Noise abatement underwater

Abstract

An underwater noise abatement system comprises an upright array of sound-absorbing panels on a supporting structure, alternating with vertical spaces between successive panels of the array. Each panel comprises a sound-absorbing matrix that contains an array of embedded voids. Each panel further comprises an acoustically reflective section that faces a reverse face of the matrix, opposed to an exposed face of the matrix. The panels are inclined relative to the horizontal so that the exposed face of the matrix faces upwardly and toward a source of underwater noise, such as an impact-driven monopile.

Description

[0001] Noise abatement underwater

[0002] This invention relates to abatement or mitigation of noise emissions in bodies of water, for example when performing construction or installation operations offshore.

[0003] 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, which will be used herein to illustrate the invention, is installation of subsea foundations by impactdriving piles into the seabed. Such foundations include monopiles for bottom-fixed offshore wind turbines.

[0004] 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.

[0005] 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. Some noise abatement solutions involve a combination of systems that involve different deployment and operational strategies. However, this increases interface risks and operational risks, and reduces operational harmonisation.

[0006] Bubble curtain systems surround a pile with a curtain of air bubbles rising 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 but usually spaced a large radial distance from the pile wall.

[0007] Shell-in-shell systems are relatively near-pile solutions, typically used in water depths of up to 40m and with pile diameters of up to 8m. They deploy a tubular barrier wall around the 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. Shell-in-shell systems offer a possibility of integrating a pile guiding system but suffer from weight, bulk and safety risks during deployment. One example of a shell-in-shell system is disclosed in EP 3240932. Another example defines the barrier wall with a tubular array of staggered upright fire hoses that are attached at the top to an air distribution ring and at the bottom to a ballast ring. Compressed air is supplied via the air distribution ring to inflate the hoses. A further example employs multi-layer sound-insulation shells that are braced and supported by an outer frame construction.

[0008] Resonator systems are also mostly near-pile solutions. They suspend an array of gas- filled 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. Typically such arrays are used in water depths of up to 45m and with piles of up to 8m in diameter. They benefit from tunable damping elements and light weight, but can present challenges in holding their position under the influence of waves and currents.

[0009] 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. The panels must be held substantially horizontal to retain air in the cavities. A similar issue arises in relation to US 9410403, in which stackable inverted open-ended resonators absorb underwater noise.

[0010] In US 9488026 and US 9812112, a collapsible frame can transition between a compact stowed configuration and a deployed configuration in which the frame is extended for use. Horizontal rows of horizontal panels of resonators are supported in vertical succession by a flexible framework comprising vertical rigging lines that can be extended or retracted in the water column, akin to a Venetian blind mechanism.

[0011] In the underwater acoustic barrier of US 8387746, multiple overlapping rows or layers each comprise multiple noise-attenuating leaves. The layers overlap to define a continuous barrier wall. A similar arrangement is employed for the acoustic panels of EP 2657410. The leaves or panels are near vertical, with only enough inclination to the vertical to allow for overlap between successive layers.

[0012] 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 soundreceiving front face of the panel. US 8887864 discloses a sound absorption panel in which a closed-cell foam resin is laminated with metal sheets on both sides. Holes penetrate the foam resin and the metal sheets to absorb sound effectively by allowing air to pass through the panel, where it encounters resistance and converts sound energy into thermal energy, effectively absorbing the sound.

[0013] Against this background, the invention resides in a sound absorbing panel that comprises a sound absorbing section and an acoustically reflective section. The soundabsorbing section comprises a matrix, such as an elastomeric mass, whose acoustic impedance substantially matches that of water. The matrix contains an array of voids embedded therein and has at least one exposed face that may be substantially planar. The voids can be closed and can be filled with air. The acoustically reflective section faces a reverse face of the matrix, opposed to the exposed face of the matrix.

[0014] The array of voids may be a regular periodic array. The matrix may contain at least one layer of voids, which may be substantially parallel to the exposed face of the matrix. If the matrix contains at least two layers of voids, those layers can be mutually parallel. The voids of those layers may be mutually aligned in a thickness direction extending between the exposed face and the reverse face of the matrix.

[0015] The acoustically reflective section may comprise a cavity that extends across the reverse face of the matrix and can also extend around edges of the matrix disposed between its exposed face and its reverse face. The cavity may, for example, be filled with air.

[0016] A rigid buffer plate, for example of steel, polymer or polymer composite, may be interposed between the matrix and the cavity. The buffer plate may be shaped with acoustic reflection formations that project from a side of the buffer plate facing the matrix.

[0017] The panel may further comprise side walls that extend from the buffer plate to a back plate that is spaced from the buffer plate across the cavity. Conveniently, the side walls can also extend from the buffer plate to embrace the matrix.

[0018] The panels of the invention are apt to be used in an underwater noise abatement system of the invention. That system comprises an upright array of sound absorbing panels on a supporting structure, wherein the panels of the array are inclined relative to the horizontal and alternate with vertical spaces defined between vertically successive panels of the array.

[0019] The array may be deployable by downward extension of the supporting structure, which can conveniently be suspended from an upper support and can extend from the upper support to a ballast weight below the supporting structure.

[0020] The inclination of the panels to the horizontal may be variable. For example, the inclination of the panels could vary from panel to panel along the array, with the inclination of lower panels of the array being greater than the inclination of upper panels of the array. More generally, the panels may be inclined at 15° to 75°, or 15° to 45°, or 30° to 60° relative to the horizontal.

[0021] A plurality of the arrays may be assembled to form an upright tubular barrier, in which case the supporting structures of the arrays can be suspended from an annular upper support such as a pile gripper. Similarly, the supporting structures of the arrays can extend from the upper support to an annular ballast weight below the supporting structures.

[0022] Where the panels of the array are panels of the invention, the sound-absorbing section may be an upper section of the panel and the acoustically reflective section may be a lower section of the panel. When placed beside or around a source of underwater noise such an an impact-driven monopile, the panels can face upwardly and toward the source.

[0023] In one sense, the invention involves sound insulation panels that comprise a rubber section including an array of internal gas-filled gaps, cavities or voids embedded in an elastomeric body or mass, and an air cavity section. The voids in the rubber section, which are not necessarily resonators in the sense described in the prior art, absorb and scatter acoustic energy whereas the air cavity section functions as a reflector of acoustic energy back into the rubber section. The rubber section and the air cavity section are brought together with a backing structure of steel, although other backing materials can be considered if dictated by weight constraints.

[0024] An array of panels of the invention are supported by a structure disposed beside or around a source of noise such as an impact-driven pile, that structure preferably being deployable vertically with stackable rows of panels, with the rubber sections of the panels facing toward the source of noise. The panels may be tilted so that the rubber sections face upwardly and toward the source of noise, for example with inclination of 30° to 60° to the horizontal. The tilt angle and the structure of the panels provide a synergistic balance of effective sound insulation and resistance to the lateral load of currents while meeting installation and stacking constraints.

[0025] In a more general example, a noise abatement system of the invention proposes a near-field solution that comprises tilted noise-mitigation panels surrounding or beside a pile or other source of noise. The panels work by a combination of: absorption by virtue of a damping matrix being a rubber section containing voids in a periodic pattern at the front of the panel, facing the pile; and reflection by virtue of a reflective layer that may be filled with air, this being an air cavity section at the rear of the panel behind the rubber section, facing away from the pile. The panel further comprises a backing or casing that can be made of steel or a reinforced polymer.

[0026] The rubber section is a noise-absorbing rubber matrix, partly inspired by phononic crystals found in nature, employs two sound-attenuating mechanisms, namely: local resonance of periodic voids such as air-filled cavities in an elastomeric mass that act as scatterers; and destructive wave interference attributed to scattering due to the periodic pattern of the voids.

[0027] The air cavity section acts as a reflector due to the large impedance mismatch between steel and air. This mismatch reflects sound waves back to the rubber section, which are then further absorbed.

[0028] As the speed of sound in water and rubber are similar, impedance matching between them means that sound waves travelling through water and impinging on the damping matrix will be effectively transmitted into the damping matrix. Conversely, there is an impedance mismatch between the rubber and the void inclusions, meaning that a sound wave is highly reflected when encountered a medium with different speed of sound properties.

[0029] Thus, sound waves travelling within the rubber matrix will be reflected by the void inclusions, maximising the damping effect. This is because multiple successive reflections of waves in a system whose properties vary periodically result in path differences, which generate constructive or destructive interference. Thus, interference mechanisms within the matrix play an important role in sound absorption, notably Bragg, Fabry-Perot of Fano interference depending on the periodic pattern of the voids.

[0030] There is also an impedance mismatch between the rubber matrix and the air cavity, including any steel or polymer wall or backing panel disposed between the rubber matrix and the air cavity. This mismatch causes sound waves to be reflected back into the rubber matrix for further absorption, hence minimising acoustic energy escaping from the side of the panel that faces away from the source of noise and hence into the surrounding environment.

[0031] An example of the invention involves a deployable cylindrical or tubular barrier structure comprising several rows of noise mitigation panels, those rows being in vertical succession and each row comprising several such panels. The structure can be deployed in the manner of a Venetian blind around a pile foundation before impact driving begins. Upright cables or ropes can extend from row to row to maintain the tubular shape of the structure and can control the inclination or tilt angle of the panels in addition to deployment of the panels.

[0032] The panels may be tilted with respect to the horizontal, which increases the angle of incidence of sound waves emanating from the pile upon the panels. The inclination of the panels determines their collective coverage of the monopile. This may influence noise mitigation performance but also affects the hydrodynamic loads of currents impinging on the structure. Typically, the shallower the inclination of the panels, the less their collective coverage of the monopile but correspondingly the lower their blockage of incident currents. Thus, those currents exert lower horizontal hydrodynamic loads upon the structure, making it less likely that deflection of the structure could cause the panels to clash with the pile surrounded by the structure.

[0033] Once the structure has been deployed around a monopile, the inclination of the panels can be adjusted depending on the hydrodynamic conditions determined by ocean currents. In one example, the structure can be arranged to apply a steeper tilt angle to panels of a deeper row, further to improve acoustic performance where currents are generally slower than at the surface.

[0034] Thus, the invention proposes a noise abatement structure comprising sound absorbent and reflective panels offering high noise mitigation performance. Preferred target absorption frequencies are in the range 1 Hz to 1 kHz as those frequencies are dominant during pile-driving operations. The panels cover a large surface area to increase their reduction of the sound exposure level. A simple deployment mechanism is suitable for mounting to current pile gripper designs of installation vessels. The structure is readily scalable and adaptable to various monopile sizes.

[0035] In summary, an underwater noise abatement system of the invention comprises an upright array of sound-absorbing panels on a supporting structure, alternating with vertical spaces between successive panels of the array. Each panel comprises a soundabsorbing matrix that contains an array of embedded voids. Each panel further comprises an acoustically reflective section that faces a reverse face of the matrix, opposed to an exposed face of the matrix. The panels are inclined relative to the horizontal so that the exposed face of the matrix faces upwardly and toward a source of underwater noise, such as an impact-driven monopile.

[0036] 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:

[0037] Figure 1 is a schematic side view of a noise abatement system of the invention comprising an array of sound-absorbing panels deployed underwater beside a noise source exemplified here by an impact-driven monopile;

[0038] Figures 2a, 2b and 2c are schematic side views of various sound-absorbing panels of the invention;

[0039] Figures 3a to 3e are schematic partial side views of other sound-absorbing panels of the invention;

[0040] Figures 4a and 4b are perspective views of sound-absorbing panels of the invention disposed beside a monopile, the panels being inclined respectively at 60° and 30° to the horizontal;

[0041] Figure 5 is a perspective view of a noise abatement system of the invention suspended from a gripper of an installation vessel before downward deployment of the array of panels around a monopile surrounded by the gripper;

[0042] Figure 6 is a detail side view of the noise abatement system of Figure 5 deployed around the monopile, with the panels inclined at 30° to the horizontal; Figure 7 is a sectional view on line VII-VII of Figure 6;

[0043] Figure 8 is a perspective view of a noise abatement system of the invention suspended from a gripper and with the array of panels deployed around a monopile surrounded by the gripper, the panels being inclined at 60° to the horizontal;

[0044] Figure 9 is a perspective view corresponding to Figure 8 but showing the system retracted upwardly with the panels remaining inclined to the horizontal;

[0045] Figure 10 corresponds to Figure 9 but shows the panels instead in a substantially horizontal orientation when the system is retracted;

[0046] Figures 11 a to 11 b are a sequence of schematic side views that show the panels reorienting from inclined to horizontal as the system is retracted; and

[0047] Figure 12 is a schematic side view of another panel arrangement of the invention.

[0048] Referring firstly to Figure 1 of the drawings, a noise abatement system 10 of the invention is shown here schematically beside a vertically elongate source of underwater noise, in this case an impact-driven monopile 12 for a bottom-fixed offshore wind turbine. By way of example, such a monopile 12 could have a length, or height, of about 100m and a width at its bottom end of about 10m.

[0049] The system 10 comprises a deployable array 14 of sound-absorbing panels 16 that are supported in the water column by a vertically-extensible supporting structure 18. The array 14 extends vertically as a column, parallel to a central longitudinal axis 20 of the monopile 12 and with the panels 16 spaced horizontally or radially from the tubular wall of the monopile 12, for example by about 2m.

[0050] Each panel 16 has a shallow, generally cuboidal body defining generally rectangular upper and lower faces 22, 24 that are substantially planar and substantially parallel. The body has a layered structure comprising an upper sound-absorbing section 26, atop a lower sound-reflecting section 28 in face-to-face relation. For example, the upper section 26 could have a thickness of 30mm out of an overall thickness of the panel 16 of about 50mm to 70mm, measured between the upper and lower faces 22, 24.

[0051] By way of further example, each panel 16 may have a length, in a direction tangential to a horizontal circle centred on the axis 20, of 3.6m, and a width, in a direction orthogonal to its length and hence intersecting the axis 20, of 0.7m to 0.9m. Thus, the length of each panel 16 is substantially greater than its width, which in turn is substantially greater than its thickness.

[0052] When the array 14 is deployed, the panels 16 are spaced apart vertically from each other, with a vertical spacing of, for example, about 1 m between successive panels 16. Thus, the vertical spaces between the panels 16 alternate with the panels 16 down the array 14. In practice, therefore, the array 14 will comprise many more panels 16 than are shown here, to extend along most of the length of the monopile 12. Similarly, the simplified view of Figure 1 shows only one array 14 of panels 16 but in practice, multiple similar arrays 14 will be arranged in a polygonal, faceted tubular arrangement to surround the monopile 12, those arrays 14 being angularly spaced around the central longitudinal axis 20. Thus, the panels 16 are arranged in columns corresponding to the arrays 14 and in rows extending from array 14 to adjoining array 14.

[0053] Each generally planar panel 16 is tilted or inclined to the horizontal at an exemplary angle or of 30°, such that an upper face 22 of the upper sound-absorbing section 26 faces upwardly and inwardly toward the central longitudinal axis 20. This inclination of the panels 16 allows the upper section 26 to receive acoustic energy more efficiently, and hence to attenuate that energy more effectively, than if the panels 16 were oriented horizontally as taught by the prior art. In this respect, the monopile 12 radiates acoustic energy downwardly and outwardly during impact driving. Consequently, Figure 1 represents an acoustic wave front 30 that radiates downwardly and outwardly from the monopile 12 at at angle 0 of, for example 16° to 18° to the vertical.

[0054] It will be apparent from the arrows in Figure 1 that the wave front 30 impinges on the upper section 26 of each inclined panel 16 at an angle that is closer to orthogonal than if the panels 16 were instead horizontal. Indeed, if the panels 16 were inclined at approximately 70° to 75° to the horizontal, the wave front 30 would impinge on the upper section 26 of each panel 16 substantially orthogonally. However, there is a tradeoff between sound absorption by the panels 16 and susceptibility of the array 14 to lateral deflection under horizontal loads applied by waves and currents, due to the current-blocking effect of the array 14.

[0055] In this respect, the supporting structure 18 conveniently suspends the panels 16 on vertical wires or cables whose length can be varied, like a Venetian blind, and is therefore somewhat flexible, to the extent that excessive deflection under horizontal loads could cause panels 16 of the array 14 to clash with the monopile 12. In view of that trade-off, it is preferred that the panels 16 have shallower inclination to reduce the blocking effect of the array 14 on incident generally horizontal water flows.

[0056] Consequently, the panels 16 are shown in this example as being at an inclination of 30° to the horizontal. That inclination offers a large reduction of current blockage while maintaining good sound-absorbing coverage around a monopile 12.

[0057] In other examples to follow, the panels 16 are shown as being at an inclination of 60° to the horizontal. More generally, various angular ranges for the inclination of the panels 16 are contemplated, such as 15° to 75°, or 30° to 60°, or 15° to 45° to the horizontal.

[0058] Figures 2a to 2c and 3a to 3e show various options for the internal structure of the panels 16. In each case, it will be apparent that the upper and lower sections 26, 28 sandwich a planar backing or buffer plate 32 between them and that the lower section 28 contains a sealed cavity 34 whose top is closed by the backing panel 16. A back plate 36 is spaced from the buffer plate 32 by a gap that defines the thickness or depth of the cavity 34. The cavity 34 is filled with air in this example, but could instead contain another gas or an aerogel or other material that supports an impedance mismatch to reflect acoustic energy back into the upper section 26.

[0059] Side walls 38 of the panel 16 bridge the gap between, and maintain the spacing between, the buffer plate 32 and the back plate 36. The side walls 38 extend upwardly and inwardly to embrace and retain a cuboidal elastomeric mass 40 of the upper section 26.

[0060] The structure of the panel 16 comprising the buffer plate 32, the back plate 36 and the side walls 38 may be of a metal, such as steel, or of a polymer or polymer composite such as glass fibre reinforced plastics. Typically, a panel 16 whose structure is of steel is less expensive but heavier than a panel 16 whose structure is of polymer or polymer composite. It will also be apparent from Figures 2a to 2c and 3a to 3e that the sound-absorbing upper section 26 contains a regular periodic array of spherical closed voids 42 dispersed within a damping matrix exemplified by an elastomeric or rubber mass 40. The voids 42, which may be filled with a gas such as air, are arranged in two or more planar layers that are parallel to the buffer plate 32, to the upper face 22 of the upper section 26, and to each other. The voids 42 of each layer are aligned, in the width and length directions, with the voids 42 of the or each other layer.

[0061] By way of example, the voids 42 may have a diameter of 5mm. The layers of voids 42 may be spaced 14mm apart, from centre to centre, in the thickness direction. The voids 42 of each layer may be spaced 20mm apart, from centre to centre, in the width and length directions. The buffer plate 32 may spaced from the centre of the closest layer of voids 42 by 8mm in the thickness direction. The cavity 34 may be 15mm across in the thickness direction between the buffer plate 32 and the back plate 36, parallel to the buffer plate 32, that defines the lower face 24 of the lower section 28.

[0062] In an example of a panel 16 with a steel structure, the buffer plate 32 is 8mm thick and the back plate 36 is 4mm thick. In an example of a panel 16 with a polymer structure, the buffer plate 32 is 22mm thick and the back plate 36 is 10mm thick.

[0063] As the speed of sound in water and rubber are similar as noted previously, sound waves travelling through water and impinging on the elastomeric mass 40 will be absorbed effectively due to impedance matching. Thus, at the upper face 22 of the panel 16, the elastomeric mass 40 is exposed to the surrounding water. The sound waves are then reflected and scattered due to an impedance mismatch upon encountering the air-filled voids 42 within the elastomeric mass 40. Local resonance of the voids 42 and destructive wave interference promoted by scattering attenuate the incoming acoustic energy.

[0064] The buffer plate 32 lies against a reverse face of the elastomeric mass 40 that is parallel to and opposed to the exposed upper face 22. By virtue of the impedance mismatch arising from the buffer plate 32 and the cavity 34 of the lower section 28, residual acoustic energy that may travel through the full thickness of the upper section 26 is reflected back into the elastomeric mass 40 for further absorption.

[0065] In all of the panels 16 shown in Figures 2a to 2c, the elastomeric mass 40 contains two layers of voids 42. In Figure 2a, the elastomeric mass 40 of the upper section 26 extends across substantially the full width of the cavity 34 of the lower section 28. Conversely, in Figure 2b, the cavity 34 has a C-shaped cross section to surround or embrace the elastomeric mass 40. Thus, respective arms of the cavity 34 extend upwardly around side edges of the mass 40 to reflect acoustic energy that could otherwise escape from those edges of the mass 40.

[0066] In Figure 2c, the buffer plate 32 has reflector formations 44 on its upper surface, facing the elastomeric mass 40, to tailor its properties of acoustic reflection. Various reflector formations 44 are possible. The reflector formations 44 shown here comprise a series of ridges or a matrix of discrete projections that impart a desired reflective texture to the upper surface of the buffer plate 32.

[0067] Figures 3a to 3e show variations in the arrangement of voids 42 in the elastomeric mass 40 and in the relative thicknesses of the upper and lower sections 26, 28 of the panel 16.

[0068] In Figures 3a and 3b, there are two layers of voids 42 as in Figures 2a to 2c, whereas Figures 3c, 3d and 3e show additional layers of voids 42. In Figure 3e, the voids 42 are substantially smaller but more numerous and more densely packed than in other examples.

[0069] In Figures 3b and 3e, the lower section 28 of the panel 16 is relatively thin and consequently the cavity 34 is narrow in comparison to the cavities 34 shown in Figures 3a, 3c and 3d.

[0070] Figures 4a and 4b show panels 16 of the invention forming part of a deployed array 14 of such panels 16 disposed around a monopile 12. The structure 18 supporting the panels 16 has been omitted from these views but it will be apparent that the panels 16 have holes to receive vertical cables or wires of that structure 18.

[0071] The panels 16 shown in Figure 4a are inclined at 60° to the horizontal whereas the panels 16 shown in Figure 4b are inclined at 30° to the horizontal. In each case, two such panels 16 are shown joined to each other end-to-end as part of a horizontal row. The panels 16 are joined by flexible or rigid joints 46 that taper inwardly toward the monopile 12. The panels 16 are angled relative to each other in plan view so that the complete row will form a continuous polygonal loop around the monopile 12. Turning next to Figures 5 to 7, these drawings show a noise abatement system 10 comprising multiple panels 16 each inclined at 30° to the horizontal when deployed. The panels 16 are arranged in vertical columns corresponding to the arrays 14 and are aligned in horizontal rows extending between the arrays 14.

[0072] When the system 10 is retracted as shown in Figure 5, the panels 16 are stacked in close proximity to, or in contact with, neighbouring panels 16 of the same column above and below. In this example, the panels 16 pivot into a substantially horizontal orientation when retracted for optimal compactness, as explained below with reference to Figures 11a to 11 d, although the panels 16 could remain inclined to the horizontal when retracted if desired. When the system 10 is deployed as shown in Figure 6, the panels 16 of each column are spaced apart vertically by, for example, 1m from panel to panel. In this example, twelve arrays 14 of the panels 16 together surround the monopile 12 in a dodecahedral tubular arrangement, as shown in sectional view of Figure 7.

[0073] As shown in Figure 5, each column of panels 16 is suspended from, and deployable like a skirt downwardly from, a pile gripper 48 that surrounds the monopile 12. The pile gripper 48 is typically mounted on an installation vessel such as a jack-up vessel (not shown). Vertical cables of the supporting structure 18 suspend the panels 16 from the pile gripper 48 and extend downwardly to a ballast ring 50 that also surrounds the monopile 12 and that imparts tension in the cables. The weight and stiffness of the ballast ring 50 lends stability to the system 10, especially to avoid clashing with the monopile 12 under the horizontal loads of currents. The cables of the supporting structure 18 are extended from the pile gripper 48 to deploy the arrays 14 of the system 10 downwardly and are retracted into the pile gripper 48 to retract the arrays 14 of the system 10 upwardly.

[0074] In Figures 8 and 9, a noise abatement system 10 comprising panels 16 inclined at 60° to the horizontal is shown in deployed and retracted states respectively. In this example, the panels 16 remain inclined to the horizontal when retracted and stacked together as shown in Figure 9. However, as shown in Figure 10, the panels 16 could instead pivot into a substantially horizontal orientation when retracted.

[0075] Figures 11a to 11 d show how the panels 16 of an array 14 can pivot from an inclined orientation into a horizontal orientation when retracted, in the manner of a Venetian blind. The supporting structure 18 comprises inner and outer cables 52, 54 to which the panels 16 are pivotably attached at longitudinal intervals. The supporting structure 18 also comprises a retractable central cable 56 around which the panels 16 can pivot. The central cable 56 extends through or around the panels 16 to the ballast ring 50 to lower and raise the ballast ring 50 during deployment and retraction.

[0076] Initially, as shown in Figure 11a, there is a vertical clearance between the ballast ring 50 and the lowermost panel 16 of the array 14. Retracting the central cable 56 lifts the ballast ring 50 into contact with the lowermost panel 16 as shown in Figure 11 b. Continued upward movement of the ballast ring 50 then tilts the lowermost panel 16 to the horizontal as that panel 16 lies upon the ballast ring 50 as shown in Figure 11c. Further upward movement of the ballast ring 50 similarly brings the lowermost panel 16 into contact with the next panel 16 of the array 14, which tilts to the horizontal in response, and so on until multiple panels 16 of the array 14 are stacked horizontally on top of the elevated ballast ring 50 as shown in Figure 11 d.

[0077] Finally, Figure 12 shows that the panels 16 of an array 14 need not all have the same inclination to the horizontal once deployed. In this respect, the consequences of current blocking, in terms of lateral loads deflecting the array 14 horizontally, are less of a concern where currents are slower. Currents are typically slower with increasing depth in the water column or with greater proximity to the seabed. Consequently, lower and hence deeper panels 16 of an array 14 could have greater inclination to the horizontal than upper and hence shallower panels 16 of the array 14, the better to align the lower panels 16 with the direction of an incident acoustic wave front 30 radiating from a monopile 12 as shown in Figure 1.

[0078] By way of example, Figure 12 shows an array 14 in which an upper panel 16 is inclined at 30° to the horizontal whereas a lower panel 16 is inclined at 60° to the horizontal. Figure 12 also shows the option of an intermediate panel 16 inclined at an intermediate angle, in this case 45° to the horizontal.

[0079] Many other variations are possible within the inventive concept. For example, the inclination of the panels 16 could be adjusted in situ, once deployed, by extending or retracting the inner or outer cables 52, 54 of the supporting structure 18. Thus, the panels 16 could be tilted to a steeper inclination if the current velocity is low, or tilted to a shallower inclination if the current velocity is high. Similarly, the panels 16 could be mounted on the supporting structure 18 via tilt mechanisms in which actuators can adjust the inclination of the panels 16 individually or collectively, for example to suit different depths or current velocities.

Claims

Claims1. A sound absorbing panel, comprising: a sound absorbing section; and an acoustically reflective section; wherein the sound-absorbing section comprises a matrix that contains an array of voids embedded therein and that has at least one exposed face; and the acoustically reflective section faces a reverse face of the matrix, opposed to the exposed face of the matrix.

2. The panel of Claim 1 , wherein the matrix is an elastomeric mass.

3. The panel of Claim 1 or Claim 2, wherein the exposed face of the matrix is substantially planar.

4. The panel of any preceding claim, wherein the array of voids is a regular periodic array.

5. The panel of any preceding claim, wherein the matrix contains at least one layer of voids.

6. The panel of Claim 5, wherein the at least one layer of voids is substantially parallel to the exposed face of the matrix.

7. The panel of any preceding claim, wherein the matrix contains at least two layers of voids.

8. The panel of Claim 7, wherein the layers of voids are mutually parallel.

9. The panel of Claim 7 or Claim 8, wherein the voids of the layers are mutually aligned in a thickness direction extending between the exposed face and the reverse face of the matrix.

10. The panel of any preceding claim, wherein the acoustically reflective section comprises a cavity that extends across the reverse face of the matrix.

11. The panel of Claim 10, wherein the cavity also extends around edges of the matrix that are disposed between the exposed face and the reverse face of the matrix.

12. The panel of Claim 10 or Claim 11 , wherein the cavity is filled with air.

13. The panel of any of Claims 10 to 12, further comprising a rigid buffer plate that is interposed between the matrix and the cavity.

14. The panel of Claim 13, wherein the buffer plate is of steel, polymer or polymer composite.

15. The panel of Claim 13 or Claim 14, wherein the buffer plate is shaped with acoustic reflection formations that project from a side of the buffer plate facing the matrix.

16. The panel of any of Claims 13 to 15, further comprising side walls extending from the buffer plate to a back plate that is spaced from the buffer plate across the cavity.

17. The panel of Claim 16, wherein the side walls also extend from the buffer plate to embrace the matrix.

18. The panel of any preceding claim, wherein the voids are closed.

19. The panel of any preceding claim, wherein the voids are filled with air.

20. The panel of any preceding claim, wherein the matrix is of a material whose acoustic impedance substantially matches that of water.21 . An underwater noise abatement system comprising an upright array of sound absorbing panels on a supporting structure, wherein the panels of the array are inclined relative to the horizontal and alternate with vertical spaces defined between vertically successive panels of the array.

22. The system of Claim 21 , wherein the array is deployable by downward extension of the supporting structure.

23. The system of Claim 21 or Claim 22, wherein the supporting structure is suspended from an upper support.

24. The system of Claim 23, wherein the supporting structure extends from the upper support to a ballast weight below the supporting structure.

25. The system of any of Claims 21 to 24, wherein said inclination of the panels is variable.

26. The system of any of Claims 21 to 25, wherein said inclination of the panels varies from panel to panel along the array.

27. The system of Claim 26, wherein said inclination of lower panels of the array is greater than said inclination of upper panels of the array.

28. The system of any of Claims 21 to 27, wherein the panels are inclined at 15° to 75°, or 15° to 45°, or 30° to 60° relative to the horizontal.

29. The system of any of Claims 21 to 28, comprising a plurality of said arrays cooperating to form an upright tubular barrier.

30. The system of Claim 29 when dependent on Claim 23, wherein the supporting structures of the arrays are suspended from an annular upper support.31 . The system of Claim 30, wherein the annular support is a pile gripper.

32. The system of Claim 30 or Claim 31 when dependent on Claim 24, wherein the supporting structures extend from the annular upper support to an annular ballast weight below the supporting structures.

33. The system of any of Claims 21 to 32, wherein the panels of the array are as defined in any of Claims 1 to 20.

34. The system of Claim 33, wherein the sound-absorbing section is an upper section of the panel and the acoustically reflective section is a lower section of the panel.

35. The system of any of Claims 21 to 34 in combination with a source of underwater noise, wherein the panels face upwardly and toward the source.

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