Patent Publication Number: US-9840909-B2

Title: Flow bypass sleeve for a fluid pressure pulse generator of a downhole telemetry tool

Description:
RELATED APPLICATIONS 
     This is a national stage application under 35 U.S.C. §371 of International Patent Application No. PCT/CA2015/050586, filed Jun. 25, 2015, which claims benefit of U.S. Provisional Patent Application No. 62/016,890, filed Jun. 25, 2014, both of which are incorporated by reference in their entireties. 
    
    
     FIELD 
     This invention relates generally to a flow bypass sleeve for use with a fluid pressure pulse generator of a downhole telemetry tool, such as a mud pulse telemetry measurement-while-drilling (“MWD”) tool. 
     BACKGROUND 
     The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to a below-surface formation or subterranean zone of interest. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. The process also involves a drilling fluid system, which in most cases uses a drilling “mud” that is pumped through the inside of piping of the drill string to cool and lubricate the drill bit. The mud exits the drill string via the drill bit and returns to surface carrying rock cuttings produced by the drilling operation. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface. 
     Directional drilling is the process of steering a well from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) the drill bit; 2) a steerable downhole mud motor of a rotary steerable system; 3) sensors of survey equipment used in logging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”) to evaluate downhole conditions as drilling progresses; 4) means for telemetering data to surface; and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars (i.e. drill pipe). MWD equipment is used to provide downhole sensor and status information to surface while drilling in a near real-time mode. This information is used by a rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, and hydrocarbon size and location. The rig crew can make intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real-time MWD data allows for a relatively more economical and more efficient drilling operation. 
     One type of downhole MWD telemetry known as mud pulse telemetry involves creating pressure waves (“pulses”) in the drill mud circulating through the drill string. Mud is circulated from surface to downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. The pressure pulses are achieved by changing the flow area and/or path of the drilling fluid as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling fluid. The pressure differentials or pulses may be either negative pulses or positive pulses. Valves that open and close a bypass stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. All negative pulsing valves need a high differential pressure below the valve to create a sufficient pressure drop when the valve is open, but this results in the negative valves being more prone to washing. With each actuation, the valve hits against the valve seat and needs to ensure it completely closes the bypass; the impact can lead to mechanical and abrasive wear and failure. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Pulse frequency is typically governed by pulse generator motor speed changes. The pulse generator motor requires electrical connectivity with the other elements of the MWD probe. 
     One type of valve mechanism used to create mud pulses is a rotor and stator combination where a rotor can be rotated relative to the stator between an open flow position where there is no restriction of mud flowing through the valve and no pulse is generated, and a restricted flow position where there is restriction of mud flowing through the valve and a pressure pulse is generated. 
     SUMMARY 
     According to a first aspect, there is provided a flow bypass sleeve for a fluid pressure pulse generator of a downhole telemetry tool, the fluid pressure pulse generator comprising a stator having one or more flow channels or orifices through which drilling fluid flows and a rotor which rotates relative to the stator to move in and out of fluid communication with the flow channels or orifices to create fluid pressure pulses in the drilling fluid flowing through the flow channels or orifices, wherein the flow bypass sleeve is configured to fit inside a drill collar which houses the telemetry tool and comprises a body with a bore therethrough which receives the fluid pressure pulse generator, the body including at least one longitudinally extending bypass channel comprising a groove longitudinally extending along an internal surface of the body or an aperture longitudinally extending through the body, wherein the bypass channel extends across at least a portion of both the stator and the rotor when the fluid pressure pulse generator is received in the bore such that the drilling fluid flows along the bypass channel in addition to flowing through the flow channels or orifices of the stator. 
     According to a second aspect, there is provided a flow bypass sleeve for a fluid pressure pulse generator of a downhole telemetry tool. The fluid pressure pulse generator comprises a stator having one or more flow channels or orifices through which drilling fluid flows and a rotor which rotates relative to the stator to move in and out of fluid communication with the flow channels or orifices to create fluid pressure pulses in the drilling fluid flowing through the flow channels or orifices. The flow bypass sleeve is configured to fit inside a drill collar which housing the telemetry tool and comprises a body with a bore therethrough which receives the fluid pressure pulse generator. The body includes at least one longitudinally extending bypass channel with an uphole axial channel inlet and a downhole axial channel outlet. The bypass channel extends across at least a portion of both the stator and the rotor when the fluid pressure pulse generator is received in the bore such that the drilling fluid flows along the bypass channel in addition to flowing through the flow channels or orifices of the stator. 
     The flow bypass sleeve may comprise a plurality of bypass channels comprising at least one groove longitudinally extending along an internal surface of the body and at least one aperture longitudinally extending through the body. 
     The body may comprise an uphole section, a downhole section and a central section positioned therebetween. The diameter of the bore in the central section of the body may be less than the diameter of the bore in the uphole and downhole sections of the body. The at least one bypass channel may comprise a channel inlet and a channel outlet. The at least one bypass channel may extend longitudinally through the central section of the body and the channel inlet may be in fluid communication with the bore in the uphole section of the body and the channel outlet may be in fluid communication with the bore in the downhole section of the body. The uphole section of the body may taper in the uphole direction. The downhole section of the body may taper in the downhole direction. The bypass channel may comprise a groove longitudinally extending along an internal surface of the central section of the body. The bypass channel may comprise an aperture longitudinally extending through the central section of the body. The flow bypass sleeve may comprise a plurality of bypass channels comprising at least one groove longitudinally extending along an internal surface of the central section of the body and at least one aperture longitudinally extending through the central section of the body. The downhole section of the body may include at least one downhole groove longitudinally extending along an internal surface thereof. The downhole groove may have an uphole axial groove inlet and a downhole axial groove outlet. The groove inlet may be fluidly connected to the channel outlet of the aperture. 
     An external surface of the body may comprise a first portion and a second portion. An external circumference of the first portion may be less than an external circumference of the second portion. The flow bypass sleeve may further comprise an outer sleeve which surrounds the first portion of the body. An external surface of the outer sleeve may be flush with an external surface of the second portion of the body. The outer sleeve may comprise a first material and the second portion of the body may comprise a second material with a thermal expansion coefficient that is different to a thermal expansion coefficient of the first material. The outer sleeve may be positioned downstream to the second portion of the body. The outer sleeve may be axially adjacent the second portion of the body. The outer sleeve may be releasably positioned on the first portion of the body. 
     The external surface of the body may further comprise a third portion with an external circumference less than the external circumference of the second portion. The third portion may be configured to be inserted in a keying ring fitted in the drill collar. A keying mechanism on an external surface of the flow bypass sleeve may be configured to mate with a keying mechanism on the keying ring to align the flow bypass sleeve within the drill collar. 
     The external surface of the body may further comprise a third portion with an external circumference less than the external circumference of the second portion, wherein the third portion is configured to be inserted in a mounting ring in the drill collar to mount the flow bypass sleeve in the drill collar. The flow bypass sleeve may further comprise an alignment mechanism configured to mate with an alignment mechanism on the mounting ring to align the flow bypass sleeve within the drill collar. 
     The third portion may be axially adjacent and upstream to the second portion of the body. 
     The flow bypass sleeve may further comprise a longitudinally extending bypass channel insert releasably positioned in the bypass channel to reduce a flow area of the bypass channel. The body may include a plurality of longitudinally extending bypass channels and a plurality of longitudinally extending bypass channel inserts may be releasably positioned in the plurality of bypass channels to reduce the total flow area of the bypass channels. 
     The bypass channel may comprise the aperture and the bypass channel insert may comprise a tubular insert with an insert aperture therethrough. The flow bypass sleeve may further comprise a longitudinally extending tubular insert releasably positioned in the aperture to reduce a flow area of the aperture. The body may include a plurality of longitudinally extending apertures therethrough and a plurality of longitudinally extending tubular inserts may be releasably positioned in the plurality of apertures to reduce the total flow area of the apertures. The tubular insert may have an uphole shoulder section with an external circumference greater than an internal circumference of the aperture and a downhole edge of the shoulder section may abut an internal surface of the body when the tubular insert is positioned in the aperture. The flow bypass sleeve may further comprise a retaining ring releasably attached to the tubular insert to releasably retain the tubular insert in the aperture. The flow bypass sleeve may further comprise a fastener to releasably retain the bypass channel insert in the aperture. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool and a plurality of flow bypass sleeves according to the first or second aspect. The plurality of flow bypass sleeves each have a different outer circumference such that each of the plurality of flow bypass sleeves can be received in a different sized drill collar. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool and a first and second flow bypass sleeve according to the first or second aspect. The first flow bypass sleeve has a greater outer circumference compared to the outer circumference of the second flow bypass sleeve such that the first flow bypass sleeve can be received in a first drill collar and the second flow bypass sleeve can be received in a second drill collar whereby the internal diameter of the first drill collar is greater than the internal diameter of the second drill collar. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool and a first and second flow bypass sleeve according to the first or second aspect. The first and second flow bypass sleeve both have corresponding internal dimensions configured to receive the fluid pressure pulse generator and the first flow bypass sleeve has a greater outer circumference compared to the outer circumference of the second flow bypass sleeve such that the first flow bypass sleeve can be received in a first drill collar and the second flow bypass sleeve can be received in a second drill collar whereby the internal diameter of the first drill collar is greater than the internal diameter of the second drill collar. 
     A total flow area of the at least one bypass channel of the first flow bypass sleeve may be greater than a total flow area of the at least one bypass channel of the second flow bypass sleeve. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool and a plurality of flow bypass sleeves according to the first or second aspect. A total flow area of the at least one bypass channel is different for each of the plurality of flow bypass sleeves. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool and a first and second flow bypass sleeve according to the first or second aspect. A total flow area of the at least one bypass channel of the first flow bypass sleeve is different to a total flow area of the at least one bypass channel of the second flow bypass sleeve. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool, the flow bypass sleeve according to the first or second aspect, and a longitudinally extending bypass channel insert that can be releasably positioned in the bypass channel to reduce a flow area of the bypass channel. 
     The body of the sleeve may include a plurality of longitudinally extending bypass channels and the kit may comprise a plurality of longitudinally extending bypass channel inserts that can be releasably positioned in the plurality of bypass channels to reduce the total flow area of the bypass channels. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool, the flow bypass sleeve according to the first or second aspect, and a longitudinally extending tubular insert that can be releasably positioned in the aperture to reduce a flow area of the aperture. 
     The body may include a plurality of longitudinally extending apertures therethrough and the kit may comprise a plurality of longitudinally extending tubular inserts that can be releasably positioned in the plurality of apertures to reduce the total flow area of the apertures. The tubular insert may have an uphole shoulder section with an external circumference greater than an internal circumference of the aperture and a downhole edge of the shoulder section may abut an internal surface of the body when the tubular insert is positioned in the aperture. The kit may further comprise a retaining ring that can be releasably attached to the tubular insert to releasably retain the tubular insert in the aperture. 
     According to another aspect, there is provided a kit comprising a fluid pressure pulse generator of a downhole telemetry tool and a flow bypass sleeve. The fluid pressure pulse generator comprises a stator and a rotor. The stator has a stator body and a plurality of radially extending stator projections spaced around the stator body, whereby adjacently spaced stator projections define stator flow channels extending therebetween. The rotor has a rotor body and a plurality of radially extending rotor projections spaced around the rotor body. The rotor projections are axially adjacent the stator projections and the rotor is rotatable relative to the stator such that the rotor projections move in and out of fluid communication with the stator flow channels to create fluid pressure pulses in drilling fluid flowing through the stator flow channels. The flow bypass sleeve comprises a sleeve body with a bore therethrough which receives the fluid pressure pulse generator. The sleeve body includes at least one longitudinally extending bypass channel with an uphole axial channel inlet and a downhole axial channel outlet. The bypass channel extends across both the stator projections and the rotor projections when the fluid pressure pulse generator is received in the bore, such that the drilling fluid flows along the bypass channel in addition to flowing through the stator flow channels. 
     The bypass channel may comprise a groove longitudinally extending along an internal surface of the sleeve body. The bypass channel may comprise an aperture longitudinally extending through the sleeve body. The sleeve body may include a plurality of bypass channels comprising at least one groove longitudinally extending along an internal surface of the sleeve body and at least one aperture longitudinally extending through the sleeve body. 
     The sleeve body may comprise an uphole section, a downhole section and a central section positioned therebetween. The diameter of the bore in the central section of the sleeve body may be less than the diameter of the bore in the uphole and downhole sections of the sleeve body. The at least one bypass channel may extend longitudinally through the central section of the sleeve body and the channel inlet may be in fluid communication with the bore in the uphole section of the sleeve body and the channel outlet may be in fluid communication with the bore in the downhole section of the sleeve body. The uphole section of the sleeve body may taper in the uphole direction. The downhole section of the sleeve body may taper in the downhole direction. The bypass channel may comprise a groove longitudinally extending along an internal surface of the central section of the sleeve body. The bypass channel may comprise an aperture longitudinally extending through the central section of the sleeve body. The sleeve body may include a plurality of bypass channels comprising at least one groove longitudinally extending along an internal surface of the central section of the sleeve body and at least one aperture longitudinally extending through the central section of the sleeve body. The downhole section of the sleeve body may include at least one downhole groove longitudinally extending along an internal surface thereof. The downhole groove may have an uphole axial groove inlet and a downhole axial groove outlet and the groove inlet may be fluidly connected to the channel outlet of the aperture. 
     An external surface of the sleeve body may comprise a first portion and a second portion. An external circumference of the first portion may be less than an external circumference of the second portion. The flow bypass sleeve may further comprise an outer sleeve which surrounds the first portion of the sleeve body. An external surface of the outer sleeve may be flush with an external surface of the second portion of the sleeve body. The outer sleeve may comprise a first material and the second portion of the sleeve body may comprise a second material with a thermal expansion coefficient that is different to a thermal expansion coefficient of the first material. The outer sleeve may be positioned downstream to the second portion of the sleeve body. The outer sleeve may be axially adjacent the second portion of the sleeve body. The outer sleeve may be releasably positioned on the first portion of the sleeve body. 
     The external surface of the sleeve body may further comprise a third portion with an external circumference less than the external circumference of the second portion. The third portion may be configured to be inserted in a keying ring fitted in the drill collar. A keying mechanism on an external surface of the flow bypass sleeve may be configured to mate with a keying mechanism on the keying ring to align the flow bypass sleeve within the drill collar. The third portion may be axially adjacent and upstream to the second portion of the sleeve body. 
     The kit may comprise a plurality of flow bypass sleeves. Each of the flow bypass sleeves may have a different outer circumference such that each of the flow bypass sleeves can be received in a different sized drill collar. A total cross sectional area for the at least one bypass channel may be different for each of the plurality of flow bypass sleeves, such that a volume of the drilling fluid that can flow along the bypass channel is different for each of the plurality of flow bypass sleeves. 
     The kit may further comprise a longitudinally extending bypass channel insert that can be releasably positioned in the bypass channel to reduce a flow area of the bypass channel. The sleeve body may include a plurality of longitudinally extending bypass channels and the kit may comprise a plurality of longitudinally extending bypass channel inserts that can be releasably positioned in the plurality of bypass channels to reduce the total flow area of the bypass channels. 
     The kit may further comprise a longitudinally extending tubular insert that can be releasably positioned in the aperture to reduce a flow area of the aperture. The sleeve body may include a plurality of longitudinally extending apertures therethrough and the kit may comprise a plurality of longitudinally extending tubular inserts that can be releasably positioned in the plurality of apertures to reduce the total flow area of the apertures. The tubular insert may have an uphole shoulder section with an external circumference greater than an internal circumference of the aperture and a downhole edge of the shoulder section may abut an internal surface of the sleeve body when the tubular insert is positioned in the aperture. The kit may further comprise a retaining ring that can be releasably attached to the tubular insert to releasably retain the tubular insert in the aperture. 
     According to another aspect, there is provided a downhole telemetry tool comprising: a pulser assembly comprising a housing enclosing a driveshaft; a fluid pressure pulse generator apparatus; and the flow bypass sleeve of the first or second aspect. The fluid pressure pulse generator comprises: a stator having a stator body and a plurality of radially extending stator projections spaced around the stator body, whereby adjacently spaced stator projections define stator flow channels extending therebetween; and a rotor coupled to the driveshaft and having a rotor body and a plurality of radially extending rotor projections spaced around the rotor body. The rotor projections are axially adjacent the stator projections and the rotor is rotatable relative to the stator such that the rotor projections move in and out of fluid communication with the stator flow channels to create fluid pressure pulses in drilling fluid flowing through the stator flow channels. The fluid pressure pulse generator is received in the bore of the flow bypass sleeve and the bypass channel extends across both the stator projections and the rotor projections, such that the drilling fluid flows along the bypass channel in addition to flowing through the stator flow channels. 
     According to another aspect, there is provided a downhole telemetry tool comprising: a fluid pressure pulse generator comprising a stator having one or more flow channels or orifices through which drilling fluid flows and a rotor which rotates relative to the stator to move in and out of fluid communication with the flow channels or orifices to create fluid pressure pulses in the drilling fluid flowing through the flow channels or orifices; and the flow bypass sleeve of the first or second aspect wherein the fluid pressure pulse generator is received in the bore of the body of the flow bypass sleeve and the bypass channel extends across at least a portion of both the stator and the rotor such that the drilling fluid flows along the bypass channel in addition to flowing through the flow channels or orifices of the stator. 
     This summary does not necessarily describe the entire scope of all aspects. Other aspects, features and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic of a drill string in an oil and gas borehole comprising a MWD telemetry tool. 
         FIG. 2A  is a longitudinally sectioned view of a mud pulser section of a MWD telemetry tool in a drill collar that includes a fluid pressure pulse generator according to a first embodiment and a flow bypass sleeve according to a first embodiment that surrounds the fluid pressure pulse generator inside the drill collar. 
         FIG. 2B  is a perspective view of the mud pulser section of the MWD tool shown in  FIG. 2A  with the drill collar shown as transparent. 
         FIG. 3  is an exploded view of the fluid pressure pulse generator of the first embodiment comprising a stator and a rotor. 
         FIGS. 4A and 4B  are perspective views of the fluid pressure pulse generator of the first embodiment with the rotor in a restricted flow position ( FIG. 4A ) and an open flow position ( FIG. 4B ). 
         FIG. 5  is an exploded view of the flow bypass sleeve of the first embodiment. 
         FIG. 6A  is a perspective view of the flow bypass sleeve of the first embodiment. 
         FIG. 6B  is a longitudinally sectioned view of the flow bypass sleeve of the first embodiment. 
         FIG. 7  is a perspective view of the downhole end of the flow bypass sleeve of the first embodiment. 
         FIG. 8  is an exploded view of a flow bypass sleeve according to a second embodiment. 
         FIG. 9A  is a perspective view of the flow bypass sleeve of the second embodiment. 
         FIG. 9B  is a longitudinally sectioned view of the flow bypass sleeve of the second embodiment. 
         FIG. 10  is a perspective view of the downhole end of the flow bypass sleeve of the second embodiment. 
         FIG. 11  is a downhole end view of the flow bypass sleeve of the first embodiment surrounding the fluid pressure pulse generator of the first embodiment with the rotor in the open flow position. 
         FIG. 12  is a downhole end view of the flow bypass sleeve of the second embodiment surrounding the fluid pressure pulse generator of the first embodiment with the rotor in the open flow position. 
         FIG. 13  is a perspective view of an uphole body section of the flow bypass sleeve of the second embodiment with tubular inserts for changing the flow area of bypass channels in the uphole body section. 
         FIG. 14  is a perspective view of the downhole end of the uphole body section of  FIG. 13 . 
         FIGS. 15A and 15B  are perspective views of a fluid pressure pulse generator according to a second embodiment comprising a rotor and a stator, with the rotor in a restricted flow position ( FIG. 15A ) and in an open flow position ( FIG. 15B ). 
         FIG. 16  is a perspective view of the rotor of the fluid pressure pulse generator of the second embodiment. 
         FIG. 17  is a perspective view of the uphole end of a flow bypass sleeve according to a third embodiment surrounding the fluid pressure pulse generator of the second embodiment with the rotor in the restricted flow position. 
         FIG. 18  is a perspective view of the downhole end of the flow bypass sleeve of the third embodiment and the fluid pressure pulse generator of the second embodiment with the rotor in the restricted flow position. 
         FIGS. 19A, 19B and 19C  are downhole end views of the flow bypass sleeve of the third embodiment and the fluid pressure pulse generator of the second embodiment with the rotor in the open flow position ( FIG. 19A ), the restricted flow position ( FIG. 19B ) and transitioning between the open and restricted flow positions ( FIG. 19C ). 
         FIGS. 20A, 20B and 20C  are downhole end views of the flow bypass sleeve of the first embodiment surrounding the fluid pressure pulse generator of the first embodiment. The flow bypass sleeves of  FIGS. 20A-20C  have the same internal dimensions which receive a one size fits all fluid pressure pulse generator of the first embodiment but a different external circumference configured to fit within different sized drill collars, with the external circumference of the flow bypass sleeve of  FIG. 20C  being greater than the external circumference of the flow bypass sleeve of  FIG. 20B  and the external circumference of the flow bypass sleeve of  FIG. 20B  being greater than the external circumference of the flow bypass sleeve of  FIG. 20A . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Directional terms such as “uphole” and “downhole” are used in the following description for the purpose of providing relative reference only, and are not intended to suggest any limitations on how any apparatus is to be positioned during use, or to be mounted in an assembly or relative to an environment. 
     The embodiments described herein generally relate to a flow bypass sleeve for use with a fluid pressure pulse generator of a downhole telemetry tool. The fluid pressure pulse generator may be used for mud pulse (“MP”) telemetry used in downhole drilling, where a drilling fluid (herein referred to as “mud”) is used to transmit telemetry pulses to surface. The fluid pressure pulse generator includes a stator with flow channels or orifices through which mud flows and a rotor which rotates relative to the stator thereby allowing and restricting flow of the mud through the flow channels or orifices to create pressure pulses in the mud. The flow bypass sleeve is configured to be fitted inside a drill collar which houses the downhole telemetry tool. The flow bypass sleeve comprises a body with a bore therethrough which receives the fluid pressure pulse generator therein. The body includes one or more longitudinally extending bypass channels and mud flows along the bypass channels in addition to mud flowing through the stator flow channels or orifices. In this way the bypass channels divert mud around the fluid pressure pulse generator and the bypass channels may be dimensioned to control the amount of mud that is diverted and thus the amount of mud that flows through the stator flow channels or orifices. 
     Referring to the drawings and specifically to  FIG. 1 , there is shown a schematic representation of MP telemetry operation using a fluid pressure pulse generator  130 ,  230  according to embodiments disclosed herein. In downhole drilling equipment  1 , drilling mud is pumped down a drill string by pump  2  and passes through a measurement while drilling (“MWD”) tool  20  including the fluid pressure pulse generator  130 ,  230 . The fluid pressure pulse generator  130 ,  230  has an open flow position in which mud flows relatively unimpeded through the pressure pulse generator  130 ,  230  and no pressure pulse is generated and a restricted flow position where flow of mud through the pressure pulse generator  130 ,  230  is restricted and a positive pressure pulse is generated (represented schematically as block  6  in mud column  10 ). Information acquired by downhole sensors (not shown) is transmitted in specific time divisions by pressure pulses  6  in the mud column  10 . More specifically, signals from sensor modules in the MWD tool  20 , or in another downhole probe (not shown) communicative with the MWD tool  20 , are received and processed in a data encoder in the MWD tool  20  where the data is digitally encoded as is well established in the art. This data is sent to a controller in the MWD tool  20  which then actuates the fluid pressure pulse generator  130 ,  230  to generate pressure pulses  6  which contain the encoded data. The pressure pulses  6  are transmitted to the surface and detected by a surface pressure transducer  7  and decoded by a surface computer  9  communicative with the transducer by cable  8 . The decoded signal can then be displayed by the computer  9  to a drilling operator. The characteristics of the pressure pulses  6  are defined by duration, shape, and frequency; these characteristics are used in various encoding systems to represent binary data. 
     Referring to  FIGS. 2A and 2B , an embodiment of the MWD tool  20  is shown in more detail. The MWD tool  20  generally comprises a fluid pressure pulse generator  130  according to a first embodiment which creates fluid pressure pulses, and a pulser assembly  26  which takes measurements while drilling and which drives the fluid pressure pulse generator  130 . The fluid pressure pulse generator  130  and pulser assembly  26  are axially located inside a drill collar  27 . A flow bypass sleeve  170  according to a first embodiment is received inside the drill collar  27  and surrounds the fluid pressure pulse generator  130 . The pulser assembly  26  is fixed to the drill collar  27  with an annular channel  55  therebetween, and mud flows along the annular channel  55  when the MWD tool  20  is downhole. The pulser assembly  26  comprises pulser assembly housing  49  enclosing a motor subassembly  25  and an electronics subassembly  28  electronically coupled together but fluidly separated by a feed-through connector (not shown). The motor subassembly  25  includes a motor and gearbox subassembly  23 , a driveshaft  24  coupled to the motor and gearbox subassembly  23 , and a pressure compensation device  48 . As described in more detail below with reference to  FIGS. 3 and 4 , the fluid pressure pulse generator  130  comprises a stator  140  and a rotor  160 . The stator  140  comprises a stator body  141  fixed to the pulser assembly housing  49  and stator projections  142  radially extending around the downhole end of the stator body  141 . The rotor  160  comprises rotor body  169  fixed to the driveshaft  24  and rotor projections  162  radially extending around the downhole end of the rotor body  169 . Rotation of the driveshaft  24  by the motor and gearbox subassembly  23  rotates the rotor  160  relative to the fixed stator  140 . The electronics subassembly  28  includes downhole sensors, control electronics, and other components required by the MWD tool  20  to determine direction and inclination information and to take measurements of drilling conditions, to encode this telemetry data using one or more known modulation techniques into a carrier wave, and to send motor control signals to the motor and gearbox subassembly  23  to rotate the driveshaft  24  and rotor  160  in a controlled pattern to generate pressure pulses  6  representing the carrier wave for transmission to surface as described above. 
     The motor subassembly  25  is filled with a lubricating liquid such as hydraulic oil or silicon oil and this lubricating liquid is fluidly separated from mud flowing along the annular channel  55  by an annular seal  54  which surrounds the driveshaft  24 . The pressure compensation device  48  comprises a flexible membrane (not shown) in fluid communication with the lubrication liquid on one side and with mud on the other side via ports  50  in the pulser assembly housing  49 ; this allows the pressure compensation device  48  to maintain the pressure of the lubrication liquid at about the same pressure as the mud in the annular channel  55 . Without pressure compensation, the torque required to rotate the driveshaft  24  and rotor  160  would need high current draw with excessive battery consumption resulting in increased costs. In alternative embodiments (not shown), the pressure compensation device  48  may be any pressure compensation device known in the art, such as pressure compensation devices that utilize pistons, metal membranes, or a bellows style pressure compensation mechanism. 
     The fluid pressure pulse generator  130  is located at the downhole end of the MWD tool  20 . Mud pumped from the surface by pump  2  flows along annular channel  55  between the outer surface of the pulser assembly  26  and the inner surface of the drill collar  27 . When the mud reaches the fluid pressure pulse generator  130  it flows along an annular channel  56  provided between the external surface of the stator  140  and the internal surface of the flow bypass sleeve  170 . The rotor  160  can rotate between an open flow position where mud flows freely through the fluid pressure pulse generator  130  resulting in no pressure pulse and a restricted flow position where flow of mud is restricted to generate pressure pulse  6 , as will be described in more detail below with reference to  FIGS. 3 and 4 . The flow bypass sleeve  170  includes a plurality of longitudinally extending grooves  173  and mud flows along the grooves  173  in addition to flowing through the fluid pressure pulse generator  130 , as will be described in more detail below with reference to  FIGS. 5 to 7 . 
     Referring to  FIGS. 3 and 4 , the first embodiment of the fluid pressure pulse generator  130  comprising stator  140  and rotor  160  is shown in more detail. The stator  140  comprises longitudinally extending stator body  141  with a central bore therethrough. The stator body  141  comprises a cylindrical section at the uphole end and a generally frusto-conical section at the downhole end which tapers longitudinally in the downhole direction. As shown in  FIGS. 2A and 2B , the cylindrical section of stator body  141  is coupled with the pulser assembly housing  49 . More specifically, a jam ring  158  threaded on the stator body  141  is threaded onto the pulser assembly housing  49 . Once the stator  140  is positioned correctly, the stator  140  is held in place and the jam ring  158  is backed off and torqued onto the stator  140  holding it in place. The external surface of the pulser assembly housing  49  is flush with the external surface of the cylindrical section of the stator body  141  for smooth flow of mud therealong. A plurality of radially extending projections  142  are spaced equidistant around the downhole end of the stator body  141 . 
     The rotor  160  comprises generally cylindrical rotor body  169  with a central bore therethrough and a plurality of radially extending projections  162 . As shown in  FIG. 2A , the rotor body  169  is received in the downhole end of the bore in the stator body  141 . A downhole shaft  24   a  of the driveshaft  24  is received in uphole end of the bore in the rotor body  169  and a coupling key  30  extends through the driveshaft  24  and is received in a coupling key receptacle  164  at the uphole end of the rotor body  169  to couple the driveshaft  24  with the rotor body  169 . A rotor cap  190  comprising a cap body  191  and a cap shaft  192  is positioned at the downhole end of the fluid pressure pulse generator  130 . The cap shaft  192  is received in the downhole end of the bore in the rotor body  169  and threads onto the downhole shaft  24   a  of the driveshaft  24  to lock (torque) the rotor  160  to the driveshaft  24 . The cap body  191  includes a hexagonal shaped opening  193  dimensioned to receive a hexagonal Allen key which is used to torque the rotor  160  to the driveshaft  24 . The rotor cap  190  therefore releasably couples the rotor  160  to the driveshaft  24  so that the rotor  160  can be easily removed and repaired or replaced if necessary using the Allen key. 
     The radially extending rotor projections  162  are equidistantly spaced around the downhole end of the rotor body  169  and are axially adjacent and downhole relative to the stator projections  142  in the assembled fluid pressure pulse generator  130 . In use, mud flowing along the external surface of the stator body  141  contacts the stator projections  142  and flows through stator flow channels  143  defined by adjacently positioned stator projections  142 . The rotor projections  162  align with the stator projections  142  when the rotor  160  is in the open flow position shown in  FIG. 4B  and mud flows freely through the stator flow channels  143  resulting in no pressure pulse. The rotor  160  rotates to the restricted flow position shown in  FIG. 4A  where the rotor projections  162  align with the stator flow channels  143  and the volume of mud flowing through the stator flow channels  143  is restricted (reduced) resulting in pressure pulse  6 . The rotor projections  162  rotate in and out of fluid communication with the stator flow channels  143  in a controlled pattern to generate pressure pulses  6  representing the carrier wave for transmission to surface. In alternative embodiments (not shown), the rotor projections  162  may be positioned uphole relative to the stator projections  142 . 
     In alternative embodiments (not shown) the fluid pressure pulse generator may be any rotor/stator type fluid pressure pulse generator where the stator includes flow channels or orifices through which mud flows and the rotor rotates relative to the fixed stator to move in and out of fluid communication with the flow channels or orifices to generate pressure pulses  6 . The fluid pressure pulse generator may be positioned at either the downhole or uphole end of the MWD tool  20 . 
     Referring now to  FIGS. 5 to 7  the flow bypass sleeve  170  of the first embodiment is shown in more detail and comprises a generally cylindrical sleeve body with a central bore therethrough and a lock down sleeve  81  surrounding the sleeve body. The sleeve body comprises an uphole body section  171   a  and an axially aligned downhole body section  171   b . The external surface of the uphole body section  171   a  has an uphole portion  172   a , a downhole portion  172   c  and a central portion  172   b  positioned between the uphole and downhole body portions  172   a ,  172   c . As shown in  FIG. 6B  the external circumference of the central portion  172   b  is greater than the external circumference of the uphole and downhole portions  172   a ,  172   c . The external surface of the downhole body section  171   b  has an uphole portion  176   a  and a downhole portion  176   b  and the external circumference of the uphole portion  176   a  is greater than the external circumference of the downhole portion  176   b . The uphole portion  176   a  of the downhole body section  171   b  has the same external circumference as the external circumference of the downhole portion  172   c  of the uphole body section  171   a.    
     During assembly of the flow bypass sleeve  170 , the uphole body section  171   a  and downhole body section  171   b  are positioned axially adjacent each other and the lock down sleeve  81  is received on the downhole end of the downhole body section  171   b  and moved towards the uphole body section  171   a  until the uphole end of the lock down sleeve  81  abuts an annular shoulder  183  provided by the downhole edge of the central portion  172   b  of the uphole body section  171   a . The lock down sleeve  81  includes an annular shoulder  82  on an internal surface of the sleeve which abuts the downhole edge of the uphole portion  176   a  of the downhole body section  171   b . The lock down sleeve  81  surrounds the downhole portion  172   c  of the uphole body section  171   a  as well as the uphole portion  176   a  and part of the downhole portion  176   b  of the downhole body section  171   b . The assembled flow bypass sleeve  170  can then be inserted into the downhole end of drill collar  27 . An annular shoulder  180  provided by the uphole edge of the central portion  172   b  of the uphole body section  171   a  abuts a downhole shoulder of a keying or mounting ring that is press fitted into the drill collar  27  as shown in  FIG. 2A . A keying notch  184  on the external surface of uphole body section  171   a  mates with a projection (not shown) on the keying ring to align the flow bypass sleeve  170  with the pulser assembly  26 . A threaded ring (not shown) threaded into the downhole end of the drill collar  27  locks the lock down sleeve  81  in position on the sleeve body with annular shoulder  82  in contact with the downhole edge of the uphole portion  176   a  of the downhole body section  171   b  so that the uphole and downhole body sections  171   a ,  171   b  maintain contact with each other. A groove  185  on the external surface of the central portion  172   b  of uphole body section  171   a  receives an o-ring (not shown) and a rubber back-up ring (not shown) such as a parbak which may help seat the flow bypass sleeve  170  and reduce fluid leakage between the flow bypass sleeve  170  and the drill collar  27 . In alternative embodiments the flow bypass sleeve  170  may be mounted or fitted within the drill collar  27  using an alternative mechanism as would be known to a person of skill in the art. In alternative embodiments, the flow bypass sleeve  170  may comprise just the uphole body section  171   a  and the downhole body section  171   b  and/or lock down sleeve  81  may not be present. 
     Referring to  FIGS. 8 to 10  a second embodiment of a flow bypass sleeve  270  is shown comprising a generally cylindrical sleeve body with a central bore therethrough and a lock down sleeve  81  surrounding the sleeve body. The sleeve body comprises an uphole body section  271   a  and an axially aligned downhole body section  271   b . The external surface of the uphole body section  271   a  has an uphole portion  272   a , a downhole portion  272   c  and a central portion  272   b  positioned between the uphole and downhole body portions  272   a ,  272   c . As shown in  FIG. 9B  the external circumference of the central portion  272   b  is greater than the external circumference of the uphole and downhole portions  272   a ,  272   c . The external surface of the downhole body section  271   b  has an uphole portion  276   a  and a downhole portion  276   b  and the external circumference of the uphole portion  276   a  is greater than the external circumference of the downhole portion  276   b . The uphole portion  276   a  of the downhole body section  271   b  has the same external circumference as the external circumference of the downhole portion  272   c  of the uphole body section  271   a.    
     During assembly of the flow bypass sleeve  270 , the uphole body section  271   a  and downhole body section  271   b  are positioned axially adjacent each other and alignment pins  282  on the uphole edge of the downhole body section  271   b  are received in recesses on the downhole edge of the uphole body section  271   a . The lock down sleeve  81  is received on the downhole end of the downhole body section  271   b  and moved towards the uphole body section  271   a  until the uphole end of the lock down sleeve  81  abuts an annular shoulder  283  provided by the downhole edge of the central portion  272   b  of the uphole body section  271   a . The lock down sleeve  81  includes an annular shoulder  82  on an internal surface of the sleeve which abuts the downhole edge of the uphole portion  276   a  of the downhole body section  271   b . The lock down sleeve  81  surrounds the downhole portion  272   c  of the uphole body section  271   a  as well as the uphole portion  276   a  and part of the downhole portion  276   b  of the downhole body section  271   b . The assembled flow bypass sleeve  270  can then be inserted into the downhole end of drill collar  27 . An annular shoulder  280  provided by the uphole edge of the central portion  272   b  of the uphole body section  271   a  abuts a downhole shoulder of a keying or mounting ring that is press fitted into the drill collar  27 . A keying notch  284  on the external surface of uphole body section  271   a  mates with a projection on the keying ring to align the flow bypass sleeve  270  with the pulser assembly  26 . A threaded ring threaded into the downhole end of the drill collar  27  locks the lock down sleeve  81  in position on the sleeve body with annular shoulder  82  in contact with the downhole edge of the uphole portion  276   a  of the downhole body section  271   b  so that the uphole and downhole body sections  271   a ,  271   b  maintain contact with each other. A groove  285  on the external surface of the central portion  272   b  of uphole body section  271   a  receives an o-ring (not shown) and a rubber back-up ring (not shown) such as a parbak which may help seat the flow bypass sleeve  270  and reduce fluid leakage between the flow bypass sleeve  270  and the drill collar  27 . In alternative embodiments the flow bypass sleeve  270  may be mounted or fitted within the drill collar  27  using an alternative mechanism as would be known to a person of skill in the art. In alternative embodiments, the flow bypass sleeve  270  may comprise just the uphole body section  271   a  and the downhole body section  271   b  and/or lock down sleeve  81  may not be present. 
     The lock down sleeve  81  may be made from the same material or a different material to the uphole body section  171   a ,  271   a . The material of the lock down sleeve  81  may have a different thermal expansion coefficient than the material of the uphole body section  171   a ,  271   a . For example, the lock down sleeve  81  may comprise beryllium copper and the uphole body section  171   a ,  271   a  may comprise Stellite. This different thermal expansion coefficient of the different materials that make up the external surface of flow bypass sleeve  170 ,  270  may result in the flow bypass sleeve  170 ,  270  being securely clamped within the drill collar  27  across a wider range of temperatures than if the flow bypass sleeve  170 ,  270  was made of the same material throughout. The lock down sleeve  81  may be protected from erosion caused by mud flow by the upstream keying ring and o-ring received in groove  185 ,  285  of the uphole body section  171   a ,  271   a . The material of the lock down sleeve  81  may therefore be chosen for its thermal expansion properties rather than having to be chosen for its ability to resist erosion caused by mud. The lock down sleeve  81  may allow the flow bypass sleeve  170 ,  270  to be reliably secured within the drill collar  27  over a wide range of temperatures than a flow bypass sleeve without the lock down sleeve and its performance may not affected by mud flow over time. 
       FIG. 2A  shows the uphole body section  171   a  of the flow bypass sleeve  170  of the first embodiment received in the drill collar  27  and surrounding the fluid pressure pulse generator  130  of the first embodiment. The diameter of the bore through the uphole body section  171   a  is smallest at a central section  177  which surrounds the stator projections  142  and rotor projections  162 . The stator projections  142  may be dimensioned such that the stator projections  142  contact the internal surface of the central section  177 . The outer diameter of the rotor projections  162  is slightly less than the internal diameter of the central section  177  to allow rotation of the rotor projections  162  relative to the uphole body section  171   a . The bore through the uphole body section  171   a  gradually increases in diameter from the central section  177  towards the downhole end of the uphole body section  171   a  to define an internally tapered downhole section  176 . The bore through the sleeve body also increases in diameter from the central section  177  towards the uphole end of the uphole body section  171   a  to define an internally tapered uphole section  179 . The taper of the uphole section  179  is greater than the taper of downhole section  176 . The uphole section  179  surrounds the frusto-conical section of stator body  141  with annular channel  56  extending therebetween. Mud flows along annular channel  56  and hits the stator projections  142  where it is channeled into the stator flow channels  143 . The downhole section  176  surrounds the rotor cap body  191 . The internal surface of the central section  177  includes longitudinally extending grooves  173  with an inlet in the uphole section  179  and an outlet in the downhole section  176 . Mud flows from annular channel  56  through the longitudinally extending grooves  173  into the bore in the downhole section  176  in addition to flowing through stator flow channels  143  of the fluid pressure pulse generator  130 . The uphole body section  271   a  of the flow bypass sleeve  270  of the second embodiment has similar internal dimensions as the uphole body section  171   a  of the flow bypass sleeve  170  of the first embodiment as shown in  FIG. 9B . 
     In the first embodiment of the flow bypass sleeve  170 , bypass flow channels are provided by the longitudinal extending grooves  173  which are equidistantly spaced around the internal surface of the uphole body section  171   a . Internal walls  174  in-between each groove  173  align with the stator projections  142  of the fluid pressure pulse generator  130 , and the grooves  173  align with the stator flow channels  143 . The flow bypass sleeve  170  is precisely located with respect to the drill collar  27  using keying notch  184  to ensure correct alignment of the stator projections  142  with the internal walls  174 . In alternative embodiments an alternative alignment mechanism may be used which provides alignment of the flow bypass sleeve  170  within the drill collar  27  such that the stator projections  142  align with the internal walls  174 . The rotor projections  162  rotate relative to the flow bypass sleeve  170  and move between the open flow position (shown in  FIG. 11 ) where the rotor projections  162  align with the internal walls  174  and the restricted flow position (not shown) where the rotor projections  162  align with the grooves  173 . The grooves  173  are semi-circular shaped, however in alternative embodiments (not shown) the grooves may be any shape and dimensioned for the desired amount of mud flow therethrough. 
     In the second embodiment of the flow bypass sleeve  270  the bypass flow channels are provided by a plurality of apertures  275  extending longitudinally through the uphole body section  271   a . The apertures  275  are circular and equidistantly spaced around uphole body section  271   a . The internal surface of the downhole body section  271   b  includes a plurality of spaced grooves  278  which align with the apertures  275  such that mud is channeled through the apertures  275  and into grooves  278 . The alignment pins  282  on the uphole edge of the downhole body section  271   b  are received in recesses  289  (shown in  FIG. 14 ) on the downhole edge of the uphole body section  271   a  to correctly align the apertures  275  with the grooves  278 . The internal surface of uphole body section  271   a  which surrounds the rotor and stator projections  162 ,  142  is uniform in this embodiment (as shown in  FIG. 12 ); therefore there is no need to align the stator projections  142  with any internal feature of the uphole body section  271   a  as with the first embodiment of the flow bypass sleeve  170  described above. The keying notch  284  or other alignment mechanism may therefore not be present and the flow bypass sleeve  270  may be inserted into a mounting ring or other mounting mechanism (without an alignment mechanism) to mount the flow bypass sleeve  270  within the drill collar  27 . Other mechanisms for fitting or mounting the flow bypass sleeve  270  within the drill collar  27  as would be known to a person of skill in the art may alternatively be used. 
     The uphole body section  271   a  generally needs to be thick enough to support the apertures  275  and the drill collar dimensions may be a limiting factor with respect to use of the second embodiment of the flow bypass sleeve  270 . As such, the second embodiment of the flow bypass sleeve  270  may be used with larger drill collars  27 , for example drill collars that are 8 inches or more in diameter. In alternative embodiments (not shown) the apertures  275  may be any shape and need not be equidistantly spaced around the sleeve body. The number and size of the apertures  275  may be chosen for the desired amount of mud flow therethrough. In further alternative embodiments (not shown) the grooves  278  may have a different shape or may not be present at all. 
     In an alternative embodiment (not shown), the sleeve body may include bypass channels comprising both internal grooves  173  and longitudinally extending apertures  275  for flow of mud therethrough. 
     A third embodiment of a flow bypass sleeve  370  is shown in  FIGS. 17 to 19  surrounding a second embodiment of the fluid pressure pulse generator  230 , however in alternative embodiments the flow bypass sleeve  370  may surround any type of fluid pressure pulse generator. The second embodiment of the fluid pressure pulse generator  230  is shown in more detail in  FIGS. 15 and 16  and comprises a stator  240  and a rotor  260 . The stator  240  comprises a longitudinally extending stator body  241  with a central bore therethrough and a plurality of radially extending projections  242  spaced equidistant around the downhole end of the stator body  241 . Mud flowing along the external surface of the stator body  241  contacts the stator projections  242  and flows through stator flow channels  243  defined by adjacently positioned stator projections  242 . The rotor  260  comprises a generally cylindrical rotor body  269  with a central bore therethrough and a plurality of radially extending projections  262  spaced equidistant around the downhole end of the rotor body  269 . The rotor projections  262  are axially adjacent and downhole to the stator projections  242  in the assembled fluid pressure pulse generator  230 . The rotor projections  262  rotate in and out of fluid communication with the stator flow channels  243  to generate pressure pulses  6 . More specifically, the rotor rotates between the open flow position shown in  FIG. 15B  where rotor flow channels  263  defined by adjacently positioned rotor projections  262  align with the stator flow channels  243  and there is unrestricted flow of mud through the pressure pulse generator  230 , to the restricted flow position shown in  FIG. 15A  where the rotor projections  262  align with the stator flow channels  243  and flow of mud is restricted generating pressure pulse  6 . The rotor projections  262  are wider than the stator flow channels  243 , such that a portion of two adjacent stator projections  242  overlie an underlying rotor projection  262  when the rotor  260  is in the restricted flow position shown in  FIG. 15A . The leading side face of each rotor projection  262  intersects the side face of one of the stator projections  242  as the rotor  260  transitions from the open flow position to the restricted flow position as shown in  FIG. 19C . 
     The rotor projections  262  each have a bypass channel  295  comprising a semi-circular groove. The bypass channels  295  have an axial inlet and an axial outlet and mud flows from the stator flow channels  243  through the bypass channels  295  when the rotor  260  is in the restricted flow position shown in  FIG. 15A . A rotor cap  290  comprising a cap body  291  and a cap shaft (not shown) releasably couples the rotor body  269  to the driveshaft  24  of the MWD tool  20 . The cap body  291  includes a hexagonal shaped opening  293  (shown in  FIGS. 18 and 19 ) dimensioned to receive a hexagonal Allen key which is used to torque the rotor  260  to the driveshaft  24  as described above in more detail with reference to  FIGS. 2 to 4 . 
     Referring to  FIGS. 17 to 19 , the third embodiment of the flow bypass sleeve  370  comprises a generally cylindrical sleeve body  371  with a central bore therethrough which receives the fluid pressure pulse generator  230 . The sleeve body  371  includes a plurality of longitudinal extending grooves  373  equidistantly spaced around the internal surface of the sleeve body  371 . The grooves  373  are semi-circular and dimensioned to correspond in width to the width of both the semi-circular grooves of the rotor bypass channels  295  in the rotor projections  262  and rotor flow channels  263 . When the rotor  260  is in the restricted flow position shown in  FIGS. 17, 18 and 19B , the grooves  373  and the rotor bypass channels  295  align to form circular bypass channels for flow of mud therethrough. When the rotor  260  is in the open flow position shown in  FIG. 19A , the grooves  373  and the rotor flow channels  263  align to form larger oval flow channels. As the rotor  260  rotates between the open flow and restricted flow positions, less mud can flow through the smaller circular bypass channels in the restricted flow position than through the oval flow channels in the open flow position, thereby generating pressure pulses  6 . In alternative embodiments (not shown) the grooves  373  may be any shape and dimensioned for desired amount of mud flow therethrough. 
     The flow bypass sleeve  170 ,  270 ,  370  may be used with any fluid pressure pulse generator comprising a stator having one or more flow channels or orifices through which mud flows and a rotor which rotates relative to the stator to move in and out of fluid communication with the flow channels or orifices to create fluid pressure pulses in the mud flowing through the flow channels or orifices. The rotor may be rotated by the driveshaft  24  of the MWD tool  20 , or it may be rotated by other mechanisms such as angled blades or turbines in the flow path of the mud flowing through the fluid pressure pulse generator. 
     The longitudinally extending bypass channels (grooves  173 ,  373  and apertures  275 ) of the flow bypass sleeve  170 ,  270 ,  370  may reduce pressure build up when the rotor  160 ,  260  is in the restricted flow position especially in high mud flow rate conditions. A build up of pressure could lead to damage of the rotor  160 ,  260  and/or stator  140 ,  240  and other components of the MWD tool  20 . By controlling the amount of mud diverted around the fluid pressure pulse generator  130 ,  230 , the flow bypass sleeve  170 ,  270 ,  370  may maintain the volume of mud flowing through the pressure pulse generator  130 ,  230  within an optimal range which provides enough of a pressure differential between the open and restricted flow positions to generate pressure pulses  6  that can be detected at surface without excessive pressure build up. 
     As the bypass channels extend through the sleeve body (i.e. apertures  275  of flow bypass sleeve  270 ) or along the internal surface of the sleeve body (i.e. grooves  173  and  373  of flow bypass sleeve  170  and  370  respectively), the external surface of the flow bypass sleeve  170 ,  270 ,  370  may be dimensioned to fit any sized drill collar  27 , for example 4¾, 6½″ or 8″ drill collars. Referring now to  FIGS. 20A to 20C , there is shown the flow bypass sleeve  170  of the first embodiment surrounding the fluid pressure pulse generator  130  of the first embodiment. Each of the flow bypass sleeves  170  of  FIGS. 20A-20C  have the same or corresponding internal dimension to receive a one size fits all fluid pressure pulse generator  130  but a different external circumference configured to fit within different sized drill collars. The flow bypass sleeve  170  of  FIG. 20A  has the smallest external circumference and is configured to fit within a smaller drill collar  27 , such as a 4¾″ drill collar. The sleeve body of the flow bypass sleeve  170  of  FIG. 20B  is thicker than the sleeve body of the flow bypass sleeve  170  of  FIG. 20A  such that the external circumference of the flow bypass sleeve  170  of  FIG. 20B  is greater than the external circumference of the flow bypass sleeve  170  of  FIG. 20A . The flow bypass sleeve  170  of  FIG. 20B  is therefore configured to fit within a larger drill collar  27  (for example a 6½″ drill collar) than the drill collar  27  which receives the flow bypass sleeve  170  of  FIG. 20A . The sleeve body of the flow bypass sleeve  170  of  FIG. 20C  is thicker than the sleeve body of the flow bypass sleeve  170  of  FIG. 20B  such that the external circumference of the flow bypass sleeve  170  of  FIG. 20C  is greater than the external circumference of the flow bypass sleeve  170  of  FIG. 20B . The flow bypass sleeve  170  of  FIG. 20C  is therefore configured to fit within a larger drill collar  27  (for example an 8″ drill collar) than the drill collar  27  which receives the flow bypass sleeve  170  of  FIG. 20B . 
     The flow rate of mud flowing along a 4¾″ drill collar will generally be lower than the flow rate of mud flowing along a 6½″ drill collar and the flow rate of mud flowing along a 6½″ drill collar will generally be lower than the flow rate of mud flowing along an 8″ drill collar. The internal grooves  173  of each of the flow bypass sleeves  170  may be configured for these different mud flow rates. In the embodiments shown in  FIGS. 20A-20C  the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20A  are shallower than the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20B  and the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20B  are shallower than the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20C , such that the total flow area of mud flowing through the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20A  is less than the total flow area of mud flowing through the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20B  and the total flow area of mud flowing through the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20B  is less than the total flow area of mud flowing through the internal grooves  173  of the flow bypass sleeve  170  of  FIG. 20C . 
     As discussed above, the flow bypass sleeve  170 ,  270 ,  370  may be releasably fitted within the drill collar  27  using a threaded ring and no screws, bolts or other fasteners are needed to fix the flow bypass sleeve  170 ,  270 ,  370  within the drill collar  27 . A kit may be provided with a one size fits all fluid pressure pulse generator  130 ,  230  with multiple different sized flow bypass sleeves  170 ,  270 ,  370  that are dimensioned to fit different sized drill collars  27 . Each of the different sized flow bypass sleeves  170 ,  270 ,  370  has the same or corresponding internal dimensions to receive the one size fits all fluid pressure pulse generator  130 ,  230  but a different external circumference to fit the different sized drill collars  27 . In larger diameter drill collars  27  the volume of mud flowing through the drill collar  27  will generally be greater than the volume of mud flowing through smaller diameter drill collars  27 , however the bypass channels of the flow bypass sleeve  170 ,  270 ,  370  may be dimensioned to accommodate this greater volume of mud as described above with reference to  FIGS. 20A-20C . The bypass channels of the different sized flow bypass sleeves  170 ,  270 ,  370  may therefore be dimensioned such that the volume of mud flowing through the one size fits all fluid pressure pulse generator  130 ,  230  fitted within any sized drill collar  27  is within an optimal range for generation of pressure pulses  6  which can be detected at the surface without excessive pressure build up. In this way, the bypass channels of the different sized flow bypass sleeves  170 ,  270 ,  370  may be dimensioned to provide optimal mud flow through the fluid pressure pulse generator  130 ,  230  rather than having to configure the fluid pressure pulse generator  130 ,  230  for optimal mud flow therethrough. 
     The bypass channels of the flow bypass sleeve  170 ,  270 ,  370  divert mud around the fluid pressure pulse generator  130 ,  230  and may be dimensioned to control the amount of mud being diverted and thus the volume of mud flowing through the stator flow channels  143 ,  243  respectively. As such, the bypass channels may be dimensioned for different mud flow rate conditions downhole. For example the total flow area of the bypass channels of a flow bypass sleeve  170 ,  270 ,  370  used in high mud flow rate conditions may be greater than the total flow area of the bypass channels of a flow bypass sleeve  170 ,  270 ,  370  used in low mud flow rate conditions, so that the total volume of mud being diverted through the bypass channels of the high mud flow rate sleeve  170 ,  270 ,  370  is greater than the total volume of mud being diverted through the bypass channels of the low mud flow rate sleeve  170 ,  270 ,  370 . A kit comprising a plurality of flow bypass sleeves  170 ,  270 ,  370  may be provided where the total flow area of the bypass channels for each of the flow bypass sleeves  170 ,  270 ,  370  is different, such that the volume of mud that flows along the bypass channels is different for each of the plurality of flow bypass sleeves  170 ,  270 ,  370 . The operator can then choose which flow bypass sleeve  170 ,  270 ,  370  to use depending on the mud flow conditions downhole. In this way, the bypass channels of the different bypass sleeves  170 ,  270 ,  370  may be dimensioned to provide optimal mud flow through the fluid pressure pulse generator  130 ,  230  in varying mud flow rate conditions, rather than having to configure the fluid pressure pulse generator  130 ,  230  for the different mud flow rate conditions experienced downhole. As the flow bypass sleeve  170 ,  270 ,  370  may be releasably fitted within the drill collar  27 , the operator may easily change the flow bypass sleeve  170 ,  270 ,  370  for different mud flow rate conditions downhole rather than having to change the fluid pressure pulse generator  130 ,  230 . Operating cost may therefore be reduced as the skill level of personal needed and time taken to change the flow bypass sleeve  170 ,  270 ,  370  may be less than that required to change the fluid pressure pulse generator  130 ,  230 . 
     The total flow area of the bypass channels of the flow bypass sleeve  170 ,  270 ,  370  may be reduced by positioning longitudinally extending inserts into the one or more of the bypass channels. Referring now to  FIGS. 13 and 14 , there is shown the uphole body section  271   a  of the flow bypass sleeve  270  of the second embodiment with longitudinally extending tubular inserts  90  positioned in the apertures  275  extending through the uphole body section  271   a . Each tubular insert  90  has an aperture therethrough and is inserted into the uphole end of one of the apertures  275  to reduce the flow area of the apertures  275 . An uphole shoulder section  91  of the tubular inserts  90  has an external circumference greater than the internal circumference of the apertures  275  such that the shoulder section  91  is not received within the aperture  275  and the downhole edge of the shoulder section  91  abuts the internal surface of the uphole body section  271   a . The downhole edge of the shoulder sections  91  is sloped (angled) to correspond with the sloped internal surface at the uphole end of the uphole body section  271   a . A retaining ring  92  received in a groove  93  near the downhole end of each of the tubular inserts releasably retains the tubular inserts  90  in position in the apertures  275 . 
     The uphole body section  271   a  with inserts  90  therein and downhole body section  271   b  may be fitted together by aligning alignment pins  282  on the uphole edge of downhole body section  271   b  (shown in  FIG. 8 ) with recesses  289  on the downhole edge of uphole body section  271   a , and the pins  282  are received in the recesses  289 . The downhole end of the tubular inserts  90  with the retaining ring  92  thereon are received in the grooves  278  in the downhole body section  271   b . The lockdown sleeve  81  may be inserted over the downhole end of the downhole body section  271   b  until the uphole end of the lockdown sleeve abuts annular shoulder  283  as described above with reference to  FIGS. 8-10 . 
     The total flow area of the bypass channels can therefore be varied without having to change the flow bypass sleeve  270 . More or less tubular inserts  90  can be used depending on the optimal total bypass flow area for different mud flow rate conditions downhole. The diameter of the aperture of the tubular inserts  90  may also be varied to vary the bypass flow area and tubular inserts  90  with different sized apertures may be used for different mud flow conditions downhole. In alternative embodiments the tubular inserts  90  may have a different external shape, for example square, oval or triangular, and/or a different shaped aperture therethrough. In further alternative embodiments the bypass channel inserts may not be tubular and may not have an aperture therethrough, for example the bypass channel inserts may be curved inserts that can be inserted into the grooves  173  of the first embodiment of the flow bypass sleeve  170  shown in  FIGS. 5 to 7  to reduce the flow area through the grooves  173 . The bypass channel inserts may be releasably retained within the bypass channels of the flow bypass sleeve  170 ,  270 ,  370  by any suitable fastener or other retaining mechanism, for example the insert may be threaded or have a threaded end which receives a nut or bolt to releasably retain the inserts within the bypass channels. 
     The bypass channel inserts may provide a relatively quick and easy way to vary the total bypass flow area of the flow bypass sleeve  170 ,  270 ,  370  fitted in the drill collar  27  to accommodate varying mud flow rate conditions downhole. A kit comprising a flow bypass sleeve  170 ,  270 ,  370  and a plurality of bypass channel inserts may be provided. 
     While particular embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modification of and adjustments to the foregoing embodiments, not shown, are possible.