Abstract:
There is provided a screen frame adapted for use in a shaker to separate solids from liquid/solid mixture and to which woven wire mesh is to be attached, comprising a plurality of intersecting elongate members ( 12 ) defining a plurality of openings ( 10 ), or cells, wherein at least one protrusion ( 26 ) in the form of an elongate rib extends partway across each opening ( 10 ).

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to sifting screens which in use are fitted to a shaker to separate solids from liquids and in particular to separate solids from liquid drilling muds brought up from downhole when drilling for oil or gas. 
       BACKGROUND TO THE INVENTION 
       [0002]    Sifting screens used to remove debris such as rock and shale from synthetic drilling muds incorporate layers of woven wire mesh to separate out the debris from the synthetic mud. The screen is vibrated within the shaker and the mesh is subjected to wear from the vibration and the mud and debris. Replacing individual screens as the mesh fails is time consuming and delays the recovery of the mud. To improve the period of time over which any given screen remains operational, the screen is typically divided up into any number of rectangular openings or cells to which the mesh is bonded. If mesh over a given cell fails, this cell can be blocked off leaving the remainder of the screen functioning. This allows the operational life of the screen to be extended, with failure of mesh in one area not compromising the integrity of the remainder of the mesh. 
         [0003]    The useful life of the screen needs to be as long as possible. The weight of the screen, the size of the cells and the exposed area of mesh all affect how much drilling mud can be recovered over a given time before the screen needs to be replaced in its entirety. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a screen frame adapted for use in a shaker to separate solids from liquid/solid mixture and to which woven wire mesh is to be attached, comprising a plurality of intersecting elongate members defining a plurality of openings, or cells, wherein at least one protrusion extends partway across at least one opening, thereby to support mesh when attached. In use mesh covering the opening is supported by the protrusion within the area defined by the opening. This reduces stress on the mesh so reducing the rate at which it wears and improving the operational life of the mesh. Whilst prior art screens have used integral partial ribs to subdivide the openings and so support the mesh, these screens have suffered from warping during manufacture as typically they are made from moulded plastics materials and when the moulded screen cools, the subdividing ribs differentially contract to the remainder of the screen, so causing warping. By having a protrusion which extends partway across the opening, and is thus free at one end, warping is avoided, whilst still providing additional support to reduce wear. 
         [0005]    Typically at least one protrusion will extend partway across each opening, so that in use the mesh covering each opening or cell is provided with additional support. 
         [0006]    Preferably the protrusions are integrally formed with the intersecting elongate members so as to cantilever from those members when extending partway across the opening. Each protrusion thus has an end secured to and extending from part of an elongate member and a free end. 
         [0007]    To provide additional support, two or more protrusions extending partway across each opening may be provided. 
         [0008]    As will be appreciated, various geometric arrangements can be employed, such that protrusions may extend from one side of an opening, may extend from opposing sides or adjacent sides. One protrusion extending from opposing sides of each opening, such that there are two protrusions for each opening, has been found particularly beneficial, but more protrusions may be provided bearing in mind the requirement that the opening needs to have a substantial free area to sift drilling mud. 
         [0009]    Desirably protrusions will be positionally staggered to support the mesh at spaced apart locations within the cell. If the protrusions extend from opposing sides of an opening, the length of the protrusions is selected to ensure they are separated by a small gap as otherwise warping may occur during manufacture. 
         [0010]    Where protrusions are provided on opposing sides of an opening, typically each protrusion extends at least halfway across the opening, and is thus at least length L/2 where L is the length of a wall of the opening parallel to the protrusion. 
         [0011]    The protrusions are preferably elongate fingers or ribs, with typically a common shape used throughout the frame. However protrusions in the form of partial ovals, partial circles or comb-like structures may be used. 
     
    
     
         [0012]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a perspective view of an opening or cell, being one of a plurality of such cells within a prior art sifting screen; 
           [0014]      FIG. 2  is a perspective view of an opening or cell in accordance with one embodiment of the present invention which incorporates a truncated rib; 
           [0015]      FIG. 3  is a perspective view of an opening or cell within a sifting screen in accordance with a second embodiment of the present invention; and 
           [0016]      FIGS. 4 to 6  relate to modelling tests carried out in relation to cells of different dimensions and with different combinations of truncated ribs. 
       
    
    
     DESCRIPTION 
       [0017]      FIG. 1  shows in detail one opening or cell  10  of a prior art sifting screen, the general type of which is disclosed in EP 1444056 where the screen comprises a rectangular frame within which a grid of intersecting elongate ribs  12  reinforced by a grid of steel wires  14  divides the filtering area into an orthogonal array of openings or cells  10 . Such screens are made by a plastics moulding process with mesh stretched over and secured to the frame so that each opening or cell is associated with an area of mesh. 
         [0018]    To extend the life of the mesh covering each cell  10 , it is known to sub-divide the cell  10  with a thin rib  16  which is half the depth of the main ribs  12 , rib  16  halving the short span of the cell associated with a shortest wall  20 . The half-depth rib  16  spans the top of the walls  20  and supports mesh covering that particular cell  10 , so reducing the load stress associated with the mesh by a factor of approximately 6 and improving the wear characteristics of the mesh. However difficulties can arise with the moulding of such screens. During moulding, GRP plastics material is injected at high temperature (approximately 250° C.) into a mould of the frame. Upon cooling the plastics material contracts, with this contraction resisted by the steel reinforcing wires  14 . However the thinner half-depth ribs  16  do not have any reinforcement and are free to contract. As they are above the neutral axis  24  of the frame, this shrinkage tends to make the frame warp. 
         [0019]      FIGS. 2 and 3  show embodiments of sifting screens in accordance with the present invention with, as for  FIG. 1 , only one cell shown in detail. Generally all cells within the screen will be identical with a regular array of cells comprising the screen. 
         [0020]    In  FIG. 2 , a truncated rib  26  integrally moulded with rib  12  cantilevers from the short wall  20  of the cell, extending partway across the cell  10  parallel to a longer wall  22 . The partial or truncated rib  26  is approximately half the depth of the main reinforced wall  20  of rib  12  with the uppermost part of the rib  26  being in the same plane as the uppermost part of the short and long walls  20 ,  22 . The truncated rib  26  extends at least halfway across the cell, and thus has a length of at least half L/2, where L is the length of the long side  22 . When moulding takes place, the truncated rib  26  is free to contract and shrink back on itself without pulling on the wall on the other side of the cell, and so the tendency for the frame to warp is avoided. In use, the mesh carried on the frame is supported by the truncated rib  26  within the area defined by the cell. 
         [0021]    Other arrangements of partial ribs are possible and  FIG. 3  shows another arrangement with partial ribs formed respectively at approximately ⅓ of the way up and down the short walls  20  in opposing short walls of the cell. These ribs extend halfway across the span of the cell, although if desired can extend more than halfway across the span. Such an arrangement reduces the stress in the wire cloth by a factor of 6, so improving wear characteristics. 
         [0022]    Analysis was undertaken in ANSYS Workbench modelling software to determine how the stress and deflection in the wire cloth varies with cell size and different configurations of partial ribs. Six different test cell structures,  1 ,  2 ,  3 ,  2 . 1 ,  2 . 2  and  2 . 3  as shown in  FIG. 4 , were generated in solid edge with the wire cloth idealised as a thin solid with a thickness of 0.25 mm. The black zone represents the wire cloth polymer bond which was supported rigidly in the finite element model.  1 ,  2  and  3  are rectangular cells with a common mesh length of 87 mm and varying mesh width with a mesh width of 27 mm for  1 , 40 mm for  2  and 54 mm for  3 .  2 . 1 ,  2 . 2  and  2 . 3  are cells of the same dimensions as  2 , namely mesh length of 87 mm and cell width of 40 mm, but with one, two or three protruding ribs respectively. 
         [0023]    When modelling, a load of 1 g was applied to each cell model so that the unsupported material of the wire cloth was accelerated relative to the rigidly supported structure. This gave results as summarised in Table 1 below and shown in  FIG. 5  which is a graph showing stress for each cell, and  FIG. 6  which is a graph showing deflection for each cell. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Stress and deflection under a 1 g load 
               
             
          
           
               
                   
                   
                   
                   
                 Deflection 
               
               
                 Open area (mm 2 ) 
                 Definition 
                 Test 
                 Stress (MPa) 
                 (mm) 
               
               
                   
               
             
          
           
               
                 2349 
                 27 × 87 
                 1 
                 0.028 
                 0.00002380 
               
               
                 3480 
                 40 × 87 
                 2 
                 0.047 
                 0.00009370 
               
               
                 3480 
                 40 × 87 1 arm 
                 2.1 
                 0.037 
                 0.00006500 
               
               
                 3320 
                 41 × 87 2 arm 
                 2.2 
                 0.022 
                 0.00001470 
               
               
                 3300 
                 42 × 87 3 arm 
                 2.3 
                 0.014 
                 0.00000695 
               
               
                 4698 
                 54 × 87 
                 3 
                 0.114 
                 0.00041390 
               
               
                   
               
             
          
         
       
     
         [0024]    As can be seen from  FIGS. 5 and 6 , generally as the unreinforced cell size increases (cells  1 ,  2  and  3 ), the stress/deflection increases. As stress and deflection increase, wear of the mesh increases and cell life decreases. 
         [0025]    Where, for a given cell size, protruding ribs are incorporated into the cell structure, see  2 ,  2 . 1 ,  2 . 2  and  2 . 3 , the stress and deflection decrease as the number of ribs increases from zero to three. It can thus be seen that reinforcing the cell with partial ribs offsets the disadvantage of increasing the cell size, and for a given cell size, the cell life increases as partial ribs are added. The benefit in adding partial ribs can be seen to level off from 2 partial ribs to 3 partial ribs, so moving to four partial ribs will be less beneficial, especially when considering the blocking effect of the ribs on mud flow through the screen. 
         [0026]    As stress and deflection decrease, so the cell life increases. Comparing test  2 . 2  with test  1 ,  2 . 2  experiences less stress and deflection for a similar percentage of open mesh versus blanked off area (test  1  has 83.3% open mesh, test  2 . 2  82.9%). Test  2 . 3  experiences even less stress and deflection but the amount of usable open mesh (80.9%) is reduced. The more partial ribs added, the more mesh that is blocked, reducing the flow rate through the sieve. 
         [0027]    The modelling results confirm that an increase in cell life is achievable by local cell reinforcement with partial ribs. The optimal number of partial ribs per cell is probably two, since the benefits of additional ribs in terms of reduced stress and deflection tend to be outweighed by the reduced usable mesh area available for increasing numbers of partial ribs.