Compressive forming processes for enhancing collapse resistance of metallic tubular products

Abstract

To provide a method for enhancing the collapse resistance of metallic tubular products.SOLUTION: The method comprises identifying the types of stress applied to change a residual stress profile of a metallic tubular product that has undergone a straightening process to a residual stress profile improved in the collapse resistance. The metallic tubular product is subjected to radial compression processing to control the residual stress profile and to enhance the collapse resistance. The radial compression processing is performed after the metallic tubular product has been subjected to the straightening process.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 62 / 458,838, filed February 14, 2017, which is incorporated by reference.

[0002] FIELD OF THE INVENTION FIELD OF THE INVENTION The present invention relates to metal tubular products, and more particularly to a treatment method for improving the collapse resistance of metal tubular products. [Background technology]

[0003] <Background information> In the manufacture of metal tubular products, straightness requirements are dictated by API, ISO, ASTM, and other standards. To meet these standards and maintain high-volume production, tubes are typically straightened at room temperature (referred to as cold straightening) using conventional rotary or gag straightening processes. This process alters the dimensional properties of the tube by bending sections in the longitudinal and / or transverse hoop directions, causing some or all of the wall fibers in those sections to yield (stress levels beyond the elastic limit). As the tube exits the straightening process, there is elastic rebound of the pipe and it straightens to its new dimensions, typically resulting in a residual hoop stress profile that reduces the tube's collapse resistance. Research reported at the American Petroleum Institute Standards Conference (P. Mehdizadeh, "Casing Collapse Performance," 1974) indicates that the minimum collapse strength properties of tubes without deleterious residual stresses would be 20-30% higher than the current API minimum collapse strength.

[0004] In conventional tubular manufacturing, a rotary cold straightening process is performed as the first operation in the tube manufacturing finishing equipment. The final tube shipped suffers from high compressive residual hoop stresses in the inner wall fiber region. Therefore, minimum collapse resistance based on these straightening processes is specified as a standard.

[0005] To improve the residual stress profile and increase the collapse resistance, additional processes must be performed after straightening to change the existing residual stresses to a profile that improves / increases the collapse resistance of the tube while maintaining the desired straightness.

[0006] Previous attempts have been made to reduce residual stresses in metal tubular products straightened by rotational straightening, for example by reheating the metal tubular product after rotational straightening. However, there remains a need for a more effective and economical process to improve the collapse resistance of metal tubular products. Summary of the Invention

[0007] The present invention provides a method for improving the collapse resistance of a metal tubular product. The method identifies types of stresses that can be applied to modify the residual stress profile of a metal tubular product, for example, after a straightening process, to obtain a residual stress profile that improves collapse resistance. The metal tubular product is subjected to radial compression processing to control the residual stress profile and improve collapse resistance. This radial compression process can be used after the tubular product has been subjected to a final straightening process.

[0008] One aspect of the present invention provides a method for improving the collapse resistance of a hollow metal tubular product, the method comprising: straightening the hollow metal tubular product to produce a straightened hollow metal tubular product having an outer diameter OD and an inner diameter ID; and radially compressing the straightened hollow metal tubular product to produce a radially compressed hollow metal tubular product having an outer diameter OD' and an inner diameter ID', the straightened hollow metal tubular product having compressive residual hoop stresses adjacent to an inner surface of the product and tensile residual hoop stresses adjacent to an outer surface of the product, the radially compressed hollow metal tubular product either (a) having a substantially reduced compressive residual hoop stress adjacent to the inner surface of the product or (b) having a tensile residual hoop stress adjacent to the inner surface of the product, and the radially compressed hollow metal tubular product either (a) having a substantially reduced tensile residual hoop stress adjacent to the outer surface of the product or (b) having a compressive residual hoop stress adjacent to the outer surface of the product.

[0009] Another aspect of the present invention is to provide a method for improving the collapse resistance of a hollow metal tubular product, the method comprising radially compressing the hollow metal tubular product to create a radially compressed hollow metal tubular product having an outer diameter OD' and an inner diameter ID', wherein at an axial location along the hollow metal tubular product, a radial compressive force acting on one side of the circumference of the hollow metal tubular product is opposed by at least one radial compressive force acting on an opposite side of the circumference of the hollow metal tubular product, the radial compressive forces being applied circumferentially around a contact area of at least 180 degrees totaling the hollow metal tubular product at the axial location.

[0010] A further aspect of the present invention provides a straightened and radially compressed hollow metal tubular product, the product including an inner surface and an outer surface, wherein the straightened and radially compressed hollow metal tubular product has either (a) a substantially reduced compressive residual hoop stress on the side adjacent the inner surface of the product, or (b) a tensile residual hoop stress on the side adjacent the inner surface of the product, and wherein the collapse resistance of the straightened and radially compressed hollow metal tubular product is greater than the collapse resistance of a product that has not been subjected to the radial compression step.

[0011] These and other aspects of the present invention will become more apparent from the following description. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a partial schematic cross-sectional view of a hollow metal tubular product straightened by rotary straightening, prior to application of a compression molding process according to an embodiment of the present invention.

[0013] [Figure 2] FIG. 2 is a partial schematic cross-sectional view of a hollow metal tubular product within a radial compression zone according to one embodiment of the present invention.

[0014] [Figure 3] FIG. 3 is a partial schematic cross-sectional view of a radially compressed hollow metal tubular product after exiting the radial compression zone according to one embodiment of the present invention.

[0015] [Figure 4] FIG. 4 illustrates a typical wall thickness stress state before, during, and after a compression molding process according to one embodiment of the present invention.

[0016] [Figure 5] FIG. 5 shows an example collapse improvement curve for a metal tubular product having a specific D / t ratio and a specific material grade according to one embodiment of the present invention.

[0017] [Figure 6] FIG. 6 is a partial schematic view of a metal tubular product in a liquid or gas compression molding chamber according to one embodiment of the present invention.

[0018] [Figure 7] FIG. 7 is a partial schematic view of a metal tubular product within a drawing die according to one embodiment of the present invention.

[0019] [Figure 8] FIG. 8 is a partial schematic view of a metal tubular product in a length forming die according to one embodiment of the present invention.

[0020] [Figure 9] FIG. 9 is a partial schematic view of a metal tubular product in a forming mill including two opposed rollers according to one embodiment of the present invention.

[0021] [Figure 10] FIG. 10 is a partial schematic view of a metal tubular product in a forming mill including three rollers according to one embodiment of the present invention.

[0022] [Figure 11] FIG. 11 is a partial schematic view of a metal tubular product in a forming mill including three sets of opposed rollers according to one embodiment of the present invention.

[0023] [Figure 12] FIG. 12 is a graph illustrating the collapse pressure of various metal tubular products, including metal tubular products subjected to a compression molding process according to an embodiment of the present invention. [Figure 13] FIG. 13 is a graph illustrating the collapse pressure of various metal tubular products, including metal tubular products subjected to a compression molding process according to an embodiment of the present invention. [Figure 14] FIG. 14 is a graph illustrating the collapse pressure of various metal tubular products, including metal tubular products subjected to a compression molding process according to an embodiment of the present invention. [Figure 15]FIG. 15 is a graph illustrating the collapse pressure of various metal tubular products, including metal tubular products subjected to a compression molding process according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] <Detailed explanation> Metal tubular products manufactured according to the controlled radial compression process of the present invention exhibit favorable residual hoop stress profiles and improved collapse resistance.

[0025] For non-heat treated metal tubular products, the raw seamless or electrically welded tubular shell may be subjected to finishing operations including cold rotary or gag straightening, surface inspection, cut-off, threading, coupling, hydrotesting, weighing, measuring, stenciling, coating, final inspection, shipping, and dispatch. In embodiments of the present invention, the radial compression process may be performed at any stage after the final cold straightening step, for example, before surface inspection, before cut-off, or before threading. In some embodiments, for non-heat treated metal tubular products, the radial compression process may be performed immediately after the cold straightening by placing a cold sizing mill after the cold straightening process.

[0026] In the case of heat-treated metal tubular products, the raw seamless or electrically welded tubular shell may be subjected to a heat treatment process including heat treatment, hot or cold sizing, and hot or cold rotary straightening, followed by finishing operations including surface inspection, cut-off, threading, coupling, hydrotesting, weighing, measuring, stenciling, coating, final inspection, shipping, and dispatch. In one embodiment of the present invention, the radial compression process may be performed at any stage after the final straightening process. For example, the radial compression process may be performed at any time during the finishing operations, such as before cut-off or threading. In some embodiments, in the case of heat-treated metal tubular products, the radial compression process may be performed immediately after the hot or cold rotary straightening by placing a hot or cold sizing mill after the hot or cold rotary straightening device.

[0027] Radially compressed hollow metal tubular products made in accordance with the present invention have been found to have favorable residual hoop stress profiles and improved collapse resistance. In some embodiments, the metal tubular products typically have improved collapse pressures of 2 percent or more, such as greater than 5 percent, greater than 10 percent, greater than 12 percent, or greater than 15 percent, or greater than 20 percent.

[0028] FIG. 1 illustrates a hollow metal tubular product 10 that has been straightened by straightening, according to one embodiment of the present invention. As used herein, the term "straightened" refers to a hollow metal tubular product that has been straightened by means such as rotational straightening, gag straightening, or any other straightening method known in the art. The straightened tube 10 includes an outer surface 12, an inner surface 14, and a wall thickness Tw. As shown in FIG. 1, the straightened hollow tube 10 can have a circular cross-section having an outer diameter OD and an inner diameter ID.

[0029] Figure 2 illustrates a straightened hollow metal tube product within a radial compression zone 10c, according to one embodiment of the present invention. The compression zone tube 10c includes an outer surface 12c, an inner surface 14c, and a wall thickness Twc. As shown in Figure 2, the compression zone hollow tube 10c can have a circular cross-section with an outer diameter ODc and an inner diameter IDc.

[0030] Figure 3 illustrates a radially compressed hollow metal tubular product 10' according to one embodiment of the present invention. The radially compressed tube 10' includes an outer surface 12', an inner surface 14', and a wall thickness T'w. As shown in Figure 3, the radially compressed hollow tube 10' can have a circular cross-section with an outer diameter OD' and an inner diameter ID'.

[0031] According to embodiments of the present invention, the outer diameter and wall thickness of the metal tubular product can vary depending on the intended use of the tube. For example, the outer diameter of the tube can typically range from 2 to 50 inches, e.g., 3 to 40 inches, or 4.5 to 24 inches. For example, the wall thickness of the tube can typically range from 0.1 to 5 inches, e.g., 0.15 to 3 inches, or 0.25 to 2 inches.

[0032] The rate at which the outer diameter OD and inner diameter ID of the straightened hollow metal tubular product 10 change relative to the outer diameter OD' and inner diameter ID' of the radially compressed hollow metal tubular product 10' after radial compression varies depending on the overall dimensions, wall thickness, D / t ratio, material grade, processing temperature, etc. As used herein, the term "D / t ratio" corresponds to the ratio of the outer diameter of the hollow metal tubular product to the wall thickness of the hollow metal tubular product. According to one embodiment of the present invention, the D / t ratio may be between 10:1 and 40:1, e.g., between 15:1 and 35:1, or between 20:1 and 30:1.

[0033] In some embodiments, the outer diameter OD' of the radially compressed tube 10' is at least 0.002 percent less than the outer diameter OD of the straightened tube 10. For example, the outer diameter OD' of the radially compressed tube 10' may typically be 0.002 to 0.2 percent less than the outer diameter OD of the straightened tube 10.

[0034] In some embodiments, the inner diameter ID' of the radially compressed tube 10' is at least 0.002 percent less than the inner diameter ID of the straightened tube 10. For example, the inner diameter ID' of the radially compressed tube 10' may typically be 0.002 to 0.3 percent less than the inner diameter ID of the straightened tube 10.

[0035] In some embodiments, radial compression can result in a wall thickness T'w of the radially compressed tube 10' that is slightly thicker than the wall thickness Tw of the straightened tube 10. For example, the wall thickness of the radially compressed tube 10' can typically be 0 to 0.5 percent greater, such as 0.0005 to 0.3 percent greater, than the wall thickness Tw of the straightened tube 10.

[0036] During a radial compression molding process according to embodiments of the present invention, the straightened tube is radially compressed to a minimum diameter, after which the tube 10 springs back to its final radially compressed state with an outer diameter OD' and an inner diameter ID'. At the point of maximum radial compression, the outer diameter ODc of the tube in the radial compression zone can be at least 0.05 percent smaller than the outer diameter OD of the straightened tube 10. For example, the outer diameter ODc of the tube in the radial compression zone can typically be 0.05 to 0.6 percent smaller than the outer diameter OD of the straightened tube 10. In some embodiments, the inner diameter IDc of the tube in the radial compression zone can be at least 0.05 percent smaller than the inner diameter ID of the straightened tube 10. For example, the inner diameter IDc of the tube in the radial compression zone can typically be 0.05 to 0.8 percent smaller than the inner diameter ID of the straightened tube 10.

[0037] 4 illustrates a typical stress state in the wall thickness of a metal tubular product before, during, and after the product is subjected to a radial compression forming process according to an embodiment of the present invention. In some embodiments, the straightened tube 10 has compressive residual stresses adjacent the inner surface 14 and tensile residual stresses adjacent the outer surface 12 before the straightened tube 10 is subjected to the radial compression process. As shown in FIG. 4, the compressive residual stresses correspond to negative hoop residual stresses adjacent the inner surface 14 of the straightened tube 10, and the tensile residual stresses correspond to positive residual stresses adjacent the outer surface 12 of the straightened tube 10.

[0038] In embodiments of the present invention, during the radial compression process, a compressive force is applied to the previously straightened tube 10 within the radial compression zone, causing a portion of the tube's wall thickness T to yield. This compressive force is at a stress level exceeding the elastic limit. In some embodiments, as a result of the effect of the radial compression process on the yield strength of the radially compressed tube 10', the final yield strength of the radially compressed tube 10' is controlled to be within a predetermined tolerance after the radial compression process. In some embodiments, the change in final yield strength caused by the radial compression process may be minimal. In some embodiments, the primary factor contributing to the improved collapse resistance of the radially compressed tube 10' is a change in the residual stress profile. In some embodiments, the compressive force applied in the radial compression zone causes the tube fibers adjacent the tube's inner surface 14 to yield. The compressive hoops of the internal fibers substantially reduce compressive residual hoop stresses, which may result in tensile residual hoop stresses in the fibers after the tube exits the radial compression zone.

[0039] In some embodiments, after the radial compression process, the radially compressed tube 10' may exhibit a substantially reduced compressive residual hoop stress adjacent the inner surface 14', resulting in a positive tensile stress. Also, the radially compressed tube 10' may exhibit a substantially reduced tensile residual hoop stress adjacent the outer surface 12', resulting in a negative compressive stress. As shown in FIG. 4, the compressive residual hoop stress corresponds to a negative hoop residual stress adjacent the outer surface 12' of the radially compressed tube 10', and the tensile residual hoop stress corresponds to a positive residual stress adjacent the inner surface 14' of the radially compressed tube 10'.

[0040] As an example, Figures 4 and 5 show the relationship between improved collapse resistance and residual hoop stress (as a percent of yield strength) in ID fibers for a metal tubular product having a particular D / t ratio and material grade according to one embodiment of the present invention. The collapse resistance is normalized by the collapse resistance of a typical hot-roller-straightened tubular product (i.e., compressive residual hoop stress in the ID fibers equals -20% of yield strength). In some embodiments, for a cold-roller-straightened tube, the compressive residual hoop stress in the ID fibers can be as much as -50% of yield strength. Straightening of a metal tubular product can produce negative residual hoop stress, i.e., compressive residual hoop stress, at the inner surface 14 of the straightened tube 10 relative to the tube's yield strength. The radial compression treatment according to one embodiment of the present invention, when performed after the straightening process, results in improved collapse resistance due to a substantial reduction in the effect of residual hoop stress on yield strength, e.g., compressive residual hoop stress, in the wall fibers adjacent the tube's inner surface. In some embodiments, the wall fibers adjacent the inner surface may exhibit a tensile or positive residual hoop stress. In some embodiments, to achieve maximum improvement in the collapse resistance of the radially compressed tube 10', the residual hoop stress at the inner surface 14' may typically be -15 to +35 percent, or -10 to +25 percent, or -7 to +20 percent, or 0 to +15 percent of the yield strength. In some embodiments, the radially compressed tube 10' may have a collapse resistance that is at least 2 percent greater than the collapse resistance of the straightened tube 10. For example, the collapse resistance of the radially compressed tube 10' is typically 3 to 20 percent greater, or 5 to 15 percent greater, or 7.5 to 10 percent greater than the collapse resistance of the straightened tube 10. In one embodiment of the present invention, there is a limit to the residual hoop stress at the inner surface 14' of the radially compressed tube 10', beyond which the product is placed in excessive tension, reducing the collapse resistance of the radially compressed tube 10'. In the example shown in Figure 5, a residual hoop stress greater than 40% of the yield strength reduces the collapse resistance.

[0041] In one embodiment of the present invention, the metal tubular product may be subjected to the radial compression process at any temperature between ambient and 1250°F. For example, a steel metal tubular product may be heated to at least 500°F, or at least 800°F, or to a higher temperature, such as 1000°F to 1200°F, and the radial compression process may be performed at these temperatures. At these temperatures, the straightened hollow metal tubular product 10 generally has a reduced yield strength, and therefore a lower radial compression force may be used in the radial compression process. Alternatively, the radial compression process may be performed at ambient or room temperature, such as 70°F.

[0042] In some embodiments of the present invention, a radial compression molding process is used to produce metal tubular products having the desired mechanical properties as described above. Several methodologies for performing the radial compression process can be used during the radial compression molding process. Examples of compression molding processes are generally shown in Figures 6-11 and are described below. Figure 6 illustrates an example of liquid or gas compression of a straightened metal tubular product 10. Figures 7 and 8 illustrate an example of radially compressing a straightened metal tubular product 10 using a compression die. Figures 9-11 illustrate an example of radially compressing a straightened metal tubular product 10 using a compression roller. In embodiments of the present invention, the straightened tube 10 does not rotate during the radial compression molding process.

[0043] In some embodiments, during the radial compression process, opposing radial compressive forces are applied at predetermined axial locations along the length of the tube, resulting in approximately equal radial compression throughout the circumference and thickness of the tube. Thus, at a particular axial location along the tube, a radial compressive force acting on one side of the tube is opposed by at least one radial compressive force acting on the remainder of the circumference of the tube. For example, a radial compressive force acting on one side of the circumference of the hollow metal tubular product is brought to bear on at least one surface area acting on the opposite side of the circumference of the hollow metal tubular product. In some embodiments, a radial compressive force acting on one side of the circumference of the hollow metal tubular product at any axial location of the tube is opposed by at least one surface area acting on the opposite side of the circumference of the hollow metal tubular product. According to one embodiment of the present invention, the radial compressive force applied in the radial compression zone is countered by a radial compressive force acting on the large circumferential tangent of the straightened hollow metal tubular product 10. A radial compressive force is mechanically applied, and the radial compressive force is applied circumferentially in two or more segments each including at least 120 degrees of the outer surface of the tube. For example, the radial compressive force applied circumferentially around the outer surface of the tube at a given axial location is at least 120 degrees in Figure 10 and 180 degrees in Figures 8, 9, and 11. According to one embodiment of the present invention, the radial compressive force can be applied circumferentially around the outer surface of the tube at a given axial location in multiple segments for a total contact area of at least 180 degrees, or at least 270 degrees, or 360 degrees.

[0044] As shown in FIG. 6 , the straightened hollow metal tubular product 10 is placed in an enclosure 20 having a chamber 22, where a compression molding process is performed by applying a liquid or gas load 360 degrees around the tube's outer surface. In some embodiments, the compressive load applied to the metal tubular product is applied until some or all of the tube's wall thickness Tw yields, e.g., in a lateral compression mode, until a stress level exceeding the elastic limit is reached. According to some embodiments, the compressive load is applied circumferentially around 360 degrees around the tube's outer surface, as shown in FIG. 6 . This allows the entire circumference and thickness of the tube to be subjected to opposing compressive forces. When the compressive load is removed and the tube is no longer in a radial compression zone, the tube elastically expands to a radially compressed tube 10′. The radial compression and subsequent expansion substantially reduces compressive residual hoop stresses and, in some cases, generates tensile residual hoop stresses in the wall fibers adjacent the inner surface.

[0045] In one embodiment of the present invention, the straightened metal tubular product 10 can have an interior volume 24. In some embodiments, a stabilization mandrel 30 can be included within the interior volume 24 prior to the hydraulic or gas radial compression molding process, as shown in Figure 6. The stabilization mandrel is sized to allow the wall thickness to yield without buckling during the radial compression molding process.

[0046] As shown in FIG. 7 , the straightened, hot or ambient temperature hollow metal tubular product 10 is subjected to a mechanical radial compression molding process at hot or ambient temperature using a pultrusion die 40 sized for the tube. The pultrusion die 40 is configured to radially compress the straightened tube 10 to form a radial compression zone along a predetermined axial length of the tube. As previously described, the radial compression zone can modify the residual stress profile of the tube, such that after exiting the radial compression zone, the wall fibers adjacent the inner surface experience a substantial reduction in compressive residual hoop stress and the wall fibers adjacent the outer surface experience a substantial reduction in tensile residual stress. Additionally, in some embodiments, the residual stress profile of the tube can be modified to result in tensile residual hoop stresses in the wall fibers adjacent the inner surface and / or compressive residual hoop stresses in the wall fibers adjacent the outer surface. According to some embodiments, the pultrusion die applies a compressive force 360 degrees around the outer surface of the tube at a predetermined axial location on the tube, as shown in FIG. 7 . This allows the entire circumference and thickness of the tube to experience opposing compressive forces. In some embodiments, after the straightened tube 10 exits the radial compression zone formed by the drawing die, the radially compressed tube 10' has smaller inner and outer diameters.

[0047] As shown in FIG. 8 , the straightened hollow metal tubular product 10 can be subjected to a mechanical radial compression molding process using a set-length forming die. In the illustrated embodiment, the forming die includes a semicircular first forming die 50 and a second forming die 52. However, any other suitable number and shape of forming dies is possible, for example, there can be one, three, four, or more forming dies around the product. The forming die is configured to sequentially radially compress each axial section of the straightened tube 10 to form a radial compression zone along a predetermined axial length of the tube. According to some embodiments, the forming die applies a compressive force circumferentially 360 degrees around the outer surface of the tube at a predetermined axial location on the tube, as shown in FIG. 8 . This allows the entire circumference and thickness of the tube to experience opposing compressive forces. As previously described, after exiting the radial compression zone, the residual stress profile of the tube can be altered, resulting in a substantial reduction in compressive residual hoop stresses in the wall fibers adjacent the inner surface and a substantial reduction in tensile residual stresses in the wall fibers adjacent the outer surface. In some embodiments, the residual stress profile of the tube is altered to create tensile residual hoop stresses in the wall fibers adjacent the inner surface and / or compressive residual hoop stresses in the wall fibers adjacent the outer surface. In some embodiments, the axial length of the first and second forming dies 50, 52 is shorter than the axial length of the straightened tube 10, so that the forming dies 50, 52 move along the axial length of the tube, sequentially forming radial compression zones along the entire axial length of the tube to form the radially compressed tube 10'. In some embodiments, after the straightened tube 10 exits the radial compression zones formed by the forming dies, the radially compressed tube 10' has smaller inner and outer diameters.

[0048] As shown in FIGS. 9-11 , the straightened hollow metal tubular product 10 is subjected to a mechanical radial compression molding process using opposing compression rollers. As shown in FIG. 9 , the compression process can include a single set of opposing compression rollers 60 and 62. In the embodiment shown in FIG. 9 , the opposing compression rollers are positioned above and below the straightened hollow metal tubular product 10. The compression rollers are configured to radially compress the straightened tube to form a radial compression zone along a predetermined axial length of the tube. According to some embodiments, each compression roller applies a compressive force to at least a 90-degree portion around the outer surface of the tube at a predetermined axial location, as shown in FIG. 9 . This allows at least both halves of the tube's circumference to experience opposing compressive forces. As previously described, the radial compression zone can alter the residual stress profile of the tube, substantially reducing compressive residual hoop stresses in wall fibers adjacent the inner surface and substantially reducing tensile residual stresses in wall fibers adjacent the outer surface after exiting the radial compression zone. In some embodiments, the residual stress profile of the tube is altered to create tensile residual hoop stresses in the wall fibers adjacent the inner surface and / or compressive residual hoop stresses in the wall fibers adjacent the outer surface. In some embodiments, after the straightened tube 10 exits the radial compression zone formed by the compression rollers, the radially compressed tube 10' has smaller inner and outer diameters.

[0049] In the embodiment shown in FIG. 10 , the compression process can include a single set of three opposing compression rollers 70, 72, and 74. As shown in FIG. 9 , the three opposing compression rollers are positioned around the straightened hollow metal tubular product 10. For example, the compression rollers can be positioned at 120-degree intervals around the hollow metal tubular product 10. The compression rollers are configured to radially compress the straightened tube 10 to form a radial compression zone along a predetermined axial length of the tube. According to some embodiments, each compression roller applies a compressive force circumferentially to at least a 60-degree portion around the outer surface of the tube at a predetermined axial location, as shown in FIG. 10 . This allows each of three segments of the tube's circumference to experience opposing compressive forces. According to one embodiment of the present invention, the compression rollers are not directly opposite each other, but the compressive force applied by one roller is opposed by the compressive force applied by the other two rollers. According to one embodiment of the present invention, the set of opposing compression rollers adjacent along the axial length of the tube can be, for example, two, three, four, five, or more. As previously discussed, the radial compression zone can alter the residual stress profile of the tube such that, after exiting the radial compression zone, the wall fibers adjacent the inner surface have a substantially reduced compressive residual hoop stress and the wall fibers adjacent the outer surface have a substantially reduced tensile residual stress. In some embodiments, altering the residual stress profile of the tube can result in tensile residual hoop stress in the wall fibers adjacent the inner surface and / or compressive residual hoop stress in the wall fibers adjacent the outer surface. In some embodiments, after the straightened tube 10 exits the radial compression zone formed by the compression rollers, the radially compressed tube 10' has smaller inner and outer diameters.

[0050] As shown in FIG. 11 , the compression process can include three sets of opposing compression rollers: 80 and 82, 90 and 92, and 100 and 102. The number of adjacent opposing compression rollers along the axial length of the tube can be, for example, two, four, five, six, or more. In the embodiment shown in FIG. 10 , two sets of opposing compression rollers are positioned above and below the straightened hollow metal tubular product 10, and one set of opposing compression rollers is positioned to the left and right of the straightened hollow metal tubular product 10. However, other suitable compression roller arrangements can be used. According to one embodiment of the present invention, by using multiple sets of opposing compression rollers, the applied radial force is less than that of a single set of two rollers, and the total radial compression force is divided among the total number of sets. The compression rollers are configured to radially compress the straightened tube 10 to form a radial compression zone along a predetermined axial length of the tube. According to some embodiments, each compression roller applies a compressive force at least 90 degrees around the outer surface of the tube at a predetermined axial location, as shown in FIG. 11 . This allows both halves of the circumference of the tube to experience opposing compressive forces. As previously described, the radial compression zone can alter the residual stress profile of the tube, such that after exiting the radial compression zone, the wall fibers adjacent the inner surface experience substantially reduced compressive residual hoop stresses and the wall fibers adjacent the outer surface experience substantially reduced tensile residual stresses. In some embodiments, altering the residual stress profile of the tube results in tensile residual hoop stresses in the wall fibers adjacent the inner surface and / or compressive residual hoop stresses in the wall fibers adjacent the outer surface. In some embodiments, after the straightened tube 10 exits the radial compression zone formed by the compression rollers, the radially compressed tube 10′ has smaller inner and outer diameters.

[0051] The following examples are intended to illustrate various aspects of the present invention, but are not intended to limit the scope of the invention.

[0052] Example 1 A 14" x 0.820" sample of 125-grade steel pipe straightened by rotational straightening was subjected to a radial compression process in accordance with an embodiment of the present invention. The collapse pressure of the resulting product is shown in FIG. 12. As shown in FIG. 12, the bottom dashed line represents the minimum collapse pressure of 9,230 psi for currently available API Q125-grade pipe, the dashed line above that represents the minimum collapse pressure of 10,530 psi for 125 high-collapse-grade pipe available three years ago, the dashed line above that represents the minimum collapse pressure of 11,580 psi for currently available 125 high-collapse-grade pipe, and the top dashed line represents the minimum collapse pressure of 125 high-collapse-grade pipe subjected to a radial compression process in accordance with an embodiment of the present invention, which is 12,540 psi. Therefore, the top dashed line in FIG. 12 corresponds to the target collapse pressure achieved by a radial compression molding process in accordance with an embodiment of the present invention. As can be seen, the collapse pressure results for all samples are significantly higher than the collapse pressure achievable by conventional methods and exceed the target collapse pressure achieved with the radial compression molding process according to one embodiment of the present invention.

[0053] <Example 2> A 16.25" x 0.817" sample of 125-grade steel pipe that had been straightened by rotational straightening was subjected to a radial compression process according to an embodiment of the present invention. The collapse pressure of the resulting product is shown in FIG. 13. As shown in FIG. 13, the bottom dashed line represents the minimum collapse pressure of 5,960 psi for currently available API Q125-grade pipe, the dashed line above that represents the minimum collapse pressure of 7,510 psi for 125 high-collapse-grade pipe that has been available for three years, the dashed line above that represents the minimum collapse pressure of 8,210 psi for currently available 125 high-collapse-grade pipe, and the top dashed line represents the minimum collapse pressure of 8,860 psi for 125 high-collapse-grade pipe that has been subjected to a radial compression process according to an embodiment of the present invention. Therefore, the top dashed line in FIG. 13 corresponds to the target collapse pressure achieved by a radial compression molding process according to an embodiment of the present invention. As can be seen, the collapse pressure results for all samples are significantly higher than the collapse pressure achievable by conventional methods and exceed the target collapse pressure achieved with the radial compression molding process according to one embodiment of the present invention.

[0054] Example 3 A sample steel pipe measuring 11.875" x 0.582" that had been straightened by rotational straightening was subjected to a radial compression process according to an embodiment of the present invention. The collapse pressure of the resulting product is shown in Figure 14. As shown in Figure 14, the bottom dashed line represents the minimum collapse pressure of 5,630 psi for currently available API Q125 grade pipe, the dashed line above that represents the minimum collapse pressure of 7,070 psi for 125 high-collapse grade pipe that has been available for three years, the dashed line above that represents the minimum collapse pressure of 8,720 psi for currently available 125 high-collapse grade pipe, and the top dashed line represents the minimum collapse pressure of 8,310 psi for 125 high-collapse grade pipe that has been subjected to a radial compression process according to an embodiment of the present invention. Therefore, the top dashed line in Figure 14 corresponds to the target collapse pressure achieved by a radial compression molding process according to an embodiment of the present invention. As can be seen, the collapse pressure results for all samples are significantly higher than the collapse pressure achievable by conventional methods and exceed the target collapse pressure achieved with the radial compression molding process according to one embodiment of the present invention.

[0055] Example 4 A 16.15" x 0.723" sample steel pipe that had been straightened by rotational straightening was subjected to a radial compression process according to an embodiment of the present invention. The collapse pressure of the resulting product is shown in FIG. 15. As shown in FIG. 15, the bottom dashed line represents the minimum collapse pressure of 4,510 psi for currently available API Q125 grade pipe, the dashed line above that represents the minimum collapse pressure of 5,650 psi for 125 high-collapse grade pipe that has been available for three years, the dashed line above that represents the minimum collapse pressure of 6,120 psi for currently available 125 high-collapse grade pipe, and the top dashed line represents the minimum collapse pressure of 6,560 psi for 125 high-collapse grade pipe that has been subjected to a radial compression process according to an embodiment of the present invention. Therefore, the top dashed line in FIG. 15 corresponds to the target collapse pressure achieved by a radial compression molding process according to an embodiment of the present invention. As can be seen, the collapse pressure results for all samples are significantly higher than the collapse pressure achievable by conventional methods and exceed the target collapse pressure achieved with the radial compression molding process according to one embodiment of the present invention.

[0056] For purposes of the foregoing description, it should be understood that the present invention is susceptible to various alternative modifications and step sequences unless expressly stated otherwise. Furthermore, except as described in the examples, all numbers expressing quantities of ingredients used in the specification and claims, for example, should be understood to be modified by the term "about" unless otherwise indicated. Accordingly, unless otherwise indicated, the numerical parameters set forth are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents, each numerical parameter should be construed in light of the number of reported significant digits by applying ordinary rounding techniques.

[0057] It should be understood that any numerical range recited herein is intended to include all subranges subsumed therein. For example, a range of "1 to 10" is intended to include all ranges between the recited minimum value of 1 and the recited maximum value of 10, where the minimum value is equal to or greater than 1 and the maximum value is equal to or less than 10.

[0058] In this application, unless expressly stated otherwise, the use of the singular includes the plural and the use of the plural encompasses the singular. Further, in this application, even if "and / or" is expressly used, the use of "or" shall mean "and / or" unless expressly stated otherwise. In this application, objects followed by the articles "a," "an," and "the" include plural objects unless expressly and unambiguously limited to one.

[0059] While several embodiments of the present invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that many changes can be made in the details of the invention without departing from the invention as set forth in the claims.

Claims

1. 1. A method for improving the collapse resistance of a hollow metal tubular product, comprising: straightening the hollow metal tubular product to produce a straightened hollow metal tubular product having an outer diameter OD and an inner diameter ID; radially compressing the straightened metal tubular product to create a radially compressed hollow metal tubular product having an outer diameter OD′ and an inner diameter ID′; the straightened hollow metal tubular product has compressive residual hoop stresses adjacent an inner surface of the product and tensile residual hoop stresses adjacent an outer surface of the product; the radially compressed hollow metal tubular product is (a) The compressive residual hoop stress adjacent the interior surface of the article is substantially reduced; or (b) having a tensile residual hoop stress adjacent the interior surface of the article; and the radially compressed hollow metal tubular product is (a) the tensile residual hoop stress adjacent the outer surface of the article is substantially reduced; or (b) having compressive residual hoop stresses adjacent the outer surface of the article.

2. The method of claim 1 , wherein the straightening is performed by rotational or gag straightening.

3. 2. The method of claim 1, wherein the outer diameter OD' of the radially compressed hollow metal tubular product is at least 0.002 percent less than the outer diameter OD of the straightened hollow metal tubular product, and the inner diameter ID' of the radially compressed hollow metal tubular product is at least 0.002 percent less than the inner diameter ID of the straightened hollow metal tubular product.

4. 10. The method of claim 1, wherein the outer diameter OD' of the radially compressed hollow metal tubular product is 0.002 to 0.2 percent less than the outer diameter OD of the straightened hollow metal tubular product, and the inner diameter ID' of the radially compressed hollow metal tubular product is 0.002 to 0.2 percent less than the inner diameter ID of the straightened hollow metal tubular product.

5. 2. The method of claim 1, wherein the straightened hollow metal tubular product has a wall thickness Tw and the radially compressed hollow metal tubular product has a wall thickness T'w, and the wall thickness T'w of the radially compressed hollow metal tubular product is greater than the wall thickness Tw of the straightened hollow metal tubular product.

6. 5. The method of claim 4, wherein the straightened hollow metal tubular product has a D / t ratio greater than or equal to 10:1 and less than or equal to 40:

1.

7. 2. The method of claim 1, wherein the radially compressed hollow metal tubular product has a residual hoop stress adjacent the interior surface of the product that is between -10 and +30 percent of the yield strength of the radially compressed hollow metal tubular product.

8. 10. The method of claim 1, wherein the radially compressed hollow metal tubular product has a collapse resistance that is at least 2 percent greater than the collapse resistance of the straightened hollow metal tubular product.

9. 10. The method of claim 1, wherein the radially compressed hollow metal tubular product has a substantially reduced compressive residual hoop stress adjacent the interior surface of the product.

10. 10. The method of claim 1, wherein the radially compressed hollow metal tubular product has a tensile residual hoop stress adjacent the interior surface of the product.

11. 2. The method of claim 1, wherein the radial compression is performed at an axial position along the straightened hollow metal tubular product, the radial compression force acting on one side of the circumference of the straightened hollow metal tubular product, and the radial compression is opposed by a radial compression force acting on an opposite side of the circumference of the straightened hollow metal tubular product.

12. 12. The method of claim 11, wherein the radial compressive force is applied circumferentially around 180 degrees or more of the contact area on the outer surface of the straightened hollow metal tubular product at an axial location along the straightened hollow metal tubular product.

13. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed by at least one set of opposed compression rollers to produce a radially compressed hollow metal tubular product.

14. 14. The method of claim 13, further comprising a plurality of opposed compression rollers at axial locations along the straightened hollow metal tubular product.

15. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed by at least one set of three compression rollers to produce a radially compressed hollow metal tubular product.

16. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed in a compression chamber to produce a radially compressed hollow metal tubular product.

17. 17. The method of claim 16, wherein a stabilizing mandrel is placed inside the straightened hollow metal tubular product before the straightened hollow metal tubular product is radially compressed.

18. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed in a drawing die to produce a radially compressed hollow metal tubular product.

19. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed in a forming die to produce a radially compressed hollow metal tubular product.

20. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed at an elevated temperature.

21. 10. The method of claim 1, wherein the straightened hollow metal tubular product is radially compressed at room temperature.

22. 1. A method for improving the collapse resistance of a hollow metal tubular product, comprising: radially compressing the hollow metal tubular product to produce a radially compressed hollow metal tubular product having an outer diameter OD′ and an inner diameter ID′; 1. A method according to claim 1, wherein at an axial location along the hollow metal tubular product, a radial compressive force acting on one side of the circumference of the hollow metal tubular product is opposed by at least one radial compressive force acting on an opposite side of the circumference of the hollow metal tubular product, said radial compressive force being applied circumferentially around at least 180 degrees of a contact area of the hollow metal tubular product at said axial location.

23. 23. The method of claim 22, further comprising straightening the hollow metal tubular product prior to radially compressing the hollow metal tubular product.

24. 24. The method of claim 23, wherein the straightening is performed by rotational or gag correction.

25. A straightened and radially compressed hollow metal tubular product, the hollow metal tubular product including an inner surface and an outer surface; (a) compressive residual hoop stress adjacent said inner surface is substantially reduced; or (b) having a tensile residual hoop stress adjacent the inner surface; 1. A product wherein the collapse resistance of a straightened and radially compressed hollow metal tubular product is greater than the collapse resistance of a straightened hollow metal tubular product that has not been radially compressed, and wherein the substantially reduced compressive residual hoop stress adjacent the inner surface of said metal annular product is less than the compressive residual hoop stress of a straightened hollow metal tubular product that has not been radially compressed.

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