US20130014992A1

US20130014992A1 - Data Transfer In A Two-Pipe Directional Drilling System

Info

Publication number: US20130014992A1
Application number: US13/410,075
Authority: US
Inventors: Richard F. Sharp; Travis W. Woodson; Floyd R. Gunsaulis
Original assignee: Charles Machine Works Inc
Current assignee: Charles Machine Works Inc
Priority date: 2011-03-01
Filing date: 2012-03-01
Publication date: 2013-01-17

Abstract

A downhole tool for a dual member drill string for detecting information and sending a signal containing that information up a wireline. The tool comprises a beacon supported on a housing for collection of orientation information. The wireline is located within a drill stem in the housing. The beacon communicates a signal to the wireline using a slip ring or a receiver for receiving a wireless signal from the beacon. The downhole tool may have a steering feature such as a deflection shoe for changing the path of the downhole tool.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional patent application Ser. No. 61/447,762, filed on Mar. 1, 2011, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a data transfer method and apparatus for a two-pipe horizontal directional drilling (HDD) system.

SUMMARY OF THE INVENTION

The present invention is directed to a downhole tool for a dual member drill string. The drill string comprises an inner member and an outer member. The downhole tool comprises a housing coupled to the outer member, an inner drive shaft coupled to the inner member, a beacon supported by the housing, a wireline disposed within the inner member, and a data relay connection. The beacon detects orientation information and transfers a signal. The wireline carries the signal. A data relay connection receives the signal from the beacon and transmits it to the wireline.
In another embodiment, the present invention is directed to a method for communication of data along a drill string. The drill string comprises an inner member and an outer member. The method comprises detecting information at a beacon wherein the beacon is rotationally coupled to the outer member, transmitting a signal containing the information from the beacon to a receiver supported on a downhole tool, wherein the receiver is rotationally coupled to the inner member, and transmitting the information from the receiver to a wireline for transmission of information along a drill string to an operator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a boring system for use with the system of the present invention.

FIG. 2A is a side sectional view of a downhole tool having the data communication system of the invention.

FIG. 2B is a side sectional view of an alternative embodiment of a downhole tool of the present invention.

FIG. 3 is a side sectional view of a downhole tool comprising a data transfer sub.

FIG. 4 is a perspective view of the downhole tool of FIG. 3.

FIG. 5A is a sectional side view of a downhole tool having a gyroscope.

FIG. 5B is a magnified side view of FIG. 5A.

FIG. 6A is a sectional side view of a downhole tool having a telemetry probe attached to an inner drive shaft.

FIG. 6B is a sectional side view of a receiver for a downhole tool.

FIG. 7 is a perspective view of a dual member drill pipe in accordance with one embodiment of the present invention.

FIG. 8 is a cutaway, sectional side view of a connection for a tool joint having a wireline in an outer member.

FIG. 9 is a sectional side view of a dual member pipe section having members which transfer electrical signals.

DETAILED DESCRIPTION OF THE DRAWINGS

Directional drilling systems which utilize a dual member drill string for transferring thrust and rotation from the drilling unit to the bit and drilling head for creating underground bores are disclosed in U.S. Pat. No. 5,490,569 issued to Brotherton et al., the contents of which are incorporated by reference. Most commonly with these systems, the housing of the dual member drill string connects to a downhole tool having a bend, or other feature, for biasing the drilling bit to one side of the bore hole to create a curved bore path when the housing is advanced forward without rotation of the housing such as the system disclosed in the Brotherton patent. The inner member of the dual member drill string is generally connected to a drill bit which cuts the soil or rock at the end of the bore hole when rotated. Such systems have proven effective in drilling a variety of ground conditions ranging from homogeneous soil, to cobbles, to solid competent rock.
With reference now to FIG. 1, a boring system comprising a drilling machine 11, a downhole tool 10 and a drill string 12. The drilling machine 11 drives the drill string 12, which in turn operates elements of the downhole tool 10 as described herein. The progress of such boring systems is generally tracked using a walkover tracking system 13 which receives data from a transmitter located within the downhole tool 10 at the terminal end of a drill string 12. The transmitter will send information regarding the orientation of the downhole tool 10, the remaining battery life, and other information related to the drilling process. Generally, this information is wirelessly relayed to the surface of the ground where an operator with a signal receiving unit picks up the transmitted signal and converts the information into a display format readable by the operator. Such walkover tracking systems are useful in many conditions, but are not desirable for conditions where the area above the downhole tool is not readily accessible. Such conditions include bores made beneath bodies of water, beneath buildings and other obstructions, and bores which are deeper than the range of the transmitter. In these situations, it may be desirable to transmit the orientation data and drilling information along the drill pipe connecting the downhole tool 10 and the boring unit 11 using a wireline 22 (FIGS. 2A, 2B).
Usually, it is advantageous to include orientation sensors and transmitters on the outer member of a drill string 12 or at an exterior of a downhole tool 10 housing. However, in order to facilitate the integrity and stability of a wireline through the drill string 12, it is advantageous to include the wireline within the inner member of a drill string 12. As the inner member and outer members are independently rotatable, it is not feasible to simply place a wireline inside the outer member, or to run a wireline from the outer member to the inner member as rotation may cause the wire to wrap around inner elements of the downhole tool 10 and drill string 12. Therefore, this invention provides a downhole tool 10 for transferring a signal sent by a beacon or transmitter on an outside of a downhole tool to a wireline on an inside of a drill string.
With reference to FIGS. 2A and 2B, embodiments of the invention which solve the above problem are shown. With specific reference to FIG. 2A, shown there in is a drilling tool 10 for a drill string 12. The drill string 12 comprises an inner member 14 and outer member 16. The drilling tool comprises an inner drive shaft 18, a housing 20, a wireline 22, a data relay receiver 24, a beacon 26, a steering feature 28, and a drill bit 30. The steering feature 28 may be incorporated into the housing 20, or alternatively, be incorporated into the drill bit 30 itself as disclosed in U.S. Pat. No. 6,827,158 to Dimitroff and Knecht. The inner drive shaft 18 of the drilling tool 10 is connected to the inner member 14 of the drill string by a torque-transmitting connection which may comprise a slip-fit or threaded connection. Likewise, the housing 20 is connected to the outer member 16 by a torque transmitting connection such as a slip-fit, threaded, geometrical, or other connection. A beacon cover 27 is preferably formed in the housing 20 to protect the beacon 26 from damage from the downhole environment.
The steering feature 28 of the downhole tool 10 biases the bit 30 to one side of a bore path. As shown, the steering feature 28, or deflection shoe, comprises an integrally formed bent sub, but also may comprise an externally attached structure or a portion of the beacon cover 27. Alternatively, a housing 20 where the longitudinal axis of the inner drive shaft 18 is offset from the longitudinal axis of the housing could serve as the biasing mechanism. Steering feature 28 may be located either in front of beacon 26 as shown in FIG. 2A, or behind the beacon. If the downhole tool 10 is advanced without rotation while the drill bit 30 is rotated, a curved path is produced. As shown, the beacon 26 is located opposite the steering feature 28. However, the beacon 26 may be located at any known angle relative to the steering feature 28 provided the angle is constant during operation of the bit. When using a bent sub as shown, the inner drive shaft 18 will have a deflection along its longitudinal axis as shown in FIG. 1. Alternatively, the inner drive shaft could contain a crowned spline joint, or other suitable constant velocity angular joint, along its length to handle the transmission of torque around the angle of the bend in the housing 20. An additional set of bearings (not shown) holding the inner drive member in axial alignment may be required.
The wireline 22 is disposed within the inner drive shaft 18 and connected to the data relay receiver 24 and is adapted to transfer information through the inner drive shaft 18 and inner member 14 to a drilling machine comprising an above ground receiver (not shown). In addition the wireline 22 may be adapted to carry electrical power from the uphole for the electronic components in data relay receiver 24, and in beacon 26 in those embodiments where an electrical connection exists between the wireline and the beacon. Alternatively, the inner drive shaft 18 and inner member 14 may themselves transfer information through the conduction of electric signals to and from the data relay receiver 24 as discussed in the text related to FIG. 9.
The beacon 26 provides information about the downhole tool 10 orientation, position, and other operational parameters of the beacon and conditions proximate the downhole tool 10. The beacon 26 is located in a side of the housing 20 of the downhole tool 10 as shown. The beacon 26 may be end-loaded or side-loaded in the housing 20. Information is sent via a data signal to a receiver, such as the data relay receiver 24 or an above ground receiver 13. In a preferred embodiment, the information is encoded on a modulated dipole magnetic field produced by the beacon 26. In each of the embodiments of the current invention, the beacon 26 may be adapted such that beacon information may be transmitted to the walkover tracking system 13 and along wireline 22 substantially simultaneously.
In addition, the beacon 26 may be configured to provide a reading of azimuth, or longitudinal heading of the downhole tool 10. To do this the beacon 26 may comprise magnetic field sensors to determine the heading of the downhole tool 10 relative to the earth's magnetic field, or to an artificially induced magnetic field created by passing a current through a loop on the surface of the ground as is known in the art of directional drilling. To provide an azimuth reading based on a magnetic field reading, the housing 20 and other components of the downhole tool 10 are preferably composed of a durable non-magnetic material such as high strength austenitic stainless steel, or a nickel-based metallic alloy.
In the embodiment shown in FIG. 2A, the data relay receiver 24 is operatively coupled to the inner drive shaft 18 and rotates with it. The data relay receiver 24 is configured to receive the information transmitted by the beacon 26. The data relay receiver 24 configures the signal, preferably provides amplification and signal format conversion, and transmits the signal along the wireline 22 and up the drill string 12. A window or slot (not shown) transparent to magnetic fields may be provided in the housing 20 proximate the beacon 26 to facilitate communication between the beacon and the data relay receiver 24.
Referring now to FIG. 2B, the beacon 26 is configured for transfer of information by transmission along a beacon wire 32 or wire pair directly. The downhole tool 10 of FIG. 2B again comprises an inner drive shaft 18, housing 20, wireline 22 and beacon 26. In this embodiment, the beacon wire 32 from the beacon 26 passes through the housing 20 to a slip ring 34. The slip ring 34 comprises two electrically conductive portions. A first portion 36 is secured to the inner drive shaft 18 and rotates therewith for secure connection to the wireline 22. A second portion 38 of the slip ring 34 is secured to an inner wall of the housing 20 and connected to the beacon wire 32. The first portion 36 and second portion 38 rotate relative to one another but contact each other across a sliding electrical contact such that the slip ring 34 provides a consistent electrical connection between the wireline 22 and beacon wire 32. Alternatively, the first portion 36 connected to the inner drive shaft 18 and the second portion 38 secured to the housing 20 may comprise wire coils arranged in a concentric or face-to-face fashion and transmit data between them by an inductive coupling rather than direct electrical contact.
Referring now to FIGS. 3 and 4, an alternative embodiment of the device is shown. In this embodiment a separate data transfer sub 50 is added to the back of the drilling tool 10. As with FIGS. 1 and 2, various methods of connecting the inner and outer drill string members may be used with this embodiment. One arrangement for this type of connection is to use a threaded connection on the outer drive member and have a slip-fit connection adapted for torque transmission on the inner drive member. Such a connection is shown in U.S. Pat. No. RE 38,418 to Deken and Sewell. Alternatively, both the inner and outer members may be connected with threaded connections. In yet another arrangement, the inner member may be connected using a threaded connection and the outer member connected using a slip-fit connection adapted to transmit torque along the outer member. As shown in FIG. 2B, the drill string 12 may comprise a “slip fit” configuration for connection to the data transfer sub 50 and drilling tool 10. The outer member 16 comprises a series of castellations 52 as shown in FIG. 4 for the transmission of torque along the outer member. The width of the individual castellations 52 may be of equal size, allowing the connection in any orientation. Alternatively, the mating castellation 52 may be of unique width such that the pipes may connect in only one unique orientation.
With continued reference to FIGS. 3 and 4, a data relay receiver 54 is contained within an appropriate cavity 56 in the data transfer sub 50. The device may receive information transmitted by the beacon 26 in the drilling tool body 10. In FIG. 3 a slip ring 34 is shown for transferring the information to the wireline 22. Alternatively, a wireless data transmission similar to that disclosed in FIG. 2A could be used. Alternatively, the data relay receiver 54 may comprise a sensor or beacon to determine characteristics of the downhole tool 10 without the use of a separate beacon 26.
Referring now to FIGS. 5A and 5B, the downhole tool 10 comprises a data transmission sub 60 attached to the rear of the drilling tool 10. The data transmission sub comprises a sensor package 62 comprising a gyroscope 64. The gyroscope 64 may be a vibratory MEMS gyroscope, a fiber optic gyroscope (FOG), or of other suitable design. An outer periphery of the gyroscope 64 may include recessed passages 66 which allow drilling fluid flowing through the inner member 14 and inner drive shaft 18 (FIG. 1) to flow around the sensor package 62 to the bit 30. The wireline 22 extends from the back of the sensor package 62 and proceeds uphole through the inner member 14.
When utilizing the gyroscope 64 located within the rotating inner drive member, a special technique may be used for mapping the bore. Gyroscopic sensors are generally limited in the rate of rotation of the instrument can undergo and still provide an accurate measurement of angle or rate of angular movement, depending on the type of gyroscope. In addition, rotation of the drill stem while mapping may provide a completely erroneous reading of position, since the axis of the bore hole is generally a linear feature with only slight curvature. One preferred technique for utilizing the gyroscope 64 with the sensor package 62 to map the bore hole is given below.
When performing rotary drilling operations in rock formations, it is common for a driller once they have completed advancing a drill string 12 to its full extent, to retract the drilling tool 10 one full joint of pipe such that it is at the same position as at the end of the forewardmost advance of the previous pipe and then thrust the bit back down to the bottom of the hole while pumping drilling fluid. A result of this action is a large surge of drilling fluid flowing back from the face of the hole. This helps to clear any cuttings which may have settled around the drilling tool 10 during the drilling operation. For mapping the bore, the driller will advance the drilling tool 10 to the extent allowed by the drill string 12. He or she will then retract the drilling tool as if performing a standard swab of the hole. Once the drilling tool 10 is retracted the full length of the drill string 12, the rotation of the inner member 14 and inner drive shaft 18 will be stopped. At this time the gyroscope 64 will be powered up and allowed to settle. The drilling tool 10 is then advanced to the bottom of the bore hole without rotation of the inner drive shaft 18. As the drilling tool 10 is advanced, angular changes, or rate changes transmitted by the gyroscope 64 are recorded. A carriage position transducer (not shown) located on boring unit 11 will simultaneously measure the distance of advance of the pipe going into the hole. The angular data, or angular rate data, from the gyroscope 64 will then be integrated with the carriage advance distance serving as the incremental value, dx, for the integration and the new position of the drilling tool at the end of the borehole can then be calculated in a stepwise fashion as the bore proceeds.
To facilitate steering of the drilling tool 10, the sensor package 62 may also provide an indication of a roll position and, thus, the orientation of the steering feature 28 of the drilling tool. One method for accomplishing this task is to have the gyroscopic sensor housed in a non-magnetic material as disclosed earlier in this text. A protrusion 67, or roll timing pin, composed of a ferrous, magnetic material may extend from the interior of the data transmission sub to a position near the outer surface of the compartment where the gyroscopic sensor is housed as shown in FIG. 5. In this manner, a magnetic sensing means housed in the gyroscopic sensor package 62 may sense the angular orientation, or clock position, of the data transmission sub 60 and drilling tool 10 with respect to the inner drilling member 14 and the sensor package 62. If an accelerometer is also included in the sensor package 62, then the acceleration of gravity can provide an indication of the orientation of the sensor package with respect to vertical. Having those two relative positions, it is then possible to calculate the roll position of the outer drill housing. This information can also be sent up the wireline 22 to allow the operator to correctly orient the housing according to whatever steering changes are required.
Alternatively, if the entire drill string 12 included outer member 16 with single-orientation castellations 54 (FIG. 4) then the position of the castellation at the drill pipe currently in contact with boring unit 11 could provide an indication of the roll position of the drilling tool down hole. By orienting the castellations in a known manner at the drilling unit, the roll orientation of the downhole drilling tool could be controlled in ground conditions where little wind up, or angular deflection, was anticipated along the length of the drill string 12.
Referring now to FIG. 6A, a receiver housing 80 is shown. The receiver housing 80 comprises a telemetry probe 81 connected to the inner drive shaft 18 of the downhole tool 10 just behind the bit 30. The receiver housing 80 is rotatable with the rotation of the inner drive shaft 18. The receiver housing 80 comprises the telemetry probe 81 and the wireline 22 extending directly from the receiver, up the drill stein 18 and uphole through the inner member of the drill string. The housing 80 may be a separate device as shown, or integrally formed with the inner drive shaft 18 of the downhole tool 10. An appropriate fluid seal 82 is preferably included to prevent unintended loss of drilling fluid out of the receiver housing 80.
As shown in FIG. 6A, telemetry probe 81 will function much the same as the data relay receiver 24 (FIG. 2A) previously discussed. That is, the telemetry probe 81 will receive the information from beacon 26, convert it into appropriate format for transmission up the wireline 22, amplify, and transmit the data along the wireline. This configuration eliminates the need for a slip ring and the receiver housing 80 may be added or removed from the downhole tool 10 as desired depending on whether wireline 22 communication is needed for a particular underground bore. A beacon 26 may be provided on the downhole tool 10 as shown, or the beacon functions can be incorporated with the telemetry probe 81. If the telemetry probe 81 is to act in place of beacon 26, a means of measuring the roll angle of the housing 20, and thus, the steering member 28 is preferably incorporated. Such a means could comprise a magnetic pickup device that senses a protruding ferrous feature (not shown) from housing 20 to indicate a certain roll position.
With reference now to FIG. 6B, an alternative embodiment of the receiver housing 80 of FIG. 6B is shown with an internal receiver coil. The housing comprises the receiver 24, operatively connected to the wireline 22. The receiver comprises a coil 84, coil bobbin 86 and electronics board 88. The coil 84 is preferably an antenna capable of receiving signals from the beacon (FIG. 6A). The coil 84 is supported by the coil bobbin 86 and disposed about the wireline 22. The signals are transmitted to the electronics board 88. The electronics board 88 configures the signal, preferably provides amplification and signal format conversion, and transmits the signal along the wireline 22. A center wire connection 90 preferably holds the wireline 22 to the electronics board 88 in a secure fashion. A fluid passage 87 in the center of the wire connection 90 allows fluid to flow forward and out of bit 30.
Referring now to FIGS. 7 and 8, an additional embodiment of the dual member drill string 12 is shown. In this embodiment, the dual member drill string 12 used comprises an outer drill pipe member 16 that connects with a sliding connection and comprises a feature for the transmission of torque, such as mating castellation features. In the disclosed embodiment, instead of passing a wire through the interior portion of the inner drive member, a passageway is prepared along the outer pipe for routing of a signal wire or wires. One preferred method as illustrated in FIG. 7 is to provide a slot 92 along an external surface of the outer member 16 deep enough to hold the wireline 22. The drill string 12 comprises tool joints 96. At the tool joints 96, a passageway is drilled to allow the wireline 22 to pass through to the shoulder of the drill string 12. The position of holes through male and female tool joint sections is consistent to allow for the wireline 22 to be self-connecting as the drill pipe 12 sections are slid together. By using a drill pipe 12 configured in this manner, a beacon 26 or probe configured to drive a data signal along a wire or wires, as shown in FIG. 2B, may be used to eliminate the need for a slip ring (FIG. 2B) or other device to transmit the signal to a wire in the center of the inner member 14. It should be understood that the wire in the slot 92 along the outer member 16 will be held in place by a suitable epoxy, acrylic adhesive, or other means to keep the wire from getting damaged during the rotational action of the drill string 12.
With reference now to FIG. 9, a pipe joint is shown wherein the inner member 14 and outer member 16 of the drill string 12 may be used in place of a wireline to transmit information. A contact 100 is provided at each end to allow the wireline (FIG. 2) or other conductive element of the downhole tool 10 or drill string 12 to transmit information to the conductive inner member 14. An inner member insulator 102 is provided at portions of the inner member 14 which are likely to come in contact with the outer member 16. Likewise, an outer member insulator sleeve 104 is provided to prevent portions of the outer member 16 from contacting the inner member 14. As illustrated in FIG. 9, the inner member 14 is a solid member and the connections for transmitting torque from one inner member 14 to the next in a pipe string are of a slip-fit variety as disclosed in U.S. Pat. No. 5,682,956 by Deken and Sewell. Alternatively, the inner member may have a hollow passageway along its interior for the conveyance of drilling fluid along its interior. Similarly, the inner member 14 connection would not have to be a slip fit connection, but alternatively could be a threaded connection. In addition, the inner member 14 may comprise a threaded connection for connection to the next inner member in a drill string 12 while the outer member 16 comprises a slip fit connection consistent with that shown in FIG. 7. In the embodiment shown in FIG. 9, it is contemplated that either the inner 14 or outer 16 pipe member will provide the electrical signal path and the other member will provide the return path to complete the circuit. To accomplish this, a slight modification to any of the configurations shown in FIGS. 2-6B could be made where the electrical connection from the data relay receiver 24, or telemetry probe 81 would be made directly to the inner pipe member rather than to the wireline.
Various modifications can be made in the design and operation of the present invention without departing from the spirit thereof. Thus, while the principal preferred construction and modes of operation of the invention have been explained in what is now considered to represent its best embodiments, as herein illustrated and described, it should be understood that the invention may be practiced otherwise than as specifically illustrated and described.

Claims

1. A downhole tool for a dual member drill string comprising an inner member and an outer member, the downhole tool comprising:

a housing coupled to the outer member;

an inner drive shaft coupled to the inner member;

a beacon supported by the housing to detect orientation information and transfer a signal;

a wireline disposed within the inner member to carry the signal; and

a data relay connection to receive the signal from the beacon and transmit it to the wireline.

2. The downhole tool of claim 1 wherein the data relay connection comprises a receiver coupled to the inner member.

3. The downhole tool of claim 1 wherein the data relay connection comprises a slip ring, the slip ring comprising:

a first portion coupled to the drill stem in electrical communication with the wireline; and

a second portion coupled to the housing and in electrical communication with the beacon;

wherein the first portion is in electrical communication with the second portion.

4. The downhole tool of claim 1 wherein the data relay connection comprises an inductive data connection between the inner and outer members.

5. The downhole tool of claim 1 wherein the downhole tool comprises a steering feature.

6. The downhole tool of claim 5 wherein the steering feature is located between the data relay connection and the beacon.

7. The downhole tool of claim 5 wherein the steering feature consists of a bend in the downhole tool.

8. The downhole tool of claim 5 wherein the steering feature is located at an opposite side of the outer member from the beacon.

9. The downhole tool of claim 1 wherein the data relay connection is located between the beacon and the drill string.

10. The downhole tool of claim 1 wherein the housing comprises a castellation for transmission of torque from the outer member.

11. The downhole tool of claim 1 wherein the beacon is located on the housing.

12. The downhole tool of claim 11 wherein the data relay connection comprises a receiver coupled to the inner member.

13. The downhole tool of claim 11 wherein the data relay connection comprises a slip ring, the slip ring comprising:

14. The downhole tool of claim 1 further comprising a gyroscope sensor.

15. The downhole tool of claim 1 further comprising a receiver housing coupled to the drill stem and extending beyond the housing, wherein the data relay connection is secured to the receiver housing.

16. The downhole tool of claim 16 wherein the beacon is secured to the receiver housing.

17. The downhole tool of claim 16 wherein the data relay connection comprises:

a coil adapted to receive the signal from the beacon; and

an electronics board adapted to configure the signal for transmission through the wireline.

18. A method for communication of data along a drill string, the drill string comprising an inner member and an outer member, the method comprising:

detecting information at a beacon wherein the beacon is rotationally coupled to the outer member;

transmitting a signal containing the information from the beacon to a receiver supported on a downhole tool, wherein the receiver is rotationally coupled to the inner member; and

transmitting the information from the receiver to a wireline for transmission of information along a drill string to an operator.

19. The method of claim 18 wherein the signal is transmitted from the beacon to the receiver wirelessly.

20. The method of claim 18 wherein the signal is transmitted from the beacon to the receiver through a slip ring, wherein the slip ring comprises a first portion rotationally coupled to the inner member and a second portion rotationally coupled to the outer member.

21. The method of claim 18 wherein the signal is transmitted from the beacon to the receiver through an inductive coupling, wherein the inductive coupling comprises a first portion rotationally coupled to the inner member and a second portion rotationally coupled to the outer member.

22. The method of claim 18 wherein the information from the beacon may be read either through an above ground receiving unit, or through a wireline within the drill string.