US2405859A - Tubular structural member - Google Patents

Description

' Aug. 13, 1.946.A H. E. sQMEs 2,405,859

TUBULAR STRUCTRAL MEMBER Filed Deo. 20. 1941 @Gl Jlcfi j I .By v

ATTORNEY v depths of between 9,000 and 12,000 feet.

Patented Aug. 13, 194s TUBULAR STRUCTURAL MEMBER Howard E. Somes, Detroit, Mich., assignor, by

mesne assignments, to Edward G. Budd Manufacturing Company, Philadelphia, Pa., a, corporation of Pennsylvania Application December 20, 1941, Serial N0. 423,739

2 Claims. 1

This invention relates to tubular articles and more particularly to an axial assemblage of tubular articles such for example, as oil well casings.

An axial assemblage of tubular articles is frequently subjected to unusual stress conditions, one of the most common examples of which is an oil well casing. An oil well casing, commonly known as a string, is comprised of a plurality of steel tubes axially connected together and extending many thousands of feet into the earth, it being not uncommon to set oil well casings to In a few instances, casings have been set to a depth of 18,000 feet. The casing string is suspended at its top and frequently is sealed at its lower end with concrete or other suitable material, and is supported at such lower end. Due to the great weight of the string, at least the upper sections thereof are subjected to severe tensile stresses acting in an axial direction especially during installation when the string is being supported from the top. At the bottom of the string, many of the lower casing sections are subjected to axial compressive stresses which arise out f the fact that the elongation and more particularly the weight of the upper sections imposes an axial compression load on such lower sections especially after installation. In addition, the hydro-static pressure existing externally of the lower casing sections exerts severe external pressures radially against the Walls thereof which set up compressive stresses in the casing wall tending to collapse the same.

Oil well casings are being set deeper and deeper and this means that the tensile stresses in the upper sections and the compressive stresses in the lower sections are becoming greater and greater, it being well known that the hydrostatic pressures increase as the depth of the casing setting increases.

From the foregoing it is clear that whereas tensile strength is of greater importance at the top of the string, compressive strength is of greater and major importance at the bottom of the string. The aim, therefore, of the present invention is to provide an oil well casing string or similar structure of improved construction wherein the casing sections forming the upper part of the string and those forming the lower part r of the string, respectively, with respect to a standard oil well casing of the same dimensions and composition, have increased resistance to axial tensile failure and increased resistance to collapse from the exertion of axial and radial compressive forces.

With the above and other objects in view, the present invention consists of certain features to be hereinafter described with reference to the accompanying drawing, and then claimed.

In the drawing which illustrates a suitable embodiment of the invention:

Figure 1 shows a fragmentary sectional -portion of a casing section undergoing a heat treating operation; and

Figure 2 is a fragmentary elevational view of an oil well casing string.

I attain the objects of the present invention by employing certain heat treating operations to provide the casing sections with an internal an nular zone or layer extending from end to end and to create within the inner and outer zones certain predetermined residual stress conditions which combine with the hardened zone to increase the strength of the casing sections, the sections to be used for the upper portion of `the casing string being provided with stress conditions different from the stress conditions existing in the casing sections employed for the lowe11 portion of the casing string.

The casing sections are rst subjected to a heat treating operation similar to that described in my Patent No. 2,281,331, April 28, 1942, wherein there is disclosed a method and apparatus for progressively hardening:A an internal layer of a tubular member by generating heat in the internal layer through magnetically induced heating currents.

In accordance with the method disclosed in my copending application aforesaid, an electro-magnetic induction heating head H and a quench head Q are arranged in coaxial relation with the casing section to undergo the heat treating operation and in such manner that relative axial movement can be effected between the casing section Il on the one hand and the heatingv and quenching heads on the other hand while preferably effecting relative rotation. The coil I2 of the heating head is connected to a high frequency source of alternating current and is provided with a laminated iron core i3 threading its center in order to assist in the concentration of heating energy at a high rate inthe internal layer or Zone which is to be hardened. The quench head Q is preferably provided wtih an annular orice Q-l arranged as closely as possible to the trailing end of the coil l2 to discharge an annular stream of quenching medium against the heated internal surface.

The construction of the heating and quenching heads H and Q, respectively, may be substantially the same as that disclosed in said copending application and of course may be of any other suitable design and construction, such as shown in Figure 1, and capable of producing the same rapid heating and quenching results.

The heat is generated annularly at a desired shallow depth in the internal zone within a period of a very few seconds and this heated 'zone is immediately quenched before any of the heat 3 generated in the same can drift to the metal of the outer zone in. any consequential amount, thus providing an inner hardened layer I4 of metal and an outer unhardened or less hard annular layer I5 of metal.

By reason of the rapid generation of heat in the inner layer I4 and the rapid quenching of the same, a state of trapped stresses is set up in which the inner hardened layer I4 is placed under residual compression and the outer less hard layer I5 is placed under residual tension, that is, when the tubular structure is under an unloaded condition. The outer layer may be placed under a state of residual tension in an amount approximately that of its yield strength.

It is well known, of course, that the tensile strength of a layer of metal is increased by hardening the same and that this increase in strength increases with the hardness.

For the lower sections B of the casing string illustrated in Figure 2 wherein compressive strength is of major importance the casing sections treated in the manner just described are employed, these sections being provided with an outer unhardened layer I5 under residual tension and an internal layer i4 under residual compression and hardened to such hardness as to increase its ultimate strength to a strength manifoldly greater than that of the unhardened layer I5 which, for example, may be three times thatof the unhardened layer. For example, the ultimate strength of a metal hardened to 60 on the Rockwell C scale may be as high as 311,000 pounds perv square inch whereas the same metal prior to hardening might have had an ultimate strength of 93,000 pounds per square inch assuming it had an original hardness of Rockwell C 10; This strength of 93,000 pounds per square inch is considerably above the yield strength which would probably be in the order of 60,000 pounds per square inch.

Assuming for example, that the yield strength of the unhardened layer of the casing section is two-thirds of the ultimate strength and that the outer layer is under a residual tension substantially equal to its yield strength, it will thus appear that an external radial load such as arises from the hydro-static pressures existing externally of the lower sections of the casing strength must rst neutralize the tension stress in outer layer I5, this, in the present case, being an amount equal to two-thirds of the ultimate strength of the outer layer. It thus will be seen that the outer layer is still capable of withstanding a compressive load equal substantially to its ultimate strength, and therefore, that since the strength of the hardened layer is in the order of three times that of the unhardened layer, the two layers combine and assist each other in resisting the compressive loading, at leastto the extent of the strength of the outer unhardened layer, Actually, however, the casing will withstand an external loading greater than the sum of two-thirds the ultimate strength (the residual tension stress) of the outer layer I5V and the strength of such loading because of the increased strength of the internal layer I4. It thus is seen that the resistance of the casing section to collapse is materially increased.

It will, of course, be obvious that the axial compressive strength of such casing section will be materially increased by the treatment described. The residual tension stress of the external layer exerts itself not only circumferentially but also axially of the section and consequently, when the 4 section is placed under axial compressive loading the external unhardened layer I5 being under considerable tension in opposition to the compression stress of the internal layer I4 assists the section in resisting axial compressive loading by the amount of the residual tension stress, since the axial loading must rst neutralize this residual tension stress.

It is obvious that in such heat treated casing sections B in which the internal layer is hardened the yield point or yield stress to which it may be carried exceeds the yield strength of the unhardened material and therefore, the internal layer may, when the casing section is subjected to an axial compressive loading, be stressed to a much higher point than the outer layer and still remain within the yield point of the material.

For the casing sections A in the upper portion of the casing string A--B wherein tensile strength is of greater importance, the present invention contemplates the embodiment of casing sections having the internal annular layer I4 hardened substantially the same as in the lower sections B but in which the internal layer is placed under a. state of residual tension and the external layer l5 is placed under a state of residual compression, that is, when the casing sections are in an unloaded condition.

In such a structure, it will thus appear that with the metal of the internal hardened layer I4 having a tensile strength far in excess of the strength of the external unhardened layer I5 and at the same time being under tension while the outer layer is under compression, an axial tension load exerted on the casing section will rst neutralize the compressive stress in the outer layer and at the same time increase the tension stress of the internal hardened layer I 4. The metal of this internal layer due to its hardness however, is of manifoldly greater strength than the metal of the external unhardened layer. Consequently, the metal of the hardened internal layer and the metal of the external layer combine and assist each other in resisting axial tensile loading and thus the axial tensile strength of the casing section is increased, the initial assistance of the external layer being the amount of its residual or trapped compression stress.

In order to bring about the stress reversal in changing the residual stress of the external layer I5 from a tension to a compression stress condition and of the internal layer I4 from a cornpression-to a tension stress condition, the internal layer I4 is expanded, but not beyond its elastic limit, an amount suiiicient to expand or stretch the metal of the external layer beyond its yield point whereby to upset the same. Upon contraction of the internal layer the external layer is placed under compression and the internal layer, due to the resistance of the external layer to compression, isv placed under tension.

It must be borne in-mind, however, that the total strength 0f the hardened layer must be greater than the total strength of the unhardened layer so that the upsetting of the external layer is accomplished before the internal layer reaches its elastic limit.

The reversed stress may be brought about mechanically or by longitudinally stressing the same through the application of opposed tension-exerting forces at its ends, or by closely confining the peripheral surface thereof throughout its length and subjecting the same to high internal hydraulic pressure. The stress reversal also may be brought'about in accordance with the disclosure of my Patent No. 2,315,558, granted April 6, 1943, wherein I proposed to thermally expand the internal layer I4, through the use of the electromagnetic induction heating head l-l shown in to expand because of the increase in compression due to thermal expansion resulting from the heat generated therein and the consequential temperature rise in the internal layer. The additional compressive stress developed in the internal hardened layer through the rapid heating thereof will be suirlcient to stress the cold external hardened layer beyond its elastic limit so that it will take a permanent set.

In practice, tubular members treated in the manner set forth will be found to have had their axial internal diameter enlarged slightly through this treatment, and too because the external layer has been given a permanent setting and stressed beyond its yield point allowing the internal layer formerly under compression to expand, the heat, however, being such that the inner layer is not expanded beyond its yield point. No quenching need be resorted to for the structural change in the section takes place immediately upon the development of heat and the increasing of compressive stresses in the internal layer sufliciently to force the outer layer to yield. The structure can thereafter be permitted to cool in air or if desired, for rapidity, it may be cooled by placement in a cooling bath, or by quenching it externally or internally.

The heating of the internal layer to a temperature of, for example, 400 degrees F. may not effect any material change in its hardness and yet the treatment described will materially increase the internal load strength of the casing section. In some instances, however, it may be desirable to carry the temperature created in the internal layer to a point such as to produce a certain amount of drawing of the original hardness of this layer, in which case the internal layer may be initially hardened to a much higher degree than that iinally desired so that the drawing action will produce an amount of reduction in the hardness of the internal layer such as will produce the hardness desired.

While a temperature of 400 degrees Fahrenheit has been suggested, the internal layer obviously may be heated to any temperature below the critical hardening temperature, the temperature chosen of course being that which is necessary to produce the desired or necessary upset in the outer layer.

The casing sections, as is well known, are threaded at their ends, as at 20, for attachment to adjacent sections through suitable couplings 2|.

It will thus be seen that I have provided an improved casing string for use in oil wells, or under conditions wherein similar conditions exist, by the use of casing sections in which the upper portion of the string, wherein tensile strength is of greater importance, have increased tensile strength, and the lower sections, wherein axial and radial compressive stress is of major importance, have increased axial and circumferential compressive strength, as compared to sections of the same size and composition. Other advantages, oi course, are manifold, particularly in the reduction in weight and saving in cost.

The number of sections at the upper and lower portions of the casing to be treated in accordance with the disclosures herein may, of course, vary depending upon the depth of the well, which as is well understood is usually determinable in advance to drilling the well. The intermediate sections may be untreated. However, in any event, the portions of all the sections may be hardened under the threaded end zones to increase the strength of the joints to compensate for loss in strength resulting from the cutting of the threads. The production of hardened zones underlying the threads 2s is preferably accomplished in the same manner as that outlined in connection with the portions forming the lower casing sections, wherein high collapse strength is desired.

Although the invention has been illustrated and described in general in connection with a speciiic embodiment, it is to be understood that the same is not limited thereto but may be practiced in various forms and ways. As many changes in the procedure and in the structure evolved therefrom may be made without departing from the spirit of the invention, as will be apparent to those skilled in the art, reference will be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. An elongated tubular metallic structure of the character subject in use throughout portions of its length t0 predominating axial compressive stresses and throughout other portions of its length to predominating axial tension stresses, having the wall of each of said portions comprised of integral inner and outer layers of metal, said inner layers having a yield strength materially greater than that of the outer layers, said inner and outer layers of said rst-mentioned portions which are subject to predominating axial compressive stresses being under residual compression and tension stresses, respectively, and said inner and outer layers of said other portions which are subject to the predominating axial tension stresses being under residual tension and compression stresses, respectively.

2. A multi-section oil well casing structure of the character wherein incident to weight and external hydrostatic pressure the lower sections thereof are subject to circumferential collapsing stresses and other compressive stresses and incident to the weight of the structure other sections thereo are subject to axial tension stresses, the wall of each section being comprised cf inner and outer annular layers of metal substantially coextensive with the length thereof, said inner layer being of materially greater hardness than the outer layer, each of said lower sections having its inner layer initially under residual compression stress and its outer layer initially under residual tension stress, and each of said other sections having its inner layer initially under residual tension stress and its outer layer initially under residual compression stress.

HOWARD E. SOMES.

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