Abstract:
A screen assembly for a shale shaker, the screen assembly including a support structure and at least one layer of screening material thereon. The support structure includes structural members having a thermal expansion coefficient and the at least one layer of screening material has a thermal expansion coefficient. The thermal expansion coefficient of the structural members is substantially equal to the thermal expansion coefficient of the at least one layer of screening material. A method for making a support structure for a screen assembly is also provided. The method includes placing at least one support member in a mold whereupon a flowable plastics material is injected into the mould, whereupon the flowable plastics material hardens, at least a portion of the flowable plastics material forming a brace.

Description:
BACKGROUND 
       [0001]    The present invention relates to a screen assembly for a shale shaker or vibratory separator and a method for making a screen assembly. 
       SUMMARY 
       [0002]    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, which is rotated to bore the borehole through a formation. 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 density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. The drilling mud contains inter alia expensive synthetic oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia the solids to be removed from the drilling mud. This is achieved by processing the drilling mud. The first part of the process is to separate solids from the solids laden drilling mud. This is at least partly achieved with a vibratory separator, such as those shale shakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328. Further processing equipment such as centrifuges and hydrocyclones may be used to further clean the mud of solids. The solids are covered in contaminates and residues. It is not uncommon to have 30 to 100 m 3  of drilling fluid in circulation in a borehole. 
         [0003]    The resultant solids, known herein as “drill cuttings” are processed to remove substantially all of the residues and contaminates from the solids. The solids can then be disposed of in a landfill site or by dumping at sea in the environment from which the solids came. Alternatively, the solids may be used as a material in the construction industry or have other industrial uses. 
         [0004]    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 over the open bottom of the basket. The screens may be substantially planar or have a slight crown. 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 separation of the drilling mud from the solids, the drilling mud passing through the screens and the solids above the screens. The shaking motion also 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. 
         [0005]    The rectangular screens may be arranged at an angle to horizontal, such as a seven degrees incline from the feed end to the discharge end of the shale shaker. The angle may be adjustable. The screens are generally fixed in the basket and the basket is adjustable to adjust the angle of the screens relative to horizontal. The flow of solids laden drilling fluid may form a pool on the inclined screens. The action of the vibratory mechanism induces solids to climb the inclined screens to the discharge end of the shaker and into the ditch or skip. 
         [0006]    Generally, a vibratory mechanism inducing a circular vibration will tend to throw solids from the screen into the air in circular motions, which is often in shale shakers having a horizontal screen deck. A vibratory mechanism inducing an elliptical motion will induce the solids to move in a direction of the longest chord of the ellipse, which is often used in shale shakers having an inclined screen deck, such that in use movement of solids up the inclined screen deck is facilitated. A shale shaker having a vibratory mechanism inducing a very thin ellipse is known as a linear shale shaker and induces fast movement of solids along the screen, although the screen tends to suffer fast degradation due to sudden deceleration of the solids as they meet the screen. 
         [0007]    The screens used in a shale shaker experience high acceleration values and heavy loading when processing drilling fluids. The high acceleration values are required in order to induce throughput through the screens and in order to convey the solids over the screen for discharge. 
         [0008]    The shale shaker vibratory drive apparatus is sized in order to induce an optimal stroke profile based upon the total mass of assembly including the vibratory drive apparatus, basket, screens and solids laden drilling fluid therein i.e. all of the vibrated components, essentially the sprung mass. The screens are clamped or otherwise fastened to the basket and the screens support the solids laden drilling fluid to be processed. 
         [0009]    The motion induced into the assembly is typically a simple harmonic motion, whereby starting from its lowest point the vibratory drive, basket, screens and solids laden drilling fluids is accelerated upwards typically in an elliptical motion, the longest chord of the ellipse set at around forty-five degrees towards the front of the basket by high positive acceleration forces reducing towards mid stroke where the acceleration values reduce to zero then negative deceleration occurs reducing the velocity to a minimum at the top of the stroke. This cycle is repeated on the downward stroke showing that high acceleration values, (positive and negative) are experienced at each tip of the stroke where the velocity is minimized and the acceleration values are zero at each mid-point of the stroke where the velocities are at their highest. 
         [0010]    As the fluids are not physically attached to the screens the screen loading varies therefore dependent upon the stroke cycle. During the upward stroke the screen is accelerated into the fluid mass thereby inducing throughput then the screen accelerates away from the fluid mass on the downward stroke only for the fluid mass to drop onto the screens again at the bottom of the stroke due to gravity and the whole cycle is then repeated typically at approximately 1800 to 2000 rpm. 
         [0011]    For optimal performance the screen overall should preferably move as one entity matching the stroke profile of the basket and any deflection should be minimized. Should the screen deflect then this can lead to far higher acceleration values being induced leading to premature wear and uneven loading across the width of the screen. Given that the motion is also utilized to convey solids across the screen, any additional deflection of the screen will result in uneven conveyance across the screen width. 
         [0012]    Prior art has shown that the screen can span the distance between the screen clamps on the sides of the basket that are utilized in order to clamp the screen to the basket, but this requires a significant amount of materials in order to provide the rigidity required. The downside of this approach is that the screens are generally bigger and heavier, so not so good for manhandling and may be more expensive to manufacture. 
         [0013]    The screens are generally of one of two types: hook-strip; and pre-tensioned. 
         [0014]    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. 
         [0015]    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 and an example of a shale shaker suitable for receiving the pre-tensioned type screens is disclosed in GB-A-2,176,424. GB-A-1,578,948 discloses a screen assembly comprising a rigid frame made from angle iron with a T-bar central support and a layer of tensioned supporting mesh and a fine screen mesh thereon tensioned to a lesser tension to create a whipping effect to help unplug plugged areas screen. U.S. Pat. No. 5,819,952 discloses a screen assembly having one layer of fine screen mesh made from stainless steel and a supporting layer of coarser mesh made from a softer material such as phosphor bronze wire or epoxy coated wire. 
         [0016]    WO 2004/035234 discloses a screen assembly for a shale shaker, the screen assembly comprising a panel and a support structure, the panel has an area provided with a multiplicity of apertures and at least one layer of screening material arranged over the multiplicity of apertures. The apertures have downwardly projecting tabs to provide rigidity to the panel. Tabs of adjacent apertures form panel ribs. The support structure comprises a plurality of transverse support ribs. Transverse panel ribs fit over the transverse support ribs. The panel is removable from the support structure. 
         [0017]    WO 03/013690 and WO 2004/069374 disclose a screen assembly comprising a screen element and a separate support. The screen element comprising a mesh panel and side inclined support members having a downwardly inclined face. The support comprises a rigid frame having side inclined surfaces at each side. When in use, the screen assembly is slid into rails of a shale shaker. A pneumatic seal arranged in the rails is activated to push the side inclined support members on to the inclined surfaces of the rigid frame to tension the mesh panel over the support. The screen element may comprise a light weight flexible apertured plate. 
         [0018]    U.S. Pat. No. 6,305,549 discloses a screen assembly for use in a vibratory machine for separating solids and fluid. The screen assembly comprises a frame having tubular sides and intermediate rods of an aluminium or aluminium alloy and a layer of stainless steel screen material on the frame. It is noted that the difference in thermal expansion between the stainless steel screen material and the aluminium frame induces tensioning in the screening material in use. 
         [0019]    A problem associated with shale shakers is that screens used therein tend to blind, especially when the solids are gummy, such as clay, or of a size close to the size of the mesh size of the screen. The latter type of blinding is known as near-sized particle blinding. A number of solutions have been proposed to tackle this problem, such as disclosed in GB-A-1,526,663 in which a screen assembly using two layers of screening material in a sandwich and allowing the layers of screening material to move independently to dislodge any near sized particles lodged in one of the screens. WO 01/76720 relating to the rotatable drum separator also discloses using air nozzles external to the perforate drum to provide positive pressure air through the perforate drum in the dry zone to ensure that the perforations do not clog with solids. 
         [0020]    It is advantageous to use fine-meshed filters to filter very small particles, for example of a size in the range of 50-200μ or more, without the filtering device clogging up with the small particles. However, it is the fine-meshed filters in particular that are prone to such undesired clogging. 
         [0021]    It is also advantageous to provide a separator which operates at low noise levels to comply with health and safety legislation. It is also advantageous to have a simple reliable separator to inhibit down-time for maintenance and repair. 
         [0022]    It is preferable in certain circumstances to retain particles, for example of a particle size in the range of 50-60μ or larger, by means of a filter. 
         [0023]    In the drilling of an oil or gas well, cracks may exist in the wall of the wellbore. Such cracks may propagate, which could cause structural problems in the wall of the wellbore and/or allow drilling fluids to escape therethorough into the formation. Furthermore, if substantial amounts of drilling fluids are lost, the pressure in the drilling fluid in the wellbore may drop, which could cause collapse of the wellbore. Accordingly, Wellbore Strengthening Materials may be added to circulated drilling fluid. The Wellbore Strengthening Materials comprise sized particles. When the drilling fluid is circulated around the wellbore wall with cracks therein, the sized particles wedge themselves in the cracks, which reduces the likelihood of the cracks propagating. It is beneficial to recover these sized particles and reuse them in circulated drilling mud. Shale shakers have thus been modified to size solids in solids laden drilling fluid. Such a shale shaker is disclosed in U.S. Ser. No. 12/490,492. A range of sizes of solids can be extracted using such a shale shaker and re-circulated as Wellbore Strengthening Material in fresh drilling fluid. 
         [0024]    In order to expedite screening of solids laden drilling mud, energy is required to be imparted to the solids laden drilling at the juncture where the filtering takes place, i.e. where the solids laden drilling mud meets the screen. The inventors have found that by vibrating a tray in the fluid bed, energy in the form of 
         [0025]    Prior art also shows that the basket configuration may be such that additional support is offered to the screen between sides of the basket, GB-A-2,206,501 thereby reducing the span of the screen hence reducing the rigidity required of the screen such that the screen can be made smaller and lighter. 
         [0026]    A shale shaker screen is subjected to high acceleration values and heavy solids loading whilst processing drilling fluids. The screen mesh requires adequate support and any deflection of the screen may compromise its performance. 
         [0027]    The manufacturing techniques employed should be suitable for mass production and result in a strong and lightweight assembly. 
         [0028]    In the drilling of an oil or gas well drilling mud mud be returned to the surface at a high temperature, such as over on hundred and twenty degrees Celsius. This is particularly common in high temperature, high pressure wells (HTHP wells). The inventors have observed that longevity of the screen mesh often reduces when screen drilling mud which has come from such HTHP wells. The inventors have noted that screen tension alters when screening drilling mud which has come from such HTHP wells. 
         [0029]    According to the present invention, there is provided a screen assembly comprising a support structure and at least one layer of screening material thereon, wherein the support structure comprises at least one structural member having a thermal expansion coefficient and the at least one layer of screening material has a thermal expansion coefficient characterised in that the thermal expansion coefficient of the structural members is substantially equal to the thermal expansion coefficient of the at least one layer of screening material thereon. Preferably, the support structure further comprises at least one brace for bracing the support members, said braces being of a material which has the same or a different thermal expansion coefficient or having parts with the same thermal expansion coefficient and other parts with a different thermal expansion coefficient. 
         [0030]    Thus the inventors have observed that by using materials having a similar coefficient of thermal expansion, tension in the screening material remains taut and at the correct tension when in use separating hot solids from solids laden drilling mud. The hot solids laden drilling mud returned from a HTHP wellbore. The inventors have also noted that braces of a differing thermal expansion coefficient may be used. Preferably, the brace comprises a corrugated member, the corrugations may comprise planar tapered portions and may be curved and may have flat peaks and flat troughs. Advantageously, the peaks and troughs are arranged longitudinally with the screen assembly, such that screened material can flow therealong and advantageously have openings therein to allow screen fluid to flow therethrough. The peaks and troughs are arranged longitudinally with the screen assembly allow a degree of relative movement between the structural members and the brace, such as with relative movement due to differences in thermal expansion coefficient. The interspaced openings along the length of the troughs also allow a degree of relative movement between the structural members and the brace, such as with relative movement due to differences in thermal expansion coefficient. 
         [0031]    Preferably, the thermal expansion coefficient of all the structural members is substantially equal to the thermal expansion coefficient of the at least one layer of screening material thereon. Advantageously, the thermal expansion coefficient of preferably at least 75% and advantageously, between 50% the structural members is substantially equal to the thermal expansion coefficient of the at least one layer of screening material thereon. 
         [0032]    Preferably, the at least one layer of screening material comprises stainless steel and advantageously, the structural members comprise stainless steel. 
         [0033]    Advantageously, the structural members comprise bars. The bars may be of any cross-section, such as rectangular, square, oblong, circular elliptical, other multi-sided shape such as T-section or L-section. The bars are preferably solid, but may be hollow or partly hollow. Preferably, the bars are arranged to form a lattice structure, such as a grid having rectangular openings. 
         [0034]    Preferably, the bars in the lattice structure have top faces which lie in the same plane, such that screening material can be stretched thereon. Advantageously, the top faces of the bars lie in an arched plane, such that the at least one layer of screening material may be tensioned thereof to form a crown. Advantageously, the crown has a peak along a centre line of the screen assembly and the crown has low points at each side of the screen assembly. 
         [0035]    Preferably, the support structure is rectangular, square or oblong having a pair of sides and a pair of ends and further comprises a side member on each side of said pair of sides. Advantageously, the side members comprise box section members, preferably fixed to at least one support member. Advantageously, the box section members are made from a material having a thermal expansion coefficient substantially equal to the thermal expansion coefficient of the structural members and the thermal expansion coefficient of the at least one layer of screening material thereon. 
         [0036]    Advantageously, the support structure further comprises bracing for bracing the structural members. Preferably, the bracing comprises a corrugated member. The corrugated member comprises corrugations having peaks and troughs. Advantageously, the corrugations are located between the structural members. Preferably, the corrugated member supports and are preferably fixed to the structural members at peaks of the corrugations and advantageously at the troughs of the corrugations. Advantageously, upstands are located between the troughs and the structural members. Preferably, the corrugated member has openings therein to allow fluid screened by the at least one layer of screening material to flow therethrough. Advantageously, the corrugations comprise at least one corrugation having tapered sides, such that, in use the tapered sides fit over a raised portion of a shale shaker or flow tray to preferably inhibit transverse or longitudinal movement of the screen assembly in the shale shaker. The inventors have noted that considerable longitudinal forces are induced in the screen assembly by the vibratory mechanism. The tapered portions preferably fit over corresponding tapered portions, thus providing a large contact area. The large contact area provides a high degree of surface tension and thus inhibits movement of the screen assembly relative to the screen deck and hence the basket, whilst still allowing easy insertion of the screen assemblies when replacing the screen assemblies. Preferably, there are a plurality of corrugations with at least one and preferably two tapered portions. Preferably, the two tapered portions are slope downwardly and away from each other. Advantageously, the corrugations may have flat or arched top portions. Preferably, the corrugated member is formed of a plastics material and is moulded on to the support members. Advantageously, the corrugated member comprises metal portions over which the plastics material is moulded to form a composite corrugated member, such that part of the structural strength of the corrugated member comes from the plastics material and part from the metal portions. Preferably, at least part of the structural support is coated in plastics material, advantageously, the structural members and side members are coated in plastics material. Advantageously, the bracing comprises straight portions at an angle to the support members and arranged therebetween or thereunder. 
         [0037]    Preferably, the screen assembly further comprises a second layer of screening material. Advantageously, the screen assembly further comprises a coarse layer of mesh to support the at least one layer of screening material. Preferably, an aperture plate is arranged between the at least one layer of screening material and the support structure. Advantageously, the aperture plate is made from a material having a thermal expansion coefficient substantially equal to the thermal expansion coefficient of the structural members and the thermal expansion coefficient of the at least one layer of screening material thereon. The layer of screening material and/or coarse layer of mesh may be fixed to the support structure by laser welding, ultrasonic welding, vibration/friction welding or other joining technique and may be fixed in one action or the coarse layer fixed first and then the screening material afterwards. 
         [0038]    Preferably, the at least one raised portion comprises at least one wear strip. Advantageously, the flow tray is made from a composite material. Advantageously, the at least one raised portion defines flow channels for directing the screened drilling fluid along the flow tray. Preferably, the raised portion forms a longitudinal wall spanning at least a major portion and advantageously the entire length of the flow tray. Preferably, the flow tray is substantially the same length as the screen deck on which the screen assembly or screen assemblies lie in use. Alternatively, the flow tray has a length which is a major portion of the length of the screen deck. Preferably, the flow channels are tapered to facilitate flow of screened drilling mud. Preferably, tapered in depth, the flow channel have a high underside at a first end and a low height at the second end at which the screened drilling fluid is discharged. 
         [0039]    Advantageously, the shale shaker comprises a basket, the flow tray fixed to the basket. Preferably, the basket comprises sides, the flow tray fixed to the sides. Advantageously, the sides each have a rail, the flow tray supported on the rail. Preferably, the flow tray is adhered, riveted, screw, glued, welded or otherwise fixed to the rail and/or sides. Alternatively, the flow tray is removable from the basket, preferably slideably removable. 
         [0040]    Preferably, the flow tray comprises a plurality of contours, at least one of the contours forms the at least one of the raised portion. 
         [0041]    Advantageously, the at least one raised portion is a plurality of raised portions. The more raised portions, the better the support for the screen assembly, although the flow channels need to be sized to remove the throughput of screened drilling mud, which can be in the order of a thousand gallons per minute. 
         [0042]    Preferably, the screen support comprises a tapered surface. Advantageously, the at least one raised portion comprises a tapered support surface to receive the tapered surface of the screen support. Preferably, the tapered surface is formed from a sheet material, which may be sheet metal folded to form a tapered surface. Preferably, the sheet material has openings therein to allow screened drilling mud to flow through the screening material. 
         [0043]    The present invention also provides a method for making a screen assembly, the method comprising the steps of constructing a plurality of support members to form a grid and placing the grid in a mould whereupon a flowable plastics material is injected into the mould, whereupon the flowable plastics material is hardened. Preferably, the hardened plastics material forms braces for said support members in said grid. Advantageously, the plastics material coats the support members. 
         [0044]    Advantageously, the support members are metal bars. Preferably, the metal bars are made from stainless steel. Preferably, the method further comprises the step of notching the bars and fitting the bars together using the notches. Advantageously, the bars are welded at their intersections. Alternatively, the grid may be cast, preferably from a stainless steel. Preferably, the method further comprises the step of removing the support structure from the mould. and the mould removed therefrom. 
         [0045]    Preferably, the thickness of the thickness of the plastics material surrounding the support members is less than 1000 micrometres. Advantageously, the plastics material forming braces for the support members has a thickness of approximately 2000 micrometres. 
         [0046]    Advantageously, at least one layer of screening material, such as wire mesh, is tensioned in a jig. Advantageously, the jig comprises pneumatic and/or hydraulic cylinders in both longitudinal and transverse axis, to tension the mesh to a predetermined tension for the particular gauge/material of the at least one layer of screening material. Preferably, the at least one layer of screening material is fixed to the support members before plastics material is injected into said mould. Advantageously, the at least one layer of screening material is fixed to the support members after plastics material is injected into said mould. Preferably, the at least one layer of screening material is fixed to the support members before being placed into said mould. 
         [0047]    The above method and screen assembly enables the same materials to be used for load bearing purposes and therefore avoids any potential coefficient of thermal expansion issues. 
         [0048]    A potentially cheaper alternative technique may be utilized where high working temperatures are not an issue whereby the load bearing structure is completely encapsulated within the carrying frame engineering plastic and the mesh is then joined predominantly to the engineering plastic typically by laser welding, ultrasonic welding, vibration/friction welding or any other joining technique. 
         [0049]    UK Patent Application No. 10 071 65.2 (unpublished at the time of filing of the present application) discloses a screen assembly arrangement in a shale shaker offering increased support to the screens by minimizing the span between supports and the use of tapered surfaces encourages the screen to wedge itself into the support structure. This in conjunction with the increased contact area between the screen and the support arrangement dictate that the friction forces to be overcome when lifting the screen from the support arrangement are higher than that required for a simple point contact arrangement. 
         [0050]    The inventors noted that for the simple point contact arrangement the reaction forces acting on the screen and support structure are vertical only such that support is given during the upward stroke of the basket only. 
         [0051]    The tapered surfaces proposed dictate that the reaction forces are angled perpendicular to the angled faces and that equivalent loads can be calculated comprising of a vertical component and a horizontal component. The horizontal component reacts against the screen and is an additional force to be overcome as the screen deflects under its own weight during the downward stroke of the basket. Maximising this force therefore will help minimizing screen deflection during the downward stroke. 
         [0052]    UK Patent Application No. 10 071 65.2 (unpublished at the time of filing of the present application) discloses an apparatus for separating solids from solids laden drilling fluid, the apparatus comprising a shale shaker and a screen assembly, the shale shaker comprising a flow tray for directing screened drilling fluid, the screen assembly comprising at least one layer of screening material, a support and a perimeter with at least one support point within the perimeter, characterised in that the flow tray comprises at least one raised portion to support the screen assembly. Thus the flow tray is preferably used as a structural component to increase the rigidity of the screen assembly. Also disclosed is a an apparatus for separating solids from solids laden drilling fluid, the apparatus comprising a shale shaker and a screen assembly, the shale shaker comprising a basket having side supports and at least one intermediate support arranged between the side supports, the screen assembly comprising a screen support supporting at least one layer of screening material, the screen support having at least two sides to be supported by the side supports and at least one support member between the sides for engaging with the at least one intermediate support, characterised in that the screen support comprises a tapered surface and the at least one intermediate support has a tapered support surface, such that, in use, the tapered surface and the tapered support surface abut. Preferably, this arrangement inhibits lateral and tangential movement of said screen assembly in said basket. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0054]      FIG. 1  shows a perspective view of an apparatus for separating solids from solids laden drilling mud; 
           [0055]      FIG. 2A  is a side cross-sectional schematic view of an apparatus for separating and sizing solids from solids laden drilling mud, the apparatus comprising a base and a basket floating thereon; 
           [0056]      FIG. 2B  is an end view of the basket shown in  FIG. 2A  showing screen decks and screen assemblies therein; 
           [0057]      FIG. 3  is a schematic end view, partly in section of a screen deck, a flow tray and a screen assembly; 
           [0058]      FIG. 3A  is a top plan view of the flow tray shown in  FIG. 3 ; 
           [0059]      FIG. 4  is a schematic end view, partly in section of a screen deck, a flow tray and a screen assembly; 
           [0060]      FIG. 4A  is a top plan view of the flow tray shown in  FIG. 4 ; 
           [0061]      FIG. 5  is a schematic end view, partly in section of a screen deck, a flow tray and a screen assembly; and 
           [0062]      FIG. 6  is a schematic end view, partly in section of the flow tray shown in  FIG. 5  spaced from the screen assembly shown in  FIG. 5 ; 
           [0063]      FIG. 6A  is a top plan view of the flow tray shown in  FIG. 5 ; 
           [0064]      FIG. 6B  is a perspective view of the screen assembly shown in  FIG. 5 ; 
           [0065]      FIG. 7  is a schematic end view in cross section of the screen assembly shown in  FIG. 5 , spaced from a flow tray; 
           [0066]      FIG. 8  is a schematic end view of a screen assembly seated on a flow tray; 
           [0067]      FIG. 9A  is a perspective scrap sectional view of a screen assembly in accordance with the present invention, the screen assembly comprising a screening mesh arrangement and a support structure therefor; 
           [0068]      FIG. 9B  shows a part of the support structure of the screen assembly shown in  FIG. 9A ; 
           [0069]      FIG. 9C  shows an exploded view of the part of the support structure shown in  FIG. 9B ; 
           [0070]      FIG. 9D  is a perspective exploded scrap sectional view of the screen assembly shown in  FIG. 9A ; 
           [0071]      FIG. 9E  shows a perspective scrap sectional enlarged view of the support structure shown in  FIG. 9A ; and 
           [0072]      FIG. 9F  is a perspective view of the support structure of the screen assembly shown in  FIG. 9A . 
       
    
    
     DETAILED DESCRIPTION 
       [0073]      FIG. 1  shows an apparatus for separating solids from a solids laden drilling mud, generally referred to as a shale shaker and identified herein by reference H. the shale shaker H comprises a base D having an open bottom R arranged above a collection receptacle (not shown) for receiving screen drilling mud. A basket B is arranged on springs C on the base D. A vibratory apparatus E is arranged on top of the basket B. The vibratory apparatus E comprises an electric or hydraulic motor M rotating offset clump weights hidden within casing S, which induce a motion in the basket D. An upper, upper middle, lower middle and lower screen assemblies A 1 , A 2 , A 3 , A 4  are arranged in the basket D and fixed thereto in rails (not shown) so that the motion induced in the basket is transferred to the screen assemblies A 1 , A 2 , A 3 , A 4 . Solids laden drilling fluid is fed on to the screen assemblies A 1 -A 4  from a feed chamber F at a feed end of the apparatus. The motion induced in the screen assemblies A 1 -A 4  facilitates separation of solids from drilling mud. Screened drilling mud passes through the screen assemblies into the collection receptacle (not shown) and solids climb along the screen assemblies A 1 -A 4  to a discharge end P of the shale shaker and into a skip, ditch or other cuttings transfer apparatus (not shown). 
         [0074]      FIGS. 2A and 2B  show a shale shaker  10  having a base  20  and a basket  30  arranged thereon on springs (not shown). The basket  30  comprising a scalping deck  11 , an upper primary deck  12  and a lower primary deck  13 . The upper primary deck  12  has a left-hand side  12   a  and a right-hand side  12   b . The lower primary deck has a left-hand side  13   a  and a right-hand side  13   b . Solids laden drilling fluid is introduced to a feed end of the shale shaker  14  from a feeder (not shown) on to a scalping screen  15  arranged in C-shaped channels  16  of the scalping deck  11 . An expandable pneumatic bladder  17  is arranged in a top part of the C-shaped channels  16  to clamp the scalping screen  15  therein. Alternatively, a wedge may be used to secure the scalping screen  15  in the C-shaped channels  16 . The scalping screen  15  comprises a screen  18  having relatively large openings for inhibiting large particles from passing though on to the primary decks, but allowing some solids and drilling mud therethrough. The scalping deck  11  and the screen  15  thereon is arranged at an uphill tilt of approximately two degrees from horizontal, although the scalping screen  15  and scalping deck  11  may be arranged horizontally, slightly downhill or at a slightly greater uphill angle. Vibratory apparatus  16   a  fixed to the basket  30  induces motion therein. The motion facilitates separation of large solids from the solids laden drilling mud and induces movement in the large solids along the scalping screen  15  from the feed end  14  to the discharge end  19  of the shale shaker. The large solids may be caught in a ditch or on a conveying means and further processed or used in other operations. The solids laden drilling fluid which passes through the scalping screen  15  falls on to a flow tray  21  which directs the solids laden drilling fluid to the feed end  14  of screen assemblies  22   a  and  22   b  in the upper primary deck  12 . A weir  23  is arranged at the feed end of the primary deck  12  to retain the solids laden drilling fluid. If the level of the solids laden drilling fluid rises beyond the height of the weir  23 , solids laden drilling fluid passes thereover into a duct  24  and on to screen assemblies  25   a  and  25   b  in the lower primary screen deck  13 . The screen assemblies  22   a ,  22   b  and  25   a  and  25   b  are preferably of the same type and have the same screen mesh thereon. 
         [0075]    A gate valve  26  in the form of a slideable tray, is in a closed position, closing off a duct  29 , to run the shale shaker in a parallel mode. Screened drilling mud falls through the screen assemblies  22   a  and  22   b  in the upper screen deck on to a flow tray  27  and over the closed gate valve  26  and into a duct  28  which runs parallel to the duct  24 . However, duct  28  leads to the bottom of the basket and directly into the collection receptacle (not shown) therebelow. Solids fall off the discharge end  31  of upper screen deck  12  and discharge end  32  of the lower screen deck  13  and into a skip or other conveying apparatus for conveying the solids for further processing or re-use. 
         [0076]    The gate valve  26  may be retracted to allow drilling mud screened by the screen assemblies  22   a  and  22   b  in the upper screen deck  12  to be further screened by screen assemblies  25   a  and  25   b  on the lower screen deck  13 . The shale shaker thus runs in a series mode. In this situation, it is preferable to use a finer screen mesh in the screen assemblies  25   a  and  25   b  than the screen mesh used in screen assemblies  22   a  and  22   b . Drilling mud screened by the screen assemblies  22   a  and  22   b  in the upper screen deck  12  flows in the flow tray  27  and into a duct  29 , which directs the screened drilling mud on to the feed end of the screen assemblies  25   a  and  25   b  in the lower screen deck  13 . Sized solids fall off the discharge end  31  of upper screen deck  12  into a conveyor (not shown) to be conveyed and mixed into a fresh batch of drilling mud for re-circulation. These sized solids are used to block cracks in the formation as hereinbefore described. Solids discharges from the discharge end  32  of the lower screen deck  13  are conveyed in a separate conveyor or added to a skip for further processing or used for other purposes. 
         [0077]    Referring to  FIGS. 3 and 3A , there is shown a screen assembly  100  and a flow tray  101 . The flow tray  101  comprises a recessed zone  102 , a closed end  103 , an open end  104  and a pair of sides  105  and  106 . The sides  105  and  106  comprise flanged portions  107  and  108 , which flanged portions  107  and  108  are slid into C-shaped channels  109  and  110  in basket  30  and fixed therein. Preferably, the flanged portions  107  and  108  are glued or otherwise adhered to the C-shaped channels  109  and  110 , but may be slideably removable from the channels and fixed with the screen assembly  100  by expandable pneumatic hose seal  111 . The recessed zone  102  may have a substantially horizontal planar base, which when fixed in said C-shaped channels  109  and  110 , assumes a slope equal to the slope of the screen assembly  100 , which is preferably seven degrees. The tray also has an opening  112  therein. The gate valve  26  of the shale shaker  10  selectively prevents and allows screened drilling mud through the opening  112 . 
         [0078]      FIG. 4  shows a screen assembly  200  and a flow tray  201 . The flow tray  201  comprises three recessed zones  202   a ,  202   b  and  202   c , a closed end  203 , an open end  204  and a pair of sides  205  and  206 . The sides  205  and  206  comprise flanged portions  207  and  208 , which flanged portions  207  and  208  are slid into C-shaped channels  109  and  110  in basket  30  and fixed therein. Preferably, the flanged portions  207  and  208  are glued or otherwise adhered to the C-shaped channels  109  and  110 , but may be slideably removable from the channels and fixed with the screen assembly  200  by expandable pneumatic hose seal  111 . The recessed zones  202   a ,  202   b  and  202   c  may have a substantially horizontal planar base, which when fixed in said C-shaped channels  109  and  110 , assumes a slope equal to the slope of the screen assembly  200 , which is preferably seven degrees. The tray also has an opening  212  therein. The gate valve  26  of the shale shaker  10  selectively prevents and allows screened drilling mud through the opening  212 . A pair of intermediate supports  213  and  214  bind the recessed zones  202   a ,  202   b  and  202   c . The supports  213  and  214  comprise upstands  215  and  216  spanning the length of the flow tray  201 . The upstands each have a T-connector  217  and  218  fixed thereto or integral therewith, with a wear strip arranged thereon. The wear strip may be any hard wearing material such as HDPE. A top of the wear strips  219  and  220  lie slightly above the top of the flanged portion  207  and  208  of the flow tray, preferably by 2 or 3 mm at the centre. The screen assembly  200  sits on top of the wear strips  219  and  220  and is held thereon by inflation of the inflatable pneumatic hose  111  in the C-shaped channels  109  and  110 . Structural rigidity of said flow tray is thus used in supporting the screen assembly  200 . 
         [0079]      FIGS. 5 to 6B  show a screen assembly  300  and a flow tray  301 . The flow tray  301  comprises six recessed zones  302   a ,  302   b    302   c ,  302   d ,  302   e  and  302   f , a closed end  303 , an open end  304  and a pair of sides  305  and  306 . The sides  305  and  306  comprise flanged portions  307  and  308 , which flanged portions  307  and  308  are slid into C-shaped channels  109  and  110  in basket  30  and fixed therein. Preferably, the flanged portions  307  and  308  are glued or otherwise adhered to the C-shaped channels  109  and  110 , but may be slideably removable from the channels and fixed with the screen assembly  300  by expandable pneumatic hose seal  111 . The recessed zones  302   a  to  302   f  may have a substantially horizontal planar base, which when fixed in said C-shaped channels  109  and  110 , assumes a slope from the discharge end  31  to the feed end  14  of the shale shaker equal to the slope of the screen assembly  200 , which is preferably seven degrees. The tray also has opening  312   a ,  312   b ,  312   c  and  312   d  therein. The gate valve  26  of the shale shaker  10  selectively prevents and allows screened drilling mud through the opening  312   a  to  312   d . Five intermediate supports  313   a ,  313   b ,  313   c ,  313   d  and  313   e  bind the recessed zones  302   a  to  302   e . Only support  313   e  will be described in detail herein, although it should be noted that all the structural support members  313   a  to  313   e  are as described for structural support member  313   e . The support  313  comprises an upstand  317  spanning the length of the flow tray  301 . The upstand  317  has a strip  315  fixed or integral with the upstand  317 . The strip  315  has two sides each having a tapered portion  318  and  319 . The taper of each tapered portion  318 ,  319  is preferably at an acute angle from the plane of the flow tray  301 , preferably between ten and eighty degrees, advantageously, forty-five to seventy degrees and most preferably sixty to seventy degrees from a horizontal plane. Each tapered portion  318  and  319  has therealong a T-shaped member  320  and  321  fixed thereto or integral therewith, with a wear strip  322  and  323  arranged thereon. The tapered portions  318  and  319  each taper at a converging angle from the flow tray  301 . The wear strips  322  and  323  may made from a hard wearing material such as HDPE. A top  325  of the strip  315  lies slightly above the top of the flanged portion  307  and  308  of the flow tray, preferably by 2 or 3 mm at the centre support  313   c,  1 to 2 mm on the adjacent supports  313   b  and  313   d  and 0.5 to 1 mm on the outer supports  313   a  and  313   e . The wear strips  322  and  323  have a chamfered portion  324 , and the top  325  of the strip  315  has a slight crown. The chamfered portions  324  provide a smooth transition between the top  325  and the wear strips  322  and  323 . 
         [0080]    The screen assembly  300  comprises a screen support  340  comprising a perforate plate  351 , such as a perforate plate of metal or perforate plastics or perforate composite material. The perforations in the perforate plate  351  may be rectangular, as shown and which is preferred or any other suitable shape, such as octagonal, hexagonal, triangular, square, round. At least one layer of screening material  299  is adhered or otherwise attached to a top surface of the perforate plate  351 . The at least one layer of screening material  351  may be two layers of screening material of equal mesh size and may be supported on a supporting layer of screening material having a larger mesh size and large gauge wires. 
         [0081]    The screen support  340  comprises outer tubular frame members  352  and  353  along each side of the screen support. Each tubular frame member  352  and  353  is of hollow rectangular cross-section. Structural support members  350   a  to  350   e  are welded or otherwise attached or integral with the perforate plate  351 . Only structural support member  350   a  will be described in detail herein, although it should be noted that all the structural support members  350   a  to  350   e  are as described for structural support member  350   a . Structural support member  350   a  is symmetrical having a first side  354  and a second side  355  which are mirror images of each other. The first side  354  has an outer tapered portion  356  arranged in a plane at approximately sixty degrees from horizontal. A top edge of the outer tapered portion  356  is welded or otherwise attached to the perforate plate  351 . An inner tapered portion  357  is linked to the other side  355  with a curved portion  358 . The inner tapered portion  357  of side  354  and the inner tapered portion  357   a  of side  355  diverge from the perforate plate  351 . The curved portion  358  is shaped such that, in use, it sits over the crowned top  325  and chamfered portions  324  of the wear strips  322  of each support  313   a  to  313   e  of the flow tray  301 . The curved portion  358  is not fixed to the perforate plate  351  and stands clear with a gap therebetween. The lower part  359  lies in a plane of approximately sixty-five degrees from horizontal to conform to an outer face of the wear strip  324  on the tapered portion  318  of each support  313   a  to  313   e . The structural support members  350   a  to  350   e  further comprise a curved portion  358  linking the two sides  354  and  355 . The structural support members  350   a  to  350   e  have a plurality of openings  360  therein to allow fluid to flow therethrough on to the flow tray  301 . The structural support members  350   a  to  350   e  may be made from sheet metal such as galvanised steel and may be formed by folding the sheet metal. The openings  360  may be punched or laser cut in the sheet metal prior to folding. Alternatively, the structural support members  350   a  to  350   e  may be formed from a composite material such as a fibre reinforced material such as KEVLAR™. 
         [0082]    In use, the screen assembly  300  is inserted into the C-shaped channels  109  and  110  above the flow tray  301 . The inner tapered portions  357  of the structural support members  350   a  to  350   e  slide along their respective wear strips  322   a  to  322   e  and  323   a  to  323   e  during insertion and then sit thereon. The screen assembly  300  is clamped in place by inflation of the inflatable pneumatic hose  111  in the C-shaped channels  109  and  110 . Inflation of the pneumatic hose  111  pushes the sides of the screen assembly  300  on to the flanged portions  307  and  308  of the flow tray  301  and the inner tapered portions  357  of the structural support members  350   a  to  350   e  slide down their respective wear strips  322  and  323  to  323   e  to achieve a tight fit. Gaps between the top  325   a  to  325   e  and the screen support are likely due to tolerances in the relative size and location of the structural support members  350   a  to  350   e  and the supports  313   a  to  313   e  of the flow tray  301 . However, the abutment of the inner tapered portions  357  of the structural support members  350   a  to  350   e  and their respective wear strips  322  and  323  provide a large contact area for supporting the screen assembly  300 . Furthermore, the screen assembly  300  is inhibited from lateral movement in the basket  30  of the shale shaker. 
         [0083]      FIG. 7  shows the screen assembly  300  shown in  FIGS. 5 to 6B  with a flow tray  401 . The flow tray  401  comprises a comprises six recessed zones  402   a ,  402   b    402   c ,  402   d ,  402   e  and  402   f  to facilitate flow of drilling fluid therealong, a far end  403 , an open end  404  and a pair of sides  405  and  406 . The sides  405  and  406  comprise flanged portions  407  and  408 , which flanged portions  407  and  408  are slid into C-shaped channels  109  and  110  in basket  30  and fixed therein. Preferably, the flanged portions  307  and  308  are glued or otherwise adhered to the C-shaped channels  109  and  110  and may by riveted or mechanically fixed, but may be slideably removable from the channels and fixed with the screen assembly  300  by expandable pneumatic hose seal  111 . The recessed zones  402   a  to  402   f  have an undulating profile which taper in depth from the far end  403  to the open end  404 , which assumes a slope from the discharge end  31  to the feed end  14  of the shale shaker greater than the slope of the screen assembly  300 . The slope of the screen assembly may be adjustable between ten degrees uphill and one degree downhill, but is preferably fixed at seven degrees uphill. The flow tray  401  also has an opening (not shown) therein. The gate valve  26  of the shale shaker  10  selectively prevents and allows screened drilling mud through the opening. Five intermediate supports  413   a ,  413   b ,  413   c ,  413   d  and  413   e  bind the recessed zones  402   a  to  402   e . The flow tray  401  is preferably formed in a mould with composite material, such as KEVLAR™. Only support  413   e  will be described in detail, although it should be noted that all the structural support members  413   a  to  413   e  are as described for structural support member  413   e . The support  413  comprises an upstand  417  moulded into the composite flow tray  401  spanning the length of the flow tray  401 . The upstand  417  has a strip  415  fixed or integral with the upstand  417 . The strip  415  has two sides each having a tapered portion  418  and  419 . The taper of each tapered portion  418 ,  419  is preferably between ten and eighty degrees, and preferably forty-five to seventy degrees and most preferably sixty to seventy degrees from horizontal. Each tapered portion  418  and  419  has therealong a T-shaped member  420  and  421  fixed thereto or integral therewith, with a wear strip  422  and  423  arranged thereon. The wear strips  422  and  423  may made from a hard wearing material such as HDPE. A top  425  of the strip  415  lies slightly above the top of the flanged portion  407  and  408  of the flow tray, preferably by 1 to 3 mm. The wear strips  422  and  423  have a chamfered portion  424 , and the top  425  of the strip  415  has a slight crown. The chamfered portions  424  provide a smooth transition between the top  425  and the wear strips  422  and  423 . 
         [0084]      FIG. 8  shows a screen assembly  500  and a flow tray  501 . The flow tray  501  comprises six recessed zones  502   a ,  502   b    502   c ,  502   d ,  502   e  and  502   f  to facilitate flow of drilling fluid therealong, a far end  503 , an open end  504  and a pair of sides  505  and  506 . The sides  505  and  506  comprise flanged portions  507  and  508 , which flanged portions  507  and  508  are slid into C-shaped channels and in basket and fixed therein, as before described. Preferably, the flanged portions  507  and  508  are glued or otherwise adhered to the C-shaped channels and may by riveted or mechanically fixed, but may be slideably removable from the channels and fixed with the screen assembly  500  by expandable pneumatic hose seal. The recessed zones  502   a  to  502   f  have an undulating profile which taper in depth from the far end  503  to the open end  504 , which assumes a slope from the discharge end  31  to the feed end  14  of the shale shaker greater than the slope of the screen assembly  500 . The slope of the screen assembly may be adjustable between ten degrees uphill and one degree downhill, but is preferably fixed at seven degrees uphill. The flow tray  501  also has an opening (not shown) therein. The gate valve  26  of the shale shaker  10  selectively prevents and allows screened drilling mud through the opening. The screen assembly comprises at least one layer of screening material  509  on a support structure  510 . The support structure comprises a frame  511 . The flow tray  501  is preferably formed in a mould with a composite material, such as KEVLAR™. 
         [0085]    In use, the screen assembly  500  is inserted into the C-shaped channels  109  and  110  above the flow tray  501 . Five support ribs  512   a  to  512   e  each having a wear strip thereon, are welded, glued or otherwise attached to flat topped crests  513   a  to  513   e  of the flow tray. A top surface of the support ribs  512   a  to  512   e  preferably lie in a plane which is between 1 and 3 millimetres above from the plane of the top of the flanges  507  and  508 . The support structure  510  of the screen assembly  500  sits on five support ribs  512   a  to  512   e . The screen assembly  500  is clamped in place by inflation of the inflatable pneumatic hose  111  in the C-shaped channels  109  and  110 . Inflation of the pneumatic hose  111  pushes the sides of the screen assembly  500  on to the flanged portions  507  and  508  of the flow tray  501 . 
         [0086]      FIG. 9A  shows a part of a screen assembly  600  comprising a screen mesh arrangement  601  and a support structure  602 . The screen assembly  600  may be used in place of screen assemblies  100 ,  200 ,  300 ,  500  and be used with flow trays  101 ,  201 ,  301 ,  401 ,  501  or any other flow tray disclosed herein. The screen assembly  600  may be used in any shale shaker and particularly but not exclusively, any shale shaker disclosed herein. The screen mesh arrangement  601  comprises one layer of fine mesh  603  superimposed on one layer of coarse supporting mesh  604 . The one layer of fine screen mesh  603  has fine shute and warp wires and small openings therebetween. The one layer of coarse supporting mesh  604  has shute and warp wires  604   a ,  604   b  of a larger diameter than shute and warp wires of the one layer of fine screen mesh  603  and larger openings than the openings in the fine screen mesh. The one layer of fine screen mesh  603  may be adhered or otherwise attached to the one layer of coarse supporting mesh  604 . The one layer of fine screen mesh  603  and one layer of coarse supporting mesh  604  are both adhered of otherwise attached to the support structure  602 . The one layer of fine screen mesh  603  and one layer of coarse supporting screen mesh  604  are preferably made from a stainless steel material, advantageously  304  stainless steel. 
         [0087]    As shown in  FIG. 9B , the support structure  602  comprises a plurality of longitudinal support bars  605 , preferably twenty-three longitudinal support bars  605  and two longitudinal end members  611  (and not shown) preferably made from a stainless steel and advantageously, a 304 stainless steel material. The preferred cross-sectional dimensions of each of the longitudinal support bars is 6 to 10 mm high and preferably 8 mm high by 1.2 mm to 2 mm wide and preferably 1.6 mm wide. The support structure  602  also comprises a plurality of transverse support bars  606 , preferably ten transverse support bars  606  preferably made from a stainless steel and advantageously, a 304 stainless steel material. The preferred cross-sectional dimensions of each of the transverse support bars is 6 to 10 mm high and preferably 8 mm high by 1.2 mm to 2 mm wide and preferably 1.6 mm wide. 
         [0088]    The longitudinal support bars  605  preferably have notches  607 , open to an underneath face  608  of the longitudinal support bars  605 , as shown in  FIG. 9C . A notch  607  is provided for each transverse support bar  606 . Each transverse support bar  606  has corresponding notches  609  in a top face  610  of each transverse support bar  605 . The longitudinal support bar end members  611  (one shown) are fixed, by for example laser welding, to ends of the transverse support bars  606 . The longitudinal support bars  605  and transverse support bars  606  slot together, preferably with an interference fit in respective notches  607 ,  609  to form a rigid grid  612  and then welded at their intersections. Preferably, the top faces  610  of the transverse support bars  606  and top faces  610   a ,  610   b  of the longitudinal support bars and longitudinal support bar end members  611  lie in the same plane, thus the rigid grid  612  has a planar top face. 
         [0089]    Preferably, box section side portions  613  and  614  are fixed by for example, moulded around the respective longitudinal support bar end members  611  and ends of the transverse support bars  606 . The longitudinal support bar end members  611  are preferably form the outermost edge of the screen. The box section side portions  613 ,  614  are preferably of oblong cross-section, although may be of any suitable cross-section, such as circular, square, rectangular or other multi-sided shape. The box section side portion  614  preferably has a core  614   a  preferably formed from an expanded foam, although the core may be hollow. An inner face  614   b  of the box section side portion preferably abuts longitudinal support bar  614   b . The box section  613  is preferably a mirror image of box section  614 . The box section side portions  613 ,  614  have a top face  615  which lies in the same plane as a top face  616  of the rigid grid  612 , such that during assembly or reconditioning the screen mesh arrangement  601  can be tensioned in a jig (not shown) and offered up to the rigid grid  612  to be attached to the top faces of the longitudinal and transverse support ribs  605  and  606  and top face  615  of the box section side portions  613 ,  614 . The box section side portions  613 ,  614  are preferably moulded from a plastics material and once removed from the mould, the core is filled with an expanded foam or a structural matrix of expanded material. 
         [0090]    A corrugated underplate  617  is arranged underneath the rigid grid  612 . The corrugated sheet  617  is moulded from a suitable plastics material separately or preferably in the same mould and in the same step as moulding the box section side portions  613  and  614 . The corrugated underplate  617  may also comprise metal elements or fibres to add structural strength thereto. The metal elements may be 304 stainless steel and pressed into a corrugated parts and the corrugated sheet moulded thereabout. The corrugated underplate  617  comprises arched portions  618 . The arched portions  618  preferably extend the entire length of the screen assembly. Preferably, the arched portions  618  may have tapered portions  618   a  and an arched top  618   b  correspondingly formed to sit over strip  415  (see  FIG. 7 ) advantageously, to inhibit lateral movement between the screen assembly  600  and the flow tray  401  and to thus use the flow tray as a stressed member to increase the stiffness of the screen assembly  600 . Arched portions  618  are interspaced by arched corrugations  619 . The arched corrugations  619  preferably run the entire length of the screen assembly  600 . A top  620  of the arched corrugations  619  extends along the length of longitudinal support bars  605  and are preferably welded, moulded or otherwise fixed to the underneath  608  of the longitudinal support bars  605 . A foot  621  of the arched corrugations  619  is fixed to a foot  622  of the arched portions  618  which, in use sits over and is supported by strip  415  of the flow tray, and thus the arch corrugations  619  help support the longitudinal support bar  605 . The rigid grid  612  is also supported at intersection areas  623  where the transverse support bars  606  intersect with the longitudinal support bars  605  above a trough  624  in the corrugated underplate  617 . Upstands  625  are arranged under the longitudinal support bars  605  at the areas of intersection  623  between the underneath  608  of the longitudinal support bars  605  and the troughs  624 . The upstands  625  are preferably made from a plastics material and advantageously, moulded in the same step as moulding the box section side portions  613 ,  614  and the corrugated underplate  617 . The upstands may also have a metal core and the core may be of 304 stainless steel. 
         [0091]    Between the upstands  625 , there are located openings  626  in the corrugated underplate  617 . The openings  626  allow screened drilling fluid to pass therethrough on to the flow tray  401  or into a receptacle or other flow means, when in use. 
         [0092]    The rigid grid  612  is encapsulated in a plastics material. The encapsulation may simply be a very thin layer of less than 1 mm, or may be a thick layer to make a composite material, wherein the thick layer of plastics material adds to the structural integrity of the support structure. The step of encapsulating the rigid grid  612  is preferably carried out in the same moulding step and in the same mould as when moulding the box section side portions  613 ,  614  and the corrugated underplate  617 . The thickness of the plastics material for the box section side portions  613 ,  614  and the corrugated underplate  617  may be in the order of 2 mm thick. The screen mesh arrangement  601  may be tensioned in a jig and offered up to the top of the support structure  602  and adhered or otherwise attached thereto. 
         [0093]    An apertured plate (not shown) may be arranged between the layer of fine screen mesh  603  and the support structure  602 , the apertured plate taking the place and function of the coarse screen mesh layer  604 . Alternatively, an apertured plate (not shown) may be arranged between the screen mesh arrangement  601  and the support structure  602 . Alternatively, an aperture plate  700  shown in  FIG. 10  may be used in place of the rigid grid  612 . The apertured plate  700  may be similar to the type disclosed in PCT Publication Number WP 2004/0035236 having punched openings  701 , the punched openings  701  having tabs  702  either side of ribs  703  which tabs  702  are folded over to form structural tabs  702  to increase the rigidity of the aperture plate  700 . The apertured plate  700  is then preferably coated in the plastics material in the same way as described above with respect to the rigid grid  612 . The corrugated underplate  617  would be arranged such that ribs of the aperture plate would be supported by the tops of the arched portions  618  and arched corrugations  619  and by upstands  625 . The aperture plate  700  is made from a material such as stainless steel and advantageously  304  stainless steel, such that the coefficient of expansion of the aperture plate and the screen mesh arrangement is substantially equal so that differential expansion is inhibited.