Patent Publication Number: US-10316594-B2

Title: Drill element and associated equipment and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to United Kingdom application GB 1320961.4, filed Nov. 27, 2013and entitled “A drill string stabiliser and associated equipment and methods”, which is hereby incorporated by reference. 
     BACKGROUND 
     a. Technical Field 
     Embodiments of the present invention relate to a drill string element such as a drill string stabiliser, and an associated bottom hole assembly, drill string, rig which may be for the drilling of an oil or gas well, method of retrofitting, computer program, and non-transitory computer readable medium. 
     b. Background Art 
     In the drilling of a wellbore for an oil or a gas well a drill bit, located at a remote end of a drill pipe (or ‘drill string’), is rotated to cut the wellbore through the ground. 
     Conventionally, rotation of the drill bit may be driven, in part, by a ‘mud motor’ which is located above the drill bit and which forms part of the drill string. The mud motor receives drilling fluid (commonly known as ‘mud’) which is delivered from the surface through the drill pipe. Movement of the drilling fluid is converted into rotational movement of the drill bit by a suitable mechanism—such as a helical flow path for the fluid through the mud motor which imparts a rotational force on a part of the mud motor which is coupled to the drill bit. 
     The drilling fluid is passed towards the drill bit through the drill string and leaves the drill string towards remote end thereof (normally at the drill bit). The drill fluid is, therefore, expelled into the wellbore. The fluid then travels up the wellbore, around the drill string, to the surface (as a result of more drilling fluid being pumped down the drill string at the surface to drive further rotation of the drill bit). 
     As is understood in the art, the drilling fluid also serves other purposes including lubrication and the transportation of cut material from the remote end of the wellbore to the surface. 
     The mud motor is, as will be appreciated, a form of ‘downhole motor’ in that the motor is part of the drill string which is located within the wellbore during use. Such motors include, for example, turbine motors (in which the fluid passes over a series of turbine blades to drive rotational movement) and positive displacement motors (in which a rotor and stator are cooperatively shaped such that the fluid pumped through the motor will cause rotation of the rotor with respect to the stator). 
     In conventional systems, the drill bit is rotated from the proximal end of the drill string (i.e. generally at or towards the ground surface)—such systems may or may not include a mud motor as well. Accordingly, the system may include one or more motors (which may be electrically powered motors) which are supported above or on the ground surface and which are mechanically coupled to the drill string. Such a system may be a top drive system in which the or each motor is suspended above the drill string and is mechanically coupled thereto. Such a system may, alternatively, be a rotary table based system in which the rotation of the drill string is driven by a rotary table which is located generally adjacent the wellbore substantially at the ground surface level (but potentially spaced apart from the ground surface by one or more other components of the system). 
     There is a need to keep the drill string in the desired position within the wellbore during the drilling operation so that the drill bit acts on the ground to cut the wellbore in the desired direction. Often this entails keeping the drill string centralised within the wellbore. 
     One or more stabilisers are provided as part of the drill string to space the drill string from a wall of the wellbore to keep the drill string in the desired position within the wellbore. These one or more stabilisers may also be used with rotary steerable tools in which a part of the drill string is spaced from the wall of the wellbore but the drill bit itself may be steerable towards the wall of the wellbore. 
     The stabilisers may take many different forms but typically comprise members which extend radially from the drill string towards the wellbore wall such that they contact the wellbore wall to restrict lateral movement of the drill string within the wellbore. 
     The drill string must be allowed axial movement, so that the drilling operation is not substantively impeded by the stabilisers, and the stabilisers must allow the flow of drilling fluid between the drill string and the wellbore wall to the surface. 
     Parts of the wellbore which do not have a casing (i.e. tubing which is typically cemented in place along the wellbore to separate the ground at the wellbore wall from the inside of the wellbore) are known as ‘open holes’. 
     The ground at the wellbore wall in an open hole is stabilised, in part, by the drilling fluid which passes between the wellbore wall and the drill string as it travels up the wellbore. This mud exerts a force against the wellbore wall to resist collapse of parts of the wellbore wall into the open hole of the wellbore. In addition, the fluid in the drilling fluid may, in places in which the ground is permeable for example, pass into the ground leaving behind an accumulation of ‘mud’ from the drilling fluid. This mud or filter cake forms a thin layer on the wellbore wall which aids in stabilisation of the wellbore. 
     Nevertheless, parts of the ground of the wellbore may partially collapse into the wellbore and may be carried by the drilling fluid up the wellbore along with cuttings excavated by the drill bit from the remote end of the wellbore. 
     To allow the passage of drilling fluid past, the stabilisers typically include elongate members which extend along a length of the drill string (as well as radially). Each such member is separated from its neighbouring member to allow the passage of drilling fluid between the members. 
     If there is an accumulation of debris adjacent the stabilisers, however, this can impede or even prevent axial movement of the drill string with respect to the wellbore. Such an accumulation may occur as a result of material from the ground collapsing into the wellbore and/or as a result of the cuttings from the action of the drill bit. The situation may be exasperated by the poor selection of an appropriate form of drilling fluid for the ground through which the wellbore is being drilled. 
     In some wellbores the ground surrounding part of the wellbore may swell (e.g. as a result of absorption of the drilling fluid). In addition, these and other materials in the ground may be pressed into the wellbore by the weight of the surrounding material—thus increasing the quantity of debris in the wellbore. Splintering of material from the wall of the wellbore may also occur as a result of over-pressured formations in the ground surrounding the wellbore, poor selection of drilling fluid, the absorption of drilling fluid by surrounding materials, and the like. 
     This is generally known a ‘packing off’ and commonly occurs in the region of the stabilisers (as the debris is packed between the drill string and the wellbore wall and inhibited from being removed by the stabilisers of the drill string). 
     The packing off of the drill string may cause the drill string to become stuck. This prevents removal of the drill string from the wellbore and/or continued drilling of the wellbore. 
     The drilling fluid, in such a situation, is often trapped and prevented from moving past one or more stabilisers by the debris. As such, any axial movement of the drill string (and hence the stabilisers which form a part of that drill string) requires the drilling fluid to be compressed or extended. This creates a hydraulic locking action which further inhibits axial movement of the drill string within the wellbore. In such events, rotation of the drill string may also be inhibited or substantially prevented—as a result of a number of different factors including increased friction at the drill bit and/or stabilisers (or elsewhere along the drill string) due to the delivery of drilling fluid being terminated (to prevent excessive fluid pressures) and/or the compression of material between the drill string and the wall of the wellbore at the site of the packing off. 
     A stuck drill string may be difficult (or potentially impossible) to remove. The time taken to remove a stuck drill string also results in significant extra costs. 
     The same problems also apply to other boreholes and the use of other drill string elements. 
     There is a need, therefore, to provide mechanisms and methods by which stuck drill strings can be more readily removed from boreholes. 
     Accordingly, the present invention seeks to ameliorate one or more problems associated with the prior art. 
     The foregoing discussion is intended only to illustrate the present field and should not be taken as a disavowal of claim scope. 
     SUMMARY 
     An aspect of the present invention provides a drill string element for use in the drilling of a borehole, the drill string element comprising: a main body; and a flow control mechanism, wherein the drill string element defines a first aperture, another aperture and a passage configured to provide a fluid flow path between the first aperture and the other aperture, the first and other apertures provide fluid communication between a volume around the drill string element and the passage, and the flow control mechanism is configured to inhibit the flow of fluid through the passage in a first operating mode and to permit the flow of fluid through the passage in a second operating mode. 
     The passage may be a fluid relief passage, such that the flow of fluid through the fluid relief passage is configured to provide a path for the flow of fluid which bypasses a portion of a volume between the wall of the borehole and the drill string element. 
     The fluid relief passage may provide a fluid flow path from a first part of the main body to a second part of the main body, the first and second parts being separated by a stabilisation member portion from which one or more stabilisation members extend. 
     The passage may include a cleaning fluid passage, such that the flow of fluid through the other aperture is configured to clean at least a part of the drill string element. 
     The drill string element may be a drill string stabiliser and the at least part of the drill string element is a part of a stabilisation member portion of the drill string element, wherein the stabilisation member portion includes the one or more stabilisation members. 
     The flow control mechanism may comprise at least one of a valve and a cover associated with the first and/or other aperture. 
     Each valve or cover may be configured to inhibit or substantially prevent the flow of fluid into the passage through the associated aperture. 
     The flow control mechanism may be actuated between the first and second operating modes by a fluid pressure adjacent the first and/or other aperture exceeding a threshold fluid pressure. 
     The flow control mechanism may comprise one or more burst discs. 
     The flow control mechanism may be a one-time actuatable mechanism such that the mechanism is actuatable from the first operating mode to the second operating mode, and cannot be returned to the first operating mode without replacement. 
     The flow control mechanism may be a multi-time actuatable mechanism such that the mechanism is actuatable from the first operating mode to the second operating mode, and can be returned to the first operating mode without replacement. 
     The flow control mechanism may be configured to be returned to the first operating mode without removal of the drill string element from the borehole. 
     The drill string element may include a further flow control mechanism and defines a further aperture and a cleaning fluid passage, wherein the further flow control mechanism may be configured to inhibit the flow of fluid through the cleaning fluid passage in a first operating mode and to permit the flow of fluid through the cleaning fluid passage in a second operating mode, and wherein the flow of fluid through the further aperture may be configured to clean at least a part of the drill string element. 
     The further flow control mechanism may comprise at least one of a valve and a cover associated with the further aperture. 
     Each valve or cover of the further flow control mechanism may be configured to inhibit or substantially prevent the flow of fluid into the cleaning fluid passage through the associated aperture. 
     The further flow control mechanism may be actuated between the first and second operating modes by a fluid pressure adjacent the further aperture exceeding a threshold fluid pressure. 
     The further flow control mechanism may comprise one or more burst discs. 
     The further flow control mechanism may be a one-time actuatable mechanism such that the mechanism is actuatable from the first operating mode to the second operating mode, and cannot be returned to the first operating mode without replacement. 
     The further flow control mechanism may be a multi-time actuatable mechanism such that the mechanism is actuatable from the first operating mode to the second operating mode, and can be returned to the first operating mode without replacement. 
     The further flow control mechanism may be configured to be returned to the first operating mode without removal of the drill string element from the borehole. 
     The drill string element may be a drill string stabiliser and the drill string element further comprises: one or more stabilisation members extending from the main body, the or each stabilisation member configured to abut a wall of the borehole. 
     The drill string element may further comprise a first attachment portion and a second attachment portion, the first and second attachment portions being configured for attachment to respective first and second further drill string elements. 
     The main body may further define a central bore. 
     The passage may extend from a first end to a second end of the main body. 
     The drill string element may include a plurality of passages between a respective plurality of first and other apertures. 
     The plurality of first apertures and/or the plurality of other apertures may be spaced around a circumference of the drill string element. 
     The passage may comprise a first passage portion and a second passage portion, wherein the first and second passage portions may be inclined at respective angles with respect to a longitudinal axis of the main body and the first and second passage portions may intersect to form at least part of the passage. 
     The passage may comprise a curved part. 
     Another aspect provides a bottom hole assembly including a drill string element. 
     Another aspect provides a drill string including a drill string element. 
     Another aspect provides a rig for the drilling of an oil or gas well, the rig including at least one drill string element. 
     Another aspect provides a method of retrofitting a drill string element for use in the drilling of a borehole, the method comprising: providing a drill string element having a main body; machining a passage through at least part of the drill string element, the passage providing a fluid flow path between a first aperture and another aperture; and fitting a flow control mechanism to the drill string element, the flow control mechanism being configured to inhibit the flow of fluid through the passage in a first operating mode and to permit the flow of fluid through the passage in a second operating mode. 
     The drill string element may be a drill string stabiliser including one or more stabilisation members extending from the main body, the or each stabilisation member being configured to abut a wall of a borehole. 
     Another aspect provides a non-transitory computer readable medium having stored thereon instructions which, when processed by a computing device, are configured to cause the computing device to: receive dimensions of a drill string element; receive one or more machining tool limitations representing limitations of a machining tool for machining the drill string element; define a first aperture location and another aperture location; and define a passage between the first aperture location and the other aperture location based on the dimensions of the drill string element and the one or more machining tool limitations. 
     The non-transitory computer readable medium may have stored thereon further instructions which, when processed by a computing device, are configured to cause the computing device to: control a machining tool to machine the defined passage in the drill string element from the first aperture to the other aperture. 
     The non-transitory computer readable medium may have stored thereon further instructions which, when processed by a computing device, are configured to cause the computing device to: define the first aperture location, the other aperture location, and/or passage based on the dimensions of the drill string element, the one or more machining tool limitations, and one or more flow control mechanism constraints, wherein the one or more flow control mechanism constraints define one or more requirements to permit the fixing of a flow control mechanism to the drill string element. 
     The non-transitory computer readable medium may have stored thereon further instructions which, when processed by a computing device, are configured to cause the computing device to: receive information about the intended use of the drill string element; and to use the received information to determine one or more attributes of a flow control mechanism and/or the passage. 
     The non-transitory computer readable medium wherein the drill string element may be a drill string stabiliser. 
     An aspect of the present invention provides a drill string stabiliser for use in the drilling of a wellbore of an oil or gas well, the drill string stabiliser comprising: a main body; one or more stabilisation members extending from the main body, the or each stabilisation member configured to abut a wall of a wellbore; and a flow control mechanism, wherein the drill string stabiliser defines a first aperture, a second aperture and a fluid relief passage configured to provide a fluid flow path between the first aperture and the second aperture, and the flow control mechanism is configured to inhibit the flow of fluid through the fluid relief passage in a first operating mode and to permit the flow of fluid through the fluid relief passage in a second operating mode. 
     Another aspect of the present invention provides a method of retrofitting a drill string element for use in the drilling of a wellbore of an oil or gas well, the method comprising: providing a drill string element having a main body; machining a fluid relief passage through at least part of the drill string element, the fluid relief passage providing a fluid flow path between a first aperture and a second aperture; and fitting a flow control mechanism to the drill string element, the flow control mechanism being configured to inhibit the flow of fluid through the fluid relief passage in a first operating mode and to permit the flow of fluid through the fluid relief passage in a second operating mode. 
     Another aspect of the present invention provides a non-transitory computer readable medium having stored thereon instructions which, when processed by a computing device, are configured to cause the computing device to: receive dimensions of a drill string element; receive one or more machining tool limitations representing limitations of a machining tool for machining the drill string element; define a first aperture location and a second aperture location; and define a fluid relief passage between the first aperture location and the second aperture location based on the dimensions of the drill string element and the one or more machining tool limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a simplified cross-sectional view of an embodiment of a drill string stabiliser; 
         FIG. 2  shows an embodiment of a drill string including a drill string stabiliser; 
         FIG. 3  shows a simplified cross-sectional view of an embodiment of a drill string stabiliser in a wellbore as a pack off event begins to occur; 
         FIG. 4  shows a simplified cross-sectional view of an embodiment of a drill string stabiliser in a wellbore in a pack off event; 
         FIG. 5  shows an embodiment of a rig including a drill string stabiliser; 
         FIG. 6  shows an embodiment of a computing device; 
         FIG. 7  shows a simplified cross-sectional view of an embodiment of a drill string stabiliser; 
         FIG. 8  shows an enlarged portion of a simplified cross-sectional view of an embodiment of a drill string stabiliser; 
         FIG. 9  shows an enlarged portion of a simplified cross-sectional view of an embodiment of a drill string stabiliser; 
         FIG. 10  shows a drill string stabiliser of an embodiment; 
         FIG. 11  shows a cross-section of a filtration member of an embodiment; and 
         FIG. 12  shows a cross-sectional view of a drill string stabiliser of an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , an embodiment of the present invention comprises a drill string stabiliser  1 . The drill string stabiliser  1  may be configured for use in the drilling of oil and/or gas wells or other drilling operations in which a fluid passes between a bore and a drill string  2  (see  FIG. 2 ) such that there is a risk of hydraulic locking if the passage of the fluid is inhibited. 
     The drill string stabiliser  1  comprises a main body  11  which may have a generally circular cross-sectional shape. The main body  11  may be elongate and is configured for attachment to one or more elements of a drill string  2 . As such, the main body  11  may include a first attachment portion  12  which is located at a first end of the main body  11 . The main body  11  may further include a second attachment portion  13  at a second end of the main body  11  (the first and second ends opposing each other across a length of the main body  11 ). 
     The first attachment portion  12  (if provided) is configured to be attached to a corresponding attachment portion of a first drill string element  21  of the drill string  2  and the second attachment portion  13  (if provided) is configured to be attached to a corresponding attachment portion of a second drill string element  22  of the drill string  2 . As such, the drill string stabiliser  1  may be integrated into the drill string  2  generally between the first and second drill string elements  21 , 22 . In embodiments, the first drill string element  21  is an element which is remote from a drill bit  23  of the drill string  2  relative to the drill string stabiliser  1  and the second drill string element  22  is an element which is proximal to the drill bit  23  relative to the drill string stabiliser  1 . In other words, in embodiments in which the drill string  2  is for use in drilling a generally vertical wellbore into the ground, the first drill string element  21  is above the drill string stabiliser  1  and the second drill string element  22  is below the drill string stabiliser  1 . 
     As will be appreciated, the first and/or second drill string elements  21 , 22  may each include a plurality of sub-elements or components. In addition, as will be understood, the first and second drill string elements  21 , 22  are depicted by way of example only and the drill string  2  may include other elements not depicted. 
     The first attachment portion  12  may comprise a female member which carries an internal thread which is configured to cooperate with a male member of the corresponding attachment portion of the first drill string element  21  (the male member carrying an external thread). The second attachment portion  13  may comprise a male member which carries an external thread which is configured to cooperate with a female member of the corresponding attachment portion of the second drill string element  22  (the female member carrying an internal thread). 
     The male member (of the second attachment portion  13  and/or the first drill string element  21 ) may be tapered such that an external diameter of a distal end of the male member is greater than an external diameter of a proximal end of the male member. The female member (of the first attachment portion  12  and/or the second drill string element  22 ) may be tapered such that an internal diameter of a distal end of the female member is greater than an internal diameter of a proximal end of the female member. 
     The male and female members may be what is known in the art as a ‘pin’ and ‘box’ respectively. 
     In embodiments, the female member of the first attachment portion  12  is configured to receive and engage a male member of the same type and configuration as the male member of the second attachment portion  13 . This configuration allows interchangeable elements of the drill string  2  (including the drill string stabiliser  1 ) to be interchanged and re-ordered with relative ease. 
     In some embodiments, the drill string stabiliser  1  has a first thread configuration at the first attachment portion  12  and a second thread configuration at the second attachment portion  13 . The first and second thread configurations may be different from each other in terms of one or more of thread angle, major diameter, minor diameter, pitch, lead, and pitch diameter. Accordingly, in some embodiments, the drill string stabiliser  1  may be used to as a converter to allow the attachment of drill string elements  21 , 22  with different attachment portions compared to other drill string elements  21 , 22  of the same drill string  2 . 
     The drill string stabiliser  1  may form part of a bottom hole assembly  3  of the drill string  2 . The bottom hole assembly  3  may include the drill bit  23  and is generally the portion of the drill string  2  which is proximal to the drill bit  23 . 
     The bottom hole assembly  3  may include a plurality of such drill string stabilisers  1 . In some embodiments, the first and/or second drill string element  21 , 22  may include another such drill string stabiliser  1 . The drill string  2  may include two or more drill string stabilisers  1 . In some embodiments, the drill string  2  includes between one and three drill string stabilisers  1 . 
     In some embodiments, the drill bit  23  is directly coupled to such a drill string stabiliser  1 . In these (and also other) embodiments, the first and second attachment portions  12 , 13  may both comprise female or male members (in other words, the drill string stabiliser  1  may have two female attachment portions  12 , 13  or two male attachment portions  12 , 13 ). 
     The main body  11  of the drill string stabiliser  1  carries a plurality of stabilisation members  14 . Each stabilisation member  14  extends from the main body  11  outwardly. This extension may be in a substantially radial direction. Each stabilisation member  14  may be elongate such that a first end of the stabilisation member  14  is towards the first end of the main body  11  and a second end of the stabilisation member  14  is towards a second end of the main body  11 . 
     In some embodiments, each stabilisation member  14  extends in a helical (or other curved) arrangement around the circumference and along a length of the main body  11 . In some embodiments, each stabilisation member  14  is a substantially straight member which extends along a length of the main body  11 . In some embodiments, each stabilisation member  14  comprises a plurality of portions and each portion extends in a different direction around the circumference of the main body  11  with respect to at least one other such portion (e.g. in a zigzag formation). 
     The first end and/or the second end of each stabilisation member  14  may be bevelled (which may ease movement of the drill string stabiliser  1  through a wellbore  4  and/or which may reduce the likelihood of the drill string  2  packing off due to the accumulation of debris at the stabilisation members  14 ). 
     The stabilisation members  14  may collectively form a stabilisation member portion  141  of the drill string stabiliser  1  which may also include one or more curved surfaces which extend between the stabilisation members  14 . The main body  11  of the drill string stabiliser  1  may, therefore, comprise a first part towards the first end of the main body  11  and a second part towards the second end of the main body  11 , the first and second parts being separated by the stabilisation member portion  141 . 
     The or each stabilisation member  14  may include an abutment surface  142  which is configured to abut against an inner wall of the wellbore  4  during use. The abutment surface  142  may be a substantially smooth surface or may include one or more surface formations. The abutment surface  142  may be configured to minimise friction between the drill string stabiliser  1  and the wall of the wellbore  4  (such that the abutment of the stabilisation members  14  and the wall of the wellbore  4  does not exert an excessive load on a mechanism  24  which is provided to drive rotation of the drill bit  23  during use). As will be appreciated, as part of the drill string  2 , the drill string stabiliser  1  rotates within the wellbore during use (the rotation being driven by the aforementioned mechanism  24  and/or by a drive mechanism provided at or towards a proximal end of the drive string  2  (e.g. above ground)). In some embodiments, it is envisaged that the drill string stabiliser  1  will generally not rotate with respect to the wellbore  4  but that the drill string stabiliser  1  may be configured for rotation with respect to one of more other drill string elements—for example, the drill string stabiliser  1  may be mounted on a section of drill pipe and may be configured to rotate about that section of drill pipe. 
     As will be appreciated, each stabilisation member  14  may be a ‘stabiliser blade’. 
     The drill string stabiliser  1  defines a central bore  15  which is configured to allow the passage of drilling fluid therethrough. The central bore  15  extends from the first end (and first attachment portion  12 ) to the second end (and second attachment portion  13 ) of the drill string stabiliser  1 . The first and second attachment portions  12 , 13  are configured such that the central bore  15  is in fluid communication with the first and second drill string elements  21 , 22  (when connected thereto) and, preferably, with central bores of those drill string elements  21 , 22 . 
     In some embodiments, the main body  11  of the drill string stabiliser  1  at least partially defines one or more fluid relief passages  16 . The or each fluid relief passage  16  extends generally from the first end of the main body  11  towards the second end of the main body  11 . The main body  11  defines a first  161  and a second  162  aperture of each fluid relief passage  16 . Each fluid relief passage  16  is configured to allow the fluid to pass into the main body  11  through the first aperture  161 , along a length of the passage  16  to the second aperture  162  and out of the main body  11  through the second aperture  162  (and vice versa). In some embodiments, the first aperture  161  is located in the first part of the main body  11  and the second aperture  162  is located in the second part of the main body  11 . 
     The fluid relief passage  16  may, in some embodiments, be at least partially defined by one or more of the stabilisation members  14 . In some embodiments, the first and/or second aperture  161 , 162  of each passage  16  is defined, at least partially, by one or more stabilisation members  14 . 
     Accordingly, the one or more fluid relief passages  16  provide a fluid communication path through at least part of the drill string stabiliser  1 . That path may traverse a part of the drill string stabiliser  1  which has the largest width or diameter. The part of the drill string stabiliser  1  which has the largest width or diameter may be the stabilisation member portion  141 . The one or more fluid relief passages  16  may, therefore, provide a path for fluid around and/or through the stabilisation member portion  141  within the drill string stabiliser  1  (which may circumvent debris which has accumulated in that region). 
     The or each fluid relief passage  16  may comprise two intersecting passage portions  163 , 164 . A first of the two intersecting passage portions may extend from the first aperture  161  along an axis which is inclined with respect to a longitudinal axis of the drill string stabiliser  1  towards the point of intersection—which may be within the main body  11 . The second of the two intersecting passage portions may extend from the second aperture  162  along another axis which is inclined with respect to a longitudinal axis of the drill string stabiliser  1  towards the point of intersection. In some embodiments, the first and second intersecting portions  163 , 164  are substantially straight. In some embodiments, the first and second intersecting portions  163 , 164  are curved or include curved parts. In some such embodiments, the first and second intersecting portions  163 , 164  may each include a curved part remote from the intersection of the two portions  163 , 164  and a substantially straight part at the intersection of the two portions  163 , 164 . In some embodiments, the first and second intersecting portions  163 , 164  form a fluid relief passage  16  with a substantially constant degree of curvature (e.g. forming an arc of a circle). 
     In some embodiments, the first and second passage portions  163 , 164  form a single passage portion which extends in a direction which is substantially parallel to the longitudinal axis of the drill string stabiliser  1  (the direction of extension may be along an axis which is, in fact, parallel to the longitudinal axis of the drill string stabiliser  1 ). The single passage  163 , 164 , may be coupled in fluid communication with the first and second apertures  161 , 162  by respective passages which extend generally perpendicular to the longitudinal axis of the drill string stabiliser  1  (and which may extend radially). 
     In some embodiments, a plurality of fluid relief passages  16  is provided. In such embodiments, the respective first apertures  161  of the plurality of fluid relief passages  16  may be located at two or more different distances from the first end of the main body  11 . Similarly, the respective second apertures  162  of the plurality of fluid relief passages  16  may be located at two or more different distances from the second end of the main body  11 . In other words, the first apertures  161  may be staggered along a length of the main body  11  and/or the second apertures  162  may be staggered along a length of the main body  11  (the plurality of fluid relief passages  16  may include fluid relief passages of different lengths). 
     In some embodiments, a plurality of fluid relief passages  16  is provided and the fluid relief passages  16  are spaced around the circumference of the drill string stabiliser  1 . In some embodiments this spacing is substantially even. 
     In some embodiments, a plurality of first apertures  161  are in fluid communication with the same fluid relief passage  16  and/or a plurality of second apertures  162  are in fluid communication with the same fluid relief passages  16 . In other words, a common fluid relief passage  16  may be provided with a plurality of first and/or second apertures  161 , 162  to allow the flow of fluid into and out of the passage  16 . 
     The fluid relief passage  16  or passages  16  and/or any portions thereof may be defined by substantially smooth internal walls to keep friction to a minimum and allow the fast flow of fluid therethrough. In some embodiments, the fluid relief passage  16  or passages  16  (or parts thereof) are lined or coated. The lining or coating may improve corrosion and/or wear resistance, and/or reduce the coefficient of friction of the passage or passages  16  to improve the flow of fluid therethrough. In some embodiments, the lining/coating is replaceable. In some embodiments, the lining is provided be a tube which is configured to be inserted into the fluid relief passage  16 . The lining/coating may be a plastic lining/coating. The lining/coating may be a tungsten carbide based lining/coating—such as a tungsten carbide tube. 
     The fluid relief passage  16  or passages  16  and/or any portions thereof may have a circular cross-sectional shape. 
     The or each fluid relief passage  16  may be associated with a flow control mechanism  17  which forms part of the drill string stabiliser  1  (see  FIGS. 2, 3 and 4  in particular). 
     The flow control mechanism  17  is configured to control the flow of fluid through the associated fluid relief passage  16 . The flow control mechanism  17  is configured to allow or inhibit (or substantially prevent), selectively, the flow of fluid through the associated fluid relief passage  16 . As such, under normal operating conditions, the flow control mechanism  17  may inhibit or substantially prevent the flow of any fluid through the associated fluid relief passage  16  (a first operating mode). However, in the event of an abnormal operating condition—such as may be caused by packing off of the drill string  2 —the flow control mechanism  17  may actuate to allow the flow of fluid through the associated fluid relief passage  16  (a second operating mode). 
     The abnormal operating condition may be a fluid pressure differential across a length of the drill string stabiliser  1 . In a packing off event, the fluid pressure towards one end of the drill string stabiliser  1  is likely to be significantly different to the fluid pressure towards the other end of the drill string stabiliser  1 . The abnormal operating condition may be a fluid pressure differential between a volume outside the fluid relief passage  16  and a fluid pressure inside the fluid relief passage  16 . The fluid pressure differential may be a differential of about 3,500 kPa (500 psi). 
     The flow control mechanism  17  may comprise a cover or valve  171  for each aperture  161 , 162  associated with a fluid relief passage  16 . Thus, in embodiments in which each fluid relief passage  16  has a first aperture  161  and a second aperture  162 , each of these apertures may be provided with a cover or valve  171  of the flow control mechanism  17 . 
     The valves or covers  171  are configured to inhibit the accumulation of debris in their respective fluid relief passage  16  which may block the passage  16  and/or inhibit the flow of fluid therethrough. Accordingly, the valves or covers  171  block their respective passage  16  from fluid communication with a volume around the drill string stabiliser  1  until such time as they are actuated to allow such communication. 
     The flow control mechanism  17  may be a mechanically operated mechanism. Thus, for example, the or each valve or cover  171  may be configured to open (to allow the passage of fluid therethrough) when it is exposed to a fluid pressure which exceeds a threshold. The threshold may be set in dependence on the expected normal operating conditions for the drive string stabiliser  1  such that the threshold is not usually exceed during normal operation but is exceeded in the event of packing off of the drive string  2 . 
     In some embodiments, the or each valve or cover  171  is a burst disc. The or each valve or cover  171  (such as a burst disc) may be secured with respect to a respective one of the first and second apertures  161 , 162 . The or each valve or cover  171  may be held in place by a respective clip—such as a circlip. For example, the or each valve or cover  171  may be at least partially received by the drill string stabiliser  1  (e.g. by the main body  11  near, adjacent, or within first or second apertures  161 , 162 ) and a clip may fit at least partially within a recess defined by the drill string stabiliser  1  (e.g. by the main body  11 ) to inhibit or substantially prevent the or each valve or cover  171  from separating from the stabiliser  1 . In some embodiments, the valve or cover  171  provides a one-time operation—in that it may be actuated from the first operating mode to the second operating mode only once before requiring a manual reset or replacement. In embodiments, the valve or cover  171  comprises a flapper-type valve/cover. In some embodiments, therefore, the valve or cover  171  is operable to operate multiple times. In some embodiments, the valve or cover  171  is intended to be replaced after a predetermined period of use in a drill string  2  in a wellbore  4  (whether or not activated during that time)—i.e. preventative maintenance. 
     In some embodiments, the valve or cover  171  may comprise a shear pin valve. 
     In some embodiments, the valve or cover  171  is configured to permit a degree of ‘float’. In particular, the valve or cover  171  may be configured such that an increase in fluid pressure across the valve or cover  171  will cause a part of the valve or cover  171  to move without the valve or cover  171  opening (i.e. with the valve or cover  171  remaining substantially closed). Thus, for example, an increase in the fluid pressure adjacent the valve or cover  171  but on the side remote from the pressure relief passage  16 , will cause a part of the valve or cover  171  to move towards or into the fluid relief passage  16 . The valve or cover  171  may, therefore, be configured to increase or decrease the fluid pressure in the fluid relief passage  16 . If such a valve or cover  171  were located at either end of the fluid relief passage  16  (as in some embodiments), then an increase in fluid pressure across the valve or cover  171  associated with one end of the fluid relief passage  16  will cause movement of a part of that valve or cover  171 . A fluid (which may be a substantially incompressible fluid) within the fluid relief passage  16  may transmit this movement to the valve or cover  171  at the other end of the fluid relief passage  16  to cause a movement of a part of that valve or cover  171  too. 
     Accordingly, the fluid pressure across a valve or cover  171  according to some embodiments of the invention may be, in part, dependent on the fluid pressure across another valve or cover  171  associated with the same fluid relief passage  16 . Therefore, the fluid pressure adjacent the valve or cover  171  which will cause that valve or cover  171  to open will be dependent, at least in part, on the fluid pressure adjacent the other valve or cover  171 . That pressure may be set by the use of a shear pin, for example. In some embodiments, therefore, a degree of float is permitted but a pressure differential across a valve or cover  171  which exceeds a threshold will still cause the valve or cover  171  to open. 
     This arrangement provides for a fluid pressure differential adjacent two different valves or covers  171  associated with the same fluid relief passage  16  be a factor in determining when, during a pack off (for example), the or each valve or cover  171  will open. 
     The fluid relief passage  16  may, therefore, be filled with a liquid such as oil (which is contained within the fluid relief passage  16  by the valves or covers  171 ). The fluid relief passage  16  may be filled with a fluid (which may be a liquid) which is under pressure (i.e. at a pressure which is greater than atmospheric pressure)—this may occur before or after the or each cover or valve  171  is attached to control the flow of fluid through the first and/or second apertures  161 , 162 . 
     As will be appreciated, an operator may fit a valve or cover  171  which is configured to actuate from the first operating mode to the second operating mode based on the expected fluid pressures for that particular drilling operation. 
     Accordingly, in an embodiment in which a fluid relief passage  16  is provided and the first and second apertures  161 , 162  are associated with valves or covers  171 , if the fluid pressure adjacent the second aperture  162  exceeds the threshold of the cover or valve  171  at that aperture  162 , the cover or valve  171  will allow flow of fluid into the fluid relief passage  16 . This will cause a corresponding increase in fluid pressure within the fluid relief passage  16  adjacent the first aperture  161  until that pressure exceeds the threshold for the cover or valve  171  at the first aperture  161 . The valve or cover  171  at the first aperture  161  will, therefore, then allow the passage of fluid through the first aperture  161 . 
     As such, in a pack off event, a passage (the or each fluid relief passage  16 ) is provided which allows the passage of fluid through a portion of the drill string stabiliser  1  to reduce the hydraulic locking action. This is shown in  FIGS. 3 and 4 . In  FIG. 3  a pack off event is about to occur as debris has accumulated between the drill string stabiliser  1  and the wall of the wellbore  4 . Fluid pressure in the wellbore on the side of the drill string stabiliser  1  which is towards the drill bit  23  will increase and create a hydraulic locking effect which inhibits the removal of the drill string  2  from the wellbore  4  and/or the further insertion of the drill string  2  into the wellbore  4  (the latter also being inhibited by the debris). The flow control mechanism  17  (in this case covers or valves  171 ) actuates to allow the flow of fluid through the fluid relief passage  16  (in this case passages  16 ). The flow path of the fluid through the fluid relief passage  16  (along with the actuated valves or covers  171 ) can be seen in  FIG. 4  which includes a dashed line indicating the flow path. On other occasions, the debris builds up above the drill string stabiliser  1  with a similar locking effect exhibited (this time with the removal of the drill string  2  being further inhibited by the debris). On such occasions, the flow of fluid through the or each fluid relief passage  16  would be the opposite to that indicated in  FIG. 4 . On some occasions, debris may build up above and below the drill string stabiliser  1 . 
     The two apertures  161 , 162  provide fluid communication between the wellbore  4  and the fluid relief passage  16  and are configured such that, in use, there is a length (a potentially packed off length) of drill string  2  between the two apertures  161 , 162 . The apertures  161 , 162  and fluid relief passage  16 , therefore, provide a secondary fluid communication path through a part of the drill string stabiliser  1 . 
     In some embodiments, the flow control mechanism  17  is not provided and one or more filtration members are provided instead. In some embodiments, the first and/or second apertures  161 , 162  are each associated with a filtration member which is configured to inhibit the passage of relatively large material into the fluid relief passage  16  with which the apertures  161 , 162  are associated. As such, the or each filtration member may inhibit or substantially prevent the movement of material into the fluid relief passage  16  which is smaller than the associated aperture  161 , 162  but larger than a predetermined size. The or each filtration member may comprise a wire mesh which covers the associated aperture  161 , 162 . In some embodiments, the or each filtration member comprises a bar or rod which extends across at least part of the aperture  161 , 162  (or a plurality of such bars or rods). The or each filtration member may, therefore, cover at least part of the aperture  161 , 162  with which it is associated. The use of a filtration member seeks to reduce the risk of the associated fluid relief passage  16  from becoming blocked. 
     In some embodiments in which one or more filtration members are provided, drilling fluid may be free to flow through the filtration member or members into and/or out of the passage  16  even when there is no packing off event occurring. In other embodiments, one or more filtration members are not provided and the apertures  161 , 162  are left open. 
     In some embodiments, the one or more filtration members are provided in addition to the flow control mechanism  17 . 
     In some embodiments (see  FIGS. 10 to 12 , for example), the one or more filtration members each comprise a main member (such as a plate)  271  defining at least one weep aperture  272 . In such embodiments, at least part of the flow control mechanism  17  may be mounted (or otherwise attached) to the main member  271 . The flow control mechanism  17  may be configured to control the flow of fluid into and/or out of the fluid relief passage  16  through the main member  271 . The main member  271  may be mounted over and/or at least partially within one of the apertures  161 , 162  (i.e. with respect to the first or second aperture  161 / 162 ). 
     The at least one weep aperture  272  provides fluid communication between a volume outside of the fluid relief passage  16  and the fluid relief passage  16 . The or each weep aperture  272  may provide restricted fluid communication between the volume outside of the fluid relief passage  16  and the fluid relief passage  16  (i.e. between the wellbore  4  and the fluid relief passage  16 , in use). This restricted flow may inhibit the passage of debris into the fluid relief passage  16 . Thus, fluid may flow into the fluid relief passage  16  during normal operation of the drill string stabiliser  1 . The flow control mechanism  17  may then operate if a pack off event occurs as discussed herein. The or each weep aperture  272 , therefore, acts to allow the fluid relief passage  16  to be primed with fluid so that the flow control mechanism  17  will operate in a pack off event. 
     In some embodiments, main members  271  are mounted with respect to respective first and second apertures  161 , 162  for the same fluid relief passage  16  (i.e. at both ends of the fluid relief passage  16 ). 
     In some embodiments, the first and second apertures  161 , 162  may be configured to receive at least part of a respective main member  271 . In some embodiments, the main member  271  is configured to fit within the first and/or second aperture  161 , 162  such that it substantially fills the aperture  161 , 162 . In some embodiments, the first and/or second aperture  161 , 162  is configured to receive substantially all of the main member  271 . In some embodiments, the main member  271 , when received by the first or second aperture  161 , 162 , will have an external surface which is substantially flush with an outer surface of the drill string stabiliser  1  adjacent the first or second aperture  161 , 162 . 
     The main member  271  may be held in place by a securing arrangement which may include one or more bolts, rivets, clips, etc. 
     An embodiment of the main member  271  is depicted in  FIG. 11 . In this embodiment (and others), the main member  271  includes an outer portion  271   a  and an inner portion  271   b . The outer portion  271   a  may have a larger cross-section than the inner portion  271   a  in at least one dimension, such that the outer portion  271   a  forms a cover or plate for the inner portion  271   b.    
     The main member  271  is depicted in  FIG. 11  received by the first or second aperture  161 , 162 . In this (and other) embodiments, the first or second aperture  161 , 162  includes a first portion which is configured (e.g. sized and shaped) to receive the outer portion  271   a  of the main member  271  and a second portion which is configured (e.g. sized and shaped) to receive the inner portion  271   b  of the main member  271 . 
     As will be appreciated, in this (and other) embodiments, the first and/or second aperture  161 / 162  is in the form of a channel which is configured to receive the main member  271 . The channel may be in fluid communication with the fluid relief passage  16 . In particular, the second portion may be in fluid communication with the fluid relief passage  16 . 
     The outer portion  271   a  of the main member  271  may define a main aperture  17   a  and a weep aperture  272 . The flow control mechanism  17  may be provided in relation to the main aperture  17   a  and may be configured to control the flow of fluid through the main member  271  to the fluid relief passage  16 . As such, the main aperture  17   a  may extend through a depth (which may be the entire depth or substantially the entire depth) of the main member  271 . The main aperture  17   a  may be coupled in fluid communication with a fluid channel  273  which may be at least partially defined by the main member  271 . In some embodiments, the fluid channel  273  is at least partially defined by the main body  11 . In some embodiments, the fluid channel  273  passes through at least part of the inner portion  271   b  of the main member  271 . The fluid channel  273  provides fluid communication between the main aperture  17   a  and the fluid relief aperture  16 . In some embodiments, the weep aperture  272  extends through a depth of the outer portion  271   a  of the main member  271  and is in fluid communication with the fluid channel  273 . 
     The weep aperture  273  may have a diameter (or width) which is smaller than the diameter (or width) of the main aperture  17   a.    
     The main aperture  17   a  (e.g. towards or within the outer portion  271   a  of the main member  271 ) may include a section with a large diameter (or width) followed by a portion of a smaller diameter (or width) towards the inner portion  271   b  such that a seat  17   b  is defined. The seat  17   b  is configured, in some embodiments, to support at least part of the valve or cover  171 . The valve or cover  171  may be configured, when seated on the seat  17   b , to seal substantially the smaller diameter section from the larger diameter section of the main aperture  17   a —thus inhibiting or substantially preventing the flow of the volume outside the drill string stabiliser  1  into the fluid relief passage  16 . 
     A recess for a clip  17   c  may be provided adjacent the seat  17   b  such that a clip  17   c  may be at least partially received by the recess to hold the valve or cover  171  in place on the seat  17   b.    
     The main member  271  may be removably fitted to the main body  11  such that it is possible to remove and replace the main member  271 . As will be appreciated, replacement of the valve or cover  17  may also possible. 
     As will be appreciated, therefore, the main member  271  of the filtration member of some embodiments provides the main aperture  17   a  (the flow of fluid through which is controlled by the flow control mechanism) and the weep aperture  272  which serves to allow the flow of fluid into the fluid relief passage  16  but to inhibit the passage of debris into the fluid relief passage  16 —see the above description regarding the operation of the filtration member. 
     In some embodiments, the drill string stabiliser  1  may include one or more further apertures  1611 —see  FIGS. 7 to 10 . The or each further apertures  1611  may be in fluid communication with a respective fluid relief passage  16  (which may be in fluid communication with the first and/or second apertures  161 / 162 . The or each further aperture  1611  may be in fluid communication with a respective fluid relief passage  16  via a cleaning fluid passage  16   a . The cleaning fluid passage  16   a  may interconnect with the fluid relief passage  16  between the first and second apertures  161 , 162 . 
     In embodiments, the or each further aperture  1611  is located in or near the stabilisation portion  141 . For example, the or each further aperture  1611  may be located between the stabilisation members  14  which form the stabilisation member portion  141  of the drill string stabiliser  1 . In some embodiments, the or each further aperture  1611  is not located through the abutment surfaces  142 . In some embodiments, the or each further aperture  1611  is located between the abutment surfaces  142  of the stabilisation member portion  141 . 
     A further valve or cover  171   a  and/or a further filtration member may be associated with the or each further aperture  1611  (such that there may be a plurality of further valves or covers  171   a  and/or further filtration members). The further valve or cover  171   a  and/or further filtration member may take the form of the valves or covers  171 , or filtration members, described herein (see above). As will be appreciated, the further valve or cover  171   a  and/or further filtration member are examples of further flow control mechanisms  17   a  which may be provided in some embodiments—each further flow control mechanism  17   a  being associated with a respective further aperture  1611 . The or each further flow control mechanism  17   a  is configured to control the flow of fluid through the associated further aperture  1611 . The or each further flow control mechanism  17   a  may be configured to control the flow of fluid from the fluid relief passage  16  through the associated further aperture  1611  and into a volume of the bore surrounding the drill string stabiliser  1 . In some embodiments, the or each further flow control mechanism  17   a  is a sub-component of the flow control mechanism  17 . 
     The or each further flow control mechanism  17   a  is configured to allow the passage of fluid from the fluid relief passage  16  through the associated further aperture  1611  on detection of commencement of a possible abnormal operating condition—such as a packing off event. This flow of fluid may be permitted before the flow control mechanism  17  permits the flow of fluid between the first and second apertures  161 , 162  through the fluid relief passage  16  in the initial stages of a packing off event. This may be achieved by the further flow control mechanism  17   a  allowing the flow of fluid at a lower pressure or pressure differential than the pressure or pressure differential at which the flow control mechanism  17  allows the flow of fluid through the fluid relief passage  16 . In order for the flow of fluid to occur through the further aperture  1611 , one or both of the first and second apertures  161 , 162  may need to have been fitted with a filtration member as described above (with or without a weep aperture  272 ). In some embodiments, one of the first or second apertures  161 , 162  includes a valve or cover  171  which is configured to allow the flow of fluid therethrough when the pressure of the fluid adjacent the valve or cover  171  exceeds a threshold (or when the pressure differential across the valve or cover  171  exceeds a threshold). 
     The flow of fluid through the or each further aperture  1611  is configured, therefore, to provide a flow of fluid (a ‘wash’) around/through the stabilisation member portion  141  which is one possible location of the accumulation which may cause a pack off event. Thus, the flow of fluid may help to dislodge the accumulating material and prevent an actual packing off event from occurring. 
     As such, in use, when a pack off event is starting to occur the further control mechanism  17   a  may permit the flow of fluid from the first or second aperture  161 / 162  through the further aperture  1611  with a view to cleaning at least a part of the stabilisation member portion  141  (e.g. the space between stabilisers  14 ). If this is insufficient to prevent the pack of event from occurring, then the pressure differential across the stabiliser  1  will increase to the point at which the flow control mechanism  17  will allow the flow of fluid through the fluid relief passage  16  between the first and second apertures  161 , 162 . 
     As will be appreciated, a plurality of further apertures  1611  may be provided. Each further aperture  1611  may be associated with one or more cleaning fluid passage  16   a ; equally, each further aperture  1611  may be one of a plurality of further apertures  1611  associated with a single cleaning fluid passage  16   a . The or each cleaning fluid passage  16   a  may be in fluid communication with a respective fluid relief passage  16 , with a plurality of fluid relief passages  16 , or a single fluid relief passage  16  may be associated with a plurality of cleaning passages  16   a —see, for example,  FIGS. 7 and 8 . 
     In embodiments with a plurality of further apertures  1611 , the further flow control mechanism  17   a  or mechanisms  17   a  may be configured to permit the commencement of the flow of fluid through the further apertures  1611  in a pre-defined sequence. The predetermined sequence may be determined by the pressure of differential pressure at or across the further flow control mechanism or mechanisms  17   a  associated with the further apertures  1611 . Accordingly, the further control mechanism or mechanisms  17   a  may permit the flow of fluid through a one of the further apertures  1611  towards the first or second end of the main body  11  before the flow of fluid is permitted through another of the further apertures  1611  which is towards the opposing end of the main body  11 . In some embodiments, the further apertures  1611  may, accordingly, open (e.g. by the bursting of a burst disc or opening of a valve) in a sequence from towards the first end of the main body  11  towards the second end of the main body  11  (or vice versa). 
     In some embodiments, the flow of fluid through a first of the further apertures  1611  is permitted at a first fluid pressure or differential pressure at or across the further control mechanism  17   a  for that first further aperture  1611 . The flow of fluid through a second of the further apertures  1611  is permitted at a second fluid pressure or differential pressure at or across the further control mechanism  17   a  for that second further aperture  1611 , and so on. The first further aperture  1611  may be located towards the first end of the main body  11  and the second further aperture  1611  may be located towards the second end of the main body  11  relative to the first further aperture  1611  (or vice versa). The second fluid pressure or differential pressure may be higher than the first fluid pressure or differential pressure. This may be achieved by the use of burst discs with different burst pressures or through the controlled operation of valves of the further flow control mechanism(s)  17   a , for example. 
     The fluid pressure may be increased during such a process, e.g. by increasing the rate at which fluid is delivered to the wellbore, to cause the sequential operation as described above. As such, some embodiments may include a pump for providing fluid to the wellbore  4  (i.e. providing the drilling fluid) and a control system for the pump which is configured to operate the pump to increase the rate at which fluid is provided by the pump to the wellbore  4 . The control system may operate to increase the rate of fluid delivery in a sequence of steps which correspond with the sequence of fluid pressures or differential pressures required to cause the aforementioned sequential operation in relation to the further apertures  1611 . The drill string stabiliser  1  may include one or more fluid pressure sensors which are in communication with the control system to provide feedback to the control system of a measured fluid pressure. The control system may use the feedback of the measured fluid pressure to control the operation of the pump to achieve the desired sequential operation. As will be appreciated, references to the pump providing fluid to the wellbore  4  are, in fact, references to the delivery of fluid to the drill string—that fluid typically flowing to or towards a remote end of the drill string where it flows out of the drill string and into the volume within the wellbore  4  around the drill string (and back towards a proximal end of the drill string). 
     The sequential operation may assist in the cleaning action in some situations. In some embodiments, only after the further flow control mechanism(s)  17   a  permit the flow of fluid through all or most of the further apertures  1611  will the flow of fluid through the other of the first or second aperture  161 / 162  be permitted (flow through one of the first and second apertures  161 / 162  already having been permitted)—by, of course, the flow control mechanism  17 . 
     In some embodiments, the or each cleaning passage  16   a  is not connected in fluid communication with a fluid relief passage  16  but, instead, forms a separate passage to a first or second aperture  161 / 162 . The first or second aperture  161 / 162  in such embodiments may not be in fluid communication with a second or first aperture  162 / 161  respectively. In other words, the cleaning passage  16   a  may be a direct passage from a first or second aperture  161 / 162  and that first or second aperture  161 / 162  may not also be connected with a fluid relief passage  16 . 
     In embodiments in which a plurality of further apertures  1611  is provided the further apertures  1611  may be staggered along a length of the stabilisation member portion  141 . In embodiments, the further apertures  1611  may be located at different positions around a circumference of a part of the stabilisation member portion  141 . In some embodiments, the further apertures  1611  flow a spiral or helical configuration between the stabilisation members  14 . 
     In use, the drill string stabiliser  1  may be secured to the drill string  2  and then inserted into a wellbore  4 . 
     The or each fluid relief passage  16  and/or the cleaning fluid passage  16   a  may be formed in the drill string stabiliser  1  at the time of manufacture or may be retrofitted thereto. 
     In embodiments, the or each stabilisation member  14  is welded to the main body  11 . As will be appreciated, in some embodiments, the drill string stabiliser  1  is formed from metal (e.g. steel) which is machined to the desired shape to form the or each stabilisation member  14 . 
     In some embodiments, the or each fluid relief passage  16  (and/or cleaning fluid passage  16   a ) may be part of the stabilisation member  14  and the first and/or second aperture  161 , 162  (and/or the further aperture  1611 ) maybe at least partially defined by the stabilisation member  14 . There is a desire, however, to position the first and second apertures  161 , 162  a greater distance apart along the length of the drill string stabiliser  1  (to improve the likelihood of both apertures  161 , 162  being clear of debris which is causing the packing off of the drill string  2 ). 
     Accordingly, in some embodiments, the or each fluid relief passage  16  (and/or the or each cleaning fluid passage  16   a ) may be drilled or cast such that it is at least partially defined by the main body  11 . In some such embodiments, the first and second apertures  161 , 162  (and/or the further aperture  1611 ) are both at least partially defined by the main body  11 . 
     In some embodiments, a method of retrofitting a drill string stabiliser  1  is provided in which dimensions of the drill string stabiliser  1  are obtained (either through measurement of the drill string stabiliser  1  or through use of recorded dimensions thereof). Entry points for a drill (or other machining tool) are located on an outer surface of the drill string stabiliser  1 —preferably two entry points which will become the first and second apertures  161 , 162  (a third entry point may become a further aperture  1611 ). The paths for intersecting first and second passage portions  163 , 164  are then determined, and the drill string stabiliser  1  is drilled (or otherwise machined) to form the fluid relief passage  16  through the drill string stabiliser  1 . Similarly, the drill string stabiliser  1  may be drilled (or otherwise machined) to form the cleaning fluid passage  16   a ). This may be repeated several times to create a plurality of such fluid relief passages  16  (and/or cleaning fluid passages  16   a ). As will be appreciated, the or each fluid relief passage  16  (and/or cleaning fluid passage  16   a ) may be cut using any suitable method or tool—including, for example, spark erosion and water cutting. 
     In embodiments, other arrangements of passage portions  163 , 164  are created to form passages for fluid communication between the two ends of the drill string stabiliser  1 . In some embodiments, a drilled or machined passage may be at least partially blocked with material (e.g. solder or welded material) to define the fluid relief passage  16  (or cleaning fluid passage  16   a ) in the desired manner. For example, a groove may be cut in at least a portion of the drill string stabiliser  1  and then part of the groove may be re-filled with material (e.g. some of the material which was removed) to form the fluid relief passage  16 . 
     In embodiments, a computer program (stored in a non-transitory computer readable medium  6 —see  FIG. 6 ) is provided which is configured to receive dimensions of a drill string stabiliser requiring retrofitting along with one or more machining tool constraints. The one or more machining tool constraints define one or more limitations of the capability of the machining tool—for example, a maximum drill depth, a range of drill angles, etc. The computer program is configured to determine one or more entry points (as set out above) and/or one or more passage portions to be machined to define one or more fluid relief passages  16  (and/or cleaning fluid passages  16   a ). The computer program may be further configured to communicate with a machining tool which is configured to perform the machining of the drill string stabiliser  1 . In some embodiments, the computer program is configured to communicate with a scanning device to determine the dimensions of the drill string stabiliser requiring retrofit and/or to register the location thereof for the machining tool. In some embodiments, the defining of the entry points is also at least partially based on one or more constraints imposed by the flow control mechanism  17  or further flow control mechanism  17   a —for example, the dimensions of a valve or cover of the flow control mechanism  17  or further flow control mechanism  17   a  which will need to be fitted to the first and/or second aperture  161 , 162  or further aperture  1611  at the entry points. 
     In some embodiments, a computer program (stored on a non-transitory computer readable medium  6 ) is provided which is configured to receive information about the intended use of the drill string stabiliser  1 —for example, one or more of: the depth of the wellbore  4  at which the drill string stabiliser  1  will be used, the material forming the ground through which the wellbore  4  is defined (e.g. allowing determination of the likely size and/or form of the debris which is likely to be present in the wellbore  4 ), the type of drilling fluid being used, the rate at which the drilling fluid will be pumped into the wellbore  4 , the fluid pressure of the drilling fluid at the expected location of the stabiliser  1  along the drill string  2 , and the components of the drill string  2 , the expected operating temperature at the stabiliser  1 . The computer program may be further configured to use the information about the intended use of the drill string stabiliser  1  to determine one or more attributes of the flow control mechanism  17  (or further flow control mechanism  17   a ) and/or the fluid relief passage  16  or passages  16  (and/or the cleaning fluid passage  16   a  or passages  16   a ) and/or any associated filtration member. For example, the computer program may be configured to determine an optimal dimension for the fluid relief passage  16  or cleaning fluid passage  16   a  (e.g. a diameter and/or length), the type of flow control mechanism  17  or further flow control mechanism  17   a  to use (e.g. the type of cover or valve), and/or the form of the filtration member (if provided). 
     Embodiments of the invention (such as depicted in  FIG. 6 ) include a computer  61  (or other computing device) configured to execute the above described computer program. Embodiments may also include the scanner  62  and/or the machining tool  63 . 
     Embodiments of the present invention include a bottom hole assembly  3  including one or more drill string stabilisers  1 . In addition, embodiments include a drill string  2  including at least one drill string stabiliser  1 . The drill string  2  may include the first drill string element  21 , and/or the second drill string element  22 , and/or the drill bit  23 , and/or the mechanism  24  to drive rotation of the drill bit  23 . Embodiments may include a rig  5  (see  FIG. 5 ) including at least one drill string stabiliser  1 . 
     In embodiments, the mechanism  24  to drive rotation of the drill bit  23  comprises a mud motor—a mechanism for converting the flow of drilling fluid into rotation of a member which is coupled to the drill bit  23 . In embodiments, the drive string  2  and/or drill bit  23  are rotationally driven from a drive mechanism which is located generally at a proximal end of the drill string  2  adjacent the wellbore  4  entrance (e.g. at or near the ground surface). This drive mechanism may include a top drive or rotary table, for example. Such a drive mechanism may be provided instead of or in addition to the mechanism  24 . 
     In the above description it will be appreciated the fluid relief passage  16  and cleaning fluid passage  16   a  are configured to allow the passage of drilling fluid therethrough. This drilling fluid will, as it passes between the drill string  2  and the wall of the wellbore  4 , typically include cutting debris and may include debris from the wall of the wellbore  4 . The drilling fluid will also include one or more additives which are present in the drilling fluid which passes through the drill string  2  and will also be present in the drilling fluid as it passes back through between the drill string  2  and the wall of the wellbore  4 . The drilling fluid may, therefore, include dissolved and suspended particulate matter as well as stones and pieces of rock and other material. The term ‘fluid’ is to be interpreted accordingly and is typically a reference to a liquid carrying one or more additives and/or debris. 
     The or each fluid relief passage  16  and cleaning fluid passage  16   a  does not provide a fluid flow path for fluid within the drill string  2  (fluid which is travelling through the string  2  to the mechanism  24 , for example) to escape into the wellbore. 
     Embodiments of the invention seek to provide a drill string stabiliser  1  in which the flow of fluid through a fluid relief passage  16  in the stabiliser  1  reduces (or substantially eliminates) the hydraulic locking effect in a packing off event and/or clears debris with a view to preventing the occurrence of a packing off event. The fluid relief passage  16  seeks to allow the flow of fluid through the stabiliser  1  from a region of high pressure to a region of low pressure to achieve such an effect. The cleaning fluid passage  16   a  allows the flow of fluid across a portion of the drill string stabiliser  1 , that portion being the likely location of accumulated debris which may cause a packing off event. Thus, the portion of the drill string stabiliser  1  may be cleaned in the initial stages of a packing off event and this may prevent those initial stages of a packing off event maturing into an actual packing off event. 
     In addition, embodiments of the invention seek to provide a drill string stabiliser  1  through which it is possible to achieve a circulation of fluid in a pack off event. Accordingly, even if the use of the drill sting stabiliser  1  of an embodiment of the invention does not allow the drill string  2  to be freed, the or each fluid relief passage  16  may allow for the circulation of drilling fluid within the wellbore  4  (i.e. from the proximal end of the drill string  2  down the drill string  2  and back up toward the proximal end). This circulation of fluid allows new forms of fluid to be circulated past (or even to) the packed off part of the wellbore  4 . The new form of fluid may be drilling fluid which includes one or more components which are intended to help to free the drill string  2 . For example, the new form of fluid may include an acid or other remedial fluid. Such remedial fluid may, in any event, be circulated even if the drill string  2  is freed by the operation of the invention. 
     In embodiments, the drill string stabiliser  1  is configured for use in vertical and/or horizontal wellbores  4 . In embodiments, the drill string stabiliser  1  is configured for use in wellbores  4  which are inclined with respect to a generally vertical axis. 
     Some embodiments of the present invention are configured for use in relation to components of a coiled tubing system. 
     Embodiments of the present invention have been described with reference to a drill drilling element which comprises a drill string stabiliser  1 . It will be appreciated that similar packing off may occur in relation to other drill string elements. As such, the above description also applies to other drill string elements which may or may not include one or more stabilisation members  14 . In other words, the fluid relief passage  16  and the associated parts (apertures, flow control mechanism, filtration members, etc) may be implemented in relation to another element of the drill string  2  other than a drill string stabiliser  1 . Equally, the cleaning fluid passage  16   a  and associated parts (further aperture, further flow control mechanism, filtration members, etc) may be implemented in relation to another element of the drill string  2  other than a drill string stabiliser  1 —preferably with the or each further aperture  1611  located near or adjacent a part of that other element of the drill string  2  at which an accumulation of debris is likely to occur (with a risk of packing off as a result of that accumulation of debris). 
     Embodiments of the invention relate to a drill string for use in drilling a wellbore  4  of an oil or gas well. As will be appreciated, such a wellbore  4  is just one example of a borehole through a ground material (e.g. earth, rock, concrete) which may be created by such a drill string. Accordingly, embodiments include a drill string element (such as a stabiliser) which may be suitable for use in a drill string for drilling a borehole (such as a wellbore  4 ). The borehole may be part of an oil or gas exploration or extraction system, for example, or could be used for the extraction of water. Equally, the borehole may be used to deliver water underground (e.g. for heating and/or storage). The above described embodiments of a drill string element (such as a stabiliser  1 ) apply equally to a drill string element for use in a borehole for all applications. 
     The drill bit  23  could take many different forms—each a conventional mechanical drill bit  23  or a bit for hydrothermal spallation. 
     When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. 
     The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.