WELL TOOL SUSPENSION CABLE HEAD WEAK POINT

Abstract: A weak point element (70) for a cable head comprises a tubular ring (82) having a circumferentially extending zone of reduced tensile strength between the ends thereof, the said zone comprising apertures 84 and interposed webs 86. The el­ ements can be incorporated in cable head assemblies, includ­ ing cable head shock isolation assemblies.

Description: This invention relates to cable heads such as are used in suspending well logging tools at the end of a "wireline" or cable in the borehole of an oil or gas well, in order to determine various parameters associated with the oil and/or gas production potential of formations intersected by the borehole, and particularly to such heads which incorporate a so-called "weak point". A cable head provides for the mechanical and electrical connection of the logging cable to the uppermost tool in the logging tool string. The mechanical connection of the cable head serves three principal functions: first, to securely fasten the logging cable to the female portion of the electrical connection between the cable and the uppermost logging tool; second, to maintain the integrity of the male to female electrical connection; and third, to ensure the failure of the mechanical connection at a predetermined tensile load level. This latter function is important because it is fairly common for cable-supported tools to become stuck in boreholes. The provision of a specific failure point at the cable head permits the operator to preserve the majority of the cable in the event the tool string becomes hopelessly stuck in the borehole. The inclusion in the cable head of a failure point having a lower tensile strength than the cable is generally referred to as providing a "weak-point". It should be noted, however, that a weak point must be strong enough to support the static weight of the tool string suspended from the cable, as well as any additional loads imparted to the tool during normal operations or in reasonable efforts to free the tool string should it become stuck. Accordingly, the required predetermined tensile failure point of the weak-point is primarily a function of logging depth. Therefore, the weak-point component is offered to the logging operator in several tensile strengths in order to accommodate the needs of logging to different borehole depths. It is, of course, desirable to provide separation of the cable from the tool string at the cable head not only to retrieve as much of the cable as possible, but to avoid leaving a loose skein of cable on top of the cable head, the presence of which could hamper subsequent efforts to "fish" out the tool string by engaging it with a fishing tool such as is known in the prior art. Known cable heads employing weak point elements suffer numerous disadvantages. For example, some of the prior art devices are unreliable in their failure points; some are difficult to inspect to ensure that the weak point element has not been partially stressed even though it remains unfailed and thus will be unpredictable in failure point; and some are hard to replace as they are located inside the cable head, and disassembly is required for replacement. US-A-3517740 describes a weak point element which comprises a substantially tubular member closed at one end and having connector means proximate each end thereof, and a circumferentially extending groove. This weak point element is designed to be sheared by detonation of an explosive charge therewithin and will be prone to many of the disadvantages listed above for applications where a predictable tensile failure strength is desired. Access to the weak point element requires disassembly. According to the present invention, there is provided a weak point element assembly for a logging tool cable head, comprising: a tubular member having connection means proximate each end thereof, the member having a circumferentially extending reduced tensile strength zone between said ends; and bending stress isolation means to isolate said reduced tensile strength zone from bending stress; characterised in that said member is a tubular ring, and said reduced tensile strength zone includes a plurality of circumferentially disposed apertures through the wall of said tubular ring, said apertures defining a plurality of webs extending between said ends, and said bending stress isolation means extend generally axially through said tubular ring, said tubular ring being connectable between a cable head and a tool with the tubular ring being disposed on the exterior of the cable head. The invention also includes a cable head for connecting a logging cable to a well logging tool, comprising: means for severably electrically connecting said cable to said tool; means for severably mechanically connecting said cable to said tool, comprising a weak point element assembly as defined above. The tensile ring may, for example, possess a plurality of substantially evenly circumferentially spaced webs separated by laterally extending circumferential apertures. The ring is preferably heat-treated to ensure that its tensile yield strength is close to the ultimate strength of the material, and to cut down on the strain hardening as the tensile ring is stressed in order to ensure greater predictability of the tensile yield point, with attendant tensile failure at the cable head and not in the logging cable. In order that the invention may be more fully understood, reference is made to the accompanying drawings, wherein: FIGURE 1 is a schematic view of a well tool suspended from a cable by one embodiment of tensile ring cable head of the present invention. FIGURE 2 is a full sectional elevation of a preferred embodiment of a tensile ring cable head of the present invention. FIGURES 2A and 2B are two variations of split connector holders suitable for use with a cable head of the present invention. FIGURE 3 is an exploded assembly of the tensile ring cable head shown in Figure 2. Figure 1 schematically depicts a borehole 2 having suspended therein logging tool string 4, comprising elements 6 and 8. Element 6 may be one or more logging tools, as may element 8 or, alternatively, element 8 may be a perforating gun run below logging tools 4. Tool string 4 is suspended in borehole 2 from logging cable 10, which passes through, and is supported in bending against kinking by,coil guard spring 12. Cable 10, which may be a standard 7/16" (11.1mm) logging cable as is well known in the art, then passes into cable head 20. Tensile ring 70 is shown on the exterior of cable head 20. Referring now to FIGS. 2 and 3 of the drawings, cable 10 is shown passing through guard spring 12, through axial bore 24 of upper connector 22 of cable head 20, and into cup 26, where it terminates. Cup 26 possesses a frusto-conical axially directed bore wall 28, which narrows toward the top of the cable head, or (looking "up hole") toward the surface. The cable end inside cup 26 is secured by outer race 32, which compresses the outer strands 34 of cable 10 between the exterior surface of outer race 32 and bore wall 28. The inner strands 36 of cable 10 are compressed between inner race 38 and outer race 32, inner race 38 also preventing relative twisting of inner strands 36 and outer strands 34 with respect to one another. The inner and outer races 38 and 32 both have the same degree of taper as bore wall 28 of cup 26, so as to cause the cable strands, races and cup to jam together tighter as the tensile load on the connection is increased, as occurs when the tool string 4 is suspended in a borehole by the cable. Cup 26 is fastened to split connector holder 40 by screws 42, split connector holder 40 housing a female connector as is well known in the art. Two examples of such holders 40 and 40' (screws 42 not shown thereon) housing different configurations of female connectors 44 and 46, respectively, are shown in FIGS. 2A and 2B. Female connector 46 is held in holder 40' by ring 48. Insulated cable conductor 50, housed within mechanically supportive cable strands 32 and 38, provides an electrical connection via female connector 44 or 46 to the logging tool(s) below cable head 20. The assembly of cup 26, races 32 and 38 and split holder 40 containing a female connector is inserted into connector sleeve 52, until leading edge 54 of sleeve 52 almost abuts lower annular shoulder 56 on upper connector 22. The female connector 44 then engages male connector 60, which extends upward into upper sleeve bore 62 from lower axial bore 66 of sleeve 52, a pressure-tight seal being effected between male connector 60 and the wall of lower sleeve bore 66 by O-rings 68. Male connector 60 is maintained in lower sleeve bore 66 by abutment of its upper portion against the bottom of bore 66, in conjunction with retainer nut 63, which threads onto the bottom of male connector 60 on lock washer 65, spacer 67 and plug 69, the latter of which maintains the axial orientation of the lower portion of male connector 60. As upper connector 22 enters sleeve 52, it is also surrounded by tensile ring 70, which has been threaded to sleeve 52 at 72, upper connector 22 then being screwed into tensile ring 70 at 74, as it is free to rotate about cable 10. As upper connector 22 screws into tensile ring 70, this assures complete engagement of female connector 44 with male connector 60, as the neck 76 of upper connector 22 is of sufficient length so that connector holder 40 butts up against the bottom 64 of upper sleeve bore 62. Tapered lock ring 78 then threads onto upper connector 22 at 80 and is tightened against tensile ring 70. The downhole end of sleeve 52 may be threaded by threads 81 to a closure head 200 at the top of the upper most logging tool in the tool string (see FIGS. 1 and 4 for closure head 200). A pressure-tight seal is effected by O-rings 83. Knurled outer surfaces on tapered lock ring 78, upper connector 22, and sleeve 52 provide gripping surfaces when cable head 20 is assembled, and made up with a closure head 200. Referring again to FIG. 3, tensile ring 70 will be described in more detail. Tensile ring 70 is of metal, preferably of AISI 416 stainless steel. The stainless steel ring is preferably heat treated, such as is known in the art, so that the yield strength is close to the ultimate tensile strength of the material, and the amount of strain hardening experienced by the material when subjected to stress is reduced. This elevation of the yield strength and provision of a greater range of elastic deformation of the tensile ring material results in a more reliable and predictable tensile failure point even after the tensile ring material has been stressed, such as during attempts to free a stuck tool string. In the event the logging tool will be exposed to H₂S in the borehole, a ring machined of AISI 17-4PH stainless steel, heat treated to resist corrosion according to NACE (National Society of Corrosion Engineers) standards, as known in the art may be substituted. The physical configuration of the preferred embodiment of tensile ring 70 is markedly different from prior art weak point elements, and comprises a tubular element 82 having female threads at each end thereof and having a plurality, preferably four, of laterally circumferentially extending windows or apertures 84 through the wall thereof, the windows 84 being separated by interposed webs 86 which provide the points of failure, or tensile members. The close sliding fit between upper bore 62 of sleeve 52 and the neck 76 of upper connector 22 carries any bending load applied to which cable head 20 is subjected, ensuring that tensile ring 70 will be substantially completely loaded in tension. This is in marked contrast to some prior art cable heads where the cable itself was employed to provide a weak point. In such prior art apparatus, cable strands were bent over a conical element, the element then being jammed into a complementary recess in a cup-shaped element, the strength of the "weak link" being determined by the number of strands wedged between the two elements; unwanted strands were merely broken off. Alternatively, some prior art cable heads have incorporated therein a single, axially oriented cylindrical rod relatively unsupported against bending loads is employed as a weak point element, rendering the element more susceptible to shear failure or defects in the weak point element material grain structure. Moreover, it is easy to determine if any undue stress has been placed upon tensile ring 70 by visual inspection after each trip into the well. Any distortion of the webs is readily visible, and a new ring 70 may be installed without reheading of the cable and in fact without internal disassembly of the cable head. In operation, when a tool string 4 becomes stuck in a borehole, and all reasonable attempts at freeing it have failed, tension is applied to cable 10 via a winch at the surface, which tension is imparted to tensile ring 70 of cable head 20 by upper connector 22 and connector sleeve 52 through threaded connections 72 and 74. When the tensile failure point of tensile ring 70 is reached, such being governed by the material and treatment thereof as well as the width and depth of the material in webs 86, ring 70 parts at webs 86. Cable 10 is then retrieved bearing upper connector 22, cup 26, races 32 and 38, split holder 40 or 40' with female connector therein, lock ring 78 and the upper half of tensile ring 70. After retrieval of the cable 10 with the referenced parts from the borehole, the top of connector sleeve 52 provides an easily engaged protrusion at the top of the tool string for subsequent fishing operations. Thus, to make up a cable head 70 to a new logging tool closure head, only a new tensile ring 70 and connector sleeve 52 (with male connector therein) need be supplied. It is evident from the above description of the preferred embodiments that a novel and unobvious means for providing a reliable weak point for a cable head, as well as novel and unobvious cable head structures, have been invented. However, while the present invention has been disclosed in terms of several preferred embodiments, it will be readily apparent to one of ordinary skill in the art that some additions, deletions and modifications may be made. For example, the ring may be threaded on its exterior and secured to other cable head elements extending over its ends; raised flanges may be provided at the ring ends, or annular shoulders on the interior thereof to provide tensile load bearing areas.

Claims: A weak point element assembly for a logging tool cable head, comprising a tubular member (70) having connection means (72,74) proximate each end thereof, the member having a circumferentially extending reduced tensile strength zone between said ends; and bending stress isolation means (52,22) to isolate said reduced tensile strength zone from bending stress; characterised in that said member is a tubular ring, and said reduced tensile strength zone includes a plurality of circumferentially disposed apertures (84) through the wall of said tubular ring, said apertures defining a plurality of webs (86) extending between said ends, and said bending stress isolation means extend generally axially through said tubular ring, said tubular ring being connectable between a cable head and a tool with the tubular ring being disposed on the exterior of the cable head. An assembly according to claim 1, wherein said connection means comprises internal threads. A cable head for connecting a logging cable to a well logging tool, comprising: means (40,60) for severably electrically connecting said cable (10) to said tool; means (70) for severably mechanically connecting said cable to said tool, comprising a weak point element assembly as claimed in claim 1 or claim 2. A cable head according to claim 3, wherein said means for mechanically connecting further includes a connector sleeve (52) having a substantially axial interior bore (62) therethrough; a connector (22) having a substantially axial bore therethrough and a neck (76) received in said connector sleeve bore; and wherein said cable head further comprises a logging cable end (50,34) extending through said connector bore; and cable strand anchor means (26,32,38) disposed in said sleeve bore below said neck and secured to said logging cable end; said connector and said connector sleeve comprise said bending stress isolation means, and said ring is secured to said connector proximate one end to said connector sleeve proximate said other end. A cable head according to claim 4, wherein said cable strand anchor means comprises a cup (26) having a frusto-conical bore therein and a plurality of races (26,32) interlocked with the strands of said logging cable disposed in said frusto-conical bore. A cable head according to any of claims 3 to 5, wherein said severable electrical connection means includes male (60) and female elements (44,46); and one (44,46) of said elements is secured to said logging cable; and the other (60) of said elements is secured to said connector sleeve. A cable head according to claim 6, which is so arranged that, upon parting of said ring at said zone thereof, said logging cable end, said connector, said cable strand anchor means, and one of said elements may be longitudinally disengaged from said connector sleeve and the other of said elements.

Inventors: BUSCH, RANDOLPH ARTHUR
Applicants: HALLIBURTON COMPANY
Application year: 1986
Main classification: E21B17
IPC codes: H01R13, E21B17