Best Practice Guide: Test protocol and approach to multiphase test facility intercomparisons utilising a MPFM transfer standard

Transcription

1 ENG58 MULTIPHASE FLOW METROLOGY IN OIL AND GAS PRODUCTION Best Practice Guide: Test protocol and approach to multiphase test facility intercomparisons utilising a MPFM transfer standard Deliverable No. D5.1.8 Work Package 1 (Multiphase Laboratory Intercomparison) Work Package 5 (Creating Impact) Version date: 15 June 2017 Main author: Gertjan Kok Contributors: Dennis van Putten, DNV GL, Groningen, the Netherlands Terri Leonard, Richard Harvey, NEL, Glasgow, United Kingdom Lev Zakharov, OneSubsea, Bergen, Norway Rick de Leeuw, Shell, Rijswijk, the Netherlands Gertjan Kok, Peter Lucas, VSL, Delft, the Netherlands The research leading to the results discussed in this report has received funding from the European Metrology Research Programme (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 1 of 11

2 Executive Summary This Best Practice Guide Test Protocol and approach to multiphase test facility intercomparisons utilising a MPFM 1 transfer standard addresses points that should be respected in order to get the best comparability between test results of multiphase flow meter test facilities. These points are both of a technical and non-technical nature. Version Date Main author Changes 1 5 May 2017 Gertjan Kok, VSL Initial version of document 2 24 May 2017 Gertjan Kok, VSL Inclusion of comments by Terri Leonard 3 9 June 2017 Gertjan Kok, VSL Inclusion of comments by Lev Zakharov and Dennis van Putten 4 15 June 2017 Gertjan Kok, VSL Inclusion of comments by NEL 1 MPFM: Multiphase Flow Meter ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 2 of 11

3 Contents EXECUTIVE SUMMARY INTRODUCTION MEASUREMENT UNCERTAINTY DETERMINATION OF TEST FACILITY UNCERTAINTY TRANSFER PACKAGE DESIGN AND MATCHING TEST CONDITIONS Transfer package design and flow pattern recognition Fluid type Flow rates, dimensionless numbers and operating conditions Single phase baseline tests FLOW METER UNCERTAINTY GENERAL POINTS RELATING TO TESTING Meter configuration Changing fluid properties Check lists for meter configuration Check list on data quality GENERAL POINTS RELATED TO ARRANGING TESTS Flow meter transportation Requirements for special approvals Scheduling of tests Training of personnel CONCLUSION LIST OF REFERENCES ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 3 of 11

4 1 Introduction Multiphase flow measurement in the oil and gas industry concerns the measurement of the individual volumetric flow rates of oil, water and gas with all three fluids flowing simultaneously through a pipeline. So-called multiphase flow meters (MPFM) can perform this difficult measurement task. The performance of this type of flow meter can be tested at multiphase flow meter test facilities. At these facilities well controlled flow rates of oil, water and gas are mixed and then offered to the flow meter. The readings of the flow meter can then be compared with the reference values provided by the test facility, which will be based on reference metering of (essentially) pure fluid flow rates and possibly some additional calculations to account for effects such as temperature and pressure changes between the reference location and at the meter under test location, and mass transfer between phases. Generally this testing activity is not called a calibration. In a calibration the meter deviation is recorded at various flow conditions, and then this is often followed by adjusting the meter in order to have in mean zero deviation with respect to the reference values (at least at that moment). In a calibration test points are repeated to determine how stable the flow meter is. The results of a multiphase meter tests are not used to adjust meter parameters. The results are rather used to determine if the meter performs within the manufacturer s performance specifications. Flow meter manufacturers may use test results to optimize the flow meter design later on. It turns out that the test results for the same flow meter tested at different multiphase test laboratories are not always as similar as what would be desirable or expected. The aim of this guide is to give guidelines on what can be done to ensure, as much as possible, comparability between measurements taken at different multiphase laboratories. These guidelines are in particular relevant when testing the same flow meter at various facilities as part of an intercomparison study between facilities. In the next sections various important aspects will be described. 2 Measurement uncertainty Comparing two measurement results of the same quantity is most useful when a so-called measurement uncertainty is associated to the measurement result. This measurement uncertainty is a number characterizing the dispersion of the values attributed to a measured quantity (see also [1]). The uncertainties U(y 1) and U(y 2) of results y 1 and y 2 are relevant when judging if the results are consistent. This can be done by comparing the difference d = y 2 y 1 with the value U(d) = [U 2 (y 1) + U 2 (y 2)]. In the case d U(d) the results are consistent, i.e. the difference is not significantly different from zero. In some cases the measured quantity is not completely stable. E.g., the flow meter deviation at a certain set point is not perfectly stable, but varies relating to the flow meter s repeatability and reproducibility. This instability e can be modelled by a parameter of nominal value 0 and uncertainty U(e). The difference d = y 2 y 1 + e now has to be compared with the value U(d ) = [U 2 (y 1) + U 2 (y 2) + U 2 (e)]. The bottom line of this discussion is that speaking about comparability between test facilities means that measurement uncertainties need to be quantified of both the test facilities and of the flow meter. If this has properly been done, one is able to say if measurement results are consistent and therefore the results of the facilities are comparable. ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 4 of 11

5 A further way of looking at and enhancing comparability is to reduce the number of uncertainty sources as much as possible. This will also be discussed in some of the following sections. 3 Determination of test facility uncertainty In this section the basic requirements for an uncertainty calculation for a multiphase testing facility will be presented. The basic requirements are as follows: 1. As a minimum, clearly state the claimed expanded uncertainties (k = 2) for the volume flow rates of gas, water and oil at the Meter Under Test (MUT) location, for a typical test condition. Preferably information is given on how these uncertainties vary for other conditions. Information on claimed uncertainties for mass flow rates and, Gas Volume Fraction and Water Liquid Ratio is desirable as well. 2. Provide information on the flow loop lay out, the reference instrumentation and the mathematical equations involved to determine final reference values at the meter under test location. 3. Provide information on how the uncertainty of the following sources of uncertainty were assessed, including quantification: i. Reference flow metering (REF) of gas, water and oil, including fluid contamination effects; ii. Pressure measurement at REF and MUT; iii. Temperature measurement at REF and MUT, including representativeness of measurement value for gas temperature at MUT; iv. Density measurement and/ or calculation at REF and MUT; v. Mass transfer effects between fluid phases in the pipe section between REF and MUT. A more detailed guideline on how to determine the uncertainty of a test facility will be available in the Guide to the expression of uncertainty in multiphase flow meter testing and calibration [2]. In a test protocol for an intercomparison of multiphase test facilities utilising a MPFM it should be requested that the test facilities provide valid uncertainty statements on all reference values. 4 Transfer package design and matching test conditions In order to make results as comparable as possible test conditions at different facilities should be matched as well as possible. In the following sections some aspects are listed. 4.1 Transfer package design and flow pattern recognition It is known that some multiphase flow meters at some operating conditions (e.g. low operating pressure) can be quite sensitive to the piping configuration and thereby induced flow pattern at the inlet of the meter. A transfer package in an intercomparison should therefore carefully be designed and this involves more than the selection of a flow meter. A possible approach is to assess, in detail, the geometrical variances existing between each participating flow loops and design a transfer package that removes as many geometrical variances as far as reasonably possible. For example, a long, straight inlet section of e.g. 100D length (this length was used in [3]) can be installed. If it is envisaged to match patterns exactly at each facility then other configurations using passive flow straighteners or active mixing devices can be considered as well. In any case best practice must ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 5 of 11

6 include utilisation of flow pattern recognition techniques e.g. tomographic devices or an optical viewing section combined with high speed video recording. This instrumentation should be installed close to the inlet of the flow meter and serves to verify that flow patterns are comparable at each flow condition. Finally, practical information on piping size, schedule and flange standard must be gathered and then a complete transfer package can be designed that suits spacing limitations and other requirements for each participant. Best practice is to match pipe inner diameter of the inlet pipe section with that of the flow meter, and to choose the length of the inlet pipe section as large as possible. 4.2 Fluid type In an intercomparison it is best practice to use as similar fluids as possible. The type of oil used in a facility is an important parameter. The viscosity of the oil can have an effect on the flow meter. Depending on the type of flow meter, the type of salt and salinity of the water may be less influential if a flow meter is properly configured. Variation in salinity often has a negligible effect on liquid Reynolds number. However, all variables should be detailed and their influences determined if possible through additional testing or previous testing and reference to previous publications or by an alternative method. Most test facilities use either nitrogen or natural gas as process gas. If both options are possible following consideration may help in the gas type selection. Natural gas dissolves much better in oil than nitrogen and phase transfer effects need to be accounted for. Phase transfer calculations may increase the uncertainty of the facility reference values for flow rates. This potential disadvantage has to be weighed against the potential advantage of using the same gas as present in an oil and gas well. Best comparability between facilities is obtained when using the same type of fluids for all three phases. 4.3 Flow rates, dimensionless numbers and operating conditions A very important part of a test protocol for an intercomparison between multiphase test facilities is to define the test matrix with phase flow rates and operating temperatures and pressures for each test point. A tolerance should be set on the set points for each condition on order to allow direct comparison of results from one facility to another. The first step is to overlay the facilities operating envelopes. Then a range of flow rates can be selected, e.g. by defining liquid flow rates, water liquid ratios and gas volume fractions. In a next step the fluid properties and operating parameters per test facility should be determined. Using the free parameters of pressure and temperature best practice is now to match dimensionless numbers. For example, it is known that in wet gas flows the Froude number and Lockhart-Martinelli parameter dominate the flow regime. The Froude numbers (for gas and liquid phases) and the Reynolds number (for oil phase) were considered in the intercomparison [3]. However note that for some types of flow meters it may be important to test at approximately equal operating pressures, at least when testing at pressures below 10 bara. This may be related to applicability ranges of the phase slip models used inside multiphase flow meters or other models limitations. Temperature differences between facilities seem to be less influential on test results. The flow meter manufacturer may be able to give information on what is more important: matching dimensionless numbers or matching operating pressure. The remaining differences in fluid properties and operating conditions should be clearly specified. If possible, additional testing should be completed in order to determine their influence. ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 6 of 11

7 4.4 Single phase baseline tests The test matrix as described in the last section should not only include multiphase test points. It is also important to include single phase test points for each of the three phases. These test points are sometimes easier to measure more accurately by both the multiphase flow meter and the test facility, and in that case a baseline for the intercomparison can be established, i.e. it gives a limit on the best comparability that can be expected for multiphase points. 5 Flow meter uncertainty In the ideal case, the flow meter uncertainty including repeatability and reproducibility values are already known prior to use as the transfer meter for intercomparison studies. In that case, checking the comparability of two test facilities will be a simple numerical exercise when reference values and uncertainties of two tests have been obtained. Inconsistent results will then indicate that a test facility uncertainty or a flow meter uncertainty was underestimated. E.g. this can be due to a larger than anticipated sensitivity of the flow meter to flow pattern variations or inability of a flow loop to perform accurate reference measurements at certain combinations of flow rates. Note that it should always be ensured that test facilities are indeed testing the flow meter under comparable conditions. In the case of inconsistent results, an effort should be made to find the root cause. A good test protocol can be helpful in this respect. In a single test the observed deviation and uncertainty is always a combination between facility and flow meter uncertainties. It is helpful to repeat some test points a few times during a test round. Both immediate repetitions and repetitions some days later can be beneficial. More formally, repeatability is defined as the closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement [4]. The repeatability of a test result at that facility can now be calculated based on the immediate repetitions. The reproducibility is defined as the closeness of the agreement between the results of measurements of the same measurand carried out under changed conditions of measurement [4]. A valid statement of reproducibility requires specification of the conditions changed [4]. This may include testing at the same facility a few days later, a year later or even at another facility provided that nominally the same test conditions can be realized. When a test point is multiple times repeated at another test facility, the reproducibility at that other facility can be calculated as well. (Note that comparing the results of nominally the same points at different test facilities is not considered here for the reproducibility analysis, as this would directly interfere with the analysis of the intercomparison results.) These measured reproducibilities can be compared with each other, with the absolute difference between the test results at the different facilities and with the specified reproducibilities of the facilities and the flow meter. If the measured reproducibility at one facility is much higher than at another facility, the cause may be attributable to the facility rather than the flow meter. However auxiliary measurements like flow pattern registration remain necessary in order to rationalise the differences in results observed. Also other parameters that remained different between the facilities after matching conditions as far as possible can have an effect on the observed flow meter reproducibility and these parameters should be listed. An apparent poorer repeatability or reproducibility at a specific facility may still not be due to the reference measurements, but to a different flow pattern or other difference in test conditions at the meter under test and can thus not completely be attributed to the facility in that case. A common way to quantify the reproducibility including drift of transfer standard plus facility (for testing at the same facility) is by performing the same tests at the start and end of the ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 7 of 11

8 intercomparison at the same facility. If the test matrix is repeated in all details (e.g. order of points, using actual flow rates of the initial test round as set points in the final test round) the influence of a change in flow conditions on the facility and flow meter results is minimized. In the ideal case both the facility operator and the flow meter manufacturer should specify which tolerances in set points are allowed so that the involved instrumentation should essentially behave the same. The additional flow meter reproducibility due to changing facility (and thus e.g. type of oil, etc.) cannot be derived from the intercomparison test data, as the goal of the intercomparison is to determine the consistency of measurement results between. If measurement data is used to determine this inter-facility reproducibility, any systematic offset between facilities would be interpreted as flow meter reproducibility effect, and intercomparison results would be consistent by construction. If other data or theoretical estimates are available to determine the inter-facility reproducibility these values can be used. Alternatively the inter-facility reproducibility of the flow meter can be set to the same value as the measured reproducibility of flow meter plus facility at the same location. This can pose a stringent condition on the consistency evaluation of the test results (i.e. lower value of consistent results may be found due to an optimistic estimate of inter-facility reproducibility), but it is less arbitrary than using some more or less well founded estimated value. Flow meter uncertainty cannot be excluded from the discussion of comparability of test facilities. As there are many parameters that can vary between test facilities (flow pattern, fluid types, etc.), and measurement results always contain a combined effect of facility and flow meter, it is difficult to identify root causes. Repeating test points (especially the ones for which and the most relevant) can help. If points for which the flow meter is known to have a good reproducibility (e.g. from results in earlier tests) are repeated, it may be possible to establish a baseline of comparability. If some points are of particular interest for some party (e.g. an industrial end user), these points deserve additional attention as well and should be repeated. Also the results from additional tests to determine the influence of each variable in turn will be critical to understand the differences between test facilities. CFD and results from previous studies can also be considered. Another option could be to test multiple flow meters at the same time. In this case attention should be paid to the fact that a flow meter installed further downstream will operate at a slightly lower operating pressure due to pressure losses upstream. One should be careful with comparing results as the flow conditions are slightly different for each meter (e.g. GVF will be higher for a meter downstream). 6 General points relating to testing In this section some general points relating to test protocol are described. 6.1 Meter configuration The set-up of the flow meter should follow a predefined protocol. Facility and flow meter clocks should be synchronized at the start of the test programme. If fluid reference measurements need to be taken, specific, numerical values should be defined in the configuration protocol, rather than imprecise terminology as sufficiently low. In latter case configuration tests (e.g. gas attenuation tests for some types of flow meters) may be unnecessarily repeated. All meter settings and options should be listed beforehand and the appropriate choices should be defined before the actual meter set-up, rather than ad-hoc by the person performing the configuration. All settings should be saved and logged so that they can be retrieved later on. Examples of flow meter settings are: type of gas (e.g. nitrogen or natural gas), type of oil (e.g. dead ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 8 of 11

9 oil or live oil). A requirement is to have a well-trained witness or a flow meter specialist from the manufacturer that ensures that the required procedures and settings are used. 6.2 Changing fluid properties In some facilities the fluid properties may change over time. For example, in facilities with an openloop design in which the gas is not recirculated the water salinity may increase with time. It should be clearly defined how to deal with this fact, and who is responsible for the associated analysis, especially on the side of flow meter data. The assessment criteria (e.g. a criterion to determine if reprocessing or repeated reference measurements for a proper configuration are needed or not) should be well-defined before starting the tests in order to avoid any later discussion as much as possible. To this purpose an optimal set of reference measurements (e.g. daily reference density measurements and single phase attenuation points in the case of using a gamma-densitometer) should be planned and performed in order to trace consistency of the fluids and to be able to reprocess data in an optimal way in the case this is deemed necessary. It can be part of the protocol that in any case a particular point on the last day of testing is reprocessed by the flow meter manufacturer using fluid property data of the last day (e.g. attenuation and density measurements in the case of using a gamma-densitometer), and that based on the observed shift in results (if any) it is decided, by comparison with an a priori defined limit value, if reprocessing of all measured data is required or not. Each type of MPFM may have its specific set of fluid parameters which should be checked on a regular basis. 6.3 Check lists for meter configuration A short check list of one or two pages should be made, listing the elements of the meter configuration, start and end of the day action, performing test points, and final actions. Highlighting a few important pages of a much longer document may not be sufficient in practice, and the risk is that it won t be followed in all details. Flow meter operators/ witnesses should be well trained before and checked during tests. In particular the check list for flow meter verification should be specified. Most often the flow meter software has some functionality to show all relevant parameters that are to be checked. The following models and modes are quite generic and need to be the same for each test: Type of flow meter Mode Fluid model Oil viscosity model Mixture viscosity model Phase fraction Flow direction Type of oil Oil PVT reference Type of water Water PVT reference Type of gas Gas PVT reference Furthermore, in general it should be checked that fluid properties and PVT data used by the flow meter is accurate. This may involve correctly configuring parameters like Oil sample density Oil sample density temperature Water sample density ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 9 of 11

10 Water sample density temperature Oil sample viscosity Oil sample viscosity temperature Each type of flow meter will in addition have its more specific hardware and software configuration settings, which should be set as similar as possible. For example for a flow meter using a gamma-densitometer there can be additional configuration settings for Oil mass attenuation reference Water mass attenuation reference Gas mass attenuation reference In this case it should as well be ensured that reference data for the fluids of the flow loop exist, and that source attenuation measurements are accurately performed for empty pipe (air), oil, water and gas phase. 6.4 Check list on data quality Some additional checks on the data may be defined before a test point is regarded as valid. Examples are: Actual test point conditions should lie within the operating envelope of the flow meter specified by the manufacturer Start time and stop time of the test facility and flow meter do not differ more than a maximum amount of time Conditions on values or (maximum) fluctuation in operating pressure, temperature, flow rates, etc. E.g. instantaneous differential pressure in a Venturi type flow meter should be higher than a minimum value specified by the manufacturer. Reference measurements are used within their calibrated ranges at all times. Once all checks regarding the flow meter settings, necessity of data reprocessing and additional data checks have been performed, the data can be further analysed and compared with results of other facilities. 7 General points related to arranging tests The points raised in this section are not of a strict technical nature relating to the comparability of test results, but are more of a general nature. 7.1 Flow meter transportation Inside the EU flow meter transport (excluding a possible radioactive source) is relatively easy, although still several administrative forms have to be filled out relating to value and possibly insurance. Importing and exporting to and from countries within the EU may require an ATA-carnet and more administrative work, especially regarding import duties. It is easy to make mistakes when completing the necessary paperwork and incomplete or incorrect documentation results in delays at customs and large increases in the invoices received from the transportation company. Some types of flow meters contain a radioactive source, e.g. a gamma-densitometer. In this case, depending on the country an announcement has to be made to the national authorities concerning the transport of a radioactive source, followed by an obligatory waiting time of several weeks. The transport company should be able to transport radioactive sources. In practice the company may say they can, but not respect all the rules. Again, it is easy to make mistakes in the administration. The ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 10 of 11

11 result will be waiting times at customs and large increases in the invoices received affecting both programme schedule and budget. 7.2 Requirements for special approvals Multiphase flow meters may contain some materials that require special attention and approvals. E.g. a flow meter may contain a radioactive source. In this case it should be made sure at an early stage that each test facility is permitted to receive, store and handle a specific radioactive source. A facility may have permission for one type of radioactive source, but not for another, for example. Regulations may exist that a facility has permission to use the source if the flow meter is operated only under the owner s supervision. Scientific or technical personnel may very well not be aware of all details. The radiation safety specialist and possibly the national authority should be consulted in order to avoid long delays or annulations at the last moment. 7.3 Scheduling of tests Often multiphase test facilities have a busy schedule with many tests that have been planned. It is therefore important to plan intercomparison test sufficiently in advance. If testing is not possible in a reserved time slot for whatever reason, it is not uncommon that a new testing time slot is available only six months later. Sufficient time must be scheduled between tests to allow flow meter transportation. A minimum of two weeks seems to be necessary. 7.4 Training of personnel The flow meter operator(s) and/ or witness(es) should be well trained and provided with sufficient documentation regarding all relevant safety precautions, flow meter operation procedures, trouble shooting, and contact information if support should be required. 8 Conclusion This document listed some aspects to pay attention to in multiphase flow testing especially in the context of an intercomparison for test facilities. These aspects were both of a technical and a practical nature. These points should lead to a maximum of comparability between multiphase test facility results. Due to the many parameters involved in multiphase flow meter testing attaining complete comparability, or finding root causes for discrepancies, remains a challenging goal. 9 List of References [1] JCGM 100:2008. Evaluation of measurement data Guide to the expression of uncertainty in measurement, Joint Committee for Guides in Metrology [2] G. Kok, e.a., Guide to the expression of uncertainty in multiphase flow meter testing and calibration, 2017 (to be finished) [3] P. Lucas, G. Kok, H. de Leeuw, T. Leonard, D. van Putten, L. Zakharov, Intercomparison between multiphase flow test facilities, Flomeko conference paper, Sydney, Sept [4] JCGM 100:2008, Evaluation of measurement data Guide to the expression of uncertainty in measurement, BIPM, IECC, IFCC, ILAC, ISO, IUPAC, IUPAP and OIML, 2008 ENG58_D5.1.8_BPG-Test-Protocol_v4.docx Page 11 of 11