Controlled weak point for wireline cable

A controllable dual weak point device for insertion in a wireline between the cable and tool. The device consists of upper and lower sections that are slidably joined together in releasable interlocking manner with the upper section secured to the cable and the lower section secured to the tool. The upper and lower sections are joined together by a concentric arrangement of central tube, mandrel and limiting sleeve which are interconnected by first and second springs and first and second shear pin arrangements, i.e., the respective first and second weak points.

BACKGROUND OF THE INVENTION 
1. Field of the Invention. 
The invention relates generally to a protective cable coupling assembly for 
use with such as surface readout oil well tools and, more particularly, 
but not by way of limitation, it relates to an improved coupling structure 
that provides a controlled first weak point where the cable support will 
shear and a second weak point which will allow tool recovery. 
2. Description of the Prior Art. 
Applicant is unaware of any prior teachings that relate to a form of 
controlled weak point that allows surface readout oil well work to be 
performed with a greater degree of safety. Surface readout service has 
implicitly involved the necessity for working under pressure at the mouth 
of a well and, for this reason, it was necessary to use the thinnest 
possible cable. In those fields where combinations of depth, pressure and 
important production were found, the balance between diameter of cable and 
mechanical resistance to the needed stress was usually so precarious that 
the work could not, for all practical purposes, be performed with an 
acceptable margin of safety. 
In prior practice, when a cable became hung within a well, its greatest 
stress was at the surface. Thus, if the testing tool could not be 
unhooked, it was necessary to stress the cable on the surface until some 
weak point allowed separation at the lower end of the cable. If the depth 
was great, the useful limit of the stress tension of the cable, less the 
weight of the vertical section of the cable (maximum pull-out), leave a 
very narrow margin for the construction of any "weak point". Keeping in 
mind that during operation the greater part of the capacity of the "weak 
point" is to carry the weight of the testing tool, which consists of the 
sinker bars plus the surface readout tool, and to open the sleeve of the E 
valve; we then find that the tolerance is so narrow that cutting can be 
effected through accidental maneuvers or even under the load produced by 
friction on the cable when flow velocity and production is high. 
Due to the fact that the consequences of these types of accident were very 
costly, it was the practice of some operators to construct the clamp from 
the strongest cable, and when they could not unhook the surface readout 
tool, they were forced to cut the cable at the surface. Such failures 
discredited the surface readout operation in some oil fields to such a 
degree that revival of the practice now is extremely difficult. 
SUMMARY OF THE INVENTION 
The present invention operates such that once a first "weak point" is cut 
or separated, a backup prevents its liberation. Meanwhile, it remains 
electrically connected, sending signals and able to resist considerable 
stresses until such time as the operator slackens the force applied to the 
cable. At this moment, a second "weak point", whose resistance can be 
calculated so as to make it either the same as or different from the 
first, takes effect for continuing the tool unhooking and recovery 
operation, or to be cut if necessary. The controllable weak point 
apparatus is mounted within two tubular steel sections that are 
interconnected with one section firmly connected to the steel cable and a 
lower section connected to the surface readout tool. The upper and lower 
tubes are joined in such a way that they can be easily separated; however, 
an internal, tubular mechanism functions to not allow any separation until 
a predetermined force shears the pins of a weak point. 
The internal mechanism consists of an inner tube carrying the cable 
therethrough and extending axially within a slotted mandrel which is 
disposed within a slotted limit sleeve which, in turn, is reciprocally 
received within upper and lower external sleeves. The inner tube is 
connected via shear pins to an upper crossover housing as an upper spring 
is compressed between the crossover housing and an adjusting nut 
threadedly received over the slotted mandrel. A support block secured 
within the lower external sleeve is secured by plural shear pins to the 
lower end of the slotted mandrel, and a lower spring is compressed between 
the lower end of the support block and an adjustable nut secured on the 
bottom of the inner tube. The upper and lower shear pin arrays provide the 
respective second and first weak points. 
Therefore, it is an object of the present invention to provide a 
controllable weak point device for enabling wider use of surface readout 
tools. 
It is also an object of the present invention to provide a connective 
device that provides greater safety in those wireline operations performed 
at greater depths. 
It is still another object of the invention to provide a greater margin of 
safety in those wireline operations where depth and pressure render 
wireline work precarious. 
Finally, it is an object of the present invention to enable the use of the 
thinnest possible cable while performing surface readout operations under 
pressure at the mouth of an oil well. 
Other objects and advantages of the invention will be evident from the 
following detailed description when read in conjunction with the 
accompanying drawings which illustrate the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1A and 1B, the weak point device 10 consists primarily 
of the tubular structure contained within bracket A and bracket B. The A 
section includes upper external sleeve 12, i.e. upper part 12a and lower 
part 12b, which is secured to a crossover housing 14 affixed to a 
cone-type cable clamp assembly 16 receiving the downfall of cable 18. 
Cable 18 may be a lighter than usual wireline on the order of 3/16" or 
even 7/32". Side bars 20, as needed, may be affixed over cable assembly 16 
by means of fasteners 22. 
The lower or B section of device 10 is housed within a lower external 
sleeve 24 which is threadedly connected to a lower crossover housing 26. 
The crossover housing 26 includes a first axial bore 28 for receiving 
sealing flanges 30 and 0-rings 32 of a cable head 34 which is threadedly 
secured through threaded axial bore 36 and secured by lock nut 38. Cable 
head 34 may then be connected to the surface readout (SRO) tool in 
conventional manner as its housing is received over external threads 40 
and sealing O-rings 41. A grease zerk 42 allows for filling the interior 
space of weak point device 10 with grease. 
Referring also to FIG. 2, the upper crossover housing 14 is formed with a 
axial bore 44 for receiving cable flexure and this narrows into an axial 
bore 46 for carrying the down fall of cable 18. A bottom end axial bore 48 
of intermediate diameter serves to receive an inner tube as will be 
further described. A plurality of dual rows of holes 50, in this 
particular case 8 holes in each row, serve to receive shear pins, as will 
be described. Externally, crossover housing 14 includes external surface 
52 which is milled to receive threads 56 at the upper end for secure 
engagement within the threaded axial bore 58 of crossover housing 16. The 
external surface 52 is then reduced to a lesser diameter surface 60 which 
provides a seating surface for receiving the upper external sleeve 
8 per row, 2. Thus, two rows of threaded holes 62, in this case are 
provided around surface 60. A collar surface 63 is then formed coaxial 
with axial bore 48. 
Referring also to FIG. 3, the upper external sleeve 12 is formed from an 
upper part 12a and a lower part 12b that are threadedly connected. Parts 
12a and 12b form an inner wall 64 and an external surface 65 is received 
over surface 60 of crossover housing 14. A plurality of screw holes 66 are 
aligned with the plurality of tapped holes 62 of housing 14 and secured by 
means of suitable bolts 68 (FIG. 1A). The external sleeve 12 is reduced in 
diameter at its lower end by means of internal shoulder 70 and external 
shoulder 72 to form a reduced diameter sleeve 74 which terminates in four 
parallel rectangular fingers 76. The fingers 76 are extensions of the 
reduced diameter sleeve surface 74 defining the same inside diameter 78. 
Referring also to FIG. 4, the lower external sleeve 24 is formed with a 
external surface 80 which is the same diameter as the external surface 65 
of upper external sleeve 12. The upper end of external sleeve 24 has an 
internal bore 82 that extends from annular end 84 down to an inner annular 
shoulder 86 which terminates in an inner cylindrical wall 88. Formed 
integrally with inner wall 88 are four equi-spaced, arcuate lands 90 which 
define slots 92 between the respective lands 90. In joinder of the lower 
sleeve 24 to the upper sleeve 12, the interior surface 82 slides over the 
exterior surface 74 (FIG. 3) as rectangular fingers 76 slide down within 
the rectangular grooves 92 and, simultaneously, the rectangular lands 90 
each interlock in a respective arcuate, rectangular slot 94 of external 
sleeve 12. The sliding relationship of this joint between upper and lower 
external sleeves 12 and 24 will be explained below in greater detail. 
The lower end of external sleeve 24 includes internal threads 96 for 
receiving the lower crossover housing 26 therein and an internal bore 
defining inner wall 98 extends upward to a downward facing annular 
shoulder 100 adjacent inner wall 88. Small bores 102, 104 directed 
radially through inner wall 98 provide diametric aligning holes for 
assembly purposes. Two rows of circularly arrayed holes 106, e.g., 8 such 
holes in each row, receive screw fasteners for securing a support block 
108 as shown in FIG. 5. 
The support block 108 is formed with an outer cylindrical surface 110, a 
base 111 and an axial bore 112 as two circular rows of tapped screw holes 
114 are formed for mating alignment with holes 106 (FIG. 4) as secured by 
a plurality of bolts 116. (FIG. 1B). The upper portion of support block 
108 is formed with a stepped mounting arrangement wherein a first shoulder 
forms a cylindrical surface 118 and a second upwardly facing shoulder 
forms a further reduced diameter cylindrical surface 120. The cylindrical 
surfaces 118 and 120 each include a plurality of radially aligned shear 
pin holes 122 and 124, respectively. In present design, 8 such pin holes 
are provided around the circumfery. 
Referring again to FIGS. 1A and IB, the internal workings of the weak point 
device 10 also include a limit sleeve 126 of the finger type which, in 
effect, constitutes a variable diameter tube. See FIG. 6. Reciprocally 
disposed within limit sleeve 126 is a slotted mandrel 128 that has an 
axial bore 130 for receiving a central tube 132. See also FIG. 7. The 
limit sleeve 126 (FIG. 6) has a bottom collar 134 (bottom as installed as 
shown in FIG. 1A and 1B) and a central bore 136. A plurality of 
longitudinal slots 138 divide the limit sleeve 126 into a plurality of six 
fingers 140 which are formed with collar portions 142 about what is the 
installed upper end collar 144. 
Next, and referring to FIG. 7, positioned within bore 136 of limit sleeve 
126 is the slotted mandrel 128. Mandrel 128 includes a central bore 146 
and upper end threads 148 as the lower portion of a cylinder 150 is slit 
into equi-sized longitudinally extending fingers 152. Fingers 152 are each 
formed with a first canted annular shoulder 154 which extends into a 
cylindrical surface 156, and a second canted annular shoulder 158 which 
extends to an outer diameter circumferal surface 160. As shown in FIG. 1B, 
the lower end of mandrel central bore 146 is formed with successive 
counterbores 162 and 164 which are received down over the stepped 
cylindrical surfaces 120 and 122 (FIG. 5) of support block 108. As shown 
in FIG. 7, a circular array of holes 166 and 168 are formed to align with 
respective pin holes 122 and 124 (FIG. 5) in order to receive a plurality 
of shear pins 170 and 172 therethrough. 
The center or inner tube 132 is then received through the central bore 146 
of the slotted mandrel 128. As shown in FIG. 8, inner tube 132 is formed 
as an elongated tube 174 having a central bore 176 for receiving the wire 
cable 18 downward therethrough. Inner tube 132 is relatively fragile and 
is pre-calculated to withstand 1000 pounds of force. The lower end of tube 
174 has external threads 178 and the upper end of tube 174 is formed with 
a receiving cup 180 having enlarged diameter sidewall 182 and including 
two rows of circularly arrayed pin holes 184 therearound. In this case, 
there are 8 such radial pin holes 184 in each row and the cup 180 is 
adapted for insertion within the bore 48 of crossover housing 14 (see FIG. 
1A) and alignment of shear pin holes 122, 124 with pin holes 50, then to 
be secured by means of a plurality of shear pins 186 (see FIG. 1A). 
An upper spring 188 is compressed between a downwardly facing shoulder 190 
of crossover housing 14 and an upper adjustment nut 192 which is 
threadedly secured about threads 148 of mandrel 128. A locking screw 194 
secures adjustable nut 192 in a pre-set position as will be further 
described. A lower nut 196 (FIG. 1B) is secured on threads 178 of inner 
tube 132 (FIG. 8) to support a lower spring 198 in compression beneath the 
base 111 of support block 108. After initial adjustment, a set screw 200 
can be tightened to maintain nut 196 in a locked position. 
The weak point device 10 must first be properly assembled and adjusted in 
order to function properly. Thus, the upper external sleeve 12 is inserted 
into the lower external sleeve 24, making sure that the anti-rotation pins 
76 are securely seated in the slots 92. The limit sleeve 126 is then 
inserted down into the end of external sleeve 24 with slotted end collar 
144 first. Then, opening the eight slots 151 of the mandrel 128 by hand, 
the mandrel 128 is inserted down within the limit sleeve 126 until 
connection is accommodated with the lower end of mandrel 128 secured over 
the support block 108 in secure alignment over the stepped faces 118 and 
120. The pin holes 166 and 168 of mandrel 128 are then aligned with pin 
holes 122 and 124, respectively, of support block 108 and pins 170 and 172 
are inserted in each of the holes of the respective circular array. The 
circular array of tapped holes 114 of support block 108 are then aligned 
with the holes 106 in lower external sleeve 24, and a plurality of 
fasteners 116 are secured therein using a suitable cement, e.g., 
LOCTITE.TM.. 
A 3.0 mm metal bar is then placed through orifice 102, 104 (FIG. 1B) in 
order to prevent the limit sleeve 126 from backing up, and the nut 192 is 
threaded onto the end of threads 148 of mandrel 128 within the external 
sleeve 12. The nut 192 is adjusted while the diameter of the upper collar 
144 of limit sleeve 126 widens, and until collar 144 is in secure contact 
with the inner wall 64 of upper external sleeve 12. The 3 mm bar may then 
be removed. 
The upper cup end 180 of inner tube 132 may then be introduced into the 
axial bore 48 of crossover housing 14 and, after matching up pin holes 50, 
the requisite number of shear pins 186 may be inserted. The spring 188, a 
40 pound No. 4 spring, is then placed adjacent the base of crossover 
housing 14, and the threaded end 178 of central tube 132 is placed axially 
within the central bore 130 of mandrel 128. The spring 198, e.g., a 
60-pound spring, is then placed over the lower end of central rod 132 
adjacent the base 111 of support block 108 and the adjusting nut 196 is 
turned onto threads 178. Then, holding down the crossover housing 14 by 
hand until the latter begins to travel on its own within the upper 
external sleeve 12, the crossover housing 14 is turned clockwise by hand 
while holding the external sleeve 12 in the other hand and until a firm 
torsion is felt. This is a sign that both ends of the upper spring 188 
have become seated in their respective anti-rotation slots thereby to 
prevent the nut 192 from becoming loosened. Then continue tightening the 
lower nut 196 until the crossover housing 14 becomes completely inserted 
down within upper external sleeve 12. At this point, the lower nut 196 
must be turned six additional turns more with tightening of the set screw 
200 to lock the nut position. The screw holes 62 in crossover housing 14 
may be lined up by turning clockwise whereupon the bolts 68 are inserted 
in tight bond to secure the assembly. 
The cone-type or other cable clamp assembly 16 may then be assembled while 
leaving about 2 feet of the electrical conductor cable 18. This slack 
section of cable 18 may then be run down through axial bore 46 of 
crossover 14 and on down through central bore 176 of central tube 132. The 
lower end of cable 18 can then be spliced into the cable head assembly 34 
using standard procedures. Finally, the crossover housing 26 can be 
threaded into the lower external housing 24 and DC-type grease compound 
can be pumped through zerk 42 until grease emits from all orifices. 
In operation, the device is first assembled and adjusted step-wise in the 
manner previously discussed and in the form shown in FIGS. 1A and 1B. The 
device 10 is then ready for service as a "weak point" as it is 
interconnected between a downhole wireline or cable 18 and the associated 
SRO tool. 
When the device 10 or cable 18 gets hung up within a well, its greatest 
stress will be at the surface. If the testing tool cannot be unhooked, it 
is necessary to stress the cable at the surface until a "weak point" 
separates on the lower end of the cable. At greater depths, the useful 
limit of the stress tension of the cable, less the weight of the vertical 
section of the cable, i.e., maximum pull-out stress, combine to leave a 
very narrow margin for the construction of a "weak point". 
If it is kept in mind that during operation the greater part of the 
capacity of the weak point is used to carry the weight of the testing 
tool, consisting of sinker bars and the SRO, you may then be operating so 
near the cutting tension that cutting may take place accidentally or even 
under load produced by friction o the cable in high velocity production 
flow situations. With weak point device 10, once the weak point is cut, a 
backup prevents its liberation and it remains electrically connected, 
sending electrical signals and is still able to resist great stresses, 
until such time as the operator slackens the tension applied to the cable. 
At this time, a second or backup weak point is available for continuing 
with the SRO unhooking and recovering operation, or to be cut if finally 
necessary. The resistance of the second weak point can be calculated so 
that it is a specific value more or less of the first weak point breakage 
point. 
In normal operation, the device 10 is in the attitude of FIGS. 1A and 1B 
wherein the cable 18 tension will be transmitted to the slotted mandrel 
128 and also to the pins 170 and 172 at the first weak point, i.e., the 
lower set of pins in the support block 108. The external sleeve 12 has a 
shoulder 70 upon which rests the uppermost collar 144 of the finger-type 
limit sleeve 126. The limit sleeve 126, in turn, rests against the 
adjustable nut 192 that has been threaded onto threads 148 of mandrel 128 
(FIG. 7). The conical surface of nut 192 keeps the limit sleeve collar 144 
spread open thereby to adjust its seating firmly against the shoulder 70 
of external sleeve 12. 
Whenever the force on cable 18 exceeds the shear strength of the lower pins 
170, 172, the pins will be sheared to allow both external sleeves 12 and 
24 to separate. This attitude is shown in FIGS. 9A and 9B. The separation 
comes between downwardly facing shoulder 72 of external sleeve 12 and the 
upper annular rim 84 of external sleeve 24, and the limit sleeve 140 will 
limit the separation to about 10 mm (0.30 inches) because the lower end 
collar 134 of limit sleeve 140 will shoulder up against the downwardly 
facing shoulder 100 of external sleeve 24. This limited movement is enough 
for the finger-type lower end of mandrel 128 (when pins are sheared) to 
collapse and thus decrease the diameter of fingers 160. In this position, 
i.e., as in FIG. 9A and 9B with pins 171 and 172 sheared, the system will 
continue to operate as device 10 supports the required operating tension 
of cable 18. Note that lower external housing 24 and lower crossover 26 
have moved downward to compress the lower spring 198. 
Referring now to FIGS. 10A and 10B, if the cable tension is removed from 
cable 18, the spring force of lower spring 198 (approximately 100 lbs.) 
will retract the upper external sleeve 12 against the lower external 
sleeve 24, i.e., a downward movement, thus again closing the gap between 
downwardly facing shoulder 72 and upper rim 84 of respective external 
sleeves 12 and 24. Since the finger-type mandrel 128 cannot return to its 
original position because the fingers 152 have collapsed, the upper spring 
188 (approximately 50 lbs. force) will compress thus retracting the nut 
192 (an upward movement) to allow the upper collar 144 of the limit sleeve 
140 to collapse inward, thus decreasing the diameter across collar 144. 
When this occurs, any tension applied to cable 18 will be transmitted 
directly to the second weak point, i.e., upper pins 186. 
As shown in FIGS. 11A and 11B, this force shears the upper pins 186 
whereupon the entire B assembly, i.e., inner tube 132, mandrel 128, 
limiting sleeve 126, lower external sleeve 24 and lower crossover 26 and 
attached test equipment will fall away breaking the electric cable 18 
(FIG. 11B). This will then allow the advantage of permitting recover of 
the sinker bars 20 along with the entire cable 18 while also preventing 
the downhole pressure from throwing the gear violently from the well. 
The foregoing discloses a novel controllable weak point tool which can be 
inserted between a downhole wireline and an SRO tool or the like to better 
manage the wireline operation while also contributing to a considerably 
safer operation. The shear pins should be of phosphorated bronze material 
calculated to support a given number of pounds each, and the upper and 
lower springs may be varied in compression value so long as complementary 
adjustment is made. In essence, the tool provides first and second weak 
points wherein a first weak point can fail while still allowing 
continuation and completion of a test as well as additional tensions 
exerted in recovery of the downhole SRO tool; and at some selected time 
the tension can be relieved so that the second weak point will shear to 
sever the cable and release a bottom portion of the connective tool for 
recovery of the wireline cable, sidebars and the like. 
Changes may be made in combination and arrangement of elements as 
heretofore set forth in the specification and shown in the drawings; it 
being understood that changes may be made in the embodiments disclosed 
without departing from the spirit and scope of the invention as defined in 
the following claims.