Abstract:
A unitary, forward buckling rupture disc is provided that is adapted to be mounted in a holder in which the disc is in closing relationship to a tubing string in an oil well and has greatest utility for testing the integrity of the connection between the sections of oil well tubing strings. The disc is designed to withstand the fluid pressure head in the string and a leak test pressure, and to then burst and fully open under a predetermined overpressure. The concavo-convex bulged section of the disc has a non-circular, continuous score line formed in the concave face thereof. The score line is of continuously varying depth around its circumference and includes an outwardly projecting cam segment which is of greatest depth that ruptures first and is in direct opposition to a lesser depth score line portion forming a hinge area for the burst region of the disc.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of The Invention 
     The present invention is broadly concerned with leak integrity testing apparatus for use in testing liquid-conveying conduits, and especially multi-sectional pipe strings, used in various industrial and oil well applications. More particularly, the invention pertains to such apparatus, and especially a frangible forward acting rupture disc forming a part thereof, wherein the rupture disc is of concavo-convex design and has a non-circular, continuous score line formed in the concave face thereof. The score line is of continuously varying depth around the circumference thereof and includes an outwardly projecting cam segment which is of greatest depth that ruptures first and is in direct opposition to a lesser depth score line portion forming a hinge area for the burst region of the disc preventing separation thereof from the flange portion of the disc upon disc rupture. The apparatus has particular utility for use in testing the integrity of the connection between the sections 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. 
     2. Description of the Prior Art 
     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. 
     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 &#39;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. 
     The preferred rupture disc design disclosed in the &#39;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 may be obtained. In addition, the disc after being ruptured must not fragment while still allowing full bi-directional liquid fluid flow therethrough. 
     SUMMARY OF THE INVENTION 
     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 to the central passage through the holder. The disc includes an outer annular peripheral flange portion having a first annular face, and an opposed second annular face and an inner circular margin, and an inner concavo-convex, generally hemispherical bulged region inboard of and projecting away from the first annular face of the flange portion. The bulged region has opposed convex and concave faces and is provided with an outer circular margin and a unitary, annular, transversely arcuate transition region integral with and joining the outer circular margin of the bulged region with the inner circular margin of the flange portion. 
     The bulged region of the disc is provided with a continuous non-circular score line in the concave face thereof adjacent the outer circular margin of the bulged region in proximal spaced relationship to the transition region. The score line has a semi-circular segment of major length, and a second generally curvilinear score line cam segment of minor length with respect to the length of said semi-circular score line segment. The cam score line segment projects outwardly of the diameter of the major score line segment and is located closely adjacent and inboard of the transition region. 
     The score line varies in depth along the length thereof and is of greatest depth along the cam score line segment and of shallowest depth in substantially direct opposition to the cam score line segment thereby defining a hinge portion of the score line. 
     The disc is openable under the influence of liquid overpressure directed against the concave face of the disc to allow flow of the liquid past the disc. The cam score line segment is of a depth relative to the remaining portion of the score line to cause the cam score line segment to rupture first under liquid pressure while the major score line segment is of a varying depth which allows the burst region to swing to a fully opened rupture position without severing of the hinge portion of the score line. Upon rupture of the bulged region and full opening of the disc, the edge of the central burst region is disposed in generally perpendicular relationship to the flange portion of the disc. 
     The tubular holder is provided with sidewall structure configured to generally complementally receive and allow sufficient full opening of the rupture disc under liquid overpressure to thereby permit full flow of liquid through the tubular holder upon rupture of the disc. It has been found that discs of this character provide improved testing results, as compared with prior designs, while allowing full bi-directional flow of liquid through the holder after rupture of the disc. 
     The provision of the continuous, non-circular 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. 
     The curvilinear cam portion of the score line, and which is of greatest depth, projects beyond the circumference of the main circular portion of the score line and acts somewhat as a cam or lever to assure reliable opening of the disc at pressures of the order of about 1500 psi to about 2000 psi. Opening pressures of that magnitude are difficult to achieve in discs having a nominal burst diameter of the order of 2 to 3 inches, which are required to block the internal diameter of conventional oil well pipe tubing strings, without the extended, deepest depression portion of the score line which projects beyond the circumference of the major circular portion of the score line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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; 
     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 allowing full bidirectional flow through the apparatus; 
     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; 
     FIG. 4 is a view similar to that of FIG. 3, but illustrating the disc in its ruptured condition; 
     FIG. 5 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; 
     FIG. 6 is a view similar to that of FIG. 5, but illustrating the disc body after bulging thereof; 
     FIG. 7 is a greatly enlarged, bottom view of the rupture disc illustrating the non-circular score line in the concave face of the bulged region of the disc wherein a cam segment of the score line projects beyond the circumference of the score line is of greatest depth so that the cam segment of the score line ruptures and opens first upon application of overpressure liquid to the concave face of the disc; and 
     FIG. 8 is a diagrammatic, vertical cross-sectional view of components that may be used to effect bulging of the central region of the disc. 
     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 
     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 or pipe 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  35   a  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. 
     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. The lowermost butt end of the inlet  38  presents a stepped configuration  47  defined by an outer annular surface  47   a  joined to an inner annular surface  47   b  which is spaced axially of and interconnected to surface  47   a  by an intermediate face  47   c  which is transverse to surfaces  47   a  and  47   b . The outlet  40  is internally threaded as at  45  adjacent its upper end for connection to inlet threading  46 ; similarly, the outlet lower end is threaded as at  53  for connection to the string section  34 . The outlet  40  is also provided with an uppermost annular surface  55  directly opposed to annular surfaces  47   a  and  47   b.    
     As is most apparent from FIGS. 2-4, 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 annular flange portion  49  of disc  42  is engaged on opposite sides thereof by surfaces  47   b  and  54 , respectively, to thereby clamp the disc  42  between the inlet  38  and the outlet  40 . 
     The burst disc  42  is best illustrated in FIGS. 4-7. The unitary disc broadly includes an outer, relatively flat annular peripheral flange  49  as well as an inner bulged, concavo-convex burst region  51  presenting a concave face  48  and an opposed convex face  50 . The burst region  51  is inboard of flange portion  49 , with an annular transversely radiused transition region  52  therebetween. The transition region  52  (FIG. 7) is defined by a pair of imaginary, concentric, circular, laterally spaced apart outer and inner imaginary margins  54 ,  56  respectively. 
     The disc  42  is also provided with a continuous score line  58  formed in the concave face  48  thereof inboard in and in proximity to the margin  56  of radiused transition region  52  of disc  42 . The score line  58  is located in close proximity to but slightly inwardly of the transition region  52 , i.e., it is spaced inwardly from transition region inner margin  56 . In addition, the score line  58  is of continuously varying depth throughout it circumferential extent. 
     Referring to FIG. 7, it is to be seen that the score line  58  is provided with a circular, circumferentially extending score line portion  60  of major length and which extends substantially around the entire periphery of the outer portion of bulged region  51  adjacent the inner margin  56  of radiused transition region  52 . Major score line portion  60  merges with a curvilinear cam score line segment  62  of minor length relative to the length of score line portion  60 . The cam score line segment  62  extends beyond the circumferential extent of score line portion  60 . Preferably, cam score line segment  62  has an outer arcuate base score line portion  66  intermediate opposed curvilinear score line leg portions  68  and  70  that merge with respective outer rectilinear end score line leg portions  72  and  74 , which join with corresponding opposite extremities  76  and  78  of score line segment  60 . 
     The midpoint of the base score line portion  70  of cam score line segment  62 , indicated as being at 180° in FIGS. 5-7, is of greatest depth indicated as x+y in FIG. 5, with respect to the circumferential extent of the score line  58 . Correspondingly, the score line portion  60  is of shallowest depth X, at the midpoint thereof, indicated as being 0° in FIG. 5, and which is thereby directly opposite the 180° point of the score line  58 . The score line  58  continuously varies in increasing depth in opposite directions from the 0° of the score line  58  until the 180° point of cam score line portion  60  is reached. 
     The score line portion  60  adjacent the 0° point indicated in FIG. 7 is of a depth on opposite sides of the 0° point to define a hinge score line portion which does not sever when the bulged region  46  is subjected to sufficient over pressure to effect rupture of the disc. Accordingly, the central bulged region  51  of the disc  42  does not release from flange  44  thereby resulting in a non-fragmenting rupture disc. 
     During manufacture of the bulged disc  42 , the score line  58  is formed in the surface of a metal blank on what will later become the concave face  48  of the disc  42 , making use of an appropriate, differential depth scoring anvil. Only after formation of the score line  58  is the disc bulged to give the concavo-convex burst region  51 . 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  52  is work hardened during bulging of the disc, thereby providing a score line hinge portion  81  of greater strength than the cam score line segment  62 . FIG. 8 is a schematic illustration of a conventional die unit  82  that is employed to effect formation of the bulged region  46  of disc  42 . Die unit  82  includes a pressure bell  84  having a central cavity  86  therein along with a passage  88  communicating with cavity  86  and adapted to be coupled to a source of high positive pressure fluid. The receiver  90  making up a part of unit  82  has a central cavity  92  which aligns with cavity  86  when pressure bell and receiver  92  are brought into closed relationship as shown in FIG.  8 . Passage  94  in receiver  90  communicates with cavity  92  and allows air within the interior of cavity  92  to escape during bulging of a disc  42 . In the use of die unit  82 , a circular flat metal blank is clamped between the outer opposed circular margins of pressure bell  84  and receiver  90  between cavities  86  and  92 . Pressurized fluid is introduced into passage  88  to deform the portion of the metal blank exposed to the fluid within cavity thus effecting bulging of the blank as shown schematically in FIG.  8 . The pressurized forming liquid directed into bell  84  is preferably at a pressure sufficiently high to bulge the disc to a point equal to about 80% of the pressure at which the bulged portion would rupture. 
     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 0.010-0.070 inches, more preferably from about 0.014-0.050 inches. The depth of the score line  58  throughout its length except for the reduced depth score line hinge portion  81  is from about 35-70% of the thickness of the disc, more preferably from about 40-60% of such thickness. The depth of the base portion  66  of cam score line segment  62  at the 180° point indicated in FIG. 7 is from about 0.001 to about 0.004 in. deeper, more preferably from about 0.015 in. to about 0.003 in. deeper, and most preferably about 0.002 in. deeper than the depth of the score line hinge portion  81  of score line portion  60  at the 0° point indicated on FIG.  7 . 
     In a representative disc  42  useful for testing the integrity of a conventional tubing string to be used within an oil well, such disc may be fabricated of an Inconel blank having a thickness of about 0.31 in. and of a diameter of about 3.25 in. The bulged region  51  of such disc may then for example have a diameter of about 2.485 in., defined by the imaginary dashed inner margin of transition region  52 . The major score line portion  60  of score line  58  is preferably located about 0.05 in. inboard of imaginary margin  56  of transition region  52 . The difference in depth of score line  58  between he 0° point of score line  58  and the 180° point of cam score line segment  62  is preferably about 0.002 in. The arcuate base score line portion  66  is preferably about 0.35 in. in length and is the radius of an arc of about 15.88° about the central axis  96  of disc  42 . Each of the score line leg portions  68  of cam score line segment  62  are preferably about 0.14 in. in length and each comprise an arc of about 15.36° from the points  98  and  100  respectively as indicated on FIG.  7 . The straight score line leg portions  72  of cam score line segment  62  are preferably each about 0.2 in. in length. 
     In the representative disc described above, in the case of a disc  42  designed to rupture at a pressure of about 2000 psi, the disc is of 0.031 Inconel material as indicated, and the score line  58  varies in depth from about 0.013 in. at the 0° point indicated at FIG. 7 to about 0.017 in. at the 180° point of FIG.  7 . For a disc designed to rupture at about 1500 psi, the score line  58  should vary in depth from about 0.015 in. at the 0° point to about 0.017 in. at the 180° point. 
     Although preferred rupture discs having a cam score line segment for effecting opening of the bulged region  46  of disc  42  at over pressures at the exemplary 1500 to 2000 psi range, it is to be understood that the cam opening effect afforded by cam score line segment  62  is equally useful in other disc opening pressure applications, as for example, in the range of 4000 psi to 2500 psi. For higher opening pressures, the varying depth score line would not be as deep as specified in the preceding examples, and the thickness of the disc could be greater than specified. 
     FIGS. 1-4 illustrate the operation of apparatus  36  when placed within a tubing string  26 . The flange portion  49  of disc  42  is clamped between the surface  55  of outlet  40  and the annular surface  47   b . In this fashion, the disc  42  is firmly sandwiched between tubular inlet  38  and tubular outlet  40 . When it is desired to conduct a leak test for the string  26 , liquid under an over pressure greater than the pressure exerted on the disc by the column of liquid in the tubing string is applied against the concave face  48  of the disc  42 . Assuming that the string  26  is essentially leak-free, at a predetermined liquid over pressure, the disc  42  will rupture along score line  58 . 
     In the case of a rupture disc  42  designed to rupture at a design over pressure of about 1500 psi, when that over pressure level is reached against the face  48  of rupture disc  42 , cam score line segment  62  will sever first commencing generally at the 180° point as illustrated in FIG.  7  and then break in opposite directions away from the 180° point toward the score line hinge portion  81  of score line  58 . As the disc ruptures along score line  58  starting at the cam score line segment  62 , the concavo-convex bulged region  51  of disc  42  will swing in the direction of liquid flow into disposition as illustrated in FIGS. 2 and 4 of the drawings with the non-fragmenting hinge portion  81  of the score line  58  preventing severing of region  51  of the disc  42  from the flange portion  49  thereof. It is noteworthy that the tubular outlet  40  has a generally semi-hemispherical inner surface  98  configured to complementally receive bulged region  51  of the disc  42  upon rupture of region  51 . The inner wall  98  of fitting  40  is configured to prevent over swinging of the region  51  which could result in tearing of the hinge portion  81  of score line  58  resulting in disengagement of bulged region  51  from the flange portion  49  of the disc  42 . In the fully open position of bulged region  51  of disc  42 , the bulged region is maintained in substantially perpendicular relationship to flange portion  49  as illustrated in FIG.  4  and out of the flow path of liquid through inlet  38  and outlet  40 . As a result of full opening of the bulged region of  51  as shown in FIGS. 2 and 4, liquid is permitted to flow bi-directionally substantially unimpeded by the opened region  51  of disc  42 . Unimpaired bi-directional flow of liquid through the integrity testing apparatus  36  upon opening of disc  42  is an important operating function of the disc  42 . 
     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. 
     In instances where it is desired that the disc  42  rupture at a liquid over pressure applied thereto of the order of 2000 psi, it has been found that such opening parameter for the disc may be realized by fabricating the disc of the dimensions and characteristics detailed above. The choice between utilization of a disc  42  designed to rupture at a liquid pressure of 1500 psi versus 2000 psi is primarily a function of the length of the pipe string in a particular well. A 2000 psi opening disc is designed to be used in deeper wells having longer pipe string and a larger total volume and therefore weight of liquid therein, whereas as 1500 psi opening disc has application for somewhat shallower wells. 
     Referring to FIG. 6, it is to be observed that the obliquely oriented score line-defining sidewalls  58   a  and  58   b  of score line  58  diverge from a lower apex upwardly to the concave surface  66  of disc  42 . When the disc  42  ruptures, movement of the burst region  46  effects relative separation between the walls  58   a  and  58   b  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. 
     The area of burst region  46  of disc  42  defined by score line segment  62  and at an imaginary continuation of the score line  58  in alignment herewith has been found to permit design of a disc of a diameter that will be received in closing relationship to a conventional pipe string, e.g., an overall diameter of 3¼ in. being typical, and that will reliably open at burst pressures of the order of 1500 to 2000 psi as compared with 4500 psi over pressure normally required to open a disc without the cam action afforded by cam score line segment  62  which is of greater depth than the hinge portion  81  of score line  58 . 
     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.