Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2000-06-29
2002-11-19
Browne, Lynne H. (Department: 3629)
Pipe joints or couplings
Particular interface
Tapered
C285S333000, C285S355000, C285S390000, C411S311000
Reexamination Certificate
active
06481760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Such connections are known, in particular for strings of casing pipes or production tubing or strings of drill pipes for hydrocarbon wells.
2. Discussion of the Background
In the remainder of the present document, the term “threaded connection for two metal pipes” will encompass both a so-called integral connection between two long pipes and a connection between a first long pipe and a second, short, pipe such as a coupling.
Thus the American Petroleum Institute (API) defines:
in specification API 5CT, metal pipes and threaded metal pipe connections for production and for casing hydrocarbon wells;
and in specification API 5B, standard tapered thread forms for such connections and standard thread forms for triangular, “round” or trapezoidal threads.
API 5B triangular or round thread forms comprise, on each of the male and female elements, two lateral flanks—a load flank and a stabbing flank—each at 30° either side of a plane normal to the axis of the connection.
At the end of the connection makeup, because of the taper of the thread forms, each of the two flanks is in contact under a metal-metal contact pressure with its mating flanks and a clearance exists between the crest and root of the mated threads, the crests and roots of the threads being very rounded in the case of round threads.
API 5B trapezoidal thread forms comprise, on each of the male and female elements, two lateral flanks—a load flank and a stabbing flank—slightly inclined with respect to a plane normal to the connection axis, a thread root and a thread crest generally parallel to the thread taper, the shape of the female threads mating perfectly with that of the male threads.
At the end of makeup of this type of connection with API 5B trapezoidal threads, because of the taper of the thread forms, the thread crest of at least one of the male or female elements is in contact, under a metal-metal contact pressure, with the root of the thread of the mated element; further, the load flanks are also in contact while a clearance exists between the stabbing flanks, at least in the most frequently encountered case where the elements of the connection are under tension from the weight of the string, for example, or from the bearing reaction of the abutments.
Such connections with an axial clearance are termed “with interfering threads” since they develop a radial interference between the mating threaded surfaces which results in a metal-metal contact pressure between these surfaces and in an increase in the makeup torque, the increase being due to the friction to be overcome. The intensity of the radial interference is measured by the value of the diametrical interference.
As is known, the term “diametrical interference” means the difference in diameter between a point on the male element and the mating point on the female element when the elements are not connected; a positive value for the diametrical interference means that a contact pressure exists in the contact zones of the connection; a zero value for the diametrical interference means a simple contact without a contact pressure, and a negative value for the diametrical interference means a radial clearance.
In other types of connections such as that described in European patent application EP-A-0 454 147, the thread form is designed so that the two lateral flanks just come into contact at the end of connection makeup; in contrast a clearance can be provided between the roots and crests of mated threads.
Such connections, termed “trapezoidal threaded with no axial clearance” in the remainder of the document, allows operation both in axial compression and in axial tension or bending.
Other connections such as those described in U.S. Pat. Nos. 2,211,179, 4,161,332 or 4,537,428 use a two-step straight thread with trapezoidal threads. Such thread forms normally do not have radial interference, contact between the threads only occurring at the load flanks, in particular the load flanks when the connection is subjected to axial tension forces.
US-RE30647 describes a variation of a threaded connection with two-step straight threads and a trapezoidal thread form known as wedge threads, the width of the male and female threads varying along the length of each step of the thread in a co-ordinated manner such that, as the male element is gradually screwed into the female element, the axial spaces between the male and female flanks existing on engaging the elements reduce to zero; the lateral flanks thus act as an abutment and to position the independent sealing surfaces.
In the remainder of the present document, the term “wedge thread” or “variable width thread” will be used for a thread with such characteristics.
US RE34467 describes an adaptation of a variable width thread on a connection comprising tapered interfering threads, with the aim of avoiding overpressures of grease between the roots and crests of the threads, which overpressures can cause erroneous measurements for the makeup torque in connections of the type described in US RE30647.
International patent application WO 94/29627 describes an adaptation of a variable width thread on a connection comprising tapered interfering threads with the aim of making up the connection to a very high makeup torque. In WO 94/29627, making up of a wedge thread is continued after the male lateral flanks of the thread have come into contact with the female flanks, the total surface of the thread flanks enabling a very high makeup torque to be exerted with no risk of plastification.
In the majority of threaded connections, in particular those with tapered threads, the makeup torque increases regularly during makeup and/or increases suddenly at a given moment but in any event it must remain below a value corresponding to that for plastification of the metal, as plastification causes permanent deformation of the metal and there is a risk of galling in the plasticized zones after several makeup-breakout operations.
If the slope of the graph of makeup torque vs number of screw turns is high, the maximum allowed torque will be reached after a relatively low rotation of one element relative to the other: this is the case, for example, in a threaded connection with threads which interfere very strongly or after the lateral flanks on a connection with trapezoidal threads with no axial clearance or with variable width threads come into contact.
Since machining tolerances cause variable slopes in the torque curves depending on matching of the male/female threads, it is not possible under such conditions to predict the final relative position of the elements of the connection for the maximum allowed value of the torque.
The above disadvantages are amplified when each male and female element also comprises a metal-metal sealing surface and an abutment such as, for example, in EP-A-0 454 147 as the action of the thread, the sealing surface and the abutment at the end of screwing must be synchronised at the end of connection makeup for all male-female element mating scenarios.
SUMMARY OF THE INVENTION
The present invention seeks to provide a connection between two metal pipes using tapered, straight or straight-tapered threads with different thread forms and connection clearances which can overcome the disadvantages described above.
The term “thread” as used in the remainder of the document means the totality of the threaded portions of an element. A thread can thus be constituted by a single threaded portion or by a plurality of threaded portions, for example two stepped straight threaded portions or by one tapered threaded portion and one cylindrical threaded portion of the type described in U.S. Pat. No. 5,437,429, the thread of that patent being termed straight-tapered.
In accordance with the invention, a threaded connection for two metal pipes comprising a male element at the end of a first pipe and a female element at the end of a second pipe, the male element comprising a male external thread, the female element comprising a female internal thread wherein each of the thread flanks i
Noel Thierry
Varenne Emmanuel
Browne Lynne H.
Dunwoody Aaron
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Vallourec Mannesmann Oil & Gas France
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