Centering apparatus

A device for centering a tool forming a part of a tool string, inside a cased well, comprises a central bar suitable for being integrated in a tool string for lowering the tool inside the tubing, two slides which are slidable over a limited range of said central bar, and a series of wheel-carrying structures of generally triangular geometry defined by arms which are hinged to said slides. The device further comprises a series of wheels with each wheel being mounted to rotate freely on a respective one of said wheel-carrying structures in a plane which is inclined relative to a radial plane through the wheel-carrying structure in such a manner that its periphery projects radially outwardly. When the wheel-carrying structures are fully retracted against the central bar, the wheels are received in recesses provided in the outer surface of the central bar. Resilient return means are provided to urge the wheels radially outwardly.

BACKGROUND OF THE INVENTION 
The present invention relates to devices for centering tools in a cased 
well, and in particular for centering tools in deviated cased wells. 
The work performed inside a well often requires that a tool which passes 
through the well be properly centered therein. This is particularly true 
when measuring the transverse dimensions of a well as described for 
example in French patent application No. 81 24021 filed Dec. 22, 1981, 
corresponding to U.S. Application No. 450,595 filed Dec. 17, 1982. 
There are technical difficulties in developing entirely satisfactory 
centering devices. Firstly, the casing of a well has localised variations 
in diameter both at joints and at the valves situated at the well head. 
The centering device must therefore be capable of adapting to such 
variations in diameter. In the industry, such an adaptive centering device 
is often called a "centralizer". Furthermore, the recent tendency, 
particularly for offshore drilling, is to drill deviated wells which 
depart substantially from the vertical on one or more occasions. As a 
result, the operation of the centering device is perturbed by the effect 
of gravity on the tools centered by the device which effect operates 
laterally on the centering device. 
Finally, to obtain proper centering, the device will by necessity bear 
against the walls of the well. In many prior art devices, sliding means 
are used to bear against the wall of the casing. This is true, in 
particular, for the centralizers described in U.S. Pat. Nos. 3,097,433, 
3,555,689, and 3,915,229. This type of bearing is no longer entirely 
satisfactory, in particular in deviated wells, since it tends to alter the 
state of the inside wall of the casing, and thereby accelerate corrosion; 
furthermore it tends to limit the critical deviation angle beyond which 
the tool will no longer advance under the effect of its own weight. 
In order to remedy the above drawbacks, a centralizer has been proposed in 
the literature in which contact with the wall of the well casing is 
provided by wheels, e.g., "Improved Technique for Logging High-Angle Well" 
by M. W. Bratovich, W. T. Bell & K. D. Kaaz, published under the reference 
SPE 6818 by the American Institute of mining Metallurgical Petroleum 
Engineers, Inc. In FIG. 3B of that article a centering device is shown 
which comprises a central bar suitable for being attached to a tubing 
string for lowering a tool in a cased well, two moving slides capable of 
sliding to a limited extent over the central bar, a series of radial 
wheel-carrying structures of generally triangular geometry defined by arms 
articulated to the two slides, a series of wheels mounted free to rotate 
on each of the radial structures in such a manner that the periphery of 
each series of wheels projects radially outwardly, and resilient return 
means which urge the wheels radially outwardly. Although this type of 
centralizer enables some work to be performed in a deviated well, it is 
not free from drawbacks. 
Firstly, its centering or bearing force on the walls of the casing tends to 
fall off too rapidly when the diameter of the casing increases. Secondly, 
in order to satisfy maximum diameter requirements when in the retracted 
position, e.g., less than 50 mm, the wheels must be of small diameter; 
they therefore rotate very quickly while a tool with the device is going 
down a well, which considerably reduces their lifetime. Finally, this type 
of centralizer is subject to severe shocks each time it passes a joint in 
the tubing, which shocks rapidly puts the device out of service. 
SUMMARY OF THE INVENTION 
The present invention seeks to solve the technical problems encountered 
with such prior art wheeled centering devices. 
A first aim of the present invention is to supply a centering device which 
has wheels of larger diameter, while still keeping within the same maximum 
outside diameter requirements when in the retracted position. 
Another aim of the present invention is to provide a device which applies a 
substantially constant radial thrust on the inside wall of casings over a 
wide range of casing diameters. 
A further aim of the present invention is to provide a device whose outer 
surface is free from sharp projections under all circumstances, thereby 
preventing the device from being put out of service by shocks during 
lowering. 
According to an essential characteristic of the present invention, each 
wheel is mounted to rotate in a plane which is inclined relative to the 
radial plane through the wheel-carrying structure on which it is mounted, 
and the central bar has external recesses to partially receive the 
wheel-carrying structures when they are fully retracted against the 
central bar. 
In a preferred embodiment, each wheel-carrying structure comprises a 
generally U-shaped fork with the outside ends of the arms articulated to 
the tines thereof, the arms being also articulated to the slides, while 
the wheel is mounted at the bottom of the U on the inside thereon in such 
a manner that the outside periphery of the wheel projects slightly beyond 
the associated wheel-carrying structure, and substantially in the radial 
plane therethrough. The bottoms of the U-shapes of the different forks are 
peripherally offset relative to the radial plane through each 
wheel-carrying structure and in the same direction when going round the 
central bar. 
Further, the resilient return means comprise firstly a pair of compression 
springs, each of which acts between a respective one of the slides and a 
corresponding shoulder on the central bar located on the side of the slide 
which is opposite to the other slide, whereby the slides are urged towards 
each other, and secondly, a curved spring blade mounted parallel with each 
arm between the slide and the associated wheel carrier, with the concave 
side of the spring blade facing inwards. 
Finally, the forks are externally shaped in such a manner that together 
they define an assembly having a substantially cylindrical periphery when 
the wheel-carrying structures are fully retracted against the central bar. 
Most advantageously, the wheels are inclined relative to the associated 
radial plane by an angle of about 10 degrees. Other characteristics and 
advantages of the invention appear from an examination of the following 
detailed description together with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, the symbol T designates overall the casing inserted in a 
deviated well which passes through successive different underground 
formations. A tool lowering string is located inside the casing and 
consists of successive components 1 and 2 which are pivotally connected to 
each other. A component 3 has universaljoints 3A and 3B at each end 
enabling it to swing angularly. The joint 3B is connected to a first 
centering device 4 which is followed by a tool 5 and then by a second 
centering device 6, and finally by a second tool 7. 
The tool 5 is, for example, a device for measuring the transverse 
dimensions of a borehole as described in the above-mentioned French Pat. 
No. 81 24021. The tool 7 comprises a reference sensor for measuring the 
propagation speed of sound waves in the medium filling the casing. 
The diagrammatic representation of the centering devices 4 and 6 given in 
FIG. 1 corresponds to the centralizer described in the above-mentioned 
article reference SPE 6818. 
Reference is now made to FIG. 2 which is a diagram of a centering device or 
centralizer in accordance with the present invention. 
A central bar 10 is provided at each end with screw threads (not shown) 
enabling two parts 12 and 13 providing facing shoulders to be fixed 
thereto. The parts 12 and 13 also serve to connect the central bar in the 
lowering string, e.g. one of them is connected to the universal joint 3B 
and the other is connected to the tool (see FIG. 1). It can also be seen 
from FIG. 1 that two centering devices are generally required. 
Two slides 20 and 30 which are described in detail below are slidably 
mounted on the central bar 10. 
The slides serve to retain three radial wheel-carrying structures 
referenced 40 and 40A (the third structure 40B which is located behind the 
parts shown is not itself shown in order to clarify the drawing). The 
three structures are identical and they are disposed at 120 intervals from 
one another around the central bar 10 and the slides 20 and 30. Only the 
structure 40A is described below. 
The structure 40A comprises a first pair of arms 71 and 72 which are hinged 
at one end to the slide 20 and a second pair of arms 73 and 74 which are 
likewise hinged at one end to the slide 30. At their other ends, the arms 
71 and 72 are hinged to one tine of a wheel-carrying fork 50. The other 
two arms 73 and 74 are likewise hinged to the other tine of the fork 50. 
The two hinge axes provided by the fork are parallel. Hinge pins 67 and 68 
can be seen in FIG. 8. 
Returning to FIG. 2, a curved spring blade 41 is fixed to one of the tines 
of the fork 50, and the other end of the spring blade is axially guided to 
slide along the slide 20. The other tine is likewise fixed to a second 
spring blade 42. 
Each of the forks such as 50 supports a wheel which is mounted to rotate 
freely thereon. 
Finally the resilient return means further include two springs 17 and 19 
which act between the end stops 12 and 13 and the slides 20 and 30. 
Reference is now made to FIGS. 3 to 7 which show the structure of one of 
the slides, e.g. the slide 20, in greater detail. The middle of the slide 
20 has three projections arranged around its periphery at 120 to one 
another. Each of these projections services to define the hinge axis for a 
corresponding one of the pairs of arms such as 71 and 72. Taking the 
projection 25 as an example, it is sufficiently large in the radial 
direction to receive a tangential bore 27 which defines the hinge axis of 
the two bars 71 and 72. An intermediate recess 26 is provided in the 
radially outer surface of the projection in order to rceive a spring blade 
such 41 when the centering device is in the fully retracted position. It 
can also be seen in FIG. 3 that the other hinge axes are referenced 27A 
and 27B. 
The slide 20 is further provided at its left hand end (as seen in FIGS. 3 
and 4) with an end piece 21 having an interior stop 28 for the spring 17. 
This stop is provided with three longitudinal recesses which lie at 120 to 
one another in the same planes of symmetry as the projections 25 (see 
FIGS. 3 to 6). On the outside there is the recess 22 which extends along 
the length of the end piece, thereby providing a housing for the end of 
the spring blade 41 when the device is fully retracted. Over a portion of 
its length starting at the free end the recess 22 is provided with side 
grooves 23. These grooves serve to slidably house a cross pin 43 (see FIG. 
9) mounted at the end of the associated spring blade 42. Over the length 
of the grooves 23, the recess 22 is in the form of a through slot 24 in 
order to facilitate machining the grooves 23. Finally, the bottom of the 
end piece 21 (as seen in FIG. 4) is provided with a hole 25 to receive a 
pin suitable for preventing the slide from rotating about the central rod, 
the pin co-operating with a matching groove in the central rod (where it 
is desired to prevent such rotation). It can further be seen that an 
orifice 29 is provided to enable the fluid contained in the tubing to pass 
into the slide, in order to equalize pressures. 
It can now be understood that the pairs of arms such as 71 and 72 are 
hinged to the slide, and also how the curved spring blade 41 is guided to 
slide on the same slide. 
FIGS. 8, 9 and 11 show the arrangement of one of the forks such as 50 in 
greater detail. As mentioned above, the pins 67 and 68 serve as the other 
pivot points for the pairs of arms 71 and 72, and 73 and 74 respectively. 
This fixing is to the tines 51 and 52 of the U-shaped fork 50. The tines 
exend in the radial plane through the associated wheel-carrier in the form 
of two plates, one of which 61 can be seen clearly in FIG. 8. The plate 61 
is provided with a projection 62 which serves as a bearing point for the 
end of the spring blade 42. The spring blade 42 is also fixed rigidly to 
the fork 50 by brasing at 64 and at 63. At this point, it may be recalled 
that the prior art centering device possesses radial structures of 
triangular geometry. In contrast, the structure in accordance with the 
invention is only slightly triangular. The person skilled in the art will 
understand that the points 67 and 68 where the arms are hinged to the fork 
are far enough apart from each other to provide, in combination with the 
role performed by the spring blades 41 and 42, a much more flexible 
suspension for the fork 50 and the wheel 80 carried thereby (the wheel is 
not shown in FIGS. 8, 9 and 11). FIGS. 8, 9 and 11 also show up another 
very important characteristic of the invention. The wheel-carrying fork 50 
receives the wheel near to the bottom 53 of its U-shape, and more 
precisely on the inside face 56 thereof as seen from the center of the 
structure. Further, the sideways offset of the fork 50 is accompanied by 
its inside face 56 being inclined and consequently the wheel plane is also 
inclined relative to the radial plane through the associated 
wheel-carrying structure. The angle of inclination in question is about 10 
degrees. 
As is shown by FIG. 2, the rolling surface of each wheel such as 80 
projects slightly beyond the outside of the associated wheel-carrying 
structure, and also beyond the inside of the U of each fork 50, i.e. 
substantially in the radial plane of the associated wheel-carrying 
structure. 
Finally, the bottoms 53 of the Us of the various forks such as 50 are 
peripherally offset in the same direction relative to the general 
direction defined by the spring blades 41 and 42 (i.e. the radial plane of 
the wheel-carrying structure), when going around the central bar 10. 
It is then highly advantageous for the outer surfaces 54 of the forks 50 to 
be shaped so that together they define a substantially cylindrical 
periphery, when the wheel-carrying structures are fully retracted against 
the central bar. In other words, the outer contour 54 of the fork is 
curved in such a manner that it gets closer to the axis of the centering 
device as it gets further from the radial plane through the associated 
wheel-carrying structure. It can be said that the fork 50 is thus offset 
sideways in a position which is slightly closed towards the interior. 
Reference is now made to FIG. 10, which is a cross section through a 
centering device in accordance with the invention in the fully retracted 
position. This figure also facilitates understanding of the way each wheel 
is mounted on the associated fork. 
Firstly it can be seen that recesses 11, 11A and 11B made in the central 
bar 10 and shown as flats, serve to receive the wheels in part, thereby 
helping to minimize the bulk of the device when in the fully retracted 
position. FIG. 10 also shows the substantially cylindrical peripheral 
contour of the device when in the retracted position. 
The central bar 10 has an inside bore 15 in order to pass conductors to the 
tool which is situated downstream therefrom. 
The mounting of a wheel 80 is now described. The inside face 56 of the fork 
50 is provided with an annular central shoulder 59. A bearing 81 is 
supported thereby. The wheel 80 rotates on the bearing 81. It can be seen 
that the wheel is hollowed out to enable it to be held by a cheek plate 83 
which is integrally mounted to a rod 82 which is received in the bore in 
the fork 50 where it is retained by a spring clip 84. Such a mounting 
makes it easy to assemble the wheel while keeping down its bulk, and thus 
keeping down the bulk of the centering device as a whole when in the 
retracted position. 
The periphery of the wheel 80 is chosen to be of suitable material to give 
it long life in the harsh conditions found inside well tubing. For the 
same reasons, it is advantageous for all of the outside portions of the 
fork 50, and the curved spring blades 41 and 42 to stand up well to the 
wear they are likely to suffer from episodic contact with the wall of the 
tubing. 
In the presently preferred embodiment of the invention as has just been 
described, the slides come into contact with, or very nearly into contact 
with, their respective end stops 12 and 13 when the wheel-carrying 
structures are in their fully retracted position. In other positions, the 
two slides are free to move together along the shaft constituted by the 
central bar 10 in opposition to the springs 17 and 19. 
When fully open the angle between the inside arms such as 71 and 72 and the 
axis of the bar is about 30. 
The fully open size of the device is defined both by the components in each 
radial wheel-carrying structure, i.e. the bars, the geometry of the curved 
spring baldes 41 and 42, and by the characteristics of the springs 17 and 
19 (taking their stops into account). 
Further, it has been observed that the disposition of the springs 17 and 19 
in accordance with the present invention makes it possible to obtain 
increasing radial force at the wheels with increasing tubing diameter. For 
their part, the curved spring blades such as 41 and 42 provide a radial 
force which, in contrast, reduces with diameter. By combining the two 
effects, it is possible to obtain a substantially constant radial or 
centering force which is indepenent of diameter over a fairly wide range 
of diameters (naturally the diameter must be smaller than the diameter 
which is reached when the two resilient systems constituted by the 
compression springs and the spring blades are in their equilibrium 
position). This characteristic is very important in highly deviated wells, 
where two of the wheel-carrying structures have to transmit the radial 
component relative to the tubing of the weight of the centering device and 
the tool with which it is co-operating. 
The Applicant is presently of the opinion that it is preferable to lock the 
slides against rotation about the central bar, as indicated above. A 
variant of the invention consists in leaving the slides 20 and 30 free to 
rotate about the central bar. It has been observed that it is sufficient 
to slightly round the edges of the flats 11, 11A and 11B provided on the 
central bar to ensure that the wheels can of their own accord take up a 
fully retracted position even if the two slides are badly placed relative 
to the flats at the moment of approach. Generally speaking it is possible 
to machine the central bar to facilitate wheel insertion in to the 
recesses thereon as he wheel-carrying structures come close to their fully 
retracted position. 
The above description relates to a single centering device. As shown in 
FIG. 2, most applications require two centering devices placed on either 
side of a tool 5, and another tool 7 may be cantelevered out from the 
leading end of a string being lowered. Naturally this arrangement lies 
within the scope of the invention.