WO2014163509A1 - System and method for determining fluid levels interfaces - Google Patents

System and method for determining fluid levels interfaces Download PDF

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Publication number
WO2014163509A1
WO2014163509A1 PCT/NO2014/050045 NO2014050045W WO2014163509A1 WO 2014163509 A1 WO2014163509 A1 WO 2014163509A1 NO 2014050045 W NO2014050045 W NO 2014050045W WO 2014163509 A1 WO2014163509 A1 WO 2014163509A1
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equivalent
fluid
vertical
optically transparent
determining
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PCT/NO2014/050045
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French (fr)
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Børre MORTENSEN
Olav SANDNES
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Univisual-Instruments As
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Publication of WO2014163509A1 publication Critical patent/WO2014163509A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
    • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
    • G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
    • G01F23/284—Electromagnetic waves
    • G01F23/292—Light, e.g. infrared or ultraviolet

Definitions

  • the invention relates to the determination of fluid level interface values in tanks comprising separated fluids. More specifically the invention is related to the determination of interface levels of separated fluids e.g. multiphase fluids, by analyzing fluids residing between optically transparent walls, such as a transparent level gauge.
  • level gauges are quite often used for determination of liquid levels in tank units.
  • Level gauges are usually divided into three categories depending on the function principle; the magnetic-, reflexion based and transparent.
  • the transparent gauges are the simplest where the liquid level inside the tube glass simply is being read up against a scale that is fixed to the tube on the outside.
  • the other two types are equipped with supporting mechanisms so that the level can be read more conveniently compared to the transparent type.
  • the latter categories are also often equipped with processor units providing the process- and control systems with level data as I/O-signals.
  • Crude oil is being separated from the water and gas by support from so called separation tank units, directly connected to the production wells at the sea bottom.
  • the operational pressure can reach as much as 3-400 bars and temperatures well above 100 °C are common.
  • separation tanks are often designed out of special alloys and with wall thickness of 10-15 cm.
  • Transparent types can be used, but only for visual purposes.
  • Other kind of metering systems must therefore be taken into consideration for reading and conversion into I/O-signals.
  • the most common alternative utilizes gamma radiation for penetration of the tanks from one side to the other.
  • DP measurements Differential pressure measurements
  • radioactive measurements have also been used, but a problem related to both the radioactive measurements and the DP-measurements is that they are very dependent on the density of the fluids, and frequent calibration is required.
  • the transparent level gauges in use today are located close to the tank and must therefore be regularly visited by operators who record the values of the different fluid interface levels.
  • Some visual systems have been disclosed to allow remote monitoring of transparent level gauges. This includes telescopes in the control room, which may work under most conditions, but are less functional due to varying light and weather conditions.
  • US patent 2,714,167 discloses a liquid level measuring apparatus with a radiation detector where a float inside the tank comprises a radioactive source floating on the liquid. Outside the tank a radiation detector can be moved up and down in parallel with the floating source.
  • European patent application EP0381894 Al discloses a sight gauge installation for the remote sensing of a liquid-level. The installation comprises a video camera and means for moving the camera up and down the sight tube. The installation comprises a monitor where a cursor line can be adjusted.
  • Japanese patent 10062231A discloses measurement of a water level by a camera that records images of a water gauge, and measures and determines a height of the water level from a reference level. The water level together with a scale can be displayed on a screen.
  • the container is irradiated by light, and the radiation which passes through the container is captured, converted into a video image and evaluated in order to determine the position of the interface in terms of height.
  • the invention is a method for determining fluid interface levels between two or more separated fluids residing between a first optically transparent wall and a second optically transparent wall , wherein the method comprises the following steps;
  • the fluid interface levels are now available from the signal processing module in real time, and can be used by other systems, such as by a remote process control and monitoring system.
  • the availability of reliable measurement values as I/O signals is important for e.g. the control of the separation process to improve the efficiency.
  • the fluid interface levels determined by the method represents the real fluid interface levels as indicated by the optically transparent walls, and will adapt to changing fluid densities, without the need for frequent recalibration.
  • the method according to the invention also solves the problems related to health risks related to prior art radio-active instruments.
  • the first optically transparent wall and the second optically transparent wall are arranged inside a fluid tank.
  • the arrangement of the transparent walls inside the tank has a number of advantages.
  • Fig. la illustrates a generic setup of a computer based fluid level monitoring system (10) according to the invention in a schematic drawing.
  • Fig. lb is an image of example separated fluids inside a tank.
  • Fig. 2 illustrates how the fluid transition levels (N l, N2,..) are determined from a set of digital images from the video cameras.
  • Fig. 3a shows by way of a flow diagram the method for determining the fluid transition levels (N l, N2,..) according to an embodiment of the invention.
  • Fig. 3b shows by way of a flow diagram the method for determining the fluid transition levels (N l, N2,..) according to an embodiment of the invention.
  • Fig. 4a illustrates in a simplified schematic drawing the computer based fluid level monitoring system (10) where a transparent level gauge external to the tank is monitored.
  • Fig. 4b illustrates in a simplified schematic drawing the computer based fluid level monitoring system (10) where the monitoring is performed inside the tank.
  • Fig. 5a illustrates in a top sectional view a screen encapsulating the cameras, the light sources and the fluid sample.
  • Fig. 5b illustrates the same as in Fig. 5a, with the addition of light sources illuminating the fluid sample from the camera side.
  • the object of the invention is to accurately determine fluid interface levels between separated fluids in the same tank or container.
  • separated fluids is used to indicate fluids with different density. Multiphase fluids from a petroleum separation process are an example of separated fluids. In this way the more dense fluids will sink to the bottom and the less dense fluids will raise. The different separated fluids will interface at certain vertical levels, defined as fluid interface levels.
  • Multiphase separation is an important process in the oil industry, and it is important to understand the internal conditions in the tank to be able to optimise the separation process.
  • crude oil from oil wells is pumped to large tanks, to separate gas, oil and water.
  • other materials, such as sand should also be separated from the final products.
  • the most common way of separating the components is by relying on gravity.
  • Fig. 1 b shows a real example of a composition of separated fluids in a separation tank, consisting of gas, oil, oil sludge, water and sand.
  • a foam layer is shown on top of the oil layer. Foaming is considered a problem in the separation process since it may limit the output capacity of the separator. To maintain full capacity, antifoaming chemicals are often added to the separation process.
  • the fluid interface levels between gas and foam, foam and oil, oil and oil sludge, oil sludge and water, and water and sand can be seen in Fig. 1.
  • FIG. 1 a generic setup of a computer based fluid level monitoring system (10) according to the invention is illustrated in a schematic drawing.
  • the sample of the separated fluids e.g. as shown in Fig. lb, is placed behind a first optically
  • Two or more video cameras (2) are arranged above each other outside the first optically transparent wall (la) with respect to the sample.
  • Each camera (2) covers a vertical section (22) of the sample that overlaps with the vertical section of a neighbouring camera (2) arranged above and/or below.
  • the first optically transparent wall (la) should be transparent for the wavelengths used by the cameras (2). In most situations this will be visible light, but may also be other wavelengths in the area from infrared to ultra violet.
  • the digital images (21) from the cameras (2) will be transferred to a signal processer module of a computer (11) that is responsible for image processing of the digital images.
  • the video transfer may be based on any video transfer protocol, wired or wireless.
  • a control module (6) in the computer (11), may in an embodiment be responsible for controlling the cameras (2).
  • the control module (6) may be implemented in a computer separate from the computer (11) that is performing the image processing of the video signals from the cameras (2).
  • Fig. 2 shows an example of how the interface levels of the separated fluids in Fig. 1 b are determined.
  • digital images (21) of the sample are captured by the cameras (2). In Fig. 2 it can be seen that they have overlapping vertical sections or ranges (22).
  • a full or combined digital image (51) is compiled from the digital images (21).
  • the combined digital image (51) is a digital image representing the sample.
  • the merge or compilation of the combined digital image (51) may be performed by edge-blending or similar available technologies applicable for merging overlapping images.
  • the combined digital image (51) is then partitioned into a number of vertical intervals (d), and the light intensity for each of the vertical intervals (d) is registered.
  • This partitioning may be performed by scanning the combined digital image (51) in vertical direction.
  • the light intensity of the vertical section (d) will depend on the darkness of the combined digital image (51) in the specific section (d).
  • the accurate determination of the light intensity is not necessary, since the intensity of one vertical section relative another vertical section can be used for the later determination of fluid interface levels. This is defined as the equivalent interval light intensity (M).
  • the fluid interface levels (N l, N2,%) may then be determined by comparing the light intensity of the vertical sections (d).
  • An example of resulting fluid interface levels can be seen in Fig. 2, where N l is the interface level between water and sand, N2 is the interface level between oil sludge and water, N3 is the interface level between oil and oil sludge, N4 is the interface level between gas and oil and N5 is the interface level between foam and gas.
  • the invention is therefore a method, as illustrated in Fig.
  • the invention is also a computer based fluid level monitoring system (10) for determining fluid interface levels (N l, N2,%) between two or more separated fluids residing behind a first optically transparent wall (la), the system comprising;
  • each the video camera (2) arranged for - capturing (102) digital images (21) of two or more overlapping vertical sections (22) of the two or more separated fluids through the first optically transparent wall (la),
  • the measurements may be considerably improved by enhancing the illumination of the sample.
  • transillumination i.e. illuminating the sample from one side, and capturing the images on the other.
  • Transillumination increases the difference in light intensity between the different fluids in the sample.
  • the two or more separated fluids reside between the first optically transparent wall (la) and a second optically transparent wall (lb), and the method comprises illuminating (101) the two or more separated fluids through the second optically transparent wall (lb) with light from one or more light sources (4) as can be seen in Fig. 1.
  • the computer based fluid level monitoring system (10) comprises a second optically transparent wall (lb) opposite the first optically transparent wall (la) relative the video cameras (2), and one or more light sources (4) behind the second optically transparent wall (lb) relative the video cameras (2), wherein the one or more light sources (4) are arranged for illuminating (101) the two or more separated fluids between the first optically transparent wall (la) and the second optically transparent wall (lb) through the second optically transparent wall (lb).
  • Another feature according to an embodiment of the invention is to find the maximum (Mmax) and the minimum light intensity (Mmin) of the combined digital image (51). In one embodiment these values are obtained by analysing each of the vertical intervals (d), and determining the lightest and darkest interval. In another embodiment these values may be found by analysing the entire combined digital image (51). Since the content of the tank is known, the types of separated fluids are also known. Thus, the equivalent light intensity values (Mr) for each fluid can be predetermined. This may not be a correct value for the light intensity, but the relative light intensity between the fluids should be correct.
  • This value is further used in the determination of the fluid level interfaces by determining an equivalent fluid phase light intensity (Me) based on the equivalent maximum light intensity (Mmax), the equivalent minimum light intensity (Mmin) and predetermined equivalent light intensity values (Mr) for each of the two or more separated fluids. This is shown to the right in Fig. 2. It thus represents an expected illumination for a specific fluid in the current sample, i.e. in the current combined digital image (51).
  • the equivalent vertical light intensities (M) can now be compared to the equivalent fluid phase light intensity (Me), and from this it may be determined which fluid they represent. I.e., if the equivalent vertical light intensity (M) of a vertical interval (d) has the same value as the equivalent fluid phase light intensity (Me), it may be concluded that the fluid at the height of the vertical interval (d) is of the type represented by the equivalent fluid phase light intensity (Me).
  • one or more vertical fluid interface levels may be determined (109) analysing where the equivalent vertical light intensities (M) suddenly changes from one value to another, i.e. from a value representing one fluid type to a value representing another fluid type.
  • the fluids are expected to have a certain order, i.e. oil sludge should be found between oil and water.
  • noise reduction is performed by suppressing, or not taking notice of equivalent vertical light intensities (M) that have values that do not follow this order.
  • the computer based fluid level monitoring system comprises, in an embodiment a screen (18) arranged behind the video cameras (2) with a height at least the distance between a lowest video camera (2) and a highest video camera (2). Please see Fig. 5a.
  • the screen (18) encloses the one or more light sources (4) and the video cameras (2) in a horizontal plane. This is illustrated in Fig. 5a, illustrating the screen ( 18) in a top sectional view.
  • the light sources (4) are arranged behind a vertical plane (17) between the first optically transparent wall (la) and the second optically transparent wall (lb) relative the cameras (2).
  • the computer based fluid level monitoring system (10) comprises an opaque shield corresponding to the vertical plane (17), to prevent backlight from the light sources (4) to reach the video cameras (2) without passing through the fluids.
  • the computer based fluid level monitoring system (10) comprises one or more front light sources (4b) arranged to illuminate the first optically transparent wall (la) from the side of the video cameras (2). This is illustrated in Fig. 5b.
  • angles a and ⁇ in Fig. 5a and 5b indicate the location of the light sources relative each other and the fluids when more than one light source is used, a is the sector between the back-lights, and ⁇ is the sector between the light sources illuminating the fluids from the front.
  • the computer based fluid level monitoring system comprises a control system (6) as shown in Fig. la.
  • the control system is connected to the light sources (4) and arranged to control the illumination of the separated fluids. In an embodiment it also controls the
  • control system may also be connected to the video cameras (2) to control camera specific parameters, such as focal length and aperture.
  • the digital images (21) and the combined digital image (51) may be in colour. And since the content of the tank is known, the types of separated fluids are also known. Thus, equivalent colour values (Cr) for each fluid can be predetermined. This may not be a correct value for the colour, but the relative colour value between the fluids should be correct.
  • the method for determining the fluid interface levels (N) comprises in this embodiment the steps of; - determining an interval equivalent colour (C) for each the vertical interval (d) of the combined digital image (51),
  • the method comprises creating a synthetic graph (61) of the transparent level gauge (1) arranged for being presented on a screen, wherein the synthetic graph (61) comprises a representation of each of the equivalent fluid phase light intensities (Me) and a visual indication of each of the more vertical fluid interface levels (Nl, N2, ).
  • This synthetic graph will be easier to read than the combined digital image, since noise is filtered out and the fluid level interfaces are indicated specifically and more clearly than in the original image.
  • the synthetic graph (61) comprises graphical effects such as different colours for each separated fluid.
  • the synthetic graph also comprises the combined digital image (51).
  • the colour of the backlight and front light may be different than the colours used when the fluids are monitored for the purpose of determining the fluid transition levels.
  • Transparent level gauges are often used to indicate the fluid interface levels of tanks, as described previously.
  • the first optically transparent wall (la) and the second optically transparent wall (lb) constitute a transparent level gauge arranged outside a fluid tank, wherein the transparent level gauge is arranged to indicate the fluid interface levels (N) in the tank.
  • This embodiment is illustrated in Fig. 4a. It can be seen that the transparent level gauge (la, lb) is arranged external to the tank, and connected to the tank in a lower and an upper end through tubes and valves.
  • the fluid interface levels (N l, N2, ...) are represented both in the tank and in the level gauge. However, rapid fluctuations in the tank may not be immediately visible in the level gauge.
  • the monitoring may also take place inside the tank.
  • the first optically transparent wall (la) and the second optically transparent wall (lb) are arranged inside a fluid tank. Consequently the cameras (2) must also be arranged inside the tank. This is illustrated in Fig. 4b.
  • the tank also comprises the light source(s) (4).
  • one or more light sources, being uniform in the vertical direction are used for illumination.
  • the light sources consist of several discontinuous light sources in the vertical direction.
  • the first optically transparent wall (la) constitutes in an embodiment a wall of a first fluid impermeable container, and the two or more video cameras (2) are arranged inside the first fluid impermeable container.
  • This impermeable container will protect the cameras from the fluids in the tank.
  • front light sources (4b) they may also be arranged inside the first fluid impermeable container.
  • the second optically transparent wall (lb) constitutes in an embodiment a wall of a second fluid impermeable container, and the one or more light sources are arranged inside the container. This impermeable container will protect the light sources from the fluids in the tank.
  • Different methods may be used for different steps when determining the fluid interface levels. Some steps may be related to the initial procedure, while other steps are related to continuous corrections. Usually, the start-up procedure will take longer time than the procedure required for smaller corrections.
  • the procedure may be tailored for this.
  • backlight illumination can be used to render the dark areas almost black, i.e. in this example sand and oil.
  • the following steps can then be performed : - scanning of the combined digital image (51) from the bottom in thin vertical sections (d),
  • the first interface level (N l) is the top of the sand, where the light intensity increases above a certain value, e.g. 5% of maximum value from last scan.
  • the lower level of the oil sludge (N2) is determined, e.g. where the light intensity becomes lower than 2% of the maximum value.
  • the higher level of the oil (N4) is determined, e.g. where the light intensity becomes higher than 10% of the maximum value.
  • the gas-foam interface may be determined after determining higher level of the oil (N4) by further scanning upwards, e.g. where the light intensity becomes higher than 30% of the maximum value.
  • the next step may be to determine the upper level of the oil sludge (N3) by scanning downwards, and looking for an increase in light intensity when in the oil layer.
  • the sludge starts where the light intensity is more than 90 % of the minimum value.
  • the back light may be adjusted to increase the contrast to obtain more accurate readings during the method for the determination of the fluid interface levels.
  • the scanning should continue in the upward direction until the sand-water interface level is found where the light intensity increases above a certain value, e.g. 5% of maximum value.
  • the new scan shows that the sand layer is lower, i.e. intensity is higher than 5% of maximum value where it used to be darker, the scanning should continue in the downward direction until the sand-water interface level is found where the light intensity decreases below a certain value, e.g. 3% of maximum value.
  • the same principle can be used for determining the other interface levels N2, N3, N4 and N5 when the system is in continuous operation to limit the number of vertical sections (d) to be scanned and limit the corresponding processing.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Thermal Sciences (AREA)
  • Fluid Mechanics (AREA)
  • General Physics & Mathematics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
  • Measurement Of Levels Of Liquids Or Fluent Solid Materials (AREA)

Abstract

A method and system for determining fluid interface levels (N1, N2,...) between two or more separated fluids residing between a first and second optically transparent walls (1a, 1b), comprising; - capturing (102) digital images (21) of two or more overlapping vertical sections (22) of the two or more separated fluids through the second optically transparent wall (1b), with respective two or more video cameras (2) arranged above each other, - combining (104) the digital images (22) into a combined digital image (51), - partitioning (105) the combinedl digital image (51) in multiple vertical intervals (d), - determining (106) an equivalent interval light intensity (M) for each the vertical interval (d) of the combined digital image (51), - determine (109) one or more vertical fluid interface levels (N1, N2,...) by comparing the equivalent vertical light intensities (M) for the intervals (d).

Description

SYSTEM AND METHOD FOR DETERMINING FLUID LEVEL INTERFACES Technical field of the invention
The invention relates to the determination of fluid level interface values in tanks comprising separated fluids. More specifically the invention is related to the determination of interface levels of separated fluids e.g. multiphase fluids, by analyzing fluids residing between optically transparent walls, such as a transparent level gauge.
Background art
The process industry utilizes a large set of instruments for collection of process data. Especially, level gauges are quite often used for determination of liquid levels in tank units. Level gauges are usually divided into three categories depending on the function principle; the magnetic-, reflexion based and transparent. The transparent gauges are the simplest where the liquid level inside the tube glass simply is being read up against a scale that is fixed to the tube on the outside. The other two types are equipped with supporting mechanisms so that the level can be read more conveniently compared to the transparent type. The latter categories are also often equipped with processor units providing the process- and control systems with level data as I/O-signals.
Some industry segments faces some extra challenges related to level metering. One such area is reading of liquid levels in tanks that are exposed for high pressure and temperature, combined with compound liquids that are separating into two or more components. This is typical for the offshore-industry with crude oil mix from the ground for separation into water, oil and gas.
Crude oil is being separated from the water and gas by support from so called separation tank units, directly connected to the production wells at the sea bottom. The operational pressure can reach as much as 3-400 bars and temperatures well above 100 °C are common. Hence, separation tanks are often designed out of special alloys and with wall thickness of 10-15 cm. For determination of water- and oil levels, the magnetic- or reflex based level gauges cannot easily be applied, since these are aimed for single phase liquids mostly. Transparent types can be used, but only for visual purposes. Other kind of metering systems must therefore be taken into consideration for reading and conversion into I/O-signals. The most common alternative utilizes gamma radiation for penetration of the tanks from one side to the other. With respect to the material dimensions, it is necessary with high radiation doses, something that again leads to health risks and complicated and costly handling procedures. Due to lack of adequate alternatives, this method is being used all over the world. Effective monitoring and control of the separation process is of great importance to oil companies, since physical space onboard offshore installations is limited, and improvements to the separation process can thus result in huge savings.
It is important to have reliable instruments to monitor the separation process and to determine interface levels between the fluids to be able to control the process based on actual values of the measurements. Accurate measurements and determination of the interface levels may also reduce pollution.
Another problem related to heterogeneous fluids is that the fluids are not always completely separated, or that the separation levels may be unclear due to the introduction of intermediate fluids, such e.g. oil-sludge or emulsion between water and oil. Transparent level glasses where the levels can be analyzed by a skilled operator are therefore used in most cases, however only for visual reading of the levels.
Differential pressure measurements (DP measurements) have also been used, but a problem related to both the radioactive measurements and the DP-measurements is that they are very dependent on the density of the fluids, and frequent calibration is required.
The transparent level gauges in use today are located close to the tank and must therefore be regularly visited by operators who record the values of the different fluid interface levels. Some visual systems have been disclosed to allow remote monitoring of transparent level gauges. This includes telescopes in the control room, which may work under most conditions, but are less functional due to varying light and weather conditions.
Cameras and video cameras has also been used to present the image of the level gauge on a screen in the control room, but due to the height of the level gauge, and the aperture of the cameras, such cameras have been placed a distance from the level gauge. The measurements will therefore be affected by infringing objects and by changes in lighting conditions. US patent 2,714,167 discloses a liquid level measuring apparatus with a radiation detector where a float inside the tank comprises a radioactive source floating on the liquid. Outside the tank a radiation detector can be moved up and down in parallel with the floating source. European patent application EP0381894 Al discloses a sight gauge installation for the remote sensing of a liquid-level. The installation comprises a video camera and means for moving the camera up and down the sight tube. The installation comprises a monitor where a cursor line can be adjusted.
Japanese patent 10062231A discloses measurement of a water level by a camera that records images of a water gauge, and measures and determines a height of the water level from a reference level. The water level together with a scale can be displayed on a screen.
International patent application WO03/060433 A2 describes a method for detecting the detecting the position of the interface between two media in a container.
According to said method, the container is irradiated by light, and the radiation which passes through the container is captured, converted into a video image and evaluated in order to determine the position of the interface in terms of height.
However, none of the disclosed applications discloses a system or method for remote, reliable determination of the fluid interface levels of tanks comprising separated fluids, where the determined fluid interface levels can be used as process control signals.
Short summary of the invention
It is an object of the invention to disclose a system and a method for remotely determining the fluid interface levels of tanks comprising separated fluids, and to provide a system level interface where the fluid level interface data in the form of I/O signals can be sent to a process control or monitoring system in real time as process signal values representing the fluid interface levels.
It is also an object of the invention to disclose a system and a method, where the operators do not have to regularly visit the spot of the transparent level gauge to read the levels. Further, it is an object to disclose a system where the handling and maintenance costs can be kept very low, as well as disclosing a system without potential negative health effects.
In an embodiment the invention is a method for determining fluid interface levels between two or more separated fluids residing between a first optically transparent wall and a second optically transparent wall , wherein the method comprises the following steps;
- capturing digital images of two or more overlapping vertical sections of the two or more separated fluids through the first optically transparent wall, with respective two or more video cameras arranged above each other,
- sending the digital images to a processor based signal processing module of the fluid level monitoring system,
- combining in the processor based signal processing module, the digital images into a combined or full digital image, - partitioning the combined digital image in multiple vertical intervals,
- determining an equivalent interval light intensity for each the vertical interval of the combined digital image,
- determine one or more vertical fluid interface levels by comparing the equivalent vertical light intensities. According to this embodiment, the fluid interface levels are now available from the signal processing module in real time, and can be used by other systems, such as by a remote process control and monitoring system. The availability of reliable measurement values as I/O signals is important for e.g. the control of the separation process to improve the efficiency. The fluid interface levels determined by the method, represents the real fluid interface levels as indicated by the optically transparent walls, and will adapt to changing fluid densities, without the need for frequent recalibration.
The method according to the invention also solves the problems related to health risks related to prior art radio-active instruments. According to an embodiment the first optically transparent wall and the second optically transparent wall are arranged inside a fluid tank. The arrangement of the transparent walls inside the tank has a number of advantages.
First of all the interior of the tank is completely dark, which gives very stable measuring conditions compared to transparent gauges arranged in open air, or in rooms where light is turned on or off. Another advantage is that the interface levels of the separated fluids inside the tank always represent the real levels, compared to the interface levels in an external level gauge that will always need a certain time to represent the real levels in the tank, since the fluid flow in the external level gauge may be slow. Short figure captions
The figures used to illustrate the embodiments of the invention are:
Fig. la illustrates a generic setup of a computer based fluid level monitoring system (10) according to the invention in a schematic drawing.
Fig. lb is an image of example separated fluids inside a tank. Fig. 2 illustrates how the fluid transition levels (N l, N2,..) are determined from a set of digital images from the video cameras.
Fig. 3a shows by way of a flow diagram the method for determining the fluid transition levels (N l, N2,..) according to an embodiment of the invention.
Fig. 3b shows by way of a flow diagram the method for determining the fluid transition levels (N l, N2,..) according to an embodiment of the invention.
Fig. 4a illustrates in a simplified schematic drawing the computer based fluid level monitoring system (10) where a transparent level gauge external to the tank is monitored.
Fig. 4b illustrates in a simplified schematic drawing the computer based fluid level monitoring system (10) where the monitoring is performed inside the tank.
Fig. 5a illustrates in a top sectional view a screen encapsulating the cameras, the light sources and the fluid sample.
Fig. 5b illustrates the same as in Fig. 5a, with the addition of light sources illuminating the fluid sample from the camera side. Embodiments of the invention The invention will now be explained in more detail with reference to the
accompanying drawings.
The object of the invention is to accurately determine fluid interface levels between separated fluids in the same tank or container. The term separated fluids is used to indicate fluids with different density. Multiphase fluids from a petroleum separation process are an example of separated fluids. In this way the more dense fluids will sink to the bottom and the less dense fluids will raise. The different separated fluids will interface at certain vertical levels, defined as fluid interface levels.
In an embodiment, determination of fluid levels in a multiphase separation tank is disclosed. Multiphase separation is an important process in the oil industry, and it is important to understand the internal conditions in the tank to be able to optimise the separation process. Usually, crude oil from oil wells is pumped to large tanks, to separate gas, oil and water. In the same separation process, other materials, such as sand should also be separated from the final products. The most common way of separating the components is by relying on gravity.
Fig. 1 b shows a real example of a composition of separated fluids in a separation tank, consisting of gas, oil, oil sludge, water and sand. In addition a foam layer is shown on top of the oil layer. Foaming is considered a problem in the separation process since it may limit the output capacity of the separator. To maintain full capacity, antifoaming chemicals are often added to the separation process. The fluid interface levels between gas and foam, foam and oil, oil and oil sludge, oil sludge and water, and water and sand can be seen in Fig. 1.
In Fig. 1 a generic setup of a computer based fluid level monitoring system (10) according to the invention is illustrated in a schematic drawing. The sample of the separated fluids, e.g. as shown in Fig. lb, is placed behind a first optically
transparent wall (la). The interface levels between the separated fluids are indicated as N l, N2, N3, N4 and N5.
Two or more video cameras (2) are arranged above each other outside the first optically transparent wall (la) with respect to the sample. Each camera (2) covers a vertical section (22) of the sample that overlaps with the vertical section of a neighbouring camera (2) arranged above and/or below. The first optically transparent wall (la) should be transparent for the wavelengths used by the cameras (2). In most situations this will be visible light, but may also be other wavelengths in the area from infrared to ultra violet.
The digital images (21) from the cameras (2) will be transferred to a signal processer module of a computer (11) that is responsible for image processing of the digital images. The video transfer may be based on any video transfer protocol, wired or wireless.
A control module (6) in the computer (11), may in an embodiment be responsible for controlling the cameras (2). The control module (6) may be implemented in a computer separate from the computer (11) that is performing the image processing of the video signals from the cameras (2).
Fig. 2 shows an example of how the interface levels of the separated fluids in Fig. 1 b are determined. First, digital images (21) of the sample are captured by the cameras (2). In Fig. 2 it can be seen that they have overlapping vertical sections or ranges (22). In the signal processing module (5), a full or combined digital image (51) is compiled from the digital images (21). The combined digital image (51) is a digital image representing the sample. The merge or compilation of the combined digital image (51) may be performed by edge-blending or similar available technologies applicable for merging overlapping images. According to the invention, the combined digital image (51) is then partitioned into a number of vertical intervals (d), and the light intensity for each of the vertical intervals (d) is registered. This partitioning may be performed by scanning the combined digital image (51) in vertical direction. The light intensity of the vertical section (d) will depend on the darkness of the combined digital image (51) in the specific section (d). For the purpose of this invention, the accurate determination of the light intensity is not necessary, since the intensity of one vertical section relative another vertical section can be used for the later determination of fluid interface levels. This is defined as the equivalent interval light intensity (M).
The fluid interface levels (N l, N2,...) may then be determined by comparing the light intensity of the vertical sections (d). An example of resulting fluid interface levels can be seen in Fig. 2, where N l is the interface level between water and sand, N2 is the interface level between oil sludge and water, N3 is the interface level between oil and oil sludge, N4 is the interface level between gas and oil and N5 is the interface level between foam and gas. In an embodiment the invention is therefore a method, as illustrated in Fig. 3a, for determining fluid interface levels (N l, N2,...) between two or more separated fluids residing behind a first optically transparent wall (la) in a computer based fluid level monitoring system (10), wherein the method comprises the following steps; - capturing (102) digital images (21) of two or more overlapping vertical sections (22) of the two or more separated fluids through the first optically transparent wall (la), with respective two or more video cameras (2) arranged above each other outside the a first optically transparent wall (la),
- sending (103) the digital images (22) to a processor based signal processing module (5) of the fluid level monitoring system (10),
- combining (104) in the processor based signal processing module (5), the digital images (22) into a full or combined digital image (51),
- partitioning (105) the combined digital image (51) in multiple vertical intervals (d),
- determining (106) an equivalent interval light intensity (M) for each the vertical interval (d) of the combined digital image (51),
- determine (109) one or more vertical fluid interface levels (N l, N2,...) by comparing the equivalent vertical light intensities (M) .
In an embodiment the invention is also a computer based fluid level monitoring system (10) for determining fluid interface levels (N l, N2,...) between two or more separated fluids residing behind a first optically transparent wall (la), the system comprising;
- two or more video cameras (2) arranged above each other, each the video camera (2) arranged for - capturing (102) digital images (21) of two or more overlapping vertical sections (22) of the two or more separated fluids through the first optically transparent wall (la),
- a processor based signal processing module (5) arranged for;
- receiving video signals from each of the video cameras, and for combining (104), the digital images (22) into a combined digital image (51),
- partitioning (105) the combined digital image (51) in multiple vertical intervals (d), - determining (106) an equivalent interval light intensity (M) for each the vertical interval (d) of the combined digital image (51),
- determining (109) one or more vertical fluid interface levels (N l, N2,...) by comparing the equivalent vertical light intensities (M) . The quality and accuracy of the measurements, as well as processing speed, often depends on the contrast level between the different fluids.
For a number of fluid samples, the measurements may be considerably improved by enhancing the illumination of the sample. Of specific importance is transillumination, i.e. illuminating the sample from one side, and capturing the images on the other. Transillumination increases the difference in light intensity between the different fluids in the sample. In this embodiment, illustrated in Fig. 3b, the two or more separated fluids reside between the first optically transparent wall (la) and a second optically transparent wall (lb), and the method comprises illuminating (101) the two or more separated fluids through the second optically transparent wall (lb) with light from one or more light sources (4) as can be seen in Fig. 1.
Similarly, according to an embodiment, illustrated in Fig. 3b, the computer based fluid level monitoring system (10) comprises a second optically transparent wall (lb) opposite the first optically transparent wall (la) relative the video cameras (2), and one or more light sources (4) behind the second optically transparent wall (lb) relative the video cameras (2), wherein the one or more light sources (4) are arranged for illuminating (101) the two or more separated fluids between the first optically transparent wall (la) and the second optically transparent wall (lb) through the second optically transparent wall (lb).
Another feature according to an embodiment of the invention, is to find the maximum (Mmax) and the minimum light intensity (Mmin) of the combined digital image (51). In one embodiment these values are obtained by analysing each of the vertical intervals (d), and determining the lightest and darkest interval. In another embodiment these values may be found by analysing the entire combined digital image (51). Since the content of the tank is known, the types of separated fluids are also known. Thus, the equivalent light intensity values (Mr) for each fluid can be predetermined. This may not be a correct value for the light intensity, but the relative light intensity between the fluids should be correct. This value is further used in the determination of the fluid level interfaces by determining an equivalent fluid phase light intensity (Me) based on the equivalent maximum light intensity (Mmax), the equivalent minimum light intensity (Mmin) and predetermined equivalent light intensity values (Mr) for each of the two or more separated fluids. This is shown to the right in Fig. 2. It thus represents an expected illumination for a specific fluid in the current sample, i.e. in the current combined digital image (51).
The equivalent vertical light intensities (M) can now be compared to the equivalent fluid phase light intensity (Me), and from this it may be determined which fluid they represent. I.e., if the equivalent vertical light intensity (M) of a vertical interval (d) has the same value as the equivalent fluid phase light intensity (Me), it may be concluded that the fluid at the height of the vertical interval (d) is of the type represented by the equivalent fluid phase light intensity (Me).
Finally, one or more vertical fluid interface levels (N l, N2,...) may be determined (109) analysing where the equivalent vertical light intensities (M) suddenly changes from one value to another, i.e. from a value representing one fluid type to a value representing another fluid type.
The fluids are expected to have a certain order, i.e. oil sludge should be found between oil and water.
In an embodiment noise reduction is performed by suppressing, or not taking notice of equivalent vertical light intensities (M) that have values that do not follow this order.
To further control the environmental conditions, the computer based fluid level monitoring system comprises, in an embodiment a screen (18) arranged behind the video cameras (2) with a height at least the distance between a lowest video camera (2) and a highest video camera (2). Please see Fig. 5a.
According to an embodiment the screen (18) encloses the one or more light sources (4) and the video cameras (2) in a horizontal plane. This is illustrated in Fig. 5a, illustrating the screen ( 18) in a top sectional view. The light sources (4) are arranged behind a vertical plane (17) between the first optically transparent wall (la) and the second optically transparent wall (lb) relative the cameras (2).
According to an embodiment the computer based fluid level monitoring system (10) comprises an opaque shield corresponding to the vertical plane (17), to prevent backlight from the light sources (4) to reach the video cameras (2) without passing through the fluids.
According to an embodiment the computer based fluid level monitoring system (10), comprises one or more front light sources (4b) arranged to illuminate the first optically transparent wall (la) from the side of the video cameras (2). This is illustrated in Fig. 5b.
The angles a and β in Fig. 5a and 5b, indicate the location of the light sources relative each other and the fluids when more than one light source is used, a is the sector between the back-lights, and β is the sector between the light sources illuminating the fluids from the front.
According to an embodiment of the invention the computer based fluid level monitoring system (10) comprises a control system (6) as shown in Fig. la. The control system is connected to the light sources (4) and arranged to control the illumination of the separated fluids. In an embodiment it also controls the
wavelength spectre of the light source. The control system may also be connected to the video cameras (2) to control camera specific parameters, such as focal length and aperture.
According to an embodiment of the invention, the digital images (21) and the combined digital image (51) may be in colour. And since the content of the tank is known, the types of separated fluids are also known. Thus, equivalent colour values (Cr) for each fluid can be predetermined. This may not be a correct value for the colour, but the relative colour value between the fluids should be correct.
The method for determining the fluid interface levels (N) comprises in this embodiment the steps of; - determining an interval equivalent colour (C) for each the vertical interval (d) of the combined digital image (51),
- determine the one or more vertical fluid interface levels (N) by comparing the equivalent vertical light intensities (M) with the equivalent phase light intensities (Me) and the interval equivalent colour (C) with the predetermined equivalent colour values (Cr) for the two or more of the separated fluids. This is illustrated in Fig. 2.
Based on the information available after determining the fluid interface levels (N l, N2,...), the method according to an embodiment of the invention comprises creating a synthetic graph (61) of the transparent level gauge (1) arranged for being presented on a screen, wherein the synthetic graph (61) comprises a representation of each of the equivalent fluid phase light intensities (Me) and a visual indication of each of the more vertical fluid interface levels (Nl, N2, ...). This synthetic graph will be easier to read than the combined digital image, since noise is filtered out and the fluid level interfaces are indicated specifically and more clearly than in the original image. In an embodiment the synthetic graph (61) comprises graphical effects such as different colours for each separated fluid.
According to an embodiment of the invention the synthetic graph also comprises the combined digital image (51). In this embodiment the colour of the backlight and front light may be different than the colours used when the fluids are monitored for the purpose of determining the fluid transition levels.
Transparent level gauges are often used to indicate the fluid interface levels of tanks, as described previously. According to an embodiment of the invention the first optically transparent wall (la) and the second optically transparent wall (lb) constitute a transparent level gauge arranged outside a fluid tank, wherein the transparent level gauge is arranged to indicate the fluid interface levels (N) in the tank. This embodiment is illustrated in Fig. 4a. It can be seen that the transparent level gauge (la, lb) is arranged external to the tank, and connected to the tank in a lower and an upper end through tubes and valves. The fluid interface levels (N l, N2, ...) are represented both in the tank and in the level gauge. However, rapid fluctuations in the tank may not be immediately visible in the level gauge.
As an alternative to the transparent level glass, or in addition to the level glass, the monitoring may also take place inside the tank. According to an embodiment the first optically transparent wall (la) and the second optically transparent wall (lb) are arranged inside a fluid tank. Consequently the cameras (2) must also be arranged inside the tank. This is illustrated in Fig. 4b. There are several ways of arranging the cameras inside the tank, but according to this embodiment they are arranged above each other. In an embodiment the tank also comprises the light source(s) (4). In an embodiment one or more light sources, being uniform in the vertical direction are used for illumination. In another embodiment the light sources consist of several discontinuous light sources in the vertical direction.
As illustrated in Fig. 4b the first optically transparent wall (la) constitutes in an embodiment a wall of a first fluid impermeable container, and the two or more video cameras (2) are arranged inside the first fluid impermeable container. This impermeable container will protect the cameras from the fluids in the tank. In an embodiment where front light sources (4b) are used, they may also be arranged inside the first fluid impermeable container. Similarly, the second optically transparent wall (lb) constitutes in an embodiment a wall of a second fluid impermeable container, and the one or more light sources are arranged inside the container. This impermeable container will protect the light sources from the fluids in the tank.
Different methods may be used for different steps when determining the fluid interface levels. Some steps may be related to the initial procedure, while other steps are related to continuous corrections. Usually, the start-up procedure will take longer time than the procedure required for smaller corrections.
At start-up it may, according to an embodiment be advantageous to determine the easiest transitions first, e.g. the transitions where the contrast in the combined digital image is high. Since the composition of fluid levels in a tank is known, the procedure may be tailored for this. As an example, for the composition shown in Fig. lb, it may be advantageous to first determine the darkest regions. In this situation backlight illumination can be used to render the dark areas almost black, i.e. in this example sand and oil. The following steps can then be performed : - scanning of the combined digital image (51) from the bottom in thin vertical sections (d),
- determination of minimum and maximum values of light intensity for each vertical section (d) by fast scan over the entire combined digital image (51) ,
- new scan from below. The first interface level (N l) is the top of the sand, where the light intensity increases above a certain value, e.g. 5% of maximum value from last scan.
- then the lower level of the oil sludge (N2) is determined, e.g. where the light intensity becomes lower than 2% of the maximum value.
- then the higher level of the oil (N4) is determined, e.g. where the light intensity becomes higher than 10% of the maximum value. The gas-foam interface may be determined after determining higher level of the oil (N4) by further scanning upwards, e.g. where the light intensity becomes higher than 30% of the maximum value.
The next step may be to determine the upper level of the oil sludge (N3) by scanning downwards, and looking for an increase in light intensity when in the oil layer. E.g. the sludge starts where the light intensity is more than 90 % of the minimum value.
The back light may be adjusted to increase the contrast to obtain more accurate readings during the method for the determination of the fluid interface levels.
In continuous operation, some of the steps may be simplified, since the changes in the levels in most cases will have a much slower variation than the scanning rate.
As an example, for the composition shown in Fig. lb, it may be advantageous to scan only the region around the top layer of the sand (N l) from the previous scan, and depending on the result, scan upwards or downwards, until the new level (N l) is found. E.g. if sand is still determined in the new scan, the scanning should continue in the upward direction until the sand-water interface level is found where the light intensity increases above a certain value, e.g. 5% of maximum value. In the other case, where the new scan shows that the sand layer is lower, i.e. intensity is higher than 5% of maximum value where it used to be darker, the scanning should continue in the downward direction until the sand-water interface level is found where the light intensity decreases below a certain value, e.g. 3% of maximum value.
The same principle can be used for determining the other interface levels N2, N3, N4 and N5 when the system is in continuous operation to limit the number of vertical sections (d) to be scanned and limit the corresponding processing.

Claims

Claims
1. A method for determining fluid interface levels (N l, N2,...) between two or more separated fluids residing between a first optically transparent wall (la) and a second optically transparent wall (lb) in a computer based fluid level monitoring system (10), wherein said method comprises the following steps;
- capturing (102) digital images (21) of two or more overlapping vertical sections (22) of said two or more separated fluids through said first optically transparent wall (la), with respective two or more video cameras (2) arranged above each other,
- sending (103) said digital images (22) to a processor based signal processing module (5) of said fluid level monitoring system (10),
- combining (104) in said processor based signal processing module (5), said digital images (22) into a combined digital image (51),
- partitioning (105) said combined digital image (51) in multiple vertical intervals (d),
- determining (106) an equivalent interval light intensity (M) for each said vertical interval (d) of said combined digital image (51),
- determine (109) one or more vertical fluid interface levels (N l, N2,...) by comparing said equivalent vertical light intensities (M).
2. A method for determining fluid interface levels (N) according to claim 1, wherein said method comprises illuminating (101) said two or more separated fluids through said second optically transparent wall (lb) with light from one or more light sources (4).
3. A method for determining fluid interface levels (N) according to claim 1, comprising;
- determining (107) an equivalent maximum light intensity (Mmax) and an equivalent minimum light intensity (Mmin), for said combined digital image (51),
- determining (108) an equivalent fluid phase light intensity (Me) based on the equivalent maximum light intensity (Mmax), the equivalent minimum light intensity (Mmin) and predetermined equivalent light intensity values (Mr) for each of said two or more separated fluids, and
- determine (109) one or more vertical fluid interface levels (N l, N2,...) by comparing said equivalent vertical light intensities (M) with said equivalent phase light intensities (Me).
4. A method for determining fluid interface levels (N) in a transparent level gauge (1) according to claim 3, comprising the steps of;
- determining an interval equivalent colour (C) for each said vertical interval (d) of said combined digital image (51),
- determine said one or more vertical fluid interface levels (N) by comparing said equivalent vertical light intensities (M) with said equivalent phase light intensities (Me) and said interval equivalent colour (C) with predetermined equivalent colour values (Cr) for said two or more of said separated fluids.
5. A method for determining fluid interface levels (N) according to claim 1, wherein said first optically transparent wall (la) and said second optically
transparent wall (lb) constitute a transparent level gauge arranged outside a fluid tank, wherein said transparent level gauge is arranged to indicate said fluid interface levels (N) in said tank.
6. A method for determining fluid interface levels (N) according to claim 1, wherein said first optically transparent wall (la) and a and a second optically transparent wall (lb) are arranged inside a fluid tank comprising said separated fluids.
7. A method for determining fluid interface levels (N) according to claim 1, wherein said first optically transparent wall (la) constitutes a wall of a first fluid impermeable container, and said two or more video cameras (2) are arranged inside said first fluid impermeable container.
8. A method for determining fluid interface levels (N) in a transparent level gauge (1) according to any of the claims 3 to 7, comprising the steps of;
- creating a synthetic graph (61) of said transparent level gauge (1) arranged for being presented on a screen, wherein said synthetic graph (61) comprises a representation of each of said equivalent fluid phase light intensities (Me) and a visual indication of each of said more vertical fluid interface levels (N l, N2, ...).
9. A computer based fluid level monitoring system (10) for determining fluid interface levels (N l, N2,...) between two or more separated fluids residing behind a first optically transparent wall (la), said system comprising;
- two or more video cameras (2) arranged above each other, each said video camera (2) arranged for - capturing (102) digital images (21) of two or more overlapping vertical sections (22) of said two or more separated fluids through said first optically transparent wall (la), - a processor based signal processing module (5) arranged for;
- receiving video signals from each of said video cameras, and for combining (104), said digital images (22) into a combined digital image (51),
- partitioning (105) said combined digital image (51) in multiple vertical intervals (d),
- determining (106) an equivalent interval light intensity (M) for each said vertical interval (d) of said combined digital image (51),
- determining (109) one or more vertical fluid interface levels (N l, N2,...) by comparing said equivalent vertical light intensities (M).
10. A computer based fluid level monitoring system (10) according to claim 9, comprising a second optically transparent wall (lb) opposite said first optically transparent wall (la) relative said video cameras (2), and one or more light sources (4) behind said second optically transparent wall (lb) relative said video cameras (2), wherein said one or more light sources (4) are arranged for illuminating (101) said two or more separated fluids between said first optically transparent wall (la) and said second optically transparent wall (lb) through said second optically transparent wall (lb).
11. A computer based fluid level monitoring system (10) according to claim 10, wherein said processor based signal processing module (5) is further arranged for;
- determining (108) an equivalent fluid phase light intensity (Me) based on an equivalent maximum light intensity (Mmax), an equivalent minimum light intensity (Mmin) and predetermined equivalent light intensity values (Mr) for each of said two or more separated fluids, and
- determine (109) one or more vertical fluid interface levels (N l, N2,...) by comparing said equivalent vertical light intensities (M) with said equivalent phase light intensities (Me).
12. A computer based fluid level monitoring system (10) according to claim 10, comprising a screen (18) arranged behind said video cameras (2) with a height at least the distance between a lowest video camera (2) and a highest video camera (2).
13. A computer based fluid level monitoring system (10) according to claim 12, wherein said screen encloses said one or more light sources (4) and said video cameras (2) in a horizontal plane.
14. A computer based fluid level monitoring system (10) according to any of the claims 10 to 13, comprising one or more front light sources (4b) arranged to illuminate said first optically transparent wall (la) from the side of said video cameras (2).
PCT/NO2014/050045 2013-04-04 2014-04-01 System and method for determining fluid levels interfaces WO2014163509A1 (en)

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