Tubular connection

Details Tubular connection Tubular connection

1998-08-10

2001-11-27

Luu, Teri Pham (Department: 3627)

Pipe joints or couplings

Particular interface

Tapered

C285S333000, C285S355000, C285S390000

Reexamination Certificate

active

06322110

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to tubular connections and more particularly to threaded tubular connections employed in joining lengths of pipe or joints of the kind commonly used in the oil and gas industry. In particular, the tubular connection of the present invention is a self-regulating, torque-resistant, threaded connection.
The trend in the oilfield is to minimize the diameter of pipe connections and to conserve borehole diameter. Two types of oilfield connections, namely integral flush joints and slim line high performance connections, have been utilized for these purposes. The outer diameter of a flush joint connection is substantially the same as the outside diameter of the body of the pipe. In other words, the connection is contained within the wall thickness of the pipe body. The outside diameter of conventional pipe couplings are typically 10 to 13% greater than the wall thickness of the pipe body. The outside diameter of a slim line high performance connection is generally 2 to 3.5% greater than that of the body of the pipe. Slim line high performance connections may be manufactured with or without cold swaged sections; hot-forged upset; and couplings. Though not undergoing as many make-up and break-out cycles as drill pipe, tubing connections must also remain serviceable after repeated makeups. So must casing connections to a lesser degree. An emerging technology, drilling with casing, requires casing connections with the added performance attributes of drill pipe tool joints.
U.S. Pat. Nos. 4,009,893; 4,538,840; 4,570,892; 4,611,838; 4,629,221; and 4,629,224 disclose various types of connections for tubular members having interfitting portions which serve to seal the connection. For example, U.S. Pat. No. 4,611,838 of Mannesmann discloses an annular end face of the pin member for opposing an annular shoulder of the box member, lying in planes transverse to the pipe axis. The pin member has an unthreaded annular bulge which engages an unthreaded frustoconical peripheral zone on the box member to form a seal.
A major deficiency in slim line and flush-type connections is their extremely low compression rating. Typically the flank angles of prior art threads are large, which results in large clearances between the compressive load bearing thread surfaces at full make up. Further as the flank angles are reduced, the clearances between the threads must be increased to permit the threads to stab into the grooves upon makeup. Thus, prior art connections provide a large clearance between the flanks of the threads. Large clearances between the threads permit movement between threads under cyclic loads and thus do not achieve a tight connection under cyclic loads. Large flank angles and thread clearances weaken the connection in compression. Prior art connections may have a 25 to 30% compression efficiency with a 60% tension efficiency. It also causes the connection to be weak in bending. Bending is compression on the inside of the connection and tension on the outside of the connection.
Square threads have substantially no flank angle and therefore are desirable because they provide good tension and compression load transfer. But in order for square threads to stab, the thread flank clearances must be so large that contact between the load flanks and stab flanks can not be achieved upon make-up. Thus, it is commonly believed that it is not possible to insert a square thread, including a tapered square thread, into an accommodating groove without having prohibitively large thread flank clearances.
Further it is commonly believed that for a thread to be “stabbable” and “machinable” a hook thread must have an included angle of 15° or more, that a non-hook thread, such as API buttress thread, must have an included angle of 13° or more and that a power thread such as a stub Acme thread or API X-line thread must have an included angle of 12° or more. Square threads cannot be easily manufactured, particularly in small diameter connections. Therefore these prior art threads require these minimum included angles to be stabbable and be machinable. The prior art connections with modified square threads use variable width threads to permit the square threads to be stabbed.
The prior art wedge thread or dove-tailed thread was developed to increase thread contact and achieve locking threads. A wedge thread is a thread having load and stab flanks which have different helix angles, i.e. different leads. Since their pitch is cut off two leads, wedge threads have a variable pitch. Wedge threads obtain their wedging by monotonically increasing the thread within the groove as one member is rotated with respect to the other member. Wedge threads mate together by rotational and then axial movement. The wedging of a wedge thread occurs along the axial length of the threads with a larger thread width being received into a smaller root opening. Connections using wedge threads produce torsional resistance between the threads due to the different leads between the load and stab flanks, i.e. a dovetail, wedge-type thread. However, a wedge thread profile requires multiple machining passes to cut the thread.
The present invention overcomes the deficiencies of the prior art.
SUMMARY OF THE INVENTION
The tubular connection of the present invention includes a pin member having tapered, external threads and a box member having tapered, internal threads, both sets of threads having the same constant helix angle, i.e. a constant lead for both stab and lead flanks, i.e. a constant pitch thread. The threads include run-in threads, full height threads, and run-out threads which extend from the nose of the pin and box members to the base of the pin and box members. The threads are square or near square threads forming accommodating grooves therebetween. The threads have crests and roots with minimal interfering contact and stab and load flanks with the stab flanks having a greater flank angle than the load flanks. Further, the stab flanks have comer chamfers for increasing the thread clearances for stabbing the threads into the grooves. The stab flanks may also include an increased thread width at the pitch line to form a cam flank extending from the comer chamfer to the regular stab flank which extends to the radius at the root of the thread.
The pin nose and box base form a metal-to-metal seal system and a primary torque shoulder. The pin nose includes an annular outer tooth-like member forming a first cylindrical surface, a first fistoconical surface, a first shoulder and a second shoulder. The box base includes an inner annular tooth-like member forming a second cylindrical surface, a second frustoconical surface, a third shoulder and a fourth shoulder. The metal-to-metal seal is formed by the engagement of the interfering first and second cylindrical surfaces and first and second frustoconical surfaces. The primary torque shoulder is fonned by the engagement of the first, second, third and fourth shoulders.
Upon assembly of the connection, the threaded pin member is stabbed into the treaded box member. In the stab position, the comer chamfers on the stab flanks engage to self-center the pin member within the box member. The corner chamfers increase the clearances between the threads and grooves. During initial assembly, the pin and box members are rotated relative to each other with the comer chamfers guiding the crests to the root openings. The cam flanks then cam the square threads on the pin and box member into the accommodating grooves on the corresponding pin and box member. As the seal surfaces on the pin nose and box base engage, a reaction force causes a shifting of thread contact from the stab flanks to the load flanks. As the rotation continues, then the crests and roots engage. The applied torque must increase to continue to force the pin and box members together. As the force increases, the pin member is placed in compression and the box member is placed in tension. This causes the clearance between the stab flanks to close. As the seal surfaces completely mate, the

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