GB2534350A

GB2534350A - Oil/water interface detection

Info

Publication number: GB2534350A
Application number: GB1421870.5A
Authority: GB
Inventors: George Clark William; Erik Berg Stein
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2014-12-09
Filing date: 2014-12-09
Publication date: 2016-07-27
Also published as: GB201421870D0

Abstract

An apparatus and method for use of a separator for an oil/water interface 26. Apparatus comprising a separation tank 12, having an inlet 16 for an oil/water mixture, an oil outlet 18 for separated oil and a water outlet for separated water. A radio transmitter 30 and receiver 32 are mounted within the separation tank. A controller 28 is configured to transmit a radio pulse, said pulse is reflected by an interface between an oil 22 and a water 24 layer in the separation tank and is received by the receiver after reflection. The radio pulse has a duration of less than 10ns or nanoseconds. The level of the interface may be based upon the reflected waveform received, including time-of-flight of the reflected radio waveform. The radio pulse may be one or more frequency components between 300 MHz and 30 GHz or an ultra-wideband radar pulse having a bandwidth of at least 500 MHz and. The sampling signal may comprise at least 100 samples at a sampling frequency of at least 500 MHz. The receiver may have a passive Vivaldi antenna, which may be further laminated in resin to reduce corrosion.

Description

OIL %WATER INTERFACE DETECTION The present invention relates to interface detection, and particularly to non-contact interface detection in an oil/water separator system, such as for an offshore oil/water separator.

In offshore oil production, water or steam is often used to extract oil from a well. Water may also be used as a transport medium or as a cleaning agent. Subsequent processing of the oil then requires the use of oil/water separators so that the water can be sent to a wastewater facility and the oil can be returned to the process stream. In the case of large separator systems, it is important to know the exact level of the oil/water interface. If the water is too high, then it will flow into the oil line, thereby contaminating production. To avoid this, the oil/water interface level is often estimated very conservatively; however, production rates are then reduced.

Existing technologies for interface detection include, amongst others, guided-and unguided-wave radar profilers. Radar profilers work in one of two ways, either by emitting a radio-frequency pulse and detecting the time taken for a return pulse, which is reflected due to the difference between the dielectric constants of the water and the oil, to be received (known as Pulse Radar), or by emitting a continuous frequency-modulated radio-frequency wave and detecting a phase difference between the transmitted and reflected waves (known as Frequency Modulated Continuous Wave (FMCVV) Radar). However, a well-Known problem with both types of existing radar level sensor is that they do not work effectively when there is an emulsion or foam present at the interface because the detecting antenna does not receive a clear reflected radar pulse.

Accordingly, existing technology does not satisfy the demands of the offshore oil industry. At least the preferred embodiments of the present invention seek to enable the interface level to be detected more accurately and quickly, which will enable production rates to be increased.

The present invention provides a separator for oil and water, the separator comprising: a separation tank for separating an oil/water mixture, the separation lank having an inlet for the oil/water mixture, an oil outlet for separated oil and a water outlet for separated water, a radio transmitter and a radio receiver, each being mounted within the separation tank; and a controller configured to transmit a radio pulse from the radio transmitter to be reflected by an interface between an oil -2 -layer and a water layer in the separation tank and to be received by the radio receiver after reflection, wherein the radio pulse has a duration of less than 10 ns. In accordance with the present invention, contactless detection of an oil/water interface in the separator can be precisely determined. It has been found that, by using a radio pulse having a duration of less than 10 ns, the interface level can be determined even in adverse conditions, such as when foam or an oil/water emulsion is formed at the interface. Prior art radar profiler systems have so far been unable to accurately detect the interface level in such circumstances.

Preferably, the controller is further configured to determine a level of the interface between the oil layer and the water' layer based on the reflected radio waveform received by the radio receiver. Whilst the controller could simply output the data to be processed elsewhere, it is preferable that this processing is performed locally and the controller directly outputs a distance to the interface or the interface level. The controller may be configured to determine the level of the interface by determining a time-of-flight of a peak value portion of the reflected radio waveform.

Preferably the or each of the radio transmitter and/or the radio receiver comprises a passive antenna, such as a Vivaldi antenna. The Vivaldi antenna may be laminated in resin to reduce corrosion. Use of a passive antenna (preferably such that the separator tank does not include any electrical components) reduces the risk of corrosion or corruption of electrical sensitive equipment and hence less maintenance is required.

The radio pulse may have one or more frequency components between 300 MHz and 30 GHz. The radio pulse may be an ultra-wideband radar pulse, which is defined as a radio signal having a bandwidth of at least 500 MHz. In preferred embodiments, the ultra-wideband radio pulse has a bandwidth of at least 2011z. These conditions have been found to be particularly effective for determining the interface levels for oil and water.

Indeed, viewed from another aspect, the present invention also provides a separator for oil and water, the separator comprising: a separation tank for separating an oil/water mixture, the separation tank having an inlet for the oil/water mixture, an oil outlet for separated oil and a water outlet for separated water; a radio transmitter and a radio receiver, each being mounted within the separation tank; and a controller configured to transmit a radio pulse from the radio transmitter to be reflected by an interlace between an oil layer and a water layer in the -3 -separation tank and to be received by the radio receiver after reflection; wherein the radio pulse is an ultra-wideband radio pulse having a bandwidth of at least 5001VIHz. The controller may be configured to emit a sequence pulses and to sample, between consecutive pulses, the signal received by the radio receiver with at least 100 (and preferably at least 200 and most preferably at least 500) samples at a sampling frequency of at least 20 GHz. This ensures that the reflected signal from the interface is accurately detected.

Viewed from another aspect; the present invention also provides a method of determining the level of an oil/water interface in a separator tank comprising an oil layer and a water layer, the method comprising: transmitting a radio pulse from a radio transmitter, wherein the radio pulse has a duration of less than 10 ns, and wherein the radio transmitter is arranged within the separator tank such that the radio pulse is reflected by the interface between the oil layer and the water layer toward a radio receiver; receiving a reflected radio waveform via the radio receiver; and processing the reflected radio waveform to determine the level of the oil/water interface.

In one embodiment, foam or an oil/water emulsion is present at the interface between the oil layer and the water laye.r. Whilst the present method may be used to detect a simple oil/water interface, the method is particularly applicable to interfaces where foam or an emulsion has formed at the interface, as existing profilers cannot accurately measure the level of such interfaces.

The level of the interface may be determined by calculating a time-of-flight of a peak value portion of the reflected radio waveform.

The radio pulse may have one or more frequency components between 300 MHz and 30 GHz. The radio pulse may be an ultra-wideband radio pulse having a bandwidth of at least 500 MHz, and preferably at least 2G11z.

The method may further comprise sampling the signal received by the radio receiver with at least 100 samples at a sampling frequency of at least 20 GHz. The method may be implemented using the apparatus as described above optionally including any or all of the preferred features thereof.

Certain preferred embodiments in the present invention will now be described in greater detail by way of example only and with reference to the accompanying figures, in which: Figure 1 shows an oil/water separator including a non-contact interface level detector; -4 -Figure 2 shows a detailed view of a transmitter antenna and a receiver antenna of the non-contact interface level detector; Figure 3 shows an exemplary reflected radio waveform received by the receiving antenna of the non-contact interface level detector; Figure 4 shows a reflected radio waveform received by the receiving antenna during a first test, in which the separator contained water and air; Figure 5 shows a reflected radio waveform received by the receiving antenna during a second test; in which the separator contained a water layer and an oil layer; Figure 6 shows a reflected radio waveform received by the receiving antenna during a third test, in which the separator contained a water layer and an oil/water emulsion layer; and Figure 7 shows a reflected radio waveform received by the receiving antenna during a fourth test, in which the separator contained a water layer and an oil layer where the oil/water emulsion layer of the third test had separated; Figure 8 shows a reflected radio waveform received by the receiving antenna during a fifth test, in which the separator contained a water layer, an oil/water emulsion layer, and an oil layer.

Figure 1 shows an oil/water separator 10 comprising a separator tank 12 for separating an oil/water mixture and an interface level detector 14 for detecting an interface level within the separator 12.

The separator tank 12 comprises an inlet 16 for receiving an oil/water mixture to be separated, a first outlet 18 for outputting separated oil and a second outlet 20 for outputting separated water. Within the separator tank 12, the oil/water mixture is separated by gravity into an oil layer 22 and a water layer 24; forming an oil/water interface 26 therebetween.

The interface level detector 14 comprises a controller 28, a radio transmitter antenna 30 and a radio receiver antenna 32. A more detailed view of the antennas 30, 32 is shown in Figure 2. The antennas 30, 32 are located proximate one another within the separator tank 12 and the controller 28 is located external to the separator tank 12. Preferably, both antennas 30, 32 are passive antenna and no electronic components of the interface level detector 14 are located within the separator tank 12. Where only the passive; but preferably extremely robust; antennas 30, 32 are mounted internally of the separator tank 12, there is no possibility of corrosion or corruption of sensitive electrical equipment. -5 -

The antennas 30; 32 are preferably positioned on the inside a flange (not shown) of the separator tank 12 A coaxial connection from each antenna 30; 32 to the controller 28 is made via high pressure electrical feed-through.

The radio transmitter and the receiver antennas 30, 32 are preferably both Vivaldi antennas (a type of tapered-slot antenna); which have been laminated in resin to protect the antenna material. A Vivaldi antenna is a passive antenna (i.e. it does not include any electrical components, which could be damaged) and so reduces the risk of corrosion or corruption of electrical sensitive equipment and hence less maintenance is required.

The controller 28 is configured to operate the radio transmitter antenna 30 to emit a radio wave pulse. Radio-frequency typically includes frequencies of between 3 kHz and 300 GHz. The frequency of the pulse is preferably in the ultra high frequency to super high frequency range, 300 MHz to 30 GHz.

The radio pulse preferably has a duration of less than 10 ns, and in a preferred embodiment has a duration of about 1ns (for example between 0.5 ns and 2ns).

The radio pulse preferably is an ultra-wideband (UWB) radio pulse, which uses a very low energy level for short-range transmission across a large radio spectrum (typically greater than 500 MHz). A valuable aspect of UWB technology is the ability for a UWB radio system to determine the time of flight" at various frequencies, which can increase the precision of distance measurements.

In a preferred embodiment, the controller 28 of the interface level detector 14 is a Nanoscale Impulse Radar Transceiver manufactured Novelda AS, a Norwegian company. The Novelda Nanoscale Impulse Radar Transceiver uses UWB radio signals and is operable at three different UWB frequency ranges: 6.0 to 9.1 GHz; 0.450 to 3.5 GI-1z; or 0.85 to 9.6 GHz.

As is known, if two media have the same dielectric constant, then there is no electromagnetic reflection at the boundary between them. On the other hand, if there is a difference in dielectric constant, then a portion of the electromagnetic wave is reflected at the boundary, which is proportional to the difference between the dielectric constants.

Air and oil both have relatively low dielectric constants; air is about 1 and oil varies from about 2 for refined oil up to about 5 for crude oil. Conversely, water has a relatively high dielectric constant; about 80 at radio frequencies. Accordingly, at the oil/water interface 26, the significant difference in dielectric constant between -6 -the oil layer 22 and the water layer 24 means that a large amount of the dio pulse will he reflected back towards the receiver 32.

Figure 3 shows an exemplary plot of the radio wave received by the receiving antenna 32 and sampled at approximately 35 Gs/s (gigasamples per second), equating to a sampling interval of approximately 30ps. In this Figure, it can be seen that 512 samples have been taken to ensure that the reflected wave is captured in the scanning window (equating to a maximum distance of approximately 4.4m).

By measuring the "time of flight" of a peak value of the received radio waveform, i.e. the time from the radio pulse being emitted from the radio transmission antenna 30 to when the portion of the radio wave reflected by the oil/water interface 26 is received by the radio receiving antenna 32, the distance travelled by the wave can be calculated. Typically, the peak value of the radio waveform received by the radio receiver antenna corresponds to the interface between the oil and the water, as this is the point where there is the most significant step change in dielectric constant.

Based on knowledge of the relative locations of the transmission antenna 30 and the receiving antenna 32, the level of the interface 26 can he determined. For example, where the antennas 30, 32 are proximate one another, the level of the interface 26 is approximately half the distance travelled by the reflected wave from transmission to reception.

Whilst unguided radio wave profilers exist already, these have traditionally operated using radio pulses in the order of milliseconds. As discussed above, whilst such devices operate well when there is a distinct boundary between the oil layer 22 and the water layer 24, when there is emulsion or foam at the interface 26 the dielectric change from oil to water is less distinct and such profilers have had difficulty in accurately detecting the interface level.

The present inventors have discovered that, by using a significantly shorter pulse width (less than 10 ns), a surprising technical effect is achieved in that the interface 26 between the oil layer and the water layer 24 can be detected even if there is an emulsion or foam layer present.

Test made using existing unguided radio wave profilers were unable to precisely detect such interface 26. However, using the interface level detector 14 described herein, the interface can be clearly identified despite the presence of the emulsion. Lab tests performed on a small-scale have confirmed that water level can -7 -be detected accurately through oil and also through emulsions. Figures 4 to 8 shows the received radio waveform detected by the receiving antenna 32 in various test cases. Details of the tests will now be provided.

Figure 4 shows the received waveform in a first test. In the first test, the separation tank 12 was filled with water to a first level of 7cm and the remainder of the separation tank 12 was filled with air. The peak value of the reflected radio waveform corresponds to the interface between the air and the water. As discussed above, by calculating the time of flight for the peak value, the distance to the water interface can be determined.

Figure 5 shows the received waveform in a second test. In the second test, the separation tank 12 was filled with water to the same first level as in the first test. Oil was filled above the water to a second level of 3cm above the first level, and the remainder of the separation tank 12 was filled with air. The peak value of the reflected radio waveform corresponds to the interface between the oil layer and the water layer. The height of this peak is substantially unchanged from the first test.

The air/oil interface can be detected and may provide useful information to an operator. However, the interface between the air and oil does not interfere with the detection of the water level detection.

Figure 6 shows the received waveform in a third test. In the third test, the separation tank 12 was filled with water to the same first level of 7cm as in the first and second tests. An oil/water emulsion comprising 45% water and 55% oil, by volume, was filled above the water to a second level of 3cm above the first level, and the remainder of the separation tank 12 was filled with air. The peak value of the reflected radio waveform corresponds to the interface between the oil/water emulsion and the water. The height of this peak is substantially unchanged from the first and second tests. The interface between the air and the oil/water emulsion can be detected and may provide useful information to an operator. However, the interface between the air and oil/water emulsion again does not interfere with the detection of the water level detection and an accurate measurement of the interface can still be made.

Figure 7 shows the received waveform in a fourth test. In the fourth test, the contents of the separation tank 12 were the same as the third test, but the oil and water had been separated into two layers having a discreet boundary at their interface. Thus, in the fourth test, the separation tank 12 contained a water layer, an oil layer and an air layer. As in the second test, the peak value of the reflected -8 -radio waveform corresponds to the interface between the oil layer and the water layer. As can be seen, the time of flight has decreased because the height of the water level has increased due to the separated water content of the emulsion. Figure8 shows the received waveform in a fifth test. In the fifth test, the separation tank 12 was filled with water to the same first level of 7cm as in the first, second and third tests. An oil/water emulsion layer comprising 45% water and 55% oil, by volume, was filled above the water to a second level of 3cm above the first level. Above the oil/water emulsion layer, an oil layer was filled to a third level of 2cm above the second level. The remainder of the separation tank 12 was filled with air. The peak value of the reflected radio waveform corresponds to the interface between the oil/water emulsion and the water. The peak value of the reflected radio waveform corresponds to the interface between the oillwater emulsion and the water. The height of this peak is substantially unchanged from the first, second and third tests. The interface between the air and the oil, and between the oil and the oil/water emulsion again does not interfere with the detection of the water level detection and an accurate measurement of the interface can still be made.

Whilst certain preferred embodiments of the present invention have been described above, it will be appreciated that these are provided merely by way of example and that various modifications may be made to the described embodiments without departing from the scope of the invention, which is defined by the following claims. -9 -

Claims

CLAIMS: 1. A separator for oil and water, the separator comprising: a separation tank for separating an oil/water mixture, the separation tank having an inlet for the oil/water mixture, an oil outlet for separated oil and a water outlet for separated water; a radio transmitter and a radio receiver, each being mounted within the separation tank; and a controller configured to transmit a radio pulse from the radio transmitter to he reflected by an interface between an oil layer and a water layer in the separation tank and to be received by the radio receiver after reflection, wherein the radio pulse has a duration of less than 10 ns.
2. A separator according to claim 1, wherein the controller is further configured to determine a level of the interface between the oil layer and the water layer based on the reflected radio waveform received by the radio receiver.
3. A separator according to claim 2, wherein the controller is configured to determine the level of the interface by determining a time-of-flight of a peak value portion of the reflected radio waveform.
4. A separator according to any preceding claim, wherein the or each of the radio transmitter and/or the radio receiver comprises a passive antenna.
5. A separator according to claim 4, wherein the passive antenna ivaldi antenna.
6. A separator according to claim 5, wherein the Vivaldi antenna is laminated in resin to reduce corrosion.
7. A separator according to any preceding claim, wherein the radio pulse has one or more frequency components between 300 MHz and 30 GHz.
A separator according to any preceding claim, wherein the controller is configured to emit a sequence pukes and to sample, between consecutive pulses, -10 -the signal received by the radio receiver with at least 100 samples at a sampling frequency of at least 50 GHz.
9. A separator according to any preceding claim, wherein the radar pulse is an ultra wide band radar pulse having a bandwidth of at least 500 MHz.
10. A method of determining the level of an oil/water interface in a separator tank comprising an oil layer and a water layer, the method comprising: transmitting a radio pulse from a radio transmitter, wherein the radio pulse has a duration of less than 10 ns, and wherein the radio transmitter is arranged within the separator tank such that the radio pulse is reflected by the interface between the oil layer and the water layer toward a radio receiver; receiving a reflected radio waveform via the radio receiver; and processing the reflected radio waveform to determine the level of the oil/water interface.
11. A method according to claim 10, wherein foam or an oil/water emulsion is present at the interface between the oil layer and the water layer.
12. A method according to claim 10 or 11, wherein the level of the interface is determined by calculating a time-of-flight of a peak value portion of the reflected radio waveform.
13. A method according to any of claims 10 to 12, wherein the radio pulse has one or more frequency components between 300 MHz and 30 GHz.
14. A method according to any of claims 10 to 13, comprising sampling the signal received by the radio receiver with at least 100 samples at a sampling frequency of at least 50 GHz.
15. A method according to any of claims 10 to 1z1, wherein the radio pulse is an ultra-wideband radio pulse having a bandwidth of at least 500 MHz.
16. A separator substantially as hereinbefore described with reference to Figures 1 to 3.

17 A method of determining the level of an oil/water interface in a separator tank comprising an oil layer and a water layer, substantially as hereinbefore described with reference to Figures 1 to 3,