NO20210843A1 - Underwater gas collector, related apparatus and method - Google Patents

Description

UNDERWATER GAS COLLECTOR, RELATED APPARATUS AND METHOD

The present invention relates to the collection and detection of underwater gas, for example, underwater gas that has leaked from an underwater industrial structure or natural source.

Underwater gas occurrences may require detection and quantification in various contexts, such as in connection with operating and installing underwater equipment or structures. Underwater gas may be released from formations of the seabed and/or may be released as leaks from underwater structures that are intended to contain gas. Gas releases over a certain size may indicate issues that may need to be remedied or otherwise addressed. The structures may include pipes, tubulars, well templates, wellheads, etc. In some circumstances, low gas rates may be acceptable. There is often a need for detection and quantification of underwater gas that has leaked from such structures etc.

Conventional devices to facilitate detecting and quantifying leaks include fluid collectors. A leak may be quantified by determining its size which may in turn be determined from the rate of production and/or amount of leaked fluid produced. Fluid collectors are often used for collecting a leaking fluid from a leak source to facilitate analysis of collected fluid and leak behaviour. These collectors typically involve collecting the fluid in a containment region of the collector and allowing the collected medium to accumulate in the containment region. They typically require operation and control of one or more flow control valves such as ball valves or needle valves or the like in order to drain the fluid from the collector for leak data to be determined.

The conventional devices are often not suitable or practical for underwater settings or for the detection of underwater gas in such settings. Seabed installations often have several facilities which may need to be monitored for gas leaks, at considerable ocean depths. There is sought improved solutions that can operate more reliably, can withstand underwater conditions, and can be easily more installed and operated.

At least one aim of the invention is to obviate or at least mitigate one or more difficulties or drawbacks of prior art.

According to a first aspect of the invention, there is provided an underwater gas collector for capturing and collecting underwater gas in one or more collecting cycles, the gas collector comprising a siphon which is operable, without moving parts, in response to sufficient accumulation of collected gas within the collector, to remove the collected gas of one collecting cycle from the collector and reset the collector to permit further gas to be collected in at least one further collecting cycle.

In this way, underwater gas can be collected through several collection cycles with the collector resetting by way of the siphon each time a certain amount of gas has been collected.

Typically, the siphon comprises a riser for removing the gas through the riser upon the accumulated gas, having sufficiently accumulated, reaching an end of the riser in a containment region of the collector, or reaching the level of the end of the riser in the containment region of the collector.

The riser typically has first and second ends, and the riser may extend from the first end at a location in the containment region to the second end located at a receiving location outside the collector.

The riser may have an open fluid passageway therethrough for providing fluid communication through the riser between the containment region and water surroundings.

The water surroundings may be the sea in which the collector may be submerged in use. The water may be the seawater in the sea. In use, the containment region may thus be in fluid communication with the sea outside of the collector through the riser.

The collector may have a containment body comprising: an outer wall structure for defining a capture area of the collector; an inner wall structure; a first containment region defined between an outer wall structure and the inner wall structure; and a second containment region between the inner wall structure and the riser, the second containment region in fluid communication with the first containment region across the inner wall structure, the end of the riser being arranged at a location in the second containment region for removing the gas from the collector.

By fluid communication between the first containment region and a second containment region through the inner wall structure, the level of gas/seawater in the first containment region during accumulation of the gas may correspond to the level of gas/seawater in the second containment region. In some embodiments, the outer wall structure may extend around and/or surround the inner wall structure. The inner wall structure may define a trajectory for fluid communication into the first end of the riser from the containment region.

The containment body may be configured to capture gas rising in use toward a ceiling of the collector through capture areas of both the first containment region and the second containment region, and the capture area of the second containment region may be spanned by a baffle plate through which gas and/or water may pass into the second containment region and travel toward the ceiling.

In some embodiments, the riser may comprise a pipe section that may extend from a ceiling into a containment body of the collector. The riser may comprise a pipe. The extent of the pipe or riser to the location in the containment region may be determined to allow the gas to collect and accumulate by a predefined amount before the level of gas reaches the location and/or the siphon operates to discharge the gas from the containment region.

Typically, the gas collector may further comprise at least one sensor for obtaining data associated with the gas. The sensor may typically be arranged to detect contents or at least one property of the contents in at least one containment region of the collector, wherein the contents may comprise either or both of collected underwater gas and water.

The sensor may be arranged between the inner wall structure and the riser. The riser may extend so that the first end of the riser is positioned at a location in the containment region, in use below, the sensor.

The sensor may be arranged to detect the contents or at least one property of the contents in the containment region, for example in the first or the second or a further containment region.

The gas collector may have: a containment body; and at least one containment region within the containment body for containing and collecting the underwater gas. The containment body may have a ceiling and one or more wall sections for trapping the gas in the at least one containment region of the collector.

The gas may collect and accumulate in a region above water in the containment region in use. More specifically, when submerged in the sea in use, the gas may accumulate above seawater in the containment region. The gas may form a body or layer of gas above the seawater in the containment region. As gas accumulates, a level of the gas or seawater in the containment region may typically progress to a lower level in the containment region.

By operation of the siphon, a siphon effect may be obtained that urges gas to be expelled from the collector, upon sufficient accumulation of the collected gas. The gas can be expelled automatically by way of the siphon, no priming or moving parts in the siphon being required.

In various embodiments, the first end of the riser may be arranged to communicate with the containment region of the collector. The riser may extend to a suitable location in the containment region. In use, the riser may be in fluid communication with the containment region. Moreover, the containment region may be in fluid communication with the sea through the riser. More specifically, the riser may be configured such that during the accumulation of the gas in use, seawater may be contained both in the containment region and in the riser, in communication through the riser with surrounding sea. Indeed, the riser may be configured to contain seawater in communication with the sea so that the seawater in the containment region and in the riser may provide a seawater lock during accumulation of the gas for hindering or preventing discharge of the gas. In use, advantageously, the gas may be expelled from the collector, and the siphon effect produced, upon the accumulated gas reaching the end of the riser in the containment region. Upon the accumulated gas reaching the first end of the riser in the containment region and gaining fluid communication with the first end of the riser in the containment region, the seawater lock in the containment region and the riser may be broken, to thereby permit the gas to be urged out of the containment region through the riser.

In use, the gas from the containment region may be communicated toward the first end of the riser in a first, e.g. downward direction, and then through the riser away from the first end in a second, e.g. upward direction in use. Thus, a U-shape path or communication of gas and/or seawater into the riser may be produced.

The second end of the riser may be arranged for discharging the gas from the riser into the surrounding water or sea.

According to a second aspect of the invention, there is provided apparatus comprising the gas collector in accordance with the first aspect of the invention, and at least one sensor for obtaining data associated with the collected gas.

The sensor may typically be arranged to detect contents or at least one property of the contents in at least one containment region of the collector, wherein the contents may comprise either or both of collected underwater gas and water. The property may comprise any one or more of: an amount or level or presence of collected gas or water in the containment region of the collector.

Typically, the sensor may be a capacitive sensor comprising a sensing head with a contact area arranged to contact the contents and which may be covered to increasing extent by collected gas upon accumulation of the gas in the containment region.

The sensor may comprise, or the apparatus may further comprise, at least one further sensor, which may be positioned outside the collector for detecting discharged gas from the collector or at least one property thereof upon resetting.

The sensor may be responsive to the contents of the containment region in contact with the sensor. The sensor may be operable to distinguish seawater from gas. The sensor may have a sensing area and may be responsive to the extent of the sensing area contacted by a particular content, such as gas or seawater. Thus, the sensor may be operable to produce data for determining or detecting the level of the gas/seawater in the containment region.

The sensor may in various embodiments be located outside the collector, and in such embodiments, the sensor may be configured to detect the discharge or removal of gas from the collector, and thereby the resetting of the collector. The sensor may be arranged to detect an effect in the water of the sea surrounding the collector from the discharge of the gas from second end of the riser. The effect may be a visual effect or a sonic effect, such as gas bubbles, turbulence, or the like.

The sensor may comprise a light sensor or an acoustic sensor, e.g. a camera sensor or microphone.

According to a third aspect of the invention, there is provided a method of detecting underwater gas or determining at least one property thereof, using the apparatus in accordance with the second aspect of the invention.

The method may typically comprise obtaining data from the sensor, using the data to determine at least one property associated with the underwater gas, wherein the property may be one or more: the presence of collected underwater gas; the rate of production of underwater gas; the amount of underwater gas.

The data may be acquired to provide one or more time series records, and the method further comprises using the time series records to determine the property associated with the underwater gas.

The method may include: using the sensor to detect an amount of gas or water contained in the containment region; producing an alarm or signal having a level corresponding to the detected amount of gas or water contained.

The method may include detecting reset events between collecting cycles from the data and using the detected reset events to determine the property of the gas.

According to a fourth aspect of the invention, there is provided a subsea structure or facility with the gas collector in accordance with the first aspect of the invention installed under water for collecting underwater gas. The subsea structure or facility may further comprise at least one sensor for obtaining data associated with the underwater gas that may be collected. The subsea structure or facility may be for example a subsea well template.

The underwater gas may be leaked underwater gas, e.g. leaked from an underwater natural source or an underwater facility or structure.

According to another aspect of the invention, there is provided an underwater gas collector for capturing and collecting underwater gas in one or more collecting cycles, the gas collector comprising a siphon which is operable, in response to sufficient accumulation of collected gas within the collector, to remove the collected gas of one collecting cycle from the collector and reset the collector to permit further gas to be collected in at least one further collecting cycle. The collector may have further features as described in relation to any of the other aspects of the invention.

Any of the aspects of the invention may have further features as described in relation to any other aspect of the invention wherever described herein.

Embodiments of the invention can be advantageous in various ways as will be apparent from throughout. The collector can reset automatically between cycles in response to the amount of accumulated gas. Merely by installing the collector in correct orientation under water, the apparatus is operational and resets by the siphon whenever the gas has collected in sufficient amounts. A riser such as fixed open pipe extending from the containment region through the ceiling to the sea above can be used. Operation of moving parts and related issues can be avoided.

There will now be described, by way of example only, embodiments of the invention, with reference to the accompanying drawings, in which: Figure 1 is a sectional representation of apparatus for collecting underwater gas, including an underwater gas collector and a sensor, during use in a start condition of a collection cycle, according to an embodiment of the invention;

Figure 2 is a sectional representation of the apparatus of Figure 1 during use in a further condition of the collection cycle, subsequent to the start condition and after a period of accumulation of collected gas;

Figure 3 is a sectional representation of the apparatus of Figure 1 and 2 in a further condition of the collection cycle, subsequent to the further condition of Figure 2 and after further accumulation of collected gas;

Figure 4 is a sectional representation of the apparatus of Figures 1 to 3 in a further condition of the collection cycle, subsequent to the further condition of Figure 3 at the moment before the accumulated gas is removed from the collector, upon further and sufficient accumulation of the collected gas;

Figure 5 is a sectional representation of the apparatus of Figures 1 to 4 during communication of the gas through a siphon of the apparatus whereby the accumulated gas is being removed and the collector reset back to the start condition for another collection cycle to be commenced;

Figure 6 is a sectional representation of apparatus for collecting underwater gas and obtaining gas data, according to an embodiment of the invention; and

Figure 7 is a perspective sectional representation of the apparatus of Figure 6 in larger scale.

Reference is made to Figures 1 to 5 depicting generally apparatus for collecting underwater gas and obtaining gas data, in the form of an underwater gas collector 1.

The underwater gas collector 1 is arranged for capturing and collecting underwater gas 7 in one or more collecting cycles. The operation of one such cycle is apparent from Figures 1 to 5. The gas collector 1 has a siphon 10 which is operable automatically, without moving parts, in response to sufficient accumulation of collected gas 7 within the collector 1, to remove the collected gas 7 from one collecting cycle of the collector 1 and reset the collector 1 to permit further gas 7 to be collected in at least one further collecting cycle of the collector 1.

Upon sufficient accumulation of the collected gas 7 in a containment region 15 of the gas collector 1, the gas 7 is automatically urged out of the containment region 15 of the collector 1 through the siphon 10 (see Figures 4 and 5). This results in resetting the gas collector 1, with seawater 8 replacing the removed gas 7 in the containment region 15, allowing a new gas collection cycle to commence (see Figure 1). Once sufficient gas has accumulated in the new cycle (see Figures 2 and 3), the siphon 10 again operates to remove the gas (see Figures 4 and 5).

The gas collector 1 in this example has a sensor 20. The sensor 20 is arranged to obtain data associated with the collected gas 7. In this example, the sensor 20 is configured for detecting a level of gas/water within the containment region 15. The sensor 20 is a capacitive sensor that has a sensor head 20h that can be used to detect the medium in contact with the head so that it differentiates between gas and seawater in the containment region 15. If the sensor head 20h is partially covered by gas/water, see Figure 2 where the hatched area of the sensor head 20h is covered by seawater 8, then a signal is obtained from the sensor 20 that is different compared to when the sensor head 20h is fully covered by seawater (see Figure 1) or fully covered by gas (see Figure 3). The signal obtained is then indicative of the accumulation and/or rate of accumulation of the gas collection.

Resetting the gas collector 1 upon removal of the gas 7 restores a level of the seawater 8 in the containment region 15 of the gas collector. The level of the seawater/gas can be detected by the sensor 20. Data from the sensor 20 can then indicate when the level has been restored, and thus that the collector 1 has been reset.

With respect to a given collection cycle, a change in signal obtained from the sensor 20 following start of the cycle can indicate that gas 7 is being collected and is accumulating in the containment region 15 of the collector. Thus, the presence of a leak can be determined from the signal from the sensor 20. The rate of accumulation / collection of the gas 7 can be detected.

Over multiple collection cycles, several resets / level restores are recorded and detectable in the data. Using recorded times of the resets from the sensor data, the rate of gas accumulation and changes in the rate can be determined.

In more detail now, the gas collector 1 has a containment body 45 comprising a ceiling section 21 and sidewall sections 22a, 22b to capture and contain the gas 7 laterally within the containment region 15 of the collector. The gas 7 is collected and contained in the containment region 15 of the containment body 45. The gas 7 is contained by and within the sidewall sections 22a, 22b laterally. In this example, upper sidewall sections 22a define an upward taper toward the ceiling 21|, and lower sidewall sections 22b extend in parallel toward the upper sidewall sections 22a. The lower sidewall sections 22a define a capture area 25 through which gas 7 to be collected is received by the collector 45. The capture area 25 in this example is wider than the ceiling. The wider capture area may allow gas to be captured from a wide region below and trapped against the ceiling in laterally narrower or more confined part of the containment region, which can enhance the vertical variation and effect upon the level of seawater/gas upon accumulation of the gas in the containment region. Thus, the level of the seawater/gas in the collector may be sensitive to even small leak amounts or small changes in leak rates.

In use, the gas 7 travels / migrates upward through seawater 8 in the containment region 15 and is trapped against the ceiling section 21. The gas 7 collects and accumulates in an upper region of the containment region 15. In particular, the gas 7 accumulates in a layer above the seawater 8 in the containment region 15. As further gas 7 enters the collector 1 through the capture area 25 and accumulates, seawater 8 is displaced. The level of seawater 8 in the containment region 15 is therefore lowered in accordance with the amount of gas 7 that has accumulated.

In this example, the containment body 45 has an outer wall structure that includes the ceiling section 21, and the containing wall sections 22a, 22b, and a an inner wall structure 14 within the outer wall structure. The inner wall structure 14 in this example divides the containment region 15 into a first, outer containment region 15a and a second, inner containment region 15b. The siphon 10 comprises a riser pipe 12 having a lower end 12a positioned within the inner wall structure 14 and at a suitable depth within the second, inner containment region 15b. The inner containment region 15b is defined around and between an outer surface of the riser pipe 12 and an inner surface of the inner wall structure 14. The riser pipe 12 and wall sections of the inner wall structure 14 extend vertically from the ceiling 21. The inner wall structure 14 is in the form of a bell providing a chamber around the riser pipe and pipe end 12 penetrating into an inside of the containment body 45. The inner wall structure 14 defines an enclosure around the riser pipe 12 and provides lateral containment for the contents of seawater/gas in the second, inner containment region 15b.

Upper sections of the inner wall structure 14 have one or more lateral openings 17 through the wall structure 14 for providing fluid communication laterally between the first, outer containment region 15a and the second, inner containment region 15b. The inner and outer containment regions 15a, 15b are therefore connected and arranged in fluid communication. By the principle of fluid equilibrium between connecting vessels, the inner and outer containment regions 15a, 15b allow communication of seawater and/or gas through the openings 17 during the collection and/or removal of gas 7. Thus, the gas 7 is collected and accumulates in the upper part of both the inner and outer containment regions 15a, 15b above seawater. Correspondingly, the level of seawater/accumulated gas in the containment regions 15a, 15b correspond and is substantially the same during the accumulation of the gas, as can be appreciated from the Figures 1 to 5.

The inner wall structure 14 has an end section 11 extending laterally across the lower end 12a of the riser pipe 12 and spaced by distance of separation 26 from the opening of the lower end 12a of the riser pipe 12. The inner wall structure 14 defines a flow region 13 between lower end 12a of the riser pipe 12 and the end section 11 of the inner wall structure 14. The wall structure 14 around the lower end 12a of the riser pipe 12 serves thus to create a U-trajectory for fluid being communicated into or out of the end 12a of the riser pipe 12 through the second, inner containment region 15b.

The siphon pipe 12 is in open fluid communication through the containment body 45 to the sea 2 surrounding the collector 1. In the initial/reset configuration of Figure 1 therefore, where the collector 1 is ready to collect gas 7, the siphon pipe 12 is open and contains seawater 8 in communication through the siphon pipe 12 with the first, inner and second, outer containment regions 15a, 15b of the collector 1.

During accumulation of the gas 7 in Figures 2 and 3, the siphon pipe 12 remains in the same configuration in communication with the seawater 8 of surrounding sea 2. The siphon 10 is arranged so that the seawater 8 in the siphon pipe 12 and in the lower part of the inner containment region 15b forms a water lock that hinders accumulating gas from escaping through the riser pipe 12 to the surrounding sea 2.

The water lock is maintained, and gas 7 continues to accumulate in the containment region 15, lowering the level of the gas/seawater along the riser pipe 12, until upon sufficient accumulation of the gas 7, the level reaches the lower end 12a of the riser pipe 12, as seen in Figure 4.

The gas 7 is communicated through the end region 13 and enters the lower end 12a of the riser pipe 12. The water lock is then broken, and the gas 7 that has accumulated is communicated upward through the riser pipe 12 to surrounding sea, as seen in Figure 5. A siphon effect is produced whereby the gas 7 is urged out of the containment region 15 and out of the collector 15.

As gas 7 is extracted through the siphon 10, the seawater level in the collector is gradually restored. The seawater 8 in the first, outer containment region 15a rises. The level of the seawater 8 rises upward along the outside of the inner wall structure 14 of the siphon toward the upper end of the inner wall structure 14. At the upper end the seawater 8 from the outer wall structure 14 enters through the openings 17 into the second, inner containment region 15b, further assisting to expel the gas 7. The seawater 8 is communicated through the second, inner containment region 15b and the riser pipe 12, re-establishing the initial state of the collector 1 as shown in Figure 1.

Thus, the gas 7 is urged out and emptied from the collector 1 automatically when sufficient gas 7 has been collected and accumulated in the containment region 15. No moving mechanical parts or valves are required to control the gas emptying operation.

The sensor 20 in the present example is arranged on an inside of the inner wall structure 14. In this way, the sensor 20 can obtain data by detecting the gas/seawater in the second, inner containment region 15b. Due to the presence of the inner structure 14 with communication between the first and second containment regions 15a, 15b possible through the openings 17 at an upper end, or optionally also through a baffle at a lower end as described further below, conditions in the inner containment region 15b may be more sheltered and calmer than in the outer containment region 15a, and thus the interface between gas 7 and seawater 8 in the inner containment region 15b more stable. The signal obtained from the sensor 20 may therefore be more accurate than if it were to be positioned to obtain a signal from the first, outer containment region 15a.

In other examples however, a sensor could be placed in the outer containment region 15a, instead of or in addition to the sensor 20 placed in the inner containment region 15b. Alternatively or in addition, a sensor could be placed on an outside of the containment body 45, e.g. to detect venting events where gas being emptied from the collector is discharged into the sea from an upper end 12b of the riser pipe 12. Further sensors can allow further data to be obtained which can enhance the data quality and accuracy. Further sensors 20 can be useful therefore when accurate determination or quantification of underwater gas leaks is required.

The collector 1 in this example is supported on a frame 31 on the seabed 3. The frame 31 can for example be a frame of a seabed facility such as a well template or the like. The collector 1 is in fixed position on the frame 31 spanning an area 25 over the seabed 3. The lower wall sections 22b of the containment body 45 are connected to the frame 31.

The containment body 45 is arranged to span an area of greater width extent than the inner wall structure 14, and therefore can facilitate capture of leaked gas from a region being monitored. The side wall sections 221, 22b can be adapted to span the area 25 required, and as described can funnel gas 7 toward an upper part of the containment body 45 and against the ceiling 21 of the collector 1 where it can be trapped in greater proximity to the siphon 10.

In other examples however, for example where the required capture span of the collector 1 is less, the siphon 10 omits the inner wall structure 14 either partially or entirely. The tapered wall sections 22a may then not be needed and omitted in favour of simply straight vertical wall sections for example. The siphon 10 then can comprise merely the riser pipe 12 extending to suitable depth within the containment region 15.

Turning to Figures 6 and 7, an example of another underwater gas collector 101 is depicted. The reference numerals in Figures 6 and 7 refer to corresponding or like features with the same numerals as used in Figures 1 to 5 but incremented by one hundred. The collector 101 has a siphon 110 formed by the riser pipe 112 within the inner wall structure 114 of the containment body 145 of the collector 101. As seen in Figures 6 and 7, the end section 111 of the inner wall structure 114 is in the form of a baffle plate 153. The baffle plate 153 has holes providing fluid communication through the baffle plate 153 into the inner containment region 115b. Upon collection of the gas 7 therefore, the gas 7 also rises toward the ceiling 121 through the inner containment region 115b. Some gas 7 may travel through the containment body 145 through the riser pipe 112, although most will accumulate in the trap formed by the containing outer wall sections 122a and ceiling 121 of the containment body 145, so that a layer of gas 7 will form and displace the seawater level within the containment body 145 as described above, until the gas 7 upon sufficient accumulation is discharged through the riser pipe 112. By allowing fluid through the baffle plate 153, the fluid conditions in the inner containment region 115b may be stabilised and calmer, for facilitating siphon 110 performance and data acquisition using the sensor 120.

In another example, a baffle plate may be provided across the capture span of the collector such that all gas 7 that is collected passes through the baffle.

While the siphon 10, 110 in various examples described above has a riser pipe 12, 112 extending from the ceiling 21, 121 as a column within the containment body 45, 145, the siphon 10, 110 can in other variants be configured differently. In one variant for instance, a riser may extend along one side of the providing a passage having a lower end arranged at depth within the containment region and an upper end in communication with surrounding sea. In such a variant, a siphon wall structure may be provided laterally inside of the opening in the containment body to divide the containment region into two and facilitate production of the siphon effect.

Clearly a great range of other automatic siphon variants are possible. The riser may be provided non-centrally and may be non-vertical.

Claims (21)

1. An underwater gas collector for capturing and collecting underwater gas in one or more collecting cycles, the gas collector comprising a siphon which is operable, without moving parts, in response to sufficient accumulation of collected gas within the collector, to remove the collected gas of one collecting cycle from the collector and reset the collector to permit further gas to be collected in at least one further collecting cycle.

2. An underwater gas collector as claimed in claim 1, wherein the siphon comprises a riser for removing the gas through the riser upon the accumulated gas, having sufficiently accumulated, reaching an end of the riser in a containment region of the collector, or reaching the level of the end of the riser in the containment region of the collector.

3. An underwater gas collector as claimed in claim 2, wherein the riser has first and second ends, and the riser extends from the first end at a location in the containment region to the second end located at a receiving location outside the collector.

4. An underwater gas collector as claimed in claim 3, wherein the riser has an open fluid passageway therethrough for providing fluid communication through the riser between the containment region and water surroundings.

5. An underwater gas collector as claimed in any of claims 2 to 4, wherein the collector has a containment body comprising:

an outer wall structure for defining a capture area of the collector; an inner wall structure;

a first containment region defined between an outer wall structure and the inner wall structure; and

a second containment region between the inner wall structure and the riser, the second containment region in fluid communication with the first containment region across the inner wall structure, the end of the riser being arranged at a location in the second containment region for removing the gas from the collector.

6. An underwater gas collector as claimed in claim 5 wherein the containing body is configured to capture gas rising in use toward a ceiling of the collector through capture areas of both the first containment region and the second containment region, and the capture area of the second containment region is spanned by a baffle plate through which gas and/or water may pass into the second containment region and travel toward the ceiling.

7. An underwater gas collector as claimed in any of claims 2 to 5, wherein the riser comprises a pipe section that extends from a ceiling into a containment body of the collector.

8. An underwater gas collector as claimed in any preceding claim, further comprising at least one sensor for obtaining data associated with the gas.

9. An underwater gas collector as claimed in claim 8, wherein the sensor is arranged to detect contents or at least one property of the contents in at least one containment region of the collector, wherein the contents comprises either or both of collected underwater gas and water.

10. Apparatus comprising the gas collector as claimed in any preceding claim and at least one sensor for obtaining data associated with the collected gas.

11. Apparatus as claimed in claim 10, wherein the sensor is arranged to detect contents or at least one property of the contents in at least one containment region of the collector, wherein the contents comprises either or both of collected underwater gas and water.

12. Apparatus as claimed in claim 11, wherein the property comprises any one or more of: an amount or level or presence of collected gas or water in the containment region of the collector.

13. Apparatus as claimed in any of claims 11 or 12, wherein the sensor is a capacitive sensor comprising a sensing head with a contact area arranged to contact the contents and which is arranged to be covered to increasing extent by collected gas upon accumulation of the gas in the containment region.

14. Apparatus as claimed in any of claims 10 to 13, wherein the sensor comprises, or the apparatus further comprises, at least one sensor which is positioned outside the collector for detecting discharged gas from the collector or at least one property thereof upon resetting.

15. A method of detecting underwater gas or determining at least one property thereof, using the apparatus of any of claims 10 to 14.

16. A method as claimed in claim 15, which comprises:

obtaining data from the sensor, using the data to determine at least one property associated with the underwater gas, wherein the property is one or more: the presence of collected underwater gas; the rate of production of underwater gas; the amount of underwater gas.

17. A method as claimed in claim 16, wherein the data are acquired to provide one or more time series records, and the method further comprises using the time series records to determine the property associated with the underwater gas.

18. A method as claimed in any of claims 14 to 17, which further comprises:

using the sensor to detect an amount of gas or water contained in the containment region;

producing an alarm or signal having a level corresponding to the detected amount of gas or water contained.

19. A method as claimed in any of claims 16 to 18, which includes detecting reset events between collecting cycles from the data and using the detected reset events to determine the property of the gas.

20. A subsea structure or facility with the gas collector as claimed in any of claims 1 to 9 installed under water for collecting underwater gas.

21. A subsea structure or facility as claimed in claim 20 being a subsea well template.