Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2002-03-15
2004-06-22
Browne, Lynne H. (Department: 3679)
Pipe joints or couplings
Particular interface
Tapered
C285S390000
Reexamination Certificate
active
06752436
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a male or female threaded tubular element of a threaded tubular connection which is particularly capable of resisting both static and cyclic stresses.
The present invention also relates to a threaded tubular connection which is particularly suitable for resisting both static and cyclic stresses.
(2) Description of Related Art
Threaded tubular connections comprise a male threaded element at the end of a first pipe and a female threaded element at the end of a second pipe which may be a great length pipe or a coupling. Such threaded connections are used in particular to constitute casing strings or production strings or drillpipe strings for hydrocarbon wells or for similar wells such as for example geothermal wells.
In its API specification
5
B, the American Petroleum Institute (API) defines threaded connections between casing pipes or between production pipes in particular with tapered threadings with trapezoidal or round triangular threads.
Other types of threaded connections are also known which use straight or tapered two-step thread: see, for example, U.S. Pat. Nos. 4,521,042 and 5,687,999.
Until recently, casing pipes or production pipes had essentially to be capable of resisting different combinations of static stresses (tension, axial compression, plane bending, internal or external pressure) despite their limited thickness resulting from the need, to be able to exploit a deep well, to insert a variety of columns of different diameters one into another.
In contrast, drillpipes, which are only used to drill wells, are subjected to substantial cyclic stresses (dynamic ones) but are not subjected to size limitations, since a single string of a given diameter is downhole at a given time.
If not strictly limited, cyclic stresses lead during operation to fatigue ruptures which start at the thread roots, generally on the side of the load flanks which are beneath load in operation, more particularly at the last engaging threads of each of the threaded elements of the drillpipes.
In the remainder of the present document, the term “first threads” means the threads which, in a longitudinal cross section passing through the axis of the threaded element, are located on the free end side of the threaded element. As a result, the last threads are those located at the other end of the threading.
The term “engaging threads” firstly means the threads of a threaded tubular connection which transfer the load from one threaded tubular element to the mated threaded tubular element.
When the threaded connection is subjected to tensile loads, the engaging threads are those where the load flanks are in contact and transfer the load from one threaded element to the mated threaded element.
By extension, the term “engaging threads of a threaded tubular element” as used in the present document means the threads intended to transfer the load to the corresponding threads of a mated threaded tubular element when these two threaded tubular elements are connected to constitute a threaded tubular connection.
The position of the engaging threads of a threaded tubular element is known from the design of the threaded element. This is a theoretical factor defined by the nominal dimensions of the threaded elements to be connected.
The position of the last or first engaging threads can thus be perfectly defined for a threaded tubular element intended for a threaded connection.
However the problem of fatigue behavior is now not only encountered in drillpipes but also for production pipe strings for certain hydrocarbon wells.
The threaded tubular connections which enable such strings to be produced must be capable of tolerating both high static stresses and cyclic stresses.
Such demands on stress behavior are now being encountered in underwater strings connecting the seabed to offshore hydrocarbon exploitation platforms.
Such columns of pipes, known to the English-speaking skilled person as “risers”, are indeed subjected to cyclic stresses caused in particular by currents which induce vibrations in the column, by waves, by tides and by possible displacement of the platforms themselves, all stresses which induce mainly cyclic bending stresses and/or tension-compression stresses.
Such demands on stress behavior are also encountered in onshore wells, in particular when dropping rotating pipes in order to cement wells in the very frequent case of wells which deviate from the vertical and have bends; the rotating dropping produces then a rotative bending.
For this reason, improvements to threaded tubular connections for casing pipes, for production pipes or for risers have been sought in order to increase their fatigue strength.
The prior art for threaded tubular or non-tubular connections (screw-nut type, for example) proposes means for improving the fatigue strength of threaded connections subjected to axial tensile loads which can vary cyclically.
patent application WO 00/06931 describes a female threaded element for a threaded tubular connection the outside peripheral surface of which is tapered and has a diameter which decreases as the female free end gets nearer so that the material thickness beneath the threading is reduced at the level of the first threads. There results a great sensitiveness to shocks of the female free end, which is very slim.
Many documents, including U.S. Pat. No. 5,779,416 and patent applications JP 04 157 280 and JP 04 157 283, use a groove in the form of a U beyond the last male engaging threads in the non threaded portion under tension of the male threaded element. Such a groove has the major disadvantage of reducing the critical cross section of the threaded connection which is the most stressed under axial tension and consequently of reducing the tensile static performances of the threaded connection.
U.S. Pat. No. 3,933,074 describes a nut for a bolted connection where the internal threading is interrupted at the first engaging threads by a plurality of axial channels in hollow regularly disposed on the periphery of the threading in order to displace the maximal transfer zone for axial tensile stress between the screw and nut from the first female engaging thread towards the middle of the axial length of the bolt.
Those channels the length of which can reach half the length of the threading and the depth of which can reach up to 80% of the height of the thread, increase the flexibility of the first engaging threads but reduce by about 20% the bearing surface of the threads in the zone where they are produced, which is a disadvantage when a high resistance to static stress is sought and when a threaded tubular connection is sought the interior and exterior of which are sealed.
Moreover, solutions as regard bolts in which the nuts are bearing on the side of the first threads against the screw casing-head (on the side of the last threads of the screw) are not necessarily directly applicable to threaded tubular connections.
Patent FR 1 317 815 describes an annular groove with the profile of a basin comparatively not very deep formed on the external peripheral surface of a female threaded element of a drill pipe.
BRIEF SUMMARY OF THE INVENTION
In the figures of this patent, the groove is disposed in the middle of the threading and has no effect on the wall at the level of the first or last threads. It enables to spread out the concentrations of stresses on the whole of the male threading while increasing the stresses at the level of the threads which are situated beneath the groove about the middle of the threading.
Nevertheless this patent does not tell to which stresses the stress fields shown in the figures (torsion, tension, compression, bending) correspond. They seem to be stresses simply resulting from the made up state of the threaded elements of the drillpipes.
It will also be noted, still in the figures of this patent, that the groove has a flat bottom parallel to the axis of the threaded connection and steep flanks, which are roughly perpendicular with respect to the groove bottom and to
Dunwoody Aaron
Vallourec Mannesmann Oil & Gas France
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