Noise abatement underwater using helmholtz resonators

Abstract

The method of underwater noise abatement comprises introducing air into inverted cups 10 that serve as Helmholtz resonators. Each cup contains a chamber 18 that has an upwardly closed volume disposed

Description

This invention relates to abatement or mitigation of noise emissions in bodies of water, for example when performing construction or installation operations offshore. The invention relates particularly to controlling the volume of air in Helmholtz resonators used in noise mitigation systems underwater. Underwater construction or installation operations are a significant source of noise pollution in the marine environment, which can cause injury to marine organisms or adversely affect their behaviour. A common example of such operations is installation of subsea foundations by impact-driving piles into the seabed. Such foundations include monopiles for bottom-fixed offshore wind turbines. As offshore wind turbines are becoming larger so too are their foundations. Consequently, percussive noise levels emitted during their installation are also increasing, to the extent that it has become difficult for installation contractors to meet noise regulations in key markets. Broadly, three types of noise abatement systems have been used, or proposed for use, in conjunction with impact-driven piles, namely: bubble curtain systems; shell-in-shell systems; and resonator systems. Bubble curtain systems surround a pile with a curtain of air bubbles that rise through the water column to define an acoustic barrier between the pile and the surrounding environment. The bubbles rise from an air distribution ring or manifold that is positioned close to the seabed, spaced radially from the pile wall. Shell-in-shell systems deploy a tubular barrier wall around a pile, close to the pile wall, to serve as an acoustic barrier. The barrier wall may conveniently be deployed and suspended from an annular pile gripper structure of an installation vessel. Resonator systems suspend an array of gas-filled tunable sound-damping resonators such as Helmholtz resonators in the water column, for example on a net or frame disposed around or beside a source of noise such as an impact-driven pile. An example of a resonator system is disclosed in WO 2015 / 095192, in which inverted cups serving as open-bottomed Helmholtz resonator cavities trap pockets of air above a body of water to absorb underwater noise. Air is added to the cups after the system has been submerged, that air being supplied by an external mechanism, such as a bubble hose disposed beneath the cups, or by an internal manifold system. Helmholtz-type noise mitigation systems dampen sound pressure in water. Their functionality is based on the resonance frequency of a mass-spring effect between the water and the air trapped in the cups. Consequently, their function depends upon the air-to-water fill ratio within the cups. In practical applications of a resonator system like that of WO 2015 / 095192, multiple Helmholtz resonator cups are grouped in panels. Rings or circumferential rows or layers of such panels can be positioned around a monopile at different depths, potentially from the seabed to above the water line, with a typical distance between the layers of about 0.7m to 1m. Other examples of resonator systems are disclosed in US 11993907 and US 11812221. The latter discloses sound-absorbing panels in which arrays of resonator cavities open downwardly to trap pockets of air within. In US 9410403, stackable inverted open-ended resonators absorb underwater noise. US 5457291 discloses a sound-attenuating panel produced by moulding concrete around pre-formed Helmholtz resonators. Each resonator comprises a hollow cavity that communicates with the exterior of the panel through a port that opens to a sound-receiving front face of the panel. Some noise abatement solutions involve a combination of systems. For example, there could be benefit in combining a bubble curtain system with a resonator system. However, this presents a challenge, namely that bubbles of air rising from a bubble curtain system can enter downwardly-opening resonator cups and over-fill the cups with air. Over-filling with air is also a challenge in systems such as WO 2015 / 095192, in which air is added to the cups after the system has been submerged. It is problematic to determine and to control the volume of air that is introduced into each and every cup. To function as Helmholtz resonators, downwardly-opening cups must contain both a volume of air serving as a spring element and a volume of water serving as a mass element. Consequently, their effectiveness as resonators reduces significantly if the cups are over-filled with air, especially if the air displaces all or most of the water from within the cups. Additionally, it has been theorised that the fill ratio of an air-water Helmholtz resonator influences the acoustic frequencies that the resonator will dampen most effectively. In accordance with that theory, the fill ratio within the cups is important for achieving appropriate tuning and even small deviations from the optimal fill ratio will reduce noise mitigation performance during impact pile driving. Against this background, the invention resides in a resonator that comprises: an inverted cup containing a chamber that has an upwardly closed volume disposed between a closed top of the cup and an intermediate level between the closed top and an open bottom of the cup; and at least one vent that opens into the chamber at the intermediate level, the or each vent being in fluid communication with an exterior of the cup. In use, the cup can contain water below the intermediate level and air above the intermediate level. The or each vent may be defined by a channel that extends upwardly from the intermediate level within the upwardly closed volume. For example, the channel may be a tube that extends to the top of the cup and is in fluid communication with the exterior of the cup through the top of the cup. The channel can be disposed centrally within the upwardly closed volume or can be at a side wall of the cup, in that case being at least partially bounded by the side wall and possibly being in fluid communication with the exterior of the cup through the side wall of the cup. The or each vent may instead be defined by at least one opening in a side wail of the cup at the intermediate level. The or each opening can extend from the bottom of the cup to the intermediate level or can be a hole that penetrates the side wall of the cup at the intermediate ievei. A resonator of the invention can be in mutual fluid communication with another, similar resonator that may or may not include a vent. The resonators can be linked for fluid communication beneath the intermediate level and can have discrete upwardly closed volumes above the intermediate level, those volumes being isolated from each other. The inventive concept embraces a panel that comprises an array of resonators of the invention, and an underwater noise-abatement system that comprises resonators of the invention and can be combined with a bubble air supply disposed beneath the resonators. The inventive concept also embraces a corresponding method of underwater noise abatement, the method comprising: introducing air into inverted resonator cups, each cup containing a chamber that has an upwardly closed volume disposed between a closed top of the cup and an intermediate level between the closed top and an open bottom of the cup; filling the upwardly closed volume with the introduced air; and allowing excess air to escape from the cup though at least one vent that opens into the chamber at the intermediate level. The air can be introduced through the open bottom of the cup. The invention provides a simple solution that mitigates overfilling of resonator cups with air and could be relevant to any system that uses resonator-type technology for noise mitigation underwater. By allowing excess air to escape resonator cups as air is added to the cups, the cups can operate closer to their optimum fill ratio. This can be achieved by, for example, providing a small hole in the side of a cup, a small slot in the side of a cup or a channel connecting the interior of a cup to the exterior of the cup. Allowing excess air to escape a cup through such an outlet opening, serving as an air vent, allows a suitable volume of water to remain in or to return to the cup, hence allowing the cup to function correctly as a Helmholtz resonator. The addition of an air vent outlet ensures that the fraction of the air content in the cup remains at or close to the optimum. This makes it possible to tune the resonator frequency to the desired natural frequency and to maintain that tuning. Without this outlet, the cups would simply fill with air from the bubble hose until the cups are full and overflowing with air. At such an extreme fill ratio, the cups would no longer contain water to serve as the mass element required to maintain Helmholtz resonator characteristics. When noise is generated during pile driving, pressure waves push water upwards into the cups. This pushes water within the cups upwardly, causing the air within the cups to compress and to act as a spring. If water in a cup can pass through the air outlet, it could disturb the build-up of air pressure and so could disturb the resonator characteristics. During pile driving, pressure waves approach the resonator cups as longitudinal waves at an angle of about fifteen degrees to the vertical. If the pressure at the top or exit of the air outlet is equal to or similar to the pressure at the bottom or inlet of the air outlet, water displacement through the air outlet could be minimal. Choosing the correct location for the air outlet could minimise this disturbance. Consequently, various options such as the embodiments shown in the accompanying drawings may be considered. Additionally, the air outlet should be as small as possible, with large wall surfaces. The best design may therefore involve a flattened tube in preference to a cylindrical tube. The greater the wall surface area, the greater the visco-thermal effects that would increase damping of any water flow through the air outlet. In summary, the invention involves introducing air into inverted cups that serve as Helmholtz resonators. Each cup contains a chamber that has an upwardly closed volume extending between a closed top of the cup and an intermediate level between the closed top and an open bottom of the cup. After filling the upwardly closed volume with the introduced air, excess air is allowed to escape from the cup though one or more vents that open into the chamber at the intermediate level. The chamber thereby contains water below the intermediate level and air above the intermediate level, with a consistent fill ratio between the air and the water being maintained even if air continues to enter the chamber through the open bottom of the cup. In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic perspective view of a first embodiment of the invention in which a vent tube is disposed centrally in a resonator cup; Figure 2 is a horizontal sectional view on plane ll-ll of Figure 1; Figure 3 is a schematic perspective view of a second embodiment of the invention in which a vent tube is incorporated in a side wall of a resonator cup; Figure 4 is a horizontal sectional view on plane IV-IV of Figure 3; Figure 5 is a schematic perspective view of a third embodiment of the invention in which a vent slit is disposed in a side wall of a resonator cup; Figure 6 is a horizontal sectional view on plane VI-VI of Figure 5; Figure 7 is a schematic perspective view of a fourth embodiment of the invention in which vent holes are disposed in a side wall of a resonator cup; Figure 8 is a horizontal sectional view on plane VIll-VIII of Figure 7; Figure 9 is a schematic perspective view of a fifth embodiment of the invention in which resonator cups are linked in mutual fluid communication and a vent slit is disposed in a side wall of one of the resonator cups; and Figure 10 is a horizontal sectional view on plane X-X of Figure 10. The drawings show embodiments of the invention in which Helmholtz resonators are defined by inverted cups 10. In use, the cups 10 are submerged in a body of water beside or around a source of noise, such as a pile being impact-driven into the seabed. Arrays of such cups 10 can be used together in an underwater noise-abatement system. Conveniently, the cups 10 can be defined by cavities formed in a body of a moulded material such as concrete or a polymer resin. Each cup 10 has a closed top 12 and an open bottom 14 and comprises a side wall 16 that surrounds an internal chamber 18. The chamber 18 has an upwardly-closed, air filled upper volume 20 that extends between the top 12 of the cup 10 and an intermediate level 22 located between the top 12 and the bottom 14 of the cup 10. The chamber 18 also contains a mass of water 24 in a lower volume between the intermediate level 22 and the bottom 14 of the cup 10. Thus, the chamber 18 is filled with air in the volume 20 above the intermediate level 22 and is filled with water 24 below the intermediate level 22. At least one vent opens into the chamber 18 of each cup 10 at the intermediate level 22 and is in fluid communication with the exterior of the cup 10 at or above the intermediate level 22. Consequently, when air is introduced into the cup 10 through the open bottom 14 or otherwise, the air gathers in the upwardly-closed volume 20 and displaces the water 24 downwardly until the water 24 reaches the intermediate level 22. Any excess air is then expelled through the vent, hence keeping the water 24 at the intermediate level 22 to maintain a desired air-to-waterfill ratio within the chamber 18. The desired fill ratio ensures that the cup 10 behaves as a Helmholtz resonator that is effective to absorb acoustic energy at a targeted frequency. In the first and second embodiments shown in Figures 1 to 4, the vent is a tube 26 or channel 28 that extends upwardly from the intermediate level 22 within the upwardly-closed, air-filled volume 20. Specifically, in the first embodiment shown in Figures 1 and 2, the vent is a central tube 26 that extends from the intermediate level 22 to the top 12 of the cup 10. The tube 26 is in fluid communication with the surrounding water 24 through an opening 30 that penetrates the otherwise closed top 12 of the cup 10. Conversely, in the second embodiment shown in Figures 3 and 4, the vent is a channel 28 at the side wall 16 of the cup 10, partially bounded by the side wall 16 in this example. Here, again, the channel 28 extends from the intermediate level 22 to the top 12 of the cup 10 and is in fluid communication with the surrounding water through an opening 30 in the top 12 of the cup 10. In other examples, the channel 28 could be in fluid communication with the surrounding water through an opening in the side wall 16 of the cup 10. In the third and fourth embodiments shown in Figures 5 to 8, the vent is defined by at least one opening in the side wall 16 at the intermediate level 22. Specifically, in the third embodiment shown in Figures 5 and 6, the vent is a slit 32 through the side wall 16 that extends upwardly from the bottom 14 of the cup 10 to the intermediate level 22. Conversely, in the fourth embodiment shown in Figures 7 and 8, multiple vents are defined by an array of circumferentially-spaced holes 34 that penetrate the side wall 16 at the intermediate level 22. Finally, the fifth embodiment shown in Figures 9 and 10 exemplifies how two or more cups 10 can be linked orsiamesed in fluid communication. Specifically, the cups 10 are linked for fluid communication of water 24 beneath the intermediate level 22. The air-filled upwardly closed volumes 22 above the intermediate level 22 remain discrete and isolated from each other. In this example, only one of the cups 10 is provided with a vent, here in the form of a slit 32 through the side wall 16 extending from the bottom 14 of the cup 10 to the intermediate level 22 as in Figures 5 and 6. However, in other examples, both cups 10 could have vents and either cup 10 could instead have vents like those shown in Figures 1 to 4 or in Figures 7 and 8.

Claims

1. A resonator, comprising:an inverted cup containing a chamber that has an upwardly dosed volume disposed between a closed top of the cup and an intermediate level between the dosed top and an open bottom of the cup; andat least one vent that opens into the chamber at the intermediate level, the or each vent being in fluid communication with an exterior of the cup.

2. The resonator of Claim 1, wherein the or each vent is defined by a channel extending upwardly from the intermediate level within the upwardly closed volume.

3. The resonator of Claim 2, wherein the channel is a tube that extends to the top of the cup and is in fluid communication with the exterior of the cup through the top of the cup.

4. The resonator of Claim 2 or Claim 3, wherein the channel is disposed centrally within the upwardly closed volume.

5. The resonator of Claim 2 or Claim 3, wherein the channel is at a side wall of the cup.

6. The resonator of Claim 4, wherein the channel is at least partially bounded by the side wall.

7. The resonator of Claim 5 or Claim 6, wherein the channel is in fluid communication with the exterior of the cup through the side wall of the cup.

8. The resonator of Claim 1, wheren the or each vent is defined by at least one opening in a side wall of the cup at the intermediate level.

9. The resonator of Claim 8, wherein the or each opening extends from the bottom of the cup to the intermediate level.

10. The resonator of Claim 8, wherein the or each vent is a hole that penetrates the side wall of the cup at the intermediate level.

11. The resonator of any preceding claim, containing water in the cup below the intermediate level and air in the cup above the intermediate level.

12. A resonator of any preceding claim being in mutual fluid communication with a like resonator that omits the or each vent.

13. Two or more resonators of any of Claims 1 to 11, being in mutual fluid communication.

14. The resonators of Claim 12 or Claim 13, being linked for fluid communication beneath the intermediate level.

15. The resonators of Claim 14, having discrete upwardly closed volumes above the intermediate level, those volumes being isolated from each other.

16. A panel comprising an array of resonators of any preceding claim.

17. An underwater noise-abatement system comprising resonators of any preceding claim.

18. The system of Claim 17, in combination with a bubble air supply disposed beneath the resonators.

19. A method of underwater noise abatement, comprising:introducing air into inverted resonator cups, each cup containing a chamber that has an upwardly closed volume disposed between a closed top of the cup and an intermediate level between the closed top and an open bottom of the cup;filling the upwardly closed volume with the introduced air; andallowing excess air to escape from the cup though at least one vent that opens into the chamber at the intermediate level.

20. The method of Claim 19, comprising introducing the air through the open bottom of the cup.12

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