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FIELD OF THE INVENTION 
     This invention pertains to flexible floating covers for covering large liquid reservoirs, and particularly it relates to a floating cover which is anchored to the perimeter walls of a reservoir and which rises and falls with the liquid level inside the reservoir. 
     BACKGROUND OF THE INVENTION 
     Floating covers are mounted over settling ponds and clarifiers to contain and collect fermentation gases of mill effluent for example. Floating covers are also mounted over water reservoirs to prevent contamination of potable water from acid rain, pollen, leaves, dust, insects, bird droppings, the effect of sunlight and from the activities of other animals. 
     The installation of a floating cover over a large liquid reservoir represents certain difficulties in that the cover is exposed to the elements and to the movement of the liquid under the cover. For example, a slight accumulation of rain over a cover creates puddles and mounds which catch the wind and promote waves along the cover and into the liquid under the cover. The movement of liquid under the cover causes tangential stresses and constant movement in the cover. These stresses could cause fatigue, localized elongation and rupture of the cover. The formation of mounds and puddles on a floating cover is amplified where the cover is installed over a reservoir that could have gases coming out of the liquid inside the reservoir. 
     Examples of floating covers of the prior art are described in the following documents: 
     U.S. Pat. No. 3,313,443, issued on Apr. 11, 1967 to H. S. Dial et al.; 
     U.S. Pat. No. 3,683,428, issued on Aug. 15, 1972 to L. Morris; 
     U.S. Pat. No. 3,980,199, issued on Sep. 14, 1976 to W. B. Kays; 
     U.S. Pat. No. 4,139,117, issued on Feb. 13, 1979 to H. S. Dial; 
     U.S. Pat. No. 4,181,986, issued on Jan. 8, 1980 to H. E. Aine; 
     U.S. Pat. No. 4,192,025, issued on Mar. 11, 1980 to C. A. Hinsperger; 
     U.S. Pat. No. 4,438,863, issued on Mar. 27, 1984 to J. V. Wilson et al.; 
     U.S. Pat. No. 4,503,988, issued on Mar. 12, 1985 to D. H. Gerber; 
     U.S. Pat. No. 4,603,790, issued on Aug. 5, 1986 to D. H. Gerber; 
     U.S. Pat. No. 4,672,691, issued on Jun. 16, 1987 to De Garie et al.; 
     U.S. Pat. No. 5,505,848, issued on Apr. 9, 1996 to Landine et al.; 
     U.S. Pat. No. 5,587,080, issued on Dec. 24, 1996 to Landine et al. 
     A common method in the prior art for supporting a floating cover over a reservoir consists of bonding float blocks to the underside of the impermeable membrane, or positioning float blocks inside pockets formed in the membrane. This method has had limited success in the past because the float blocks and pockets cause obstructions which catch the liquid movement under the cover and apply tearing stresses along the surface of the cover. Where the cover is installed over a clarifier or a pond and scum tends to form at the surface of the pond, the wind-induced movement in the liquid of the pond and associated scum movement under the cover generate forces that can rip a float block or a pocket away, and tear the strongest impermeable membrane. 
     Therefore, it is believed that there is a need in the industry for a better flexible floating cover which is adapted to minimize the formation of puddles and mounts thereon and which is less susceptible of generating destructive stresses from wind-induced liquid movement under the cover. 
     SUMMARY OF THE INVENTION 
     In the present invention, there is provided a floating cover for liquid reservoir wherein the structure of the cover is particularly flexible to follow the movement of the liquid inside the reservoir without generating excessive tangential stress in the water-impermeable membrane of the cover. Specific segments of the cover become quickly submersed during a rainstorm to keep the cover membrane taut and to limit the formation of randomly spaced puddles that can deform the cover and create stresses in the cover membrane. 
     In a first aspect of the present invention there is provided a liquid reservoir having a floating cover mounted thereon. The floating cover has a floating grid anchored to the perimeter walls of the reservoir. The floating grid floats over the liquid inside the reservoir. The floating grid comprises a keel member and an array of buoyant beams affixed to the keel member and extending away from the keel member. A water-impermeable membrane is affixed to the perimeter wall and is loosely laid over the floating grid. There is also provided an array of weight lines anchored to the perimeter walls and loosely laid over the impermeable membrane. Each of the weight lines is laid at about halfway between an adjacent pair of the buoyant beams. 
     The primary advantage of this structure is that the floating grid, the impermeable membrane and the array of weight lines constitute three separate layers that are loosely laid over each other. These three separate layers are therefore free to slide upon each other and flex to follow the movement of the liquid inside the reservoir without generating any destructive tangential stress in the impermeable membrane. 
     In another feature of the present invention, the buoyant beams and the weight lines are flexible longitudinally whereby a relative movement of the impermeable membrane between the floating grid and the array of weight lines does not generate any point of concentrated shear stresses in the impermeable membrane. 
     In still another feature of the present invention, the impermeable membrane has segments that are quickly submersed under the liquid level during a rainstorm. The submersed segments extend along the keel member and along the weight lines. These submersed segments are advantageous for keeping the impermeable membrane in a taut condition during a rainstorm, and for reducing the formation of puddles and mounds thereon. 
     In accordance with yet another feature of the present invention, the impermeable membrane has a series of drain holes therein. The drain holes are located in a central one third portion of the width of the impermeable membrane. Due to the location of these drain holes, the submersed segments remain present on the impermeable membrane for extended period of time following a rainstorm. Furthermore, the drain holes in the cover of the present invention represent a distinct advantage over the traditional use of hazardous electrical sump pumps. 
     Other advantages and novel features of the present invention will become apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention is illustrated in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which: 
     FIG. 1 is a partial perspective view of a floating cover according to a preferred embodiment of the present invention installed over a liquid reservoir; 
     FIG. 2 is a partial cross-section view of the floating cover; 
     FIG. 3 is a partial top view of the floating grid supporting the impermeable membrane; 
     FIG. 4 is a top view of a rectangular reservoir having the floating cover according to the preferred embodiment mounted thereon; 
     FIG. 5 illustrates a cross-section of one of the drain holes through the impermeable membrane of the floating cover. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will be described in details herein one specific embodiment of the present invention, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and is not intended to limit the invention to the embodiment illustrated and described. 
     A preferred embodiment of the flexible floating cover according to the present invention is partly illustrated in FIGS. 1 and 2. The floating grid supporting the impermeable membrane of the floating cover is partly illustrated in FIG.  3 . These partial drawings are provided herein for clarity. These drawings are believed to be sufficient for illustrating the concept and principles of the present invention. Numerous other structural details or variations may be included in a complete cover installation. However, these additional structural details and variations are known to those skilled in the art. The floating cover according to the present invention is an improvement to the floating cover described in U.S. Pat. No. 4,672,691 of which the first named inventor is also the inventor of the present invention. 
     The floating cover according to the preferred embodiment comprises a flexible water-impermeable membrane  20  which is anchored to the perimeter wall  22  of a reservoir. A flat bar  24  and a series of anchor bolts  26  are used for clamping the impermeable membrane  20  to the top edge of the wall  22 . The impermeable membrane  20  has sufficient surface to cover the reservoir at its lowest operating level. The flexible impermeable membrane  20  is preferably made of a stretch-resistant nylon-based pliable sheet material. 
     The impermeable membrane  20  is loosely supported at the surface of the reservoir by a floating grid  30 . The floating grid is also anchored to the perimeter wall  22  by means of tie cables  32  and connector plates  34  mounted to the anchor bolts  26 . The floating grid  30  is made of a series of buoyant beams  36  attached to and extending from a keel member  38  set along the centre of the floating grid  30 . Each buoyant beam  36  has an inside end attached to the keel member  38 . The outside end of each buoyant beam  36  is retained to the perimeter wall by a tie cable  32 . 
     Each buoyant beam  36  has a bag-like shape and is filled with chunks of foam or similar buoyant material. The envelope of each buoyant beam  36  is preferably made of a different material than the material of the impermeable membrane  20 . The envelope of each buoyant beam  36  is preferably made of a stretch-resistant polyethylene pliable sheet material. The difference in material between the impermeable membrane  20  and each buoyant beam  36  ensures that the two materials do not fuse together in use, when exposed to excessive heat from the sun&#39;s rays. 
     Because of the polyethylene sheet material, each buoyant beam  36  is somewhat flexible lengthwise and widthwise to follow to a certain degree the wave movements of the liquid under the cover. Moreover, it has been found that the coefficient of friction between the buoyant beams  36  and the impermeable membrane  20  in the presence of a water is very low, whereby a relative movement of the impermeable membrane  20  over one of the buoyant beams  36  does not apply any significant tangential stress in the impermeable membrane  20 . It is believed that the flexibility of the buoyant beams  36  is also a contributing factor for providing a low stress contact between the impermeable membrane  20  and the buoyant beams  36 . 
     The central keel member  38  is made of several plies of the same material as the buoyant beams  36 , that is a stretch-resistant polyethylene pliable sheet material. The keel member  38  is thereby relatively flexible and has a surface which offers a low coefficient of friction against the surface of the impermeable membrane  20 . The keel member  38  comprises a trough  40  and opposite horizontal flaps  42 ,  44  extending from the trough. The buoyant beams  36  are attached to the flaps  42 ,  44  and extend substantially at right angle with the keel member  38 . A series of perforations  45  through the bottom segment of the trough  40 , evacuate the liquid that may be trapped inside the trough between the bottom segment of the trough and the central portion of the impermeable membrane  20 . 
     The height ‘A’ of the trough  40  constitutes spare surface for accommodating the widthwise extension and contraction of the floating grid  30  and of the impermeable membrane  20 , as the liquid level changes inside the reservoir. When the liquid level rises in the reservoir to its upper level as shown in FIG. 2, the central portion of the impermeable membrane  20  accumulates in the central trough  40 . 
     Optional transverse cables  46  may also be used between the outside ends of the buoyant beams  36  to retain the buoyant beams in a parallel orientation with each other. The use of transverse cables  46  is advantageous for stabilizing a floating grid  30  over a larger reservoir. A grommet  48  on the outside end of each buoyant beam  36  is used to retain cables  32  and  46 . 
     The floating cover according to the preferred embodiment also comprises an array of lateral weight lines  50  laid over the impermeable membrane  20 , each being laid at about halfway between an adjacent pair the buoyant beams  36 . The lateral weight lines  50  are linked to a central weight line  52  which is laid inside the trough  40  of the keel member  38 . The lateral weight lines  50  and the central weight line  52  are made of a plurality of pipe sections filed with sand or concrete for example. The pipe sections in the lateral weight lines  50  and the central weight line  52  are linked to each other by rope  54  or light cable, such that each weight line is longitudinally flexible to follow the movement of the membrane with any wave action in the covered liquid. The outside end of each lateral weight line  50  is anchored to the perimeter wall  22  of the reservoir by means of an anchor cable  56  attached to an anchor tab  58  mounted to one an anchor bolt  26  above the membrane clamping flat bar  24 . 
     Because the weight lines  50 ,  52  and the buoyant beams  36  are relatively flexible longitudinally, their movement relative to the impermeable membrane  20  do not apply significant concentrated shear stress in the impermeable membrane. 
     The function of the central weight line  54  is to cause the flexible trough  40  to sink below the level of liquid inside the reservoir and to entrain the central portion of the impermeable membrane  20  inside the trough  40 . 
     A first function of the lateral weight lines  50  is to cause lateral depressions on the membrane surface around several drain holes  60  through the impermeable membrane  20 . These drain holes  60  are located between the buoyant beams  36 , and in the central portion ‘B’ of the cover, as shown in FIG.  4 . This central portion ‘B’ represents about one third of the width ‘W’ of the cover. The ropes  54  of the weight line above a drain hole  60  are preferably attached to the drain hole to retain the weight line to that drain hole. When rainwater is considered a contaminant relative to the content of the reservoir, the drain holes  60  are connected to each other and to one or more drain pipes  62  which are routed outside the reservoir. 
     Referring again to FIG. 4, it will be better understood that the flexible trough  40  accumulates a spare surface of the impermeable membrane  20  for accommodating extension and contraction of the impermeable membrane  20  across the width ‘W’ of the reservoir, when the liquid level changes inside the reservoir. Similarly, a second function of the lateral weight lines  50  is to cooperate with the buoyant beams  36  and form peaks and valleys across the length ‘L’ of the reservoir to accumulate a spare surface of the impermeable membrane  20  along the length ‘L’ of the reservoir, to accommodate for the lengthwise extension and contraction of the impermeable membrane  20  as the liquid level change inside the reservoir. The lengths of the anchor cables  56 , of the tie cables  32  and of the intermediate ropes  54 , and the height ‘A’ of the trough  40  are selected to allow unrestricted vertical movement of the impermeable membrane  20  over the expected level variations of the liquid inside the reservoir. 
     FIG. 4 also illustrates a typical accumulation of rainwater over the cover. As mentioned before, the buoyant beams  36  cause transverse ridges in the impermeable membrane  20 , and the lateral weight lines  50  causes depressions in the membrane between the ridges. Rainwater accumulates inside the trough  40  and in the depressions much like according to the illustrated contour line  64 . As rainwater is evacuated through the drain holes  60  the size of the puddles recedes toward the central trough  40  until the water line is within the central region of the cover such as illustrated by label  66 . The rainwater remaining inside and along the central trough  40  is slowly evacuated by evaporation. 
     The advantages of this installation is that rainwater has a stabilizing effect on the cover during a rainstorm by submersing the central segment  68  of the membrane and a series of rib-like lateral segments  70 . Because of these submersed segments  68 ,  70 , the impermeable membrane  20  is kept taut and the surface of the membrane exposed to uplifting wind forces is greatly reduced. Because of the position of the drain holes  60  in the central one third portion of the impermeable membrane  20 , the submersed segments  68 ,  70  are still present when the rainwater recedes to a low level  66  beyond the drain holes  60 . The stabilizing effect is therefore maintained during and after a rainstorm. 
     Additionally, the floating grid  30 , the impermeable membrane  20  and the array of weight lines  50 ,  52  are free to move relative to each other. Therefore, any liquid movement under the membrane  20  is less susceptible of applying excessive tangential stress in the membrane. 
     The ridges created by the buoyant beams  36  still offer gas passages under the membrane  20  whereby any off-gas generated by the content of the reservoir can be evacuated along the buoyant beams  36  and toward the perimeter wall  22  of the reservoir, such as illustrated by arrows  72 . 
     Referring now to FIG. 5, there is illustrated therein the structural arrangement of a preferred drain hole  60 . Each drain hole  60  has an inverted Y-shaped fitting  80 , the legs  82  of which retain segments of the drain hose  62 . The fitting  80  has a flange  84  which is bolted to a pair of washers  86 , one on each side of the impermeable membrane  20 . The washers  86  have an inside diameter ‘D’ which is larger than the maximum width ‘C’ of the fitting  80 , across the legs  82 . The fitting  80  is removable from the washers  86  by removing a series of bolts  88 . The fitting  80  and the drain hose segments  62  are retrievable from under the impermeable membrane  20  through the opening ‘D’, for inspection, repair or replacement of the drainage system, without removing the cover. 
     While one embodiment of the present invention has been illustrated in the accompanying drawings and described hereinabove, it will be appreciated by those skilled in the art that various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. Therefore, the above description and the illustrations should not be construed as limiting the scope of the invention which is defined by the appended claims.

Summary:
The floating cover has a floating grid anchored to the perimeter walls of the reservoir, and floats over the liquid level inside the reservoir. The floating grid comprises a flexible keel member and an array of flexible buoyant beams affixed to the keel member. A flexible impermeable membrane is affixed to the perimeter wall and is loosely laid over the floating grid. An array of flexible weight lines is anchored to the perimeter walls and is loosely laid over the impermeable membrane. Each weight line is laid at about halfway between an adjacent pair of buoyant beams. The floating grid, the impermeable membrane and the array of weight lines constitute three separate layers that are movable relative to each other without generating destructive stress in the impermeable membrane.