Device for measuring stresses transmitted by a shaft, in particular to a drill bit

The device for measuring stresses applied to a drilling tool comprises a removable instrument socket (2) secured by means of protuberances (15, 16) thereon to the wall of a compartment formed thereby in a shaft (OM) such as to make fast said socket and said shaft in two transverse planes (12, 13) and as to transmit to said socket the strains occurring in the shaft in response to the stresses to be measured. Strain gauges (9) are fitted to the socket to measure said strains, and accordingly said stresses.

FIELD OF THE INVENTION 
This invention concerns a device for measuring stresses transmitted by a 
shaft and especially those stresses being transmitted to a drill bit in a 
drill string. 
BACKGROUND OF THE INVENTION 
There are basically two methods of drilling now in use. 
The first method, termed rotary drilling, is illustrated by FIG. 1. A 
drilling tool or bit TR driven by a tubular shaft or drill pipe OR 
equipped, from the bit upwards, with first and second stabilizers SA and 
SB. The shaft is rotatively driven from ground level. Drilling mud 
descends through the shaft to the bit and goes back up around the shaft. 
The second method, termed downhole motor drilling, is illustrated by FIG. 
2. The bit TM is driven by a shaft OM, which is itself driven by a 
downhole motor MF--usually a turbine--and operated by the drilling mud 
descending under pressure through an axial channel in a drill stem GM. The 
shaft is held in relation to the stem by means of a certain number of 
friction bearing disks DA. The drilling mud penetrates into an axial 
channel CA of the shaft below the disks and motor through special openings 
LM. 
Regardless of the drilling method used, the values of real stresses exerted 
on the drilling tool down in the hole (the torque, the weight and the 
lateral stresses on the tool) are only very approximately known on the rig 
floor in the absence of downhole measurements. Indeed, the torque applied 
to the drill stem at the surface by the rotary table arrives at the tool 
diminished by all the losses due to the friction arising between the rods 
and stabilizers and the walls of the well, all of which losses are highly 
variable and difficult to evaluate. For the same reasons, knowledge of the 
total weight on the tool affords a roughly worthwhile indication of the 
actual axial thrust received by the tool only for near-vertical holes, and 
the lateral stress on the tool as well as the rod bending stresses are a 
complete mystery. 
Given the conditions surrounding operation of the downhole parts and their 
shapes and dimensions, it is moreover difficult to envisage placing strain 
gauges on such shafts, either by bonding or deposition as taught by the 
latest techniques. Yet such strain gauges are known to be well suited to 
the measurement of the stresses under consideration. 
SUMMARY OF THE INVENTION 
The present invention is directed to a measuring device for obtaining exact 
knowledge of these effective stresses which device manufacture, 
installation and maintenance whereof are easily effected even when the 
measurement must be made in such unfavorable conditions as obtain at the 
bottom of a drill hole. 
Specifically, the invention is a measuring device for measuring the 
stresses transmitted by a shaft or drill pipe, in particular to a drilling 
tool, said device comprising: 
at least one measuring sensor mounted in a shaft length of known stiffness 
to supply an output signal representative of the elastic deformation of 
said length in response to said stresses which it transmits, such that 
said output signal serves as a measurement signal of said stresses, the 
main improvement of said device being that said sensor is carried in a 
removable instrument socket, arranged coaxially in a compartment formed in 
said measurement length of shaft, said socket being drawn out and its main 
axis being longitudinal, and being elastically deformable and fitted with 
a plurality of anchoring means with external faces adapted to anchor 
themselves in the wall of said compartment, each of said anchoring means 
being radially mobile between a retracted position where it does not 
project from the outer surface of said socket, to enable the removal 
thereof, and a thrust out position where it does project from said 
surface, to anchor itself in said wall, said anchoring means being 
angularly distributed in two anchoring planes perpendicular to said main 
axis so that the socket and the shaft interlock in both of said planes 
when said anchoring means are thrust out or positively engaged, the length 
of said socket between said two planes constituting a measurement area 
wherein said sensor is attached, 
and reversible manouvering means for working said anchoring means to lock 
or unlock said socket after it has been placed in said compartment. 
At least one sensor is attached to the socket in said measurement area to 
supply an output signal reflecting the deformations affecting said socket, 
such that the signal serves as the stress measurement signal when said 
anchoring means have been thrust out into positive engagement or locking 
position. 
Moreover, the device according to the invention can be given further 
improvements as follows: 
The said manouvering or control means comprise a plunger operable to slide 
longitudinally between a position anchoring said socket and a position 
where said socket is slidable, said plunger having a plurality of control 
inclines respectively cooperating, at least indirectly, with said socket 
anchoring means such that the longitudinal displacement of said plunger 
from its sliding position to its catching position radially thrusts out 
each of said anchoring means into positive engagement with said shaft 
wall. 
Said device comprises an oblong sheath having a fastening area equipped 
with means for coaxially attaching it in a removable manner within said 
compartment in the shaft, part of the length of said sheath lacking said 
fastening area being coaxially arranged within said measuring socket with 
some play, said socket itself having a fastening area, outside its 
measurement area, by which it is attached to said sheath so as to be able 
to elastically deform without deforming said sheath, said socket or said 
sheath being provided with elastic means for pulling back said catching 
means to set the latter in their retracted position when the plunger is in 
sliding position, said sheath being transpersed by radially extending 
guiding channels and being equipped with transmission parts operable to 
move radially within said channels in front of inside faces of said 
anchoring means, such that, as the said plunger moves to its anchoring 
position, said manouvering or control inclines first thrust out said 
transmission parts then thrust out said anchoring means into positive 
engagement position. 
Said transmission parts can consist of balls, in which case, behind said 
inclines on said plunger (ie. following said plunger in its direction of 
motion), there are cylindrical bearing surfaces bearing a coaxial relation 
with said socket which make contact with said balls when the plunger 
reaches its anchoring position, said anchoring means also having on their 
inside faces similar coaxial cylindrical bearing surfaces in contact with 
said balls, and said guide holes having a larger diameter than said balls, 
such that when the plunger is in anchoring position said anchoring means 
are able to move slightly relative to the plunger thanks to the balls 
rolling between bearing surfaces on the plunger and the anchoring means. 
Said elastic pull-back means of the anchoring means of the socket consist 
of tabs, each carrying one anchoring means and each extending lengthwise 
in one direction of the surface of said socket, extending widthwise in 
another direction of same said surface, perpendicular to said lengthwise 
dimension, and having a radial thickness less than said length and said 
width. At least one longitudinal end of each tab is attached to the socket 
away from said anchoring means, making the tab flexible lengthwise and 
rigidly operable to block the movements of said anchoring means relative 
to the socket in parallel relation to the surface of said socket. 
The long side of the tab may be arranged parallel to the axis of the 
socket, if the main stresses applied are parallel to said axis. Or, if the 
directions of the loads were mainly circumferential, the long sides of the 
tabs would also be circumferentially disposed. 
Said sheath has an enlarged, hollow back end, which is provided with an 
outside thread for the purpose of screwing it into the shaft compartment 
and an inside thread to receive a control screw operable to push and pull 
said plunger. 
An extension of said control screw is made longitudinally fast with the 
plunger by a radial pin and circumferential groove combination and angular 
movement of the plunger relative to the sheath is prevented by a radial 
pin and longitudinal groove combination. 
Said plunger and said control screw are both given an annular shape to 
allow axial passage of a fluid such as a drilling mud wetting a tool on 
the end of said shaft. 
Said anchoring means have anchoring points on their outside faces. 
The instrument socket at least is made of steel. But more often the 
measuring device as a whole is made of steel, as are the devices which 
will be described hereinafter by way of example. 
The socket's stiffness is selected to be less than 1% of the stiffness of 
the measurement lengths of shaft so that the stresses exerted upon said 
anchoring means are low. 
It is another object of the invention to provide a method of measuring the 
stresses exerted on a drilling tool in operation, by a transmission shaft 
having a long axis, said method consisting in inserting a device according 
to the invention coaxially into a cylindrical compartment coaxially formed 
in a section of said shaft, between the drilling tool or bit and the 
nearest area (ZA) where another stress is likely to be applied to said 
shaft. 
In accordance with the invention, a separate and removable measuring device 
is inserted into the part strained by the stresses to be measured, namely 
in the above-mentioned section fo the shaft, to supply the desired stress 
information. Moreover, two rigid, independent portions of the device are 
made fast with two separate portions of the stressed part such that the 
relative displacements of said two device portions when stressed provide 
an indication of the relative movements of the separate portions of the 
part, said movements being directly proportional to the stresses and thus 
indicating the amount of strain. 
Several preferred embodiments of the invention will now be described with 
reference to the appended drawings. It should be understood that the 
elements specifically shown and described could be replaced by other 
elements serving the same technical functions without departing from the 
scope and spirit of the invention. 
In the drawings, like items appearing in different figures are given the 
same references throughout.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In these figures, the axis (11, 68) of the sockets of the measuring devices 
according to the invention is shown vertically and said devices are 
inserted from the bottom, into the corresponding compartments such that 
the previously mentioned front and back ends or forward and behind 
directions become the top and bottom ends, respectively, in the drawings. 
As shown in FIG. 3, a center bore or axial channel CM has been made in the 
shaft OM of the downhole motor drill illustrated in FIG. 2 for passage of 
an electrical transmission cable usd to transmit the measurements 
according to the invention. The bottom end of said channel is enlarged to 
form an axial compartment 10 upstream from the place where the motive 
fluid passes from the outside to the inside of the shaft OM, through 
openings LM therein, to reach the tool. 
Said compartment is calibrated to accept the measuring device of FIGS. 4, 5 
and 6, which forms a fluid-tight housing. The latter comprises the 
following basic components: 
a sheath 1 which screws into the shaft compartment 10, 
a socket 2 loosely fitted around sheath 1, 
a plunger 3 slidably mounted in sheath 1, 
and a screw plug 4 which pushes up plunger 3 as it is screwed into sheath 
1. 
Sheath 1 has a cylindrical midsection and tapers in at the top to form a 
neck 5 adapted to receive a connector. The other end of the sheath tapers 
out and is given an outside thread 6 and an inside thread 7. The 
cylindrical midsection has drilled cylindrical guide channels 23 having a 
radial axis. In said channels 23, transmission parts--in this case, balls 
8--are arranged. Said channels are located in two mutually parallel 
anchoring planes 12 and 13 which are perpendicular to device axis 11. 
Socket 2 carries several strain gauges such as 9 on its inside wall 
providing full intelligence about the mutual displacement of two rigid 
circular sections of the socket, namely those sections defined by planes 
12 and 13. The socket is connected to sheath 1 by means of a single-point 
fastener 14 consisting of a radial pin on the outside of a measurement 
section extending longitudinally from plane 12 to plane 13. It is 
provided, in the same said planes 12, 13 where the balls are located, with 
protuberances 15 and 16 shaped as cones or diamond points. These 
protuberances themselves constitute the anchoring means. They are carried 
by tabs 17 which are elastic and flexible in the radial direction but 
rigid in the directions of the plane which is tangent at this point to the 
cylinder surrounding the socket. 
Plunger 3 is given two circular notches 18 in its outside surface, which 
are coaxial with shaft 11. The shape of said notches is made up of a 
quarter torus at the top, a truncated cylinder in the middle and an acute 
truncated cone toward the bottom, to form a driving or control incline 
merging at the bottom with the regular cylindrical surface 18a of the 
plunger, which latter surface forms a bearing surface for said balls 8 
near said incline. 
The plug 4 serving as a control screw is given either a square or a 
hexagonal drive socket 21. Plug 4 screws into the inside thread 7 of 
sheath 1 and is provided with a circular slot 19 engageable by a pin 20 on 
plunger 3. As the plug 4 is driven in by means of its drive socket 21, the 
inward driving motion is transmitted to plunger 3 via plug shoulder 22. At 
this time, the balls 8 in sheath 1 channels 23, initially located in the 
notches 18, are gradually pushed out by the truncated cone incline in said 
notches and come to radially press against the tabs 17 right behind 
protuberances 15 and 16. The latter's very fine points dig into the shaft 
OM and rigidly fix, in planes 12 and 13, the sections of said shaft to the 
sections of socket 2 which carries the strain gauges. When stresses are 
exerted on the shaft, these sections move (in or out in response to axial 
stresses, obliquely in response to bending stresses, and rotationally 
about the shaft axis in response to twisting stresses). The instrument 
socket 2 is provided with grooves arranged so that the movements or 
strains in sections 12 and 13 bring about only very limited strains in 
anchors 15 and 16 and so that virtually all of said movement is 
transmitted to the area supporting the measuring gauges 9. The strain 
gauge leads 24 are taken back to a common commercial connector 28 in part 
5 of sheath 1 for connection to the above-mentioned signal cable, which 
has not been specifically identified in the drawings. The screwbase of 
socket 1 is provided with a groove 40 containing an O-ring seal not shown. 
Installation of the device in the shaft OM is easily carried out according 
to the following procedure. 
After fitting connector 28, the device is arranged in its configuration as 
per FIG. 4, with plug 4 unscrewed. It is then screwed into compartment 10 
thanks to threads 6 until it abuts with the sealed shoulder 29 on shaft 
OM. The next step simply consists in screwing in plug 4 as previously 
described until its shoulder 22 comes against the bottom 30 of the hole in 
sheath 1. The device is then operational. 
Removing the device is as easy as its installation. Plug 4 must first be 
unscrewed to the point where the end 26 of slot 25 of plunger 3 makes 
contact with the pin 14 on sheath 1. Balls 8 resume their initial 
positions in notches 18, thus allowing the flexible tabs 17 to release 
protuberances 15 and 16 from their anchored positions in shaft OM. 
The device can then be removed simply by unscrewing sheath 1 using notches 
27 provided therefor in its base. 
The device specified in the foregoing can easily fit in a diameter of 35 to 
40 mm. However, it can also be adapted for large diameter drill pipes. 
Instrument sockets some 10 to 20 mm thick can be used with very large 
diameters on the order for example of 100 to 400 mm. 
The device used in the case for example of the rotary drill of FIG. 1 can 
be the same as the first, or can be made according to FIG. 7. 
In the latter case, the sheath 51 is axially inserted into a body 50 being 
part of the shaft OR. A keyway 53 is used to angularly locate the sheath 
51 in body 50. Nut 54 is then screwed home against shoulder 55, after 
which screw 56 can be driven into nut 54, axially driving a plunger 
consisting of a sliding socket 52. Said latter socket has two functions: 
anchoring the protuberances 61 (on flexible, elastic tabs 62) in body 50 
by pressing against bulbs 58, and sealing off the device from its 
surroundings. The measurement area is shown under the reference 64, the 
control inclines under reference 66, and the overall axis under reference 
68. Shaft OR is given an inside skirt 69 having a groove 70 for a sealing 
ring bearing against sliding socket 52. Said skirt 69 protectively closes 
off a compartment 80 housing the electronics associated with the strain 
gauge 9. 
In the device represented in FIG. 8, said transmission parts 71 radially 
mobile in guiding channels 23 of sheath 1 are elongated in the device's 
radial direction and are given along part of their lengths either 
cylindrical or prism-shaped guiding surfaces 72 to cooperate with said 
guiding channels in preventing any swivelling of said parts about axes 
parallel to the circumferential direction of the device, such that said 
parts exert only a radial force on the inside faces of said anchoring 
means 16 of the instrument socket 2.