Cutting bed impeller

A cutting bed impeller comprises a body portion and a plurality of paddles projecting outwardly from the body portion, at least one of the paddles comprising a radially outer face extending between a leading edge and a trailing edge in a normal direction of rotation of the impeller about an axis of rotation of the cutting bed impeller, a leading face, and a trailing face. The radially outer face extends between the leading face and the trailing face. The leading face is inclined at a first angle .alpha. to a first tangent T.sub.1 to the radially outer face at the leading edge. The trailing face is inclined at a second angle .beta. to a second tangent T.sub.2 to the radially outer face at the trailing edge, the second angle .beta. being less than the first angle. The radially outer face of one or more of the paddles may be provided with one or more replaceable wear elements.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to equipment for drilling operations.
 2. Description of the Prior Art
 In conventional drilling operations, mud or other drilling fluid is pumped
 down a hollow bore in the drill string and is ejected from the drill bit
 to lift the drill cuttings out of the bore-hole.
 In an inclined well-bore it is been found that at a certain deviation or
 sail angle, some of the drill cuttings being transported back to the
 surface by the drilling fluid fall out of the main flow and settle on the
 lower portion of the bore-hole. These cuttings interfere with the drilling
 process and especially with the rotation of the rotating drill-pipe which
 also lies on the low side of the bore-hole.
 The flow of returning drilling fluid which carries the cuttings is not
 uniform across the diameter of the bore-hole. On the low side of the
 bore-hole the flow is at a minimum and the capacity of the drilling fluid
 to transport drilling cuttings and solid particles is reduced.
 To overcome this problem it is known to fit one or more cutting bed
 impellers to the drill-pipe. The impellers are integrally formed with a
 length of drill-pipe and comprise a body portion having a central
 longitudinal bore and a plurality of paddles in the form of single spiral
 blades which project radially outwardly from the body portion. These types
 of blade are similar in profile to those used on down-hole drilling
 stabilizers.
 As the cutting bed impeller rotates with the drill-pipe, it disturbs and
 agitates the settled cuttings and other particles and moves them upwards
 into the path of the main flow of cutting fluid on the upper side of the
 bore-hole. Although these tools have proved reasonably effective they have
 been found to create extra down-hole torque.
 SUMMARY OF THE INVENTION
 According to the present invention there is provided a cutting bed impeller
 comprising a body portion and a plurality of paddles projecting outwardly
 from the body portion, at least one of the paddles comprising:
 a radially outer face extending between a leading edge and a trailing edge
 in a normal direction of rotation of the impeller about an axis of
 rotation of the impeller;
 a leading face; and
 a trailing face, the radially outer face extending between the leading face
 and the trailing face, the leading face being inclined at a first angle to
 a first tangent to the radially outer face at the leading edge, the
 trailing face being inclined at a second angle to a second tangent to the
 radially outer face at the trailing edge, the second angle being less than
 the first angle.
 Each paddle is preferably substantially straight.
 A recess comprising a depression may be formed in the leading face of the
 paddle. Each recess is preferably from 1/4" to 2" (6.4 to 51 mm) deep.
 Preferably the cutting bed impeller comprises part of a drill string sub,
 rather than a complete length of drill-pipe. Preferably the wall thickness
 of the sub is reduced on one or both sides of the cutting bed impeller.
 This reduced thickness portion accommodates bending due to high side
 forces which may be generated on the sub.
 The provision of the cutting bed impeller on a sub allows the tool to be
 run in conjunction with or immediately between bearing devices or torque
 reduction tools. This is not possible with a conventional cutting bed
 impeller, which is integrally formed with a length of drill-pipe.
 The radially outer face of the paddles may be provided with replaceable
 wear elements. These wear elements may comprise nylon inserts fitted into
 openings in the radially outer faces of the paddles. The nylon inserts may
 be cylindrical and may fit within blind bores in the paddles. In another
 embodiment, the wear elements comprise wear pads which fit within slots
 formed through the paddles. Alternatively, the replaceable wear elements
 may comprise any appropriate shape or size of element of elements which
 may be used to protect the cutting bed impeller from abrasion with the
 wall of the bore-hole and/or which reduce the down-hole torque.
 Preferably the wear elements comprise approximately 60% of the total area
 of the radially outer face of the or each paddle.
 The recesses defined between respective paddles act as scoops to lift
 cuttings and solid particles from the lower portion of the bore-hole into
 the main flow of cutting fluid in the upper portion of the bore-hole. The
 effectiveness of the impeller is governed by the size of the clearance
 between the radially outer faces of the paddles and the bore-hole wall and
 the profile of the recess.
 According to another aspect of the present invention there is provided a
 cutting bed impeller comprising a body portion and a plurality of paddles
 projecting from the body portion, replaceable wear elements being provided
 on the radially outer faces of one or more of the paddles.
 Preferably, replaceable wear elements are disposed equidistantly around the
 circumference of the impeller to ensure an even bearing in the bore-hole.
 For example, they may be provided on oppositely disposed pairs of paddles.
 A plurality of cutting bed impellers may be fitted to a drill string. The
 cutting bed impellers are preferably spaced apart at 90 m to 150 m (300
 ft. to 500 ft.) intervals.
 The invention consists of certain novel features and a combination of parts
 hereinafter fully described, illustrated in the accompanying drawings, and
 particularly pointed out in the appended claims, it being understood that
 various changes in the details may be made without departing from the
 spirit, or sacrificing any of the advantages of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIGS. 1 and 2, a drill string sub 2 comprising a hollow
 cylindrical sleeve 4 has a male connector or pin 6 at one end and a female
 connector or box 8 at the other end. A cutting bed impeller 10 is
 integrally formed with the sleeve 4 at an intermediate point along its
 length. On either side of the cutting bed impeller 10, the sleeve 4 has a
 reduced external diameter which provides bending zones 12, 14 which enable
 the drill string sub to accommodate the reverse bending forces which are
 generated as the sub 2 rotates in the curve of a deviated bore-hole.
 As best shown in FIG. 1, the cutting bed impeller 10 comprises a
 substantially cylindrical body portion into which are machined five
 substantially straight grooves 18 which extend parallel to the rotational
 axis CL of the drill string sub.
 Respective pairs of adjacent grooves 18 define respective straight paddles
 24 which also extend parallel to the rotational axis CL and project
 substantially outwardly. Each paddle 24 has a radially outer face 26 (seen
 best in FIGS. 2 and 4) which is received with some play in the bore-hole.
 Referring to FIGS. 3 and 4, each paddle 24 is further bounded by a
 respective leading face 23 and a respective trailing face 25, the terms
 "leading" and "trailing" being defined with respect to the normal
 direction of rotation, depicted by arrow R in FIGS. 3 and 4, of the
 cutting bed impeller about the axis CL. The line of intersection between
 the leading face 23 and the radially outer face 26 of each paddle 24
 defines a respective leading edge 27 and the line of intersection between
 the trailing face 25 and the radially outer face 26 of each paddle 24
 defines a respective trailing edge 29.
 In an illustrated embodiment, all of the paddles 24 are identical and each
 is configured as follows. The leading face is inclined at a first angle
 .alpha. to a first tangent T.sub.1, to the radially outer face 26 at the
 leading edge 27, and the trailing face 25 is inclined at a second angle
 .beta. to a second tangent T.sub.2, to the radially outer face 26 at the
 trailing edge 29. As best shown in FIG. 4, the second angle .beta. is less
 than the first angle .alpha..
 It will be appreciated that respective grooves 18 are defined by a trailing
 face 25 of a first paddle 24 and a leading face of the next paddle 24 and
 act as the "buckets" or "scoops" of the cutting bed impeller 10.
 The angle .alpha. is preferably between about 60 and about 120.degree. and
 is most preferably greater than or equal to about 70.degree. to provide an
 enhanced "scooping" action when the cutting bed impeller 10 rotates in the
 direction R. The angle .beta. is always less than the angle .alpha., and
 is preferably less than about 90.degree. and is most preferably between
 about 20 and about 40.degree..
 In the illustrated embodiment, which provides a particularly effective
 "scooping" action, the angle .alpha. is approximately 80.degree., so that
 a plane P.sub.1 containing the leading face 23 does not intersect the axis
 of rotation CL, but rather is offset from it and forms an angle of
 approximately 10.degree. with a plane P.sub.2 extending between the
 leading edge 27 of the respective paddle 24 and the axis of the rotation
 CL. The offset (d), as measured along a radial line perpendicular to
 P.sub.1 is preferably less than or equal to about 2".
 Referring to FIG. 4, the height H.sub.1 of each paddle 24, as measured from
 the first tangent T.sub.1 to the portion of groove 18 closest to the axis
 CL, is preferably one quarter to one ninth of the radius H.sub.2 of the
 cutting bed impeller. A particularly effective compromise is achieved when
 H.sub.1 is about one sixth of H.sub.2. At this size ratio, the cutting bed
 impeller 10 provides an effective scooping action, but is unlikely to
 become jammed by large pieces of debris which might be entrained in the
 fluid flow around the impeller.
 FIG. 5 shows another embodiment of paddle 24 which is provided with a
 plurality of blind bores 28. Respective cylindrical replaceable wear
 elements 30 are located in each bore 28, such that they project slightly
 from the radially outer face 26 of the paddles 24. The wear elements 30
 provide a replaceable bearing surface which protects the cutting bed
 impeller 10 from abrasion against the wall of the bore-hole and reduces
 down-hole torque.
 FIG. 6 shows another embodiment of the paddle 24 which is provided with cut
 outs 32 in which are located square replaceable wear elements 34.
 FIG. 7 shows a final embodiment of paddle 24 in which are machined
 transverse slots 36. Rectangular replaceable wear elements 38 are located
 in the slots and are held in place by adhesive, by screws or by any other
 appropriate fixing means.
 The replaceable wear elements 30, 34, 38 are preferably made of nylon, but
 any other appropriate material may be used and any shape of wear element
 is contemplated.
 In use, the drill string sub 2 is fitted to a drill pipe and is rotated in
 the direction R indicated by arrow R in FIGS. 3 and 4, as drilling
 proceeds. Drilling fluid is pumped down the hollow interior of the
 drill-pipe and is ejected at the drill bit to force cuttings and other
 solid particles up and out of the bore-hole.
 As the cutting bed impeller 10 rotates, cuttings and other solid particles
 lying on the lower portion of the bore-hole are caught against the leading
 faces 23 of the paddles 24 and are scooped upwards into the main flow of
 drilling fluid where they become entrained in the flow. The zones 12, 14
 act as clearance areas for the turbulence created by the paddles 24 in
 lifting the debris to the high side of the hole. Consequently, the
 bore-hole is kept cleaner with less debris accumulating on the low side of
 the hole, so that there is less sliding friction when picking up or
 lowering the drill string.
 The asymmetrical cross-section of the recesses 18 defined by respective
 pairs of adjacent paddles 24 results in a very efficient blade profile
 which enhances the scooping/pumping action of the impeller.
 In the illustrated embodiment, the recesses 18 which scoop up the cuttings
 and other solid particles are defined between the trailing face 25 of a
 first paddle 24 and the leading face 23 of the next paddle 24. However, it
 is contemplated that these recesses could be extended by depressions
 formed in the leading face 23 of each paddle 24, thereby enhancing the
 scooping action.
 If the paddles 24 are provided with replaceable wear elements, which can be
 renewed periodically, the service life of the cutting bed impeller 10 is
 greatly increased. Furthermore, the replaceable wear elements reduce the
 drag on the walls of the bore-hole, thereby reducing the down-hole torque.
 If the cutting bed impeller 10 is carried on a short drill-string sub it
 can be run in conjunction with or immediately between bearing devices or
 torque reduction tools, so that a further reduction in down-hole torque is
 possible. It is however contemplated that the cutting bed impeller 10
 could also be formed on a length of drill-pipe, particularly as the
 provision of replaceable wear elements provides a torque reduction
 function.
 Though in all the above-illustrated embodiments the paddles of each
 impeller are identical to one another, it is contemplated that only some
 of the paddles of an impeller may provide a scooping action and/or be
 provided with replaceable wear elements.
 While particular embodiments of the present invention have been shown and
 described, it will be appreciated by those skilled in the art that changes
 and modifications may be made without departing from the invention in its
 broader aspects. Therefore, the aim in the appended claims is to cover all
 such changes and modifications as fall within the true spirit and scope of
 the invention. The matter set forth in the foregoing description and
 accompanying drawings is offered by way of illustration only and not as a
 limitation. The actual scope of the invention is intended to be defined in
 the following claims when viewed in their proper perspective based on the
 prior art.