Note: Descriptions are shown in the official language in which they were submitted.
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ADJUSTABLE STABILISER FOR DIRECTIONAL DRILLING
The present invention relates to down-hole tools and
particularly to stabilisers for drill strings, especially
near-bit stabilisers.
Directional drilling is either sophisticated,
expensive and unreliable or simple, reliable but rather
limited. For the most part, the latter type meets all
requirements. This type relies entirely on gravity and
can only adjust the inclination of a hole, rather than
its horizontal direction.
An adjustable stabiliser has a base diameter larger
than the drill string, but not as large as the hole bore
being drilled. It prevents the drill string from
contacting the sides of the bore. When actuated however,
its diameter increases and so the drill string is
constrained to run concentric with the hole being
drilled. Thus an adjustable stabiliser near the drill
bit steers the drill bit depending on its actuation.
Down-hole motors are frequently used in drilling.
The string itself is not rotated. Instead, the motor
near the end of the string rotates just the bit at the
end. The motor is hydraulically driven by drilling mud
pumped from the surface. The down-hole motor should be
as close to the drill bit as possible, but a stabiliser
can be interposed between them in order to provide
steerage.
Thus a short stabiliser is called for. Down-hole
stabilisers have been actuated in a number of different
ways.
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In EP-A-0251543, a fairly short-stabiliser is
disclosed, but it involves using mechanical compressive
forces on the drill string to set and unset it.
In US-A-4951760, a stabiliser is hydraulically
operated, employing fluid pressure of pressurised
drilling mud to actuate the stabiliser by a piston
mandrel moving axially in a bore of the body of the
stabiliser and having ramps or cams which move a
stabiliser bar radially outward. A long, and strong,
spring returns the stabiliser to a deactivated position
when the fluid pressure is released.
In the same patent a throttle member increases the
pressure drop across the tool, serving both to accelerate
movement of the mandrel for actuation of the tool and to
signal to the surface the state of actuation of the tool.
It is an object, at least in one aspect of the
present invention, to provide a down-hole tool which is
relatively short and does not suffer the disadvantages of
the prior art, or at least mitigates their effects.
In another aspect, it is an object of the present
invention to provide a down hole tool which minimises
mechanical contact between components in order to reduce
opportunity for jamming,~as well as wear.
In accordance with the present invention there is
provided a down-hole tool comprising:
a body having a through-bore;
a mandrel, being axially slidable in the body bore to
actuate and de-actuate the tool; and
a valve to control hydraulically the movement of the
mandrel by drilling mud pumped under pressure along said
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body bore.
Preferably said valve controls the drilling mud to
drive the mandrel hydraulically both to actuate and de-
actuate the tool. Thus, by hydraulically driving the
mandrel in both directions, the need for a strong return
spring is avoided.
Preferably, the mandrel has a through bore and the
valve comprises a control piston slidable in the mandrel
bore, against the force of a return spring, by drilling
mud pressure from a low-pressure position to a pressure
position.
Said pressure position may alternately be one of an
actuate position and a de-actuate position axially spaced
along the mandrel bore from said actuate position, the
tool being actuated by mud pressure when the piston is in
said actuate position and de-actuated when in said de-
actuate position. The actuate position may be between
the de-actuate and low-pressure positions of the piston.
A return step is preferably formed in the body and
mandrel to define annular chambers between them on either
side of the return step, one chamber, when pressurised
with mud, serving to actuate the tool while the other
serves to de-actuate the tool.
Said control piston may have an axially disposed
passage and a seal against the mandrel at both ends of
the passage, the mandrel having two ports communicating
each of said annular chambers with the mandrel bore and
an intermediate port venting said passage, the piston in
the actuate position connecting one chamber with the
passage and the other chamber with the piston bore beyond
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the seals, and vice-versa in said de-actuate position.
Preferably, the mandrel bore and piston are stepped,
the annular piston chamber formed by said step between
them being vented so that pressure of drilling mud in the
body moves the piston along the mandrel to close said
piston chamber.
The piston and mandrel between them preferably
define a barrel cam so that the piston rotates on axial
movement thereof relative to the mandrel, the cam
permitting different strokes of the piston in dependence
upon the angular position of the piston in the mandrel.
The piston and mandrel may have inter-digitating
castellations which, when they oppose one another in a
first angular position of the piston with respect to the
mandrel, as determined by the barrel cam, permit the
piston to move to one of said actuate and de-actuate
positions and, when they inter-digitate, permit the
piston to move to the other of said actuate and de-
actuate positions.
The barrel cam may comprise a pin in a track and the
track is arranged so that rotation of the piston with
respect to the mandrel is complete before the
castellations engage one another. The track may be on
the piston and the pin on the mandrel.
The track is preferably so arranged that the
castellations abut in either the actuate or de-actuate
positions and transmit axial hydraulic forces between the
piston and mandrel before the pin reaches the end of the
track.
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Preferably, the return step is inward of the body
and comprises two rings interconnected and captivating
between them ring sectors received in an annular groove
in the body.
5
A passage through the return step may be vented and
communicate with said intermediate port of the mandrel,
the mandrel being sealed to the return step on either
side of said passage and intermediate port.
The diameter of the chambers are preferably
different, the chamber serving to actuate the tool when
pressurised having the larger diameter.
Additionally, or alternatively, the diameter of the
mandrel in the body on the sides of the chambers remote
from the return step is larger on the side where
hydraulic pressure moves the mandrel to actuate the tool.
Both these differences serve to increase the force
with which the tool is actuated which, in the case of a
stabiliser, may be necessary if the drill string is not
already central in the hole being drilled.
A choke may be activated when the tool is actuated,
such activation to change the pressure drop of the
drilling mud across the tool so as to signal the states
of actuation of the tool. The piston may carry a piston
restrictor plate across the piston bore in face to face
contact with a mandrel restrictor plate, the restrictor
plates being angularly fixed with respect to the piston
and mandrel respectively and restricting mud flow through
the tool in dependence upon their relative angular
position.
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Preferably, the mandrel restrictor plate is
angularly fixed with respect to the body, the body being
angularly fixed with respect of the mandrel.
Preferably, the restrictor plates have a central
aperture in register with one another and alternating
sector spaces and sector lobes so that, when the lobes on
the piston and mandrel plates are in register with one
another, mud flows through both the central aperture and
spaces, and when the lobes and spaces are in register,
mud flows through the central aperture.
In a different aspect of the present invention, said
mandrel is moved to actuate the tool by hydraulic
pressure of said drilling mud when said valve permits
bleeding of a bleed chamber. Preferably, in this event,
the mandrel is moved to de-actuate the tool, on release
of said hydraulic pressure, by a mandrel return spring.
Said bleed chamber may be formed by a step between
the mandrel and body.
Said piston may serve to open and close a port
between said bleed chamber and the body bore.
Preferably, said piston, when it moves from said low-
pressure position to said pressure position, only opens
said port when it moves to said actuate position.
Preferably, said body defines, with the ends of said
piston and mandrel, a valve chamber, said choke
comprising a path between said piston and body which is
opened when said piston moves to said actuate position
and the mandrel moves to actuate the tool, and which is
restricted when the piston moves to said de-actuate
position.
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In one application of the present invention, the
tool is a stabiliser and comprises members radially
disposed in the body and pressed outwardly during
actuation of the tool to increase the effective diameter
of the stabiliser.
Indeed, the invention provides a drill string
comprising a drill bit, a near-bit stabiliser as defined
above.
The invention is further described hereinafter, by
way of example, with reference to the attached drawings,
in which:
Figures la and lb are a longitudinal section when
joined end to end along lines X-X in each drawing through
a stabiliser in accordance with the present invention;
Figure lc is am end view of a restrictor plate;
Figures 2a to Of are longitudinal sections through
the stabiliser of Figure 1 in different states of
actuation;
Figure 3 is a schematic diagram of the tool
actuating arrangement of a tool in accordance with the
present invention;
Figure 4 is a schematic diagram of an alternative
arrangement;
Figure 5 is a schematic illustration similar to
Figures 3 and 4 of a further preferred embodiment
corresponding with the arrangement shown in Figures 1 and
2 above;
Figure 6a is a side profile of the barrel cam track
employed on a control piston in accordance with the
present invention, Figure 6b comprising an enlarged
section through the control piston;
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Figure 7 is an illustration of a down-hole drill
string; and
Figures 8 a and b are sections through an
alternative arrangement of present invention.
Referring to Figures la and lb, a drill string
stabiliser 10 comprises a body 12 in two parts 12a, 12b.
A mandrel 14 is slidable in bore 16 of the body 12. The
mandrel likewise comprises two parts 14a, 14b. A control
piston 18 is slidable in a bore 20 of the mandrel 14.
The body 12 has an enlarged stabiliser bars 22
comprising spirally formed bars in which pistons 24 are
disposed in radially directed bores 26 through the wall
of the body 12. Springs 28 acting on cross pins 30
(fixed in the pistons 24) and studs 32 (fixed in the
stabiliser bars 22), press the piston 24 radially
inwardly against wedge surfaces 34 formed on the mandrel
14. When the mandrel 14 moves leftwardly in Figure 1,
the pistons 24 are pressed radially outwardly to increase
the effective diameter of the stabiliser 10. The angular
position of the mandrel 14 is fixed by a pin 36 in the
body 12 engaging a slot 38 in the side of the mandrel 14.
A return spring 40 is disposed in the bore 20 of the
mandrel 14 and bears on a shoulder 42 of the mandrel at
one end and, through a thrust-bearing 44, on the piston
18.
The piston 18 has its own through-bore 46 so that a
clear passage extends from an upstream end 10a to a
downstream end 10b of the tool 10. The piston 18 is
sealed to the bore 20 of the mandrel 14 through ring
seals 50, 52 which, it will be noted, are of different
diameters consequently, since piston chamber 54 is vented
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(as explained further below), any increase in'hydraulic
pressure in the bore 46 will result in leftward movement
of the piston as shown in Figure 1.
Piston chamber 54 communicates with intermediate
port 60 of mandrel 14, which in turn communicates with
passage 62 in step ring 64, and then with aperture 66 in
ring sectors 68 and finally vent port 70 in the wall of
the body 12.
Thus, as is well known in the art, drilling mud
pumped under pressure down the drill string and through
the stabiliser 10. It returns under reduced pressure
around the outside of the drill string and stabiliser 10.
Consequently, when the drill string is pressurised with
drilling mud the piston 18 moves leftwardly in the
drawing compressing the spring 40. A barrel cam 72 is
formed on one end of the piston 18, the mandrel 14 being
provided with pins 74 whose ends engage the barrel cam
72.
Turning to Figure 6a, the barrel cam 72 is shown
having a cam track 76. The arrows in the drawing show
the movement of the pins 74 as the piston moves backwards
and forwards in the axial direction. If the pin 74
starts in the position Z as shown in Figure 6a, then as
the barrel cam moves downwardly in the drawing, the pin
will impact the side of track 76 at point A, whereupon
further axial movement of the piston will result in
rotation of the piston until point B is reached. The
piston can continue axial movement until the pin reaches
point C. When the mud pressure is reduced, the spring 40
returns the piston, which moves axially until the pin 74
it impacts the wall of the track 76 at D, whereupon the
piston rotates in the same direction as when moving from
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A to B until it reaches point E. The next time the mud
pressure is increased again, the piston will move axially
until the pin 74 strikes the wall 76 of the track at
point F, where again the piston will be turned to rotate
5 in the same direction and until the pin reaches point G.
On the next cycle, when the mud pressure is again
reduced, the pin strikes the track 76 at H before turning
the piston once more until the pin 74 reaches point I
which is equivalent to the start position.
The piston 18 is provided with castellations 78a on
an external surface thereof and which castellations match
internal castellations 78b in the mandrel 14. Indeed,
the castellations 78b may be provided on a separate
element 80 bolted to the base of a step 82 in the mandrel
14, which step 82, in fact, defines the piston chamber
54.
The track 72 is so arranged in relation to the
castellations 78a on the piston 18, and the castellations
78b in the mandrel 18 are so arranged in relation to the
pins 74, that, when the pin reaches position B in the
track 76, the castellations 78a, 78b inter-digitate, as
shown in Figure 6a. This means that the piston can move
down the bore 20 of the mandrel until chamfered edge 84
of the castellations 78a contact and about chamfered base
86 of the castellations element 80. Indeed, the track 76
is so arranged that contact between the castellations
occurs before the pin contacts the end of the track 76 at
C, so that when further load is imposed between the
piston 18 and mandrel 14, it is transmitted through the
more substantial abutments between castellation surfaces
84, 86 than through the pin 74 and track 76. On the
other hand, when the pin is in position G, the
castellations 78a, 78b face one another, so that the
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piston can only advance until chamfered edge 84 abuts
chamfered face 88 of the castellations 78b. Likewise,
the castellations 78a, 78b abut one another before the
pin 76 impacts'the base of the track 76 at G.
There are preferably three castellations 78a, 78b
around the circumference of the piston and mandrel
respectively, and likewise three repetitions of the cycle
Z to I described above, so that a complete cycle
represents a rotation of the piston in the mandrel of
120 , and a difference between inter-digitation and mutual
opposition of the castellations 78a, 78b of 60 .
The internal return step 64 of the body 12 is
provided in the bore 16, of the body 12 by two rings 90, 92
bolted together by evenly spaced bolts 94 around the
peripheries of the rings 90, 92. An internal grove 96 is
formed in the body 12 and three ring sectors 98, each of
about 120 of arc, are captivated in the grove 96 by
clamping together the step rings 90, 92. Shims 100 can be
inserted on either side of the ring sectors 98 in order to
adjust the axial position of the step 64 in the bore 16 of
the body 12.
The step 82 in the mandrel 14 creates a first annular
chamber 102. After assembly of the piston 18 in the
mandrel part 14b and insertion thereof in the bore 16, and
after fixing of the step rings 64 in the bore 16, the
second part 14a of the mandrel is connected to the first
part 14b. This is effected by ring sectors 106 and pins
108 retained in engagement with inset holes 110 in the
surface of the mandrel 14 by ring 112 retained on flange
14c of the mandrel part 14a by means not shown.
Mandrel part 14a defines with the step 64 a second
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annular chamber 104. The step 82 is-a return step because
the chambers 102, 104 oppose one another.
The mandrel 14 is provided with a first port 120
which communicates the bore 20 of the mandrel with the
first annular chamber 102. The mandrel 14 has a second
port 130 which communicates the second annular chamber 104
with the bore 20 of the mandrel.
However, with respect to the first chamber 102, the
port 120 opens into the piston annular chamber 54 between
the seals 50, 52 on the piston 18. Therefore, chamber 102
is isolated from the bore 46 of the piston 18 and the
pressure of the drilling mud therein. In fact, by virtue
of intermediate port 60 in the mandrel 14, which is vented
to the outside through passages 62, 66, 70, (and isolated
by seals 64) the annular chamber 102 is likewise vented to
the outside. On the other hand, chamber 104 is in
communication with the drilling mud under pressure in bore
46 of the piston 18 by virtue of the second port 130 and a
number of slots 122 in the piston 18.
Thus, from the position shown in Figure la, when the
pressure of the drilling mud increases, the pressure in
chamber 104 rises and begins to urge the mandrel 14
leftwardly in the drawing. The mandrel 14 is sealed at
both ends to the bore 16 of the body 12 by seals 132, 135.
The diameter of the bore 16 in body part 12b is slightly
greater than the diameter of the mandrel in body part 12a.
Therefore, there is net leftward force on the mandrel 14
which moves in that direction since the annular space 134
formed by the step between the body parts 12a, 12b is
vented by radial outward movement of the piston members
24. At the same time the piston 18 also moves leftwardly
with respect to the mandrel, and if the piston is in such
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a position that the castellations 78a. 78b oppose one
another and abut through chamfered faces 84, 88, this
leftward movement of the mandrel persists. In that event,
the inclined surfaces 34 of the mandrel 14 press the
piston members 24 radially outwardly until ring 112 abuts
the end of the body part 12a. Indeed, the final diameter
of the stabiliser 10 when actuated is determined by the
axial extent of permitted movement of the mandrel 14, and
this can be controlled by shimming out the ring 112.
However, if the castellations 78a, 78b are in a de-
actuate position in which they inter-digitate, then the
piston 18 continues leftward movement, and in this event
two hydraulic switches occur. The first is that the seal
50 passes the first port 120 so that instead of
communicating the first annular chamber 102 with a vent
through intermediate port 60, the annular chamber 102 is
connected to mud pressure behind the piston 18. Secondly,
the seal 52 at the other end of the piston passes the
second port 130 in the mandrel 14, so that, instead of the
second annular chamber 104 being connected to mud pressure
inside the bore of the piston 18, that chamber is instead
put in communication with the intermediate port 60 and,
thereby, the vent 62, 66, 70 to outside. There is,
therefore, a reversal of the hydraulic forces acting on
the mandrel 14 and it moves to the position shown in
Figure la where the pistons 24 are fully retracted and the
stabiliser 10 is de-actuated.
Figures 2a to 2f show the sequence of cycling. In
Figure 2a the position is as shown in Figures la and lb.
In Figure 2b fluid pressure has moved the piston
rightwardly in the drawing until the castellations 78a,
78b abut one another. In this position, the first chamber
102 is vented while second chamber 104 is connected to
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higher pressure. Therefore, the mandrel 14 moves
rightwardly in the drawing to the position shown in Figure
2C. Here, second annular chamber 104 is fully developed
and first annular chamber 102 is now closed. Moreover,
the pistons 24 are now radially extended. In Figure 2D
fluid pressure in the drill string has been switched off
so that spring 40 returns piston 18 to its position in the
mandrel it has in Figure 2a. However, because the spring
40 is acting between the piston and mandrel, the mandrel
does not move in the body 12.
In Figure 2e, fluid pressure in the drill string is
once again reinstated and accordingly. the piston 18 moves
rightwardly in the drawing and this time the castellations
78a, 78b inter-digitate so that the piston 18 moves
further rightwardly in the mandrel than it did in the
previous half-cycle (as shown in Figure 2C). There is
therefore the reversal mentioned above in that the second
annular chamber 104 is now vented and the first annular
chamber 102 is connected to fluid pressure. In this
event, the mandrel 18 moves leftwardly in the drawing to
the position shown in Figure 1 where the pistons 24 are
fully withdrawn and the stabiliser 10 has a minimum
diameter. The spring 40 is nevertheless fully compressed.
Returning to Figure 1A, a piston restrictor plate 150
is fitted in the mouth of the bore 46 of the piston 18. A
sleeve 153 is a sliding fit, without rotation, in bore 11
of the body 12. In the end of the sleeve 153 facing the
piston restrictor plate 150 is a mandrel restrictor plate
152. Figure lc is an end view of a restrictor plate which
is circular but has three 60 open sectors 154. Both
restrictor plates 150, 152 have identical profiles so that
when they are perfectly aligned, a central bore 156 is
open, as well as the 60 sectors 154. However, by rotating
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the piston through 60 with respect to the mandrel, and
hence the body 12 and mandrel restrictor plate 152, open
segments 154 of the restrictor plate coincide with closed
segments 158 of the other restrictor plate 152.
5 Consequently, in this arrangement, the only passage
through the restrictor plates 150, 152 is the central
opening 156. There is therefore a marked pressure
difference across the restrictor plates which is
detectable at the surface. Since an increased pressure
10 difference increases the leftward forces on the piston 18,
and hence on the mandrel 14, the restrictor plates 150,
152 are arranged so that they are out of phase with one
another (ie when only the passage 156 exists through them)
when the piston castellations 84 abut the tips of the
15 mandrel castellations 88 and in which the mandrel is urged
leftwardly to its actuated position.
Figure 3 is a schematic representation showing the
principle of operation of the tool shown in Figures 1 and
2. Phantom line 200 is the centre line of the down-hole
tool 10'. Body 12 is provided with vent aperture 62
extending through return step 64. Mandrel 14 receives
piston 18 and has first and second ports 120, 130. First
and second annular chambers 102, 104 are here labelled a,
b. Intermediate port 60 communicates vent port 62 with
passage 54 between piston 18 and mandrel 14. The piston
is shown into axial position 18a, 18b. In position 18a
annular chamber 104(b) is vented to atmosphere through
second port 130, passage 54, intermediate port 60 and vent
62, while first annular chamber 102(a) is connected to
main pump pressure (P1) through first port 120.
In Figure 3, the area of the mandrel 14 under the
step 64 is A2, which is dependent on the diameter of the
step 64. Likewise, the areas Al of the mandrel under the
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chambers 102, 104 is determined by the diameter of those
chambers. In Figure 3, the chambers have the same
diameter. Thus, the forces acting on the mandrel 14 when
the piston 18 is in the position 18a is given by
Fa = P1 (A1 - A2)
On the other hand, when the piston is in position
18b, then the situation is reversed and it is second
annular chamber 104(b) which is connected to high
pressure, whereas first annular chamber 102 is vented.
Thus, the force (Fb) acting on the mandrel 14 is given by
Fb = -P1 (A1 - A2)
Thus, the value of the force on the mandrel 14 is the
same in both positions of the piston 18, except that it is
reversed in direction.
Figure 4 shows an alternative arrangement in which
'the step 64' is formed as part of the mandrel 14' so that
the chambers 102', 104' are in a recess of the body 12'
rather than in a recess of the mandrel 14'. While this
creates different issues of construction, the operation is
in principle identical with that described above in
relation to Figure 3.
Figure 5 illustrates a preferred arrangement in which
the forces acting in the direction of actuation (arrow F
in Figure 5) is greater than in the reverse. In Figure 5,
F. = P1 (A1 - A3 + A2 - A4) , whereas
Fb = - P1 (A3 - A4) , where
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Fa and Fb are the forces acting in the direction of
the arrow F when chambers a and b are respectively
pressurised with mud pressure P1. In this scenario Fa is
in the direction of the arrow F because (A1 + A2) is
greater than (A3 + A4), whereas Fb is in the opposition
direction because A3 is greater than A4. However, the
value of Fa is much larger than the value of Fb, which is
desirable because the potential force required to push the
pistons radially outward is much larger than that
potentially required to release them.
Figure 7 is a schematic diagram of a drill string 200
in a well bore 210, the drill string terminating in a
drill bit 220 driven by a down-hole motor 230 which is
spaced from the drill bit by a near-bit stabiliser 240. A
remote stabiliser 250 is spaced some distance from the
motor 230. If the stabiliser 240 is de-actuated, then the
weight of the motor 230 and drill bit 220 tends to drop
the drill string vertically so that the drill tends to
vertical. On the other hand, if the stabiliser 240, is
actuated, then the drill string tends to follow a straight
line.
Finally, Figures 8 a, b show an alternative
arrangement, being a half longitudinal section through a
tool 10'. Here actuation of the tool 10' is effected by
movement of the mandrel 14', which slides in a stepped
body bore 16' of the body 12'.
Figure 8b shows a de-actuated position of the piston
18' relative to the mandrel 14'. The piston 18' is
positioned between a body cup 12b forming an annular valve
chamber 151. One or several apertures 152 connect the
body bore up-stream (16a) and down-stream (16b) of the
body cup 12b. When hydraulic pressure of drilling mud in
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the bore 16a rises, piston 18 is pressed leftwardly in the
drawing against the force of return spring 40' to the
position shown in Figure 8b. However, here, a sealing
ring 153 has not passed over a bleed port 155 in the wall
of the mandrel 14'. Therefore a bleed chamber 157 cannot
be vented, it being sealed at its ends by seal rings 132'
and 135', and possibly intermediate seal ring 160.
Piston 18' is prevented from moving further than
shown in Figure 8b by a barrel cam arrangement 72' similar
to that described above. When the hydraulic pressure is
lowered sufficient to permit the return spring 40' to urge
the piston 18' to its low pressure position (not shown)
then it rotates, as described above. Thus, when the
hydraulic pressure again rises, the piston moves on the
mandrel 14', under hydraulic action, to the position shown
in Figure 8a.
In this position, port 155 is exposed, so that
hydraulic pressure urges the mandrel leftwardly in the
drawing and pressurises bleed chamber 157. The fluid in it
escapes into valve chamber 151 and permits the mandrel to
move to the position in the body 12' shown in Figure 8a.
Here, the piston members 24 are pressed outwardly.
Furthermore, the piston 18' clears internal edge 162
of the body cup 12b so that the fluid flow passage formed
between the two is substantially enlarged and so that the
pressure drop across the arrangement is substantially
reduced. Such reduction in pressure drop, and maintenance
of a high pressure drop in the case of the de-actuated
position in Figure 8b, informs the user of the state of
actuation of the tool 10'.
Nevertheless, while mud pressure remains high, the
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pressure drop across throat 162 is sufficient to keep the
piston in the position shown. However, when the pressure
drops, the piston moves rightwardly in the drawings. The
mandrel likewise moves rightwardly, driven by a mandrel
return spring 164. However, it may be possible for the
piston 18' to cover the port 155 before the mandrel has
moved all the way to the de-actuated position of Figure
8b. Consequently a non-return valve 166 may be provided
in the end of the mandrel 14' to permit mud to enter the
bleed chamber 157.
While this arrangement employs a mandrel return
spring and is therefore necessarily longer than the
previous embodiment, nevertheless it removes the necessity
of employing mechanical detent means which must be shifted
between the mandrel and body to permit and restrain the
movement of the mandrel.
