Patent Publication Number: US-2009223876-A1

Title: Screen Assembly for a Shale Shaker

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a division of U.S. patent application Ser. No. 10/530387 filed on Oct. 16, 2003 and incorporated by reference herein for all it contains. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screen assembly, for a shale shaker, a panel for a screen assembly, a support structure for a screen assembly, a shale shaker comprising a screen assembly, a shale shaker comprising a support structure and a method for fitting a screen assembly into a shale shaker. 
     2. Description of the Related Art 
     In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string and is rotated to bore the borehole. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The drilling mud contains expensive synthetic oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires the solids, to be removed from the drilling mud. This is achieved by processing the drilling fluid. The first part of the process is to separate the solids from the solids laden drilling mud. This is at least partly achieved with a shale shaker, such as those disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328. 
     Shale shakers generally comprise an open bottomed basket having one open discharge end and a solid walled feed end. A number of rectangular screens are arranged in the basket, which are held in C-channel rails located on the basket walls, such as those disclosed in GB-A-2,176,424. The basket is arranged on springs above a receptor for receiving recovered drilling mud. A skip or ditch is provided beneath the open discharge end of the basket. A motor is fixed to the basket, which has a drive rotor provided with an offset clump weight. In use, the motor rotates the rotor and the offset clump weight, which causes the basket and the screens fixed thereto to shake. Solids laden mud is introduced at the feed end of the basket on to the screens. The shaking motion induces the solids to move along the screens towards the open discharge end. The recovered drilling mud is received in the receptor for further processing and the solids pass over the discharge end of the basket into the ditch or skip. 
     The screens are generally of one of two types: hook-strip; and pre-tensioned. 
     The hook-strip type of screen comprises several rectangular layers of mesh in a sandwich, usually comprising one or two layers of fine grade mesh and a supporting mesh having larger mesh holes and heavier gauge wire. The layers of mesh are joined at each side edge by a strip which is in the form of an elongate hook. In use, the elongate hook is hooked on to a tensioning device arranged along each side of a shale shaker. The shale shaker further comprises a crowned set of supporting members, which run along the length of the basket of the shaker, over which the layers of mesh are tensioned. An example of this type of screen is disclosed in GB-A-1,526,663. The supporting mesh may be provided with or replaced by a panel having apertures therein. 
     The pre-tensioned type of screen comprises several rectangular layers of mesh, usually comprising one or two layers of fine grade mesh and a supporting mesh having larger mesh holes and heavier gauge wire. The layers of mesh are pre-tensioned on a rigid support comprising a rectangular angle iron frame and adhered thereto. The screen is then inserted into C-channel rails arranged in a basket of a shale shaker. An example of this type of screen is disclosed in GB-A-1,578,948. 
     A further example of a known rigid support is disclosed in WO 01/76719, which discloses, amongst other things, a flat panel like portion having apertures therein and wing portions which are folded to form a support structure, which may be made from a single sheet of material. This rigid support has been assigned the Trade Mark “UNIBODY” by the applicants. 
     European Patent Publication Number 1 002 588, discloses a panel comprising a plurality of groups of perforations, each group comprising six generally equally triangular apertures arranged with their apices facing a central portion, wherein the apices of two opposing ones of said triangular apertures are spaced apart further than the apices of opposed ones of the remaining triangular apertures. 
     The layers of mesh in the screens wears out frequently and therefore needs to be easily replaceable. Shale shakers are generally in the order of 5 ft wide and 10 ft long. A screen of dimensions 4 ft wide by 10 ft long is difficult to handle, replace and transport. It is known to use two, three, four or more screens in a single shale shaker. A standard size of screen currently used is of the order of 4 ft by 3 ft. A pre-tensioned type of screen is generally easier and faster to replace than the hook strip type, as the layers of screening material do not need to be tensioned in the shale shaker. A pre-tensioned type of screen is especially easily and quickly replaceable when used in a shale shaker having rails provided with inflatable bladders, such as those disclosed in GB-A-2,176,424 to clamp the pre-tensioned type of screen in place. 
     BRIEF SUMMARY OF THE INVENTION 
     The inventor has noted that the support structure for the screen assembly has to be very rigid. It is known to strip the layers of mesh off used screen assemblies and to replace the worn layers of mesh. However, this is a time consuming process conducted in a workshop. The inventor has also noted that all of the screen assembly need not be replaced. In one aspect, the present invention attempts to provide a screen assembly to replace a known screen assembly of the pre-tensioned type. 
     The present invention also attempts to provide a panel for a screen, which will increase the life of layers of screening material arranged thereon. 
     The present invention also attempts to retain rigidity in the screen assembly, whilst being easy to replace. 
     In accordance with the present invention, there is provided a screen assembly for a shale shaker, the screen assembly comprising a panel and a support structure, the panel having an area provided with a multiplicity of apertures and at least one layer of screening material arranged over the multiplicity of apertures, the panel further comprising at least one support rib characterised in that said panel is removable from said support structure and wherein at least one member is arranged between said panel and said support structure within the perimeter of said panel over which said at least one support rib and said panel is deflectable, such that at least two spans are defined by said panel. The layers of screening material are the most likely components of a screen assembly to fail in use. A screen assembly of the present invention allows replacement of the panel with layers of screening material attached thereto, without having to replace the entire screen assembly. It has been noted that a replaceable screen support is friendlier to the environment, as only the panel and worn layers of screening material need be sent for recycling and the screen support be reused on site. 
     Preferably, the support structure is removable from said shale shaker. Advantageously, the screen assembly is insertable into a clamping mechanism of a shale shaker. Advantageously, the panel has a perimeter, at least part of which, in use is arranged in the clamping mechanism and is pushed on to the support structure when operated. 
     Preferably, the at least one member is arranged substantially centrally such that the panel has at least two distinct screening areas, the at least two distinct areas have half the free span between fixing points, which in the case of the VSM 300 brand shale shaker sold by Varco limited, is between the two side rails. By reducing the span by half, the rigidity of the screen assembly can be reduced proportionally by a much greater amount. Accordingly, the screen assembly can be made much lighter, as less of the same material is required. Hence a reinforced screen panel is suitable in combination with a rigid support structure providing the support member. 
     Advantageously, at least one of the support structure and the panel comprises the member over which the panel is deflectable in use. Most preferably, the member is rigidly fixed support structure and/or the panel. Preferably, the support structure comprises a structural support member and the panel comprises a corresponding support member, which engage or co-operate to form a member over which the panel is deflectable in use. Advantageously, one of the structural support member and the support member has a convex rounded profile and the other has a corresponding concave rounded profile. The support member and structural support member may be formed to co-operate to allow slight movement to facilitate deflection of the panel over the support member and/or to facilitate location of the panel on to the support structure. It is important to provide means for the user to facilitate location of the panel over the support structure accurately so that downward force provided by the fixing means in the shale shaker, preferably an inflatable bladder or wedges, will properly fix the panel in the shale shaker. An interface is provided at the front end and back end of the panel, which may facilitate proper location of the panel over the support structure by abutting an adjacent screen or an interface means provided in the shale shaker. The interface may also facilitate sealing between panel to inhibit particles passing between screens and through interfaces. 
     Advantageously, the structural support member comprises a bar or tube extending across a substantial portion of the structural support. The structural support preferably extends along the entire length of the panel. The structural support member may comprise a square, oblong, triangular or circular section bar or tube. 
     Preferably, the support member comprises portions having openings therein. The openings advantageously allow fluid and small particles to flow through the openings. The openings also reduces the overall weight of the panel. 
     The panel may take the form of a rectangle or a circle. Preferably, the panel is rectangular having a pair of opposing sides and a pair of opposing ends, wherein the part of the perimeter is the two opposing sides. 
     Advantageously, the member is arranged equidistant the two opposing sides and is arranged substantially parallel to the two opposing sides. The member is arranged substantially centrally such that the panel has at least two distinct areas, the at least two distinct areas having half the free span between fixing points. 
     Preferably, two support members are arranged between the two opposing sides and are arranged substantially parallel to the two opposing sides. Each of the two members is preferably arranged approximately a third the way along the free span between the two sides, such that the panel has at least three distinct areas, the at least three distinct areas having a third the free span between fixing points. 
     Advantageously, the structural support comprises an outer frame and cross members. The outer frame and cross members are preferably made from steel tubing of square or circular cross section and are advantageously welded together at the junctures. 
     Preferably, the panel comprises a perforate plate, the multiplicity of apertures therein. Advantageously, the panel comprises a flat plate which may be of mild steel, aluminium or a plastics material. The apertures may be punched out, drilled, cast or cut out with a laser or saw. 
     Preferably, the panel comprises at least one support rib. In a rectangular screen, the ribs are arranged between sides preferably to increase rigidity across the screen, although the ribs are considerably smaller than for a screen assembly spanning the full distance between sides. The inherent rigidity of the panel must be equal or greater than the rigidity of a standard screen assembly designed to be held free between the two opposed side rails when arranged in the shaker in use, but can be much less rigid when not in use: between a third and a quarter as rigid when a single centrally mounted member is used and between a ninth and a sixteenth of the rigidity with two members arranged at approximately evenly spaced intervals across the width of the panel. Advantageously, the support rib is fixed to the perforate plate. Preferably, a multiplicity of the support ribs extend across the panel. Advantageously, the perforate plate comprises a series of panel ribs formed in the perforate plate, the support ribs aligned with and underneath the panel ribs. 
     Preferably, the panel comprises folded portions. Advantageously, the folded portions are perimeter portions. The folded perimeter portions may be located along the sides of a rectangular panel, which may increase the rigidity of the panel. The folded portions may be located at the ends of a rectangular panel, which may be folded to increase the rigidity of the panel and also to provide an interface between adjacent panels or to provide a holder for a seal for an interface between adjacent panels. 
     Advantageously, the folded portions form the apertures. The folded portions may form flanges which increase the overall rigidity of the panel, especially if all or a substantial number of the apertures are formed in this way. 
     Preferably, the panel has side portions, which are not provided with apertures. In a shale shaker provided with inflatable bladders or wedges as means for fixing the screen assembly in the shale shaker, the side portions are blinded by the means. 
     Advantageously, the at least one layer of screening material is adhered to the side portions of the perforate plate. Preferably, the at least one layer of screening material is adhered to at least a portion of the perforate plate. Advantageously, the at least one layer of screening material is adhered to the area provided with apertures. Preferably, the panel further comprises a second layer of screening material of substantially the same mesh size. Advantageously, a coarse mesh backing screen is arranged between the at least one layer of screening material and the perforate plate. The coarse mesh backing screen may have larger openings and larger wires to support the screening material. 
     Preferably, the support structure comprises a plurality of support ribs on which, in use the panel is pushed on to. Advantageously, the support structure has a crowned profile and the panel is pushed down over the support structure by a clamping mechanism at an outer perimeter of the panel. Preferably, the panel is semi-flexible, preferably such that the panel may change shape when a force is applied to it by the clamping mechanism of the shale shaker. The clamping mechanism may provide a tonne of force over the side edges of the screen assemblies arranged in the shale shaker, which may cover 3 to 12 m over 1 to 2 cm in width through a pneumatic hose. Advantageously, the panel is flexible, wherein it is easy to apply the layers of screening material to the panel and a tension in the layers of screening material is held by the panel, advantageously, such that the panel does not bend under then tension in the layers of screening material. Advantageously, wherein it is easy to transport the panel with at least one layer of screening material arranged thereon. 
     The present invention also provides a shale shaker comprising a screen assembly of the invention, the shale shaker comprising a basket, a vibratory mechanism and a clamping mechanism for fixing the screen assembly to the basket. Preferably, the clamping mechanism firmly fixes the panel to the support structure. Advantageously, the clamping mechanism comprises a pneumatic means. Preferably, the pneumatic means comprises a pneumatic hose. Alternatively, a hydraulic hose could be utilized and preferably, provided with an accumulator. 
     The present invention also provides a method for fitting a screen assembly in a shale shaker, the screen assembly comprising a panel having at least one layer of mesh thereon and a support structure, the panel further comprising at least one support rib arranged, the method comprising the steps of inserting the screen assembly into a clamping mechanism of a shale shaker, operating the clamping mechanism wherein at least part of a perimeter of said panel of said screen assembly is pushed down over at least one member arranged within the perimeter of the panel such that the at least one support rib and the panel is deflected over the at least one member to define at least two spans. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a rear end view of a screen assembly in accordance with the present invention, partly in cross-section and arranged in clamping rails of a shale shaker, the screen assembly comprising a panel, a support structure and a pull down member; 
         FIG. 1A  is a cross-sectional view of the panel shown in  FIG. 1 ; 
         FIG. 1B  is an end view of the pull down member shown in  FIG. 1 ; 
         FIG. 1C  is an end view of the support structure shown in  FIG. 1 ; 
         FIG. 1D  is a top plan view of the panel shown in  FIG. 1A  fitted to the pull down member shown in  FIG. 1B ; 
         FIG. 1E  is an underneath view of the panel shown in  FIG. 1A  fitted to the pull down member shown in  FIG. 1B ; 
         FIG. 1F  is a top plan view of the support structure as shown in  FIG. 1C ; 
         FIG. 1G  is an enlarged top view of part of the panel shown in  FIGS. 1 and 1D ; 
         FIG. 1H  is a top plan view of a blank used in the construction of a panel in accordance with the present invention; 
         FIG. 1I  is a template used in the construction of the panel of the present invention; 
         FIG. 2  is an exploded view of a screen assembly in accordance with the present invention, the screen assembly comprising layers of screening material, a panel and a support structure; 
         FIG. 2A  is an end schematic view of part of the screen assembly shown in  FIG. 2 , the screen assembly arranged in a clamping rail of a shale shaker; and 
         FIG. 2B  is an end schematic view of part of the screen assembly shown in  FIG. 2 , the screen assembly clamped in a clamping rail of a shale shaker; 
         FIG. 3  is a perspective view of a screen assembly in accordance with the invention in a shale shaker; 
         FIG. 4  is an top plan view of a panel in accordance with the invention, with parts cut away to show; 
         FIG. 4A  is a cross-sectional view of the panel shown in  FIG. 4  taken along line IV A-IV A; 
         FIG. 4B  is an end view of the panel shown in  FIG. 4 ; 
         FIG. 4C  is a perspective view of a support structure in accordance with the present invention, for use with the panel of  FIG. 4 ; 
         FIG. 4D  is an end schematic view of a screen assembly comprising the panel shown in  FIG. 4  and the support structure shown in  FIG. 4C , the screen assembly shown in a clamping rail of a shale shaker; 
         FIG. 4E  is an end schematic view of the screen assembly shown in  FIG. 4D , clamped in a clamping rail of a shale shaker; 
         FIG. 5  is a perspective view of a support structure in accordance with the present invention; 
         FIG. 5A  is an end schematic view of a screen assembly comprising the support structure shown in  FIG. 5  and a panel, the screen assembly shown in a clamping rail of a shale shaker; 
         FIG. 5B  is an end schematic view of the screen assembly shown in  FIG. 5A , clamped in a clamping rail of a shale shaker; 
         FIG. 5C  is an enlarged end view of a preferred structural rib; 
         FIG. 5D  is an end view of a preferred support member; and 
         FIG. 5E  is a side view of part of the preferred support member shown in  FIG. 5D . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , there is shown a screen assembly, generally identified by reference numeral  100 . The screen assembly  100  comprises a panel  101 , a support structure  102  and a pull down member  103 . In use, the panel  101  would have at least one layer of screening material adhered or otherwise attached thereto. Typically, each layer of screening material comprises a layer of wire mesh. Typically, the panel  101  would have three layers of screening material lying one over the other, the lowermost layer of screening having larger openings and larger wires. In use, the screen assembly  100  is arranged in clamping rails  104  and  105  of a shale shaker. 
     Referring to  FIGS. 1A ,  1 D and  1 G, the panel  101  is made from a 1.5 mm mild steel plate. The panel  101  comprises an area  106  provided with a plurality of apertures, a left side portion  107  provided with no apertures and a right side portion  108  provided with no apertures. The plurality of apertures in area  106  comprises a plurality of triangular apertures and a plurality of circular openings. 
     The panel  101  is formed from a blank shown in  FIG. 1H . Lines  110  and  111  and fold lines  112  and  113  indicate the boundary of area  106  which will be provided with the plurality of apertures. The area  106 , the left side portion  107  and right side portion  108 , all lie in the same plane to form a flat top surface. Left side portion  107  and right side portion  108  extend the entire length of the panel  101 . Wing portions  114  and  115  approximately  1  cm wide extend the entire length of the panel  101 . The wing portions  114  and  115  are folded downwardly to stand approximately at right angles to the top surface. The forward end of the panel  101  has a forward end portion  116  extending the width of the panel  101  and is folded downwardly along fold line  112  to be perpendicular to the top surface of the panel  101 . The trailing end of the panel  101  has a rear end portion  117  folded downwardly along fold line  113 , such that the rear end portion  117  lies perpendicularly to the top surface of the panel  101 . A screen interface, such as those disclosed in PCT Publication Number WO 01/97947 may be used at both the front and rear of the panel. The folded wing portions  114  and  115  and the folded end portions  116  and  117  meet at their respective side edges, at which they may be welded together, soldered or otherwise joined. 
     The area  106  of the blank shown in  FIG. 1H  has a plurality of apertures including a plurality of triangular apertures and a plurality of circular apertures formed therein. One of the triangular apertures is identified by reference numeral  118  and one of the circular openings is identified by reference numeral  119 . The triangular aperture  118  is formed by first punching, laser cutting, sawing, drilling, milling or casting the blank with an opening  120 , in the shape shown in the template shown in  FIG. 1I . The shape comprises three semi circular ends  121 ,  122  and  123  each arranged within and close to where a respective vertex  124 ,  125  and  126  of the triangular aperture  118  is to be formed, as shown in  FIG. 1G ; and a small triangular opening  127  concentric with the triangular aperture  118  to be formed and slots  128 ,  129  and  130  link the semi circular ends  121 ,  122  and  123  to form structural portions  131 ,  132  and  133 . The structural portions  131 ,  132  and  133  are folded downwardly along fold line  134 , over a form tool (not shown) having a similar profile to the fold line  134 . The structural portions  131 ,  132  and  133  are folded by the form tool to an angle of approximately 65.degree. to the surface of the panel  101  to form edges  131   a,    132   a,  and  133   a.  The areas  135 ,  136  and  137  of panel  101  bounding the semi circular ends  121 ,  122  and  123  are also folded downwardly. 
     Referring back to  FIGS. 1D and 1G , triangular apertures, such as triangular aperture  118 , are arranged in ten full sets of rows in the panel  101  and one further row of a set. A first set  138  comprises a first row  139  having a rearwardly pointing triangular aperture  118  and a forwardly pointing triangular aperture  140  adjacent thereto, such that folded structural portion  132  and a folded structural portion  141  of the forwardly pointing triangular aperture  140  form a panel rib  142 , approximately 2.3 mm wide. An apex  143  of the forwardly pointing triangular aperture  140  is rearwardly offset by approximately 2.3 mm?? from a base edge  131   a  of the rearwardly pointing triangular aperture  118 . The first row  139  comprises twelve forwardly pointing triangular apertures interspaced by twelve rearwardly pointing triangular apertures. The first set  138  also comprises a second row  144 , which is a mirror image of the first row  139  about line A-A. A structural portion  145  of forwardly pointing triangular aperture  140  of the first row  139  and a structural portion  146  of a rearwardly pointing triangular aperture  147  of the second row  144 , form a panel rib  148 . The underside of panel rib  148 , the structural portion  145  and the structural portion  146  form a channel. The panel rib  148  is in line with panel ribs  149  to  159  in the first set  138 , the undersides of which form a channel which extends the width of the panel  101 . Circular opening  119  is drilled, punched, laser cut or otherwise formed in the panel  101  between vertices  125 ,  160 ,  161  and  162  of rearward pointing triangular aperture  118 , forward pointing triangular aperture  163 , forward pointing triangular aperture  140  and rearward pointing triangular aperture  147  respectively. A segment opening  164  arranged between rearward pointing triangular aperture  118 , forward pointing triangular aperture  163  and circular hole  119  is punched, laser cut or otherwise formed in the panel  101 , having a straight portion following line  110  of the blank, shown in  FIG. 1H  and a curved portion extending toward the rearward pointing triangular aperture  118 , forward pointing triangular aperture  163  and circular opening  119 . 
     Similarly, circular opening  165  is drilled, punched, laser cut or otherwise formed in the panel  101  between vertices  166 ,  167 ,  168 ,  169 ,  170  and  171  of forward pointing triangular aperture  140 , rearward pointing triangular aperture  172 , forward pointing triangular aperture  173 , rearward pointing triangular aperture  174 , forward pointing triangular aperture  175 , and rearward pointing triangular aperture  147  respectively. 
     Referring to  FIG. 1A , the panel  101  further comprises two inverted T-shape rails  176  and  177 , arranged longitudinally from the forward end portion  116  to the rear end portion  117 . The inverted T-shape rails  176  and  177  are spaced at intermediate the left side and right side of the panel  101 , preferably, each located at a third of the width between the left and right sides. The inverted T-shape rails  176  and  177  are welded to the panel  101  at the root of the T. 
     Referring to  FIGS. 1 ,  1 B,  1 D and  1 E, the pull down member  103  comprises twelve substantially identical ribs  178  to  189 . Rib  178  is made from 3 mm steel plate. The rib  178  has a body portion  190 , a left arm  192  extending along a top of the body portion provided with a head  193 ; and a right arm  194  extending along a top of the body portion provided with a head  195 . A left side runner  196  is welded to the head  193  and a right side runner  197  is welded to the head  195 . The left side runner  196  and right side runner  197  extend the entire length of the screen assembly  100 . Two receiving rails  198  and  199  are welded in respective recesses  200  and  201  in the body portion  190 , intermediate the rib  178 , preferably, each located at a third of the length of the rib from either end thereof. The receiving rails  198  and  199  are of a C-shape cross-section to receive the inverted T-rails  176  and  177 . The eleven other ribs  179  to  189  have corresponding heads, which are welded at intervals therealong to the left side runner  196  and right side runner  197  respectively and corresponding recesses in which receiving rails  198  and  199  are welded. The rib  178  is at a rear end; rib  179  is arranged slightly less than two intervals from rib  178 ; rib  180  is arranged two intervals from rib  179 ; rib  181  is arranged two intervals from rib  180 ; rib  182  is arranged two intervals from rib  181 ; rib  183  is arranged two intervals from rib  182 ; rib  184  is arranged two intervals from rib  183 ; rib  185  is arranged two intervals from rib  184 ; rib  186  is arranged two intervals from rib  185 ; rib  187  is arranged two intervals from rib  186 ; rib  188  is arranged two intervals from rib  187 ; rib  189  is arranged slightly less than one interval from rib  187 . An interval being equal to the width of a row  139 ,  144  in the panel  101 ; and two intervals being equal to the width of a set of rows  138  in the panel  101 . 
     Referring to  FIGS. 1 ,  1 C and  1 F the support structure  102  comprises twelve substantially identical support ribs  202  to  213 . Support rib  202  is made from 3 mm steel plate. The support rib  202  has a body portion  214 , a left arm  215  extending from the body portion having a bottom face  216 , and a right arm  217  having a bottom face  218 . A left side support bar  219  is welded in recess  220  in the left side of the body portion  214  and a right side support bar  221  is welded in recess  222  in a right side of the body portion  214 . The left side support bar  219  and right side support bar  221  extend the entire length of the screen assembly  100 . Two recesses  223  and  224  in the body portion  214  are located intermediate the ends of the rib  202 , preferably, each located at a third of the length of the rib  214  from either end thereof. The top edge  225  of the support rib  202  is provided with a chamfer. The eleven other ribs  203  to  213  are welded into corresponding recesses  220  and  221 , at intervals along the left side support bar  219  and right side support bar  221  respectively. The support rib  202  is at a rear end of the screen assembly  100 . Support rib  203  is arranged one interval from support rib  202 ; support rib  204  is arranged two intervals from support rib  203 ; support rib  205  is arranged two intervals from support rib  204 ; support rib  206  is arranged two intervals from support rib  205 ; support rib  207  is arranged two intervals from support rib  206 ; support rib  208  is arranged two intervals from support rib  207 ; support rib  209  is arranged two intervals from support rib  208 ; support rib  210  is arranged two intervals from support rib  209 ; support rib  211  is arranged two intervals from support rib  210 ; support rib  212  is arranged two intervals from support rib  211 ; support rib  213  is arranged two intervals from support rib  212 . An interval being equal to the width of a row  139 ,  144  in the panel  101 ; and two intervals being equal to the width of a set of rows  138  in the panel  101 . 
     The screen assembly  100  is assembled by sliding the inverted T-shape rails  176  and  177  of the panel  101  into the receiving rails  198  and  199  of the pull down member  103 . The pull down member  103  is located in the support structure  102 . The ribs  178  to  189  are inserted into support ribs  202  to  213 . End ribs  178  and  189  are inserted inside support ribs  202  and  213 . Rib  179  is arranged one interval from support rib  203  and one interval from support rib  204 ; Rib  180  is arranged one interval from support rib  204  and one interval from support rib  205 ; rib  181  is arranged one interval from support rib  205  and one interval from support rib  206 ; rib  182  is arranged one interval from support rib  206  and one interval from support rib  207 ; rib  183  is arranged one interval from support rib  207  and one interval from support rib  208 ; rib  184  is arranged one interval from support rib  208  and one interval from support rib  209 ; rib  185  is arranged one interval from support rib  209  and one interval from support rib  210 ; rib  186  is arranged one interval from support rib  210  and one interval from support rib  211 ; rib  187  is arranged one interval from support rib  211  and one interval from support rib  212 ; rib  188  is arranged one interval from support rib  212  and one interval from support rib  213  and slightly less than one interval from rib  189 . The support ribs  203  to  212  align underneath the lines of panel ribs  226  to  235  between structural portions folded to form the edge of the apertures. Support rib  202  aligns with line of panel ribs  236  and support rib  213  aligns with line of panel ribs  237 . 
     The panel  101  has at least one layer screening mesh arranged thereon. The layer of screening mesh may be tensioned and adhered to the outer perimeter of the panel  101  and to all of the panel ribs. Preferably, at least three layers are applied. The layers may be of the same mesh grade or of different mesh grades. Preferably, a layer of screening mesh having larger openings and larger wires lies beneath layers of fine mesh. 
     In use, the screen assembly  100  having layers of mesh (not shown) arranged on the panel, is slid into clamping rails  104  and  105  of a shale shaker. The clamping rails  104  and  105  comprise a C-shape rail  240  and  241  having a bottom surface  242  and  243  on which the support structure  102  of the screen assembly  100  rests. The C-shape rail  240  and  241  also has a pneumatically inflatable bladder  244  and  245  fixed to an upper part  246  and  247  of the C-shape rail. The inflatable bladder  244 ,  245  is inflated which pushes down on side portions  107  and  108  of the panel  101 , pushing the panel  101  on to the top edges  225  of the twelve supporting ribs  202  to  213 . The pneumatic bladder also engages side runners  196  and  197  of the pull down member  103 , which pushes the pull down member  103  downwardly, pulling the inverted T-shape rails downwardly within recesses  223  and  224 . The panel  101  is pulled down along the inverted T-shape rail to pull the panel  101  down on to the supporting ribs  202  to  213 . The supporting ribs  202  to  213  lie underneath the circular openings  119 ,  165 , which partially blinds the openings, however, this is not significant as the ribs are below the level of the top surface of the layers of screening mesh. 
     The downwardly folded wings  114  and  115  of the panel  101  locate over the ends of the supporting ribs  202  to  213  and forward end portion  116  and rear end portion  117  are located over supporting rib  213  and  202 . 
     Drilling mud having solids entrained therein is introduced at a feed end of the shale shaker and is shaken along the layers of mesh on the screen assembly. Fluid and small particles pass through the layers of mesh on the screen and through the triangular apertures and the circular openings in the panel  101  and past the pull down member  103  and the support structure  102  and into a receiver (not shown). The larger solids pass over the layers of screening material and out of a discharge end of the shale shaker into a skip or ditch. 
     The most likely component to wear out or fail first, is the layers of screening material arranged on the panel  101 . The screen assembly  100  is removed from the C-shape rails  104  and  105 . The panel  101  having layers of worn out screening mesh thereon and the pull down member  103  may be lifted from frictional engagement with the support structure  102 . The panel  101  is slid out from receiving rails  198  and  199  and replaced with a new panel having layers screen mesh thereon. The rails of the new panel are slid into the receiving rails of the pull down member  103 . The pull down member  103  with the new panel is placed on the original support structure  102  and slid back into the shale shaker. 
     It is envisaged that the panel may be of any known type, such as 1.5 mm to 3 mm steel, aluminium or plastics material plate with a multiplicity of apertures punched therein or perforated plate, not having folded edges to the apertures. The apertures may be oblong, pentagonal, hexagonal, heptagonal, octagonal, circular or any other shape. 
     Referring to  FIG. 2 , there is shown a screen assembly comprising at least one layer of screening material  300  overlying a panel  301  and a support structure  302 . The panel  301  comprises a flat 3 mm mild steel plate. The panel  301  has left and right side portions  304  and  305  which are not provided with apertures and a central portion  307  provided with a multiplicity of apertures and openings arranged in the same configuration as described above with reference to panel  101  shown in  FIGS. 1 ,  1 D and  1 E. It should be noted that the left and right side portions  304  and  305  are wider than the left and right side portions  107  and  108  in the panel  101 . 
     The support structure  302  comprises a left side plate  308  and a right side plate  309  and twelve substantially identical crowned ribs  310  to  321  welded to the left and right side plates  308 ,  309 . The crowned rib  310  is made from  3  mm mild steel plate having a crowned top edge  322 , whose central point  323  is approximately 5 mm above a horizontal line joining two top corners  324  and  325  of the crowned rib  310 . 
     The crowned ribs  310  to  321  are spaced along the left and right side plates at a distance equal to two intervals, an interval as defined with reference to  FIGS. 1 ,  1 D and  1 E above with reference to the panel  101 , as being equal to the width of a row of apertures in the panel  301 ; and two intervals being equal to the width of a set of rows (two rows) in the panel  301 . 
     In use, the panel  310  having layers of mesh  300  adhered thereto, is laid on to the top of the crowned ribs  310  to  321 . Preferably, in-line panel ribs  326  to  329  (others not shown) lying parallel to the rear edge  306  of panel  301 , each lie over the crowned ribs  310  to  321 , such that, the crowned ribs do not substantially occlude the apertures and openings. As shown in  FIG. 2A , the screen assembly is slid into clamping rails  330  (only one shown) arranged on each side of a basket of a shale shaker. The clamping rail  330  comprises a C-shape rail  331  having a bottom surface  332  on which the support structure  302  of the screen assembly rests. The C-shape rail  330  also has a pneumatically inflatable bladder  333  fixed to an upper part  334  of the C-shape rail  330 . The C-shape rail  330  is fixed to the side of a left side wall  335  of the basket to receive the left side of the screen assembly. A further C-shape rail (not shown) is fixed to a right side wall (not shown) of the basket to receive the right hand side of the screen assembly. Once the screen assembly is slid into the C-shape rails  330  and (not shown), the pneumatically inflatable bladder  333  is inflated which pushes down on left and right side portions  304  and  305  pushing and holding the panel  301  over the crowned ribs  310  to  321 , rigidly fixing the panel  301 , as shown in  FIG. 2B . 
     Drilling mud having solids entrained therein is introduced at a feed end of the shale shaker and is shaken along the layers of mesh on the screen assembly. Fluid and small particles pass through the layers of mesh  300  and the triangular apertures and the circular openings in the panel  301  and past the support structure  302  and into a receiver (not shown). The larger solids pass over the layers of screening material and out of a discharge end of the shale shaker into a skip or ditch. 
     The most likely component to wear out or fail first, is the layers of screening material  300 . The screen assembly may be removed from the C-shape rails  330  and the panel  301  having layers of worn out screening mesh arranged thereon and replaced with a new panel having layers screen mesh thereon. The new panel is placed on the original support structure  302  and slid back into the shale shaker. 
     A further embodiment of a screen assembly is shown in  FIG. 3 . The screen assembly  400  comprises a panel  401  on which layers of screening material (not shown) are arranged, and a support structure  402 . The support structure is substantially identical to the support structure  402 , save for the left and right side plates  404  and (not shown), which are arranged in a recesses  405  and (not shown) near to the ends of the crowned ribs  406 . A portion  407  has been removed from each crowned rib  405 , which amongst other things, facilitates insertion of the screen assembly in clamping rails  408 ,  408   a  of a shale shaker  409 . 
     The panel  401  is of the type shown in  FIGS. 1 ,  1 D and  1 E, save for the inverted T-shape rails, which are omitted, and larger left and right side portions  410  and  410   a  provided with no apertures or openings. The panel  401  has folded left wing portion  411  and folded right wing portion (not shown), folded front end (not shown) and a folded rear end  412 . 
     In use, the panel  401  has layers of mesh adhered thereto, and is laid on to the top of the crowned ribs  406 . Preferably, in-line panel ribs lying parallel to the folded rear end  412  of panel  401 , each lie over the crowned ribs like crowned rib  406 , such that, the crowned ribs do not substantially occlude the apertures and openings. The screen assembly is slid into clamping rails  408 ,  408   a  arranged on each side of a basket  413  of a shale shaker  409 . The clamping rails  408 ,  409  comprise a C-shape rails each having a bottom surface on which the support structure  402  of the screen assembly rests. Each of the C-shape rails also has a pneumatically inflatable bladder  414  fixed to an upper part  334  of the C-shape rail  330 . Once the screen assembly  400  is slid into the clamping rails  408 ,  408   a,  the pneumatically inflatable bladder  414  is inflated which pushes down on left and right side portions  410  and (not shown) pushing and holding the panel  401  over the crowned ribs, rigidly fixing the panel  301 . The folded left wing portion  411  and folded right wing portion (not shown), folded front end (not shown) and a folded rear end  412  fit about the support structure  406 . 
     Referring to  FIGS. 4 ,  4 A and  4 B, there is shown a panel  500  comprising a perforate plate  501  having at least one layer of screening material  502  adhered to panel ribs  503  defining apertures  504  in the perforate plate  501 . The perforate plate  501  may be made from flat 1 mm to 3 mm mild steel plate. The apertures  504  are preferably triangular and are preferably arranged in the configuration as described above with reference to panel  101  shown in  FIGS. 1 ,  1 D and  1 E. The apertures  504  in the perforate plate  501  may be punched or cut with a laser in the shape of a triangle. Alternatively, the apertures  504  are cut in the pattern described with reference to  FIG. 1I  and flanges are pressed as described with reference to  FIG. 1G . 
     The panel  500  further comprises supporting ribs  505 . The supporting ribs  505  are arranged between sides  506  and  507  of the panel  500  and are spaced along the panel  500  at a distance equal to one interval, an interval as defined with reference to  FIGS. 1 ,  1 D and  1 E above with reference to the panel  101 , as being equal to the width of a row of apertures in the perforate plate  501 . The supporting ribs  505  are thus aligned with the panel ribs  503   a  between the triangular apertures running between sides  506  and  507 , such that the support ribs  505  do not substantially blind the apertures  504  in the perforate plate  501 . The ribs comprise tabs (not shown) along their lengths, which are folded at approximately right angles or preferably, at between 120.degree. and 60.degree. to facilitate a lazy-7 profile. The tabs are approximately 1 cm long and 0.4 cm wide, such the flat part of the tab is substantially the same width as the width of the panel ribs  503   a.    
     Side portions  508  and  509  of the perforate plate  501  are not provided with apertures  504  and are folded to form substantially vertical sides and are folded back over the bottoms  510  of the ribs  505  and may be welded thereto. The ribs  505  preferably have a cross-section similar to a lazy-7 and are preferably between 0.5 and 2.5 cm deep, and most preferably 1 to 1.8 cm deep and most preferably 1.1 cm and advantageously made from 0.5 to 1.5 mm thick steel plate. However, the ribs  505  may simply be vertical strips welded to the perforate plate. End portion  511  of the perforate plate  501  is folded to form a ledge  513  and an upwardly angled portion  514  to form a longitudinal recess  515  to receive a seal member (not shown). End portion  512  of the perforate plate  501  is folded to form a ledge  516  and an end face  517 . 
     Support members  518  and  519  are spaced along the length of the panel  500 . The support members  518  and  519  may be spaced equidistant along the width of the panel  500 , or preferably, equidistant between the unsupported width when arranged in a shale shaker. The support members  518  and  519  preferably take the form of rails having a curved channel  520  extending along the length of the panel  500 . The support members are preferably made from sheet metal folded into a lazy-W, or formed from cut box section steel tubing and pressed to form the curved recess  520 . 
       FIG. 4C  shows a support structure  600  comprising box-section steel tubing rectangular perimeter  601  having six structural ribs  601  also of box-section steel tubing arranged between two side members  603  and  604  of the perimeter  601 . Two structural support members  605  and  606  of circular-section steel tubing arranged parallel to the two side members  603  and  604  and spaced between them to align with the corresponding support members  518  and  519  of the panel  500 . The circular-section steel tubes  605  and  606  are welded to ends  607  and  608  of the perimeter  601  and to the structural ribs  602 . 
     In use, the support structure  600  is slid into side rails  650  and  651  of a shale shaker (not shown) and rests on surfaces  652  and  653  of the rails. The shale shaker may be of the type sold under the brand name VSM 300 by Varco Limited. The panel  500  is then slid into the rails  650  and  651  such that the support members  518  and  519  run along respective structural support members  605  and  606  of the support structure  600 . The distance between the bottom of the panel  500  and the top of the box-section steel tubing perimeter  601 , is preferably 1 mm to 30 mm, and most preferably 5 mm to 10 mm. Inflatable bladders  654  and  655  are arranged in the rails  650  and  651 . Upon inflation of the inflatable bladders  654 ,  655 , the sides  506  and  507  of the panel  500  are pushed down on to the box-section steel tubing perimeter  601 , deflecting the panel  500  over the circular-section steel tubing  605  and  606 . 
       FIG. 5  shows a support structure  700  generally similar to the support structure  600  shown in  FIG. 4C , save for the structural support member  701 , of which there is only one arranged substantially equidistant between sides  703  and  704  of the support structure  700 . 
     In use, the support structure  700  is inserted in side rails  750  and  751  of a shale shaker (not shown) and rests on surfaces  752  and  753  of the rails. A panel  800 , is generally similar to the panel  500  shown in  FIG. 4 , save for the support member  801 , of which there is only one arranged substantially equidistant between sides  802  and  803  of the panel  800 . The panel  800  is then slid into the rails  750  and  751  such that the support member  801  runs along the structural support member  701  of the support structure  700 . The distance between the bottom of the panel  800  and the top of the box-section steel tubing perimeter  701 , is preferably 1 mm to 30 mm, and most preferably 5 mm to 10 mm. Inflatable bladders  754  and  755  are arranged in the rails  750  and  751 . Upon inflation of the inflatable bladders  754 ,  755 , the sides  802  and  803  of the panel  800  are pushed down on to the box-section steel tubing perimeter  705 , deflecting the panel  500  over the circular-section steel tubing  605  and  606  such that the bottom of the sides  802  and  803  of the panel  800  abut the top of the box-section steel tubing perimeter  701 . 
       FIG. 5C  shows a preferred supporting rib  505  for use in panel  500  or  800 . The supporting rib  505  is formed from a strip of steel and folded to form a ledge  523  which is spot welded to the panel ribs  503   a.  Alternatively or additionally, the supporting rib  505  may be adhered, soldered, otherwise welded or otherwise attached to the panel ribs  503   a.  A lower portion  524  is folded back against main body  525  of the supporting rib  505 . During manufacture of the panel  500 ,  800 , the ledges  523  of the supporting ribs  505 , the panel ribs  503   a,  and the rest of the top surface of the perforate plate  501  are coated in a powder coating, preferably by being dipped in powder coating. The perforate plate  501  and the supporting ribs  505  are placed in a press and at least one, preferably two or three layers of mesh  502 , at least one of which is a screening mesh and one may be a supporting or backing mesh, are placed on the perforate plate  501 . Preferably, the at least one layer of mesh  502  is tensioned. The press presses the at least one layer of mesh  501 , the perforate plate  501  and the supporting ribs  505  together and heat is applied sufficient to melt the powder coating to adhere the at least one layer of mesh  502  to the panel ribs  503 , 503   a  and to the rest of the top surface of the perforate plate  501  and adheres the ledges  523  of the supporting ribs  505  to the panel ribs  503 , 503   a  of the perforate plate  501  and to side portions  508 ,  509 . The ledges  523  are slightly wider than the width of the panel ribs  503   a  to provide a large surface area for the powder coating to adhere the panel ribs  503   a  to the supporting ribs  505 . The ledges are preferably 1 mm wider than the width of the panel ribs  503   a,  which are preferably 3.8 mm wide. Preferably, at least one of the at least one layer of mesh  501 , the perforate plate  501  and the supporting ribs  505  is preheated. 
       FIG. 5D  shows a preferred supporting member  518  for use in the panel  500  or  800 , the supporting member  518  having splayed sides  526  and  527  splayed at an angle of 15.degree. The sides stand approximately 1.4 cm high. The supporting member a curved channel  520 . The spayed sides  526  and  527  have slots  528  spaced at points equal to the spacing between the supporting ribs  505  such that the slots fit over the supporting ribs  505 . Openings  529  are made in tabs  530  formed between the slots  528 . The curved channel  520  extends along substantially the entire length of the panel  500 . 
     The layers of mesh used in any of the embodiments shown herein and in any embodiment of the invention, may be pre-tensioned and adhered, bonded or otherwise attached to the panel. The layer of mesh may be bonded using a heat activated powder. 
     In accordance with the present invention, there is provided a screen assembly for a shale shaker, the screen assembly comprising a panel and a support structure, the panel having an area provided with a multiplicity of apertures and at least one layer of screening material arranged over the multiplicity of apertures, wherein said panel is removable from said support structure. The layers of screening material are the most likely components of a screen assembly to fail in use. A screen assembly of the present invention allows replacement of the panel with layers of screening material attached thereto, without having to replace the entire screen assembly. 
     Advantageously, there is a friction fit between the panel and the support structure. The panel may be provided with wing portions which fit over the support structure to provide a friction fit, such that the panel may be aligned thereon. 
     Advantageously, the screen assembly further comprises a pull down member located within the panel for pulling the panel on to the support structure. Preferably, the pull down member is linked to said panel at at least two intermediate points. Preferably, the pull down member is releasably connected to the panel. Advantageously, the pull down member comprises a rail and preferably, the panel comprises a rail, which co-operate to enable the pull down member to pull on said panel. Preferably, the panel is rectangular and the pull down member is located between sides of the rectangular panel Advantageously, the pull down member is operated by the clamping mechanism preferably, such that, in use, the clamping mechanism pushes down on the pull down member, which pulls the panel on to the support structure. Most advantageously, at least a portion of the perimeter of the panel is, in use, arranged in the clamping mechanism, such that the perimeter of the panel is pushed on to the support structure by the clamping mechanism. Preferably, the pull down member comprises at least one rib, which advantageously extends between sides of the rectangular panel. Advantageously, the at least one rib has two ends each having a top face which, in use is contactable by said clamping mechanism. Preferably, the pull down member comprises a plurality of ribs linked by a side runner on each of said two ends to form said top face which, in use is contactable by said clamping mechanism. Advantageously, the support structure comprises a plurality of support ribs on which, in use the panel is pushed or pulled on to. Preferably, each support rib has a top edge which is flat, in use the panel is pushed or pulled on to the flat top edge. 
     The present invention also provides a panel for the screen assembly of the invention, the panel having a perimeter comprising a multiplicity of apertures and a member arranged inside said perimeter for reception with a pull down member to pull said panel on to a support structure. 
     The present invention also provides a support structure for a screen assembly comprising a plurality of substantially parallel support ribs having top edges, characterised in that said top edges are flat. 
     The present invention also provides a shale shaker comprising a basket, a vibratory mechanism and a set of support ribs arranged across said basket characterised in that said ribs have flat top edges and a clamping mechanism arranged about the basket. Preferably, the clamping mechanism comprises a pneumatic bladder. 
     A method for fitting a screen assembly in a shale shaker, the screen assembly comprising a panel having at least one layer of mesh arranged thereon and a support structure, the method comprising the steps of inserting the screen assembly into a clamping mechanism of a shale shaker, operating the clamping mechanism wherein at least part of a perimeter of said panel of said screen assembly is pushed down on to said support structure, the screen assembly further comprises a pull down member, and the method further comprises the step of operating the clamping mechanism depresses a pull down member, pulling intermediate parts of said panel on to said support structure. 
     Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.