Hydraulic drill string jar

The present invention accordingly relates to hydraulic jars which include elongated telescopically arranged mandrel and housing members relatively movable between contracted and extended positions such members respectively having a hammer and an anvil with the anvil being spaced from the hammer when the members are in the contracted position and arranged for contract when the members are in the extended position. The jar includes hydraulic means for retarding movement of the mandrel relative to the housing member for a given time during a jarring stroke when the jar is tensioned. The hydraulic means includes a fluid-filled chamber having adjoining first and second bores, the first bore being of lesser diameter than the second bore and a piston on the mandrel member being receivable in the first bore with the piston having a slightly smaller diameter than the first bore to restrict the flow of fluid past the piston as it moves relative to the first bore during the jarring stroke. When the piston exits from the first bore, the members move rapidly to the extended position so that the hammer delivers a blow to the anvil. The piston is of annular form and is positioned around the mandrel member for movement therealong between first and second axially spaced stops on the mandrel member. The piston has a first end adapted to co-operate with the first stop to permit free-flow of the fluid between the piston and mandrel member as the members move toward the contracted position. The piston also has a further portion adapted to co-operate with the second stop to provide sealing engagement therewith during movement of the piston through the first bore during the jarring stroke.

BACKGROUND OF THE INVENTION 
The present invention relates in general to well drilling equipment and 
particularly to hydraulic jars for releasing drill strings or tools 
trapped in well bores. 
As is well known in the art, during the drilling of a well, the drill pipe 
or well tools sometimes become lodged or stuck in the well bore. To assist 
in the recovery of a stuck tool, a hydraulic jar is commonly included in 
the drill string. As is well known in the art, hydraulic jars operate on 
the principle of a hydraulically delayed longitudinally moving hammer 
operatively coupled to the drill pipe, the hammer being released after a 
time delay to strike an anvil operatively attached to the parts stuck in 
the well bore. To provide the longitudinal movement and the resulting 
impact forces, tension is applied to the drill string via the usual 
hoisting equipment. The initial relative longitudinal movement in the jar 
is retarded by a hydraulic mechanism to permit the desired tension to be 
applied. 
A typical hydraulic mechanism for providing the time delay in the jar 
comprises a piston moving in a bore to compress a hydraulic fluid in a 
chamber, the delayed rate of movement of the piston being controlled 
either by a fluid metering orifice bypassing the piston or alternatively 
by sizing the piston relative to the bore so that the fluid leaks 
therebetween at a selected rate. The jar is provided with a release bore 
of larger diameter than the compression bore to provide a relatively large 
fluid bypass and sudden release of the piston in the chamber. The 
compression of the hydraulic fluid produces large magnitude forces within 
the jar. The tension which can be applied to the jar is often limited by 
the capability of the jar notwithstanding the hydraulic forces produced 
therein. These forces sometimes develop a pressure of well in excess of 
40,000 lbs. per square inch. 
While metering orifices bypassing the piston as described above are well 
known and quite widely accepted, there is the danger of the metering 
orifice becoming partially or fully blocked by foreign matter during the 
course of operation thus impairing or disabling the jar. When a well tool 
is so stuck in the well bore that tension alone will not release it it is 
vital that a jar, if used, be in perfect working order. A jar that has 
failed is of utterly no use and it is probable that the stuck tool will be 
lost if the drilling string breaks under tension. 
The present invention is concerned with the second variety of hydraulic jar 
referred to above, i.e. one wherein the necessary time delay is provided 
by sizing the piston relative to the compression bore so that the fluid 
leaks therebetween at a selected rate during the jarring stroke. 
In order to provide for uniformity and consistency in the time delay in a 
jar of this nature it is of course well known that the outside diameter of 
the piston must be carefully selected in relation to the inside diameter 
of the compression bore of the jar. The effect on the time delay of wear 
on these surfaces and of any changes in surface shape which may occur in 
the piston as a result of the pressures encountered during use are quite 
well known and have been dealt with elsewhere. However, there is another 
vital area of concern and this comprises the surfaces of the inner mandrel 
and the piston which must come into sealing engagement with one another to 
prevent bypass of hydraulic fluid therebetween during the jarring stroke. 
It has been found that unless these surfaces are extremely accurately 
machined that some leakage therebetween will occur thus making it 
difficult to maintain a uniform release time from one jar to the next. In 
addition, during normal use, a certain amount of wear of the contacting 
surfaces occurs which, particularly when coupled with the presence of any 
foreign materials in the hydraulic fluid, causes wear and degeneration of 
the mating surfaces such that the time delay period will vary during 
normal usage. 
SUMMARY OF THE INVENTION 
It is accordingly a principal object of the present invention to provide 
improvements in hydraulic jars which are capable of alleviating the 
problem referred to above. 
The present invention accordingly relates to hydraulic jars which include 
elongated telescopically arranged mandrel and housing members relatively 
movable between contracted and extended positions such members 
respectively having a hammer and an anvil with the anvil being spaced from 
the hammer when the members are in the contracted position and arranged 
for contract when the members are in the extended position. The jar 
includes hydraulic means for retarding movement of the mandrel relative to 
the housing member for a given time during a jarring stroke when the jar 
is tensioned. The hydraulic means includes a fluid-filled chamber having 
adjoining first and second bores, the first bore being of lesser diameter 
than the second bore and a piston on the mandrel member being receivable 
in the first bore with the piston having a slightly smaller diameter than 
the first bore to restrict the flow of fluid past the piston as it moves 
relative to the first bore during the jarring stroke. When the piston 
exits from the first bore, the members move rapidly to the extended 
position so that the hammer delivers a blow to the anvil. The piston is of 
annular form and is positioned around the mandrel member for movement 
therealong between first and second axially spaced stops on the mandrel 
member. The piston has a first end adapted to co-operate with the first 
stop to permit free-flow of the fluid between the piston and mandrel 
member as the members move toward the contracted position. The piston also 
has a further portion adapted to co-operate with the second stop to 
provide sealing engagement therewith during movement of the piston through 
the first bore during the jarring stroke. 
In accordance with a principal feature of the invention, one of said 
further portion and said second stop includes a smooth annular wall 
portion which is sloped relative to the longitudinal axis of the jar while 
the other includes a smooth annular surface having a convexly arcuate 
contour as seen in a longitudinal section, with said sloped annular wall 
portion and said annular convexly arcuate surface being relatively 
disposed to come into said sealing engagement during the jarring stroke. 
As a further feature of the invention, said further portion of said piston 
is located interiorly of the annular piston intermediate said first end 
and the opposite end thereof. Preferably, said further portion of the 
piston is located nearer to said first end than it is to the opposite end 
of the piston. 
In the preferred form of the invention, the sloped wall portion comprises a 
frustro-conical wall portion, the latter being located on the annular 
piston while the annular convexly arcuate surface is located on the 
mandrel member. In a preferred embodiment of the invention the convexly 
arcuate surface has a semi-circular outline as seen in longitudinal 
section. 
In a typical embodiment of the invention, the mandrel member has a splined 
connection with the housing member thus permitting non-rotative relative 
movement between the extended and contracted positions. 
It has been found that hydraulic jars constructed in accordance with the 
principles of the present invention can exhibit substantially longer 
service lives than otherwise conventional designs with there being less 
change in the average release time over this service. At the same time, 
there is less variation in the release time from one hydraulic jar to the 
next assuming that they all have been produced utilizing the same 
manufacturing techniques and with the same machining and manufacturing 
tolerances.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference firstly to FIG. 1, reference numeral 10 designates the 
hydraulic jar, the latter, in turn, being constructed from two basic 
sub-assemblies namely a mandrel 12 and an outer housing 14. The mandrel 12 
is telescopingly arranged within housing 14 and is relatively movable with 
respect to the latter along the longitudinal axis of the jar between 
contracted and extended positions. 
The mandrel 12 is comprised of three main sections, namely an upper section 
12a, an intermediate section 12b, and a lower section 12c. These three 
sections are secured together by mutually engaging male and female screw 
thread portions which provide for ease of assembly and disassembly. The 
upper section of the mandrel 12 is provided with a connector head 20 
provided with a tapered internal thread 22 to provide for connection to an 
external thread on the pin end of a drill string. Thus, when placed in a 
drill hole, the connector head 20 would be the uppermost component of the 
hydraulic jar 10. 
The upper section 12a of the mandrel is also provided with a plurality of 
longitudinally directed splines 24 spaced around the circumference of 
same, the use of which will be discussed later. The intermediate mandrel 
section 12b is provided with an annular hammer 26 which will be discussed 
in further detail later on while the lower mandrel section 12c passes 
through, among other things, a hydraulic chamber provided adjacent the 
lower end of housing 14. The entire mandrel 12 includes a central 
longitudinally extending bore 30 extending therethrough for passage 
therealong of drilling fluid for purposes well known in the art. 
The housing 14 is also comprised of a plurality of sections 14a, 14b, 14c, 
and 14d. The uppermost housing section 14a includes a plurality of splines 
32 which interengage with splines 24 on the mandrel thereby permitting 
only non-rotative relative movement of the mandrel 12 relative to the 
housing 14 between extended and contracted positions. The interengaging 
splines of the housing and mandrel are shown in further detail in the 
cross-section view of FIG. 6. 
The next section of the housing, i.e. 14b, may be referred to as the anvil 
section in that it includes an annular step 34 formed in the inner wall of 
same, which step is adapted to cooperate with the annular hammer 26 formed 
on the adjacent mandrel section. When the mandrel 12 moves relative to 
housing 14 from the contracted position to the extended position, the 
hammer 26 is capable of delivering a heavy blow to anvil 34 for purposes 
of providing the desired jarring force. 
In order to prevent entry of contaminants into the interior of the jar, the 
upper end of housing 14 is provided with suitable sealing elements 36 
provided in a series of annular grooves provided in the inner wall of the 
housing. The seals may be of any suitable variety capable of withstanding 
the high fluid pressures involved, e.g. the well known "Parker" seals. In 
order to further guard against entry of contaminants into the housing 14, 
and particularly to guard against entry of such contaminants into the 
regions between the interengaging splines 24 and 32 of the mandrel and 
housing, the intermediate mandrel section 12b is provided with an annular 
compensating piston 38 (best seen in FIG. 2) which sealingly engages with 
both the interior wall of the housing and the exterior wall of the 
mandrel. This compensating piston is provided with both external and 
interior annular grooves which serve to retain therein suitable sealing 
rings 40 and 42. The housing 12 is provided adjacent the lower end of its 
intermediate section 12b with a plurality of radially directed ports 46 
which allow fluids in the well bore to enter into the casing behind the 
compensating piston 38. Thus, as mandrel 12 is extended relative to 
housing 14, the compensating piston moves along the interior of housing 14 
with drilling fluids entering via the ports 46. Thus the interior pressure 
is balanced with the exterior pressure at all times thus helping to ensure 
that the lubricants used to prevent wear of interengaging splines 24 and 
32 are not contaminated by foreign material such as well bore cuttings. 
Section 14c of the housing may be referred to as the hydraulic section 
since it is in this portion of the housing that the hydraulic means are 
provided which serve to retard the movement of the mandrel 12 relative to 
the housing 14 for a given time between a contracted and extended position 
during a jarring stroke when the jar is tensioned. Connected to section 
14c of the housing is the lowermost section 14d which is the connector 
section in that it includes internal tapered threads 50 adapted for 
connection to the threaded pin portion of a tool, such as a drill bit. 
The housing section 14c is provided with three adjoining bores 52, 54 and 
56. Bore 54 is of lesser diameter than bores 52 and 56. The entire chamber 
in which the three bores 52, 54 and 56 are located is filled with a 
working fluid such as a silicone-based fluid which exhibits small 
viscosity changes over a wide temperature range. 
An annular piston 60 is mounted on mandrel section 12c and is axially 
movable relative thereto between first and second stops 62 and 64 provided 
on adjoining mandrel sections 12b and 12c respectively. The annular piston 
60 has an outside diameter which is just slightly smaller than the 
diameter of bore 54 thereby to restrict the flow of the working fluid past 
the piston 60 as it is moved through the first bore 54 during a jarring 
stroke until piston 60 exits from bore 54 and enters into the larger bore 
52 whereupon the mandrel and housing move rapidly to the relatively 
extended position thus causing hammer 26 to deliver a heavy blow to anvil 
34. This overall action need not be described in great detail since it is 
generally well known in the art. 
During the return stroke it is desired that there be free flow of the 
working fluid between the piston 60 and the mandrel 12 as the mandrel is 
moved toward the contracted position relative to the housing. In order to 
achieve this, the upper end portion 66 of piston 60 is provided with a 
plurality of radially directed grooves 68 as best seen in FIG. 4, such 
grooves 68 preferably being of semi-circular cross-section as best seen in 
FIG. 5. Thus, during movement toward the contracted position, the stop 62 
buts against end portion 66 of piston 60 with the working fluid passing 
between the mandrel 12 and piston 60 via the radially directed grooves 68; 
this is particularly the case when the piston 60 is moving through the 
reduced diameter bore 54. The other relatively enlarged diameter bore 56 
need not be described here in detail; it being sufficient to state that 
the opposite of a jarring action may be effected by virtue of the 
structure shown, i.e. a bumping action may be effected as the housing and 
mandrel are moved toward the relatively contracted position. The annular 
piston 60 also rests in this second enlarged diameter bore 56 when the 
mandrel and housing are in the relatively contracted position, i.e. prior 
to commencement of a jarring stroke. 
As best seen in FIGS. 2 and 3, the annular piston 60 has a portion 70 which 
is shaped to co-operate with the second stop 64 on the mandrel to provide 
sealing engagement therewith during movement of the piston through bore 54 
during a jarring stroke. As shown in the drawings, this portion 70 of the 
piston is in the form of a smooth annular wall portion which is sloped 
relative to the longitudinal axis of the jar. As shown in the drawings, in 
the preferred form of the invention, this sloped wall portion comprises a 
frusto-conical wall portion and it is preferably disposed at about a 
45.degree. angle to the jar axis although this angle is not particularly 
critical. The stop 64 on the mandrel is in the form of a smooth surfaced 
annular member having a convexly arcuate contour as seen in a longitudinal 
section. In the preferred form of the invention the convexly arcuate 
surface has a semi-circular outline centered at point C as best seen for 
example in FIG. 3. Both of the above described surfaces are machined to a 
smooth surface and preferably they are provided with a hard chrome surface 
to improve their wearing capabilities. 
During the jarring stroke, the high pressures inserted on annular piston 60 
by the working fluid cause the convexly arcuate annular surface of stop 64 
to come into close sealing engagement with the sloping annular wall 
portion 70. A "wedging" effect is achieved and it has been found that by 
virtue of this combination that a good sealing effect can be achieved over 
a very long period of use as compared with prior art devices thus 
effecting a great saving in overall maintenance costs and down-time. The 
sealing capabilities of the abovedescribed surfaces are not adversely 
affected even after a reasonable amount of wear has taken place. The 
overall result is that the fundamental characteristics of the hydraulic 
jar do not deteriorate as rapidly during use as is the case with 
comparable prior art devices. Furthermore it has been found easier to 
achieve substantially uniform characteristics from one hydraulic jar to 
the next when manufactured in accordance with or using basically the same 
tolerances. 
The overall mode of operation of the hydraulic jar described above will be 
readily apparent to the person skilled in the art as will also be its 
manner of use as a jarring and bumping tool. Hence, no further description 
of these operations need be provided. 
It will be understood that the invention is not to be limited to the 
specific embodiment described above by way of example but that the 
invention may be provided in various ways and within the scope of the 
appended claims.