Abstract:
A fixture for evaluating a metal-to-metal seal. A first metal seal surface and a second metal seal surface are brought into initial contact by engagement of a male connection of a first tubular component and a female connection of a second tubular component to form a metal-to-metal seal. A contact pressure regulator is provided to alter the contact load acting in the interfacial region between the first metal seal surface and the second metal seal surface in a predictable manner An inlet passage is provided to communicate test fluid to an inlet side of the metal-to-metal seal. An outlet passage is provided on an outlet side of the metal-to-metal seal. In accordance with the method, a capacity of the metal-to-metal seal is expressed as the pressure differential required to just induce leakage given a measure of absolute contact load greater than zero.

Description:
FIELD 
       [0001]    This invention pertains to a fixture for evaluating a metal-to-metal seal between tubular components, and in particular, the quality of seals for threaded connections used to complete wells drilled in earth materials. 
       BACKGROUND 
       [0002]    Testing systems used in the prior art tend to test the sealing performance of the entire connection system and do not characterize the sealing performance of the metal-to-metal seal alone. It will be understood that the term metal-to-metal seal is known in the art to include annular seals formed in a region where an interval of continuous contact is provided between mating solid components of similar stiffness wherein a thin film of material may be disposed in the interfacial region of the contacting interval, the thin film of material typically comprised of lubricant film, compressed thread compound paste [and/or] surface coating carried by one or both of the mating components. 
         [0003]    By way of example, with U.S. Pat. No. 3,653,254 (Simon) entitled “Leak-testing internal seals in pipe joints” the test fluid media is applied externally with a driving differential test pressure that must force sufficient test fluid to pass through the connection thread to first reach the internal metal-to-metal seal and subsequently continue to flow so as to increase the differential pressure across the seal to substantially that of the driving pressure, so as to meet the condition for detection of leakage under a known pressure differential. The connection threads are coated with a thread compound paste to provide lubrication and prevent galling. This thread compound paste typically contains solids and has plastic and thixotropic fluid phase properties. When trapped between thread surfaces it can thus restrict or impede the flow of the externally applied fluid when the clearance between thread surfaces is small. Thus, this method may not reliably evaluate the sealing performance of the metal-to-metal seal separate from the sealing performance of thread compound trapped within the thread, i.e., the driving pressure for leakage past the metal-to-metal seal may not be reliably known as it can depend on the pressure drop of fluid across the thread due to both time dependent viscous characteristics or blockage. 
         [0004]    This and other similar test methodologies characteristically provide pass/fail characterization of seal integrity and do not lend themselves to the quantitative definition of seal capacity relative to applied loads. 
       SUMMARY 
       [0005]    According to one aspect of the present invention, there is provided a fixture for evaluating a metal-to-metal seal between tubular components. A first tubular component is provided carrying a first metal seal surface and having a male connection. A second tubular component is provided carrying a second metal seal surface and having a female connection. The first metal seal surface and the second metal seal surface are brought into initial contact by an engagement of the male connection of the first tubular component and the female connection of the second tubular component to form a metal-to-metal seal. A contact pressure regulator is provided to control load acting between the first tubular component and the second tubular component to alter the contact load acting in the interfacial region between the first metal seal surface and the second metal seal surface in a predictable manner. An inlet passage is provided to communicate test fluid to an inlet side of the metal-to-metal seal. An outlet passage is provided on an outlet side of the metal-to-metal seal. 
         [0006]    According to another aspect there is provided a method for evaluating a metal-to-metal seal between tubular components. A first step involves providing a fixture as described above. A second step involves engaging the male connection of the first tubular component and the female connection of the second tubular component to bring the first metal seal surface and the second metal seal surface into initial contact. A third step involves directing a test fluid under pressure through the inlet passage to the inlet side of the metal-to-metal seal to apply a pressure differential across the metal-to-metal seal. A fourth step involves monitoring the outlet passage for leakage of test fluid while inducing sufficient pressure to just cause the onset of leakage and determining a measure of contact load acting between the first metal seal surface and the second metal seal surface. A fifth step involves calculating a measure of a capacity of the metal-to-metal seal as a pressure differential required to just induce leakage given a measure of absolute contact load greater than zero. 
         [0007]    The most common form of engagement between tubular components is a rotational engagement, typically, a threaded engagement. However, as the functioning of the metal-to-metal seal is being isolated from other aspects of the coupling, the actual form of engagement used is irrelevant to the operation of both the fixture and the method. As is known to persons skilled in the art there are a wide variety of engagements from which to choose. 
         [0008]    There are various means that can be provided as a contact pressure regulator to control load acting between the first tubular component and the second tubular component to alter the contact load of the first metal seal surface and the second metal seal surface in a predictable manner independent of the engagement between the male connection of the first tubular component and the female connection of the second tubular component. Those load control means can be incorporated internally within the fixture or applied externally to the fixture. There are various mechanical means that are workable. It is preferred that the means to control load be independent of the interference between the male connection of the first tubular component and the female connection of the second tubular component. In the description which follows, a preferred means which will be described is a pressure chamber positioned in an annulus between the first tubular component and the second tubular component. The pressure chamber reduces the contact stress by causing relative movement of the first tubular component and the second tubular component when fluid pressure is applied to the pressure chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein: 
           [0010]      FIG. 1  is a plot of the rate of fluid leakage past the metal-to-metal seal versus the contact intensity between the two sealing surfaces. 
           [0011]      FIG. 2  is a plot of the metal-to-metal seal contact intensity versus the pressure applied in the control chamber determined by mathematical modeling of the structural response. 
           [0012]      FIG. 3  is a plot of the circumferential (hoop) strain of the apparatus versus the pressure applied in the control chamber. 
           [0013]      FIG. 4  is a longitudinal cross-section of the invention in the preferred embodiment. 
           [0014]      FIG. 5  is a longitudinal cross-section of the invention in a first alternate embodiment. 
           [0015]      FIG. 6  is a is a longitudinal cross-section of the invention in a second alternate embodiment 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A fixture for evaluating a metal-to-metal seal between tubular components will now be described with reference to  FIG. 1 through 6 . 
         [0017]    This invention aims to overcome the disadvantages of the prior art and provide a better method of evaluating the efficacy of metal-to-metal seals used in threaded connections. In such pipe connections, a standard practice is to apply a thread compound to the threads and to the metal-to-metal seal surfaces to prevent galling during make-up/assembly. A thin layer of the thread compound is trapped between the two seal surfaces during make-up and becomes an integral interfacial component of the metal-to-metal seal. Other interfacial seal components would include any coatings applied to the sealing surfaces prior to make-up such as, for example, manganese phosphate. In the process of developing this invention, it was learned and demonstrated that these interfacial components strongly affect seal performance; thus, the term metal-to-metal seal implicitly includes any interfacial component. 
         [0018]    To provide an effective barrier against fluid leakage, no gap can exist between the two sealing surfaces of a metal-to-metal seal unless such gap is filled by an interfacial component. The physical limit at which a theoretically perfect metal-to-metal seal can prevent fluid leakage is where the contact force between the two sealing surfaces at any location reaches zero, with no gap between the surfaces. This invention provides a method of quantitatively evaluating the efficacy of a practical seal relative to this physical limit for a theoretically perfect seal by measuring the relationship between the rate of fluid leakage past the seal and the contact intensity between the two sealing surfaces, where the contact intensity at any circumferential location along the seal is defined as the total contact force acting between the sealing surfaces per unit of circumferential length. 
         [0019]    The invention quantitatively evaluates the efficacy of metal-to-metal seals relative to a deterministic physical limit for perfect seals and provides a means to investigate specific parameters relevant to good seal function independently of one another. The physical limit of sealing for perfect seals is the point of initial separation of the two sealing surfaces, where the minimum contact intensity between the two sealing surfaces at any location reaches zero with no gap between the surfaces. Sealing performance is quantified by measuring the relationship between the rate of fluid leakage past the seal and the contact intensity between the two sealing surfaces as illustrated in  FIG. 1 . Curve  1  shows the relationship between the rate of fluid leakage and the contact intensity for a theoretically perfect seal. No leakage past the seal occurs unless the contact intensity of the seal equals zero. Curve  2  shows an example of a measured relationship between the rate of fluid leakage past a typical physical metal-to-metal seal and the contact intensity between two sealing surfaces with interfacial components. Leakage past the seal occurs when the contact intensity of the seal is greater than zero. The rate of leakage increases as the contact intensity decreases. Curve  3  shows another example relationship for a typical physical metal-to-metal seal with interfacial components. The shape and position of a seal performance curve depends on the statistical repeatability of the measurement and on seal design variables such as seal surface geometry, surface finish, and the properties of any interfacial components such as surface coatings and thread compound. 
       Preferred Embodiment 
       [0020]      FIG. 4  shows the preferred embodiment of the apparatus for evaluating the efficacy of metal-to-metal seals. Male component  10  with a sealing surface  11  of the metal-to-metal seal is mated by threads  1201  to female component  20  with a facing sealing surface  21  of the metal-to-metal seal with a predictable contact load distribution that can be either circumferentially uniform or non-uniform. Male component  10  is assembled to female component  20  by applying sufficient relative rotation between the two components so that the desired initial contact force between the seal surfaces is achieved. Any means that provides accurate relative rotation of the two components can be used. In the preferred embodiment of the invention apparatus, female component  20  is held stationary to fixed base  30  by one or more rods inserted through holes  31  and  22 . The male component  10  is held to coupling  40  by one or more rods inserted through holes  41  and  12 . Coupling  40  and male component  10  are rotated by applying a torque to coupling  40 . In the preferred embodiment, the torque is created by applying a force tangential to the axis of thread  1201  at a selected distance from the axis via a suitable lever inserted through hole  42 . Use of coupling  40  is optional and torque can be applied directly to the male component  10  to effect the desired rotation relative to female component  20 . 
         [0021]    An annular space  1202  is formed by male component  10 , female component  20  and plug  50 . To test the fluid barrier formed by sealing surfaces  11  and  21 , a test fluid is applied through port  51  located in plug  50 . Plug  50  is mated to female component  20  by threads  2501 . One or more holes  13  ensure that the test fluid from port  51  reaches annular space  1202  next to the metal-to-metal seal. Any leakage of the test fluid past the metal-to-metal seal is detected by connecting an appropriate fluid flow measurement device, such as a gas bubble counter, to fitting  70  that seals with an o-ring to port  23  located in female component  20  behind the metal-to-metal seal and held in place by ring  80 . 
         [0022]    The apparatus incorporates a contact pressure regulator which has means of reducing the seal contact stress after make-up (assembly). Typically this is achieved by applying a fluid pressure in a control chamber located near the test seal. A mechanical means could also be used to reduce the seal contact stress. In the preferred embodiment of the invention, control chamber  1203  is formed by exterior surface  14  of the male component  10 , interior surface  24  of female component  20 , and sealing elements  1204  and  1205 . Sealing element  1204  is held in place by spacer  90  which is grooved to ensure fluid leaking past the seal can reach port  23 . Sealing element  1205  is held in place by ring  60  which is mated to female component  20  by thread  2601 . Fluid pressure is applied to control chamber  1203  through port  25  in female component  20 . A second function of sealing element  1205  is to ensure that any test fluid leaking past the metal-to-metal seal enters port  23  and subsequently, the flow measurement device. The preferred embodiment of the apparatus allows ring  60  and sealing elements  1204  and  1205  to be removed and reinstalled any time before or after assembly of male component  10  to female component  20 . Plug  50  may be installed in female component  20  prior to assembly of male component  10  to female component  20  to help align the two components. 
         [0023]    The size and position of control chamber  1203  relative to the location of the metal-to-metal seal is selected such that the contact stress reduces in a predictable manner when fluid pressure is applied to the chamber. The relationship between the pressure applied to control chamber  1203  and the contact intensity of the metal-to-metal seal can be determined by mathematical modeling of the system structural response, one such method is known in the art as finite element analysis. Curve  4  in  FIG. 2  indicates the relationship for a specific version of the preferred embodiment of the invention. A number of means are available for determining the separation point between seal surfaces, when the contact intensity reaches zero, as the control pressure is increased, which is the physical limit for sealing of a theoretically perfect seal. One such means is the aforementioned finite element analysis. Another means is to observe the change in the stiffness of the apparatus when separation occurs. The separation point can be identified from measurements of circumferential (hoop) strain on the male or female component and the control pressure as shown by Curve  5  in  FIG. 3 . When seal separation occurs there is a distinct change in the relationship between the circumferential strain and the control pressure as indicated by point  6 . 
         [0024]    First Alternate Embodiment 
         [0025]      FIG. 5  shows a first alternate embodiment of the apparatus for evaluating the efficacy of metal-to-metal seals. Male component  10  with a sealing surface  11  of the metal-to-metal seal is mated by threads  1201 , ring  60 , and thread  2601  to female component  20  with a facing sealing surface  21  of the metal-to-metal seal. The apparatus is assembled by threading ring  60  on to male component  10  until shoulders  61  and  15  lightly touch. Sealing elements sealing elements  1204  and  1205  are placed in female component  20  and on male component  10 , respectively. Then, ring  60  and male component  10  are threaded together in to female component  20  via threads  2601  until shoulders  62  and  26  firmly touch. At this point in the assembly, there is no contact between sealing surfaces  11  and  21 . The final step in the assembly is to apply sufficient relative rotation between male component  10  and ring  60 , which at this point is fixed relative to female component  20 , so that the desired initial contact force between the seal surfaces is achieved. Any means that provides accurate relative rotation of the two components can be used. The means used in the alternate embodiment of the invention apparatus shown in  FIG. 5  is similar to the means used in the preferred embodiment. Female component  20  is held stationary to fixed base  30  by one or more rods inserted through holes  31  and  22 . The male component  10  is held to a coupling similar to that used in the preferred embodiment by one or more rods inserted through holes in the coupling and holes  12 . The coupling and male component  10  are rotated relative to female component  20  in a manner similar to that in the preferred embodiment. 
         [0026]    To test the fluid barrier formed by sealing surfaces  11  and  21 , a test fluid is applied through port  51  and/or  52  located in plug  50  leading to annular space  1202  next to the metal-to-metal seal. Plug  50  is mated to female component  20  by threads  2501 . Any leakage of the test fluid past the metal-to-metal seal is detected by connecting an appropriate fluid flow measurement device, such as a gas bubble counter, to fitting  70  that seals with an o-ring to port  23  located in female component  20  behind the metal-to-metal seal and held in place by ring  80 . 
         [0027]    In the alternate embodiment of the invention apparatus shown in  FIG. 5 , control chamber  1203  is formed by exterior surface  14  of the male component  10 , interior surface  24  of female component  20 , and sealing elements  1204  and  1205 . Sealing element  1204  is held in place by spacer  90  which is grooved to ensure fluid leaking past the seal can reach port  23 . Sealing element  1205  is held in place by ring  60  which is mated to female component  20  by thread  2601 . Fluid pressure is applied to control chamber  1203  through a port located in female component  20 . In the alternate embodiment shown in  FIG. 5 , the port for applying fluid pressure to control chamber  1203  is not visible because it is not located in the plane of the cross-section. A second function of sealing element  1205  is to ensure that any test fluid leaking past the metal-to-metal seal enters port  23  and subsequently, the flow measurement device. The size and position of control chamber  1203  relative to the location of the metal-to-metal seal is selected such that the contact stress reduces in a predictable manner when fluid pressure is applied to the chamber. The relationship between the pressure applied to control chamber  1203  and the contact intensity of the metal-to-metal seal can be determined as described previously for the preferred embodiment of the apparatus. A number of means are available for determining the separation point between seal surfaces as described previously for the preferred embodiment of the apparatus. 
         [0028]    Second Alternate Embodiment 
         [0029]      FIG. 6  shows a second alternate embodiment of the apparatus for evaluating the efficacy of metal-to-metal seals. Male component  10  with a sealing surface  11  of the metal-to-metal seal is mated by threads  1201  to female component  20  with a facing sealing surface  21  of the metal-to-metal seal. Male component  10  is assembled to female component  20  by applying sufficient relative rotation between the two components so that the desired initial contact force between the seal surfaces is achieved. Any means that provides accurate relative rotation of the two components can be used. The means used in the alternate embodiment of the invention apparatus shown in  FIG. 6  is similar to the means used in the preferred embodiment. Female component  20  is held stationary to fixed base  30  by one or more rods inserted through holes  31  and  22 . The male component  10  is held to a coupling similar to that used in the preferred embodiment by one or more rods inserted through holes in the coupling and holes  12 . The coupling and male component  10  are rotated relative to female component  20  in a manner similar to that in the preferred embodiment. 
         [0030]    To test the fluid barrier formed by sealing surfaces  11  and  21 , a test fluid is applied through port  51  located in plug  50  leading to annular space  1202  next to the metal-to-metal seal. Plug  50  is held in place by threaded rod  53 , cap  54 , and nut  55 . Ring  56  is seated within female component  20 . The axial position of plug  50  relative to ring  56  and thus, female component  20  and male component  10  can be adjusted with threads  57 . The axial position of sealing element  1502  relative to plug  50  and thus, ring  56 , female component  20  and male component  10  can be adjusted by changing the axial length of ring  58 . Any leakage of the test fluid past the metal-to-metal seal is detected by connecting an appropriate fluid flow measurement device, such as a gas bubble counter, to fitting  70  that seals with metal-to-metal seal to port  23  located in female component  20  behind the metal-to-metal seal and held in place by ring  80 . Sealing element  1204  acts to ensure that any test fluid leaking past the metal-to-metal seal enters port  23  and subsequently, the flow measurement device. 
         [0031]    In the second alternate embodiment of the invention apparatus shown in  FIG. 6 , fluid pressure applied to annular space  1202  through port  51  acts not only to drive leakage through the metal-to-metal seal but also acts to reduce the seal contact intensity. The selected axial position of sealing element  1502  relative to the location of the metal-to-metal seal controls the relationship between the seal contact intensity and the applied fluid pressure. The position of sealing element  1502  is selected so that the contact intensity reduces in a predictable manner when fluid pressure is increased. The relationship between the pressure applied to annular space  1202  and the contact intensity of the metal-to-metal seal can be determined as described previously for the preferred embodiment of the apparatus. A number of means are available for determining the separation point between seal surfaces as described previously for the preferred embodiment of the apparatus. 
         [0032]    It will be apparent to one skilled in the art that the present invention can be practiced with non-uniform tubular elements as long as they are substantially axisymmetric so as to respond to changes in pressure in a predictable manner. 
         [0033]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
         [0034]    The following claims are to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and what can be obviously substituted. Those skilled in the art will appreciate that various adaptations and modifications of the described embodiments can be configured without departing from the scope of the claims. The illustrated embodiments have been set forth only as examples and should not be taken as limiting the invention. It is to be understood that, within the scope of the following claims, the invention may be practiced other than as specifically illustrated and described.