GB2562925A

GB2562925A - Device for detecting gas in fluids

Info

Publication number: GB2562925A
Application number: GB1811306.8A
Authority: GB
Inventors: Marx Stefan
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-01-26
Filing date: 2017-01-25
Publication date: 2018-11-28
Anticipated expiration: 2037-01-25
Also published as: GB2562925B; GB201811306D0; WO2017129187A1; DE102016201041A1; NO20180976A1; DE102016201041B4

Abstract

The invention relates to a device for detecting gas (130) in a fluid. Said device has a sensor arrangement (52) which has at least one gas sensor (54) and is in direct connection to a gas contained in the fluid. Moreover, the device (2) according to the invention has a second sensor arrangement (90) which has a gas sensor (92) that is identical to the gas sensor (54) of the first sensor arrangement (52) and is in connection to the environment of the device (2) via a buffer chamber (112).

Description

(71) Applicant(s):

Stefan Marx

Stoltenberg 3, D-24248 Monkeberg, Germany (72) Inventor(s):

Stefan Marx (74) Agent and/or Address for Service:

Baron Warren Redfern

1000 Great West Road, BRENTFORD, TW8 9DW, United Kingdom (51) INT CL:

G01N 33/18 (2006.01) G01N 33/00 (2006.01) (56) Documents Cited:

WO 1987/006009 A1

- FUKASAWAT ET AL, Dissolved Methane Sensor for Methane Leakage Monitoring in Methane Hydrate Production, OCEANS 2006, IEEE, PI, (20060901), doi: 10.1109/OCEANS.2006.307110, ISBN 978-1-4244-0114-7, pages 1 - 6, XP031046360

- BOULART C ET AL, Sensors and technologies for in situ dissolved methane measurements and their evaluation using Technology Readiness Levels, TRAC TRENDS IN ANALYTICAL CHEMISTRY, ELSEVIER, AMSTERDAM, NL, vol. 29, no. 2, doi:10.1016/J.TRAC.2009.12.001, ISSN 0165-9936, (20100201), pages 186-195, (58) Field of Search:

INT CL G01N

Other: EPO-lnternal (54) Title ofthe Invention: Device for detecting gas in fluids

Abstract Title: Device for detecting gas in fluids (57) The invention relates to a device for detecting gas (130) in a fluid. Said device has a sensor arrangement (52) which has at least one gas sensor (54) and is in direct connection to a gas contained in the fluid. Moreover, the device (2) according to the invention has a second sensor arrangement (90) which has a gas sensor (92) that is identical to the gas sensor (54) of the first sensor arrangement (52) and is in connection to the environment of the device (2) via a buffer chamber (112).

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Description

The invention relates to a device for detecting gas in liquid.

Great significance is given to the detection of gases present in liquids in different technical fields. Offshore exploration and offshore conveying of petroleum and natural gas is mentioned by way of example. Leakages at wells, conveyer plants and conveyer pipelines located underwater are shown there first of all in an increased content of hydrocarbon gases in the immediate water surroundings of these devices. It is thus of particular interest to detect there in good time hydrocarbon gases located in the water to be able to immediately take measures against a leakage detected in this manner.

A device provided in water with precedence for detecting hydrocarbon gases is known from WO 96/34285 At. This device has a semiconductor sensor arranged in a measuring chamber for the detection of hydrocarbons, wherein the measuring chamber is completely encapsulated, apart from an opening to the water surroundings ofthe device. The opening is closed by a gas-permeabie, but otherwise impermeable membrane. Hence, only the gases located in the surrounding water ofthe device may reach, by diffusion through the membrane, the measuring chamber, in which then the concentration of the hydrocarbon gases is detected by the sensor.

However, a disadvantage of this known device consists in that the hydrocarbon concentration ascertained on the basis of the sensor signals may be encumbered with considerable inaccuracies, since different physical and/or chemical effects may lead to a measured value drift of the semiconductor sensor.

Against this background, the object ofthe invention is to provide a device for detecting gas in liquid which does not have the disadvantage ofthe previously known device.

This object is achieved by a device having the features indicated in claim 1. Advantageous developments of this device can be seen from the sub-claims, the description which follows and the drawing. The features indicated in the sub-claims may thus contribute advantageously in the indicated combination, but also, provided technically useful, in themselves or in a different combination to the design ofthe invention.

The device of the invention serves for detecting gas in a liquid. Not exclusively, but with precedence it is provided for inspecting plants located under water in offshore exploration and offshore conveying of petroleum and natural gas. In this context the device may be used in conjunction with a remotely operated underwater vehicle (ROV), an autonomous underwater vehicle (AUV) or in conjunction with a long-term monitoring station arranged in the Immediate vicinity of an exploration device or conveyer device located under water to detect leakages at the exploration devices and conveyer devices.

The device has a sensor arrangement which is connected to the surroundings ofthe device and thus also to a gas present in the surroundings. This sensor arrangement is equipped with at least one gas sensor which delivers a corresponding measuring signal in the presence of a gas to be detected, Such a gas sensor, which provides an electrical measuring signal, is preferably provided as the gas sensor. However, a measured value drift may occur particularly in these gas sensors so that the measuring signal provided by the gas sensor viewed alone is not very meaningful since it changes constantly.

The invention takes this fact Into account since the device of the invention has a second sensor arrangement which has a gas sensor which is identical to the gas sensor ofthe first sensor arrangement, wherein the second sensor arrangement is connected to the surroundings ofthe device via a buffer chamber.

Importance is thus given to the buffer chamber upstream ofthe second sensor arrangement to form a reservoir for liquid which keeps away the gas to be detected from the at least one gas sensor of the second sensor arrangement, wherein the gas sensor of the second sensor arrangement is however otherwise exposed to the same physical and/or chemical influences as the gas sensor ofthe first sensor arrangement. This takes place effectively in that the buffer chamber is already filled with liquid, which dues not contain the gas to be detected, before using the device.

Since the gas sensor of the second sensor arrangement is designed tc have the same structure as the gas sensor of the first sensor arrangement and apart from the fact that it does not come in contact with the gas to be detected, it is exposed to the same ambient conditions as the gas sensor of the first sensor arrangement, its measuring signal is influenced by these ambient influences in the same manner as the measuring signal of the gas sensor of the first sensor arrangement, that is, since the gas sensors of the two sensor arrangements have identical sensor properties, the measured values ascertained by the two gas sensors drift with comparable rise in the same direction, in this respect the gas sensor of the second sensor arrangement delivers a reference signal to the measuring signal of the gas sensor of the first sensor arrangement which makes it possible to eliminate a measured value drift from the measuring signal of the gas sensor of the first sensor arrangement. This preferably takes place in that the measuring signals of the gas sensors of the two sensor arrangements are constantly compared with one another in an electronic evaluating device and only the difference in the measured values of the two gas sensors is evaluated, wherein this differential value then forms the basis of the calculation of concentration of the gas detected. This procedure leads to the concentration of a gas also being able to be ascertained with a very high accuracy for a measured value drift caused by changing ambient conditions.

The buffer chamber is preferably designed to be open towards the liquid surrounding the device. Therefore an overflow possibility from the external surroundings of the device to the gas sensor exists for a surrounding liquid of the device via the buffer chamber. The overflow of liquid into the buffer chamber and to the gas sensor of the second sensor arrangement or the filling of the buffer chamber with liquid associated therewith is thus effected effectively when the gas to be detected is not present in the liquid. If the device of the invention is located in an environment in which the liquid contains the gas to be detected, it should be ensured that the gas sensor of the second sensor arrangement, when a gas located in the liquid is detected by the gas sensor of the first sensor arrangement, does not come in contact with this gas. For this purpose, the shape and the dimensions of the buffer chamber should effectively be selected so that the gas located in the liquid cannot reach the gas sensor ofthe second sensor arrangement during detection ofthe gas by the gas sensor of the first sensor arrangement,

As an alternative to a design, in which the buffer chamber is designed to be open towards the liquid surrounding the device, such a design may also be advantageous, in which the buffer chamber is closed by a resilient membrane towards the liquid surrounding the device. This resilient membrane is typically designed to be both liquid-impermeable and gas-impermeable. The buffer chamber is thus filled with a liquid, the physical and chemical properties of which effectively correspond to those ofthe liquid in which the device ofthe invention is used. The membrane thus allows the pressure and the temperature of the surrounding liquid of the device to be transferred to the liquid located in the buffer chamber and at the same time reliably prevents gas located in the surrounding liquid from penetrating into the buffer chamber and being able to reach the gas sensor of the second sensor arrangement.

In particular in a buffer chamber which is open towards the liquid surrounding the device, this buffer chamber is advantageously formed by a pipe wound several times, A design of the pipe is thus preferred In which the latter is wound like a helix, however, the pipe may optionally also have a different labyrinth-like winding, The plurality of windings of the pipe brings with it the advantage that the pipe may have a relatively long length for a comparatively small installation space and hence forms a correspondingly long forerun path from the liquid surroundings of the device to the gas sensor of the second sensor arrangement. Hence, it has been shown that when detecting methane in water, the methane requires at least two days for a preferred internal diameter of the pipe of 0.5-10 mm and a preferred length of the pipe of 10 - 15 cm due to diffusion of methane In water to reach the gas sensor of the second sensor arrangement from the pipe end directly connected to the external surroundings ofthe device. With corresponding increase in the pipe length, this time span may be increased considerably more so that in any case it is ensured that the gas located In the water does not reach the gas sensor of the second sensor arrangement during detection ofthe gas by the gas sensor ofthe first sensor arrangement.

According to a further preferred development ofthe invention, the two sensor arrangements are connected In each case to a measuring chamber which Is closed by means of a gas-permeable membrane. This is understood to mean such an arrangement of the two sensor arrangements, in which the sensors of the sensor arrangements are in contact with the interior of the respective measuring chamber, wherein the sensors of the sensor arrangements may also advantageously be arranged completely within the measuring chambers. Furthermore, provision is made in this development in that the membrane which closes the measuring chamber connected to the first sensor arrangement forms a liquid-tight barrier directly to the external surroundings of the device and the membrane which doses the measuring chamber connected to the second sensor arrangement forms a liquid-tight barrier to the buffer chamber. Due to the liquid impermeability ofthe membranes, the measuring chambers connected to the two sensor arrangements comprise in each case only a gas phase, The gas present in the surrounding liquid of the device and to be detected diffuses through the membrane which doses the measuring chamber connected to the first sensor arrangement into the measuring chamber connected to the first sensor arrangement. The liquid itself may thus not pass through the membrane. Diffusion ofthe gas lasts as long as it takes for a thermodynamic equilibrium to be set between the gas and the liquid. So that this takes place as quickly as possible, the internal volume ofthe measuring chamber is kept as small as possible.

The two membranes which dose the measuring chambers may generally be made from all suitable membrane materials, wherein it is to be ensured only that the membrane on the one hand is gas-permeable and on the other hand liquidimpermeable. According to the invention, the membranes are however preferably silicone membranes. In particular when the device is used at high liquid pressures, such as for example when using the device in the field of offshore exploration and offshore conveying of petroleum and/or natural gas at considerable water depths, provision may advantageously be made in that the membranes are supported in each case on a correspondingly pressure-resistant, gas-permeable plate. This plate may consist, for example of sintered metal, sintered ceramic or ceramic produced in a different way or of glass.

The gas sensors of the first and of the second sensor arrangement may advantageously be formed by semiconductor sensors. Such semiconductor sensors have a gas-sensitive semiconductor layer, which is usually made from a metal oxide. The semiconductor sensors are exposed to voltage, Under a certain influence of gas, the conductivity ofthe semiconductor sensor changes and leads to a change in resistance which forms the basis of the measuring signal delivered by the semiconductor sensor. The use of semiconductor sensors is advantageous inasmuch as they can be obtained comparatively cost-effectively and their energy consumption is relatively low. The last-mentioned property makes It suitable in particular for use in autonomously operated devices for detecting gases, in which the energy supply of the device is arranged within the device.

Alternatively to the use of semiconductor sensors as gas sensors, the gas sensors of the first and ofthe second sensor arrangement may also be formed by optical sensors. Provision is thus preferably made in that the measuring chambers of the two sensor arrangements are arranged between a light source in the form, for example of a tuneable laser beam source operating according to the TDSL principle (TDLS ~ Tuneable Diode Laser Spectroscopy), or an infrared light source and a detector detecting the light intensity which converts an optical signal, such as for example infrared radiation, into an electrical signal. Starting from the light source, light beams are sent through the measuring chamber, wherein light beams are also used, the wavelength of which is absorbed specially by the gas to be detected. This is detected by the detector and further processed to form a measuring signal in conjunction with electronics downstream ofthe detector. The detector may be a photodetector utilising the inner or outer photoelectric effect or in the case of using an infrared light source, a thermal detector, such as for example a pyroelectric detector or thermopile detector, which converts the heat energy of the infrared radiation into an electrical or mechanical signal.

Each of the two sensor arrangements further preferably has at least one second gas sensor. These second gas sensors of the two sensor arrangements form a redundancy to the two first gas sensors of the sensor arrangements and thus detect the same gas as the first gas sensors, Also the second gas sensors of the first and second sensor arrangement are effectively designed to have the same structure and are preferably connected in each case to a measuring chamber which is closed by means of a gas-selective membrane, wherein the membrane of the first sensor arrangement forms a liquid-tight barrier directly to the external surroundings of the device and the membrane of the second sensor arrangement forms a liquid-tight barrier to the buffer chamber. The design ofthe second gas sensors and ofthe measuring chambers connected thereto may advantageously be completely identical to the design of the first gas sensors and of the measuring chambers connected thereto. Furthermore, the first gas sensors and the second gas sensors of the two sensor arrangements may also be connected in each case to the same measuring chamber.

As already noted, the device of the invention is provided with precedence for inspecting plants located under water in offshore exploration and offshore conveying of petroleum and natural gas and thus serves for early discovery of leakages at the exploration devices and conveyer devices located under water which are rendered perceptible first of ail by an increased content of hydrocarbon gases in the immediate water surroundings of these devices. According to a further advantageous development ofthe invention, the gas sensors are therefore designed for selective detection of hydrocarbon gases.

For example when only the concentration of a special gas is to be ascertained using the device of the invention, wherein the gas sensor used for this does not have the corresponding selectivity, it is effective if only this special gas comes in contact with the gas sensor. This guarantees a further advantageous development of the invention, according to which in each case a filter, which is permeable only to a certain gas, is arranged at least on the inlet side of the gas sensors of the first sensor arrangement. If the gas sensors are designed for selective detection of hydrocarbon gases, the filter may be designed, for example so that it allows through only a certain hydrocarbon gas, such as for example methane, to the gas sensor. In the preferred design, in which the gas sensors are connected to a measuring chamber which is closed by means of a gas-permeable membrane, provision is preferably made in that the filter is formed by a corresponding layer which is attached to the membrane.

If a corresponding filter is not arranged on the membrane, oxygen located in the liquid may also reach the measuring chamber connected to the gas sensors of the first sensor arrangement by diffusion. Since greatly changing oxygen concentrations in the measuring chambers possibly have influence on the measured values of the gas sensors of the two sensor arrangements provided for detecting a gas than other oxygen, it is useful to take into account the oxygen located in the measuring chamber when ascertaining the concentration ofthe gas actually to be detected. According to a further preferred design, each ofthe two sensor arrangements therefore additionally has a gas sensor for selective detection of oxygen.

Also the absolute humidity in the measuring chambers connected to the gas sensors of the two sensor arrangements has an effect on the measured values of the gas sensors. Hence, for example an increased absolute humidity in the measuring chambers results in an increase in conductivity ofthe semiconductor sensor and hence going along with that falsification of its measured value when using semiconductor sensors, in this respect, there is fundamentally interest in determining the absolute humidity in the measuring chambers and allowing it to flow into ascertaining the concentration ofthe gas to be detected. In this respect according to a further preferred design, a sensor for determining the absolute humidity in the measuring chamber is arranged in the measuring chamber of the two sensor arrangements.

In order to be able to keep constant the absolute humidity in the measuring chambers, a temperature sensor and a heater are advantageously arranged in the measuring chamber of the two sensor arrangements, wherein a temperature regulating device is provided which regulates the temperature in the measuring chamber to a constant value.

Furthermore, it is also necessary in ascertaining the concentration ofthe gas to be detected to have knowledge of the gas pressure in the measuring chambers of the two sensor arrangements. In this context provision is preferably made In that a gas pressure sensor is arranged in the measuring chamber of the two sensor arrangements. This gas pressure sensor is effectively signal-connected to the electronic evaluating unit for determining the concentration of the gas to be detected, hi addition, it is also useful to also let the conductivity, the temperature and the pressure of the surrounding liquid of the device of the invention flow into the determination ofthe concentration ofthe gas to be detected. The invention takes this into account in that a sensor which detects the conductivity of the surrounding liquid ofthe device is preferably provided, a sensor which detects the temperature ofthe surrounding liquid of the device is further preferably provided and furthermore, a sensor which detects the pressure ofthe surrounding liquid ofthe device is provided. Also these said sensors are effectively signal-connected to the electronic evaluating unit for determining the concentration of the gas to be detected.

The invention is illustrated in more detail below using exemplary embodiments shown in the drawing. The drawing shows schematically simplified and partly on different scales:

Figure 1: a device for detecting gas in a liquid in a sectional view,

Figure 2: possible use scenarios of the device according to Figure 1,

Figure 3: a gas sensor of the device according to Figure 1 according to a first embodiment,

Figure 4: a gas sensor of the device according to Figure 1 according to a second embodiment and

Figure 5: a remotely operated underwater vehicle shown in Figure 2.

The device 2 shown in detail in Figure 1 is provided for underwater surveying of devices located below the water surface of offshore plants for conveying and for transporting petroleum and natural gas and serves to detect hydrocarbon gases optionally emerging there. Possible use scenarios of the device 2 associated therewith are shown in Figure 2.

Figure 2 shows an offshore plant far conveying petroleum arranged in a stretch of water. This offshore plant, in addition to an offshore platform arranged for the most part above a water surface 4, also has a few conveyer devices arranged below the water surface 4. Figure 2 shows in this context by way of example an underwater structure 10 arranged on the base ofthe stretch of water 8 with conveyer pipelines 12 and 14 starting therefrom, wherein a so-called subsea tree 16 is connected to one end of the conveyer pipeline 14.

If leakages, which are rendered perceptible by hydrocarbon gases 20 emerging into the water 18, occur at the conveyer devices arranged below the water surface 4, the device 2 serves to discover these leakages in that it detects the hydrocarbon gases 20 then emerging into the water 18 at the leakage point.

In order to be able to position the device 2 in the vicinity of a possible leakage point, it may be arranged in or on an underwater vehicle. As can be seen from Figure 2, the underwater vehicle may be a remotely operated underwater vehicle 22 (ROV) or an autonomous underwater vehicle 24 (AUV), Whereas the autonomous underwater vehicle 24 operates essentially self-sufficiently, the remotely operated underwater vehicle 22 in the present case is remotely controlled by a surface ship 26. Hence, the underwater vehicle 22 is connected to the surface ship 26 via a cable 28. The cable 28 serves, in addition to the energy supply ofthe underwater vehicle 22, to transfer control signals from the surface ship 28 to the underwater vehicle 22 and for data exchange between the device 2 and devices communicating therewith on the surface ship 26.

Furthermore, the possibility exists of monitoring the individual components ofthe conveyer devices arranged below the water surface 4 by means of devices 2 which are arranged to be fixed in the immediate vicinity ofthe monitored object. In this context Figure 2 shows a long-term monitoring station 30 fitted with two devices 2, and with which the subsea tree 16 is monitored.

Detailed reference is made below to Figure 1. The device 2 shown there has a housing 32. In view of a possible use ofthe device 2 at considerable water depths, the housing 32 is designed to be pressure-resistant and components of the device 2 which are not designed to be pressure-resistant are encapsulated to be pressureresistant in the housing 32. The housing 32 has a base body 34 which forms a hollow body which is open towards two sides which are directly opposite one another. An interior 36 of the base body 34 is closed by two walls 38 and 40 which are designed to be pressure-resistant and are arranged in the interior of the base body 34 to be slightly offset to the open ends of the base body 34. hi the interior of the base body 34, in each case a shoulder 42 is formed on both open ends of the base body 34 of the housing 32. A plate 44 lies on each of these shoulders 42. The plates 44 are designed in each case to be gas-permeable and pressure-resistsnt as sintered parts made of metal or other materials.

A membrane 46 likewise engaging in the shoulder 42 rests on the plate 44, which is arranged directly on the outside of the wall 38, on the side of the plate 44 facing away from the wail 38. The membrane 46 is designed to be gas-permeable, but not liquid-permeable. The membrane 46 ends flush with the end of the base body 34 with its side facing away from the plate 44, The membrane 46 and the plate 44 are fixed in the shoulder 42 of the base body 34 positively and optionally also nonpositively by means of a fastening ring 48 which is screwed to the base body 34 of the housing 32.

A free space, which is designated below as the measuring chamber 50, is located between the plate 44 and the wall 38. If the device 2 is being used, no water 18 may reach the measuring chamber 50 due to the liquid-impermeable design of the membrane 46. The situation is different with gases located in the water 18, such as hydrocarbon gases 20 emerging with precedence at a leakage point, which diffuse through the membrane 46 and thus reach the measuring chamber 50 via the likewise gas-permeable plate 44, Diffusion of the gas lasts as long as it takes for a thermodynamic equilibrium to be set between the hydrocarbon gas 20 and the water

18. The molar concentration of the hydrocarbon gas 20 dissolved in the water 18 is then in thermodynamic equilibrium with the partial pressure of the hydrocarbon 20 in the gas phase in the measuring chamber 50.

A first sensor arrangement 52 of the device 4 is arranged in the measuring chamber 50. The sensor arrangement 52 comprises a first gas sensor 54 for ascertaining the hydrocarbon gas concentration in the measuring chamber 50, a second gas sensor 56 forming a redundancy to the first gas sensor 54, a gas sensor 58 for ascertaining the oxygen concentration in the measuring chamber 50, a sensor 60 far ascertaining the absolute humidity in the measuring chamber 50, a temperature sensor 62 for ascertaining the gas temperature in the measuring chamber 50 and a pressure sensor 64 for ascertaining the gas pressure in the measuring chamber 50.

in addition to the first sensor arrangement 52, a heater 66 is also arranged in the measuring chamber 50. The heater 66, together with the temperature sensor 62 and a temperature regulating device not shown explicitly in the drawing, serves to keep constant the temperature in the measuring chamber 50.

All sensors of the first sensor arrangement are connected to an electronics module 68, which is arranged in the Interior 36 ofthe base body 34 ofthe housing 32, by means of leads. The electronics module 68 serves as the electrical energy supply of all electrical or electronic devices of the device 2 and for evaluating the measuring signals delivered by the sensors ofthe device 2. The electronics module 68 is equipped with a battery 70, which is connected to a charging connection 74 via a charge regulator 72, for self-sufficient energy supply ofthe device 2. Evaluation of the measuring signals provided by the sensors of the device 2 is effected in an evaluating device 76 of the electronics module 68. Furthermore, the electronics module 68 has a data store 78 which can be lead-connected to a peripheral device not shown via a data input and data output unit 80 and a data transmission cable 80.

A membrane 84 engaging in the shoulder 42 rests on the plate 44, which is arranged directly on the outside of the wall 40, on the side of the plate 44 facing away from the wall 40. Like the membrane 46, the membrane 84 is also designed to be gaspermeable but not liquid-permeable. The membrane 84 ends flush with the end of the base body 34 with its side facing away from the plate 44. The membrane 84 and the plate 44 are fixed in the shoulder 42 positively and optionally also non-positively by means of an element 86 which is screwed to the base body 34 of the housing 32.

A free space, which is designated below as the measuring chamber 88, is also located, as between the plate 44 and the wall 38, between the plate 44, on which the membrane 84 rests, and the wail 40. A second sensor arrangement 90 ofthe device 2 is arranged in this measuring chamber 88. This second sensor arrangement 90, in addition to a first gas sensor 92, which is identical to the gas sensor 54 ofthe first sensor arrangement 52, has a second gas sensor 94, which is identical to the second gas sensor 56 of the first sensor arrangement 52, and forms a redundancy to the first gas sensor 92 of the second sensor arrangement 90. Furthermore, the second sensor arrangement 90 comprises a gas sensor 96 for ascertaining the oxygen concentration in the measuring chamber 88, which is identical to the gas sensor 58 ofthe first sensor arrangement 52, a sensor 98 for ascertaining the absolute humidity in the measuring chamber 88, which Is identical to the sensor 60 of the first sensor arrangement 52, a temperature sensor 100 for ascertaining the gas temperature in the measuring chamber 88, which is identical to the temperature sensor 62 of the first sensor arrangement 52, and a pressure sensor 102 for ascertaining the gas pressure in the measuring chamber 88, which is identical to the pressure sensor 64 of the first sensor arrangement 52. All sensors of the second sensor arrangement 90 are likewise connected to the evaluating device 76 of the electronics module 68 via leads. In addition to the second sensor arrangement 90, a heater 104, which together with the temperature sensor 100 and a temperature regulating device not shown explicitly in the drawing, serves to keep constant the temperature in the measuring chamber 88 at the temperature value of the measuring chamber 50, is also arranged in the measuring chamber 88.

The element 86 screwed to the base body 34 of the housing 32 and which fixes the membrane 84 and the plate 44 in the shoulder 42 ofthe base body 34 positively and optionally non-positlvely, is designed to be like a bowl and forms a hollow cavity 106 which is open towards the membrane 84. A hole, into which one end of a pipe 110 wound several times like a helix engages, is formed on an end plate 168 ofthe element 86 aligned parallel to the membrane 84.

Together with the hollow cavity 106 of element 86, the pipe 110 forms a buffer chamber 112 to receive water 18, which may flow from the surroundings of the device 2 via the free end of the pipe 110 designed to be open into the pipe 110 and from there into the hollow cavity 106 ofthe element 86. In particular the length and Internal cross-sectional dimensions ofthe pipe 110 thus ensure that gases located In the water 18, at least during use ofthe device 2, do not reach the vicinity ofthe membrane 84 and from there the measuring chamber 88, whereas the remaining ambient conditions ofthe measuring chamber 88 and ofthe second sensor arrangement 90 arranged therein correspond to the ambient conditions ofthe measuring chamber 50 and the first sensor arrangement 52 arranged therein. Changes in the measuring signals ofthe gas sensors 92, 94 and 96 ofthe second sensor arrangement 90 can therefore only be traced back to a measuring signal drift of these gas sensors 92, 94 and 96, which correspond to the measuring signal drift of the gas sensors 54, 56 and 58 of ths first sensor arrangement 52. This circumstance is utilised in the evaluating device 76 ofthe electronics module 68 to calculate the drift-related proportions ofthe measuring signal changes ofthe gas sensors 54, 56 and 58 of the first sensor arrangement 52 used for detecting hydrocarbon gases 20 and oxygen.

The corrected measuring signals of the gas sensors 54, 56 and 58 of the first sensor arrangement 52 form the basis for calculating a hydrocarbon gas concentration in the water 18 in the surroundings ofthe device 2 in the evaluating device 76 ofthe electronics module 68. However, to calculate this hydrocarbon gas concentration it is also necessary to have knowledge of the temperature, the pressure and the conductivity of water 18 in the surroundings ofthe device 2. Therefore the device 2, in addition to the sensors of the first sensor arrangement 52 and the sensors of the second sensor arrangement 90, also has a temperature sensor 114 directly connected to the external surroundings ofthe device 2, a pressure sensor 116 directly connected to the external surroundings ofthe device 2 as well as a conductivity sensor 118 directly connected likewise to the external surroundings of the device 2.

The gas sensors 54, 56 and 58 of the first sensor arrangement 52 and the gas sensors 92, 94 and 96 of the second sensor arrangement 90 may be designed as semiconductor sensors. Such a semiconductor sensor 120 is shown greatly simplified schematically in Figure 3. The semiconductor sensor 120 has a base body 122, on which a sensor layer 124 of a semiconductor material is arranged. An electric voltage is applied to the sensor layer 124 via contacts 126 and 128. The semiconductor material ofthe sensor layer 124 is selected so that, if it comes in contact with a certain gas 130, it reacts thereon in the form of a change in electrical conductivity ofthe sensor layer. So that a good Intrinsic conductivity ofthe sensor layer 124 starts at ail, the latter must be heated to a certain temperature. Hence, a heating element 132 is arranged in the base body 122 ofthe semiconductor sensor 120.

Alternatively to using semiconductor sensors as gas sensors, the gas sensors 54, 56 and 58 of the first sensor arrangement 52 and the gas sensors 92, 94 and 96 of the second sensor arrangement 90 may also be formed by optical sensors. Such an optical sensor 134 is shown in Figure 4 in a diagrammatic sketch. This sensor 134 has a light source 138 controlled by an electronics module 136. The light source 138 may be a tuneable laser, a tuneable laser diode or an infrared light source. In the ray path ofthe light source 138 there is a detector 140 detecting the light intensity which is arranged spaced from the light source 138. If a certain gas 130 is located in the gap 142 between the light source 138 and the detector 140, wavelengths ofthe light emitted by the light source 138 which are typical for this gas 130 are absorbed by the gas 130, which is detected by the detector 140. An electronic signal processing device 144 signal-connected to the detector 140 then generates a correpsondlng measuring signal In the form of an electric voltage value.

As already noted in connection with Figure 2, the device 2 may be used in conjunction with a remotely operated underwater vehicle 22 (ROV). Figure 5 shows the underwater vehicle 22 represented in Figure 2 in an enlarged individual drawing. The underwater vehicle 22 has an upper part 146, to which a substructure 148 Is connected. Whereas the upper part 146 contains the float of the underwater vehicle 146, the device 2 is arranged in its substructure 148, wherein the end region ofthe device 2, in which the first sensor arrangement 52 is arranged, projects from the substructure 148. A housing 150 is arranged on the outside ofthe fastening ring 48 on this end ofthe device 2 projecting from the substructure 148. The housing 150 is designed to be open towards the fastening ring 48, wherein the cross-section of an interior 152 ofthe housing 150 corresponds to the inner cross-section ofthe fastening ring 48. A hole, into which a hose pipe 154 engages, is formed on the housing 150 on its side facing away directly from the fastening ring 48. A funnel 156 is formed on the end ofthe hose pipe 154 facing away from the housing 150. Water 18 is conveyed from the surroundings ofthe underwater vehicle 22 via the funnel 156 and the hose pipe 154 into the Interior 152 ofthe housing 150 by means of an underwater pump 158 integrated in the hose pipe 154, There the water 18 comes in contact with the membrane 46, through which hydrocarbon gas 20 possibly present in the water 18 diffuses into the measuring chamber 50 ofthe first sensor arrangement 52 so that the concentration of hydrocarbon gas 20 may be ascertained in the device 2 in the manner already described, While the underwater pump operates, excess water 18 may leave the interior 152 of the housing 150 via two outlets 160 formed on the housing 150. In order to be able to align the funnel 156 precisely to a possible leakage point, the underwater vehicle 22 has a multi-hinged manipulator arm 162. The hose pipe 154 is attached to this manipulator arm 162 in the region ofthe funnel 156,

Reference numbers

Device

Water surface

Offshore platform

Base of stretch of water

Underwater structure

Conveyer pipeline

Subsea tree

Water

Hydrocarbon gas

Underwater vehicle

Surface ship

Cable

Long-term monitoring station

Housing

Base body

Interior

Wall

Shoulder

Plate

Membrane

Fastening ring

Measuring chamber

Sensor arrangement

Gas sensor

Senser

Temperature sensor

Pressure sensor

Heater

Electronics module

Battery

Charge regulator

Charging connection

Evaluating device

Data store

Data input and data output unit

Data transmission cable

Membrane

Element

Measuring chamber

Sensor arrangement

Gas sensor

Sensor

Temperature sensor

Pressure sensor

Heater

Hollow cavity

End plate

Pipe

Buffer chamber

Temperature sensor

Pressure sensor

Conductivity sensor

Semiconductor sensor

Base body

Sensor layer

Contact

Gas

Heating element

Sensor

Electronics module

Light source

Detector

Gap

Signal processing device Upper part

Substructure

Housing interior Hose pipe

Funnel Underwater pump

Outlet

Claims

1. Device (2) for detecting gas (130) in a liquid, having a first sensor arrangement (52), which has at least one gas sensor (54) and is connected to the surroundings ofthe device (2), and having a second sensor arrangement (90), which has a gas sensor (92) which is identical to the gas sensor (54) of the first sensor arrangement (52) and is connected to the surroundings of the device (2) via a buffer chamber (112).

2. Device (2) according to claim 1, in which the buffer chamber (112) is designed to be open towards the liquid surrounding the device (2).

3. Device (2) according to claim 1, in which the buffer chamber (112) is closed by a resilient membrane towards the liquid surrounding the device (2).

4. Device (2) according to one of the preceding claims, in which the buffer chamber (112) is formed at least partly by a pipe (110) wound several times.

5. Device (2) according to one of the preceding claims, in which the two sensor arrangements (52, 90) are connected in each case to a measuring chamber, which is closed by means of a gas-permeable membrane (46, 84), wherein the membrane (46) which closes the measuring chamber (50) connected to the first sensor arrangement (52) forms a liquid-tight barrier directly to the external surroundings ofthe device (2) and the membrane (84) which closes the measuring chamber (88) connected to the second sensor arrangement (90) forms a liquid-tight barrier to the buffer chamber (112).

6. Device (2) according to one of the preceding claims, in which the gas sensors (54, 56, 92, 94) of the first (52) and of the second (90) sensor arrangement are formed by semiconductor sensors (120).

7. Device (2) according to one of claims 1 to 5, in which the gas sensors (54, 56, 92, 94) ofthe first (52) and ofthe second (90) sensor arrangement are formed by optical sensors (134).

8. Device (2) according to one ofthe preceding claims, in which the gas sensors (54, 56, 92, 94) are designed for selective detection of hydrocarbon gases (20).

9. Device (2) according to one ofthe preceding claims, characterised in that each of the two sensor arrangements (52, 90) additionally has a gas sensor (58, 96) for selective detection of oxygen.

10. Device (2) according to one of claims 5 to 9, in which a sensor (60, 98) for determining the absolute humidity in the measuring chamber (50, 88) is arranged in the measuring chamber (50, 88) of the two sensor arrangements (52, 90).

11. Device (2) according to one of clams 5 to 10, In which a temperature sensor (62, 100) and a heater (66, 104) are arranged in the measuring chamber (50, 88) ofthe two sensor arrangements (52, 90), wherein a temperature regulating device is provided which regulates the temperature in the measuring chamber (50, 88) to a constant value.

12. Device (2) according to one of claims 5 to 11, in which a gas pressure sensor (64, 102) is arranged in the measuring chamber (50, 88) of the two sensor arrangements (52, 90).

13. Device (2) according to one ofthe preceding claims, In which a sensor(118) which detects the conductivity of the surrounding liquid of the device (2) is provided.

14.Device (2) according to one ofthe preceding claims, in which a sensor (114) which detects the temperature of the surrounding liquid of the device (2) is provided.

15.Device (2) according to one ofthe preceding claims, In which a sensor(116) which detects the pressure ofthe surrounding liquid ofthe device (2) Is provided.