Diamond Bit Unit

Abstract

A diamond drill bit unit including a diamond bit, drill collar means operatively associated with said bit, and vibration reducing stabilizing means provided on said drill collar means.

Description

The invention relates to drilling apparatus, and more particularly, to a diamond bit unit comprising a diamond bit, a drill collar and stabilizing means.

Diamond bits are in particular applied when drilling holes through rock material of poor drillability which is often required when drilling in subsurface formations for searching and/or recovering oil or other valuable products.

These bits are of the rotary type and the diamonds thereof exert a scraping action on the rock through which a hole is being drilled, while the bit is loaded in an axial direction and rotated around its central axis.

Bits of this type are especially useful for drilling through deep abrasive hard formations, since their life time when drilling through such formations is greater than with any other type of rotary rock bit such as roller bit. Consequently a diamond bit has to be replaced less frequently than a bit of another type. The higher material and manufacturing costs of a diamond bit with respect to other types of bits are easily compensated by the advantages obtained as a result of the reduction in time required to replace the worn bits during drilling operations. Known diamond bit unit designs, however, are subject to vibrations during drilling with possible resulting damage.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a diamond bit unit which will be less liable to be damaged by vibrations during drilling than known diamond bit units. As a consequence thereof the life time of the diamond bits and stabilizing means used in the diamond bit unit according to the invention will be much longer than that of known diamond bit units. The life time of the diamond bits in the new diamond bit unit will further also be longer than the life time of the diamond bits used in combination with a drill collar only.

According to the invention, the stabilizing means as used in the diamond bit unit comprises stabilizing elements mounted on the collar, these stabilizing elements having outer surfaces which are conically arranged with respect to the central axis of the diamond bit unit. The surfaces lie on cones which are arranged with the tops thereof in the direction in which the bit is axially loaded during operation. Diamonds touching the conical surfaces are set in the stabilizing elements, each diamond being set in a cutting position in a direction tangential to a helical line passing through the location of the diamond and having an axis coinciding with the central axis of the diamond bit unit, this tangential direction being taken at the location of the relevant diamond. Further each diamond is set in a blunt position in a direction radial to the central axis of the diamond bit unit. The greatest cutting diameter of the diamonds set in a conical surface directly adjacent to the diamond bit is at least greater than the greatest cutting diameter of the diamonds set in the diamond bit.

The smallest cutting diameter of the diamonds of the stabilizing means may be equal to the largest cutting diameter of the diamonds of the diamond bit.

The conical surfaces may have a top angle between 10' and 40'.

The conical surfaces may be placed in groups, which groups are distributed over the length of the drill collar.

The smallest cutting diameter of the diamonds set in a first conical surface not directly adjacent to the diamond bit may be equal to the largest cutting diameter of the diamonds set in a second conical surface which is situated between the first conical surface and the diamond bit.

The stabilizing elements having surfaces which are conically arranged with respect to the central axis of the diamond bit unit may be of wear-resisting material, this material enclosing the diamonds.

The drill collar may have an outer diameter which is at least smaller than the smallest cutting diameter of the diamonds of the stabilizing means and at least one channel substantially extending in longitudinal direction may be arranged between each pair of stabilizing elements having the outer surfaces thereof lying on a common cone.

At least one stabilizing element may comprise further diamonds which are set in the stabilizing element in a blunt position in directions tangential to as well as radial to the central axis of the diamond bit unit, these diamonds touching outer surfaces of the body element which border the conical surface thereof at at least one side and lie within the cone on which the conical surface is located.

The invention further relates to a drill collar suitable for use in rotary drilling operations in subsurface formations by means of diamond bits. According to the invention, the drill collar has mounted thereon stabilizing elements having outer surfaces which are arranged on at least two noncoinciding conical planes having their central axes coinciding with the central axis of the drill collar, diamonds being set in the stabilizing elements, these diamonds touching the conical surfaces. The largest cutting diameter of the diamonds lying in a first conical plane is greater than the largest cutting diameter of the diamonds lying in a second conical plane. The second conical plane envelopes the first conical plane. Each diamond is set in a cutting position in a direction tangential to a helical line passing through the location of the diamond and having the central axis thereof coinciding with the central axis of the drill collar, the tangential direction being taken at the location of the relevant diamond. Further, each diamond is set in a blunt position in a direction radial to the central axis of the drill collar.

Other objects, purposes and characteristic features of the present invention will be obvious from the accompanying drawing and from the following description of the invention. In describing the invention in detail, reference will be made to the drawing in which like reference numerals designate corresponding parts throughout several laws in which:

FIG. 1 is a longitudinal sectional view of a diamond bit unit comprising a diamond bit, a drill collar and stabilizing means;

FIG. 2 shows a cross section over the drill collar and part of the stabilizing means of the example according to FIG. 1, this cross section being taken in the direction as indicated by arrows 2-2;

FIG. 3 is a cross-sectional view illustrating a detail of FIG. 1, but on a larger scale than FIG. 1;

FIG. 3A is a longitudinal sectional view of an alternative construction of the stabilizing element as shown in FIG. 3;

FIG. 4 is a side view of part of the stabilizing means and the drill collar in the direction as indicated by arrow 4 in FIG. 3;

FIG. 5 is a cross-sectional view of part of the stabilizing means and the drill collar as indicated by section 5-5 in FIG. 4;

FIG. 6 is an enlarged view of the diamond which is located in the intersection of section 3-3 and section 5-5 in FIG. 4;

FIG. 7 shows a cross section over the diamond shown in FIG. 6 in the direction of the arrows 7-7;

FIG. 8 shows a cross section of the diamond shown in FIG. 6 in the direction of the arrows 8-8;

FIG. 9 shows a longitudinal section of part of a stabilizing means and a drill collar, wherein the stabilizing means is of a design different from the design of the stabilizing means in FIG. 3;

FIG. 10 shows a side view of part of the stabilizing means and of the drill collar according to FIG. 9, taken in the direction of arrow 10;

FIG. 11 shows a cross section of part of the stabilizing means and of the drill collar according to FIG. 10, as indicated by section 11-11 in FIG. 10; and

FIG. 12 shows a cross section over a drill collar and stabilizing means according to the invention, wherein the stabilizing means comprises groups consisting of three stabilizing elements.

The diamond bit unit as shown in FIG. 1 comprises a diamond bit 1 of the known type, which consists of a body 2 carrying diamonds 3 near the outer surface thereof and a stem 4 provided with a screw thread. The unit further comprises a drill collar 5 which is a tubular member with an extremely thick wall 6 defining a central passage 7. The drill collar 5 is provided at one end with an inner screw thread 8 matching the screw thread of the stem 4 of the diamond bit, and at the other end with an outer screw thread cut on the pin 9. The central passage 7 communicates with the central passage 10 through the bit 1.

Drill collars are usually utilized to load the bit during operation. To this end a number of known drill collars is screwed together (only one of which being shown as indicated by reference numeral 11) and the string thus obtained is at one end screwed to the upper end of the drill collar 5 and at the other end to the lower end of a drill string (not shown).

The drill collar 5 as shown in FIG. 1 of the drawing is provided with stabilizing means comprising stabilizing elements 12.

The relative position of the stabilizing elements 12 in a radial sense with respect to the central axis of the drill collar 5 can best be seen in FIG. 2. The stabilizing elements 12 are mounted in grooves present in side extensions 13 of the drill collar wall 6.

As can be seen from FIG. 3 which shows an enlargement of detail A of FIG. 1, the body element 12 has an outer surface 14, and diamonds 15 are set in the body element 12 in such a manner that they touch this outer surface 14. This outer surface is conically arranged with respect to the central axis of the diamond bit unit, which axis is at the same time the central axis of the drill collar 5 as well as the central axis of the diamond bit 1, since the screw threads connecting the drill collar 5 and the bit 1 have been manufactured with great precision.

The stabilizing element 12 consists of any suitable wear-resisting material, and is mounted by suitable means in the groove 16 cut in the side extensions of the wall 6 of the drill collar 5.

The cones not shown of which the conical surfaces 14 form part are arranged with the tops thereof downward in the position of the diamond bit unit as shown in the drawing. Thus the top of the cones point in the same direction as the direction in which the bit 1 is axially loaded during operation.

It will be understood that the surface 14 of the stabilizing element 12 is in the cross section according to FIG. 3 indicated by a straight line which runs under an angle different from zero with the central axis of the drill collar 5. This surface 14 is indicated in the cross section according to FIG. 5 by part of a circle, this circle having its center in the central axis of the drill collar 5 and in the plane in which section 5-5 has been taken.

The exact position of the diamonds in the stabilizing element 12 will now be described in the reference to the FIGS. 6, 7 and 8, which show a diamond 17 which corresponds to the diamond in FIG. 4, which is located in the intersection of the sectional plans 3-3 and 5-5.

FIG. 6 is an enlargement of this diamond in the situation as shown in FIG. 4, and FIGS. 7 and 8 are cross sections thereof, respectively, along sectional planes 7-7 and 8-8 in FIG. 6.

From FIGS. 6, 7 and 8 it can be seen that the diamond, which is cube-shaped, is located with a flat plane thereof in the conical surface 14 (vide FIGS. 7 and 8). This means that the diamond is set in a blunt position in a direction radial to the central axis of the diamond bit unit. Thus, the diamond cannot exert any cutting action as a result of load on the diamond 14, exerted in a direction passing through this central axis and in a plane perpendicular thereto.

It can further be seen that in the position as shown in FIG. 6, the lower plane of the diamond 17 makes an angle B with the horizontal line 18. Consequently, on movement of the diamond 17 in the direction of arrow 19 (being the movement of rotation of the diamond bit unit) the tip 20 of the diamond 17 is in a cutting position, when load is exerted thereon in a direction axial to the central axis of the unit.

The angle between the conical surface 14 of the stabilizing element 12 and the central axis of the diamond bit unit is indicated by C in FIG. 7. Two times the value of C equals the top angle of the cone of which the surface 14 forms part.

The relationship between the cutting diameters of the diamonds of the diamond bit 1 and the two groups of stabilizing members which are axially distributed along the drilling collar 5 will now be described.

Under the expression "cutting diameter" of a diamond there is to be understood the diameter of a circle in a plane perpendicular to the central axis of the diamond bit unit (or drill collar) in which the relevant diamond is located and along which circle the cutting point of the diamond travels during rotation of the unit (or the drill collar) around its central axis.

In the unit as shown in FIG. 1, the smallest cutting diameter of the diamonds in the stabilizing elements 12 directly adjacent to the diamond bit 1 is equal to the greatest cutting diameter of the diamonds in the diamond bit 1. Further, the smallest cutting diameter of the diamonds in the group of stabilizing elements 13, which group is located above the first-mentioned group of stabilizing elements is equal to the greatest cutting diameter of the diamonds in the stabilizing elements 12 of this first-mentioned group.

It will be clear from the above that the hole which is being drilled by the diamond bit, will be enlarged in diameter by the first group of stabilizing elements, and be further enlarged by the second group of stabilizing elements. Since the diamonds which are present in the stabilizing elements are set in a blunt position in a direction radial to the central axis of the diamond bit unit, a hole results from this drilling, where in all the stabilizing elements of both groups of the stabilizing means are over the total area of the outer surface thereof in contact with the wall of the borehole. All loads exerted by the drill string and the drill collar string in directions other than the axis of the diamond bit unit will be taken up in the contact planes between the conical surfaces of the stabilizing elements and the conical wall portions of the hole which is being drilled, without any cutting action occurring in a direction radial with respect to the central axis of the diamond bit unit. Thus, the contact between all the outer surfaces of the stabilizing elements and the relevant portions of the wall of the borehole will be maintained under all circumstances. This means that any nonaxial load and any lateral vibrational action which would be exerted on the diamond bit by the action of the drill string if no stabilizing means according to the invention were applied, will now be taken up by the stabilizing elements which are in firm contact with the borehole wall, and will not reach the diamond bit 1, which is thus freed from undesired loads, that are loads which are exerted in a direction other than the direction of the central axis of the diamond bit. The lateral forces exerted on the diamond bit by the cutting action of the diamonds will be taken up by the contact surfaces between the stabilizing elements and the relevant portions of the borehole wall. Consequently, the lateral load on the bit will be of insufficient magnitude to create an oversize hole, which prevents the occurrence of vibrational movements of the bit which would otherwise shorten the life of the diamonds thereof.

It will be understood that the above advantages cannot be obtained by the application of drill collars provided with stabilizers or reamers having cutting elements, such as diamonds, which can carry out cutting actions when under influence of nonaxial loads. These cutting elements will enlarge the diameter of the hole which is being drilled to a value which is greater than the cutting diameter of the stabilizer or reamer, which means that the contact between the stabilizer or reamer and the wall of the borehole is only local. The diamond bit unit can then move in lateral directions in the hole which is being drilled, as a result of which nonaxial loads and vibrations are passed to the diamond bit. These extra loads on the bit over the axial load required for cutting the hole, are detrimental for the diamonds, which will be overloaded and either be broken out of the matrix of the bit body or burn.

It will be further appreciated that the efficiency of the diamond bit unit according to FIGS. 1--7 can still further be improved by placing or setting the diamonds 3 in the sidewall of the diamond bit 1 in the same manner as has been described with reference to the diamonds set in the stabilizing elements 12. In the same way as has hereinbefore described with reference to FIGS. 6, 7 and 8, the sidewall of the diamond bit 1 may be provided with a conical surface, in which each diamond is set in a cutting position in a direction tangential to a helical line passing through the location of the diamond and having an axis coinciding with the central axis of the diamond bit. This tangential direction is taken at the location of the relevant diamond (compare FIG. 6). Each diamond further touches the conical plane in which the conical surface of the sidewall of the bit is located (compare FIGS. 7 and 8) and is set in a blunt position in a direction radial to the central axis of the diamond bit (compare FIG. 8).

It will further be appreciated that the form of the sidewall of the drill bit is not restricted to a cone. With equal favorable results, this sidewall may be formed by a rotational symmetric plane obtained by rotating a slightly curved line around the central axis of the bit. However, the sidewall has to have a positive curvature, and the diameter of the bit body in cross section perpendicular to the central axis of the bit has to increase in upward direction. To prevent heavy axial loads on the diamonds in the sidewall, the projection of this sidewall on the bottom of the hole should be relatively small. This means that the top angles of the cones which can be arranged tangentially to this sidewall should be small, and be in the order of 10'-40'.

What has been discussed above with regard to the form of the sidewall of the diamond bit, also applies to the form of the outer surfaces of the stabilizing elements 12. However, no particular advantages are obtained by deviating the outer surfaces of these elements 12 from a conical plane. For reasons of protection, however, the definition of cones in which the conical surfaces of the body elements 12 and the sidewall of the diamond bit may be located, includes rotational symmetric planes which can be placed in tangential relationship with the said conical planes.

For purposes of illustrating the above, the stabilizing element 12 as shown in FIG. 3, which element has a conical outer surface 14, has been shown in FIG. 3A with an outer surface 14' which is slightly curved in longitudinal section. The rotational symmetric plane on which the surface 14' is located, has a tangential conical conical plane 14". The top angle of this plane (and any other conical planes which can be tangentially arranged with respect to the surface 14') is between 10' and 40'.

FIGS. 9, 10 and 11 show sections over a stabilizing element for use in the diamond bit unit according to FIG. 1, and of a design different from the design of the stabilizing element as shown in FIGS. 3, 4 and 5.

The outer surface 21 of the stabilizer element 22 is divided into two parts which parts are bounded by the lines D-E and E-F (vide FIG. 10). The part of the outer surface 21 having a rectangular shape lies on a conical plane, of which the characteristics correspond to those of the conical plane as described with reference to FIG. 3. It will be understood that the angle H as indicated is equal to half the value of the top angle of the conical plane. The L-shaped part of the outer surface 21, however, is not located on this conical plane but is situated within this cone. This part can consequently be regarded as a guiding plane of the stabilizing member 22. The part J of this guiding plane borders the conical plane at a level where the diameter is equal to the minimum cutting diameter of the stabilizing member 21, and is of conical nature and located with respect to the first conical plane under an angle K.

The other part of the guiding plane is indicated by the reference M in FIG. 11. This part is also located within the conical plane on which the surface 21 of the stabilizing member 22 is located.

The diamonds as set in the parts J and M of the guiding plane are set blunt in axial as well as in radial direction with respect to the central axis of the diamond bit unit.

It will be appreciated that the invention is not limited to the examples as have been described hereinabove with reference to the drawing, but that various modifications may be introduced in these examples.

Some of such modifications will be mentioned hereinbelow.

Although the stabilizing members have been shown to have outer surfaces of rectangular nature, it will be appreciated that there are many other forms of outer surfaces which will be suitable for the purpose.

The invention is not limited to the number of diamonds as shown in the examples.

Further the number of stabilizing members is not limited to eight, as shown in FIGS. 1 and 2. If desired, a single group of stabilizing members consisting of four of these members may be applied. Further, a group of three of these members has been illustrated by way of example in FIG. 12, which shows a cross section of a drill collar at the level where the stabilizing members are mounted. The drill collar 23 comprises a wall portion 24 enclosing a central passageway 25, and is locally provided with three side extensions 26 on each of which a stabilizing member 27 is mounted in a groove 28. Each member 27 consists of a wear-resisting body in which diamonds 29 are set in the manner as described hereinabove with reference to FIGS. 6, 7 and 8. The cutting diameter of the diamonds as located in the cross section shown in FIG. 12 is indicated by the reference letter P. The wall of the hole which will be cut by the diamonds as shown in the cross section according to FIG. 12 is indicated by the broken line 30. As can be seen from FIG. 12 channels 31 exist between the side extensions 26 of the drill collar 23, which channels form together with the central passageway 25 in the drill collar 23 a way via which drilling fluid can be circulated along the surfaces of the bit and the bottom of the hole which is being drilled.

The shape of the side extension 13 as shown in FIG. 4 is mostly but not necessarily adapted to the shape of the stabilizing member 12. The most convenient manner of mounting the stabilizing members on the drill collar is by soldering these members in grooves cut in the side extensions. The members may be manufactured of tungsten carbide powder or grains. The diamonds are placed in the required positions at the required locations in graphite moulds of the elements, whereafter the remainder of the space within the mould is filled with the tungsten carbide grains. A desired amount of binder material is then placed on the grains to be sintered and the whole is subjected to a required sintering temperature.

The top angle of the cones on which the outer surfaces of the stabilizing members are located may be between 10' and 40'. If two or more groups of stabilizing members are used, which groups are axially distributed along the length of the drill collar, the cones on which the outer surfaces of the stabilizing members are arranged may have top angles which are of equal value.

Although in the examples as described above, the minimum cutting diameter of the group of stabilizing members arranged just above the diamond bit is chosen equal to the largest cutting diameter of the bit, it will be appreciated that this minimum cutting diameter may also be chosen smaller, or even slightly larger than the largest cutting diameter of the bit. The first construction operates in the same manner as described, whereas the second construction creates a borehole having a stepped diameter.

As shown in FIGS. 6, 7 and 8, the blunt position of a diamond with respect to loads exerted in a direction radial and perpendicular to the central axis of the diamond bit unit is obtained by placing a substantially flat plane of the diamond in the conical plane. However, this blunt position may also be obtained by placing the diamond in a "dragging" position which means that the point of contact between the diamond and the conical plane is the trailing part of the diamond when looking in the direction of movement of the diamond along the wall of the borehole. This means that the sense of rotation of the diamond bit unit has to be taken into account when placing the diamonds. This sense can be derived from the type of screw thread used. Normally, a right-handedly wound screw thread is applied on the bit, the drill collars and the drill pipes which means that the diamond bit unit is rotated clockwise when looking into the hole (vide arrow 19 in FIG. 6). The angle R in FIG. 8 indicates the angle between the diamond plane and the conical plane, under which the diamond can be "dragged" along the wall of the borehole, without cutting in radial direction.

The diamonds as shown in the stabilizing elements 12 (FIG. 4) and 21 (FIG. 10) need not be placed such that all the diamonds belonging to a horizontal group are located at the same level. If desired these diamonds may be arranged at different levels.

Although the diamonds are shown in the drawing to which the above examples refer are all cube-shaped, it will be appreciated that also diamonds of other shape can be used, provided that they are set in the position that they are cutting a direction tangential to a helical line passing through the location of the diamond and having an axis coinciding with the central axis of the diamond bit unit, this tangential direction being taken at the location of the relevant diamond, and that they are blunt in a direction radial to the central axis of the diamond bit.

Further, the guide planes which are arranged in the stabilizing element 22 in FIG. 10 at two sides of the conical surface thereof, may, if desired be extended to the other sides thereof as well. The advantage of a stabilizing member provided with a conical surface which at all sides thereof is provided with guide planes, lies in the fact that the movement of the drill string will not be hampered in any direction.

If desired stabilizers and/or reamers of known design may be combined with the diamond bit unit according to the present invention. These known stabilizers may be of rubber or hard-faced steel and are placed at some distance above the uppermost stabilizing member of the diamond bit unit according to the invention. They will restrict the oscillating movements of the drill string and their liability to wear will be reduced owing to the presence of the stabilizing members according to the invention. The known reamers, if applied, are also located above the stabilizing means of the diamond bit unit according to the invention. Since these known reamers have the characteristic feature of being able to cut in directions radial to the central axis of the reamer, the hole is cut to a diameter greater than the outer diameter of the reamer due to the vibrations occurring in the drill string. However these vibrations do not reach the diamond bit since in the diamond bit unit according to the invention stabilizing elements are placed between these known reamers and the diamond bit which stabilizing elements are, over the total outer surface thereof in contact with the borehole wall. An advantage of the oversized hole as results from the use of the known reamers in combination with the diamond bit unit according to the invention is that the diamond bit unit can be lifted very easily from the hole, or run in.

Claims

I claim:

1. A diamond drill bit unit for performing rotary drilling operations in subsurface formations and having a central axis, said unit comprising:

a diamond bit;

drill collar means operatively associated with said diamond bit;

stabilizing means comprising stabilizing elements mounted on said drill collar means, said stabilizing elements having outer surfaces which are conically arranged with respect to the central axis of the diamond bit unit said surfaces lying on cones which are arranged with the tops thereof in the direction in which the bit is axially loaded during operation; and

a plurality of diamonds touching said conical surfaces and set in said stabilizing elements each of said diamonds being set in a cutting position in a directions tangential to a helical line passing through the location of the diamond and having an axis coinciding with the central axis of the diamond bit unit this tangential direction being taken at the location of each diamond, each diamond further being set in a blunt position in a direction radial to the central axis of the diamond bit unit with the greatest cutting diameter of the plurality of diamonds set in a conical surface directly adjacent to the diamond bit being at least greater than the greatest cutting diameter of the diamonds set in the diamond bit.

2. The diamond bit unit according to claim 1, wherein the smallest cutting diameter of the diamonds of the stabilizing means is equal to the largest cutting diameter of the diamonds of the diamond bit.

3. The diamond bit unit according to claim 1, wherein the conical surfaces have a top angle between 10' and 40'.

4. The diamond bit unit according to claim 1, wherein a plurality of conical surfaces are distributed over the length of the drill collar means.

5. The diamond bit unit according to claim 4, wherein the smallest cutting diameter of the diamonds set in a first conical surface of the stabilizing means not directly adjacent to the diamond bit, is equal to the largest cutting diameter of the diamonds set in a second conical surface of the stabilizing means which surface is situated between the first conical surface and the diamond bit.

6. The diamond bit unit according to claim 1, wherein the stabilizing elements having surfaces which are conically arranged with respect to the central axis of the diamond bit unit are of wear-resisting material enclosing the diamonds.

7. The diamond bit unit according to claim 1, wherein the drill collar has an outer diameter which is at least smaller than the smallest cutting diameter of the diamonds of the stabilizing means, and wherein at least one channel substantially extending in longitudinal direction is arranged between each pair of stabilizing elements having the outer surfaces thereof lying on a common cone.

8. The diamond bit unit according to claim 1, wherein at least one stabilizing element comprises additional diamonds which are set in the stabilizing element in a blunt position in directions tangential to as well as radial to the central axis of the diamond bit unit, these diamonds touching outer surfaces of the stabilizing element which border the conical surface thereof at at least one side and lie within the cone on which the conical surface is located.

9. A drill collar suitable for use in rotary drilling operations by means of diamond bits in subsurface formations;

stabilizing elements mounted on said drill collar having outer surfaces which are arranged on at least two noncoinciding conical planes having their central axes coinciding with the central axis of the drill collar;

diamonds set in the stabilizing elements, and touching the conical surfaces, the largest cutting diameter of the diamonds lying in a second conical plane, and the second conical plane enveloping the first conical plane, each diamond being set in a cutting position in a direction tangential to a helical line passing through the location of the diamond and having an axis coinciding with the central axis of the drill collar, said tangential direction being taken at the location of each diamond, and each diamond being set in a blunt position in a direction radial to the central axis of the drill collar.