Patent Publication Number: US-3878684-A

Title: Devices for protecting the bases of structures immersed in a volume of water, against undermining

Description:
United States Patent [1 1 Lamy 1 Apr. 22, 1975 DEVICES FOR PROTECTING THE BASES [56] References Cited 0F STRUCTURES IMMERSED IN A UNITED STATES PATENTS VOLUME OF WATER, AGAINST 2.474.786 6/1949 Humphrey 61/4 UNDERMINING 3.280.569 10/1966 Wosenitz [75] Inventor: Jacques Edouard Lamy, fg g Fomenay-aux&#39;Roses- France 3:386:25O 6/1968 ra -3.5;... [73] Assigneez Doris Compagnie Generale 3,387,458 6/1968 Jarlan 61/3 pour les Developpements Operationnels des Richesses Primary E.\&#39;aminrPhillp Karma&#34; S M -i P i France Attorney, Agent, or Firm-Wigman &amp; Cohen [22] Filed: Aug. 24, 1973 [57] ABSTRACT PP 391,465 A device for protecting the bases of structures im- Related Application Data mersed in a volume of water, against undermining, by I I I means of a wall or curtain constructed in proximity of [63] gg Md) the said structure and comprising distributed holes for water transfer purposes, the disposition of the perforated wall and of its holes being such that in a water [30] Forelgn &#39;jpphcanon Priority Data trajectory directed towards the structure, in the zone Sept. 15, 197.. France 72.32847 of the base thereof and returning tOwdrds the Open water at a higher level, the pressure drop experienced [52] U.S. Cl. by the water is Smaller than would be the case for a 4 5 37 trajectory in the opposite direction.  
 13 Claims, 6 Drawing Figures PATENTEDAPRZZIQYS sumep z 2828228 DQEN DEVICES FOR PROTECTING THE BASES OF STRUCTURES IMMERSED IN A VOLUME OF WATER, AGAINST UNDERMINING This is a Continuation-in-Part of copending U.S. Pat. application Ser. No. 358,500, filed May 9, 1973, now U.S. Pat. No. 3,846,988.  
  As stated in above-mentioned U.S. Pat. application Ser. No. 358,500, in order to protect structures such as dykes or breakwaters or again maritime installations such as reservoirs, against the effects of a swell or waves, it has already been proposed that at a certain interval in front of the wall of the structure, and in a substantially parallel position in relation thereto, at the side facing the volume of water in which the swell develops another wall should be erected which contains multiple holes both above and below the mean water level, the top of which wall is at a higher level than the crests of the largest waves, so that it always stands out of the water. These two high walls thus form a kind of caisson which, by filling up through said holes, absorbs the waves climbing along the perforated wall, and subsequently restores the water through the holes, when the perforated wall is uncovered by the wave trough.  
  The prime objective here is to break kown the reflected waves and render them uncoordinated so that a major part of the wave energy is transformed into heat by friction and turbulence.  
  In an embodiment described in above-mentioned U.S. Pat. application Ser. No. 358,500, the holes are provided by ducts the profiles of which have a flared form with quite a steep gradient of sections so that when the water is flowing in the direction in which the section of the duct increases, the streamlines break away from the duct wall, generating energy dissipating eddies.  
  However, it has been found that the device furthermore has the advantage of preventing undermining of the base of the structure by the water, the structure being the assembly constituted by the structural work itself and the said perforated wall, when both are erected upon a common foundation as is generally the case. This tendency towards banking up at the base of the structure, instead of the normal undermining tendency which is otherwise encountered in maritime structures, constitutes a supplementary advantage, indeed a very important one, of the known perforated wall device.  
  In seeking to explain this phenomenon, it has been found that a mean unidirectional water circulation develops in which the water tends on average to enter the structure through the holes located towards the bottom of the perforated wall, whilst it tends on average to leave the structure through holes located at the top of the wall. Consequently, the silt or sand being displaced by the water tends to move towards the bottom of the caisson.  
  The invention stems from this observation. One of its objects is to reinforce the banking up tendency which has been observed in the known wave-breaking device.  
  One characteristic of the invention is an arrangement of the perforated wall and its holes, such that during a water trajectory extending towards the structure in the zone of the base thereof and returning towards the volume of open water at a higher zone, the pressure loss providing the holes with profiles having a flared form A with quite a steep gradient of sections, in the direction in which the flow of water is desired to experience the greater pressure loss. The greater pressure loss is produced by the energy dissipating eddies referred to in above-mentioned U.S. Pat. application Ser. No. 358,500 of which the present Patent application is a Continuation-in-Part.  
  Another object of the invention, in a more general way, is to protect any submerged foundation or any submerged work or device, even if located at great depths, against undermining by water currents, whether these result from the wave action or from any other cause; In this context the invention is characterised by the arrangement on the bed of the volume of water, around the foundation or work being protected, of&#39;a wall pierced by holes, projecting only a small height above the said bed of the volume of water, and completely submerged. The holes in the bottom part of the wall, close to the foundation, can have a flared form, convergent in the direction of the trajectory of the water approaching the foundation, and divergent in the opposite direction, or again a discontinuous form producing the desired asymmetry in the pressure drops.  
  The foregoing and further features of the invention may be more readily understood from the following description of some preferred embodiments thereof, by way of example, with reference to the accompanying drawings. in which: i  
  FIG. 1 is a vertical section through a structure and through a perforated wall in accordance with an embodiment of the invention;  
 FIG. 2 illustrates in section another embodiment:  
  FIG. 3 illustrates a vertical section through another embodiment;  
  FIG. 4 is a transverse schematic section, illustrating a pipe, for example part of a pipe-line installed upon the sea bed and protected against undermining by the application of the invention;  
  FIG. 5 illustrates in transverse section a variant embodiment in which the pipe rests upon spaced blocks; and  
 FIG. 6 is a planview of the FIG. 5 arrangement.  
  In FIG. 1, the reference I indicates a concrete wall or masonry wall, for example, a harbour jetty which is to be protected against the effects of the swell moving in from the right in the figure, in particular against undermining.  
  To do so, a wall 2 of a certain thickness, for example in the order of one metre, is erected at a certain interval from the wall 1, and parallel thereto, on a foundation 3 common to both walls.  
  The wall 2 contains multiple holes or conduits 4 distributed over its surface from its base to its top, so that the interior of the caisson formed by the wall 1 which is a solid wall, the perforated wall 2 and the foundation 3, communicates with the open water through the holes in the wall 2.  
  When the crest of a wave arrives at said perforated wall, the water passes through the wall both through the top holes which are momentarily covered by the wave and through the lower holes right down to the base of the wall, by virtue of the hydrostatic pressure, the reverse taking place when the wave trough is in contact with the perforated wall.  
  In reality, however, as the applicants have observed, the phenonenon is not symmetrical and in the holes at the bottom of the perforated wall. a mean unidirectional current towards the interior of the caisson develops. whilst in the holes at the top part of the wall. a mean current moving away from the caisson develops. The arrows marked in the figure illustrate this phenonenon.  
  To reinforce this mean current and. consequently. to improve the protective effect against undermining. the holes 4a close to the base of the wall. in the embodiment shown in FIG. 1, have a shape which is convergent towards the interior of the caisson.  
  The result of this arrangement is that the water flowing through the holes 4a at the base of the wall. towards the caisson is accelerated without substantial pressure loss. whilst on return from the caisson towards the open water. the same holes 4a then present a divergent form and if the angle of divergence is quite large. the water streamlines break away from the conduit wall. producing eddies and a pressure loss which limits the flow in this direction. The holes 4a. convergent for entry into the caisson and divergent there from. act as detectors vis-a-vis the oscillating flow which passes through them alternately in one direction and then the other.  
  This promotes the development of a mean flow towards the caisson.  
  The holes 4b in the highest part of the wall. in FIG. I have a shape which is this time convergent towards the open water; in this case. these holes promote the current from the caisson towards the open water. their action thus being added to that of the holes 4a at the base. in order to create a mean current flowing from the open water towards the interior of the caisson near the base of the wall. and from the caisson to the open water at the top of the wall.  
  The holes. in the simplest case. can be conical with an apex angle the value of which will generally range between and 45, preferably between 34 and 38, the radius of smallest circular section (throat) being in the order of 0.3 to 0.5 metres.  
  In the interval between the top and bottom zones. the holes 40 in the wall-2 can have an arbitrary form. for example being cylindrical or again convergent/divergent. in accordance with one of the examples described in the above-mentioned U.S. Pat. application No. 358.500. filed May 9. I973.  
  The holes 4a are provided over a height of the wall I. which depends on its depth of immersion. which may be in the order of several metres.  
  The holes 412 are provided over a height of the same order below the mean level of the open water or its lowest level (tidal waters).  
  Again. it is possible to provide only one of the two arrangements hereinbefore described and for example to restrict the system to the provision of convergent holes in the base zone of the wall, while the rest of the wall being equipped with cylindrical orifices 4c as shown in FIG. 3, or conversely to restrict the system to providing in the top zone. holes convergent towards the open water. the holes in the base zone being cylindrical. The essential factor as far as the desired effect is concerned. is that the shapes of the holes in the base zone of the wall and in a higher part should combine in such a way that in a trajectory extending towards the caisson through a hole in the base zone of the perforated wall. and returning towards the open water through a hole in the top zone of the wall. the water should pass through at least one convergent conduit.  
 Numerous variant embodiments are conceivable.  
  For example. the convergent orifices may not be defined by geometric solids of revolution or may have a form other than conical, being for example constituted between two flat walls. or pillars as in the copending U.S. Pat. application referred to hereinbefore.  
  In the drawing. the perforated wall and the structure being protected. are built upon a common foundation of concrete or masonry this being the most advantageous design. and the protection against undermining then extends to this foundation as well.  
  But the perforated wall and the work or structure could have independent foundations, the protective effect then being exercised both vis-a-vis the base of the perforated wall and that of the structure.  
  Instead of having a continuous flared form. as shown in the drawing. the holes may have a discontinuous form.  
  For example. FIG. 2 illustrates a hole shape comprising two cylindrical portions 5, 6 of different dimensions. the two cylinders being connected with one another for instance via a radiused surface 7. It will be understood that with a hole of this shape the pressure loss experienced by the water when flowing in the direction of the arrow is less than that which it experiences in the reverse direction. Any other hole shape giving this kind of asymmetry and pressure loss. can equally well be utilised for the purpose of the invention.  
  FIG. 3 illustrates too, that to secure the objective of the invention the perforated wall can also have a lower height than that of the structure whose foundation is to be protected. and may even be completely immersed, defence of the top of the structure then being assigned. if required. to some other device.  
  It will be observed. too. that if the perforated submerged wall projects only a small distance above the level of the foundation. that is to say makes up only a fraction of the depth of the water. the holes in the wall can all be convergent or have some other shape producing asymmetrical pressure drop. as for example the shape shown in FIG. 2, or may for that matter all be cylindrical.  
  The invention is applicable quite generally to the protection of any foundation against undermining by currents at depth. whether these currents are produced as a consequence of swell action or some other cause.  
  The foundation to be protected can have an arbitrary perimeter. It may be that of a structure built on the shore or at a distance therefrom in the case of a lighthouse. a beacon or an oil drilling rig.  
  By way of example. FIG. 4 illustrates the protection of a pipe 8, such for example as a pipeline arranged on the sea bed. Over the length of said pipeline. for which undermining might occur. it is surrounded by two parallel walls 9. 10. arranged at either side of it. preferably having heights slightly greater that the pipe diameter and pierced by holes 11 of convergent form towards the pipe. or cylindrical form. The two walls 9, 10 can be connected by a foundation 12 on which the pipe rests. The latters cross-section can have the desired. known. shape in order to reduce the effects of hydrodynamic lift which tend to lift it from the sea bed under the effect of current flow.  
  FIGS. 5 and 6 relate to a variant embodiment in which the pipe 8 rests upon blocks 13, for example of concrete. regularly distributed over the sea bed. To protect these blocks against undermining. a cage 14 perforated with multiple holes and open at the top, is erected around each of the blocks. In this case, again the holes in each cage will advantageously be convergent towards the interior of the cage but may also be cylindrical.  
  In these embodiments, the perforated walls can be of plastic material instead of masonry, this having the advantage that it possesses a surface polish which makes it more difficult for living organisms, barnacles, algae, etc., to cling there.  
  The embodiments described are purely examples and could be modified, in particular by the substitution of equivalent techniques, without in so doing departing from the scope of the invention.  
 I claim:  
  1. A device for protecting the base of a structure immersed in a volume of water, against undermining, comprising a construction forming a curtain in proximity of the structure, and a plurality of passages for the water through said curtain, said passages being disposed in order to produce a pressure drop which is smaller for water flowing towards the structure in the region of the base thereof and flowing away from it at a higher level, than it is for water flowing towards the structure in the region at a higher level, and flowing away from it in the region of the base of the structure.  
  2. A device as claimed in claim 1, in which the passages located in the base region of the curtain are arranged to produce a smaller pressure drop in a water flow directed towards the structure, than in a water flow directed away from it.  
  3. A device as claimed in claim 2, in which the passages located in the region of the base of the curtain have a flared form in the direction away from the structure. with a sufficiently large gradient of section to produce breakaway in the streamlines ofa water flow moving in said direction.  
  4. A device as claimed in claim 2, in which the passages located in the base region of the curtain have a stepped profile and a section which increases in the direction away from the structure.  
  5. A device as claimed in claim 1, in which the passages located in the upper region of the curtain are arranged to produce a smaller pressure drop in a water flow moving away from the structure, than in a water flow moving towards it.  
  6. A device as claimed in claim 5, in which the passages located in the upper region of the curtain have a flared form in the direction towards the structure. with a sufficiently large gradient of section to produce breakaway in the streamlines of a water flow passing in said direction.  
  7. A device as claimed in claim 5, in which the passages located in the upper region of the curtain have a stepped profile and a section increasing in the direction towards the structure.  
  8. A device as claimed in claim 1, in which said con struction comprises a wall completely submerged in the water, and said passages are disposed in order to produce a pressure drop which is smaller for water flowing towards said structure through said passages, than it is for water flowing away from said structure through said passages.  
  9. A device as claimed in claim 8, in which said passages have a flared form in the direction away from said structure, with a sufficiently large gradient of section to produce breakaway in the streamlines of a water flow moving in said direction.  
  10. A device as claimed in claim 8, in which said wall has a height of a fraction of the depth of the volume of water.  
  II. A device for protecting the base of a structure or piece of equipment immersed in a volume of water, against undermining, comprising a wall constructed upon the bed of the volume ofwater and extending parallel along at least part of said base, said wall being completely submerged in the water. and a plurality of passages for the water through said wall. said passages being disposed in order to produce a pressure drop which is smaller for water flowing towards said base through said passages, than it is for the water flowing away from said base through said passages.  
  12. A device as claimed in claim 11, in which the passages have a flared form in the direction away from the base, with a sufficiently large gradient of section to produce breakaway in the streamlines ofa water flow moving in said direction.  
  13. A device as claimed in claim 11, in which the wall has a height of a fraction of the depth of the volume of water.