Full opening bulged forward acting rupture disc having variable depth score line

Abstract

A leak integrity testing apparatus (36) is provided which is adapted to be inserted within a liquid-delivery line, as for example an oil well tubing string (26), to permit in situ testing of the line or string (26) without the need for removal or inspection of the line. The apparatus (36) is a tubular body preferably including an interconnected tubular inlet (38) and outlet (40); a liquid pressure-responsive, metallic burst disc (42) is located between the inlet (38) and outlet (40). The disc (42) includes a peripheral flange (62) and an inner concavo-convex burst region (64). A score line (82, 86) is located on the concave face (66) of the burst region (64), and is of differential depth, having a reduced-depth hinge area (84, 88) serving to prevent detachment of the burst region (64) upon actuation of the disc (42). The score line (82) preferably is located entirely inboard of the transition region (76) between the flange (62) and burst region (64); alternately, the score line (86) may have the hinge area (88) thereof within the transition region (76). In use, liquid under pressure less than the burst pressure of the disc directed against the concave face (66) of the disc (42) thereby confirming the integrity of the line or string (26). Increasing the pressure of the liquid against the concave face of the disc to a level which effects rupture of the disc, the central ruptured section of the disc gives way and moves to an open position allowing substantially unimpeded flow of liquid through the opening in the disc defined by the ruptured section.

Description

RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No. 09/626,585, filed Jul. 27,2000, entitled BULGED SINGLE HINGED SCORED RUPTURE DISC HAVING CIRCULAR SCORE LINE OF LESSER DEPTH IN HINGE AREA.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is broadly concerned with leak integrity testing apparatus for use in testing liquid-conveying conduits in various industrial and oil well applications. More particularly, the invention pertains to such apparatus, and the frangible rupture discs forming a part thereof, wherein the rupture discs are of concavo-convex design and have a continuous score line formed on the concave faces thereof; the score lines are of differential depth in order to define a region of reduced depth forming a hinge area for the burst region of the disc preventing separation thereof upon disc rupture. The apparatus has particular utility for use in testing of oil well tubing strings, but also may be used in petroleum refining and petrochemical operations, as well as other uses in which a liquid is conveyed under pressure through a pipe or conduit.

[0004] 2. Description of the Prior Art

[0005] In order to place an oil well in service, an elongated, sectionalized tubing string is lowered into the well casing, with the tubing string housing a sucker rod and pump assembly. In deep wells, the tubing string may extend thousands of feet from grade down to an oil formation. Leaks in the joints between string sections have a significant impact on pumping efficiency and oil well production. In the past, it has sometimes been necessary to remove the pipe string, locate joint leaks, and repair the string. This can represent a very substantial expense both in terms of repair costs and well downtime.

[0006] U.S. Pat. No. 5,996,696 describes a method and apparatus for leak integrity testing of oil well tubing within the well casing, thus eliminating the need for string removal for such testing. The apparatus of the '696 patent includes a housing interposed between string sections (usually adjacent the lower end thereof close to the pump and well formation). The housing is equipped with a metallic rupture disc in closing, flow-blocking relationship. When it is desired to test the string, predetermined fluid pressure is applied against the rupture disc. If the tubing string is sufficiently leak-free, the rupture disc will burst at or about the predetermined burst pressure. On the other hand, if substantial leaks are present, insufficient pressure will be developed within the string to burst the disc.

[0007] The preferred rupture disc design disclosed in the '696 patent is a metallic, concavo-convex disc having a discontinuous score line formed in a face of the bulged rupture portion thereof. The discontinuity in the score line serves as a hinge region for the disc. While the apparatus and methods described in this patent represent a significant breakthrough in the art, it has been found that sometimes the disc design is not optimal from a performance standpoint. As can be appreciated, a useful rupture disc in this context must reliably burst at desired burst pressures, or inaccurate test results maybe obtained.

SUMMARY OF THE INVENTION

[0008] The present invention overcomes the problems outlined above and provides an improved leak testing apparatus for liquid-conveying line applications, and especially oil well tubing strings, giving increased testing reliability. Broadly speaking, the apparatus of the invention includes a tubular holder adapted for coupling to an oil well tubing string with a rupture disc within the holder in normally closing relationship thereto. The disc has a peripheral flange and a concavo-convex burst region inboard of the flange. The disc also has a continuous score line formed on the concave face of the disc (i.e., the face of the disc subjected to liquid pressure during a testing operation) wherein the score line is of varying depth along the length thereof to define a region of reduced score line depth; this region serves as a hinge area for the disc preventing detachment of the concavo-convex burst region upon opening of the disc. It has been found that discs of this character provide improved testing results, as compared with prior designs.

[0009] The provision of the continuous score line on the concave face of the disc permits reliable disc rupture and opening owing to the fact that when rupture occurs, the adjacent interconnected score line-defining wall surfaces of the disc separate from each other. This is to be contrasted with the more usual situation where the score line(s) on rupture discs are formed in the face thereof remote from contact with the product and/or pressure. In such a situation, the score line-defining wall surfaces of the disc move toward each other during rupture.

[0010] In one embodiment, the continuous, variable depth score line is closely adjacent to but not within the radiused transition region between the flange and concavo-convex burst region of the disc. Alternately, a hinge area of increased strength may be provided by scoring the disc so that the hinge area extends along the transition region with the remainder of the score line spaced from the transition region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a fragmentary vertical sectional view illustrating a typical oil well including a casing and internal tubing string, the later having the integrity testing apparatus of the invention installed therein;

[0012] FIG. 2 is a greatly expanded, fragmentary vertical sectional view illustrating the integrity testing apparatus within the tubing string, where the rupture disc of the apparatus is in its open, ruptured condition;

[0013] FIG. 3 is a fragmentary vertical sectional view of an oil well tubing string, with the testing apparatus of the invention installed therein, and with the disc in blocking relationship to liquid flow through the string;

[0014] FIG. 4 is a view similar to that of FIG. 3, but illustrating the disc in its ruptured condition;

[0015] FIG. 5 is an elevational view of the preferred rupture disc of the invention, viewing the concave face thereof;

[0016] FIG. 6 is a side elevational view of the disc shown in FIG. 5;

[0017] FIG. 7 is an elevational view of the rupture disc of FIGS. 5-6, viewing the convex face thereof;

[0018] FIG. 8 is a fragmentary, enlarged view illustrating the position of the score line formed on the concave face of the rupture disc;

[0019] FIG. 9 is a perspective view of a rupture disc body after formation of the peripheral flange and continuous score line thereon;

[0020] FIG. 10 is a perspective view similar to that of FIG. 9 but viewing the opposite face of the disc body;

[0021] FIG. 11 is a perspective view similar to that of FIG. 9 but illustrating the concave face of the disc after formation of the concavo-convex burst region inboard of the peripheral flange;

[0022] FIG. 12 is a perspective view similar to that of FIG. 11 but illustrating the convex face of the rupture disc;

[0023] FIG. 13 is a greatly enlarged, fragmentary cross sectional view depicting the disc body after formation of the flange and score line but prior to bulging thereof, and further illustrating the differential depth of the score line along its length;

[0024] FIG. 14 is a view similar to that of FIG. 13, but illustrating the disc body after bulging thereof;

[0025] FIG. 15 is a greatly enlarged, fragmentary cross sectional view of the marginal flange portion of the rupture disc and which is positioned between the tubular inlet and the tubular outlet of the apparatus; and

[0026] FIG. 16 is a greatly enlarged, fragmentary cross sectional view similar to FIG. 15 and illustrates the configuration of the marginal flange portion of the rupture disc when the disc has been subjected to sufficient pressure to effect opening of the central part of the disc.

[0027] FIG. 17 is a schematic elevational view of the concave face of another rupture disc in accordance with the invention, wherein the hinge region of the score line is formed in the radiused transition between the peripheral flange and burst region of the disc; and

[0028] FIG. 18 is a greatly enlarged view of the hinge region and portions of the remainder of the score line of the embodiment depicted in FIG. 17.

[0029] The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Turning now to the drawings, and particularly FIG. 1, a conventional oil well 20 is shown, having an upright, continuous casing 22 extending from grade downwardly to an oil formation 24. A tubing string 26 is located within the casing 22 and includes a plurality of end-to-end connected sections such as the sections 28, 30, 32, and 34; it will of course be appreciated that an operating well would have many such sections in its tubing string. A sectionalized, conventional sucker rod 35 with a lowermost pump assembly 35a are within the string 26 in order to effect pumping of oil from the formation 24. The majority of the string sections are interconnected via conventional couplers 29. However, at a selected point along the length of the string 26 (usually near the lower end thereof adjacent the formation 24) integrity testing apparatus 36 is inserted between a pair of tubing sections.

[0031] Referring to FIG. 2, it will be seen that the lower end of section 32 is threaded, as is the upper end of section 34. The apparatus 36 is threadably coupled between these tubing sections, and includes a tubular inlet 38 and a tubular outlet 40, with a burst disc 42 interposed between the inlet and outlet. In detail, the inlet 38 is internally threaded as at 44 adjacent its upper end, and is externally threaded as at 46 near its lower end. Viewing FIGS. 15 and 16, it can be seen that the lowermost butt end of the inlet 38 presents a stepped configuration 48 defined by an outer annular surface 48a joined to an inner annular surface 48b which is spaced axially of and interconnected to surface 48a by an intermediate face 48c which is transverse to surfaces 48a and 48b. The outlet 40 is internally threaded as at 50 adjacent its upper end for connection to inlet threading 46; similarly, the outlet lower end is threaded as at 52 for connection to the string section 34. The outlet 40 is also provided with uppermost surfaces defining stepped configuration 54 presented by an outer annular surface 54a, as well as an inner annular surface 54b which is out of the plane of 54a and is joined to the latter by intermediately transversely-extending face 54c.

[0032] As is most apparent from FIG. 15, when the disc 42 is placed between inlet 38 and outlet 40 and the inlet 38 is rotated in the direction to cause the inlet 38 to move toward the outlet 40, the relative movement of the inlet 38 and outlet 40 is arrested when the outer flange portion 70 of disc 42 is engaged on opposite sides thereof by surfaces 48a and 54a, respectively, to thereby clamp the disc 42 between the inlet 38 and the outlet 40. Surfaces 48b and 54b of inlet 38 and outlet 40, respectively, are disposed such that upon clamping of the outermost annular flange portion 70 of disc 42 between surfaces 48a of inlet 38 and surface 54a of outlet 40, an annular gap 51 is presented between the surface 48b of inlet 38 and the directly opposed face of the inner flange portion 72 of disc 42. Gap 51 is generally of the order of from about 0.001 in. to about 0.060 in., and preferably from about 0.003 in. to about 0.010 in. The underside of flange 54 presents an outwardly extending, angular, chamfered surface 58 extending in a direction away from inlet 38 which joins with radiused edge 60 of flange 54. It can be seen from FIGS. 15 and 16 that the diameter of the opening defined by flange 54 of outlet 40 is greater than the internal diameter of the cylindrical suface 38a of inlet 38.

[0033] The burst disc 42 is best illustrated in FIGS. 5-8. The disc broadly includes an outer peripheral flange 62 as well as an inner concavo-convex burst region 64 presenting a concave face 66 and an opposed convex face 68. The flange 62 is of stepped configuration in cross section and has an outermost essentially flat flange portion 70 and an inner essentially flat flange portion 72. These flange portions are interconnected via a radiused transition region 74. As will be observed in FIG. 6, the outer and inner flange portions 70, 72 are vertically offset from each other and interconnected by an integral, intermediate, transversely extending annular portion 73.

[0034] The burst region 64 is inboard of flange portion 72, with a radiused transition region 76 therebetween. The transition region 76 (FIG. 8) is defined by a pair of concentric, laterally spaced apart outer and inner margins 78, 80.

[0035] The disc 42 is also provided with a continuous score line 82 formed on the concave face 66 thereof. In the present embodiment, the score line 82 is located entirely inwardly of the transition region 76, i.e., it is spaced inwardly from transition region inner margin 80. In addition, the score line 82 has a varying depth along its length to define a hinge area 84 of reduced depth. Referring to FIGS. 5-8, the hinge area 84 is centered about the 180° mark and extends for a length from about 5-10° on either side thereof. The purpose of hinge area 84 is to prevent detachment of the burst region 64 from flange 62 upon bursting of the disc 42. To this end, and as best seen in FIGS. 13-14, the score line 82 at the 0° mark is formed to leave an unscored depth of metal “X” in., whereas at the 180° mark, the hinge area 84 of the score line is of lesser depth, leaving an exemplary unscored depth of metal of “X+Y” in., where “Y” for example is equal to about 0.003 in.

[0036] Preferably, the disc 42 is formed of a metal selected from the group consisting of Inconel (an alloy of 76% Ni, 17% Cr and 7% of Fe) and nickel, and has a thickness of from about 0.010-0.060 inches, more preferably from about 0.014-0.050 inches. The depth of the score line 82 throughout its length except for the reduced depth hinge area 84 is from about 35-70% of the thickness of the disc, more preferably from about 40-60% of such thickness. The depth of the score line at the reduced depth area 84 is from about 0.001-0.005 inches (more preferably from about 0.002-0.004 inches) less than the depth of the score line 82 throughout the remainder thereof.

[0037] During manufacture of the disc 42, the stepped peripheral flange 62 is initially formed, prior to bulging of the central region of the disc. The flange is formed by a conventional crimping die under pressures sufficient to form the desired stepped configuration. Next, the score line 82 is formed on what will later become the concave face 66 of the disc, making use of an appropriate, differential depth scoring anvil. Only after formation of the flange 62 and score line 82 is the disc bulged to give the concavo-convex burst region 64. While it would be possible to first bulge the disc followed by scoring, from a manufacturing point of view, it is greatly preferred to initially score, followed by bulging. The metal along the transition region 76 is work hardened during bulging of the disc, thereby providing a hinge region of greater strength at the .

[0038] FIGS. 3-4 illustrate the operation of apparatus 36 when placed within a tubing string 26. The disc 42 rests upon the flange 54 of outlet 40, with the stepped flange 62 in mating, face-to-face contact with the complementally stepped upper face 56 of the flange. When the inlet 38 is threaded into the upper end of outlet 40, the stepped butt end 48 of the inlet comes into contact with the upper surface of the flange 62. In this fashion, the disc 42 is firmly sandwiched between the butt end 48 and upper flange surface 56. When it is desired to conduct a leak test for the string 26, pressurized well liquid is applied against the concave face 66 of the disc 42. Assuming that the string 26 is essentially leak-free, at predetermined liquid pressure, the disc 42 will rupture along score line 82. However, owing to the presence of the reduced depth hinge area 84, the burst region 64 will remain attached to the flange 62 and not be carried downward into the pump area of the string.

[0039] Again referring to FIG. 14, it will be observed that the obliquely oriented score line-defining sidewalls 82a and 82b diverge from a lower apex upwardly to the concave surface 66. When the disc 42 ruptures, movement of the burst region 64 effects relative separation between the walls 82a and 82b without any compressive forces exerted against these sidewall surfaces. This is to be contrasted with the more usual approach of providing a score line on the convex face. In such a situation, rupture of the disc causes the score line-defining sidewalls to move towards each other, thereby compressing these surface together.

[0040] It is further to be observed from enlarged FIGS. 15 and 16, that because of the gap 51 presented between surface 58b of inlet 38 and the opposed face of flange portion 72 of disc 42, during bulging of the central portion of disc 42 under liquid pressure thereagainst, the intermediate flange portion 73 of the disc is deformed so that it essentially lines up with flange portion 72 and the inner flange portion 70 of the disc as best illustrated in FIG. 16. This straightening out of the stepped flange portion of the disc 42 causes the annular score line 82 to move inwardly a greater distance from the edge 60 of flange 54 of outlet 40 to provide more assured opening of the central part of the disc along score line 82.

[0041] The slightly radiused edge 60 of flange 54 of outlet 40 and the receding chamfered surface 58 of flange 54 cooperate to facilitate full opening of the central part of disc 42 as best illustrated in FIG. 2 for substantially unimpeded flow of liquid through the opening in the disc 42.

[0042] FIGS. 17-18 illustrate another type of score line 86 which may be used in the discs of the invention. In this case, the majority of the score line 86 is inboard of the transition region 76 between the inner margin of the flange and the burst region. However, at the hinge area 88, the score line crosses the transition region inner margin 80 and extends along the length of the transition region 76.

[0043] As illustrated in FIG. 18, the hinge area 88 within the transition region is defined by a pair of relatively short arcuate segments 90 and 92 of relatively small diameter, and a longer central arcuate segment 94 of relatively large diameter. The score line 86 also has a pair of crossover regions 96 and 98 on opposite sides of the segments 90 and 92 where the score line crosses the inner margin 80 of the transition region 76. Placement of the hinge area 88 within the transition region 76, with the remainder of the score line 86 inboard of the transition region, provides a stronger hinge than in the case of the first embodiment. That is, the metal along the transition region 76 is work hardened during bulging of the disc, thereby providing a region of greater strength.

[0044] Although the invention has been described with reference to the preferred embodiment illustrated in the attached drawing figures and particularly when used in testing the integrity of an oil well pipe string, it is to be understood that the apparatus has utility for testing a variety of different liquid-conveying pipes or lines in petrochemical, petroleum refining, and other similar industrial applications. It is also to be noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.

Claims

1. A unitary, forward buckling, rupture disc adapted to be mounted on a tubular rupture disc holder in closing relationship to a central fluid passage through the holder, said disc comprising:

an outer peripheral flange portion having a first annular face and an opposed second annular face,

an inner concavo-convex, generally hemispherical bulged region presenting a concave face and an opposed convex face,

an annular, circumferentially extending, cross-sectionally arcuate transition region integral with the peripheral flange portion and the bulged region respectively, said transition region having a first cross-sectionally arcuate annular surface joining the first face of the flange portion to the convex face of the bulged region and a second cross-sectionally arcuate annular surface joining the second face of the flange portion to the concave face of the bulged region, said first arcuate surface being defined by a pair of circumferentially extending, concentric, laterally spaced apart, inner and outer first margins integral with the first annular face of the flange portion and with the convex face of the bulged region respectively, said second arcuate surface being defined by a pair of circumferentially extending, laterally spaced apart, inner and outer second margins integral with the second annular face of the flange portion and with the concave face of the bulged region respectively, said first arcuate surface having a smaller radius than the radius of the second arcuate surface, and

said bulged region being provided with a continuous, generally circular score line formed in the concave face of the bulged region inboard of and spaced inwardly of the transition region, said score line defining a central rupture segment of the bulged region having a generally circumscribing edge located in proximal relationship to the inner second margin of the transition region,

said score line varying in depth along the length thereof presenting a first reduced depth line portion defining a hinge section of the score line and defining a second score line portion of greater depth than said reduced depth score line portion, said second score line portion being of a depth to rupture and allow the central rupture segment of the bulged region to move to a fully open position when a pre-determined pressure is applied to the concave face of the rupture disc, said second score line portion of the score line being located in sufficiently close relationship to the inner second margin of the transition region and of a depth such that the central rupture section of the bulged disc region opens to said fully open position without severing of the hinge section of the disc and with the edge of the central rupture disc segment disposed in at least about substantially perpendicular relationship to said flange,

said central rupture segment of the disc when moved to the fully open position thereof being located out of the flow path of fluid through the rupture disc holder so as to not present any substantial impediment to bi-directional flow of fluid through said central passage of the rupture disc holder

2. A unitary, forward buckling rupture disc as set forth in claim 1, wherein said disc has a thickness of from about 0.010 in. to about 0.060 in.

3. A unitary, forward buckling rupture disc as set forth in claim 2, wherein said disc has a thickness of from about 0.014 in. to about 0.050 in.

4. A unitary, forward buckling rupture disc as set forth in claim 1, wherein said second line portion of the score line is from about 35% to about 70% of the thickness of the disc.

5. A unitary, forward buckling rupture disc as set forth in claim 4, wherein said second line portion of the score line is from about 40% to about 60% of the thickness of the disc.

6. A unitary, forward buckling rupture disc as set forth in claim 1, wherein the depth of said first score line portion of the score line is from about 0.001 in. to about 0.005 in. less than the depth of the second score line portion of the score line throughout the extent of the first score line portion.

7. A unitary, forward buckling rupture disc as set forth in claim 1, wherein the depth of said second score line portion of the score line is from about 0.002 in. to about 0.004 in. less than the depth of said first score line portion of the score line.

8. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing and thereafter permitting full bi-directional flow of liquid through the pipe string, said apparatus comprising:

a tubular rupture disc holder presenting a central liquid passage therethrough and having opposed end segments, one of said holder end segments being adapted for attachment to an end of one of the pipe sections of the pipe string and the other end segment of the holder being adapted to for attachment to an end of an adjacent pipe section of the pipe string, said holder being provided with a circumferentially extending enlarged central section of greater diameter than the diameter of each of the opposed end segments of the holder; and

a unitary, forward buckling, rupture disc mounted on the rupture disc holder in normal closing relationship to said central liquid passage through the holder, said disc provided with

an outer peripheral flange portion having a first annular face and an opposed second annular face,

an inner concavo-convex, generally hemispherical bulged region presenting a concave face and an opposed convex face,

an annular, circumferentially extending, cross-sectionally arcuate transition region integral with the peripheral flange portion and the bulged region respectively, said transition region having a first cross-sectionally arcuate annular surface joining the first face of the flange portion to the convex face of the bulged region and a second cross-sectionally arcuate annular surface joining the second face of the flange portion to the concave face of the bulged region, said first arcuate surface being defined by a pair of circumferentially extending, concentric, laterally spaced apart, inner and outer first margins integral with the first annular face of the flange portion and with the convex face of the bulged region respectively, said second arcuate surface being defined by a pair of circumferentially extending, laterally spaced apart, inner and outer second margins integral with the second annular face of the flange portion and with the concave face of the bulged region respectively, said first arcuate surface having a smaller radius than the radius of the second arcuate surface, and

said bulged region being provided with a continuous, generally circular score line formed in the concave face of the bulged region inboard of and spaced inwardly of the transition region, said score line defining a central rupture segment of the bulged region having a generally circumscribing edge located in proximal relationship to the inner second margin of the transition region,

said scored line varying in depth along the length thereof presenting a first reduced depth line portion defining a hinge section of the score line and defining a second score line portion of greater depth than said reduced depth score line portion, said second score line portion being of a depth to rupture and allow the central rupture segment of the bulged region to move to a fully open position when a pre-determined pressure is applied to the concave face of the rupture disc, said second score line portion of the score line being located in sufficiently close relationship to the inner second margin of the transition region and of a depth such that the central rupture section of the bulged disc region opens to said fully open position without severing of the hinge section of the disc and with the edge of the central rupture disc segment disposed in at least about substantially perpendicular relationship to said flange,

said central rupture segment of the disc when moved to the fully open position thereof being located in the enlarged central section out of the flow path of liquid through the rupture disc holder so as to not present any substantial impediment to bi-directional flow of liquid through said central passage of the rupture disc holder.

9. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing as set forth in claim 8, wherein said disc has a thickness of from about 0.010 in. to about 0.060 in.

10. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing as set forth in claim 9, wherein said disc has a thickness of from about 0.014 in. to about 0.050 in.

11. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing as set forth in claim 8, wherein said second line portion of the score line is from about 35% to about 70% of the thickness of the disc.

12. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing as set forth in claim 11, wherein said second line portion of the score line is from about 40% to about 60% of the thickness of the disc.

13. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing as set forth in claim 8, wherein the depth of said first score line portion of the score line is from about 0.001 in. to about 0.005 in. less than the depth of the second score line portion of the score line throughout the extent of the first score line portion.

14. Apparatus for testing the integrity of the connections between end to end pipe sections of a liquid-delivery pipe string line within an oil well casing as set forth in claim 8, wherein the depth of said second score line portion of the score line is from about 0.002 in. to about 0.004 in. less than the depth of said first score line portion of the score line.