Abstract:
An apparatus for separating solids from solids laden drilling fluid, the apparatus comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate the basket, the basket having at least one screen deck for receiving at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, the apparatus further comprising a hood encompassing at least a discharge end of the basket and at least one extraction outlet arranged at the discharge end to remove fume laden air from the basket.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of GB Patent Application No. 1112193.6, filed on Jul. 15, 2011, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The invention relates to an apparatus and method for separating solids from a solids laden liquid and particularly, but not exclusively an apparatus and method for separating solids from a solids laden drilling mud. 
     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 the 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. 
     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. 
     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 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. Drilling mud passes through the screens. 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 and an example of a shale shaker suitable for receiving the pre-tensioned type screens is disclosed in GB-A-2,176,424. 
     Drilling rigs often have space restrictions for mud processing equipment, particularly, but not exclusively on off-shore drilling rigs. Shale shakers have a foot print defined by a skid. Instead of having separate scalping shakers and primary screening shakers, shale shakers may incorporate a scalping deck and a primary screening deck to reduce the number of skids required on a drilling rig. An example of such a shale shaker is the VSM 300® supplied by NATIONAL OILWELL VARCO®. Shale shakers may include multiple primary screening decks inter alia to screen more solids laden drilling fluid per skid. Furthermore, primary, secondary and tertiary decks each with different screening material thereon may be incorporated on a single skid to inter alia provide a filtered drilling fluid with finer solids therein or to filter out graded solids for re-circulating in the well. 
     Solids laden drilling fluid may have chemicals therein which give off noxious fumes, fumes detrimental to the environment and fumes which may potentially be explosive. Such fumes may include hydrogen sulphide and oil vapours. Accordingly, it is often desirable to remove or contain such fumes. 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. 
     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. 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. 
     It is known to house shale shakers in rooms on or near derricks of a drilling rig. These rooms are known as shaker rooms. A ventilation system may be used in such shaker rooms to draw off fumes given off by the solids laden drilling fluid when being processed by the shale shakers. 
     US 2008/0078699 discloses a shale shaker for separating solids from solids laden drilling mud, the shale shaker comprising a basket with a fume hood arranged thereon. 
     WO 2006/098638 discloses an apparatus for separating solids from solids laden drilling fluid, the apparatus comprising an inlet screen and upper and lower horizontal endless belt screen cloths energized with acoustic pulses, the endless belt screen cloths having a screen cleaning system using jets of air and the endless belt screen cloth having a discharge end and a negative pressure ventilation system thereabove. 
     The inventors have observed that the quantity of fumes given off by the shale shaker is inter alia related to the screening area of the screens in the shale 15 shakers. Thus with the popularization of shale shakers with multiple-decks, more fumes are given off per skid. The inventors also observed that there is a need for a more efficient apparatus and method for drawing off fumes produced by the shale shakers. 
     SUMMARY 
     According to the present invention, there is provided an apparatus for separating solids from solids laden drilling fluid, the apparatus comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate the basket, the basket having at least one screen deck for receiving at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, the apparatus further comprising a hood encompassing at least a discharge end of the basket and at least one extraction outlet arranged at the discharge end to remove fume laden air from the basket. Such fumes may include hydrogen sulphide and oil vapours. 
     There is also provided an apparatus for separating solids from solids laden drilling fluid, the apparatus comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate said basket, the basket having at least one screen deck for receiving at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, the apparatus further comprising a hood encompassing at least a discharge end of said basket and at least one extraction outlet arranged at said discharge end to remove fume laden air from the basket, the apparatus further comprising a sump arranged beneath said basket said sump having a sump discharge opening, wherein the at least one replacement air opening is the sump discharge opening supplying replacement air to replace fume laden air removed through said at least one extraction outlet. 
     Preferably, the hood is a complete housing, advantageously the housing encompasses a substantial portion of the basket and preferably all of the basket, having at least one replacement air opening sized to facilitate control of flow of replacement air to replace fume laden air removed through the at least one extraction outlet. The complete housing needn&#39;t be completely air tight and may have a few small gaps allowing a small amount of air to pass from the atmosphere into the complete housing. Negative pressure induced by suction through the extraction outlet ensures that air is sucked in through any small gap in the housing and does not allow fume laden air to escape. Preferably, the complete housing covers the feed end of the basket up to the base of the vibratory mechanism arranged on the discharge end of the basket, enclosing the sides of the basket and fixed but not necessarily, although advantageously, sealed  1  to the skid on which the shaker is mounted. The complete housing also encloses the discharge end of the basket and a void above the trough in which the solids fall with extraction outlets arranged thereabove and preferably the extraction outlets at a higher level than the top screen deck of the basket. Preferably, the hood extends over the entire basket of shaker to inhibit fumes escaping the shale shaker. 
     Advantageously, the replacement air opening is arranged underneath the at least one screen deck and at a feed end of the basket, and in a multiple screen deck arrangement, under the lowermost screen deck. This is preferable, such that at least a large part of the replacement air is through, around and over the screens in the screen decks, thereby evacuating fume laden air from the entire basket. 
     Preferably, a sump is arranged beneath the basket the sump having a sump discharge opening, wherein the at least one replacement air opening is the sump discharge opening supplying replacement air to replace fume laden air removed through the at least one extraction outlet. Preferably, the hood is a complete housing, thus replacement air comes mainly from controlled sources, i.e. the sump discharge opening and preferably, the solids discharge opening. The sump outlet opening can control the flow of replacement air passing into the hood and may influence the rate of fume laden air being evacuated from the hood. 
     Advantageously, a solids removal apparatus for conveying solids away from the discharge end of the basket, wherein the at least one extraction outlet is arranged above the solids removal apparatus. 
     Preferably, the solids removal apparatus comprises a ditch or further comprise the outside of the foot print of the skid, preferably above a ditch located to catch the solids discharging from the discharge end of the shale shaker. Advantageously, the at least one extraction outlet is arranged in a top of the hood, drawing fumes vertically through a duct attached or integral with the at least one extraction outlet. 
     Preferably, the hood extends over the entire basket to inhibit fumes escaping the shale shaker. Advantageously, the hood comprises an opening about a base of the vibratory mechanism. Advantageously, the vibratory mechanism is mounted on top of the basket, preferably close to the discharge end of the basket. This enables the hood size to be kept to a minimum while still enclosing the basket and allowing access to the vibratory mechanism for inspection, repair and cooling. Advantageously, a seal is arranged between the base of the vibratory mechanism and the hood. Preferably, the seal is a bellows type seal, which allows relative movement between the basket and the hood. Preferably, the at least one extraction outlet is arranged above said discharge end and advantageously, the outside of the foot print of the skid, preferably above a ditch located to catch the solids discharging from the discharge end of the shale shaker. Advantageously, the at least one extraction outlet is arranged in a top of said hood, drawing fumes vertically through a duct attached or integral with the at least one extraction outlet. 
     Advantageously, the hood is fixed to the skid, allowing the basket to vibrate relative thereto. The hood may also be attached to any other static part of the shale shaker, such as the feed chute. 
     Preferably, the basket comprises a scalping deck and one or two primary decks spaced vertically one above the other. The primary decks may also be split horizontally into two sides, each having rails into which screens can be slid. The rails preferably use a pneumatic hose sealing arrangement, such as the arrangement produced and sold by NATIONAL OILWELL VARCO® under the trade mark PNEUMOSEAL™. 
     Preferably, the apparatus further comprises a ventilation system for drawing fume laden air through the at least one extraction outlet, such as a roots type fan for producing negative pressure to suck fume laden air from the hood enclosing the basket. The ventilation system may comprise an HVAC unit. 
     The present invention also provides a bank of shale shakers each shale shaker comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate the basket, the basket having at least one screen deck for receiving at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, each shale shaker further comprising a hood encompassing at least a discharge end of the basket and at least one extraction outlet arranged at the discharge end to remove fume laden air from the basket, each at least one extraction outlet linked by a duct with a ventilation system for drawing fume laden air from each basket of each shale shaker. 
     The present invention also provides a bank of shale shakers each shale shaker comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate the basket, the basket having at least one screen deck for receiving at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, each shale shaker further comprising a hood encompassing at least a discharge end of the basket and at least one extraction outlet arranged at the discharge end to remove fume laden air from the basket, the shale shaker having a sump arranged beneath said basket said sump having a sump discharge opening, wherein the at least one replacement air opening is the sump discharge opening supplying replacement air to replace fume laden air removed through said at least one extraction outlet, each at least one extraction outlet linked by a duct with a ventilation system for drawing fume laden air from each basket of each shale shaker. 
     The present invention also provides a method of separating solids from solids laden drilling mud using a shale shaker comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate said basket, the basket arranged over a sump, the basket having at least one screen deck having at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, the shale shaker further comprising a hood encompassing at least a discharge end of said basket and at least one extraction outlet arranged at said discharge end, the method comprising the steps of drawing fume laden air from the basket through the at least one extraction outlet and replacement air drawn through said sump outlet. 
     The present invention also provides a method of separating solids from solids laden drilling mud using a shale shaker comprising a skid and a basket resiliently suspended thereto and a vibratory mechanism to vibrate the basket, the basket having at least one screen deck having at least one screen, the basket having a feed end at which solids laden drilling fluid is introduced and a discharge end at which solids are discharged through a solids discharge opening, the shale shaker further comprising a hood encompassing at least a discharge end of the basket and at least one extraction outlet arranged at the discharge end, the method comprising the steps of drawing fume laden air from the basket through the at least one extraction outlet. Preferably, the shale shaker comprising a sump and a sump outlet, wherein the method further comprises the step of drawing replacement air through the a sump outlet. 
     The method, system and apparatus of the invention may use any combination of the above features and steps. 
     Some shale shakers have screen decks and/or a basket which have an adjustable rake to alter the pool depth on the screens. The rake adjustment may be between five degrees downhill and twenty five degrees uphill, although more popularly up to seven or ten degrees. A hood of the present invention can be arranged around the basket with enough clearance to allow for such movement of the basket. 
     Preferably, replacement air flows in from the underside of the shale shaker, advantageously, through the skid, preferably, through a ditch located beneath the discharge end. Preferably, the replacement air is drawn up through a drilling fluids collection sump arranged beneath the basket. 
    
    
     
       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. 1A  shows a side view of a known apparatus for separating solids from solids laden drilling mud; 
         FIG. 1B  shows a perspective view of the apparatus shown in  FIG. 1A ; 
         FIG. 1C  shows a front view of the apparatus shown in  FIG. 1A ; 
         FIG. 2A  shows a side view of an apparatus for separating solids from solids laden drilling mud in accordance with the present invention; 
         FIG. 2B  shows a perspective view of the apparatus shown in  FIG. 2A ; 
         FIG. 2C  shows a front view of the apparatus shown in  FIG. 2A ; 
         FIG. 3A  is a side view of an apparatus for separating solids from solids laden drilling mud in accordance with the present invention; and 
         FIG. 3B  shows a front view of the apparatus shown in  FIG. 3A . 
         FIG. 3C  shows a front view of the apparatus shown in  FIG. 3A ; 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A to 1C  show a known shale shaker for separating solids from a solid laden drilling mud, generally identified by reference numeral  1 . The shale shaker  1  comprises a skid  2  which is fixed to a rig floor (not shown). A basket  3  is resiliently mounted on the skid  2  on four springs  4 . A vibratory mechanism  5  is arranged on a top front portion of the basket  3 . The vibratory mechanism comprises two motors  6 , each having a shaft provided with offset clump weights (not shown). The motors  6  drive the shafts rotating the offset clump weights to induce an oscillating motion to the basket  3 . The skid  2  is substantially isolated from the oscillations by the springs  4 . A scalping deck  7  and upper and lower primary decks  8  and  9  are arranged in the basket  3 . The scalping deck  7  is provided with rails  10  into which scalping screens are located (not shown). The scalping deck may extend the complete width of the basket  3  or part thereof. Upper and lower primary screen decks  8  and  9  each have two sets of rails  11 ,  12  and  13 ,  14  arranged next to each other in the same plane, each set for receiving primary screens (not shown). The scalping deck  7  is arranged at an uphill angle of preferably minus  2  to plus three and advantageously one degree from the feed end to the discharge end of the basket and the upper and lower primary decks  8  and  9  are arranged at an uphill angle of between two and ten degrees and preferably seven degrees. 
     The primary screens (not shown) on the upper and lower primary decks may include screening material of the same mesh grade, which would generally be used if the upper and lower primary decks  8  and  9  were used in a parallel mode or the screening material used in the lower screen deck may be of a finer mesh grade, which would generally be used if the upper and lower screen decks were used in a series mode. Parallel and series mode can be selected by changing the flow paths of the underflow of solids laden drilling fluid from the scalping deck and the underflow of drilling fluid from the upper primary deck. 
     A feed chute  15  is arranged above the rear of the basket  3  at a feed end  16 . In use, solids laden drilling fluid flows through the feed chute  15  on to scalping screens (not shown) in the scalping deck  7 . Large solids are conveyed inter alia by the motions induced in the basket  3  by the vibratory mechanism  5  to a discharge end of the basket  3 . Scalped solids laden drilling mud flows through the scalping deck on to a flow back pan  18  to the rear of the basket and is directed on to the upper and lower primary decks  8  and  9  in parallel mode or solely to upper primary deck  8  in series mode, as desired. Ponds of drilling mud form on each of the upper and lower primary decks  8  and  9 . Oscillatory motions induced in the screens in the primary decks cause solids to “walk out” of the ponds towards and over a discharge end  17  of the basket  3 . 
     The discharged solids fall into a ditch opening  19  of ditch  20  or other conveyor or conveying means (not shown). The ditch  20  has an auger (not shown) running therealong to move the solids to a discharge chute  21 . A grate  22  is seated over the ditch  20  to inhibit fouling of the auger (not shown) in the ditch  20 . 
     Screened drilling mud flows through screens in the upper and/or lower primary decks  8  and  9  and into a sump  23 . The sump  23  has an sump opening  24  through which screened drilling mud can flow back to a mud pit or on to be further processed. 
       FIGS. 2A to 2C  show the shale shaker in accordance with the present invention. A shale shaker such as the shale shaker shown in  FIG. 1A to 1C  is shrouded by a hood generally identified by reference numeral  101 . Reference will now be made to  FIGS. 2A to 2C  with further reference to parts shown in  FIGS. 1A to 1C . The hood  101  comprises a front cover  102  extending from the vibratory mechanism  5  at the top of the basket to the bottom of the skid  2  covering the ditch  20  across the width of the skid  2 . The front cover  102  has a vertical end wall  103  with an access opening therein covered by access doors  104  hinged on hinges  106  at each outer side, with handles  108  provided to facilitate opening of the access doors  104 . Curved side portions  105  wrap around the ditch opening  19  to front  17  of the basket  3 . A top curved portion  107  joins the vertical wall  103  to the rear cover  110 . The top curved portion  107  has a fume duct outlet  111  located at each side. Each fume duct outlet  111  is provided with a flange  112  for connecting to an air removal system such as an HVAC system. A curved access hatch  109  is provided in a top center portion of the front cover  102 . 
     The rear cover  110  has a top cover  113  extending from the rear of the basket  3  over the feed chute  15  and the top of the basket to meet the top of the front cover  102 . The hood  101  is spaced from the basket  3  to allow the basket  3  to oscillate within the hood  101 . An opening  114  is provided in the top cover  103  to allow the vibratory mechanism to project therethrough. An elastomeric bellows type seal  115  is arranged between the edge of the top cover defining the opening  114  and a plate  116  on which the vibratory mechanism  5  is seated. Walls  117  are arranged between the outer perimeter of the skid  2  and the outer perimeter of the top cover  113 . A face-to-face seal is used between the walls  117  and the outer perimeter of the skid  2 . Hatches  118  are provided between the walls  117  and the top cover to allow visual inspection and maintenance of the basket  3  therewithin. A handle  119  is provided to facilitate opening of the hatch  118 . A rear portion end wall (not shown) is provided on the rear end of the shale shaker attached to the skid  2  to complete the housing shrouding the basket  3 . 
     A cover plate  120  is arranged over the portion of the ditch  20  extending beyond the side of the front cover  103  to inhibit escape of fumes rising from solids in the ditch  20  and to control the quantity of replacement air flowing through the ditch  20 . 
     A sump cover plate  121  is placed over sump opening  24  in the rear portion of the skid towards the rear of the shale shaker. The sump cover plate  121  is provided with a sump discharge pipe stub  125  having a flange  126  for connection to a sump discharge pipe or hose  127  for returning drilling mud to a mud system, holding tank or mud pit (not shown). The sump discharge pipe stub  125  may have an internal diameter of approximately 250 mm (10 inches). The sump discharge pipe stub  125  is located close to the rear of the screens in the upper and lower screen decks  8  and  9 . The bottom of the skid  2  is preferably provided with a plate to inhibit air from rising through the shale shaker although a solid floor on which the shale shaker may be placed may be sufficient to inhibit air from flowing from under the shale shaker in an uncontrolled way. The plate may have an opening therein preferably at a rear end, to control the flow of replacement air through the basket  3  of the shale shaker. 
     Solids augured along ditch  20  fall through outlet  21  which may be provided with a flange for connection to another pipe or allow solids to discharge directly into a solids movement apparatus such as a positive pneumatic pressure conveying system such as the system provided by NATIONAL OILWELL VARCO® under the trade mark FREEFLOW™. 
     In use, solids laden drilling mud flows through the feed chute  15  into the basket  3  on to screens in the scalping deck  7 . Large solids are scalped-off, passing over the scalping deck  7  and into ditch  20 . Drilling mud having small solids therein passes on to fine screens in the upper and lower decks  8  and  9 , as previously described. Screened solids pass over the upper and lower decks into the ditch  20 , although a graded portion may be returned to the sump  23 . Screened drilling mud passes through the fine screens into the sump  23 . Fumes may develop as a result of the fluid being spread thinly across the screens, increasing the surface area open to the atmosphere within the basket  3 . Furthermore, the drilling mud may return hot from the well, up to approximately 120 Celsius and with further energy imparted to the drilling mud, the drilling mud may be kept hot. Thus there is a likelihood of fumes rising from the drilling mud on the screens. The fumes flow naturally or are sucked under negative pressure through the fume duct outlets  111  into an air conditioning system such as an HVAC system (not shown) for safe removal of the fumes from the shale shaker. Replacement air is drawn through the ditch  20  and through the sump discharge pipe  127 . It should be noted that the sump discharge pipe  127  is sized so that drilling mud flows therethrough with a large head of air thereabove, thus a large diameter pipe is used. Replacement air flows from the sump discharge pipe  127  and is drawn across the screens in the upper and lower decks  8  and  9  to the space defined by the ditch  20  and front cover  103  allowing fumes to be collected and drawn off into the fume duct outlets  111 . Any fumes rising from solids within the ditch  20  will flow with the replacement air into the space enclosed by the front cover  103  and rise into fume duct outlets  111  and into the air conditioning system (not shown). 
       FIGS. 3A to 3C  show the shale shaker in accordance with the present invention. Reference will now be made to  FIGS. 3A to 3C  with further reference to parts shown in  FIGS. 1A to 1C . A shale shaker such as the shale shaker shown in  FIGS. 1A to 1C  is shrouded by a hood generally identified by reference numeral  201 , save for a solids collection chute  220  in place of the ditch  20 . The hood  201  comprises a front cover  202  extending from the vibratory mechanism  5  at the top of the basket  3  to the bottom of the skid  2  covering the ditch  20  across the width of the skid  2 . The front cover  202  has a vertical end wall  203  with an access opening therein covered by access doors  204  hinged on hinges  206  at each outer side, with handles  208  provided to facilitate opening of the access doors  204 . Curved side portions  205  wrap around a solids collection chute opening  219  to front  17  of the basket  3 . A top curved portion  207  joins the vertical wall  203  to rear cover  210 . The top curved portion  207  has a fume duct outlet  211  located at each side. Each fume duct outlet  211  is provided with a flange  212  for connecting to an air removal system such as an HVAC system (not shown). A curved access hatch  209  is 15 provided in a top center portion of the front cover  202 . 
     The rear cover  210  has a top cover  213  extending from the rear of the basket  16  over the feed chute  15  and the top of the basket  3  to meet the top of the front cover  202 . The hood  201  is spaced from the basket  3  to allow the basket  3  to oscillate within the hood  201 . An opening  214  is provided in the top cover  213  to allow the vibratory mechanism to project therethrough. An elastomeric bellows type seal  215  is arranged between the edge perimeter of the top cover defining the opening  214  and a plate  216  on which the vibratory mechanism  5  is seated. Walls  217  are arranged between the outer perimeter of the skid  2  and the outer perimeter of the top cover  213 . A face-to-face seal is used between the walls  217  and the outer perimeter of the skid  2 . Hatches  218  are provided between the walls  217  and the top cover  213  to allow visual inspection and maintenance of the basket  3  therewithin. A handle  219  is provided to facilitate opening of the hatch  218 . A rear cover end wall (not shown) is provided on the rear end  16  of the shale shaker attached to the skid  2  to complete the housing shrouding the basket  3 . 
     A sump cover plate  221  is placed over sump opening  24  in the rear portion of the skid towards the rear of the shale shaker. The sump cover plate  221  is provided with a sump discharge pipe stub  225  having a flange  226  for connection to a sump discharge pipe or hose  227  for returning drilling mud to a mud system, holding tank or mud pit (not shown). The sump discharge pipe stub  225  may have an internal diameter of approximately 250 mm (10 inches). The sump discharge pipe stub  225  is located close to the rear of the screens in the upper and lower screen decks  8  and  9 . The bottom of the skid  2  is preferably provided with a plate to inhibit air from rising through the shale shaker although a solid floor on which the shale shaker may be placed may be sufficient to inhibit air from flowing from under the shale shaker in an uncontrolled way. The plate may have an opening therein preferably at a rear end, to control the flow of replacement air through the basket  3  of the shale shaker. 
     The solids collection chute  220  converges to an outlet  222  which may be provided with a flange for connection to another pipe or to allow solids to discharge directly into a solids movement apparatus, such as a positive pneumatic pressure conveying system, such as the system provided by NATIONAL OILWELL VARCO® under the trade mark FREEFLOWM™. The solids collection chute  220  may be designed to achieve mass flow of the solids into the outlet  222  to inhibit bridging or rat holing of the solids. 
     In use, solids laden drilling mud flows through the feed chute  15  into the basket  3  on to screens in the scalping deck  7 . Large solids are scalped-off, passing over the scalping deck  7  and into solids collection chute  220 . Drilling mud having small solids therein passes on to fine screens in the upper and lower decks  8  and  9 , as previously described. Screened solids pass over the upper and lower decks into the solids collection chute  220 , although a graded portion may be returned to the sump  23 . Screened drilling mud passes through the fine screens into the sump  23 . Fumes may develop as a result of the fluid being spread thinly across the screens, increasing the surface area open to the atmosphere within the basket  3 . Furthermore, the drilling mud may return hot from the well, up to approximately 120 Celsius and with further energy imparted to the drilling mud, the drilling mud may be kept hot. Thus there is a likelihood of fumes rising from the drilling mud on the screens. The fumes flow naturally or are sucked under negative pressure through the fume duct outlets  211  into an air conditioning system such as a HVAC system (not shown) for safe removal of the fumes from the shale shaker. Replacement air is drawn through the sump discharge pipe  227  and the solids collection chute  220 . It should be noted that the sump discharge pipe  227  is sized so that drilling mud flows therethrough with a large head of air thereabove, thus a large diameter pipe is used. Replacement air flows from the sump discharge pipe  227  and is drawn across the screens in the upper and lower decks  8  and  9  to the space defined by the solids collection chute  220  and front cover  202  allowing fumes to be collected and drawn off into the fume duct outlets  211 . Any fumes rising from solids within the solids collection chute  220  will rise into fume duct outlets  211  and into the air conditioning system (not shown). 
     The flow rate of air an fumes being drawn off through the fume duct outlets is between 0.5 and 3 cubic meters per second, and preferably between one and two cubic meters per second, and most preferably one cubic meter per second. 
     A bank of shale shakers shown in  FIGS. 2A to 3C  may be used on a drilling rig, with each of the fume duct outlets  111 , 211  linked by at least one duct with a ventilation system such as an HVAC system, using a screw fan or roots type fan to create negative pressure to draw fume laden air from the hood enclosing the basket.