1. Field of the Invention
The present invention relates to tubular connections for application to heavy wall, high performance casing in high pressure, critical service oil and gas wells. More particularly, the present invention relates to such a connection having an improved taper and buttress type threads.
2. Description of the Prior Art
In recent years, oil and gas wells that have been drilled for exploration and production have commonly extended to depths in the range of 15,000 to 35,000 feet, where pressures and temperatures may exceed 15,000 psi and 250.degree. F. respectively. These conditions not only require tubulars (commonly known as Oil Country Tubular Goods (OCTG)) having higher strength, higher reliability and better resistance to failure under higher loads and corrosive applications, such as are disclosed in commonly assigned U.S. Pat. No. 4,354,882, but also require tubular connections which have higher strength and better resistance to severe stress applications. There are three basic types of OCTG's, each of which makes its own contribution to the drilling and completion of oil and gas wells. These types of tubulars are drill pipe, casing and tubing. This invention is primarily intended as a superior connection for heavy wall casing.
Typically, the lengths of tubulars are threaded at each end and are joined together by an internally threaded coupling. The threads of both the tubes and the couplings must be able to carry the weight of the tubulars and couplings extending below it into the well and must also be able to withstand the high internal and external pressure encountered during drilling and producing of the wells. Since the casing commonly in use will have a weight of from 10 to in excess of 100 pounds per foot, the tensile loads which the threads in the uppermost casing connection must be able to withstand when there are 15,000 to 35,000 feet of tubulars and connections below it are tremendous.
With sufficiently high tensile loads, connections may fail due to rupture of the base metal within the connection or by sudden disengagement of the mated male and female threads. The latter of these failures is termed the "pull-out" mode of failure.
The casing connection must, of course, be leak-proof and a means of sealing must be provided. The threaded connections must be capable of being made up and disassembled without galling and without the danger of stripping threads or cross-threading. It is desirable after the joint is "stabbed" that it can be made up with a relatively small number of turns.
Two types of casing connections are in common use, coupled and integral joint. Coupled connections employ male threads on each end of the tube which are joined by a female threaded connector called a coupling. Tubes with male threads on one end and female threads on the other are referred to as integral joints since no couplings are required.
The following patents and publications illustrate the current state of the art.
Eaton U.S. Pat. No. 2,062,407 discloses a threaded pipe joint of the integral type which employs a buttress type thread having broad crests and roots and which is disclosed to have a reduced galling tendency (page 6, column 2, lines 48-50); easier and faster make-up (page 7, column 2, lines 28-40); and improved hoop strength at the end of the female section of the joint (page 3, column 2, lines 36-46).
Frame U.S. Pat. No. 2,204,754 discloses a threaded pipe joint of the integral type having a modified acme type thread wherein the thread crests and roots are flat and parallel to the axis of the joint (page 2, column 1, lines 34-42). Five threads per inch are provided and are tapered at both ends, but not in the middle of the thread length, between 3/4" and 13/4" per foot (page 2, column 2, lines 21-27).
Rollins U.S. Pat. No. 2,885,225 discloses a drill pipe coupling employing a double thread having three to five threads per inch (column 2, lines 37-43) which is tapered about 11/2" per foot (column 1, lines 63-67). This taper is said to result in easier and rapid make-up and to promote optimum stabbing (column 1, lines 65-67). The thread is provided with flat crests and roots which are parallel to the taper (column 4, lines 11-20).
Kloesel, Jr., et al., U.S. Pat. No. 3,355,192 discloses a pipe joint of the integral type having a modified buttress type double thread which is tapered 11/2" per foot (column 3, lines 40-47, and see column 5, lines 40-45).
Yocum U.S. Pat. No. 3,346,278 discloses a tapered V-thread for pipe joints of the integral type wherein the external threads have a basic taper of between 20 percent and 80 percent of the basic taper of the internal threads (column 2, lines 25-28). This arrangement is said to produce a seal between the leading external threads and the innermost internal threads irrespective of whether either thread is at the extreme limits of the commercial taper tolerance (column 3, lines 34-40).
Nowosadko U.S. Pat. No. 3,427,707 discloses a non-threaded soldered pipe joint wherein the female member is machined to a larger taper than the male member so as to assure a seal at the leading end of the male member (column 2, lines 37-41).
Blose et al. U.S. Pat. No. 3,224,799 discloses a threaded tube joint of the integral type which employs a buttress type thread tapered about 1" per foot to permit deeper stabbing and more rapid make-up (column 2, lines 51-54; see also column 3, lines 44-47). U.S. Pat. No. 3,224,799 also suggests that the tolerance be such that the actual taper of the male threads will tend to be slightly deeper than that of the female threads (column 3, lines 47-55).
Blount et al. U.S. Pat. No. 3,307,860 discloses a pipe coupling having an internally thickened central region and shoulders against which a resilient sealing ring is compressed by a shoulder on the ends of the pipe (column 6, lines 36-51; see also column 4, lines 49-55). The threads are of a tapered modified buttress type (column 3, lines 13-15).
Blose et al. U.S. Pat. No. 3,572,777 discloses a pipe coupling thickened in the central region and provided with an internal groove for a resilient sealing ring which is compressed by the ends of the pipe (column 3, lines 1-18). The thread is of the acme or buttress type and is provided with a relatively steep taper (column 1, lines 5-16).
American Petroleum Institute (API) Standard 5B (March, 1979) discloses a tubular connection having a tapered buttress casing thread. The API tubular connection consists of a male joining element on the end of the pipe and a coupling member provided with cooperating female joining elements. The threads are on a taper of 3/4" per foot on the diameter on sizes 133/8" OD and smaller. The male joining element taper has a tolerance of from +0.0035 inch per inch to -0.0015 inch per inch. The female joining element taper has a tolerance of from +0.0045 inch per inch to -0.0025 inch per inch. The maximum disadvantageous taper spread (i.e., minus on coupling, plus on the pipe end) is 0.006 inch per inch. The API buttress threads have a substantially rectangular form with rounded corners and parallel crests and roots, a 0.200 inch thread pitch, a 3.degree. load flank, 10.degree. trailing flank, and a 0.062 inch thread height with a tolerance of plus or minus 0.001 inch.
The thread crests on the API male joining element threads have a length of 0.100 inch with a tolerance of from +0.000 inch to -0.003 inch (+0.000 inch to -0.005 inch on sizes 85/8" and larger). The rounded corner on the load flank end of the crest has a radius of 0.008 inch with a tolerance of +0.002 to -0.000 and the rounded corner on the trailing flank end of the crest has a radius of 0.030 with a tolerance of from +0.002 to -0.000 inch. The roots have a length of 0.100 inch with a tolerance of from +0.003 to -0.000 inch (+0.005 to -0.000 on sizes 85/8" or larger). The rounded corners on either end of the root have a radius of 0.008 inch with a tolerance of from +0.000 inch to -0.002 inch.
The crests of the API female joining element threads have a length of 0.099 inch with a tolerance of from 0.000 inch to -0.003 inch. The rounded corners at each end of the crest have a radius of 0.008 inch with a tolerance of from +0.002 inch to -0.000 inch. The roots have a length of 0.101 inch with a tolerance of +0.003 inch to -0.000 inch. The rounded corner of the load flank end of the root has a radius of from 0.008 inch with a tolerance of +0.000 inch to -0.002 inch. The rounded corner of the trailing flank end of the root has a radius of 0.030 inch with a tolerance of from +0.000 inch to -0.002 inch.
These API buttress thread tolerances result in a thread form that makes contact on the root and crest of the thread and the 3.degree. load bearing flank, but which permits a clearance on the trailing 10.degree. flank of as much as 0.007" on smaller diameter tubulars. This clearance forms a leakage path that must be sealed by the solids in the thread lube. The API taper of 3/4" per foot results in 1.984" of imperfect (non-sealing threads) and, for a 7", nominal, outside diameter casing corresponds to a buttress thread length of 4.200 inches.
The present invention represents an improvement on this API tubular connection and, as compared to that connection, provides a reduced maximum mated thread clearance, combines less imperfect threads with an increase in overall thread length to provide a greatly increased leakage path, and an advantageous taper mismatch which provides high bearing pressure and sealing within the made-up joint adjacent the narrow end of the male member where the coupling is strongest.