AU709929B2

AU709929B2 - Swell-attenuating device

Info

Publication number: AU709929B2
Application number: AU52778/96A
Authority: AU
Inventors: Jean Bougis; Alex Degaie
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-03-17
Filing date: 1996-03-18
Publication date: 1999-09-09
Anticipated expiration: 2016-03-18
Also published as: WO1996029475A1; EP0815327A1; FR2731724A1; AU5277896A; EP0815327B1; DE69614839D1; ES2163619T3; FR2731724B1; TR199700970T1

Description

It would be desirable to provide a swell-attenuating device that substantially overcomes at least one of the above disadvantages.

The invention in a broad form provides a swell-attenuating device, with said swell, termed incident swell, having a particular mean direction of propagation, the device comprising breaking means capable of recovering the energy of the incident swell and of generating, downstream of the device, a forced swell with a phase shift relative to said incident swell and means for retaining said breaking means in place relative to the mean direction of propagation of the incident swell by leaving the device unconstrained, wherein: the breaking means, is semi-submersible and comprises a heavy plate oriented transversely to the mean direction of propagation of the incident swell; the device comprises a semi-submerged transverse wall above the plate, which wall delimits in the direction of the incident swell, an upstream chamber, and, on the other side, a downstream chamber; the means for retaining the breaking means is configured to leave the device unconstrained only in its vertical movements.

Such a swell-attenuating device advantageously lets the marine currents pass and thus allows to maintain the ecological and sedimentological balances, the removal of the contaminating materials returned into the protected stretch of water and the integrity of 20 the marine environment. In the event of tides, the device advantageously follows the tide *°level. The device has a small draught, which advantageously allows to maintain the aesthetic character of the site. The swell-attenuating device also has a low building cost and is simpler to install and maintain. The swell-attenuating device can also be removed at a low cost and without causing any damage to the site and its environment.

•o A preferred form of the present invention will now be described by way of example only with reference to the accompanying drawings, wherein: Figure 1 schematically describes the principle of a swell-attenuating device S.according to the invention.

Figure 2 shows, in a cross-sectional view, an example of a swell-attenuating device according to the invention.

Figure 3 is a front view of the swell-attenuating device shown in figure 2.

Figure 4 is a cross-sectional top view, according to line IV-IV shown in I D -\YLlBhlll\392989 docViP figure 3, of the swell-attenuating device shown in the foregoing figure 2.

Figure 5 is a top view showing an example of use of the swell-attenuating device according to the invention.

This invention relates to a swell-attenuating device. Generally, it will find its application viz. in the field of the control or use of stretches of water, rivers, coasts or other maritime sites.

In a stretch of water the swell generally propagates in a given direction, viz. defined by the prevailing winds on open sea, the wave guides formed by eventual dykes, the path followed by large ships the track of which generates swell, or the like. This is in fact a mean direction, the various incident swells having a direction substantially spread according to a Gaussian trend, e.g. over more or less 450 about this fictive general direction.

As shown in figure 1, it seems to be theoretically possible to consider a device which allows to completely attenuate a swell, called incident swell 1, having a given mean direction of propagation 2 as defined above.

If there is generated, at a point a swell, then called forced swell 4, shown in long dotted lines, with the same direction 2 and the same wavelength as the transmitted incident swell, shown in short dotted lines, but in opposition of phase with respect to this latter, their effects will be cancelled and a dead calm will be achieved, as symbolized by the straigth line designated by In the event of reflection, at the level of point O of the incident swell 1, as shown by the arrow designated by 6, there should be noticed that there will then be observed, upstream of point O and in the vicinity of same, a pure lap, i.e. in an area designated by 7, a locally stationary phenomenon.

As shown in figure 2, the example of swell-attenuating devices according to the invention is based on this principle. Thus, it comprises semi-submerged breaking means 8 capable of recovering the energy of the incident swell 1 which one wants to attenuate and of generating, downstream of the device, a forced swell phase-shifted with respect to said incident swell 1.

Thanks to the breaking means 8, there is thus generated a swell the effects of which will, according to the above-stated principle, counterbalance those of the incident swell. By approaching the opposition of phase, there will then be tended towards getting a dead calm. In addition, such a device has the advantage of being autonomous. It is indeed the breaking means 7 themselves which by recovering the energy of the incident swell 1 ensure, through oscillating vertically, the generation of the forced swell.

The notions of downstream side and upstream side are defined according to the direction of propagation of the incident swell 1, this latter extending, according to the arrow designated by 2, from the upstream side to the downstream side.

Furthermore, the word semi-submerged means that the breaking means are foreseen to be floating close to the surface of the water, as shown in figure 2, and are not fixed to the bottom.

In addition, the swell-attenuating device according to the invention comprises means 9 for maintaining said breaking means 8 in place with respect to the mean direction of propagation of the incident swell 1, while keeping the device free in its vertical motions.

They thus allow to keep the attenuator back, viz. from its horizontal translations and from rotations about a vertical axis.

The device in fact acts as a "springy mass" system the motions of which can be put into an equation, through a projection on a vertical axis, as follows: (M+Ma) A (Av+Ak+Ar) V X F (t) where M represents the mass of the device, Ma represents the added mass of water, A represents the acceleration of the device, Av represents the viscous damping, Ak represents the damping due to the frictions, such as viz. the frictions between the breaking means 8 and the means 9 for maintaining same in place, Ar represents the gravity-wave damping due to the radiation corresponding to the radiated energy in the generated swell, V represents the speed of the device, 0 H represents the hydrostatic stiffness, K represents the stiffness of the means 9 for maintaining in place, X represents the displacement of the device, o F represents the excitation the incident swell would exert on the device if same were fixed.

In order for the system to be oscillating, i.e. for the breaking means to generate swell, we should have: (Av+Ak+Ar) 2 M 4. (H K) Since the radiation damping should be as large as possible, as it takes into consideration the energy of the generated swell, and the stiffness of the device must, as will be explained below, remain limited, it thus seems necessary for the device to oscillate vertically that the sum M+Ma be high.

Therefore, the breaking means 8 viz. comprise a heavy plate 10. This latter is e.g. oriented transversally to the mean direction of propagation of the incident swell 1.

In order to make same heavier, the heavy plate 10 has, according to the particular embodiment viz. shown in figures 2 and 3, ballast tanks 11, whereby same may be more or less filled in order to adapt the mass of the device.

They eventually can also serve to adjust its draught according to the amplitude and/or the direction of the swell, or the like. However, care should be taken in order to avoid the risks of free surface.

Another advantage of the ballast tanks 11 consists in that they allow to bring the device to the site where it has to be used by causing same to stably float.

As shown, the heavy plate 10 is viz. rectangularly shaped. According to other embodiments, it can e.g. also be square, oval or round.

In the case of an oval plate 10, this latter is also oriented transversally to the mean direction 2 of propagation of the incident swell 1.

By transversally should be understood that the heavy plate 10 is substantially horizontally arranged, its largest dimension being perpendicular to the direction of propagation of the swell. Generally, the plate 10 is furthermore so oriented that one of its edges be hit by the incident swell according to a mean angle of 900.

To contribute to the putting into oscillation of the breaking means 8, the device also comprises e.g. means 12 for increasing the added water mass Ma.

These are viz. transversal sidewalls 13 provided for under the plate 10, in the vicinity of its large sid.'s and/or eventually of its small sides.

These two sidewlls 13 thus delimit a volume of liquid 14 which, for being confined under the swell attenuator, allows to increase the efforts in phase with the acceleration and thus the added water mass.

The sidewalls 13 also have the advantage viz. of attenuating the swells of small wave-lengths.

Thus, even if these small swells do not have the energy necessary to put the breaking means 8 into oscillation, they are nevertheless attenuated by the device and this viz. by reflection.

This being said, the incident swells one wants to attenuate generally have, like for the direction, a mean wave length 3. In order to generate oscillations of a maximum amplitude, an own period of oscillation is chosen for the device, as close as possible to said mean period of the incident swell 1. Nevertheless, care should be taken for the own period of the device to remain slightly higher then said mean period, in order to prevent the generated swell 4 from undergoing a reversal and from being in phase with the transmitted incident swell.

For this purpose, the device comprises e.g. means for adapting its own period of oscillation, viz. as a function of the mean period of the incident swell 1.

They are viz. formed by the heavy plate 10 which is foreseen submerged, in order to limit the hydrostatic stiffness of the device by reducing its surface in the vicinity of the water/air interface.

The own period of the device is indeed given by the formula: Tp 27c M+Ma

SH+K

To adapt the own period of the device as developped above, it is thus necessary to limit H, i.e. the stiffness of the system. Since same diminishes with the surface of the device at the level of the water/air interface, the heavy plate is submerged.

However, as we have seen above, the stiffness of the device should neither be too low if one wants same to be put into oscillation.

Thus, the device in addition comprises e.g. a semi-submerged transversal, viz. vertical thin wall 16 above the plate As shown, the thin wall 16 may be provided for substantially in the centre of said plate 10. It defines, on the side of the incident swell 1, a chamber called upstream chamber 17, and, on the other side, a chamber called downstream chamber 18.

According to other embodiments, the thin wall 16 can be provided for either more set back or more set forth, so as to modulate the respective area of the upstream chamber 17 and the downstream chamber 18.

The thin wall 16 also has the advantages viz. of also contributing to stop the small swells. In addition, it allows to break the speed and pressure fields of the incident swell 1. There can be observed that the sidewalls 13 and the thin wall 16 also allow to contribute to the attenuation of the swell through viscous dissipation.

Thanks to the position of the thin wall 16, the taking off of energy necessary for the putting into oscillation between the upstream chamber 17 and the downstream chamber 18 is optimally spread.

By way of an example, though good results have been achieved without the presence of thin walls 16 for swells with periods varying between five and six seconds, the performances are improved thanks to said thin walls 16 for the swells the periods of which vary between seven and eight seconds.

In addition, it can be observed that the downstream chamber 18 forms a buffer tank. The water it stores indeed allows to deepen the peaks and to fill the downstream recesses.

The substantially vertical thin wall 16 may viz. have slanted thin walls 19 in the vicinity of its edges.

In addition, viz. in order to reduce the horizontal efforts exerted by the incident swell 1, it can be provided in its upper portion with not shown holes.

As can be seen more in particular in figure 4, the device in addition comprises e.g. semi-submerged side plates 20 perpendicular to the transversal thin wall 16, in the vicinity of the transversal edges of the plate They contribute to ensure the hydrostatic stability of the device. In addition, they allow to protect the means 9 for maintaining the device in place and attenuate the edge effects. They also limit the passing-through of the swell.

The vertical edges of the side plates 20 viz. have bevels 21. They contribute, like the slanted thin walls 19, by smoothing the current lines, to the hydrodynamic efficiency of the device.

As described above, the device has a semi-submerged oscillating structure. According to the laws of hydrodynamics, there thus exists a period for which the excitation transmitted by the incident swell 1 is minimum. This is the vanishing period Te defined by the vertical dimensions of the device and the relationship of the submerged surfaces.

For this period, since the excitation passes at a minimum, the generated forced swell also passes at a minimum. In order for the device to remain efficient over the full range from the small wave-lengths to the wave-lengths corresponding to its own period Tp, a small vanishing period is chosen, so that same be as close as possible to the periods for which the attenuator operates by reflection and/or dissipation, i.e. the small wave-lengths.

Therefore, the device in addition comprises e.g. means for lowering its vanishing period viz. formed by the control of the relationship between the surface of the device at the liquid/air interface and the surface of the plate By way of an example, for a device the transversal thin wall 16 of which is about 2 meters high and the draught is of about 1 meter, the relationship between the floating surface shown in hatching and the horizontal surface of the plate 10 is in the range of 26%.

Still according to figure 4, one observes that the means 9 for maintaining the device in place are formed e.g. by posts 23, not shown in figures 2 and 3, on which the device is substantially vertically slidingly mounted.

The poles 23 are viz. vertical and end through the plate 10. They are e.g.

secured to the bottom. There can also be provided for a flexible anchoring, for safety's sake.

When referring anew to figures 2 and 3, one observes that according to a particular embodiment the device comprises, above the thin wall 16, a substantially horizontal panel 24. It allows to form a pontoon. Its dimensions must however be adapted to the requirements of static stability and of putting into oscillation of the device.

The panel 24 has the advantage of limiting the effectcs of the columns of water generated by the upstream splashing.

In order to further improve the hydrodynamic performances of the swell attenuator according to the invention, there can eventually be considered to provide same e.g. with not shown bilge keels.

As shown in figure 5, in order to protect an entire stretch of water, several swell-attenuating devices according to the invention can be associated.

According to this application, they are viz. joined two by two, side by side and vertically sliding viz. along substantially parallel poles 23. The overall direction of the attenuators is viz. perpendicular to the mean direction 2 of the incident swell 1.

By way of an example, good results have been achieved for a swell with a mean period varying from seven to eight seconds with an attenuator having breaking means 8 with a width of five meters and a length of twelve meters, the height of the thin wall 16 being in the range of the hollow peak amplitude of the incident swell, in the case considered, in the range of 2 to 3 meters.

The own period had been established at about 10 seconds and the vanishing period at about 3 seconds.

Under such circumstances, the amplitude of the swell is indeed divided by two.

In order to still further improve the result, attenuating devices according to the invention can be associated in series behind each other.

Of course, further embodiments within reach of the specialist could have been considered without therefore departing from the framework of this application.

The claims defining the invention are as follows: 1. A swell-attenuating device, with said swell, termed incident swell, having a particular mean direction of propagation, the device comprising breaking means capable of recovering the energy of the incident swell and of generating, downstream of the device, a forced swell with a phase shift relative to said incident swell and means for retaining said breaking means in place relative to the mean direction of propagation of the incident swell by leaving the device unconstrained, wherein: the breaking means, is semi-submersible and comprises a heavy plate oriented transversely to the mean direction of propagation of the incident swell; the device comprises a semi-submerged transverse wall above the plate, which wall delimits in the direction of the incident swell, an upstream chamber, and, on the other side, a downstream chamber; the means for retaining the breaking means is configured to leave the device unconstrained only in its vertical movements.

2. The swell-attenuating device according to Claim 1, further comprising means for increasing the added water volume of the device so as to contribute to making .the breaking means oscillate.

3. The swell-attenuating device according to Claim 2, wherein the means oo°for increasing the added water volume of the device is formed by transverse skirts 20 provided under the plate.

4. The swell-attenuating device according to Claim 1, further comprising means for adjusting the period of its eigenoscillations.

5. The swell-attenuating device according to Claim 4, wherein the means for adjusting the period of its eigenoscillations are formed by said plate which is provided 25 semi-submerged so that the hydrostatic resistance of the device is limited by decreasing its surface near the water-air interface.

6. The swell-attenuating device according to Claim 1, further comprising semi-submerged side walls which are perpendicular to the transverse wall and situated near the transverse edges of the plate.

7. The swell-attenuating device according to Claim 1, further comprising means for reducing its extinction period, which means are formed by adjustment of the ratio between the surface of the device toward the liquid/air interface and the surface of e plate.

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VP

Claims

8. The swell-attenuating device according to Claim 1, wherein the means for retaining the device in place is formed by posts on which the device is substantially vertically movably mounted.
9. A swell-attenuating device substantially as hereinbefore described with reference to the accompanying drawings. Dated 21 June, 1999 Alex Degaie Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON fe 9 .9 ft I\DAYLIB'ibII\392989.doc VJP 12 ABSTRACT This invention relates to a swell-attenuating device, said swell, called in- cident swell having a given mean direction of propagation According to the invention, the swell-attenuating device comprises semi-submerged breaking means capable of recovering the energy of the incident swell and of generating, downstream of the device, a forced swell phase-shifted with respect to said incident swell and means for maintaining said breaking means in place with respect to the mean direction of propa- gation of the incident swell by keeping the device free in its vertical mo- tions. Figure 2.