities of 1.5m/s at slightly higher oil superficial velocities of about 0.07 m/s. The operational zone (>30%) is similar to the oil/gas tests, extending to gas superficial velocities over 4 m/s at low oil superficial velocities (<0.1 m/s) and to gas superficial velocities of m/s at high oil superficial velocities of 0.4m/s. Project No: FEMU04 Page 6 of 48

29 Figure 6 Pulse-Echo Operation Rate Combined System Water / Gas Test 7. Gas Meter Performance The velocity measurements of each operational path were recorded during the test programme. Due to the presence of the oil or water in the meter body, not all paths operated. Figures 7 and 8 show how many paths were operating during the oil/gas and water/gas tests respectively. Superficial Liquid Vel (m/s) Superficial Gas Vel (m/s) Figure 7 Flow Map of Gas Meter Operational Paths Oil / Gas Test Project No: FEMU04 Page 7 of 48

30 Water Superficial Vel (m/s) Gas Superficial Vel (m/s) Figure 8 Flow Map of Gas Meter Operational Paths Water / Gas Test Figures 6 and 7 show that the gas meter operated throughout both tests with at least two operational paths. It was also observed that all four paths operated at least for a time during both tests. This may have been due to a false reading that occurred infrequently on the path nearest the gas / liquid interface. It was observed that this path would seem to be operating; however the speed of sound measurement was significantly different from the other paths. 7.3 Liquid Meter Performance Prior to commencing the multiphase tests, functional checks were performed. These were carried out in single phase water and oil. The recorded path velocities from these tests are shown in Figures 9 and Path Velocity (m/s) Path 1 Path Path 3 Path Reference Water Velocity (m/s) Figure 9 Path Velocities Recorded in Water Single-Phase Functional Check Project No: FEMU04 Page 8 of 48

31 In the water-phase functional check, the path velocities trends were as expected. The paths velocities were arranged in pairs, the middle paths and 3 recording a higher velocity than the outer paths 1 and 4..5 Meter Path Velocity (m/s) Path 1 Path Path 3 Path Reference Oil Velocity (m/s) Figure 30 Path Velocities Recorded in Oil Single Phase Functional Check This does not occur in the oil-phase functional check, the path 4 velocity reading (the lowest path) under-reads path 1 by up to 0.19 m/s. A review of the meter diagnostics revealed that during the oil single-phase check the signal-to-noise ratio of path 4 was significantly lower than that of the other paths. This was not repeated in the water single-phase check. This may be due to residual water in the oil test. Due to the design of the multiphase facility, there is always a background level of water in the oil phase (typically to 4%). These results are in line with the effects observed on the -phase water/oil tests reported in Reference 1. Figure 30 shows the maximum number of paths that were functional during the oil / gas tests. Due to the presence of the gas in the two-phase tests, not all of the paths on the liquid meter operated. It shows that no more than two out of the four paths operated during the testing. The paths that did operate were, as would be expected, the lower paths, 3 and 4. It was also observed that the liquid meter failed to operate at all (i.e. operational paths = 0) over a significant portion of test matrix at the higher gas and oil velocities. This was mainly due to the low level of oil in the pipeline. Project No: FEMU04 Page 9 of 48

32 Figure 31 Flow Map of Liquid Meter Operational Paths Oil / Gas Test The operation of liquid meter is shown in more detail in Figures 3 and 33, these show the path reject level diagnostic (RL p3/p4 ) for paths 3 and 4 respectively. Figure 3 Oil / Gas Path 3 reject Level Figure 3 shows that path 3 operated (rejection level < 80%) at the low gas superficial velocities, less than m/s, when the liquid level was high enough. Project No: FEMU04 Page 30 of 48

33 Figure 33 Oil / Gas Path 4 reject Level Path 4 is located at a lower position in the pipe and so can operate in lower liquid levels (Figure 33). It continued to operate at up to 4 m/s gas superficial velocity at low liquid velocities (< 0.1 m/s) and up to m/s at higher oil velocities. However a significant anomaly occurred at low gas and liquid superficial velocities. Path 4 failed to operate at two test conditions and only operated partially at three others. The effect observed at 0.5 / 0.05 and 1 / 0.05 gas / oil superficial velocities resulted in path 4 (the lowest path) failing whilst path 3 (the path directly above) continued to operate. The signal-to-noise ratio on path 4 dropped to zero, whilst it remained at normal levels on path 3. This confirms that the path 4 signal was degraded during transmission. It is considered likely that this effect was caused by the background level of water in the oil (-4%). The two-phase mixture running along the bottom the pipe could have diminished the signal mainly by diffraction at the phase boundaries. This could have resulted in the transmitted signal missing its receiver. Project No: FEMU04 Page 31 of 48

34 Figure 34 Flow Map of Liquid Meter Operational Paths Water / Gas Test Figure 34 shows the number of operational paths on the liquid meter during the water / gas test. As with the oil/gas test, no more than two paths were operational, these being the lower paths 3 and 4. However it is noted that the zone of meter failure was significantly smaller in the water / gas test. There were only two test locations, at the highest water and gas velocities, were the meter failed to operate. Figure 35 Water / Gas Path 3 reject level Project No: FEMU04 Page 3 of 48

35 Figures 35 and 36 show the rejection-rate diagnostic data for paths 3 and 4 respectively. As in the oil/gas tests, path 3 only operated at low gas velocities when the liquid level was high enough. However, whereas path 3 continued to operate to almost m/s gas superficial velocity in the oil/gas test, in water/gas it generally only functioned up to 1 m/s gas superficial velocity. It is thought this may be due to the lower viscosity of water, which allows a wavier water / gas interface. Figure 36 Water / Gas Path 4 reject level In the water / gas test path 4 operated over a large area of the test matrix. Unlike in the oil/gas tests, it continued to operate at very low gas and liquid velocities and its operation zone extended to higher gas and liquid velocities. At water superficial velocities below 0.15m/s it continued to operate up to gas superficial velocities of over 4 m/s. At gas superficial velocities below m/s it operated at the maximum water superficial velocity tested. This suggests that in the oil/gas tests path 4 was being affected by the background water level in those tests. 7.4 Multiphase Operational Zones By overlaying the operational zones of the pulse-echo level system and the respective meters it is possible to view on a flow map the zones where liquid and gas measurements could be made. Liquid Measurement Figure 37 shows the operational zone for oil flow measurement; this is indicated by the white background. The grey area indicates where operation was inhibited by failure of the liquid meter and the hashed zone shows where the pulse-echo level system was not operational. It shows that the oil measurement zone is limited by the operation of liquid meter. The positions of the diamonds indicate the superficial velocities for each test. Project No: FEMU04 Page 33 of 48

36 Figure 37 Oil Measurement Zone of Operation Figure 38 shows the operational zone for water measurement; again indicated by the white background. It shows the main limiting factor for operation to be the level measurement system. Figure 38 Water Measurement Zone of Operation Project No: FEMU04 Page 34 of 48

37 Gas Measurement Figure 39 shows the zone in which gas measurement can be made whilst running with oil. The gas meter operated over the full test range and the measurement was therefore limited by the operational limits of the level-measurement system. Figure 39 Gas measurement zone of operation (with Oil) Figure 40 shows the zone in which gas measurement could be made whilst running with water. Again the operational zone was limited by the limits of the level measurement system. Project No: FEMU04 Page 35 of 48

38 Figure 40 Gas Measurement Zone of Operation (with Water) 7.5 Multiphase Results This section describes the final results of the concept meter, in terms of its measurement of oil, gas and water. The results are shown in three forms: a graph of reference flowrate against meter flowrate, a graph of reference flowrate against meter error (as percentage of reading) and as a contoured flow map, with locations given by reference-meter-derived superficial velocities. The graphs show only the results from within the operational zones as described in the previous section. The contoured flow maps are derived from the averaged results at each flow condition. These averaged values are labelled on the maps. Results from the full test matrix are shown on the flowmaps, non-operational areas are cross hatched. All results have been determined as per the data analysis section. Oil / Gas Test Results Figures 41, 4 and 43 show the gas measurement results of the concept meter when operating in two-phase oil and gas flow. Project No: FEMU04 Page 36 of 48

39 Meter Flowrate (l/s) Limits shown +/-1 l/s or +/- 0% Ref Gas Flowrate (l/s) Figure 41 Gas Measurement Results (Oil/Gas Test) Error (% of reading) Ref Gas Flowrate (l/s) Figure 4 Gas Measurement Results % Error (Oil/Gas Test) The results show that, at gas flowrates below 35 l/s the concept meter performs to within a band of 11% (+7% - 4%). Two points at a gas flow of 10 l/s are out with this band. At these points it was observed that path 4 (the lowest path) of the gas meter operated periodically. However, from its measurement (about zero) and its velocity of sound measurement (which was significantly lower than the other paths), it is obvious that the readings were false. Forcing path 4 into the non-operational state removes its contribution to the average gas velocity and results in gas measurement errors of 0.5% and -0.8%, well within the band stated. At flowrates between 35 and 76 l/s the error band is %, (+19% -%). Again, two points lie outside this band, at a gas flowrate of 55 l/s. The error at these points was caused by a false level measurement. At these points the standard level transducer systematically underread, due to an extended Perspex reflection. Removing these Project No: FEMU04 Page 37 of 48

40 false measurements gives measurements with errors of 14.8% and 17.8%, which are again within the band stated. Figure 43 shows the gas measurement results in the form of a contoured flow map. It shows that +/-5% measurement is achievable across a large zone of the test matrix. Figure 43 Gas Measurement Flowmap (Oil / Gas Test) Figures 44, 45 and 46 detail the results of the oil flow measurement in the oil / gas tests. Concept meter Flow (l/s) Limits shown +/-0.75 l/s or +/- 40% Ref Oil Flowrate (l/s) Figure 44 Oil Measurement Results (Oil / Gas Test) Project No: FEMU04 Page 38 of 48

41 Oil Flow Error (% of reading) Ref Oil Flowrate (l/s) Figure 45 Oil Measurement Results % Error (Oil / Gas Test) At flowrates above.5 l/s, the average error is approximately -0% with all results falling between -10 and -30%. Below this flow rate, errors increase substantially to a range from +40% to -65%. Overall, most results lie within the limits shown in Figure 44, of +/-0.75 l/s or +/- 40%. Figure 46 Oil Measurement Flowmap (Oil / Gas Test) Project No: FEMU04 Page 39 of 48

42 Water / Gas Test Results Figures 47, 48 and 49 show the results of the gas measurement in two-phase water and gas flow. Meter Flowrate (l/s) Limits shown +/-1 l/s or +/- 0% Ref Gas Flowrate (l/s) Figure 47 Gas Measurement Results (Water/Gas Test) Error (% of reading) Ref Gas Flowrate (l/s) Figure 48 Gas Measurement Results - % error (Water/Gas Test) These results show that over its operational range the meter performs to within a error band of 5%, (+18% to -7%). The results have a generally positive bias across the gas flowrate range of approximately + 6%. Project No: FEMU04 Page 40 of 48

43 Figure 49 Gas Measurement Flowmap (Water / Gas Test) The flow map of the gas results (Figure 49) shows the gas measurement performance to be within +/- 5% over a substantial area of the flow matrix and almost all points to be within +/- 15%. It shows that performance in the defined nonoperational area (i.e. the hatched area) is similar to that of the operational range. The operation range is defined by the 30% operation rate contour of the level measurement system. This shows that level-system operation rates below 30% can still result in reasonable flow measurement performance. Figures 50, 51 and 5 show results of the oil measurement in the two-phase oil / water flow tests Limits shown +/-0.75 l/s or +/- 40% Meter Flow (l/s) Ref Water Flow (l/s) Figure 50 Water Measurement Results (Water/Gas Test) Project No: FEMU04 Page 41 of 48

44 Error (% of reading) Ref Water Flow (l/s) Figure 51 Water Measurement Results % Error (Water/Gas Test) Figure 51 shows the large scatter of the results at all flows. The results are offset by approximately -%. Most results lie within the limits, shown in Figure 50, of +/-0.75l/s or +/- 40%. Figure 5 Water Measurement Flowmap (Water / Gas Test) Project No: FEMU04 Page 4 of 48