The invention relates to jar mechanisms and, in particular, a jar
mechanism for an upstroke jar.
Wireline is a method of lowering specialised equipment into an oil
or gas well, or raising specialised equipment from an oil or gas well. The principle
of wireline is to attach a workstring or toolstring to the end of a reel of wire
and by reeling out the wire the toolstring is lowered into the well. By either
reeling in or reeling out the wire, the toolstring can be made to perform simple
The toolstring consists of a variable combination of individual tools
screwed together to form a working unit. A toolstring typically comprises a rope
socket, a stem or sinker bar, an upstroke jar, a spang jar and a pulling and running
tool. The rope socket attaches the toolstring to the wire. The stem or sinker bar
comprises lengths of bars screwed together to give the toolstring weight and also
provides a mass for jarring operations. The upstroke jar is a mechanical or hydraulically
operated device which allows a delayed partial release of the upper portion of
the toolstring to enable an upwardly directed high impact force to be generated
by the toolstring. The sprang jar is a very simple device and similar to an elongated
chain link and allows immediate movement of the upper portion of the toolstring.
Spang jars are used to allow upward or downward impact forces to be generated by
the toolstring. At the end of the toolstring there are pulling tools and running
tools. These various tools are available to be screwed directly to the end of the
toolstring. These are used for either pulling equipment out of the well or setting
and leaving equipment in the well.
Conventionally, there are two distinct types of upstroke jar available
on the market. The first is a hydraulic jar and the second is a mechanical or spring
jar. Both types of jar have different attributes and disadvantages.
The hydraulic jar is activated only when the bottom end of the jar
is anchored and the top end is subjected to a constant pulling force. For simplicity
the jar can be regarded as being a piston located in a cylinder which is filled
with hydraulic oil. The piston, commonly known as the jar rod, is normally at the
bottom end of its stroke within the cylinder, where the two are close fitting.
Very limited fluid by-pass around the piston means that it takes considerable
force and time to move the piston up the cylinder. The time factor allows a desired
pull force to be reached before the piston reaches the point where the internal
diameter of the cylinder opens out. When the piston reaches the opened out portion
of the cylinder, the pulling force accelerates the piston to the top of its stroke
where it will deliver an impact force upwardly when it is stopped by the jar housing
itself. The piston usually contains a small check valve to enable a fast return
stroke into the small internal diameter portion of the cylinder by allowing greater
fluid by-pass in that direction only.
The advantages of hydraulic jars are that they are very versatile
in use because a small pulling force will result in a small jar force and similarly
a large pulling force will result in a large jar force. In addition, there is
no need to remove these jars from the toolstring to adjust the release setting,
as is necessary with mechanical jars. Hydraulic jars will also fire whatever the
value of the pulling force that is used or is available.
However, hydraulic jars still have a number of disadvantages. As
there is a seal around the jar rod itself, the ability of the jar to function depends
on the life time of this seal. This seal is subjected to considerable wear and
tear due to the violent motion of the jar rod. To ensure relocation of the piston
back into the lower reduced internal diameter the jar rod is usually fairly short
and this compromises the resulting jarring force available. Also, the whole tool
is full of hydraulic oil which makes maintenance of the tool difficult.
Mechanical jars contain no hydraulic oil. The jar therefore has no
seals. Again the jar can be regarded as a piston within a cylinder however this
time the piston is held at the bottom end of its stroke by various mechanical
mechanisms which are usually dependent on the manufacturers. Usually the mechanism
comprises a coil spring or spring washer stack arrangement as part of the mechanism.
The spring is used to pull against to allow the piston to be released and travel
up its full stroke within the main housing of the jar when a certain known pull
force is reached. This value is usually dependent on the spring rate.
The advantages of the mechanical jar are that there is no seal around
the jar rod and there is an unhindered travel of the jar rod up to its full stroke,
ie. there is no hydraulic oil to be by-passed. It is also possible to obtain a
larger jar rod stroke than can be achieved with a hydraulic jar.
However, there are also a number of disadvantages associated with
mechanical jars. Mechanical jars must be removed from the toolstring in order to
be adjusted to the desired pull force for activation downhole and the pull force
at which the jar is set to fire must be applied to the jar before the jar will
work. This value is often difficult to predict especially when it is used deep
downhole. There is also a difficulty in maintenance due to the large number of
parts which comprise the jar.
Coil tubing operations are similar to wireline operation and also
use jar mechanisms to enable high impact forces to be generated by the toolstring
during the coil tubing operation. However, with coil tubing operations there is
the additional complexity that it is desirable to pump fluid through the toolstring
during the operations, and this feature has been difficult to combine with conventional
In accordance with the present invention a jar mechanism comprises
a piston movably mounted in a fluid chamber; a jar member releasably coupled to
the piston by a release device such that when the piston is in a first position
in the fluid chamber, the jar member is coupled to the piston by the release device
for movement therewith and whereby a first force exerted on the jar member moves
the piston to a second position within the fluid chamber against the resistance
of the fluid, whereby the action of the first force applied to the jar member
actuates the release device to enable the jar member to be uncoupled from the piston.
The invention mitigates the problems of the prior art jar mechanisms
by combining a jar member which is releasably secured to a piston with a fluid
chamber in which the piston is mounted so that the jar member is not in contact
with the fluid.
Preferably, the piston includes a one way valve which closes and
prevents fluid flow through the piston when the piston moves from the first to
the second position, but which opens and allows fluid to flow through the piston
when the piston moves from the second to the first position. In one example, the
one way valve comprises a chamber which communicates with the fluid on either
side of the piston and inside the chamber is located a spherical member such as
a ballbearing which prevents fluid passing through the chamber when the piston
moves from the first position to the second position, but which permits fluid to
pass through the chamber when the piston moves from the second position to the
In another example, the one way valve comprises at least one fluid
channel which communicates with the fluid on one side of the piston and an annular
sleeve movable between a first position, in which the at least one fluid channel
is prevented from communicating with the other side of the piston, and a second
position in which the at least one fluid channel communicates with the other side
of the piston, to enable fluid to pass from the one side of the piston to the other
side of the piston. Typically, the external surface of the annular sleeve forms
the surface of the piston which abuts against the surface of the fluid chamber.
Preferably, in the second example of the one way valve there are
four fluid channels.
Preferably, the release device is movably mounted on the piston for
movement between an engagement position and a release position and the release
device is typically biased to an intermediate position, between the engagement
and the release positions, and whereby the jar member may be uncoupled from the
piston when the release device is in the release position and the piston is in
the second position and whereby the jar member may be recoupled to the piston when
the release device is in the engagement position and the piston is in the first
Typically, the release device moves to the intermediate position
when the jar member is uncoupled from the piston.
Typically, the release device is biased to the intermediate position
by a spring, such as a helical spring.
Preferably, when a force opposite to the first force is applied to
the jar member, the jar member causes the release device to move to the engagement
position and the piston is moved from the second to the first position so that
the release device couples the piston to the jar member.
Alternatively, the jar mechanism may comprise means to retain the
piston in the second position when the jar member is uncoupled from the piston.
In this example the means to maintain the piston in the second position comprises
a biasing means such as a helical spring.
Typically, the jar mechanism may be used as a wireline jar for wireline
operations, or as a pump through jar for coil tubing operations in a borehole.
An example of a jar mechanism in accordance with the invention will
now be described with reference to the accompanying drawings, in which:-
- Fig. 1 is a cross sectional view of a first example of an upstroke jar showing
the jar mechanism in the primed position;
- Fig. 2 is a cross sectional view similar to Fig. 1 but with the mechanism at
the point at which the jar rod is released;
- Fig. 3 is a cross sectional view similar to Figs. 1 and 2 but with the mechanism
at an intermediate position during re-priming of the mechanism;
- Fig. 4 is a cross sectional view of a second example of an upstroke jar;
- Fig. 5 is a cross sectional view along the line A-A in Fig. 4;
- Fig. 6 shows a first portion of a piston for use in the upstroke jar shown
in Fig. 4;
- Fig. 7 shows a second portion of a piston for use in the upstroke jar shown
in Fig. 4; and,
- Figs. 8A to 8D are schematic diagrams showing the sequence of positions for
a release device for use in the upstroke jar shown in Fig. 4.
Fig. 1 shows an upstroke jar 1 for use in wireline operations which
comprises a jar rod 2 which is releasably secured via a latch key 3 and a latch
sub 4 to a piston 5. The piston 5 comprises a piston top 6 and a piston body 7
and the latch sub 4 is secured to the piston top 6 by means of a roll pin 8.
The piston top 6 and the piston body 7 are secured together within
a fluid chamber 9 located in a piston housing 10. The fluid chamber 9 contains
a fluid 11, such as hydraulic oil although any other suitable gas or liquid could
be used. The piston 5 comprises a chamber 12 within which is located a one way
valve which comprises a helical spring 13 and a ball 14. Fluid may enter into
the chamber 12 via the two passage ways 15, 16 on either side of the piston 5 and
which communicate with the fluid chamber 9 and the internal piston chamber 12.
There are also three O ring seals 17, 18, 19 which prevent leakage
of the fluid 11 from the fluid chamber 9. The O ring seal 17 prevents leakage of
fluid 11 between the shaft of the body portion 7 of the piston 5 and a bottom
sub 20. The O ring seal 18 prevents leakage of the fluid 11 between the piston
housing 10 and the shaft of the piston top 6 and the O-ring seal 19 prevents leakage
of the fluid 11 from between the piston housing 10 and the bottom sub 20.
A bleed screw 21 is located in the piston housing 10 and this is
used to prevent an air lock forming in the fluid chamber 11 when the jar 1 is being
assembled. The jar 1 also comprises a main body housing 22 which is attached to
the piston housing 10 by means of a locking screw 23. The bottom sub 20 is connected
to the piston housing 10 by a locking screw 24.
When an upward jarring force is to be exerted by the jar 1 the jar
rod 2 is pulled in the direction shown by the arrow 25 in Fig. 1. The pulling force
exerted on the jar rod 2 is transmitted to the piston via the latch key 3 and
the latch sub 4 so that the piston 5 is moved through the fluid chamber 9 against
the resistance of the fluid 11. This is achieved by the restricted by-pass of
the fluid 11 between the piston body 7 and the piston housing 10 and by preventing
fluid passing through the passage ways 15, 16 and chamber 12 in the piston 5 by
blocking the passage way 15 with the ball 14.
As the movement of the piston 5 and the jar rod 2 is slow due to
restricted fluid by-pass, time is available to pull up to a desired pull force
before the piston 5 reaches the other end of the fluid chamber 9. This position
is shown in Fig. 2 and at this point continuation of the pulling force in the direction
of the arrow 25 on the jar rod 2 forces the latch key 3 out of engagement with
the jar rod 2 and into engagement with the main housing 22 so that the jar rod
2 is released from the piston 5 and rapidly accelerates in the direction of the
arrow 25 until it hits the top end (not shown) of the main body housing 22. When
this occurs an upward jarring force is exerted on the toolstring to which the
jar 1 is attached.
After the jarring force has been produced the jar rod 2 is returned
to the latch sub 4 by application of a downward force to the jar rod 2 in the direction
shown by the arrow 27 in Fig. 3. The latch sub 4, the latch key 3 and the piston
5 are maintained in the release position by means of helical spring 26 which enables
the jar rod 2 to be inserted back into the latch sub 4.
Continued application of the downward force in the direction of the
arrow 27, as shown in Fig. 3, forces the latch key 3 to re-engage with the jar
rod 2 and forces the piston 5 to return to the primed position against the action
of the helical spring 26.
When the piston 5 is being returned to the primed position the force
of the fluid entering into the passage way 15 in the piston body 7 forces the ball
14 against the action of the spring 13 into the middle of the chamber 12 so that
fluid may pass through the chamber 12 into the passage way 16 and into the chamber
9 on the other side of the piston 5. Hence, the ball 14 and spring 13 act as a
one way valve so that the resistance against movement of the piston is high when
the piston moves from the primed position to the unprimed position but is very
low when the piston moves from the unprimed position to the primed position. This
enables the piston 5 to be easily returned to the primed position after the jarring
force has been produced.
In addition, the seals 17, 18 are of the same dimensions and this
allows the pressure around the piston housing 10 to be balanced at all times.
By constructing the jar mechanism so that the jar rod 2 and the piston
5 are separable mitigates the disadvantages of conventional jar mechanisms by
locating the fluid 11 only in the vicinity of the piston and avoiding the need
for fluid seals around the jar rod 2. This configuration also avoids the disadvantages
of a mechanical jar as it is not necessary to remove the toolstring from the borehole
in order to adjust the jarring force. The jarring force exerted by the jar rod
2 is dependent on the force with which the jar rod and piston 5 are pulled from
the first position to the second position and therefore is only dependent on the
maximum pulling force available on site at the oilfield.
Fig. 4 shows an example of an upstroke jar for use in coil tubing
operations. The upstroke jar 50 works in a similar manner to the upstroke jar 1
and the parts of the upstroke jar 50 which are similar to the upstroke jar 1,
shown in Figs. 1 to 3 have the same reference numerals.
However, the upstroke jar 50 has a bore 65 through its entire length
which enables fluid to be pumped through the jar 50 so that the jar may be used
in coil tubing operations.
Another difference between the upstroke jar 50 and the upstroke jar
1 is the design of the piston 5. In the upstroke jar 50 the piston 5 comprises
two piston sections 36, 37 and a by-pass sleeve 42. The piston section 37 has
four channels 44 equidistantly spaced around the external surface of the portion
of the piston section 37 on which the by-pass sleeve 42 is located. The channels
44 are shown in more detail in Figs. 5 and 7.
The piston section 36 also has four channels 43 which coincide with
the channels 44 in the piston section 37. The channels 43 are shown in more detail
in Fig. 6. The threaded male portion 51 of the piston 36 is screwed into the threaded
female portion 52 of the piston section 37, so that the channels 43, 44 are coincident
with each other.
The upstroke jar 50 also has a modified release and re-engagement
mechanism 66 for connecting the piston 5 to the jar rod 2. In the upstroke jar
50 this mechanism 66, shown schematically in Fig. 8A, comprises a latch housing
64 within which is slidably mounted the piston section 36 which is attached to
a spring bush 38 and locked onto the retainer 38 by a locking screw 53. The latch
housing 64 is threaded onto a spring retainer 54 and held in position by a locking
screw 56. A helical spring 40 is located between the spring bush 38 and a shoulder
55 of the latch housing 4, and a second helical spring 39 is located on the other
side of the spring bush 38 between the spring bush 38 and the spring retainer
In use, when a force is applied to the jar rod 2 of the upstroke
jar 50 in the direction shown by the arrow 70, the pressure exerted by the jar
rod 2 is released from the latch housing 64. This allows the spring 40 to expand
and push the latch housing 64 and the spring retainer 54 to the position shown
in Fig. 8B while the piston 5 and the spring bush 38 remain static relative to
the main body housing 22 and the piston housing 10. As the jar rod 2 is pulled
further in the direction of the arrow 70, the spring 39 is compressed by the spring
retainer 54 and the piston 5 is pulled along the piston chamber 9. As the piston
5 is pulled along the piston chamber 9 the movable by-pass sleeve 42 is forced
against the shoulder 57 of the piston section 37 which prevents fluid in the piston
chamber 9 flowing through the channels 43, 44 to the other side of the piston.
Hence, a large force is required on the jar rod 2 to move the piston 5 from one
end to the other end of the piston chamber 9.
When the piston 5 reaches the other end of the piston chamber 9 the
latch housing 4 is in the position shown in Fig. 8C and the force exerted by the
jar rod 2 pushes the latch key 3 out of engagement with the jar rod 2 to enable
the jar rod 2 to be released from the housing 4. This causes the jar rod 2 to move
rapidly upwards to exert an upward impact force on the top of the upstroke jar
After the jar rod 2 has been released, the latch housing 4 moves
to the position shown in Fig. 8D due to the biasing action of the helical spring
39 against the spring retainer 34 and the latch keys 3 are within the reduced
diameter section 60 of the main housing 22.
To re-latch the jar rod 2 with the latch housing 4, the jar rod 2
is pushed against the latch key 3 and the latch housing 4, and this causes the
latch housing 4 and the spring retainer 54 to move relative to the piston 5 and
the spring bush 38 which causes the helical spring 40 to compress. This compression
exerts a force on the piston 5, via the spring bush 38 which urges the piston
5 towards its initial position. As the piston moves towards its initial position
the shoulder 67 on the piston section 36 abuts against the by-pass sleeve 42 so
that the channels 44, 43 communicate with both sides of the fluid chamber 9 and
fluid may pass from one side of the fluid chamber to the other side via the channel
44 and the channel 43. Hence, there is very little resistance to movement of the
piston 5 as it returns to its initial position. When the piston 5 returns to its
initial position the piston 5 and the spring bush 38 stop moving relative to the
piston housing 10 and the main body housing 22 but the force exerted by the jar
rod 2 on the latch housing 4 causes the latch housing 4 to move to the position
shown in Fig. 8A and the latch keys 3 move out of the reduced diameter section
60 of the main body housing 22 so that the jar rod 2 may re-engage with the latch
housing 4 and the latch keys 3, as shown in Fig. 4.
Both the piston assembly 5 and the engagement and release mechanism
as described above for the upward jar 50 could of course be used in an ordinary
wireline jar and in fact the wireline upstroke jar 1 performs better when the
release and engagement mechanism 66 is used instead of the simple release mechanism
shown in Figs. 1 to 3.
Modifications and improvements may be incorporated without departing
from the scope of the invention.