Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2000-04-28
2003-06-24
Sandy, Robert J. (Department: 3677)
Pipe joints or couplings
Particular interface
Tapered
C285S333000, C285S355000, C285S390000
Reexamination Certificate
active
06581980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to threaded connections for securing together the ends of tubular bodies. More specifically, the present invention relates to a threaded connection for connecting together tubular pipe bodies used in the construction of wells.
2. Description of the Prior Art
The pipe used in drilling and completing oil and gas wells and other wells employed in extracting minerals from the earth is typically in the form of a long string of pipe segments secured together by threaded connections provided at the end of each pipe segment. The connections serve the dual function of holding the adjoining segments together and providing a pressure seal at the connection. Stresses are encountered in the threaded connection that result from forces applied to the connection during its initial assembly into a string, forces associated with the placement of the string into the well and forces resulting from the pressure differentials acting across the engaged connection. When the stresses acting on the connection are either excessive or inadequate, the connection can fail, causing separation of the string or disrupting the pressure seal within the connection.
During the makeup and subsequent running and utilization of a typical string of casing or tubing employed in a well, the forces acting on the connection may alternate between high tension and high compression forces. Tension forces are imposed, for example, when the connection is part of a heavy string that is suspended from the well surface. Compression forces occur during the makeup of the connection and during the process of placing the connection into a well where the well bore is deviated such that the pipe string must be bent around a curve in the well bore. In the latter situation, the portion of the connection at the inside of the curve is stressed in compression relative to the portion of the connection on the outside of the curve. Concentration of excessive compressive forces within the connection can cause the connection to be permanently deformed or to fail when the compressive forces exceed design limitations of the connection.
Current drilling and completion applications require increasing compression ratings for strings being used in deeper and more deviated wells. The well string designs for these critical conditions often require the use of connections having external radial dimensions that are the same as those of the pipe or only slightly larger than the pipe body. Reducing the volume of material employed in forming the connection in an effort to reduce the pipe diameter increases the stress required to be sustained by the remaining material of the connection. In general, these reduced diameter connections have a low elastic compression rating as compared with the connections having larger outside diameters. Use of the smaller diameter connections in critical wells increases the probability that the connection will be exposed to stresses that exceed the elastic limits of the elements of the connection. Exceeding the elastic limit of the components of the connection changes the characteristics of the connection, which increases the likelihood of failure of the connection.
Conventional threaded connections fall generally into the category of interfering or non-interfering, or a combination of both. Threads that do not interfere are sometimes referred to as “free running.” A connection having interference threads has dimensions such that the threads of one component interfere with the threads of the adjoining component to cause a mechanical deformation of the material of the engaged threads. Threads in a free running, non-interference-type connection may be engaged without causing any mechanical thread deformation in the made-up connection.
Some thread connections may include a combination of both interference and non-interfering, or free-running, threads. An important component of the makeup of a free running thread connection is a mechanical limit, such as a torque shoulder, that permits the connection to be tightened. In many cases, the torque shoulder also provides a sealing surface between the engaged pipe sections. Some prior art designs using free running threads provide a radial seal adjacent the torque shoulder by tapering the internal surfaces extending to the torque shoulder and forcing the tapered surfaces together during the makeup. The sealing between interference fit threads is normally obtained by mechanical engagement of the threads assisted by a void filling thread compound.
FIG. 1
illustrates a conventional prior art connection using two-step, free-running threads
11
and
12
separated by a central torque shoulder
13
. The torque shoulder is formed by the engagement of circumferential shoulders in each of the members of the connection.
FIG. 2
illustrates details of the torque shoulder of the connection of
FIG. 1. A
typical shoulder area of a threaded connection with free-running threads has radial gaps
14
and
15
between the surfaces of the female component of the connection, or the box
16
, and the male component of the connection, or the pin
17
. The gaps
14
and
15
result from the large machining tolerances permitted in order to make the manufacture of the connection easier and less expensive. The presence of the gaps
14
and
15
also contributes to the ease of assembly of the connection.
The torque shoulder
13
is formed in the engaged contact area indicated at
18
. The radial dimension of the contact area
18
is less than the radial dimension of the respective elements of the torque shoulder formed on the pin and box sections by an amount equal to the radial dimension of the gap
14
or
15
. In some conventional connections, the area of contact represented by the bearing surface
18
may be as little as 70% of the total available surface area of the torque shoulder.
In a connection such as illustrated in
FIG. 2
, the compressive forces exerted against the torque shoulder during the makeup or other compressive loading of the connection can cause the areas of the torque shoulder with the smallest cross-sectional dimensions to be plastically deformed as indicated in FIG.
4
. The plastic deformation is accommodated in the gaps
15
and
14
adjacent the torque shoulder
13
. The deformation of the pin shoulder is indicated at
19
, and the deformation of the box shoulder is indicated at
20
.
FIG. 4
illustrates that, under the influence of compression loading, the corners of the torque shoulder
13
will flex and distort into the open radial gaps, which permits yielding to occur at a load that is less than that theoretically sustainable by a torque shoulder having full engagement of the contacting surfaces of the torque shoulder
13
. As may be appreciated, a connection such as illustrated in
FIGS. 1-4
is limited in its compressive capabilities to the compressive forces that cause yielding of the weakest point of the torque shoulder that occurs at the unrestrained smallest cross-sectional area at the outside, extreme corner of the torque shoulder.
The thread design of the conventional connector illustrated in
FIGS. 1-4
also plays a part in the compressive strength of the connection. As illustrated in
FIG. 3
, in such connections a gap
21
exists between the stab flanks of the threads
22
of the pin
17
and threads
23
of the box
16
. As with the gaps formed about the torque shoulder
13
, the gap
21
results from large machine tolerances that contribute to simplifying the manufacture and assembly of the connection. When the connection becomes sufficiently loaded in compression, the gap
21
is closed and the stab flank of the threads can begin to share the compressive loading exerted on the torque shoulder. However, the degree of applied compressive force necessary to close the gap
21
can exceed that required to produce the deformation of the torque shoulder indicated in FIG.
4
. The net result is that the compressive loading rating for the connection illustrated in
FIGS. 1-4
is limited to a
Costa Darrell Scott
DeLange Richard W.
Eason Reji
Evans Merle Edward
Browning & Bushman P.C.
Grant Prideco, L.P.
Lugo Carlos
Sandy Robert J.
Torres Carlos A.
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