The present invention relates to a threaded pipe coupling, and in particular to a threaded connection having special thickness relationships between the threaded portion of a pin portion of the pipe, and a threaded portion of a box portion of a coupling.
In the prior art, various designs of threaded pipe joints have been proposed for producing strings or lines of pipes for the oil industry. One goal of the oil industry is to reduce the diameter of the down hole of an oil well. Normally, oil well pipes having threaded joints on both ends, and the pipes are connected together through female couplings linking with the opposing males ends of adjacent pipes. The outer diameter of the female coupling is the largest so it is this diameter that controls the diameter of the down hole.
Various techniques and designs have been proposed to minimize the down hole diameter. One example of such a proposal is taught in U.S. Pat. No. 5,687,999 to Lancry et al., herein incorporated by reference. This patent describes a threaded joint for tubes, wherein each of the male and female element of a coupling have two shifted frustoconical threaded sections with vanishing threads at the two ends as a result of convergences of frustoconical envelop surfaces and cylindrical surfaces. The joint also has a central abutment that cooperates with negative flank threads to reinforce the male/female connection and two internal and external ring-shaped metal/metal tightness zones that prevent the penetration of fluid into the threaded zones.
One drawback of the design of the joint disclosed in the Lancry et al. patent is that it is necessary to swage or expand the edge of the pipes to extend the diameter of the edge, see col. 9, lines 60 to col. 10, line 18. While this swaging is not as much of a problem when using low alloy steels as the material for the pipe, swaging becomes much more difficult when employing a highly alloyed material such as steel containing 13% chromium, because the swaging may compromise the corrosion resistance of the pipe.
Therefore, a need exists to provide improved pipe joint designs that reduce the diameter of the coupling without compromising other properties or characteristics of the joint that are important to drilling.
One object of the present invention is an improved threaded pipe joint.
Another object of the present invention is a threaded pipe joint that includes a male end section and a female end section that are especially configured to minimize the diameter of the female end section, thus allowing for the durability of the threaded pipe joint to external pressure without a loss in sealing performance and anti-galling characteristics.
Other objects and advantages of the invention will become apparent for the following description.
preferred embodiment of the invention is illustrated in the accompanying representations in which:
a-e show a joint of
The present invention is an improvement in threaded pipe joints, and particularly, threaded pipe joints that are designed to minimize the outer diameter of the female coupling so as to minimize the diameter of a down hole for oil drilling.
Referring now to
For couplings or boxes, the critical section for bearing the tensile force is at the inner end region of the thread (at Sc), because at this region, only the coupling bears the force and the cross section of the coupling becomes smallest.
It is normal in the prior art for the cross section of the pipe (Sp) to be same as the cross section of the critical section (Sc). This is so because reducing the outer diameter of the coupling, i.e., making Sc less, results in reducing the bearing tensile force. But for compression force, the shoulder 9 located at the tip 11 of the pin also bears the force, so the coupling with less Sc can still bear compression force adequately. Near the bottom of down holes, the tensile force to the coupling is not so large because the number of connected pipes below it is small. On the other hand, compression force may be still high at the bottom because such force originates from the circumstance of the pipes. Since the tensile forces are not so high near the bottom of the down holes, it is not necessary that Sc match Sp, as would be the case where tensile forces are high.
Thus, a coupling can be made wherein Sc is 80% of Sp, or Sc=0.80 Sp. This can be accomplished by merely making the box 3 with a smaller outside diameter. However, reducing the diameter more, e.g., Sc=0.60 Sp, causes the thickness Se to be very small, and this reduced thickness region does not bear the hoop stress by the internal pressure and interference of threads. A solution to this problem would be to control the interference of the threads 5 and 7, but doing so requires severe control of the thread dimensions, and such is not cost effective or practical.
The invention proposes an alternative approach to obtain a joint with a reduced diameter of the box, e.g., the thickness of the box Sc equals 0.60 Sp. This is accomplished by reducing the diameter of the tapered male end or pin of the pipe. This is illustrated in
A preferred relationship between Sc (special clearance) and Sp, and Sp1 and Sp are shown in
a-e show in more detail how the pin 21 and box 23 interface. The box 23 is shown with threaded portion 26 in
Referring to
By practicing the invention, the goal of reducing the outside diameter of the box or male end of the coupling is achieved without a loss in joint performance by unduly minimizing the box end thickness. This is accomplished by controlling both the thickness of the box end and the pin end to satisfy the relation as outlined above for Sp and each of Sc and Sp1. In this way, the disadvantages of making Sc too thin when seeking to reduce the overall diameter of the joint are avoided. It is believed that the invention can be used for any type of pipe, including different materials, and different sizes and application. It is preferred that the pipe is a corrosion resistant pipe for oil drilling as discussed above.
As such, an invention has been disclosed in terms of preferred embodiments thereof which fulfills each and every one of the objects of the present invention as set forth above and provides a new and improved threaded pipe joint. Of course, various changes, modifications and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.
Number | Date | Country | |
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60701519 | Jul 2005 | US |