EP2677082B1 - Method of damping underwater noise - Google Patents

Description

The invention relates to a method for damping underwater noise.

When constructing offshore wind farms, the towers of the wind turbines to be built must be connected to the seabed. These are regularly rammed into the ground. Ramming causes considerable pile-up noise which poses a potential hazard, especially to marine mammals such as harbor porpoises.

Is known from the DE 10 2008 017 418 B4 and the EP 2 441 892 A2 to dampen the ram noise by means of a bubble curtain. Such a bubble curtain is created by feeding compressed air into a loop, the loop having openings. Air flows through the openings, creating bubbles. It is currently assumed that the bubble diameters must be as large as possible so that the first omnidirectional natural frequency of the bubbles corresponds to the frequency of the ramming sound. It then comes to the resonance of the sound waves in the bubbles, which leads to a particularly high attenuation.

A disadvantage of known bubble generation devices is the relatively low damping of the Rammschalls. If, for example, a single mast, which is also referred to as monopile, are rammed, it must be rammed with such a large impulse that the resulting ramming noise can not be shielded sufficiently adequately by a single bubble vaicing device according to the prior art. It must then be built at least two bubble curtain, which is expensive.

From the JP 2011-125781 , of the EP 2 476 653 A1 , of the US 2010/0224541 A1 . DE 90 13 851 U1 , of the US 2007/0096346 A1 , of the DE 84 15 461 U1 and the US 2012/0061 862 A1 Membrane aerators for introducing atmospheric oxygen in water are known. In these systems, particularly small air bubbles are sought in order to achieve a particularly large diffusion of oxygen into the water, aspects of sound insulation are irrelevant.

From the JP 2011 125 781 a ventilation device is known, by means of which air can be introduced into a liquid. To prevent sludge from entering the pipe when the supply of compressed air is stopped, the pressure pipe is surrounded by a membrane. The membrane is slotted so that the slot is at an angle to the normal. The ambient pressure closes the slot safely.

The invention has for its object to improve the sound insulation.

The invention solves the problem by a method according to claim 1.

An advantage of the invention is that a particularly high noise reduction can be achieved. Recent research shows that, unlike what is expected, a bubble veil of a variety of small bubbles is more effective than a bubble veil of large bubbles. One possible reason for this is that small bubbles rise more slowly. For a given length-specific flow rate of compressed air, a higher air density in the bubble curtain can thus be achieved. The ram sound therefore sees a larger number of objects that make it difficult to pass through the bubble curtain, which improves the reduction of the sound.

Another advantage is that the large number of small bubbles lead to more scattering of the sound waves of the Rammschalls and to a stronger reflection of the Rammschalls. It has surprisingly been found that by scattering and reflection of the Rammschall can be more effectively prevented from passing through the bubble curtain than by resonant absorption in large air bubbles.

Another advantage of the invention is that when the air supply is cut off (for example, by switching off the compressors), the pores of the membrane close and thus no water and sediment can enter the system. This reduces the start-up time (time until the bubble curtain is fully developed) when the system is restarted (switching on the compressors). In previous systems, sediment can penetrate into the pressure line, with the risk of clogging of the nozzles. Thus, the reliability is increased by the invention.

Another advantage is the low consumption of compressed air, since, as already stated above, the rate of rise of the air bubbles is particularly low due to their small size.

In the context of the present description, the pressure line is understood to mean, in particular, a flexible pressure line which can be made of plastic, for example. It is favorable if the pressure line only has a cross section of at least 50 mm, so that the pressure loss due to internal friction is as low as possible.

Under the pores are understood in particular recesses defined shape in the membrane, which have been introduced, for example, mechanically or by laser. It is favorable if the membrane has a multiplicity of pores. According to a preferred embodiment, the membrane has at least 1000, in particular at least 5000, and particularly preferably at least 10000 pores per meter of continuous metering line.

The bubble curtain is preferably closed. That is, the bubble curtain has no regions in which the bubble density is so low that sound can pass substantially unattenuated.

It is possible, but not necessary, that the apertures in the pressure line have a circular cross-section. It is also possible that the openings are, for example, slit-shaped. It is important that they provide a sufficiently high flow resistance to the escaping compressed air, so that the pressure difference between the interior and the surrounding water drops by no more than 20% above the membrane.

Under the pressure line is in particular a pipe or a hose understood, that is, the pressure line can also be flexibly formed as a tube, whereas training as a flexible hose is particularly advantageous because it facilitates laying the bubble curtain generating device.

Under the membrane is in particular a slit film, in particular a slotted rubber hose understood. In particular, the membrane is designed so that the apertures open only when an air bubble emerges, wherein they remain closed when no bubble escapes, so that no water can penetrate into the membrane.

The term "sound attenuation" is understood to mean, in particular, a reduction of the sound at a location under water outside the bubble curtain. Under the steaming so in particular not only a dissipation of sound energy is understood, but also a scatter, provided that it reduces the sound. In addition to damping, so reducing, Rammlärm the invention is for example also suitable for reducing explosion noise. It can therefore also be spoken of a method for noise protection in sound-intensive construction work in waters. Since the reduction of sound underwater is relevant, could also be spoken of a method for reducing underwater noise.

According to a preferred embodiment, the pressure line extends at least in sections along a longitudinal direction of the pressure line and the membrane is connected to the pressure line such that propagation of compressed air in a gap formed between the pressure line and the membrane is prevented along the longitudinal direction for more than a predetermined distance. In particular, the predetermined distance has a length of at most 5 m. In other words, it is not possible for an imaginary air volume to move more than this predetermined distance along the longitudinal direction along the pressure line. This ensures that the bubble curtain generator can also be used on uneven seabed. An uneven seabed causes the pressure line on its outer side depending on the position experiences different high water pressure. By attaching the membrane to the pressure line ensures that these different water pressures do not cause the air bubbles emerge only at the point with the lowest water pressure.

Preferably, the bubble curtain generating device includes a plurality of pressing members connecting the diaphragm to the pressure line. For example, the pressing elements are formed by hose clamps. This has the advantage that a tubular membrane can be used. By means of the pressing elements, this tubular membrane is then connected to the pressure line so that the distance between the two pressing elements is smaller than the predetermined distance. As described above, it is then ensured that a substantially uniform release of air takes place along the longitudinal direction of the bubble curtain generating device. The pressing elements in the form of hose clamps are particularly simple, so that the bubble curtain generating device is easy to produce.

Preferably, the pressure line is flexible, in other words, the pressure line is preferably a pressure hose. The pressure hose can be reinforced, for example by means of fibers. It is also possible that the pressure hose is stabilized against radial forces and / or transverse forces, so that a safer Discharge of air bubbles over the entire length of the pressure line is ensured.

Preferably, the pressure line has a length of at least 10 meters, in particular 40 meters, preferably at least 100 meters. Support tubes encased in a membrane are known for aerating, for example, clarifiers, but these always have only short lengths in order to keep the pressure loss along their length sufficiently small. The carrier tubes serve to secure the position of the membrane and not the transport of compressed air over long distances. Also, only low air pressures (water pressure + 0.1 MPa maximum) are used. They are therefore not suitable for use as a bubble curtain generating device.

A ramming method according to the invention is carried out in particular by producing the bubble curtain by means of a bubble curtain generating device described above, wherein the pressure line is supplied with an air pressure of at least 0.5 MPa, in particular at least 1 MPa, and wherein a pressure of at most 0, 3 MPa drops.

In order to determine the breakthrough cross-sections so that the requirement is met that at most the specified percentage of the 30% of the air pressure across the membrane drops, first a pressure line with small breakthroughs of, for example, 1 mm is produced. On this, the membrane is raised, which contains a predetermined number of pores. The bubble curtain generator is then brought to the depth of water in which it is to be used to reduce the sound. Subsequently, compressed air is fed into the pressure line and the air pressure at, for example, three equidistant points over the length of the pressure line measured both in the pressure line and between the pressure line and the membrane.

The total decreasing air pressure is the pressure difference between the air pressure in the pressure line and the water pressure immediately outside the membrane. If the pressure difference between the membrane pressure and the water pressure is more than the specified percentage, either the number of pores is increased or the breakthrough cross sections are reduced until the requirement is met.

To increase the density of air bubbles in the bubble curtain, the number of pores can be increased. Alternatively or additionally, the air pressure in the pressure line can be increased. The adaptation of the breakthrough cross sections is carried out as indicated above. It is beneficial if all breakthrough cross sections are the same.

It is particularly favorable if a cross section of the pressure line, the air pressure and the breakthrough cross sections are selected such that the air pressure at an air pressure introduction point of the compressed air into the pressure line is at most twice the air pressure at the point furthest from the Compressed air inlet point is spaced.

Figur 1

eine schematische Ansicht einer Blasenschleiererzeugungsvorrichtung beim Durchführung eines erfindungsgemäßen Verfahrens und

Figur 2

eine Teilansicht einer Blasenschleiererzeugungsvorrichtung zum Durchführung eines erfindungsgemäßen Verfahrens.

In the following the invention will be explained in more detail with reference to the accompanying drawings. It shows

FIG. 1

a schematic view of a bubble curtain generating device in carrying out a method according to the invention and

FIG. 2

a partial view of a bubble curtain generating device for carrying out a method according to the invention.

FIG. 1 shows a bubble curtain generating device 10 having a pressure line 12 for conducting compressed air. The pressure line 12 is shown schematically as a loop which surrounds a pile 14 to be rammed, which is rammed into a seabed 16. The pressure line 12 is from a compressed air source 18, for example on board a ship 19, supplied with compressed air. The pressure pipe 12 extends along a longitudinal direction L formed by the tangent at each point.

The pressure line 12 is what's in FIG. 1 is not visible, surrounded by a membrane through which small air bubbles 20 are discharged. In particular, the membrane is chosen so that the air bubbles have an air bubble size distribution whose average diameter is between 0.1 mm and 10 mm near the membrane. The air bubbles 20 form a bubble curtain 22, which is shown only in sections.

FIG. 2 shows a part of the pressure line 12. It can be seen that this pressure line 12 has a wall 24 which surrounds an interior 26. The wall 24 has a plurality of openings 28.1, 28.2,..., Through which compressed air 30 can escape into a gap 32.

The space 32 is formed between the outside of the wall 24 and a membrane 34. The membrane 34 is formed in this case by a slotted rubber hose and has a plurality of pores 36.1, 36.2, ... on. Through these pores, the compressed air 30 can escape into the environment 38.

FIG. 2 shows that the membrane 34 is connected by means of a plurality of pressing elements 40.1, 40.2, ... with the pressure line 12, so that diaphragm sections 42.1, 42.2, ... between the individual pressing elements 40 (reference numerals without Zählsuffix the object respectively such). The pressing elements 40 are designed so that a transfer of compressed air is prevented from one membrane section in the next. The membrane 34, the pressure line 12 and the pressing elements 40 are part of a membrane aerator 43.

The breakthrough cross sections have respective breakthrough cross sections Q. These breakthrough cross sections are selected in comparison to the pore sizes such that, with respect to a position x along the longitudinal direction L, an air pressure p ₂₆ in the interior 26 and a pressure p _{32 present} in the interspace ₃₂ form; for which the following relationship applies: $\frac{\left(p_{32}-p_{\mathit{Surroundings}}\right)}{\left(p_{26}-p_{\mathit{Surroundings}}\right)}\le 0.\mathrm{Third}$

In other words, the air pressure of the compressed air drops to at most 30% across the membrane, the rest of the pressure drops over the wall 24 of the pressure line.

Preferably, p ₂₆ ≥ 2 · p _environment , that is, the air pressure p ₂₆ in the interior is at least twice the ambient pressure, that is, the water pressure. It is favorable if the relationship of 0.03 MPa ≦ p ₃₄ ≦ 0.06 MPa is satisfied, that is to say that a pressure of between 0.3 bar and 0.6 bar drops across the membrane.

The membrane 34 is, as stated above, a slotted rubber hose, wherein the slots may have between 0.6 and 2 mm slots. Although larger or smaller slots are possible, it has been found that slots produce a particularly advantageous effect in this interval. It is possible, but not necessary, for all slots to be the same length.

The individual membrane sections 42 have a respective length I, which corresponds to a predetermined distance. In particular, this predetermined distance is shorter than 5 meters.

In order to carry out a method according to the invention for damping sound in water in the form of a piling method, the pile 14 is turned from one into one FIG. 1 not shown in detail piling device driven into the seabed 26. By means of the bubble curtain generation device 10 according to the invention, the bubble curtain 20 is produced. A cross section, in particular a diameter d of the pressure line 12 is selected so that, conveniently, a pressure drop between a point of maximum pressure p ₂₆ and a point of minimum pressure p _{26 is} at most 50% of the maximum pressure. In FIG. 1 is the point at most pressure, the point of feed, in which compressed air is fed via a feed line 44 in the pressure line 12 on the one hand, and the opposite point.

LIST OF REFERENCE NUMBERS

10

Bubble curtain generating device

12

pressure line

14

stake

16

Seabed

18

Compressed air source

19

ship

20

bubbles

22

bubble curtain

24

wall

26

inner space

28

breakthrough

30

compressed air

32

gap

34

membrane

36

pore

38

Surroundings

40

presser

42

membrane portion

43

membrane diffusers

44

supply

Q

Breakthrough section

L

longitudinal direction

d

diameter

I

Distance between pressing elements

Claims (5)

1. Method for dampening sound in water, wherein a bubble curtain (22) is generated by means of a bubble curtain generation device with

   (a) a pressure line (12) for conducting compressed air (30)

   - that has a wall (24) which encloses an interior space (26),

   - the wall (24) comprising a number of openings (28), each of which has an opening cross-section (Q),
   characterised by the fact that the bubble curtain generation device

   (b) has a membrane (34) that

   - comprises a multitude of pores (36), each of a particular pore size, and

   - that is designed such that compressed air (30) can be passed through the membrane (34) into the surrounding water in the form of small bubbles (20) through the pores (36),

   (c) wherein the wall (24) is enclosed by a membrane (34), at least in the area surrounding the openings (28), that is designed such that compressed air (30) can be passed out of the interior space (26) through the openings (28), then through the membrane (34) in the form of small bubbles (20) and then through the pores (36) into the surrounding water (38),

   (d) wherein the opening cross-sections (Q) and the pore sizes are selected in such a way that an air pressure (p₂₆) forms in the interior space (26) and an applied pressure (p₃₂) forms in a space (32) between the pressure line (12) and the membrane (34) for which the following applies: $\frac{\left(p_{32}-p_{\mathit{Umgebung}}\right)}{\left(p_{26}-p_{\mathit{Umgebung}}\right)}\le 0,3,$

   

   wherein p_Umgebung represents the ambient pressure.
2. Method according to claim 1, characterised by the fact that an air pressure (p) in a pressure line (12) of the bubble curtain generation device has a value of at least 0.3 MPa.
3. Method according to one of the claims 1 or 2, characterised by the fact that

   - the pressure line (12) is supplied with compressed air (30) with an air pressure (p) of at least 0.5 MPa, in particular at least 1 MPa, and

   - a maximum drop in pressure of 0.3 MPa occurs across the membrane.
4. Method according to one of the above claims, characterised by the fact that the pressure line (12) is at least 10 metres, especially 40 metres, long.
5. Method according to one of the above claims, characterised by the fact that the membrane (34) is connected to the pressure line (12) such that a spread of compressed air (30) in the space (32) between the pressure line (12) and the membrane (34) along the length (L) of the system over more than a predetermined distance (l) is prevented.

Images