1. Field of the Invention
The invention relates to threaded tubular connections.
2. Discussion of the Related Art
Threaded tubular connections can include a male threaded element and a female threaded element with trapezoidal threads located at the end of pipes to be connected; the threaded elements can be disposed as well as the end of great length pipes as at the end of short length pipes such as couplings.
Such threaded tubular connections are in particular used to constitute casing strings or tubing strings or drillpipe strings for hydrocarbon wells or for similar wells such as geothermal wells.
They have, for example, been defined in American Petroleum Institute (API) specifications API 5B and 5CT as tapered threadings with trapezoidal threads termed “buttress” threads.
The trapezoidal threads comprise a stabbing flank on the side of the threads directed towards the free end of the threaded element under consideration, a load flank on the side opposite the threads, a thread crest with a non zero width and a thread root also with a non zero width, the load flanks and the stabbing flanks being oriented substantially perpendicular to the axis of the threaded element (inclination at +3° for the load flanks, at +10° for the stabbing flanks for example in the case of the API Buttress threads).
Trapezoidal threads have thus been defined in the present document by opposition to other types of threads such as defined in the above mentioned API specifications, i.e. triangular or triangular rounded threads (“round”) the load and stabbing flanks of which are highly inclined with respect to the normal to the axis of the threaded element (at 30° for example) and the thread crests and roots of which have a substantially zero width. Compared to the triangular or round trapezoidal threads present important advantages with respect to the unacceptable risk of jumping out.
API type threaded tubular connections with trapezoidal threads constitute the subject matter of a number of developments; in particular to improve their strength to the very wide variety of stresses resulting from the operating conditions (axial tension, axial compression, bending, torsion, internal or external pressure . . . ) and their tightness against fluids circulating inside or outside such connections under such stresses. These improvements have, for example, been described in the following documents: EP 0 488 912, EP 0 707 133, EP 0 454 147 and International patent application WO 00/14441.
Other types of threaded connections with trapezoidal threads described for example in patents U.S. Pat. No. 4,521,042 or U.S. Pat. No. 4,570,982 and designed for the same purpose use straight threadings, in particular two-step threadings.
Generally speaking the faces of the trapezoidal threads of the threaded connections of the prior art, i.e., the flanks of the threads and the crests and roots of the threads, appear rectilinear in axial cross section except for the junctions between the faces, which junctions generally have a junction radius or a chamfer; for this reason, such faces will be termed “rectilinear” in the present document.
In all those types of threaded tubular connections, when the threaded elements have been made up into each other, contact is developed with a contact pressure between at least one male thread face and the corresponding face of the female thread. Depending on the threading type, that contact pressure can be developed between mating thread crests and roots, between load flanks, between stabbing flanks or between a plurality of such faces.
Developing a contact pressure between corresponding trapezoidal thread faces (and possibly other surfaces such as sealing surfaces and transverse abutment surfaces disposed in a corresponding manner on the threaded elements) results from the necessity to make up the threaded connection with a substantial make-up torque.
Once the threaded connection is connected with a given makeup torque, it has been noted, in known threaded connections, that it is sufficient to apply a torque the absolute value of which is only slightly higher than the makeup torque to, depending on the direction of that torque, overtorque or breakout the threaded connection.
Overtorquing can in particular occur when a pipe string is rotated and descended to the bottom of a deviated or even horizontal hydrocarbon well and cause an offset in the relative positioning of the male and female elements, with the deleterious result of a risk of leakage of the threaded connections.
Accidental breakout in a well can have even more severe consequences.