Abstract:
A novel underwater noise abatement and deployment system are described. The system uses inverted open-ended resonators (e.g., Helmholtz resonators) to absorb underwater noise. The system includes stackable resonator cavity embodiments arranged to surround a noisy environment or act nearby it. The system can be deployed from a ship or barge or similar structure, and can be stored when not in use.

Description:
RELATED APPLICATIONS 
       [0001]    The present application derives from and claims priority to U.S. Provisional Application No. 61/917,343, filed on Dec. 17, 2013, bearing the present title. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to reduction of noise in noisy underwater environments including sea-faring vessels, oil rigs and other industrial and military applications. 
       BACKGROUND 
       [0003]    Some human activities cause underwater noise that is transmitted from the source of the underwater noise to the surrounding environment, sometimes many miles away. The underwater noise generated by oil and gas drilling platforms, ships and other human activities and machinery is generally considered undesirable. Some studies conclude that underwater noise pollution can adversely affect marine life, and it may be disruptive to other human activities such as scientific, meteorological and military activities. This is especially true for noise generating activities that result in large amplitude acoustic emissions (loud sounds) and transmissions at frequencies to which human and oceanic life is sensitive. 
         [0004]    Ships that operate in environmentally sensitive or highly regulated regions can be limited in the manner or time in which they can operate due to the noise generated by the ship. This occurs in the oil and gas field, where noise from mobile drilling ships limits drilling time due to the effect that the noise can have on migrating bowhead whales in Arctic regions. When bowhead whales are sighted, operations may be halted until they have safely passed, and this process can take many hours. 
         [0005]    As mentioned above, there is some concern over the effect that shipping and other man-made noise has on marine mammals. Some studies suggest that man-made noise can have a significant impact on the whale&#39;s stress hormone levels, which might affect their reproduction rates, etc. 
         [0006]    Known attempts to reduce noise emissions from surface ships include the use of a so-called Prairie Masker, which uses bands of hoses that produce small freely-rising bubbles to mitigate ship&#39;s noise. However, small freely-rising bubbles are usually too small to effectively attenuate low-frequency noise. In addition, Prairie Masker systems require continuous pumping of air through the system, a process itself that produces unwanted noise, and also consuming energy and requiring a complex gas circulation system that is costly and cumbersome to the other operations of the ship. Finally, such systems cannot operate below a given depth due to hydraulic forces and back pressures. 
         [0007]    One principle that is useful in approximating or understanding the acoustic effects of gas pockets in liquid (e.g., air pockets or bubbles or enclosures in water) is the behavior of spherical gas bubbles in liquid. The physics of gas bubbles is relatively well known and has been studied theoretically, experimentally and numerically. 
         [0008]      FIG. 1  illustrates a model of a gas (e.g., air) bubble  10  in a liquid  15  (e.g., water). One model for studying the response of gas bubbles is to model the bubble of radius “a” as a mass on a spring system. The mass is “m” and the spring is modeled as having a spring constant “k”. The radius of the bubble  10  will vary with pressures felt at its walls, causing the bubble  10  to change size as the gas therein is compressed and expands. In some scenarios the bubble  10  can oscillate or resonate at some resonance frequency, analogous to how the mass on spring system can resonate at a natural frequency determined by said mass, spring constant and bubble size. 
         [0009]    Continuing efforts to mitigate the effects of underwater noise continue. While some solutions can actually reduce the amount of noise generated by a source other solutions seek to reduce the effect of the noise by surrounding or partially surrounding the noise-making source with something that absorbs or otherwise attenuates the propagated noise. 
       SUMMARY 
       [0010]    The present disclosure is directed to reduction of the severity of noise emissions from the vicinity of a noise generating object or activity. The present concepts can be applied to man-made noise but also more generally to any noise generated from a source under water (e.g., in the seas, coastal areas, drilling fields, lake beds, and so on). 
         [0011]    Gas trapped in the pockets under or around an object in the water can act as free bubbles and/or Helmholtz-like resonators and thus work to abate noise in much the same way as a resonant bubble. To give an example of how this would work in on a ship, a panel with hemispherical, cylindrical, conical (or similar shape) cavities could be attached to its hull, and while submerged the pockets could be filled with gas via an external mechanism or an internal manifold system. The properties of these pockets would be chosen so that the gas trapped within each pocket resonates at or near the frequencies that we wish to attenuate (e.g., between about 30 Hz to about 200 Hz including about 110 Hz), thus maximizing their efficacy. For the example of pile driving, sheets or panels containing a plurality of these resonators can be deployed to fully surround the whetted portion of the pile. As in the previous example, the properties of the pockets would be chosen to maximize the efficacy of the system. 
         [0012]    The system is customizable and can attenuate noise to the amount desired (e.g., 10 dB or more). The system can also be produced to specifically target frequencies that are particularly loud. In other aspects, the present invention provides added thermoacoustic absorption of sound by selective application of a permeable mesh over an open end of the resonator. 
         [0013]    In an aspect, the system includes a resonator with articulated sidewalls that reduce a length of the resonator in a storage configuration. In another aspect, the system includes resonators that are stackable in a storage configuration to reduce space during transportation, storage, and stowage on board a pile-driving vessel, for example. In yet another aspect, the system includes a first resonator in fluid communication with a second resonator through a conduit. The first resonator can receive a gas through an inlet where the gas can fill the interior volume of the first resonator and the second resonator through the conduit. 
         [0014]    This system may allow the operator to work for longer periods of time and in areas previously unavailable due to noise regulations. This system is also much more effective at reducing noise than current technology because each gas cavity is built so that the gas trapped inside will maximally reduce the targeted underwater noise. In addition it does not require power or expensive support equipment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a fuller understanding of the nature and advantages of the present concepts, reference is made to the following detailed description of preferred embodiments and in connection with the accompanying drawings, in which: 
           [0016]      FIG. 1  illustrates a model of a gas bubble in a liquid according to the prior art; 
           [0017]      FIGS. 2A and 2B  illustrate cross sections of a collapsible resonator according to an embodiment; 
           [0018]      FIGS. 3A and 3B  illustrate cross sections of a collapsible resonator according to an embodiment; 
           [0019]      FIGS. 4A  and B illustrate a noise abatement system 
           [0020]      FIG. 5A  illustrates an exemplary resonator system in a deployed configuration; 
           [0021]      FIG. 5B  illustrates an exemplary resonator system in a stacked configuration 
           [0022]      FIG. 6  illustrates a panel of resonators according to an embodiment; 
           [0023]      FIGS. 7A-7C  illustrate mechanical details of a gas-filled resonator according to an embodiment, 
           [0024]      FIGS. 8A and 8B  illustrate a noise abatement apparatus arranged in stackable strips according to an embodiment; and 
           [0025]      FIG. 9  illustrates an exemplary deployment system for a water noise abatement system. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Gas trapped in the pockets under or around an object in the water can act as free bubbles and/or like Helmholtz (or similar) resonators (e.g., Minnaert resonators and/or Church resonators) and thus work to abate noise in much the same way as a resonant bubble. 
         [0027]    The height of the interior volume of the cavity and its volume are configurable to suit the purpose at hand. The hydrostatic pressure around the resonators varies with depth below the surface, the cavities&#39; size and/or shape can vary according to their location with respect to the water line on the face of the panel. Thus, the cavities may be designed to accommodate the change in water pressure felt at the neck of the cavities due to the depth to which they are submerged, as (in the analogy of  FIG. 1 ) their spring constants can change according to the density and depth of water around them. 
         [0028]    In some embodiments, a mesh or other solid screen such as a metal screen (e.g., copper screen) can be placed over the face of the panels. This can act to stabilize the air in the cavities. This can also act as a heat sink to dissipate thermal energy absorbed by the resonating volume of the cavity and potentially improve its performance. 
         [0029]    In some embodiments a hemispherical or spherical section or spheroidal section cavity is suitable for damping noise in a useful frequency range. 
         [0030]      FIGS. 2A and 2B  illustrate cross sections of an embodiment of a collapsible resonator  20 . The resonator  20  in  FIG. 2A  is shown in collapsed form as it would be stored and transported when not deployed in water  25 . The resonator  20  has a hollow body  200  including an optional circumferential portion  220  connected to segmented sidewalls  230 . The hollow body  200  has a closed end  240  and an open end  250 . The closed end  240  generally corresponds to the segmented sidewalls  230  and optional circumferential portion  220 . 
         [0031]    As illustrated, the segmented sidewalls  230  are folded (e.g., similar to an accordion) in a first direction  260  to reduce a length of the segmented sidewalls  230  in a second direction  270 . The second direction  270  is orthogonal to the first direction  260 . It is noted, however, that other relative orientations of the first direction  260  and second direction  270  fall within the scope of the invention and are a matter of design choice. The segmented sidewalls  230  include a first sidewall  232  and a second sidewall  234 . The first sidewall  232  is shorter than the second sidewall  234  to reduce the length of the segmented sidewalls  230  along the first direction  260 . The first direction  260  can be parallel to the first sidewall  232  when the resonator  20  is in the collapsed or storage configuration. The first sidewall  232  can have an equal or greater length than the second sidewall  234  in some embodiments. The segmented sidewalls  230  can be formed of a rigid material or can have a rigid frame (e.g., aluminum) with a flexible material (e.g., neoprene) on the walls defined by the frame. Alternatively, the segmented sidewalls  230  can be a flexible material. 
         [0032]    The resonator  20  in  FIG. 2B  is shown in expanded form as it would be when deployed in water  25 . As the resonator  20  is submerged in water  25 , the resonator  20  traps air or a buoyant fluid in an interior  290  of the hollow body  200 . In addition or in the alternative, a gas can be introduced into the hollow body  200  from a gas source (not shown), such as a gas tank. The buoyancy of the air (or buoyant fluid) in the interior  290  of the hollow body  200  creates a force on the segmented sidewalls  230  causing them to unfold in the second direction  270  thus increasing the length of the segmented sidewalls  230  in the second direction  270 . As the segmented sidewalls  230  increase in length in the second direction  270 , like a parachute, a volume of the hollow body  200  increases as well. The volume is filled with the air but at a reduced pressure due to the increased volume of the hollow body  200 . Alternatively, the volume is filled with a fluid having a higher buoyancy than the water  25 . 
         [0033]    As illustrated, the resonator  20  in  FIG. 2B  looks like an inverted cup with an interface  295  between the water  25  and the air (or buoyant fluid) in the cup. The interface  295  is near the open end  250  of the hollow body  200 . The resonator  20  can act like a Helmholtz resonator (or other resonator such as a Minnaert resonator and/or a Church resonator) and can have a resonance frequency as discussed above. The interior  290  of the resonator  20  can have a volume of approximately (i.e., within 10%) 2670 cubic centimeters. 
         [0034]      FIGS. 3A and 3B  illustrate another exemplary embodiment of the resonator of the present invention similar to the one described above with respect to  FIGS. 2A and 2B . However, a mesh  310  that is substantially permeable to fluid flow has been added to the open end  350  of the resonator  30 . The mesh  310  can be constructed of a screen having thermally conductive properties as mentioned above. 
         [0035]      FIGS. 4A  and B illustrate a noise abatement system  40  including a plurality of collapsible inverted cup-like resonator volumes  400 , each having a downward-facing open end  410 . Therefore, each of the resonators  400  can be designed as shown above with respect to  FIGS. 2 and 3 . When the system  40  is stored, transported or in the air above water (e.g., as illustrated in  FIG. 4A ) the resonators are in their collapsed state. Then, upon deployment in the water  25  (e.g., as illustrated in  FIG. 4B ) the plurality of resonators  400  expand to their operational size and shape as the resonators  400  fill with buoyant air. The plurality of resonators  400  can be formed on or in a panel  420  (e.g., as an array of resonators  400 ) in a way similar to a venetian blind, so as to simplify deployment. The resonators  400  can be formed of a rigid material or can have a rigid frame (e.g., aluminum) with a flexible material (e.g., neoprene) on the walls defined by the frame. Alternatively, the resonators  400  can be formed from a flexible material. 
         [0036]      FIG. 5A  illustrates an exemplary resonator system  50  in a deployed configuration. The resonator system  50  has a plurality of stacked or stackable resonator bodies  500 A,  500 B,  500 N (referred to in general as resonator body  500 ) in the form of a cone. It is noted that the resonator bodies  500 A,  500 B,  500 N can be other shapes (e.g., pyramid, semi-spherical, etc.) and that the cone shape illustrated in  FIGS. 5A and 5B  is merely illustrative. At least one coupling  510  connects adjacent resonator bodies (e.g.,  500 A and  500 B). The coupling  510  is articulated to flexibly connect one resonator body (e.g.,  500 A) to another (e.g.,  500 B). In some embodiments, the coupling  510  is flexible, telescoping, and/or segmented. Alternatively, the coupling  510  can be rigid. 
         [0037]    The resonator body  500  has an open end  520  and a closed end  530 . The resonator body  500  is hollow and is generally tapered from the open end  520  to the closed end  530 . The open end  520  has a first width (e.g., a diameter)  525  and the closed end  530  has a second width (e.g., a diameter)  535 . As the resonator body  500  is shaped like a cone, the first width  525  is greater than the second width  535 . In some embodiments, however, the first width  525  is less than the second width  535 . Thus, in general, the first width  525  is not equal to the second width  535 . The resonator body  500  can be formed of a rigid material or can have a rigid frame (e.g., aluminum) with a flexible material (e.g., neoprene) on the walls defined by the frame. Alternatively, the resonator body  500  can be formed from a flexible material. The resonator  500  can have an internal volume of about (i.e., within 10%) 220 cubic centimeters. 
         [0038]      FIG. 5B  illustrates the resonator system  50  in a stacked or collapsed configuration. In this configuration, the open end  520  of a first resonator body  500 A is stacked and/or nested on top of the closed end  530  of a second resonator body  500 B while the coupling  510  is in a folded/bent configuration. The first resonator body  500 A partially covers the second resonator body  500 B. This configuration is advantageous for storage as the resonator system  50  is more compact along a central axis  590  than the resonator system  50  in the deployed configuration ( FIG. 5A ). The central axis  590  passes through the open end  520  and the closed end  530  of the resonator body  500  and forms an angle  570  (i.e., other than 180 degrees) with a tapered sidewall  580  of the resonator body  500 . 
         [0039]    The first resonator  500 A and the second resonator  500 B have respective resonance frequencies, as discussed above. In some embodiments, the first resonator  500 A has a first resonance frequency that is different than a second resonance frequency of the second resonator  500 B. Alternatively, the first resonator  500 A and the second resonator  500 B can have the same or substantially the same (i.e., within 10%) resonance frequency. The resonance frequencies can be between about 30 Hz and about 200 Hz including about 110 Hz. 
         [0040]    In some embodiments, one or more conduits  540 A,  540 B,  540 N (referred to in general as conduit  540 ) are defined on or in the stackable resonator bodies  500 A,  500 B,  500 N, respectively. A lower open end  502  of the conduit  540  (e.g., a spill hole) is disposed at or near the open end  520  of the resonator body  500 . An upper open end  504  of the conduit  540  is disposed at or near the closed end  530  of the resonator body  500  and below the adjacent resonator  500 . In operation, gas (e.g., air) bubbles into the open end  520  of the hollow resonator body  500 N. The gas can be supplied from a gas source (e.g., a pressurized gas tank). The gas bubbles rise to the closed end  530  of the hollow resonator body  500 N and then fill the hollow resonator body  500 N from the closed end  530  to the open end  520  thereof. When the hollow resonator body  500 N is filled with gas, the gas is at or near the open end  520  of the hollow resonator body  500 N. The gas then flows into the conduit  540 N on the resonator body  500 N from the lower open end  502  to the upper open end  504  of the conduit  540 N. The gas then bubbles into the next resonator body  500 B immediately above resonator body  500 N. The same process can repeat until all resonator bodies  500  along a vertical axis are filled with gas. 
         [0041]      FIG. 6  illustrates a panel  60  of resonators  600  in an embodiment. The resonators  600  are configured in an array of X resonators  600  horizontally and Y resonators vertically (e.g., in a column). In some embodiments, the array includes an additional dimension of Z resonators  600  along a direction orthogonal to the horizontal and vertical directions. Each resonator  600  has a first end  610  and a second end  620  and has a hollow body as discussed above. The resonator  600  is generally in the shape of an inverted bulb (e.g., a light bulb) but it can be in any shape appropriate to catch and contain gas. The first end  610  can be open or partially open to the surrounding water  25  environment. The resonators  600  can be formed of a rigid material or can have a rigid frame (e.g., aluminum) with a flexible material (e.g., neoprene) on the walls defined by the frame. Alternatively, the resonators  600  can be formed from a flexible material. 
         [0042]    A conduit  630  connects adjacent resonators  600  (through respective first ends  610 ) along a vertical direction as illustrated in  FIG. 6 . Through the conduit  630 , a first resonator  600 A is in fluid communication with a second resonator  600 B where the second resonator  600 B is disposed below the first resonator  600 A. A gas can be introduced into the first end  610  of the first resonator  600 A through an inlet  640 . The inlet is connected to a manifold  650 , which in turn is connected to a gas source  660 . Alternatively, the inlet  640  is directly connected to the gas source  660 , which can be a source of compressed gas. 
         [0043]    The first resonator  600 A and the second resonator  600 B have respective resonance frequencies, as discussed above. In some embodiments, the first resonator  600 A has a first resonance frequency that is different than a second resonance frequency of the second resonator  600 B. Alternatively, the first resonator  600 A and the second resonator  600 B can have the same or substantially the same (i.e., within 10%) resonance frequency. The resonators  600  across the array can be the same, substantially the same, or different than each other. 
         [0044]    In operation, the gas (e.g., air) is pumped or otherwise introduced into the inlet  640  of the first resonator  600 A through the manifold  650 . The gas fills the hollow body of the first resonator  600 A and displaces the fluid (e.g., water) in the hollow body. The fluid flows through the conduit  630  to the second resonator  600 B. Alternatively, the fluid flows through a vent or valve in the first end  610  of the first resonator  600 A. After the gas creates a threshold pressure in the first resonator  600 A, the gas displaces the fluid in the conduit  630  and in the second resonator  600 B thus filling the second resonator  600 B with the gas. This process continues for the Y conduits  600  in the vertical direction (i.e., through resonators  600 C,  600 D, and  600 E). In this orientation, the gas will naturally flow vertically towards a surface  35  of the water  25  due to the buoyancy of the gas. The fluid in the resonators  600 A,  600 B, etc. displaced by the gas can be expunged into the water  25  through a valve or similar means. 
         [0045]      FIGS. 7A-7C  illustrate mechanical details of a gas-filled resonator  700  in a panel  710  adapted for supporting a plurality of resonators to abate underwater noise, for example as described with respect to  FIG. 6 .  FIG. 7A  shows a cutaway cross-section of the hollow body  770  of the resonator  700 . An inlet  740  and an outlet/conduit  730  are optionally connected to another such resonator (not shown).  FIG. 7B  illustrates a first perspective view of the resonator  700  in a support panel  780 , while  FIG. 7C  illustrates yet another perspective view of the same. 
         [0046]    In some embodiments, a wall  720  of the resonator  700  is soft and/or flexible while the panel  710  is rigid. The soft and/or flexible wall  720  permits the resonator  700  to be collapsible during storage. For example, the panel  710  (which can include an array of resonators  700 ) can be stored by stacking multiple panels  710  on top of each other or by rolling the panel  710  around a drum. In either case, the panel  710  can be stored more efficiently and/or compactly if the wall  720  of the resonator  700  is collapsible. 
         [0047]    This invention is not limited to use in surface or sub-surface ships and vessels, but may be used by oil and gas companies drilling in the ocean (e.g., on rigs and barges), offshore power generation activities (e.g., pile driving activities from the installation of wind farms), as well as in bridge and pier construction or any other manmade noise-producing structures. 
         [0048]    As far as applications of the current system, one can prepare panels similar to those described above for attachment to submerged structures or vessels. The panels can include a plurality of gas (e.g., air) cavities where the buoyancy of the air in the water environment causes the air to remain within the cavities. The cavities can be filled by the act of inverted submersion (i.e., the open side of the resonator is oriented down towards the ocean floor) of the panels or structure. Alternatively, the cavities can be actively filled using an air source disposed beneath the cavities so that the air from the source can rise up into and then remain in the cavities. The cavities may need to be replenished with gas from time to time. 
         [0049]    In some embodiments, gas other than air may be used to fill the cavities. The temperature of the gas in the cavities may also affect their performance and resonance frequencies, and so this can also be modified in some embodiments. 
         [0050]      FIGS. 8A and 8B  illustrate exemplary side view and top view sections, respectively, of a noise abatement apparatus  80  arranged in stackable strips that can be deployed from a sea-faring platform by a deployment system. The noise abatement apparatus  80  comprises conical resonators  800  that are coupled to one another in a stackable fashion by a gas line  810 . Each resonator  800  has a flexible resonator and stainless steel expansion ring  820 . The stack can also be equipped with air, power, communication and other fluid and electrical signaling lines  840 . A smooth outer sheath  850  houses a stack of resonators. Stiffeners  830  (e.g., fire hose like tubes or inflatable structures) can provide mechanical rigidity to the system. Lift cables  860  can be included as shown to provide counter weighting if necessary. 
         [0051]      FIG. 9  illustrates an exemplary deployment system  90  for the water noise abatement system  900 . The system  90  can be deployed from a barge boom  910  supporting a resonator strip  920  on a guide of belts and rollers  930 . The resonators are stored and deployed from a roll  940  that can be collapsed to about 8 ft×16 ft in an exemplary embodiment. A ballast  950  can be used if necessary to assist the lowering of the noise abatement resonator system  900  into the water. A steerable counter weight base, air supply, cameras, thrust units and other assemblies for moving and positioning the system (collectively referred to as  960 ) are included and coupled to a platform tower structure. 
         [0052]    Many other designs can be developed for noise abatement and damping purposes. In other embodiments, the resonating cavity may be filled with a liquid fluid instead of a gas fluid. For example, if the system is to be operated at extreme depths in the ocean, a liquid other than water having a compressibility different than that of sea water could also be used, as would be appreciated by those skilled in the art. 
         [0053]    Those skilled in the art will appreciate upon review of the present disclosure that the ideas presented herein can be generalized, or particularized to a given application at hand. As such, this disclosure is not intended to be limited to the exemplary embodiments described, which are given for the purpose of illustration. Many other similar and equivalent embodiments and extensions of these ideas are also comprehended hereby. The claims are intended to cover such modifications.