WO2016051197A1

WO2016051197A1 - Measurement system and methods

Info

Publication number: WO2016051197A1
Application number: PCT/GB2015/052888
Authority: WO
Inventors: Steven Morrison; Laurie Linnett; Thomas D. Marshall; Stuart Clarke
Original assignee: Bios Developments Limited
Priority date: 2014-10-03
Filing date: 2015-10-02
Publication date: 2016-04-07
Also published as: GB201417539D0

Abstract

In some examples, there is described a measurement system, methods and other apparatus and devices used in measuring fluid, and for example fluid density. Some examples describe a system having a measurement cell (110) for fluid to be measured. The cell, together with the fluid to be measured, may have a particular frequency response characteristic within a band of frequencies. An excitation device (160) may also be used that is configured to communicate an excitation signal to the cell. Such an excitation signal can comprise a plurality of frequencies within a selected band, and that selected band may be based on the particular frequency response characteristic of the cell and fluid to be measured. Further, a response sensor may be used to obtain a spectral response from the measurement cell across some or all of the selected band. In some examples, in communication with the response sensor, a controller (300) can be configured to calculate a resonant frequency response characteristic of the cell and fluid based on some or all of the obtained spectral response, and calculate the density of fluid in the measurement cell based on the calculated resonant frequency response characteristic.

Description

Measurement System and Methods

Technical Field Some described examples relate to measurement systems, particularly for determining properties of fluids, such as fluid densities and the like.

In some described examples, the system(s) may be incorporated into a structure such as a pipeline, conduit, flow loop, or the like, and may form part of that structure (e.g. in an inflow manner). In some examples, the system(s) may be used in the oil and gas industry and may be used to determine properties, such as the density of fluids flowing in an oil and gas structure (e.g. hydrocarbons, water, etc.).

Background

Significant innovation and technology development has occurred in recent years in relation to the measurement of fluids, particularly in the oil and gas industry. Accurate measurement of those fluids can be vital to the safety and/or overall performance of equipment, or indeed to fiscal metering and custody transfer of such fluids. It will be appreciated that fluids may include different phases such as gases, liquids and combinations thereof, including in some cases those which contain solids (e.g. liberated fines, solidified hydrates, etc.).

Existing systems and devices for measuring fluid properties, such as vibration-based systems, can have difficulty in accurately measuring low flow rates (e.g. fewer than 100 bbl/day), and/or multi-phase flows. In particular, such systems and devices often have difficulty accurately measuring flows having significant gas-volume fractions (e.g. in excess of 40%). This is particular true of existing systems or devices that try to measure density, sometimes indirectly, such as existing Coriolis-based systems and devices. In the alternative, systems or devices that require specialist training to install or use restricted materials (e.g. radioactive materials, such as those used in Gamma densitometers) can be expensive to operate and maintain, as well as difficult and costly to manufacture and transport. This background serves only to set a scene to allow a skilled reader to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. One or more described aspects/embodiments of the invention may or may not address one or more of the background issues.

Summary

Some examples describe measurement systems and methods for measuring fluid properties, such as fluid density and the like.

The described measurement systems and methods may permit quicker and/or more cost effective and/or more accurate determination of fluid properties, such as density, particularly multiphase flows and/or fluids having a low flow rate (e.g. fewer than 100 bbl/day). In some examples, such systems and methods may permit the determination of further properties, such as flow regimes, or the like. In some examples there is described a measurement system comprising a measurement cell for fluid to be measured.

The cell, together with, for example, the fluid to be measured may have a particular frequency response characteristic within a band of frequencies (e.g. an expected or anticipated frequency response, based on conditions). The system may additionally comprise one or more excitation devices, which may be configured to communicate an excitation signal to the cell (e.g. directly and/or indirectly). In some described examples, such an excitation signal may have - or comprise - a plurality of frequencies within a selected band. Further, that selected band may be based on the particular frequency response characteristic of the cell and fluid to be measured.

The system may comprise one or more response sensors configured to obtain a response, such as a spectral response, from the measurement cell across some or all of the selected band.

In some particular examples, the system may comprise a controller, for example in communication with the response sensor. Such a controller may be configured to calculate (or otherwise determine or compute) a resonant frequency response characteristic of the cell and fluid based on some or all of the obtained response (e.g. spectral response). The controller may be configured to calculate, or otherwise determine/compute, the density of fluid in the measurement cell based on the calculated resonant frequency response characteristic. The controller may be provided by a processor and memory, arranged in a known manner. For example, the controller may be provided by an application specific integrated circuit, field programmable gate array, or the like. The controller may be provided by hardware or firmware and software. The system may comprise a user interface, configured to permit input and/or output at the controller. The controller and/or user interface may be positioned at the measurement cell, or indeed remote from the cell.

The controller may be configured to calculate the density of fluid by using, for example comparing, the calculated resonant frequency response characteristic together with one or more reference frequency response characteristics. The reference frequency response characteristics may be derived from calibration methods and/or theoretical methods or models, or the like, (e.g. previous calibration methods, under static and/or flow conditions). The controller may be configured to calculate the density of fluid, in the measurement cell, by using further parameters, such as (i) measured or expected/estimated temperature of the fluid and/or cell, (ii) measured or expected/estimated pressure, (iii) measured or expected/estimated flow rates, or the like. The response may provide spectral data representing a plurality of discrete frequency values, having frequency intervals and amplitudes.

The controller may be configured to compute statistically the resonant frequency response characteristic using the spectral response or spectral data derived from the measurement cell. In similar words, the controller may be configured to compute, from a plurality of observed/determined frequency values within the spectral response/data the likely, or most likely, resonant frequency response characteristic of the measurement cell (e.g. measurement cell and fluid to be measured). Or put differently, the controller may be configured to interpolate and/or extrapolate, from a plurality of observed/determined frequency values within the spectral response/data the likely, or most likely, resonant frequency response characteristic of the measurement cell (e.g. measurement cell and fluid to be measured). In some examples, the computed resonant frequency response characteristic may provide data relating to the mode of frequency response. In some examples, the resonant frequency response characteristic may include the peak frequency response of the system/cell. In some examples, that computed peak resonant frequency response from the system may not correlate with, or be the same as, the maximum amplitude of observed frequency value within the spectral response/data.

In some examples, the controller may use regressive analysis of the response (e.g. use regressive analysis of the plurality of frequency values within spectral data) to compute the resonant frequency response characteristic. In some particular examples, the controller may use a least-squares analysis of the spectral data (e.g. least squares analysis of a plurality of frequency values). The controller may be configured to determine the resonant frequency response characteristic (e.g. the peak frequency response) by using a statistical analysis (e.g. least-squares analysis) of most or all observed frequency values within the spectral data. The excitation device may be configured to communicate a periodic excitation signal to the measurement cell. That is to say, the excitation signal may comprise a plurality of frequency components, within the selected band, where each frequency component may be selected (e.g. frequency and/or phase) such that, when combined, the excitation signal maintains a signal period. Each frequency component of the excitation signal may have the same, or similar, signal strength (e.g. amplitude). The excitation device may be configured to communicate each of the frequency components simultaneously (e.g. as a complex signal). The response sensor may be configured to obtain the spectral response, and so each frequency value within the spectral response, of the fluid cell simultaneously. The controller may be configured to sample the response sensor and obtain frequency values simultaneously.

The controller may be configured to sample the response sensor (or observed spectral response) at defined intervals, for example, intervals relative to each periodic iteration of the excitation signal. The controller may be configured to calculate, for each periodic iteration of the excitation signal, the response of the measurement cell.

The controller may be configured to calculate the fluid density for each periodic iteration of excitation signal. The calculated fluid density may be taken to be an absolute, or instantaneous, value of the fluid density at each particular sample interval.

The measurement cell may be specifically configured (or constrained) to have the particular frequency response characteristic within a band of frequencies (e.g. configured to have an expected modal response). In some examples, the cell may be specifically configured based on anticipated measurement conditions, such as anticipated fluids being measured, pressures, and/or temperatures. In some examples, the cell may be configured to have a single resonant frequency response characteristic within a band of frequencies (e.g. a single degree of freedom system). In such examples, the system may exhibit two regions of limited, or no vibrational amplitude in response to excitation, and one region with maximum response to excitation.

The cell may be specifically configured such that the particular resonant frequency response characteristic may lie within the band of frequencies, irrespective of whether the fluid within the cell is gas or liquid, or combination thereof (including, in some cases, expected solids). In similar words, the cell may be configured such that the lower end of the band of frequencies is provided based on the densest expected fluid condition (e.g. cell fully containing water), whereas an upper end of the band of frequencies may be provided based on the a least dense expected fluid condition (e.g. cell fully containing gas).

The particular frequency response characteristic may be based not only on the expected fluids, but also expected pressures and/or temperature in use.

The system may comprise one or more constraints, which may in some examples be considered as nodal constraints. Such constraints, or nodal constraints, may be configured to inhibit, or otherwise mitigate, aspects of movement of the measurement cell in use. The constraints may be configured to mitigate on otherwise inhibit movement at particular locations or positions at the measurement cell. In some examples, the system comprises two constraints, a first positioned at or near the inlet of the measurement cell and a second positioned at or near the outlet of the measurement cell. Those constraints may comprise one or more mass elements. The mass element(s) may be attached to the measurement cell (e.g. attached at or near intended nodal positions). The mass elements may be integral with the cell.

Those two constraints may be considered primary constraints, for example, defining first intended nodal positions. The system may comprise two or more secondary constraints. Those two or more secondary constraints may be spaced outwardly, e.g. towards the structure, and along the cell, from the primary constraints. The system may comprise an equal number of constraints, either side of the measurement cell. The system may comprise one or more dampening elements. Such dampening elements may be specifically selected or configured to assist in provided a particular frequency response (e.g. a relatively broad peak resonance, compared to having no dampening elements).

In some examples, the system may have dampening elements in addition to the one or more constraints. In some such cases, one, some or all the constraints (e.g. comprising mass element(s)) may be attached to the measurement cell via one or more dampening elements. In other words, the dampening elements may interface between constraints, or otherwise mass elements, and the measurement cell.

The excitation device may be mechanically coupled to the measurement cell. The excitation device may be configured to communicate the excitation signal mechanically to the measurement cell. In other words, the excitation device may be configured to impart a force/pressure to the measurement cell in order communicate the excitation signal to the cell. Such mechanical communication may be considered different from, for example, inducing or otherwise providing the signal in a non-contact manner. The excitation device may be coupled to the measurement cell between an expected node and expected antinode of the cell. The excitation device may be coupled substantially closer to a node than to an antinode of the cell (e.g. coupled near a constraint). The excitation device may be coupled to the cell via a retaining element. The retaining element may permit coupling and/or removal of the excitation device from the measurement cell. In some examples, the retaining element is configured as an interference coupling (e.g. comprising a threaded stud and a complimentary threaded recess). The excitation device may comprise one or more signal actuators (e.g. mechanical shaker(s)). That signal actuator(s) may be configured to communicate the excitation signal to the measurement cell. The excitation device may additionally comprise an actuation sensor, configured to observe the signal communicated, or force imparted, from the signal actuator to the measurement cell. In some examples, the actuation sensor may be positioned between the signal actuator and measurement cell.

The excitation device may comprise a housing. The housing may fully, partially or substantially surround the signal actuator and/or actuation sensor. In some examples, the housing fully surrounds and encloses the shaker, etc. In such examples, the excitation device may be configured to comply with explosion-proof criteria (e.g. may be rated Ex-d, or the like). In some examples, the response sensor may be coupled to the measurement cell at an expected antinode of the cell. Similarly, the response sensor may be coupled to the cell via a retaining element (e.g. permitting coupling and/or removal of the response sensor). In other examples, the response sensor may be provided together with the excitation device (e.g. within the housing of the excitation device). The response sensor may be configured to sense the rate of change of movement of the measurement cell (e.g. provided by one or more inertial sensors, such as accelerometers).

The system may be configured such that, when coupled to the measurement cell, the excitation device (e.g. the housing of the excitation device) exhibits a response node at the excitation device, when communicating an excitation signal to the cell. In similar words, in use, the excitation device may exhibit a region of limited, or no movement, in during, or in response to, excitation (e.g. even though being coupled to an excited measurement cell). The system may be configured such that wired or wireless interconnectors, such a signal cabling and/or power supplied, etc. are attached or coupled to the excitation device at the response node. Such interconnector(s) may be used for signalling, power, etc., between the excitation device and/or response sensor and the controller.

The measurement cell may comprise a fluid inlet together with a fluid outlet. A fluid conduit may extend between the inlet and outlet, the conduit being configured for passage of fluid from the inlet to the outlet. Both the inlet and the outlet of the measurement cell may be configured to couple to a structure, such as a pipeline, flow loop, or the like. The measurement cell may be configured as an inline flow cell. In other words, intended to be incorporated with the structure, rather than simply clamped on to it. The measurement cell may be configured essentially tubular.

The system may comprise one or more couplings (e.g. flexible or deformable couplings). The coupling(s) may be arranged to mount or otherwise couple the inlet and/or outlet of the measurement cell with a structure, such as a pipeline or flow loop. The coupling(s) may be configured to isolate, such as vibrationally isolate or the like, the measurement cell from an attached structure. The coupling(s) may provide an impedance interface (e.g. an impedance mismatch) between the measurement cell and a structure. The or each coupling may additionally/alternatively be configured to mitigate or avoid communication of mechanical strain between the cell and the structure.

The or each coupling may comprise a flexible conduit, or otherwise flexible interface, configured to be arranged between the measurement cell and a fluid/flow structure. The flexible coupling may comprise braided, or woven, tubing, or the like. The coupling(s) may permit passage of fluid from/to the structure to/from the measurement cell. The system or indeed the or each coupling, may comprise a conditioning mechanism, configured to impart a particular mechanical condition to the system/coupling. In some examples, the conditioning mechanism may be configured to impart a particular stress regime in the system/coupling, such as tension or compression. The conditioning mechanism may be adjustable. The conditioning mechanism may be configured as one or more braces or struts, for example extending along the coupling. The conditioning mechanism may be configured to be adjustable so as to impart a particular mechanical condition depending on one or more of: expected fluids or flow regimes, expected pressures, expected temperatures. The conditioning mechanism may be set, based on expected flow conditions, and then adjusted based on revised flow conditions. The conditioning mechanism may be considered to be pre-stress the coupling. The conditioning mechanism may be comprised with a housing for the system,

The system may be configured such that the one or more constraints are spaced from the constraints along the measurement cell.

The system may comprise one or more restrictions (e.g. regions of reduce cross- sectional flow area). The or each restriction may be provided at, or close to the inlet of the measurement cell. The restriction may be configured to impart, or assist in providing, a particular flow of fluid entering the measurement cell. For example, the restriction may be configured to assist in homogenising fluid entering the measurement cell. The restriction may assist in providing a lower flow pressure in the measurement cell, compared to the flow in the structure to which the system is otherwise connected with or coupled. The restriction may comprise one or more frustoconical portions.

The measurement cell may be configured to have a particular frequency response between 10 Hz and 5000 Hz. In other words, the frequency response (or frequency response function) may be substantially between 10 Hz and 5000 Hz, or even 100 Hz and 1000 Hz. The excitation device may be configured to communicate an excitation signal having frequency components within such ranges. In other words, the selected band may correspond to the band of expected frequency response. The response sensor may be configured to obtain a spectral response across some or all of the band between 10 Hz and 5000 Hz (e.g. between 100 Hz and 1000 Hz).

The system may be configured for measuring density of water, oil and/or gas. The system may be configured to measure the density of multi-phase fluids. The system may be configured to be integrated into an oil and gas structure, such as a pipe line, flow loop or the like. When integrated, the system may be considered to be vibrationally isolated, or substantially vibrationally isolated from the structure.

In some examples, there is described a method, for example, for calculating fluid properties, such as the density of fluid, in the measurement cell.

The method may comprise communicating an excitation signal to a measurement cell. The cell together with a fluid to be measured may have a particular (e.g. expected) frequency response within a band of frequencies. The excitation signal may comprise a plurality of frequencies within a selected band. In some examples, that selected band may be based on the expected frequency response of the cell and fluid to be measured. The method may comprise obtaining a response, such as a spectral response, from the measurement cell across some or all of the selected band. The method may comprise calculating a resonant frequency response characteristic (e.g. of the cell and fluid) based on some or all of the obtained response. The method may comprise calculating the density of fluid in the measurement cell based on the calculated resonant frequency response characteristic.

The method may comprise calculating the density of fluid by using, for example comparing, the calculated resonant frequency response characteristic together with one or more reference frequency response characteristics. The reference frequency response characteristics may be derived from calibration methods, theoretical methods or models, or the like, (e.g. previous calibration methods, under static and/or flow conditions). The method may comprise calculating the density of fluid by using further parameters, such as measured or expected temperature of the fluid and/or cell, measured or expected pressure, measured or expected flow rates, or the like.

The method may comprise obtaining, as the spectral response, data representing a plurality of discrete frequency values, which may have particular frequency intervals and/or amplitudes.

The method may comprise computing, statistically, the resonant frequency response characteristic using the spectral response/data from the measurement cell. In similar words, the method may comprise computing, from a plurality of observed frequency values within the spectral response/data the likely, or most likely, resonant frequency response characteristic of the measurement cell (e.g. measurement cell and fluid to be measured). Or put differently, the method may comprise interpolating and/or extrapolating, from a plurality of observed/determined frequency values within the spectral response/data the likely, or most likely, resonant frequency response characteristic of the measurement cell (e.g. measurement cell and fluid to be measured).

In some examples, that computed resonant frequency response characteristic may provide data relating to the mode of frequency response. In some examples, the resonant frequency response characteristic may include the peak frequency response of the system/cell. In some examples, that computed peak resonant frequency response from the system may not correlate with, or be the same as, the maximum amplitude of observed frequency value within the spectral response/data.

In some examples, the method may comprise using regressive analysis of the spectral response (e.g. using regressive analysis of the plurality of frequency values within the spectral response) to compute the resonant frequency response characteristic. In some particular examples, a least-squares analysis of the spectral response may be used (e.g. least squares analysis of a plurality of frequency values).

The method may comprise determining the resonant frequency response characteristic by using a statistical analysis (e.g. least-squares analysis) of all observed frequency values within the spectral response.

The method may comprise communicating a periodic excitation signal to the measurement cell. That is to say, the excitation signal may comprise a plurality of frequency components, within the selected band, where each frequency component may be selected (e.g. frequency and/or phase) such that, when combined, the excitation signal maintains a signal period. Each frequency component of the excitation signal may have the same, or similar, signal strength (e.g. amplitude).

The method may comprise communicating each of the frequency components simultaneously (e.g. as a complex signal). The method may comprise obtaining each frequency value within the spectral response/data of the fluid cell simultaneously. The method may comprise sampling a response sensor and obtaining frequency values simultaneously. The method may comprise sampling the observed spectral response at defined intervals, for example relative to each periodic iteration of excitation signal. The method may comprise calculating, for each periodic iteration of the excitation signal, the observed spectral response/data of the measurement cell. The method may comprise calculating the fluid density for each periodic iteration of excitation signal. The calculated fluid density may be taken to be an absolute, or instantaneous, value of the fluid density at that sample interval.

The method may comprise providing, or otherwise constraining, the measurement cell or constrained) to have a particular frequency response characteristic within a band of frequencies (e.g. an expected modal response). In some examples, the cell may be specifically configured based on anticipated measurement conditions, such as anticipated fluids being measured, pressures, and/or temperatures. In some examples, the cell may be configured to have a single resonant frequency response characteristic within a band of frequencies (e.g. a single degree of freedom system). In such examples, the system may exhibit two regions of limited, or no vibrational amplitude in response to excitation, and one region with maximum response to excitation. The method may comprise providing a cell having a single particular/expected resonant frequency response characteristic lying within the band of frequencies, irrespective of whether the fluid within the cell is gas or liquid, or combination thereof (including, in some cases, expected solids). In similar words, the cell may be provided such that the lower end of the band of frequencies is provided based on the densest expected fluid condition (e.g. cell fully containing water), whereas an upper end of the band of frequencies may be provided based on the a least dense expected fluid condition (e.g. cell fully containing gas).

The particular frequency response characteristic may be based not only on the expected fluids, but also expected measurement conditions, such as pressure and/or temperature in use.

The measurement cell may be configured to have an expected frequency response characteristic between 10 Hz and 5000 Hz. In other words, the frequency response characteristic (or frequency response function) may be substantially between 10 Hz and 5000 Hz, or even 100 Hz and 1000 Hz. The excitation signal may have frequency components within such ranges. In other words, the selected band may correspond to the band of expected frequency response characteristic. The method may comprise obtaining a spectral response across some or all of the band between 10 Hz and 5000 Hz (e.g. between 100 Hz and 1000 Hz). The method may be configured for measuring density of oil and/or gas. The method may measure the density of multi-phase. The method may be for use with an oil and gas structure, such as a pipe line, flow loop or the like. In some particular examples, there is described a measurement system comprising: a measurement cell for fluid to be measured, the cell together with the fluid to be measured having a particular frequency response characteristic within a band of frequencies;

an excitation device configured to communicate an excitation signal to the cell, such an excitation signal comprising a plurality of frequencies within a selected band, that selected band based on the expected particular frequency response characteristic of the cell and fluid to be measured;

a response sensor configured to obtain a spectral response from the measurement cell across some or all of the selected band;

a controller, in communication with the response sensor, configured to calculate a resonant frequency response characteristic of the cell and fluid based on some or all of the obtained spectral response, and

the controller further configured to calculate the density of fluid in the measurement cell based on the calculated resonant frequency response characteristic.

In some particular examples, there is described a controller configured to obtain (e.g. calculate, receive and/or store) spectral data derived from the response of a measurement cell, that spectral data being across some or all of a selected band, wherein that selected band is based on a particular (e.g. expected) frequency response characteristic of a measurement cell and fluid to be measured in response to communication of an excitation signal to a cell and fluid comprising a plurality of frequencies within the selected band; and wherein

the controller is further configured to calculate a resonant frequency response characteristic of a cell and fluid based on some or all of the obtained spectral response/data, and to calculate the density of fluid in a measurement cell based on the calculated resonant frequency response characteristic.

In some examples, there is described a method comprising,

communicating an excitation signal to a measurement cell, that cell together with a fluid to be measured having a particular frequency response characteristic within a band of frequencies, wherein the excitation signal comprises a plurality of frequencies within a selected band, and wherein that selected band is based on the particular (e.g. expected) frequency response characteristic of the cell and fluid to be measured;

obtaining a spectral response from the measurement cell across some or all of the selected band;

calculating a resonant frequency response characteristic of the cell and fluid based on some or all of the obtained spectral response, and

calculating the density of fluid in the measurement cell based on the calculated resonant frequency response characteristic.

In some examples, there is described a method comprising,

obtaining a spectral response from a measurement cell across some or all of a selected band, that selected band being based on the particular frequency response characteristic of the measurement cell and fluid to be measured in response to communication of an excitation signal to the cell and fluid comprising a plurality of frequencies within the selected band;

calculating the density of fluid in the measurement cell based on the calculated resonant frequency response characteristic. In further examples, there is described a measurement system comprising:

a measurement cell for fluid to be measured, the cell having a fluid inlet and fluid outlet,

one or more isolation couplings, the or each coupling arranged to mount or otherwise couple the inlet and/or outlet of the measurement cell with a structure and configured to assist in vibrationally isolating the measurement cell from an attached structure.

The coupling(s) may provide an impedance interface (e.g. an impedance mismatch) between the measurement cell and a structure.

The or each coupling may comprise a flexible conduit, or otherwise flexible interface, configured to be arranged between the measurement cell and a fluid/flow structure. The flexible coupling may comprise braided, or woven, tubing, or the like. The coupling(s) may permit passage of fluid from/to the structure to/from the measurement cell.

The system (e.g. the or each coupling) may comprise a conditioning mechanism, configured to impart a particular mechanical condition to the system (e.g. the/each coupling). In some examples, the conditioning mechanism may be configured to impart a particular stress regime in the coupling, such as tension or compression. The conditioning mechanism may be adjustable. The conditioning mechanism may be configured as one or more braces or struts, for example extending along the coupling. The conditioning mechanism may be configured to be adjustable so as to impart a particular mechanical condition on the coupling depending on one or more of: expected fluids or flow regimes, expected pressures, expected temperatures. The conditioning mechanism may be set, based on expected flow conditions, and then adjusted based on revised flow conditions. The conditioning mechanism may be considered to be pre- stress the coupling.

In further examples, there is described a measurement system comprising:

a measurement cell for fluid to be measured,

an excitation device mechanically coupled to the measurement cell and configured to communicate an excitation signal to the cell, and wherein

the system is configured such that, when coupled to the measurement cell, the excitation device exhibits a response node at the excitation device, when communicating an excitation signal to the cell.

In similar words, in use, the excitation device may exhibit a region of limited or no movement in response to excitation, even though being coupled to an excited measurement cell. The system may be configured such that interconnectors, such a signal cabling and/or power supplied, etc. are attached or coupled to the excitation device at the response node. Such interconnector(s) may be used for signalling, power, etc., between the excitation device and/or response sensor and the controller. In further examples, wireless couplings (e.g. transponders, inductors) may be provided at the response node for communication and/or power to/from the excitation device.

In further examples, there is described a measurement system comprising:

a measurement cell for fluid to be measured,

two or more nodal constraints configured to inhibit, or otherwise mitigate, movement of the measurement cell in use, wherein

at least a first nodal constraint is positioned at or near the inlet of the measurement cell and at least a second nodal constraint is positioned at or near the outlet of the measurement cell. The nodal constraints may comprise one or more mass elements. The mass element(s) may be attached to the measurement cell (e.g. attached at or near intended nodal positions).

The system may comprise one or more dampening elements. Such dampening elements may be specifically selected or configured to assist in provided a particular frequency response (e.g. a relatively broad peak resonance, compared to having no dampening elements).

In some examples, the system may have dampening elements in addition to the one or more constraints. In those cases, the constraints (e.g. comprising mass element(s)) may be attached to the measurement cell via one or more dampening elements. In other words, the dampening elements may interface between the constraints, or otherwise mass elements, and the measurement cell.

In some examples, there is provided a computer program product or computer file configured to at least partially (or fully) implement the system and methods as described above. In some examples, there is also provided a carrier medium comprising or encoding the computer program product or computer file. In some examples, there is also provided processing apparatus when programmed with the computer program product described. Some of the above examples may implement certain functionality by means of software, but also that functionality could equally be implemented solely in hardware (for example by means of one or more ASICs (application specific integrated circuit) or Field Programmable Gate Arrays (FPGAs)), or indeed by a mix of hardware and software (e.g. firmware). As such, the scope of the present invention should not be interpreted as being limited only to being implemented in software.

Aspect of the inventions described may include one or more examples, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It will be appreciated that one or more embodiments/examples may be useful with measuring fluids. The above summary is intended to be merely exemplary and non-limiting.

Brief Description of the Figures

A description is now given, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 shows a simplified representation of an exemplary oil and gas structure comprising a measurement system;

Figure 2a shows a detailed representation of the measurement system of Figure 1 , while Figure 2b shows an exemplary frequency response characteristic of a measurement cell of the system;

Figures 3a and 3b show inlets, and couplings, of the measurement system, while Figure 3c shows an example of a conditioning mechanism for the system;

Figure 4a shows a cross-sectional view of a measurement cell of the measurement system, Figure 4b shows a lifting mode response of the cell, and Figure 4c shows a section through A-A in Figure 4a; Figure 5 shows an example of an excitation signal; Figures 6a and 6b show an excitation device of the system;

Figures 7a and 7b show exemplary spectral data and frequency response characteristics of the cell when measuring oil and water, respectively;

Figures 8a, 8b and 8c show similar exemplary spectral data and frequency response characteristics of the cell, but having varying gas-volume fractions;

Figure 9 shows steps in calculating density; and

Figure 10a, 10b and 10c show a further example of a system having a plurality of spaced constraints.

Description of Specific Embodiments

While some of the following examples are described with reference specifically to systems, devices and methods for oil and gas measurement, it will nevertheless be appreciated that certain aspects of the described examples may equally be used beyond the oil and gas industry, for example within industries in which it is important - or helpful - to accurately and/or quickly and/or cost effectively measure fluids (e.g. measure fluid densities). In addition, while in some examples the fluids being measured may include hydrocarbons, water, etc. - and in some cases multi-phase flows - it nevertheless will be appreciated that in further examples the systems and methods described herein may be used to measure alternative fluids. Figure 1 shows a simplified representation of an exemplary oil and gas structure 50, which in this case can be considered to be an oil and gas pipeline 50 (e.g. a upstream/midstream pipeline, or the like). Integrated with the structure 50, in an inflow or inline manner, is an example of a measurement system 100. In other words, the described system 100 is configured to permit passage of fluid, from the pipeline 50, therethrough. The direction of expected fluid flow, F_in to F_out, is shown in Figure 1. Figure 2 shows a more detailed representation of the system 100. Here, the system 100 is specifically configured to measure the density of oil and/or gas flows (e.g. including multi-phase flows) passing through the system 100, and so through the pipeline 50, as will be further explained. As such, the measurement system 100 may be considered to be a densitometer. Of course, further fluid properties may be measured by the system 100, or indeed derived from any determined fluid density as will be appreciated.

As shown more clearly in Figure 2a, the measurement system 100 comprises a measurement cell 1 10 for fluid to be measured. The cell 110, together with fluid to be measured (e.g. oil, water and/or gas) can be considered to have a particular or expected frequency response characteristic within a band of frequencies. That is to say that the measurement system 100 can be considered to be specifically configured such that the cell 1 10, across a range of expected measurement conditions (such as expected flow rates and conditions, pressures, temperatures, etc.), has a frequent response function (FRF), and peak resonant frequency, that is expected to lie principally within a certain band of frequencies. Figure 2b show an exemplary spectral response 103 having a particular frequency response characteristic 105 of the cell 1 10 within such a band of frequencies. Here, the spectral response and band of frequencies span 530 Hz to 680 Hz and the peak resonant response 107, which is provided at a particular flow composition/condition, has a peak or maximum amplitude response occurring at around 578 Hz. This spectral response 103 will differ, such as increase/decrease in the frequency at which the peak resonant frequency 107 occurs, depending on condition at the cell 110, and this can be correlated with density of fluid in the cell 110, as will be further described below.

In this example, the system 100 is configured such that measurement cell 110 exhibits a lifting mode response. In some cases, this may be the only mode of response during use, i.e. the cell 1 10 may not, or not significantly, exhibit a compressive mode response. In other words, in use and upon excitation the cell 1 10 may be expected to flex substantially axially, but not radially. Such axial flexing is exemplified in Figure 4b.

The system 100 comprises a fluid inlet 112 together with a fluid outlet 1 14, whereby a fluid conduit 1 16 can be considered to extend between the inlet 1 12 and outlet 114, that conduit 1 16 being configured for passage of fluid from the inlet 1 12 to the outlet 114. Both the inlet 112 and the outlet 1 14 of the system 100 are in fluid communication with the pipeline 50. In the example described, the measurement cell 110 and flow conduit 1 16 can be considered to be essentially tubular or cylindrical, permitting passage of fluid therethrough.

Here, the cell 110 is attached to the structure 50 via couplings 120, which in this example can be considered to be isolation couplings 120. The couplings 120 are arranged to mount, or otherwise couple, the inlet and outlet of the cell 110 to the pipeline 50. For ease of reference, Figure 3a provides a representation of the inlet 112 shown in Figure 2a. Each coupling 120 is specifically configured, and intended to assist with, isolating (e.g. mechanically and/or vibrationally isolating) the measurement cell 1 10 from the attached structure 50. For example, the each coupling 120 may be configured to vibrationally isolate the cell 1 10 from the structure 50 at least over the frequency band within which the frequency responses 105 of the cell 110 are expected. Additionally/alternatively, the coupling(s) 120 may be configured to mechanically isolate the cell/structure (e.g. preventing communication of strain, or other induced forces, from the structure 50 or remainder of the system 100 to the cell 110). In some examples, the coupling 120 may in addition be considered to provide an impedance interface, or impedance mismatch, between the measurement cell 1 10 and the structure 50. In such a way, signals communicated with the cell 110 (e.g. injected into the cell), such as acoustic excitation signals, are inhibiting or otherwise mitigated from leaking into the surrounding pipeline 50. In addition, acoustic emissions or vibrations emanating from the pipeline 50 are less likely to be communicated to the measurement cell 1 10 (e.g. as noise). Further, the response of the cell 1 10 itself is less likely to be attenuated via energy dispersion to the pipeline 50, given a better signal-to-noise ratio.

In the example shown, each coupling 120 comprises a compliant or flexible conduit 122, or otherwise compliant or flexible interface. Here, the coupling 120 comprises central flexible conduit, for example a bellowed tubing, covered by a braided, or woven, tubing, or the like. The central flexible conduit permits passage of fluid from/to the pipeline 50 to/from the measurement cell 110.

In some examples, the mechanical condition of the system, or indeed the coupling 120 shown in Figure 3a, may differ when in use, particularly during occurrences of higher than expected flow pressures, and/or significant temperature variations (e.g. when used in desert environments, or the like). In such cases, stresses developed within the pipeline 50 or coupling 120 may be unhelpfully communicated to the measurement cell 110, augmenting the particular resonant frequency response characteristic for a given density of flow condition. If such situations are expected, it may be appropriate to precondition, or condition during use, the system 100 or indeed in this case the coupling 120 so as to impart a particular mechanical condition to the system 100/coupling 120.

Figure 2b shows an example of the system 100 comprising a conditioning mechanism 125, which in this case is for use with the coupling 120. Here, the conditioning mechanism 125 can be considered to impart a particular mechanical condition, or stress regime, in the system/coupling 120, such as tension or compression. In this example, the conditioning mechanism 125 may be considered to be able to modify or vary the mechanical condition (e.g. stiffness) of the coupling 120. While other ways of achieving this are envisaged, in the present example the conditioning mechanism 125 comprises a plurality of braces 127 or struts, positioned radially outward of the coupling 120, and circumferentially spaced around and extending along the coupling 120 (e.g. parallel to a central axis of the coupling 120). The braces 127 or struts are adjustably fixed to conditioning mounts 129 at either end of the coupling 120. In use, the conditioning mechanism 125 (and in particular struts 127) are configured to be adjustable so as to impart a particular mechanical condition on the coupling 120 depending on one or more of: expected fluids or flow regimes, expected pressures, expected temperatures. In some cases, the conditioning mechanism 125 may be set, based on expected conditions, and then adjusted based on revised conditions. In such a way, the particular expected frequency responses of the measurement cell 110 can be maintained, or maintained within appropriate limits. Of course, in some examples, a similar conditioning mechanism 125 may be employed across some or all of the remainder of the system 100. For example, consider further the embodiment shown in Figure 3c. Here, the conditioning system 125 is used across the measurement cell 110. In this particular case, the conditioning system is also used across the isolation couplings 120. It will be appreciated that in some cases, such a conditioning system 125 may be used to impart a particular mechanical condition on the system, but may also be used as a protective casing or the like for the system 110.

As is shown in Figure 2a, and Figures 3a, 3b and 3c, the system 100 in this particular example also comprise a restriction 130, e.g. a region reducing the cross-sectional flow area compared to that of the pipeline 50 (e.g. reduced diameter). Here, the restriction 130 is provided at, or close to the inlet 112 of the measurement cell 1 10 (a corresponding expansion 135 is provided at the outlet 1 14). In the example shown, the inlet restriction 1 12 comprises a frustoconical portion.

In the example shown, the restriction 130 reduces the diameter of the inlet to between 1 and 2 inches (e.g. roughly between 2.5 cm to 5 cm), which corresponds to the cross- section of the measurement cell 110. It will be appreciated that a reduced cross- sectional measurement cell 1 10 (including fluid to be measured) will have a higher inherent peak resonant frequency, compared to a larger cross-sectional cell 1 10. That higher peak frequency response can assist is increasing any subsequent measurement band and so the resolution of the cell 1 10 can be improved. Further, higher frequencies generated at the cell 1 10 are less likely to transmit to the remaining structure 50, and so signal loss can be reduced. Of course, it will be appreciated that in some examples the restriction 130 may not be provided (e.g. depending of the cross-section of the structure 50). The restriction 130 may also assist with, or be configured to, impart a particular flow of fluid entering the measurement cell 1 10. For example, the restriction 130 may be configured to assist in fully or partially homogenising fluid entering the measurement cell 110 (e.g. for a mixed multiphase flow regime).

As mentioned briefly above, the system 100 in this example is configured to exhibit a lifting mode response (e.g. axially flexing). Figure 4a and 4b show a simplified cross- section of the measurement cell 1 10 together with an exaggerated amplitude response 190 of the cell 1 10 when excited, respectively. Figure 4c shows a section of the cell 1 10 through A-A shown in Figure 4a.

In this example, based on expected conditions and excitation, the system 100 exhibits two response nodes 192, 194, or regions of limited or no movement in response to excitation, together with an antinode 196, or region of maximum movement when excited. Here, the cell 1 10 is specifically configured such that the particular resonant frequency response characteristic, and in particular the peak resonant frequency response (mentioned above), lies within a known or expected band of frequencies, irrespective of whether the fluid within the cell is gas or liquid, or combination thereof (including, in some cases, expected solids). Achieving this can be helped by selecting the geometry and material of the cell 1 10 based on expected conditions and fluid to be measured. A lower peak resonant frequency response may be apparent when the densest expected fluid conditions exist (e.g. cell fully containing water), whereas an upper peak resonant frequency response may be provided based on the least dense expected fluid condition (e.g. cell fully containing gas). The cell 110 may be configured such that the particular frequency response characteristic expected, or possible over the measurement range, is based not only on the expected fluids, but also expected pressures and/or temperature in use. To assist in provided a particular mode of frequency response, the system 100 shown in Figure 4a further comprises one or more constraints 140 - in this example two constraints 140. In this particular example, those constraints 140 may be considered as primary constraints or nodal constraints. The first nodal constraint 140 is positioned at or near the inlet of the measurement cell 110, while the second is positioned at or near the outlet of the measurement cell 1 10. The nodal constraints 140 comprise one or more mass elements (e.g. mass elements intended to provide inertia resistance to movement of the cell 1 10). As shown comparing Figure 4a and Figure 4b, the constraints 140 are attached to the measurement cell 110, and define, intended nodal positions 192, 194. Here, the measurement cell 1 10 has a diameter of 1 inch (roughly 2.5 cm) and the nodal constraints 140 are positioned approximately 0.515 m (i.e. 51.5 cm) apart.

In some examples, as is shown here, the system 100 can comprise dampening elements 150 in addition to the one or more constraints 140. In those cases, the constraints 140 (e.g. comprising mass element(s)) may be attached to the measurement cell 1 10 via one or more dampening elements 150. In other words, the dampening elements 150 interface between the constraints 140, or otherwise mass elements, and the measurement cell 110. Here, the dampening elements 150 are compressible and so additionally configured to alleviate issues of mechanical stress between the constraints 140 and the cell 110 associated with varying measurement conditions (e.g. pressures and/or temperatures). However, ideally the dampening elements 150 themselves may exhibit as little as possible hysteresis effects over temperature variation, which may otherwise unduly affect the resonant properties of the system/cell 100/1 10. While rubbers, or the like, present good dampening properties, in this example, dampening elements 150 comprising deformable metal is provided (e.g. lead). Not only does the dampening of the system 100 in such a manner assist with vibrational isolation of the cell 1 10 from the structure/pipeline 50, but also when used with a single degree of freedom response, as described, the dampening elements 150 can be specifically selected to provide, or configured to assist with, a particular frequency response having a relatively broad peak resonance. Such spreading of spectral response 103 can reduce noise effects and assist when calculating a resonant frequency response 107 from a spectral response (e.g. plurality of frequencies) of the cell 110, as will be described in more detail below. Of course, in other examples, the system/cell 110 may be dampened in additional/alternative ways.

The measurement system 100 also comprises an excitation device 160, which communicates an excitation signal 200 to the cell 110. In this case, the signal 200 is communicated essentially directly to the cell 110, but in other situations that need not be the case and the signal may be induced or otherwise communicated indirectly with the cell 110. Inducing the signal within the measurement cell can assist with reducing any mass effects from the excitation device 160 upon the cell 110.

However, the excitation device 160 in this example is configured to impart a force/pressure to the measurement cell 1 10 in order communicate the excitation signal to the cell 1 10. Such mechanical communication may be considered different from, for example, inducing or otherwise providing the signal in a non-contact manner and can, in some cases, permit an increased strength of excitation signal, thus improving signal- to-noise ratio.

Here, the system 100 is configured such that the any excitation signal 200, generated by the excitation device 160, will have - or comprise - a plurality of frequencies within a selected band (e.g. a band spanning 530 Hz to 680 Hz in the example in Figure 2b). That selected band can be specifically based on the particular frequency response characteristic of the cell 1 10 and fluid to be measured (e.g. expected single degree peak resonant response lying between 560 Hz and 660 Hz). In other words, any excitation signal 200 is expected to comprise a band of frequencies overlapping the expected peak frequency response of the measurement cell 1 10.

The excitation device 160 is configured to communicate each of the frequency components simultaneously (e.g. as a complex signal). This may be considered very differently from an incrementally varying signal applied to the cell 110 (e.g. starting by communicating a signal at one particular frequency, and then increasing the signal frequency over time)ln addition to communicating a plurality of frequencies, the excitation signal 200 in this example is periodic. That is to say, the excitation signal 200 comprises a plurality of different frequency components, within the selected band, where each frequency component may be selected (e.g. frequency and/or phase) such that, when combined, the excitation signal 200 maintains a signal period. Further, in some examples, the excitation signal 200 may have a randomised or distributed phase offset. In such cases, the relative phase offsets may provide an excitation signal with relatively uniform amplitude over the time duration of any pulse. This can assist in maintaining a peak amplitude over an entire pulse. Alternatively, the excitation signal may have a zero phase offset, however, this can provide an impulse having a strong output excitation for a brief time period in a pulse, but not the whole pulse. Figure 5a shows an example of a complex periodic excitation signal 200 comprising a plurality of different frequency components with a zero-phase offset. Whereas Figure 5b shows an example of a complex periodic excitation signal 200 comprising a plurality of different frequency components with a randomised or distributed phase offset. In each cases, each individual frequency component of the excitation signal 200 may have the same, or similar, signal strength (e.g. amplitude).

Figure 6a shows an exploded representation of the excitation device 160, comprising a housing 170, for mechanically coupling to the measurement cell 1 10. As is shown in Figure 4a, the excitation device 160 is coupled to the measurement cell 1 10 between an expected node 192 and expected antinode 196 of the cell 1 10. However, the device 160 is coupled substantially closer to the node 192 than to the antinode 196 (e.g. coupled near the constraint 140). In such a way, the effects of mechanically coupling device 160 to the cell 110, and in particular the effect of the mass of the device 160 on any response spectra of the cell 1 10, in use, can be minimised. In this example, the excitation device 160 is coupled to the cell 1 10 via a retaining element, which is provided by a threaded stud on the cell 1 10 (not shown) and a complementary threaded recess 162 at the housing 160. Such a retaining element can permit coupling and/or removal of the excitation device 160 from the measurement cell 110.

Here, the excitation device 160 comprises a signal actuator 164, which in the example given is provided by a mechanical shaker 164. In this particular example, while the signal actuator 164 is used to communicate the excitation signal 200 to the measurement cell, an actuation sensor 166 observes the signal communicated, or force imparted, from the signal actuator 164 to the measurement cell 110. Here, the actuation sensor 166 is positioned between the signal actuator 164 and measurement cell 110. In such a way, in some cases, data representative of the force imparted can be derived from the actuation sensor 1 16 and used to normalise (or otherwise remove) effects of the excitation device on the cell 110, when observing a response. Of course, in other example, no force sensor may be provided. In this example, a controller 300 is in communication with and used to operate the excitation device 160.

As is shown, the housing 170 of the device 160 fully surrounds the signal actuator 164 and in this case, the excitation device 160 is configured to comply with explosion-proof ratings (e.g. may be rated Ex-d, or the like).

Further, and as is shown in Figure 6b, the device 160 (and system 100) is arranged such that, when coupled to the measurement cell 100, the excitation device 160 (e.g. the housing 170 of the excitation device 160) exhibits a response node 180 at the excitation device 160, when communicating an excitation signal to the cell 110. In similar words, in use, the excitation device 160 itself exhibits a region of limited, or no movement, during, or in response to, excitation of the cell 1 10 (e.g. even though the device/housing 160/170 may be coupled to an excited measurement cell 110). Such a response node 180 means that wired or wireless interconnectors, such a signal cabling and/or power supplied, etc. can be attached or coupled to the device/housing 160/170 at the response node 180, without adversely affecting of the vibration of the system 100, or in the case of wireless interconnection, without unduly affecting an inductance gap, or the like.

Of course, it will be appreciated that in some systems 100, the excitation device 160 may be positioned close to, or at, the expected antinode of the cell 1 10. In those cases, the excitation device 160 may be configured to mechanically, or indeed inductively communicate, the excitation signal 200 with the cell 1 10. For example, in situations whereby the system 100 comprises an outer enclosure (e.g. similar to Figure 3c, with or without capabilities of the conditioning mechanism 125), any outer enclosure may act as a housing, or the like, which may be explosion proof. A skilled reader will readily be able to implement such further embodiments.

Here, the system 100 further comprises one or more response sensors 190 configured to obtain a response, such as a spectral response, from the measurement cell 110 across some or all of the selected band (i.e. the band associated with the excitation signal 200).

While it will be appreciated that a plurality of sensors may be used, here, one response sensor 190 is provided and configured to sense the rate of change of movement of the measurement cell (e.g. provided by an inertial sensor, such as an accelerometer).

While in some examples, that sensor 190 may be coupled to the measurement cell 110 at region having an expected antinode 196, or the like, of the cell 110, in other situations - as is the case here - the response sensor 190 may be provided together with the excitation device 160 (e.g. within the housing 170 of the excitation device 160).

In such a way, any signalling, etc. is containing with the housing 170, which can be made safe for use in hazardous environments.

The response sensor 190 is configured to obtain a spectral response of the cell 110, when excited. A controller 300, comprising a processor 310 and memory 320 arranged in a known manner, is used to sample and obtain that spectral response of the cell 1 10 using the sensor 190, during excitation. In some cases, the controller 300 may comprise an application specific integrated circuit, field programmable gate array, or the like, and may be implemented by hardware or firmware and software. In some examples, the system 100 further comprises a user interface 330, configured to permit input and/or output at the controller 300. While the controller 300 and/or user interface 330 may be positioned at the measurement cell 110, indeed they may be positioned remotely from the cell, and may be configured to communicated via wired and/or wirelessly with the remainder of the system 100.

In use, and as will be explained, the controller 300 is configured to calculate (or otherwise determine or compute) the resonant frequency response characteristic of the cell 1 10 and fluid - which may be, or include, the peak resonant frequency of the cell 1 10 and fluid - based on some or all of the obtained response (e.g. some or all of the spectral response observed by the response sensor 190). Further, the controller 300 is configured then to calculate, or otherwise determine/compute, the density of the fluid in the measurement cell 110 based on that calculated resonant frequency response characteristic.

In use, when fluid - such as oil and gas to be measured - is within the cell 1 10 (e.g. flowing through the cell 110), a complex excitation signal 200 can be communicated to the cell 110. In this example, the controller 300 is used to operate the excitation device 160. Depending on the properties of the fluid, the cell 1 10 will exhibit a particular spectral response. That response can be observed by the response sensor 190, and periodically sampled by the controller 300, to provide spectral data representing a plurality of discrete frequency values, having frequency intervals and amplitudes (e.g. using Fast Fourier Transform techniques at the controller 300).

Figure 7a shows an exemplary spectral response 303 to an excitation signal 200 for a particular signal period when only oil is flowing in the cell 1 10. Here, spectral data 310 is obtained by the controller 300 for a plurality of discrete frequency values, having frequency intervals and amplitudes. From that data, the controller 300 is configured to compute statistically, for example using regressive analysis (e.g. a least-squares approach) the resonant frequency response characteristic, and in this case the peak resonant frequency response 307 of the measurement cell 110. In this example, the controller subsequently calculates the density of fluid by using, for example comparing, the calculated resonant frequency response characteristic (e.g. peak resonant frequency response) and/or spectral data together with one or more reference frequency response characteristic. Those reference frequency response characteristics may be derived from calibration methods and/or theoretical methods or models, or the like, (e.g. previous calibration methods, under static and/or flow conditions). In some examples, the controller 300 additionally uses further parameters, such as (i) measured or expected/estimated temperature of the fluid and/or cell, (ii) measured or expected/estimated flow pressures, (iii) measured or expected/estimated flow rates, or the like, when calculating the density. Figure 7b shows an exemplary spectral response 403 to an excitation signal 200 for a particular signal period when only water is flowing in the cell 1 10. While the fluid being measured in the example in Figures 7a and 7b was simply oil or water, respectively, in some cases - particularly in the oil and gas industry - the fluids may be multi-phase flows (e.g. comprising liquids and gases).

Consider now Figures 8a, 8b and 8c which show exemplary spectral response 503, 603, 703 for a particular signal period for various flows having different gas-volume fractions (22.5%, 37.5% and 68.0%, respectively).

Again, spectral data 510, 610, 710 is obtained by the controller 300 for a plurality of discrete frequency values, having frequency intervals and amplitudes. Unlike the spectral data provided in Figures 7a and 7b, here the data 510, 610, 710 does not immediately provide a smooth polynomial relationship across the band of frequencies. This is particularly true as the gas-volume fraction increases. However, as above, from that data 510, 610, 710, the controller 300 is configured to compute statistically, for example using regressive analysis (e.g. a least-squares approach) resonant frequency response characteristics (e.g. peak frequency response 507, 607, 707) of the measurement cell 1 10. Again, from this, the controller 300 subsequently calculates the density of fluid by using, for example comparing, the calculated resonant frequency response characteristic and/or spectral data together with one or more reference frequency responses. It will be appreciated that in some cases, the controller 300 may not actually calculate the absolute peak resonant response, but rather compute an overall resonant frequency response mode or the like as the characteristic from the spectral data, that mode usable to subsequently compute density. A skilled reader will readily be able to implement such further systems.

Figure 9 shows a flow diagram of the above steps. Firstly, the excitation signal 200 is communicated 1000 to a measurement cell 110 using the excitation device 160. As mentioned above, the cell 1 10 together with the fluid being measured have a particular frequency response within a band of frequencies, and the excitation signal 200 comprises a plurality of frequencies within a selected band based on the particular (e.g. expected) frequency response of the cell and fluid to be measured. Secondly, a spectral response 1100 is obtained from the measurement cell across some or all of that selected band. Thirdly, using the methods described above, a resonant frequency response characteristic (e.g. peak response, modal shape, or the like) of the cell and fluid based on some or all of the obtained spectral response can be calculated 1200 (e.g. using a least-squares approach). Lastly, the density of fluid in the measurement cell 1 10 can be calculated 1300 based on the calculated resonant frequency response characteristic. This can be achieved by, for example, comparing the calculated resonant frequency response characteristic (and in some examples some or all of the calculated frequency response function over the measurement band) with models derived from calibration.

It will be appreciated that in some examples, the density of fluid may be calculated remotely from the measurement cell/system 110/100. In some cases, the method may comprise calculating density simply from one or more data sets comprising previously obtained or collected spectral data. Further, it will be appreciated that, based on density, further properties of the fluid may be calculated, such as flow rates, etc.

It should also be noted (particularly at higher gas-volume fractions), that the calculated peak resonant frequency response of the cell 1 10 may be different from that of the maximum observed frequency response 708 from the response sensor 190 - see Figures 8a, 8b and in particular Figure 8c. In some cases, this difference can be significant such that observing or tracking only the maximum response from the cell 1 10 may lead to erroneous results. In the described examples, the controller 300 can compute, from the plurality of observed/determined frequency values within the spectral response/data the likely, or most likely, resonant frequency response of the measurement cell (e.g. measurement cell and fluid to be measured). Or put differently, the controller interpolates and/or extrapolates, from a plurality of observed/determined frequency values within the spectral response/data the likely, or most likely, resonant frequency response of the measurement cell (e.g. measurement cell and fluid to be measured). That computed resonant frequency (and in some cases the spectral response) can be compared with correlated reference responses in order to calculate/confirm the density of fluid at that time interval until a maximum response is observed. Firstly, in such Coriolis systems this is time consuming for fast changing flow conditions, but secondly as shown in Figure 8c, such tracking of the frequency can lead to erroneous results, when operating with higher gas-volume fractions. Consequently, the above described system and methods can be quicker and more accurate than such Coriolis devices. In addition, due to the accurate measurement at each sample interval, the described methods and systems additionally are better suited to identifying changes in flow regime, or the like.

Further, it will be appreciated that due to use of spectral data 510, 610, 710, and the periodic nature of the excitation signal 200, the density of the fluid can be calculated at each sample interval, and so averaging of multiple calculated responses need not be performed. In other words, each sample interval provides an accurate representation of the density of fluid in the cell 110 at that time. As such, accurate analysis and identification of particular flow regimes may be performed (e.g. slugging). In addition, the analysis time can be not only accurate, but also quick (e.g. one spectrum per calculation).

However, in some cases, for example under flow conditions that are not expected to vary significantly, such averaging may indeed be performed by the controller 300. Further, in some examples where signals are communicated to a mixed flow, a certain amount of decoupling may occur, i.e. different phases responding (and moving) relatively differently to those signals. However, in this case, the use of multiple excitation signals obviates or at least minimising any difference in relative motion. It is also worth re-iterating, that when the cell 1 10 is suitably dampened (e.g. using dampening elements 150), the frequency response function, and in particular the half- power bandwidth of the response is increased, reducing susceptibility to broad- spectrum noise and improving accuracy of any best-fit analysis at higher gas-volume fractions.

However, in some circumstances, such as those having higher gas-volume fractions, the provision of the fluid to measured, such as gas, in the flow may in fact act to dampen the cell 1 10/system 100. In those cases, and in different circumstances, no dampening elements 150 may be provided. For example, consider the system 1000 shown in Figure 10, which is similar to that shown in Figure 4a and 4c. Again, the system 1000 is configured to exhibit a lifting mode response (e.g. axially flexing). Figure 10a and 10b show a simplified cross-section of a measurement cell 1 100 together with an exaggerated amplitude response 1900 of the cell 1100 when excited, respectively. Figure 10c shows a section of the cell 1100 through A-A shown in Figure 10a.

Again, based on expected conditions and excitation, the system 1000 exhibits two response nodes 1920, 1940, or regions of limited or no movement in response to excitation, together with an antinode 1960, or region of maximum movement, when excited. To assist in provided a particular mode of frequency response, the system 1000 again comprises constraints 1400, 1410. However, unlike Figure 4a, in this example four constraints are shown 1400, 1410. Here, primary constraints 1400 are attached to, or formed integrally with, the measurement cell 1100 to define first intended nodal positions 1920, 1940, whereas secondary constraints 1410 are likewise attached to, or formed integrally with, the measurement cell but positioned outwardly, and axially along the cell 1 100, from the primary constraints 1400. Put differently, a constraint spacing 1420 (e.g. an axial constraint spacing) exists between primary and secondary constraints 1400, 1410, each side of the measurement cell 1100. In this example, no dampening elements 150 are provided, but of course that need not always be the case. Further, the primary and secondary constraints 1400, 1410 in the example shown are formed integrally with the measurement cell 1 100, but of course, they may otherwise by clamped on as before.

In this case, the provisional of the additional constraints 1410 act to further isolate and attenuated any vibration beyond the measurement cell 1 100. In other words, the additional secondary constraints (e.g. spaced from the primary constraints) act to filter and remove unwanted noise. Of course, while only four constraints 1400, 1410 have been exemplified here, it will readily be appreciated that more may be provided (e.g. further secondary - or indeed tertiary - constraints may be provided).

It will be appreciated that any of the aforementioned structures, systems, devices, etc. may have other functions in addition to the mentioned functions, and that these functions may be performed by the same structures/systems/devices.

Further, it will readily be appreciated that aspects of the described embodiments may be used in alternative measurement systems. For example, the couplings 120 and/or constraints/dampening elements, etc., may be used of further excitation devices, not using spectral data, but still nevertheless requiring isolation or the like from the structure.

Further, while the above example describes a single degree of freedom system - which assists with ease of regressive analysis of the data, in other examples, that need not be the case, and further different modes of vibration may be utilised. The described measurement systems and methods permit quicker and/or more cost effective and/or more accurate determination of fluid properties, such as density, particularly multiphase flows (e.g. having a gas-volume fractions greater than 40%) and/or fluids having a low flow rate (e.g. fewer than 100 bbl/day). The systems and methods can also permit the determination of further properties, such as flow regimes, or the like.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the spirit and scope of the invention.

Claims

CLAIMS:

1. A measurement system comprising:

a measurement cell for fluid to be measured, the cell together with the fluid to be measured having a particular frequency response characteristic within a band of frequencies;

an excitation device configured to communicate an excitation signal to the cell, such an excitation signal comprising a plurality of frequencies within a selected band, that selected band based on the particular frequency response characteristic of the cell and fluid to be measured;

2. The system according to claim 1 , wherein the controller is configured to calculate the density of fluid by comparing a calculated resonant frequency response characteristic together with one or more reference frequency response characteristics, those reference frequency response characteristics derived from calibration methods and/or theoretical methods or models.

3. The system according to claim 1 or 2, wherein the controller is configured to obtain spectral data from the spectral response, that data representing a plurality of discrete frequency values having frequency intervals and amplitudes.

4. The system according to any of the claims 1 to 3, wherein the controller is configured to compute, using regressive analysis, the resonant frequency response characteristic using the spectral response derived from the measurement cell.

5. The system according to claim 4, wherein the controller uses a least-squares analysis of the spectral response.

6. The system according to any of the claims 1 to 6, wherein the controller calculates the peak resonant frequency response of the cell and fluid based on some or all of the obtained spectral response, and calculates the density of fluid in the measurement cell based on that peak resonant frequency response.

7. The system according to any of the claim 1 to 6, wherein the excitation is configured to communicate a periodic excitation signal to the measurement cell.

8. The system according to claim 7, wherein the controller is configured to calculate, for each periodic iteration of the excitation signal, the response of the measurement cell and to calculate the fluid density for each periodic iteration of excitation signal.

9. The system according to any of the claims 1 to 8, wherein the excitation device is configured to communicate each of the plurality of frequencies simultaneously as a complex signal.

10. The system according to any of the claim 1 to 9, wherein the cell is specifically configured, or constrained, to have a single resonant frequency response within a band of frequencies.

1 1. The system according to any of the claims 1 to 10, further comprising two constraints, a first positioned at or near an inlet of the measurement cell and a second positioned at or near an outlet of the measurement cell, the constraints configured to inhibit, or otherwise mitigate, aspects of movement of the measurement cell in use.

12. The system according to claim 11 comprising one or more dampening elements, the constraints being attached to the measurement cell via the one or more dampening elements.

13. The system according to claim 11 or 12 wherein the two constraints are primary constraints defining first intended nodal positions, and the system comprises two secondary constraints, spaced outwardly and axially along the cell, from the primary constraints.

14. The system according to any of the claims 1 to 13, wherein the excitation device is mechanically coupled to the measurement cell, and configured to communicate an excitation signal mechanically to the measurement cell.

15. The system according to claim 14, wherein the excitation device is coupled to the measurement cell between an expected node and expected antinode of the cell.

16. The system according to claim 15, wherein the excitation device is coupled substantially closer to a node than to an antinode of the cell.

17. The system according to any of the claims 1 to 16, wherein the excitation device comprise one or more signal actuators configured to communicate an excitation signal to the measurement cell, and further wherein the excitation device additionally comprises an actuation sensor, positioned between the signal actuator and measurement cell, and configured to observe the signal communicated, or force imparted, from the signal actuator to the measurement cell.

18. The system according to any of the claims 1 to 17, wherein the excitation device comprises a fully surrounding housing.

19. The system according to claim 18, wherein the response sensor is provided as an inertial sensor within the housing of the excitation device.

20. The system according to any of the claims 1 to 19, wherein the excitation device and/or response sensor is positioned at an expected antinode of the measurement cell.

21. The system according to claim 18 or 19, wherein the housing of the excitation device exhibits a response node exhibiting limited, or no movement, at the excitation device, when communicating an excitation signal to the cell, and wherein wired or wireless interconnectors are attached or coupled to the excitation device at the response node.

22. The system according to any of the claims 1 to 21 , further comprising one or more couplings arranged to mount or otherwise couple an inlet and/or outlet of the measurement cell with an oil and gas structure in an inflow manner.

23. The system according to claim 22, wherein the coupling(s) are configured to vibrationally isolate the measurement cell from an attached structure.

24. The system according to claim 22 or 23, wherein the or each coupling comprises a compliant conduit, or otherwise flexible interface, configured to be arranged between the measurement cell and a fluid/flow structure.

25. The system according to any of the claims 22 to 24, comprising a conditioning mechanism, configured to impart a particular mechanical condition to the system, the conditioning mechanism being adjustable so as to impart that particular mechanical condition depending on one or more of: expected fluids or flow regimes, expected pressures, expected temperatures.

26. The system according to claim 25, wherein the conditioning mechanism is provided at the or each coupling, and configured to impart a particular mechanical condition to the coupling.

27. The system according to any preceding claim, comprise one or more restrictions, the or each restriction provided at, or close to an inlet of the measurement cell.

28. The system according to any preceding claim, wherein the measurement cell is configured to have a particular frequency response between 10 Hz and 5000 Hz.

29. The system according to any preceding claim, wherein the system is configured to measure density of water, oil and/or gas.

30. The system according to any preceding claim, wherein the system is configured to be integrated, in an inflow manner, into an oil and gas pipeline.

31. The system according to claim 30 wherein, when integrated, the system can be considered to be substantially acoustically isolated from the structure.

32. A controller:

the controller configured to obtain spectral data derived from a response of a measurement cell, that spectral data being across some or all of a selected band, wherein that selected band is based on a particular frequency response characteristic of a measurement cell and fluid to be measured in response to communication of an excitation signal to a cell and fluid comprising a plurality of frequencies within the selected band; and wherein

33. A method comprising:

communicating an excitation signal to a measurement cell, that cell together with a fluid to be measured having a particular frequency response characteristic within a band of frequencies, wherein the excitation signal comprises a plurality of frequencies within a selected band, and wherein that selected band is based on the particular frequency response characteristic of the cell and fluid to be measured;

obtaining a spectral response from the measurement cell across some or all of the selected band; calculating a resonant frequency response characteristic of the cell and fluid based on some or all of the obtained spectral response, and

34. A method comprising:

35. A measurement system comprising:

36. A system according to claim 35, wherein the or each coupling comprises a compliant conduit, or otherwise flexible interface, configured to be arranged between the measurement cell and a fluid/flow structure.

37. The system according to claim 35 or 36, wherein the system comprises a conditioning mechanism, configured to impart a particular mechanical condition to the system, and wherein the conditioning mechanism is adjustable so as to impart a particular mechanical condition depending on one or more of: expected fluids or flow regimes, expected pressures, expected temperatures.

38. The system according to claim 37, wherein the conditioning mechanism is provided at the or each coupling, and configured to impart a particular mechanical condition to the coupling.

39. A measurement system comprising:

a measurement cell for fluid to be measured,

40. A system according to claim 39, wherein interconnectors, such a signal cabling and/or power supplies, are attached or coupled to the excitation device at the response node.

41. A measurement system comprising:

a measurement cell for fluid to be measured,

two or more constraints configured to inhibit, or otherwise mitigate, movement of the measurement cell in use, wherein at least a first constraint is positioned at or near the inlet of the measurement cell and at least a second constraint is positioned at or near the outlet of the measurement cell.

42. The system according to claim 41 , comprising one or more dampening elements, the constraints being attached to the measurement cell via one or more dampening elements.

43. The system according to claim 40 or 41 wherein the two constraints are primary constraints defining first intended nodal positions, and the system comprises two secondary constraints, spaced outwardly and axially along the cell from the primary constraints.

44. A computer program product configured to implement the method of claims 33 or 34.