GB2176604A - Acoustic detection of gas leaks - Google Patents

Abstract

In order to detect gas leaks from gas containers, particularly but not exclusively, underwater gas pipelines 4, 5, either a passive or active sonar detection system is mounted externally of the pipeline. In a passive system leaking gas passes through a sensor beam pattern which effectively "envelopes" the pipeline. In an active system both transmitters and receivers 24, 25 are used and a canopy 20 may be provided over the pipeline 4 so that any leaking gas collects at an apex. A test system is also provided which simulates a gas leak to test the detection system. <IMAGE>

Description

SPECIFICATION Detecting gas leaks This invention relates to detecting gas leaks, particularly but not exclusively to detecting underwater gas leaks, for example from undersea gas supply pipelines.

Gas which is recovered from gas fields under the sea bed, for example under the North Sea, is pumped through a pipeline from the gas drilling rig to a gas production platform.

Should a leak occur in this pipeline a highly explosive gas/air mixture can quickly engulf the platform leading to a high likelihood of an explosion and a fire.

An existing technique of detecting such a leak involves the use of acoustic sensors located within the pipeline and sensitive to a noise characteristic of a leak occurring, to shut down a main supply valve, thereby minimising the risk. An alarm is also sounded.

However this system has a number of drawbacks. For example it is difficult for the sensor to distinguish between the noise of a gas leak and other noises within the pipeline, such as valve-hiss, and valves opening and closing and other disturbances transmitted along the metallic pipeline structure. Also the equipment is difficult to test, service or repair due to its location.

It is an object of the present invention to provide an improved gas leak detection system which is easier to test, less prone to false alarms, and easier to install both as original equipment when the gas-carrying pipeline or container is installed and as an enhancement to existing installations.

According to the present invention there is provided a method of detecting a gas leak from a gas container, such as a gas pipeline, comprising providing a sensor externally of but in the vicinity of the container and so arranging that any gas which should leak from the container will be detected by the sensor.

According to another aspect of the present invention there is provided apparatus for detecting gas leaks from a gas container, such as a gas pipeline, comprising a sensor adapted for mounting on or adjacent the container, externally thereof, and so arranged that any gas leaking from the container will be detected by the sensor.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Fig. 1 shows diagrammatically an undersea gas supply pipeline with a gas leak detection system according to an embodiment on the present invention; Fig. 2 shows part of the equipment of Fig.

1 viewed along the pipeline; Fig. 3 shows schematically a modification of the system of Figs. 1 and 2; Figs. 4A and 4B show another modification of the embodiment of Figs. 1 and 2; Fig. 5 shows another modification of the embodiment of Figs 1 and 2, this time for rising pipelines; Fig. 6 shows an alternative arrangement to the Fig. 1 embodiment, and Fig. 7 shows a test system for testing the leak detection system.

In the drawings the same reference numerals indicate the same or at least functionally similar parts.

Referring to Fig. 1 a production platform 1 mounted on the sea bed 2 by legs such as 3 has a gas supply pipeline having a horizontal section 4 and a rising, vertical section 5.

Mounted on the pipeline at a location indicated by the reference letter A is a pair of passive acoustic array detectors, shown in greater detail in Fig. 2 viewed along the pipeline 4.

As seen in Figs. 1 and 2 together, the passive acoustic arrays, designated in Fig. 2 by the reference numerals 6 and 7, and located at A in Fig. 1 generate beam patterns indicated by the reference letter a arranged to give complete coverage over the pipeline 4.

At location B is located an active acoustic array 8 which monitors the correct operation of the passive arrays 6 and 7 and/or monitors the propagation paths between the locations A and B to check that this region is free of bubbles/gas volumes and jets.

Shown at reference letter C is a downwardlooking passive acoustic array, similar to the arrays 6 and 7, which monitor the region between the platform 1 and the surface S of the sea, for gas leaks.

The system shown in Figs. 1 and 2 can be modified as shown in Fig. 3. Referring to Fig.

3 which is a cross-section of a pipe such as the pipe 4 of Fig. 1, the pipe 4 is covered by a canopy 10 which is free flooding and has small vent holes 11 and 12 at intervals along the apex 13 of the canopy 10. This modification has the effect of increasing the discrimination of the system shown in Figs. 1 and 2, which would be incorporated as shown in Figs. 1 and 2 but located beneath the canopy, and this will help to reduce the possibility of false alarms. Furthermore the modification also has the effect of isolating the detection system from spurious acoustic sources and interferences from equipment and personnel involved in pipeline maintenance. The small vent holes 11 and 12 prevent the build up of very small amounts of gas which escape naturally from the sea bed 2.A significant leakage of gas from the pipeline 4 would produce a gas pocket near the apex 13 which would be detected by the acoustic array system described earlier. Alternatively the presence of a gas pocket could be detected by a more conventional liquid-level-type gauge, including very simple acoustic sensors, as will be described in the following.

Referring to Figs. 4A,and 4B there is now described an acoustic level detection system within a canopy similar to the one shown in Fig. 3. However in this embodiment the pipeline 4 is covered by a canopy 20 having small vent holes 21 and 22 provided in gas traps 23, clearly seen in Fig. 4B. The gas traps 23 prevent a small leakage of gas from dispersing throughout the length of the canopy 20. An acoustic transmitter/detector pair designated by the reference numerals 24 and 25 are arranged to monitor the presence of a gas pocket which will collect in the gas traps when a significant leak occurs. As shown in Fig. 4A the formation of a liquid-gas interface 20 would change significantly the acoustic path between transmitter and receiver 24 and 25.

Referring now to Figure 5 there is shown a modification of the system shown in Figs. 4A and 4B, as applied to a rising, vertical pipeline, such as the vertical pipeline portion 5 shown in Fig. 1. Referring to Fig. 5 a series of cylindrical canopies 30, 31 and 32 are mounted on the pipeline. Canopies 31, 32 and 33 have small vent holes 31a, 32a, 33a and each has an acoustic projector 34 and an acoustic detector such as a single hydrophone, 35. The principle of operation is the same as that described with reference to Fig.

4 where an accumulation of gas will occur when a significant leak develops. Otherwise small amounts of gas generated through natural causes, such as the production of gases from the sea bed or from corrosion of the metal pipeline or canopies, will seep through the vent holes and not be produced quickly enough to form a gas pocket. As shown a fault 5a in pipe 8 has caused a gas pocket to collect in canopy 30 which is sensed and causes shut down of valve 9 (Fig. 1) and raises an alarm.

The embodiments described propose a method of detecting the leakage of gas from gas pipelines and tanks, and can be applied to other storage and transport media, using passive or active acoustic arrays or sensors. The passive arrays are used to detect radiated acoustic energy from the gas jet, whether underwater or in air. The active array is an alternative technique for detecting escaping gas in air or water whereby the gas alters the transmission loss between an acoustic projector and a detector.

It is proposed to use the arrays so as to focus the reponse of the detector into a relatively narrow, two-dimensional beam in embodiments where the canopy arrangement is not used. By this means the gas detection is restricted to a region close to the surface of the pipe and the resulting directivity reduces the possibility of spurious acoustic signals triggering the leakage detection system.

The passive or active arrays are arranged to "look" along the length of the pipeline, and by a suitable choice of acoustic frequency the escaping gas can be detected over considerable distances. The possibility of false alarms is reduced by the use of "signature analysis" of the signals produced by escaping gas. The reliability of the system depends upon the ability to discriminate between escaping gas and other acoustic sources. The gas will tend to produce significant amounts of acoustic energy at frequencies above 20kHz. By restricting the detection to high frequencies, only very powerful, localised sources will be detected due to the considerable attenuation to high frequency sound under water.

The positive detection of a gas leak will be the signal to close down the emergency valve, which is indicated by the reference numeral 9 in Fig. 1. The build up of hazardous volumes of inflammable gas in the vacinity of the production platform is thereby avoided. The emergency valve will operate as part of a "failsafe" system in which any failure of communication with the acoustic detection system will cause the valve to close. The valve is designed so that it fails "closed".

For gas pipelines which are buried under the sea the same system will respond to gas bubbles or jets emerging from the sea bed.

Similarly the system is suitable for monitoring the very high frequency components of sound emitted from gas escaping into air for land line systems.

It is proposed to use purely fibre optic links to the detection system and the emergency shutdown valve, so that the system is intrinsically safe should gas escape in the region of the production platform 1.

With the canopies disclosed in the embodiments of Figs. 3, 4 and 5, the size of the vent holes are calculated by determining what is to be regarded as the minimum detectable leak. For example a leak at the rate of 1Kg of gas per second will become 1.5m3/s of CH4 at atmospheric temperature and pressure. The volume will vary according to the local hydrostatic pressure. For example, in a 100m of water the local pressure is about 10 bar.

Therefore 1 gas is equivalent to about 1501/s. Those volume flows will be readily detectable as a gas pocket as long as the vent holes are correctly designed.

For the embodiment described using passive sensors, two kinds of passive sensors are envisaged: either a mechanically-focussed array of piezoelectric transducers or an electronically-focussed array of piezoelectric tranduers.

The latter type of array could comprise already-available transducer elements, and the application to gas leak detection would involve appropriate signal processing to form beams and to interpret the signals received.

The active array (source) for the active sensor embodiment described would be a mechanically-focussed array of piezoelectric transducers, designed to project a narrow beam of sound in the frequency range up to about 50KHz.

Fig. 6 shows an alternative to Fig. 1. Here a string of single hydrophones 40 having an omnidirectional response in a linear array are distributed along the length of the pipeline 4.

The array is linked by cable 41. As an alternative to the cable 41 an acoustic telemetry system could be used. Two approaches to deployment of the passive sensors are possible. Single hydrophones 40 can be suspended close to the pipeline, on suitable mechanical support 42 as shown, and connected by cable 41 to the platform and/or to the emergency shut-down valve 9. Alternatively, the hydrophones 40 may be incorporated within an acoustically-transparent enclosure (e.g. a polyurethane tube), to form a linear array. This array could then be suspended above, or laid alongside the pipeline, as if it were a cable. Connections would be made to the platform or the emergency valve.This method of deploying the hydrophones would make the system easy to recover and replace (for maintenance), and would be more suitable for monitoring buried pipelines where attachment of single hydrophones might prove difficult.

Operating techniques: Option 1.-Passive detection only:- Passive arrays (planar arrays) or linear arrays of single hydrophones would be used to detect acoustic characteristics of leaking gas. e.g. amplitude and frequency thresholds would be pre-set, and the alarm triggered when acoustic energy is detected above the preset levels.

Option 2.-Passive detection of "signature" of leaking gas. This would be physically the same arrangement of sensors as Option 1 but with signature analysis (frequency spectrum) as opposed to purely amplitude and frequency thresholds. This technique would offer additional discrimination over Option 1 and so reduce the possibility of false alarms.

Option 3.-As Option 1 but with active array producing calibration and test signals characteristics of the amplitude and frequency of escaping gas.

Option 4.-As Option 2 but with active array producing calibration and test signals of the same spectrum as a gas leak. This would, again, provide greater system discrimination.

It is also proposed to use a light pressure gas (air) pipeline (of small diameter) running adjacent to the main pipeline to generate a gas jet test at intervals to check the leak detection system. Referring to Fig. 7 gas jets 50A would be generated by releasing a valve, controlled from the platform, which would ad mit high pressure air into the test line 52. The test line 52 would have orifices such as 50 at a spacing of about 50m and the leak detection system is checked for correct operation by activating the test line.

Claims (14)

1. A method of detecting a gas leak from a gas container, such as a gas pipeline, comprising providing a sensor externally of but in the vicinity of the container and so arranging that any gas which should leak from the container will be detected by the sensor.

2. A method as claimed in claim 1, wherein the sensor is responsive to a sound signal generated by the leaking gas.

3. A method as claimed in claim 1 wherein the container is an undersea gas pipeline, the method comprising establishing a sonar beam pattern which extends along the pipeline and effectively envelops the pipeline from above and along the sides, any leaking gas causing an interruption of the beam pattern.

4. A method as claimed in claim 1, wherein the container is covered with a canopy arranged to constrain and collect any leaking gas, and wherein the sonar signal is directed into the collected gas.

5. A method as claimed in claim 4 wherein the container is constituted by a rising pipeline underwater, and wherein the canopy extends around the rising pipeline.

6. A method as claimed in any preceding claim, comprising a gas shut-down valve connected to the container, and wherein the detection of gas automatically closes the shutdown valve.

7. Apparatus for detecting gas leaks from a gas container, such as a gas pipeline, comprising a sensor adapted for mounting on or adjacent the container, externally thereof, and so arranged that any gas leaking from the container will be detected by the sensor.

8. Apparatus as claimed in claim 7, wherein the sensor is part of a sonar detection system which provides a beam so shaped that the beam itself provides an envelope around the container.

9. Apparatus as claimed in claim 7, comprising a canopy adapted for locating over the container and said detector so that any gas leaking from the container will be collected, and detected.

10. Apparatus as claimed in claim 8, wherein the container is an underwater gas pipeline, and the canopy has raised portions at discrete intervals along its lenght, at which said gas will collect.

11. Apparatus as claimed in claim 9, wherein the container comprises a rising pipeline, said canopy comprising a circumferentially continuous structure located around the rising pipeline.

12. Apparatus as claimed in claim 8, comprising beam-forming sonar projectors, said beams being inclined with respect to each other whereby to provide a "beam canopy".

13. Apparatus as claimed in claim 7, wherein the sensor comprises a string of hy drophones for distribution about the container.

14. A method or apparatus for detecting gas leaks, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.