US3489445A

US3489445A - Threaded sucker rod joint

Info

Publication number: US3489445A
Application number: US676108A
Authority: US
Inventors: Archer W Kammerer Jr
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-10-18
Filing date: 1967-10-18
Publication date: 1970-01-13
Anticipated expiration: 1987-01-13
Also published as: NL6814892A; FR1576076A

Description

Jan. 13, 1970 .A. wjKAMMERER, JR 3,489,445

TI'UREADED` SUCKER ROD JOINT Filed Oct. 18. 1967 Liam? 6l smv/N 6,46 #a pkesrkcss 44./00PS/ g-a/l Ps1 c le rroeA/EV.

United States Patent 3,489,445 THREADED SUCKER ROD JOINT Archer W. Kammerer, Jr., 1900 Yucca Ave., Fullerton, Calif. 92632 Filed Oct. 18, 1967, Ser. No. 676,108 Int. Cl. F16b 7/18 U.S. Cl. 287--117 15 Claims ABSTRACT OF THE DISCLOSURE A prestressed threaded joint, such as a sucker rod joint used in pumping oil and similar wells and subject to repeated stress reversals during pumping of the well, comprising a pin threaded into a companion box with a washer interposed and in compression between the pin shoulder and end of the box, the washer having a substantially greater modulus of elasticity than the pin and box to significantly lower the stress added to the prestressed joint upon imposition of the pumping load thereto.

The present invention relates to threaded joints, and more particularly to the threaded joints of sucker rods which are threadedly secured to one another to form a string of rods running between a suitable power mecha nism at the top of the well bore and a reciprocating pump in the well bore set at a desired location therein.

In connection with deep well pumps actuated through the reciprocation of a string of sucker rods extending from the pump to the top of the well bore, the rods of such string are intermittently loaded upon their upward movement as a result of the pump load imposed thereon, required to lift the entire height of the fluid column above the pump. Thus, a cyclic or intermittent loading and unloading on the sucker rod pins and boxes occurs during operation of the well. A theory has been extant that if the threaded joints in the string of sucker rods were initially stressed to a degree greater than the load applied cyclically, no additional loading would be applied on the joints when the pump load was applied thereto.

It has been demonstrated through actual tests that this theory of prestressing, to avoid the addition of the deadweight load of the iiuid column to the threaded pins during pumping, is in error. These tests showed that about 4() percent of the deadweight pumping load was added to the stress in the threaded pin in its relief section during the upstroke of the sucker rods and the pump. Such stress reversal in the sucker rod joints, and other similar connections, shortens the endurance limit of the connections, reduces the safe load limit that can be applied to the joints, or causes failure.

By virtue of the present invention, the stresses added to the sucker rod joints, as a result of the pumping load applied thereto, is reduced materially by placing a material of a signilicantly higher modulus of elasticity in the connection between the pin thread shoulder and the box connection. In the event that a coupling is used for interconnecting adjacent sections of sucker rods, the member of significantly higher modulus of elasticity would be placed between the pin thread shoulder and the opposed end of the coupling. In effect, the coupling constitutes a box portion of the threaded connection. It is found that the use of a member having a higher modulus of elasticity than the material of the pin and box between the pin shoulder and the adjacent box end lowers the added load imposed on the pin upon each stress reversal to a significant extent, as compared to the prior threaded connection. The material used may, for example, be a sintered tungsten carbide formed into a washer, which will be compressed between the pin shoulder and the opposed end of the box upon prestressing of the threaded 3,489,445 Patented Jan. 13, 1970 ICC joint. However, other materials can be used so long as the modulus of elasticity is substantially higher than that of the pin and box portions of the threaded joint. In fact, in a general sense, if the pin and box are made of materials other than steel, such as brass, the interposition of a cast iron or steel washer between the pin thread shoulder and the box will accomplish significant reduction in the stress in the pin of the prestressed joint resulting from the application of load thereto. By virtue of the present invention, a much lesser load is imposed on the connection, which accomplishes applicants desirable objectives that include increasing the endurance limit of the connection, increasing the safe load limit that can be carried thereby, and reducing failure of the connection.

This invention possesses many other advantages, and has other purposes which may be made more clearly apparent from a consideration of several `forms in which it may be embodied. Such forms are shown in the drawings accompanying and forming part of the present specification. These forms will now be described in detail for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Referring to the drawings:

FIGURE 1 is a longitudinal sectional and elevational View of a threaded connection embodying the invention;

FIG. 2 is a longitudinal section through a threaded connection used in its testing, and embodying a specifically different connection than disclosed in FIG. 1; and

FIG. 3 is a longitudinal section through a prior art threaded connection.

In FIG. 3, a typical prior art pin and box threaded connection or joint 10 is disclosed for interconnecting adjacent

sucker rod sections

11, 12 used in a pumping oil well. Thus, one of the sucker rods 11 has a pin portion 13 projecting longitudinally from the main body of the rod, having external threads 14 and an undercut 15 extending from the threads to a pin shoulder 16. The externally threaded portion of the pin is threadedly received within the internal threads 17 of the box portion 18 of the connection that has a counterbore 19 extending from its thread to its end 20. The threaded pin-box joint is tightened or prestressed to cause the end 20 of the box to abut the pin shoulder 16, thereby placing the undercut portion 15 of the pin in tension and the portion 21 of the box provided with the counterbore in compression. The tension applied to the undercut portion 15 of the pin will exceed to a substantial extent the pumping load to which the string of sucker rods is to be subjected, the theory, which has been proved erroneous, being that no additional loading will be applied to the undercut portion of the pin so long as the cyclic loading there-on is less than the initial tensile load or stress applied thereto as a result of preloading the pin and box connection 10.

Prestressing a sucker rod connection, such as disclosed in FIG. 3, only partially accomplish the desired result of reducing the deadweight load added to the pin stress during the pumping action. The box portion 21 opposite the pin relief may be considered to be a spring under cornpression and is exerting a load through the threaded connection on the undercut portion 15 of the pin. Thus, the load of the compressed box portion 21 is constantly being applied to the undercut portion 15 of the pin, and when the dead load applied to the pump on the upstroke of the sucker rods is added to this force, it increases the initial tensile force applied to the undercut portion 15 of the pin beyond that of its prestressed value. Accordingly,

an intermittent or cyclic load is being added to the undercut portion 15 of the pin, such cyclic loading imposing repeated reversals of stress on the pin which shorten its useful life and eventually introduces failure of one of the connections in the sucker rod string.

By virtue of the present invention, the load added to the undercut portion of the pin is considerably reduced by replacing the part 21 of the box member opposite the pin undercut 15 with a material having a higher modulus of elasticity than the material, such as a suitable steel, of which the pin and box are formed. As disclosed in FIG. 1, the box 18a hasbeen terminated substantially at the end 22 of its internal thread 17 and a washer 21a interposed between the pin shoulder 16 and the end of the box, this washer being made of a material having a higher modulus of elasticity than the material of the pin 13 and box 18a.. As an example, the washer 21a can be made of a sintered tungsten carbide which has a modulus of elasticity in excess of about 80 million. In fact, depending upon the particular sintered tungsten carbide employed, the modulus of elasticity of the washer 21a may range from about 67 to about 97.5 million. Sintered tungsten carbide is specieally mentioned for reasons of economy, and since it is believed to be best suited for the purposes of the present invention. Any suitable material having a modulus of elasticity substantially exceeding that of the material of the pin 13 and box 18a Would also achieve the objectives of the invention. As an example, assuming the sucker rods and their threaded pin and box portions to be made of a suitable steel having a modulus of elasticity of about 29 million, the intervening member or washer 21a can be made of a material having a modulus of elasticity substantially in excess of 29 million. Thus, tungsten carbide has a modulus of elasticity of about 79 million, tungsten of about 50 million, and chromium of about 36 million. In eifect, the Washer 21a of the higher modulus of elasticity provides a higher spring rate in the region between the end 22 of the box 18a and the pin shoulder 16, which -will have a lesser effect of continuing to impose its compressive force on the undercut portion 15 of the pin as loading is added to the string of sucker rods. If a material were available that was incompressible, the making of the washer 21a therefrom would enable a system to be provided that approaches a 100 percent prestress condition, that is to say, the prestressing of the threaded joint to an extent greater than the maximum loading to be applied thereto during the pumping action would result in no additional loading being applied to the pin undercut 15.

Actual tests have demonstrated the signicant advantages of applicants interposition of a washer of relatively high modulus of elasticity between the pin shoulder 16 and the end 22 of the box. FIG. 2 discloses a sucker ro-d connection on which a series of tests were conducted. Short sucker rod sections 11a, 12a were used, the sucker rods being of the hollow type, The adjacent sucker rods both had threaded

pins

25, 26 thereon interconnected by an intervening threaded coupling 30. As disclosed in the drawings, the coupling 30 had a lower counterbore 19b opposite the lower pin undercut 31, the lower end 20h of the coupling bearing against the pin shoulder 16 in the manner of a prior art connection disclosed in FIG. 3; whereas, the upper end 22a of the coupling terminated substantially at its upper internal thread, a sintered tungsten carbide washer 21a bearing against its end 22a and also against the upper pin shoulder 16, the washer being disposed opposite the upper pin undercut 32. Dimensions in inches of the various parts are given in FIG. 2. The only variations in dimensions between Tests A, B and C, reefrred to below, are in connection with the outside and inside diameters of the sintered tungsten carbide washer 21a. These dimensions for each of Tests A, B and C are set forth in FIG. 2.

The hollow sticker rods 11n, 12a and the coupling 30 interconnecting the same are standard parts, the dimensions given being standard. The only variation in FIG. 2

4 is in connection with the upper portion of the coupling, which was removed, and the washer 21a substituted in its place.

In connection with Tests A, B and C specifically referred to below, two standard hollow oil well sucker rod pony members 11a, 12a and a standard coupling 30 were purchased off the shelf. The coupling was machined for the substitution and insertion of the tungsten carbide washer 21a referred to above. Electronic strain gages Nos. 2 and 3 were cemented in the pin thread bores at the thread reliefs or undercuts 32, 31, respectively. Electronic strain gage No. 1 was cemented on the outside of one of the sucker rods 11a above the threaded joint to record the tensile load applied to the assembly.

Both joints threaded

pins

25, 26 were prestressed by appropriately threading the pins within the coupling 30 to the extent at which strain gage No. 2 showed a prestress corresponding to 44,100 p.s.i., and strain gage No. 3 a prestress also corresponding to 44,100 p.s.i.

Test A In Test A, an axial load or tensile pull of 13,800 lbs. was applied to the sucker rods 11a, 12a, which strain gage No, 1 disclosed as being equivalent to 27,600 p.s.i. The electronic indicator connected selectively to strain gages No. 2 and No. 3 were previously adjusted to zero with the 44,100 p.s.i. prestress applied thereto. The applied axial load of 13,800 lbs. showed an average stress at strain gage No. 2, where the tungsten carbide washer 21a was located, of 5,325 p.s.i., strain gage No. 3 showing an average stress of 8,115 p.s.i where the tungsten carbide washer was not used, but, instead, the prior art pin and box or coupling arrangement was employed. Therefore, there was a 34.4 percent decrease in stress at the pin undercut portion 32 where the tungsten carbide Washer was employed, as compared to the other connection, where it was not employed.

Test B In Test B, a dimensionally different washer 21a was used, in which its outside diameter was less than that employed in Test A, and its inside diameter was greater than that employed in Test A. The same prestress was applied, namely, 44,100 p.s.i. at the pin undercuts 32, 31 where strain gages No. 2 and No. 3 were located, and the same total axial load of 13,800 lbs. imposed on the assembly. Strain gage No. 1 showed the same unit stress of 27,600 p.s.i. The average axial load added to the pin undercut 32, where the tungsten carbide washer 21a was located, was 5,600 p.s.i.; whereas, the average axial load at the pin undercut 31, where the prior art box or coupling arrangement was provided, was 8,000 p.s.i. Thus, the axial load added to the pin undercut 32 was about 30 percent less than the load added at the pin undercut where the prior art type of connection was employed.

Test C In Test C, the only dimensional difference, as compared with Test A, was in the slight reduction in the outside diameter of the tungsten carbide Washer 21a. Again, the two threaded joints were prestressed to provide the prestressed load of 44,100 p.s.i. where strain gage No. 2 was located, and also 44,100 p.s.i. where strain gage No. 3 was located. The entire assembly was again placed in tension by imposing a 13,800 lbs. total tensile pull thereto. The axial load at the pin undercut 32 where the tungsten carbide vvasher was employed increased by 4,500 p.s.i., as transmitted to strain gage No. 2; whereas, the axial load at the undercut 31 where the prior art connection was employed, namely, at strain gage No. 3, increased by 8,000 p.s.i. Thus, strain gage No. 2 where the tungsten carbide Washer is used showed the addition of a 43 percent less stress to the undercut 32 than added to the other pin undercut 31.

The above actual tests disclose that the stress reversals in a threaded connection will be significantly reduced by placing a material of a substantially higher modulus of elasticity in the connection between the pin thread shoulder and the box connection. The term box connection is being used herein as including the threaded box 18a integral with the sucker rod member, as disclosed in FIG. 1, or a coupling 30 as disclosed in FIG. 2. The cyclic or intermittent applications of loading as a result of the operation of the sucker rod string adds a much lesser load or tensile force to the pin of each sucker rod joint, enhancing the endurance limit of each threaded joint, increasing the safe load that can be imposed on the sucker rod string, and minimizing fatigue or other types of failure of the threaded joint or connection.

4I claim:

1. In a combination in which sections are secured together, such sections being reciprocated in the performance of work and being subject to repeated intermittent tensile loading as a result of such reciprocation; said sections being metallic; one of said sections being elongate and having an externally threaded pin and shoulder; a metallic box threadedly receiving said pin and spaced axially from said shoulder; and an annular member in compression between said shoulder and box, said annular member having a modulus of elasticity at least about 25% greater than the vmodulus of elasticity of said one of said sections and its pin and the modulus of elasticity of said box.

2. In a combination as defined in claim 1; said pin having an undercut portion between said shoulder and external thread, said annular member surrounding said undercut portion.

3. In a combination as deiined in claim 1; said box, pin and said one of said sections being of steel; said annular member being a hard carbide.

4. In a combination as defined in claim 3; said hard carbide being sintered tungsten carbide.

5. In a combination as delined in claim 2; said box, pin and said one of said sections being of steel; said annular -member being of sinteredI tungsten carbide.

6. In a device for securing sucker rod sections together which are used in a well bore for pumping liquids therein to the surface: a sucker rod having an externally threaded pin and a shoulder; a box threadedly receiving said pin and spaced axially from said shoulder; and an annular member in compression between said shoulder and box, said annular member having a modulus of elasticity at least about 25% greater than the modulus of elasticity of said sucker rod and its pin and the modulus of elasticity of said box.

7. In a device as defined in claim 6; said pin, box and member in compression being prestressed when assembled together to a substantially greater value than the maximum pumping load to be applied to the rod sections in the Well bore.

8. In a device as defined in claim 6; said pin having an undercut portion between said shoulder and its externally threaded portion; said annular member surrounding said undercut portion.

9. In a device as delined in claim 6; said sucker rod and its pin and said box being of steel; said annular member being a hard carbide.

10. In a device as delined in claim 8; said sucker rod and its pin and said box being of steel; said annular member being of sintered tungsten carbide.

11. In a device as defined in claim 6; said box being integral with an adjacent sticker rod.

12. In a device as dened in claim `6; said box being provided in a coupling adapted to be secured to an adjacent sucker rod.

13. In a device for securing sucker rod sections together which are used in a well bore for pumping liquid.q therein to the surface: a steel sucker rod having an externally threaded pin, a shoulder and an undercut portion between said shoulder and externally threaded pin; a steel box threadedly receiving said pin and spaced axially from said shoulder; an annular sintered tungsten carbide member surrounding said undercut portion and engaging and compressed between said shoulder and box; said pin, box and member in compression being prestressed when assembled together to a substantially greater value than the maximum pumping load to be applied to the rod sections in the well bore; said undercut portion being in tension when said pin, box and member are assembled together.

14. In a device for securing sucker rod sections together which are used in a well bore for pumping liquids therein to the surface: a sucker rod having an externally threaded pin, a shoulder and an undercut portion between said shoulder and externally threaded pin; a box threadedly receiving said pin and spaced axially from said shoulder; an annular member surrounding said undercut portion and engaging and compressed between said shoulder and box, said annular member having a modulus of elasticity at least about 25 greater than the modulus of elasticity of said sucker rod and its pin and the modulus of elasticity of said box; said pin, box and annular member being prestressed when assembled together to a substantially greater value than the maximum pumping load to be applied to the rod sections in the well bore; said undercut portion being in tension when said pin, box and member are assembled together.

15. In a device as defined in claim 14; said sucker rod and its pin and said box being of steel; said member in compression being a hard carbide.

References Cited UNITED STATES PATENTS 254,477 3/1882 Hardy 285-333 X 2,045,520 6/ 1936 Davison 285-333k X 2,206,166 7/1940 Dunn 285-334 X 2,539,057 1/1951 Brown 285-115 X 2,955,847 10/1960 McKenna 285-286 X 1,059,560 4/1913 Parker 286-117 2,834,625 5/1958 Stanley et al 286-125 FOREIGN PATENTS 871,991 l/1942 France.

1,157,568 11/1963 Germany.

THOMAS F. CALLAGHAN, Primary Examiner U.S. Cl. X.R.