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Reliable Pipe Threads
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Reliable Pipe Threads
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Why do so many pipe threads loosen and leak in service?

Conventional tapered pipe threads are normally tightened to a given torque and as affected by numerous pin and box tolerances, the thread dope used, the torque measurement accuracy, and the field temperature, they hopefully stop within a desired position range that will seal and develop rated connection strength. However if for any reason, the torque is not within the narrow range or changes while in service, then over tightening may prevent reuse, and under tightening may cause leakage and/or pin jump out.

Old Style Wedging Threads have too low a ratio of “radius-change-per-turn”, to “thread-width-per-turn”, so they trap dope between mating flanks when tightened, and like API Buttress threads, the first compressive axial load, causes relative axial movement, which in turn, causes loosening and leakage. They also have weak, sharp corners between crests and flanks positioned at less than 90 degrees to each other, such that they often suffer handling and stab damage that is not perceptible under field conditions, causing galling and/or improper makeup, which in turn, causes loosening, leakage and fatigue failure.

Any pipe thread, whose configuration allows relative movement between box and pin under service loads such as bending or compression, will leak because the thread dope seal is destroyed by such movement.

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Is a connection design OK, if make-up torque loads the pin end against a shoulder?

If torque is closely controlled such that the makeup stress on the pin end is well below the yield stress of the metal, and no compression or bending loads are expected on the connection, then pin end loads are acceptable. However should makeup be too loose or too tight, or should bending or compression loads occur, then the potential for the pin end to fail plastically exists, which will cause loosening and leakage because the compressive axial stress due to makeup, plus compression loads plus bending loads are all additive, which compound the compressive stress on the pin ends so as to cause plastic failure, loosening and leakage.

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What effect does a pin shoulder makeup against a box face have on a connection?

The pin neck is stressed in tension in proportion to the makeup torque applied, and the average neck stress is increased at the juncture of the neck and the shoulder by the “stress concentration factor” as is well known. If both the juncture radius and the applied torque are controlled carefully such that the concentrated stress from makeup is substantially less that the yield stress but greater that the sum of combined neck stress from all operating loads, then improvement in fatigue strength may occur. However if not, then rupture in service may occur.

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What’s wrong with using a “pin-nose-seal if it doesn’t shoulder”?

“Pin-Nose-Seals”, also known as “metal-to-metal seals”, are extremely susceptible to damage, and a single “ding” on a nose or its seat too small to see under field conditions, can cause leakage, even though extreme care was taken during assembly. Although a “ding” can be filed down so you can’t feel it, the sealing capability cannot be restored in the field, and no adjacent seal as exists in thread seals, is close by to compensate. The most important function for a pin-nose-seal is to reduce the pressure area of a connection to the minimum, which becomes an advantage for such as a flush- joint connections when a 100% pressure rating is required, and when used, threads that seal should be present to back it up. Also, improperly designed pin-nose seals may fail after minimum corrosion.

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Which thread forms can utilize RPT Patented features?

Virtually any thread Form can benefit from one or more RPT Thread Features, but not all RPT Features apply to all thread forms.^Top of Page

How can RPT Features increase my profits?

Prevention of down-hole leaks reduces costs directly and indirectly, which benefits all who make, furnish or use threaded pipe connections. The direct cost of minor remedial work on a well may be relatively inexpensive, but the associated costs of lost production and the demand on management resources, accumulates rapidly. When a down-hole leak threatens the loss of a well, then damage to the environment and/or loss of life, costs may spiral uncontrollable out of hand.

Joint lay-down and replacement of damaged joints saves double purchase and double shipping costs, but more importantly, it costs wasted rig time incurred to handle and run the pipe and possibly, unscheduled wait-time.

Insurance companies are very interested as to whether a reliable seal is furnished on each-and-every pipe connection, which in one way or the other, will affect your insurance costs.

In most cases, the use of expensive “Premium Connections” multiplied by the different pipe sizes, times grades, can be eliminated which not only reduces connection purchase costs, but also reduces your pipe inventory investment required to cover projected usage, but it can improve pipe delivery as afforded by fewer types of connections to maintain in inventories.

The use of full-strength flush and near-flush joints that seal and don’t loosen, in place of collar type joints, can reduce the weight of pipe for a given well by 50%, which saves not only purchase and shipping costs for pipe but saves rig time and platform storage space as well, and saves rigs rate in that smaller rigs with lower lift capacities are required for smaller size pipes.

For a company to know for the first time what quality level is sufficient to prevent leaks, a worry that has hovered over the industry since 1939 is eliminated. Because Go-NoGo Inspection decisions can now be firm, production and inspection test costs can be reduced.

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Which sizes and grades of pipe can utilize RPT Thread Features?

 RPT Features can be used with any size or grade of pipe, but Feature Advantage may vary with the application or with the pipe size or grade. For instance, the strength of a large pipe connection may be increased with our self-swaging feature, whereas, higher strength of a smaller size connection might best be accomplished with a double pin coupling.

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Can TopTorq® Threads compete with the capabilities of old style wedging threads?

The torque capacity of both threads are theoretically the same, however, TopTorq® threads do not suffer probability of incomplete makeup, loosening or leakage as do such old style threads.

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What good is the wide range torque capability of TopTorq® Threads?

You may set your tongs to any makeup torque within its wide torque range that best suits a particular application, such as a low torque for use in a shallow straight hole, a medium torque for use in a deep straight hole, a high torque for use in a deep and/or deviated well so you can rotate the pipe right or left to prevent sticking, while lowering or pulling. A TopTorq® connection anywhere in a string may be tightened to a somewhat lower torque than the other connections to provide tiebacks because TopTorq® Connections can be stabbed blind, within a relatively wide range of misalignment.

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Why are TopTorq® threads so much less susceptible to stab damage?

TopTorq® threads have wider crests and stab flanks and larger radii that reduce dents and dings, and have faster tapers that allow wider angular misalignment, whereas, other threads require stabbing guides and a slow tedious lowering procedure to prevent damage that occurs especially to such as old style wedging type threads and cylindrical two-step threads. Also, TopTorq® pins stab deep, unlike API pins, which insures alignment without effort, and several RPT Patents have features such as differential tapers, steep tapers, and proportional thread form dimensions to prevent cross-threading.

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Why are RPT threads less subject to handling and makeup damage?

For all thread forms, the pin crest is the thread element most often damaged, and many pin crests get “dinged” by bumping, by dragging, by cross-threading and such, but when RPT threads are made-up tight, there is always a gap between the pin crest and the box root that prevents crest galling and false makeup, as can happen for instance, with the API Buttress thread form. Also, TopTorq® threads have the added features of wide crests between relatively large radii that minimizes tendency to such damage.

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Why don’t TopTorq® threads wear like other tapered threads?

Most tapered pipe threads connections require two to six power turns to either make or break, which causes wear and tangential stress between the box and pin threads particularly, when they are over-tightened, which casts doubt as to whether they can be safely inspected or used again, because the pin end may be yielded. Conversely, makeup of a TopTorq® connection requires less than one power turn, with no fear of over-tightening. Therefore, thread burning and galling does not occur, so phosphate can serve in place of plating.

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How do you field inspect a TopTorq® thread?

Clean the pin and make a visual and hand "feel inspection" of the crest and outer half of the stab flank, for “stab drop” damage. You will find that the open included angle allows for easy cleanup and inspection. If it has been long used in a drill string, then check for wear by callipering the crest widths perpendicular to the flank of two threads several pitches apart in the same plane (ten preferably), subtract one width from the other, and then divide by the number of pitches between those two thread turns. The result is the “width change per turn”, which should be within the allowable range listed for that connection.

The pin end will not be yielded inwardly as occurs with conventional tapered threads, because TopTorq® tightening is stopped by axial flank wedging, instead of by tension hoop stresses in the box against compressive hoop stresses in the pin, as in conventional pipe threads.

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How do you field questions not shown above?

E-mail us at, and we will get back to you ASAP.

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