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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE DISCLOSURE 
     1. Field of Disclosure 
     The present disclosure relates to vibrating tools in general, and in particular to a method and apparatus for vibrating a downhole tool in a drill string. 
     2. Description of Related Art 
     In the field of drilling, friction may frequently impair the ability of the drill string to be advanced within the hole. For example, highly deviated holes or horizontal drilling cannot rely on the weight of the drill pipe alone to overcome friction from the horizontal pipe resting against the wall of the hole. 
     Conventional vibration tools have alternatingly increased the pressure of the drilling fluid within the drill string by cyclically blocking and unblocking the flow of the drilling fluid within the drill string. Such devices accordingly cyclically increase the pressure of the drilling fluid within the drill string and then release it. Such devices disadvantageously require a high supply pressure over and above the supply pressure for the drilling fluid. This increases cost and complexity of the machinery required to support this operation. In addition, many conventional vibration tools involve complex downhole systems and devices which may be more prone to breakage. 
     Many such conventional vibration tools also create backpressure in the drilling fluid supply. This has the negative consequences of requiring supply pumps of greater capacity and also reduces the supply pressure to the drilling bit. Still other apparatuses have utilized blunt mechanical impacts which increases the wear life and the complexity of the design. 
     SUMMARY OF THE DISCLOSURE 
     According to a first embodiment there is disclosed a method of vibrating a downhole drill string. The method comprises pumping a drilling fluid down the drill string and cyclically venting the drilling fluid through a valve in a side wall of the drilling string so as to cyclically reduce the pressure of the drilling fluid in the drill string. 
     The method may further comprise rotating a rotor within a tubular body located in-line within the drill string wherein the venting comprises intermittently passing the drilling fluid through a rotor port in the rotor and a corresponding tubular body port in the tubular body. The rotor may be rotated by the drilling fluid. 
     The method may further comprise separating the drilling fluid into a central bypass portion and an annular rotor portion, passing the bypass portion past the rotor and rotating the rotor with the rotor portion. The bypass portion and the rotor portion may be combined after the rotor portion rotates the rotor wherein the rotor port and the tubular port pass the combined rotor portion and the bypass portion therethrough. 
     According to a further embodiment there is disclosed an apparatus for vibrating a downhole drill string. The drill string is operable to have a drilling fluid pumped therethrough. The apparatus comprises a tubular body securable to the drill string and having a central bore therethrough, a valve in the tubular body for venting the drilling fluid out of the drill string and a valve actuator for cyclically opening and closing the valve. 
     The valve may comprise a radial tubular body port in the tubular body and a rotor located within the central bore having a radial rotor port wherein the rotor port is selectably alignable with the tubular body port as the rotor rotates within the central bore. The valve actuator may comprise at least one vane on the rotor for rotating the rotor as the drilling fluid flows therepast. The rotor may include a central bypass bore therethrough and a plurality of vanes radially arranged around the central bypass bore. 
     The apparatus may further comprise a separator for separating the drilling fluid into a bypass portion and a rotor portion secured within the central bore, the rotor portion being directed onto the plurality of vanes so as to rotate the rotor, the bypass portion being directed though the bypass bore of the rotor. The separator may include a central bypass port and an annular rotor passage therearound. The separator may be located adjacent to the rotor such that the central bypass port of the separator directs the bypass portion of the drilling fluid though the bypass bore of the rotor and wherein the rotor passage of the separator directs the rotor portion of the drilling fluid onto the plurality of vanes of the rotor. The rotor passage of the separator may include stator vanes for directing the rotor portion of the drilling fluid onto the plurality of vanes. 
     The apparatus may further comprise a plurality of rotor ports selectably alignable with a plurality of tubular body ports. Each of the plurality of rotor ports may be selectably alignable with a unique tubular body port. 
     The tubular body may be connectable inline within a drill string. The tubular body may include threaded end connectors for linear connection within a drill string. 
     The bypass port of the separator may include an inlet shaped to receive a blocking body so as to selectably direct more drilling fluid through the rotor passage. The inlet may have a substantially spherical shape so as to receive a spherical blocking body. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate embodiments of the invention wherein similar characters of reference denote corresponding parts in each view, 
         FIG. 1  is a perspective view of the vibrating downhole tool located within a drill string. 
         FIG. 2  is a partial cross-sectional perspective view of a vibrating downhole tool according to a first embodiment. 
         FIG. 3  is a perspective view of a separator of the apparatus of  FIG. 2 . 
         FIG. 4  is a perspective view of a rotor of the apparatus of  FIG. 2 . 
         FIG. 5  is a cross sectional view of the apparatus of  FIG. 2  taken along the line  5 - 5  with the rotor at a first position. 
         FIG. 6  is a cross sectional view of the apparatus of  FIG. 2  taken along the line  5 - 5  with the rotor at a second position. 
         FIG. 7  is a perspective view of the flow separator of the apparatus of  FIG. 2  according to a further embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a drill string  10  is illustrated down a bore hole  8  in a soil or rock formation  6 . The drill string includes a drill bit  12  at a lower end  14  thereof and an apparatus according to a first embodiment shown generally at  20  for vibrating the drill string within the bore hole  8 . The apparatus  20  may be located proximate to the lower end  14  of the drill string  10  or at an intermediate portion  16  of the drill string  10 . It will also be appreciated that a plurality of apparatuses  20  may be located at a plurality of locations along the drill string. 
     Turning now to  FIG. 2 , the apparatus  20  comprises a tubular body  30 , a flow separator  60  and a rotor  80 . The tubular body  30  has a cylindrical wall  31  having inner and outer surfaces  32  and  34 , respectively extending between inlet and outlet ends,  36  and  38 , respectively. The inner surface  32  defines a central bore  40 . The tubular body  30  includes at least one radial tubular body port  42  extending therethrough. The tubular body port  42  may be formed as a bore through the wall  31  or may optionally be located within a tubular body port insert  44  as illustrated in  FIG. 2 . The use of a tubular body port insert  44  facilitates the interchangability of tubular body port  42  of differing sizes as will be further described below. 
     As illustrated the tubular body port insert  44  may be threadably secured within the wall  31  or by any other suitable means, such as by way of non-limiting example, compression fit, latches, retaining clips or the like. As illustrated, the tubular body port  42  may have a throttling cross section such that the tubular body port  42  is wider proximate to the interior surface  32  of the tubular body than proximate to the exterior surface  34 . The use of a throttling cross section will assist in controlling the volume of drilling fluid vented therethrough. The tubular body port insert  44  may be sealed to the tubular body  30  with an o-ring to prevent washout and backed with a snap ring to prevent the tubular body port insert  44  from backing out. 
     The inlet and outlet ends  36  and  38  of the tubular body  30  may include interior and exterior threading  46  and  48 , respectively, for securing the tubular body in-line with the drill string  10 . It will be appreciated that the interior and exterior threading  46  and  48  will be of a conventional type, such as a pin/box type to facilitate ready connection with the drill string  10 . The tubular body  30  is of steel construction and is surface hardened for durability and abrasion resistance. 
     The flow separator  60  comprises a disk shaped body having a central bypass passage  62  and a plurality of rotor passages  64  distributed radially around the bypass passage. The flow separator  60  is sized to be located within the central bore  40  of the tubular body as illustrated in  FIG. 2 . Turning now to  FIG. 3 , the flow separator  60  comprises an outer cylinder  66  and an inner cylinder  68  with a plurality of radial support arms  70  extending therebetween. The outer cylinder  66  includes an outer surface  72  sized to be securely received within the central bore  40  of the tubular body  30 . The inner cylinder includes an inner surface  74  defining the bypass passage. The inner cylinder  68 , outer cylinder  66  and the support arms  70  define the rotor passages  64 . 
     With reference to  FIG. 4 , the rotor  80  comprises a substantially cylindrical body having inlet and outlet sections,  82  and  84 , respectively and a turbine section  86  therebetween. The rotor inlet section  82  of the rotor comprise an outer sleeve  90  and a bypass cylinder  88  defining an annular rotor passage  92  therebetween. The outer sleeve  90  includes an outer surface  104 . The bypass cylinder  88  defines a bypass passage  94  therethrough and as a distal end  96  extending substantially into the turbine section  86  as illustrated in  FIG. 4 . The turbine section  86  comprises a plurality of vanes  98  extending angularly from the inlet to outlet sections  82  and  84 . Proximate to the inlet section  82 , the vanes  98  extend between the outer sleeve  90  and the bypass cylinder  88  so as to provide support for the bypass cylinder. The vanes  98  include an exterior surface  106  corresponding to the outer surface  104  of the outer sleeve  90 . The outlet section  84  comprises an outlet sleeve  100  having a rotor port  102  in a sidewall thereof. The outlet sleeve  100  has an outer surface  108 . The outer surfaces of the outer sleeve  90 , the vanes  98  and the outlet sleeve  100  act as a bearing surface to permit the rotor  80  to freely rotate within the central bore  40  of the tubular body  30 . The rotor  80  may be formed of any suitable material such as steel and may be surface hardened for resistance to impact and surface abrasion. The rotor may be machined as a single component. Alternatively, the rotor may be formed of a plurality of components which are fastened, welded or otherwise secured to each other. 
     The apparatus  20  may be assembled by rotatably locating the rotor  80  and fixably locating the fluid separator  60  within central bore  40  of the tubular body. The rotor is located such that the rotor port  102  is alignable with the tubular body port  42  and the flow separator  60  is located adjacent to the inlet section of the rotor  80 . The rotor passages  64  of the separator direct drilling fluid into the rotor passage  92  of the rotor while the bypass passage  62  of the flow separator  60  directs a bypass portion of the drilling fluid through the bypass passage  94  of the rotor. The rotor portion of the drilling fluid passed through the rotor passage  92  of the rotor will encounter the vanes  98  thereby causing the rotor to rotate. As the rotor  80  rotates within the tubular body  30 , the rotor port  102  will be intermittently aligned with the tubular body port  42  so as to intermittently jet a portion of drilling fluid therethrough. Each ejection of drilling fluid through the rotor port  102  and tubular body port  42  causes a reduction of the pressure of the drilling fluid within the drill string and a corresponding low pressure wave through such drilling fluid. The intermittent ejection of the drilling fluid will create a resonant frequency to be established within the drilling fluid from the multiple low pressure pulses. The multiple pulses causes a vibration to be transmitted from the drilling fluid to the drill string  10  so as to vibrate the drill string  10  within the bore hole  8 . 
     With reference to  FIG. 2 , the central bore  40  of the tubular body  30  may have an inlet section  110  sized to receive the flow separator  60  snugly therein. The inlet section  110  may end at a first shoulder  112  for retaining the flow separator within the inlet section of the central bore  40 . The flow separator may also be retained against the first shoulder  112  by a snap ring  114  or other suitable means. The flow separator  60  may also be sealed within the inlet section  110  by an o-ring  116  or other suitable means. The central bore  40  also includes a rotor portion  120  sized to rotatably receive the rotor  80  therein. The rotor portion  120  ends in a second shoulder  122  for retaining the rotor  80  within the rotor section  120 . The flow separator  60  serves to retain the rotor  80  against the second shoulder. The apparatus may also include a wear ring  124  sized to abut against the second shoulder  122  and provide an enlarged surface to retain the rotor  80  within the rotor section  120 . The wear ring  124  may be sealed within the rotor section by an o-ring  126  or the like. As shown in  FIG. 2 , the wear ring  124  functions as a thrust bearing against the rotor  80 . The wear ring  124  is easily replaceable and expendable. Grooves in the bearing surface help prevent debris from collecting on the bearing surface, thus improving the wear rate. Multiple material types can be used depending on the application. Alternative bearing types such as rolling element bearings are also applicable. The rotor  80  and the flow separator  60  may be inserted into the tubular body  30  through the inlet end  36  of the apparatus and are sized to fit through the internal threading  46 . 
     As described above, the flow separator  60  is a flow distributing device which directs a prescribed amount of drilling fluid flow through to the vanes  98  of the rotor  80 . As illustrated in  FIG. 2 , drilling fluid is pumped downwards within the drill string  10  and therefore through the apparatus  20  as indicated generally at  142 . By correctly sizing or adjusting the rotor passage  64  the flow separator will direct sufficient flow through the rotor  80  to allow the rotor to spin at the desired rotational speed. The remaining flow is directed through the bypass passage  62  and subsequently through a bypass passage  94  of the rotor  80 . The diameter of the bypass passage  62  can be adjusted to allow for variations in fluid flow rate and fluid properties. The bypass passage  62  of the flow separator  60  may also be included in a threaded orifice plug (with or without a centre bore) in the centre of the flow separator  60  to permit the bypass passage  62  size can be adjusted without replacing the flow separator. 
     The rotor  80  is designed to spin at a set rotational speed. To achieve this, the rotor is designed to be free spinning and rotate at its runaway speed. As the flow enters the rotor  80  through the rotor passage  92  and is then directed onto the vanes  98 . The angle of the vanes  98  determine the runaway speed of the turbine for a given flow rate. Closing the bypass passage  94  entirely (i.e. sending all available flow through the rotor passage  92 ) will allow the rotor to maintain its intended rotational speed should the flow rate be reduced by 50%. As the rotor  80  rotates, drilling fluid is jetted through the rotor port  102  and the tubular body port  42  once per revolution when the rotor port and tubular body port are aligned. As illustrated in  FIG. 5 , the rotor  80  is illustrated in a first or closed position within the tubular body  30 . As illustrated, the rotor pot  102  and the tubular body port  42  are not aligned and therefore no drilling fluid is passed therethrough. Turning now to  FIG. 6 , the rotor is illustrated in a second or open position within the tubular body  30 . In the open position, the rotor port  102  and the tubular body port  42  are aligned and therefore the drilling fluid is passed therethrough as indicated generally at  140 . The second position is generally referred to herein as a jetting event. 
     The width of the rotor port  102  determines the duration of the jetting event and can be varied depending on the demands of the application. The diameter of the tubular body port  42  may also be sized to vary the volume of drilling fluid ejected during a jetting event and thereby to vary the impulse delivered to the apparatus  20  by that jetting event. Although one tubular body port  42  is illustrated, it will be appreciated that a plurality of tubular body ports  42  may be utilized. Such plurality of tubular body ports  42  may be located to jet drilling fluid at a common or a different time as desired by the user. Furthermore, the plurality of tubular body ports  42  may be located at different lengthwise locations along the tubular body  30 . The rotor port  102  may therefore have a variable width from the top to the bottom such that when a specific tubular body port  42  is selected, the apparatus  20  will have a jetting event length corresponding to the width of the rotor port  102  at that location. All other tubular body ports  42  will therefore be plugged. In other embodiments, a plurality of rotor ports  102  may be utilized each having a unique length and a corresponding tubular body port  42  to produce a jetting event of a desired duration. 
     With reference to  FIG. 7 , inlet to the bypass passage  62  of the flow separator  60  may also be shaped to allow a blocking body (not shown) to land therein so as to partially block the bypass passage  62  thereby altering the flow distribution and the rotational speed of the turbine. The blocking body may comprise a spherical body although it will be appreciated that other shapes may be useful as well. This allows the torque capacity/speed of the apparatus to be adjusted during operation, without returning the apparatus to surface. In a further embodiment, the support arms  70  of the flow separator may be shaped to act as turbine stator blades, thereby increasing the torque capability of the rotor  80 . This additional torque may be required for heavy or viscous mud conditions. 
     With reference to  FIG. 7 , inlet to the bypass passage  62  of the flow separator  60  may also be shaped to allow a blocking body (not shown) to land therein so as to partially block the bypass passage  62  thereby altering the flow distribution and the rotational speed of the turbine. The blocking body may comprise a spherical body although it will be appreciated that other shapes may be useful as well. This allows the torque capacity/speed of the apparatus to be adjusted during operation, without returning the apparatus to surface. In a further embodiment, the support arms  70  of the flow separator may be shaped to act as turbine stator blades, thereby increasing the torque capability of the rotor  80 . This additional torque may be required for heavy or viscous mud conditions. 
     The apparatus  20  creates pressure fluctuations that induce vibration in a drill string  10  and create a time varying WOB (weight on bit) with a cycling frequency of approximately 15-20 Hz (the natural frequency of the drill string). This vibration or hammering effect reduces wall friction and improves the transfer of force on to the drill bit. The rotor port  102  and the tubular body port  42  function as a valve that is cyclically opened and closed by the rotation of the rotor. It will be appreciated that such a valve function may be provided in another means for venting the drilling fluid from the drill string such as through the use of common valves as known in the art. It will also be appreciated that the tubular body port  42  may be selectably opened by a wide variety of methods. By way of non-limiting example, the tubular body port  42  may be cyclically opened by a solenoid valve or other suitable means or through the use of a motor for rotating the rotor  80 . It will be appreciated that in such embodiments, the flow separator  60  and rotor  80  will not be necessary. 
     While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.

Summary:
Disclosed is an apparatus for vibrating a downhole drill string operable to have a drilling fluid pumped therethrough. The apparatus comprises a tubular body securable to the drill string and having a central bore therethrough, a valve in the tubular body for venting the drilling fluid out of the drill string and a valve actuator for cyclically opening and closing the valve. The method comprises pumping a drilling fluid down the drill string and cyclically venting the drilling fluid through the valve so as to cyclically reduce the pressure of the drilling fluid in the drill string. The valve may comprise a tubular body port and a corresponding rotor port selectably alignable with the tubular body port as the rotor rotates within the central bore. The valve actuator may comprise at least one vane on the rotor for rotating the rotor as the drilling fluid flows therepast.