Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2002-02-22
2004-05-04
Browne, Lynne H. (Department: 3679)
Pipe joints or couplings
Particular interface
Tapered
C285S334000, C285S355000, C285S390000
Reexamination Certificate
active
06729658
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a male or female threaded tubular element of a threaded tubular connection which is particularly able to resist 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.
BACKGROUND OF THE INVENTION
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 5B, 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 threads: see, for example, U.S. Pat. No. 4,521,042.
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 and 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 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 root of the threads, generally on the side of the load flanks which are under load.
This preferred location for initiation of fatigue cracks results from a stress concentration at the junction between the load flank and the thread root.
To improve the resistance to cyclic stresses, it is necessary to reduce the maximum level of the stresses by reducing the general level of stresses on the load flank and by producing the least possible angularity of the junction between the load flank and the thread root.
API specification 7D defines drillpipes with robust tapered threads which are adapted to operational stresses. API specification 7D threads are triangular in shape and very rounded with load and stabbing flanks which are each disposed at 30° with respect to the normal to the axis of the threaded element.
The load flank is that which is disposed on each thread on the side opposite the free end of the element. This definition will be used throughout the present document.
The thread root is rounded in an arc of a circle with a radius of 0.97 mm (0.038″) centred on the axis of the thread root; this arc of a circle joins tangentially with the flanks.
The angle of 60° between the thread flanks resulting from the triangular shape of the threads enables the radius of the arc of a circle to be substantial.
The thread crests are truncated so as to avoid any radial interference between the thread crests and the thread roots of the mated threading.
The height of the truncated threads is 3.08 mm (0.121″) which corresponds to twice the height of API 5B threaded connections.
These means can, however, prove to be insufficient since U.S. Pat. No. 4,549,754 describes a threading profile which is modified with respect to API specification 7D for drillpipes which renders it capable of further reducing stress concentration.
The thread of U.S. Pat. No. 4,549,754 shows in cross section a root which is not symmetrical but comprises a rounded zone the centre of which is offset towards the stabbing flank (opposite the load flank) and with the radius increased by about 50% with respect to the API radius, namely 1.45 mm (0.057″).
This rounded zone joins the load flank tangentially while it joins with the stabbing flank in a less critical profile: a simple straight segment or a radius of 0.81 mm (0.032″) followed by a straight segment.
The thread root is thus more undercut than an API thread and hence requires a large starting pipe thickness in order to cut the threads.
Such a disposition cannot be envisaged for strings of pipes for exploiting wells since these are subjected to both static and dynamic stresses.
Such demands on stress resistance are now being encountered in underwater strings connecting the sea bed to offshore hydrocarbon exploitation platforms.
Such columns of pipes, known to the English-speaking skilled persons as “risers”, are 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.
Such demands on stress resistance 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.
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.
Patent application WO 98/50 720 describes such an improved threaded tubular connection.
The threadings described in that document have trapezoidal threads derived from the threads of API specification 5B known as “buttress” threads.
The trapezoidal shape of the threads limits the risk of deformation of the threaded elements which can lead to them dislodging during coupling, in particular by overtorquing.
The thread roots are substantially rectilinear and join to each of the flanks via a rounded zone the radius of which is comprised between 10% and 50% of the total width of the thread root (and preferably between 16% and 26% of that total width), the rounded zone terminating tangentially to the flank and to the thread root.
The thread heights are such that radial interference between the root of one thread and the crest of the corresponding thread of the mated threading is completely avoided by maintaining a radial clearance of at least 0.25 mm (0.010″).
Taking into account the threadings given in the example, the radii at the thread root are of the order of 0.5 mm as opposed to 0.15 mm for the radii specified in API 5B.
Such radii may appear low if they are compared with those of drillpipes but the trapezoidal form of the threads used does not allow as large radii to be formed as in the case of triangular threads unless it is acceptable to drastically reduce the bearing surface of the contacting flanks.
The threadings of document WO 98/50720 are also not adapted to interfering type threads which have a radial interference between the thread crests of one threading and the corresponding thread roots of the mated threading. The threads shown are “wedge” type threads with a variable width, like those shown in U.S. Re. Pat. No. 30,647.
SUMMARY OF THE INVENTION
The aim of the present invention is to produce a male or female threaded tubular element for threaded tubular connections, which are particularly resistant both to:
a) static stresses, in particular axial tension, axial compression, bending, torsion, internal or external pressure, dislodging during connection, either simple or combined (for example tension+internal pressure);
b) cyclic stresses.
In the remainder of the present document, such a threaded element will be described as having an anti-fatigue profile.
The present invention also aims to ensure that the threaded tubular element of the invention can be formed with all types of threadings: tapered, straight, straight-tapered combinations, with one or more steps, with trapezoidal or triangular threads, which may be interfering or non-interfering; non interfering threadings are, for example, of the type described in European patent application EP 0 454 147 with simultaneous contact of the two flanks with those of the mated thread (also called “rugged
Browne Lynne H.
Hewitt James M.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Vallourec Mannesmann Oil & Gas France
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