Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2000-07-11
2002-07-02
Browne, Lynne H. (Department: 3629)
Pipe joints or couplings
Particular interface
Tapered
C285S355000, C285S333000, C285S390000
Reexamination Certificate
active
06412831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to threaded connections for two metal pipes with a tapered thread and with trapezoidal threads.
Such connections are known, in particular for strings of casing pipes or production tubing or drillpipe strings 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 an integral connection between two long pipes and a connection between a first, long, pipe and a second, short, pipe such as a coupling.
The American Petroleum Institute (API) defines:
in specification API
5
CT, metal pipes and threaded metal pipe connections for production and for casing hydrocarbon wells;
and in specification API
5
B, standard tapered thread forms for such connections.
The threads of such API thread connections can be trapezoidal and then comprise, on each of the male and female elements, a thread root, a thread crest and two flanks, namely a load flank and a stabbing flank.
The thread roots and thread crests are normally parallel to the taper of the thread.
The load flanks are so termed because on bearing against each other when the connection is subjected to tensile forces, for example due to the weight of the pipes, they enable the connection to tolerate such tensile forces. The load flanks are located on the threads opposite the stabbing flanks.
When making up such an API connection, depending on the taper of the threads, at a given moment corresponding to a given relative position of the male and female elements, the thread roots of one of the elements come into contact with the thread crests of the other element.
If screwing of the male element is continued into the female element beyond that position, the male threads start to interfere radially with the female elements, which leads to an expansion of the female element and a contraction in the male element; such an interference must be limited so as not to develop excessive stresses or deformation.
The diametrical interference between the mated points of two surfaces of revolution which radially interfere is generally defined as the difference in the diameter of the half cross section of the surfaces at those points, the difference being measured before connection and taken to be positive when the two surfaces, once connected, exert a contact pressure between the mated points.
To limit such stresses or deformations, an annular bearing surface which is orientated substantially transverse with respect to the axis of the connection can be provided on each of the male and female elements, the bearing surfaces being positioned such that they come into abutment with each other at a given moment during makeup thus precisely defining a makeup completion position.
The position at the end of connection makeup is, for example, determined by the torque required to arrive at that position.
The use of abutting bearing surfaces to position the connection has other advantages:
placing the load flanks of the connection threads under tension which are then ready to tolerate the tensile stresses to which the connection is subjected during service;
precise positioning of the male and female elements thus guaranteeing, when each of the male and female elements of the connection comprises a sealing surface which radially interferes with that located on the mating element, a high metal-metal contact pressure between the surfaces with no risk of plastification thereof;
reduced risk of accidental breakout because of the makeup torque which has to be overcome before being able to break out the connection, this torque being well defined and always above a minimum value.
European patent EP-A-0 488 912 describes such a connection with tapered threads screwed one into the other, a pair of radially interfering metal-metal sealing surfaces and a pair of abutting bearing surfaces, namely a concave tapered surface at the end of the male element and a convex annular surface forming an internal shoulder on the female element.
Such a threaded connection can be made up with high nominal makeup torques which can be up to 34 kN.m (25000 lbf.ft), for example, which is sufficient in the majority of cases.
However, it may be necessary to make up the connection with even higher torques, in particular for casing pipes for multiple deviated wells or horizontal wells enabling a wide zone to be exploited from a single site.
The use of techniques for rotating the string comprising drillpipes at their end (drilling liner) also permits better cementing of horizontal wells but necessitates pipe connections made up with torques which are higher than the rotational torque of the string if it is desired to prevent the threaded elements from rotating with respect to each other when the string is rotated, which rotation between elements can modify the characteristics of use of the connections, in particular their sealing properties.
Table 1 below gives an idea of the desired makeup torques for such applications.
TABLE 1
Desired level of makeup torque
External pipe diameter
Level of makeup torque
(mm)
(″)
(kN · m)
(lbf · ft)
101.6-139.7
4″-5″½
20-34
15000-25000
168.3-177.8
6″⅝-7″
27-41
20000-30000
244.5
9″⅝
54-88
40000-65000
The abutting bearing surfaces can only tolerate such torques without deterioration if the radial width of the abutment surfaces is increased, but then much thicker pipes have to be used which may be incompatible with service requirements.
Thus other means have to be used than abutting bearing surfaces to absorb high makeup torques.
International patent application WO 94/29627 describes a threaded connection with a tapered thread and trapezoidal threads known as wedge threads in the general form of a dovetail and more particularly a half dovetail.
Such threads are known as wedge threads or threads with a variable width since the width of the male and female threads varies from one end of the thread to the other in a manner which is coordinated between the male and female threads.
Such threads are termed “half dovetail” since they overhang the thread roots on one side only, either on the load flank side, or on the stabbing flank side, and because the angle between the load flank and the normal to the connection axis and that between the stabbing flank and said normal is such that the thread width is higher at the crest than at the root.
When the male element is engaged in the female element in accordance with WO 94/29627, the narrowest thread crests face the widest thread roots and there is a large axial clearance between the mating flanks of the threads.
As the male element is screwed into the female element, the axial clearance reduces to a position where the two male flanks come into contact with their female mates.
Beyond that position, the female flanks interfere with the male flanks and there ensues a very rapid increase in the curve of the makeup torque as a function of rotation.
Such a connection in accordance with WO 94/25627 can certainly tolerate a high makeup torque due to the developed surface of the threads but it suffers from a number of important disadvantages.
Firstly, variable width wedge threads are expensive to machine and difficult to inspect.
Further, the acute angles of dovetails or half dovetails disposed on the load flank side and/or on the stabbing flank side, constitute sharp angles which are sensitive to cuts and flash from such cuts are deleterious to the function of the connection.
Such sharp angles also notch the thread roots and as a result the threads are more fragile during use.
SUMMARY OF THE INVENTION
The present invention seeks to provide a threaded connection which can be made up to a high makeup torque T which is free of such disadvantages and in particular a threaded connection which is economical to machine and which can be readily manipulated on-site.
We have also sought to provide a threaded connection whereby the desired makeup torque is obtained after considerable rotation, for example of the
Noel Thierry
Varenne Emmanuel
Dunwoody Aaron
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Vallourec Mannesmann Oil & Gas France
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