US20090008087A1 - Threaded joint for an oil-well tubing - Google Patents

Abstract

The engineering problem of the claimed invention consists in the increase of operating reliability of the threaded joint, whose coupling has the coating applied by the diffusion galvanizing method, by way of ensuring a proper standard tightness. The technical result of the increase of the inlet cylindrical counterbore length in the internally threaded element by the value of the thread tightness increment caused by galvanizing and determined from the claimed empirical formula consists in increased reliability of the joint under service conditions as the probability of overtightening the joint for the sake of attaining the nominal tightness, as well as damage to the end thread turns projected from the coupling on service are ruled out. This result is attained without consumption of time and funds for re-training of the personnel, publication and delivery of new normative materials to each drilling site, manufacture of new calibrating tools and repair mountings. In its turn, this makes it possible to immediately begin supplying the suggested joints and increase the service life of standard threaded joints of oil-well tubings from 6÷8 to at least 50 screwing/unscrewing cycles. Besides, in case of forced screwing with a standard torque the joint reliability also increases because the threaded part of the externally threaded element is nearly completely hidden inside the counterbore due to the increased counterbore length and thus protected against the mechanical effect of the well medium.

Description

RELATED APPLICATIONS
• This application is a Continuation of PCT application serial number PCT/RU2007/000529 filed on Oct. 3, 2007, which in turn claims priority to Russian Patent Application No. 2006135967 filed on Oct. 11, 2006, both of which are incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
• The invention proposal relates to tapered threaded joints mainly utilized in drilling equipment and more particularly to threaded connection assemblies of oil-well tubing or drill tubes in expendable, oil, or gas wells.
BACKGROUND OF THE INVENTION
• The terms and expressions used below have the following interpretations:
• “Joint or tubing joint” is the knockdown assembly comprising the externally threaded element, for example, tube or reducing coupling sometimes called “union nipple” in the standard materials, and the internally threaded element, for example, coupling or reducing coupling hereinafter referred to as “coupling”.
• “Axial thread tightness” or “tightness” is the value measured by the distance between the plane passing through the thread runout end on the nipple and the plane of the coupling face (GOST 633-80). The tightness is positive if the thread end is outside the coupling, zero if the thread end and coupling face coincide, and negative if the thread end is inside the coupling. According to the same GOST, to check the thread usability, the tightness value is measured in the joint coupled finger tight.
• “Thread runout end” or “thread end” (according to GOST 633-80) is the crosspoint of the vanish cone generator with the generator of the cylinder, whose diameter is equal to the external diameter of the nipple (tube).
• Other terms used in the present description: thread pitch, thread, etc. relate to threaded joints of oil-well tubings (hereinafter referred to as OWT).
• Oil-well tubings are operated under heavy conditions of impact and sign-changing loads, under high pressure, often at the increased temperature and in aggressive media. At that the threaded joints of such tubings should be reliable and air-tight at pressures of up to thousands of atmospheres. Irrespective of the use of sealing and thread lock compounds of various kinds, application of high tightening forces close to the limit, past which thread stripping occurs, remains the basic method of ensuring reliability and tightness of the joints. Owing to this the service life of the threaded elements does not exceed but several screwing/unscrewing cycles.
• Known is the tapered threaded joint of the oil-well tubing comprising a tubing joint that has a cylindrical boring and a mated nipple, for example, tube or reducing coupling (GOST 633-80, Dwg 6). According to above GOST the coupling thread should have a zinc or phosphate coating. The standard does not regulate the method of coating application, but at the time of its development use was made of only electrolytic coatings with a thickness in the order of 10 μs, which were comparatively soft and non-wear-resistant and intended for thread protection against corrosion during storage and transportation of threaded elements. The hot zinc plating method by dipping a coupling with liquefied zinc is not applied due to irregularity of the coating thickness on the thread and zinc sags on the thread hindering the check with gages. The electrolytic zinc coating features low mechanical strength and are susceptible to hydrogen embrittlement under the conditions of the corrosive environment in an oil or gas well. It is more expensive than the phosphate one and at present is used only on the threads of casing tubes, where just one or two screwings with subsequent casing cementing. At present all Russian tube-rolling mills employ only phosphate coating of the couplings of oil-well tubing. By virtue of the properties of above coatings the standard requirements to provision of threaded joint tightnesses are based on the most widely spread phosphate coating of the coupling thread in the order of 10 μs.
• The shortcoming of the known joint consists in its low service life. Thus, in “Operating Instructions for Oil-Well Tubing” RD 39-136-95, Para. 7.15 states: “The number of screwing/unscrewing cycles of the threaded joints should be recorded in the course of operation of oil-well tubing. According to the performed studies durability of the threaded joints is retained for up to 6-8 cycles”. This is a very small value if one takes into account the fact that even at the operated wells the tubes are elevated for cleaning or replacement of the pumping equipment. Therefore, the main task of the developers consists in the increase of the durability of the oil-well tubing joints.
• At a zero tightness, when the external thread is manually screwed in the coupling to the last thread turn, the joint is rejected. The joint wear is often manifested not in the reduced profile of the thread turns with the tightness reduction, but in the form of increased tightness in excess of the above value because of appearance of scorings on the contacting surfaces. If tightness exceeds the standard value, the joint is also rejected.
• Known is the threaded element for the tube threaded joint with a high limit endurance, in which at least some threads contain a spiral groove reaching the crest (RF Patent No. 2261395 according to class F16L15/06). Owing to pliability of the threads, which are weakened at the crest by the groove, more even load distribution among the thread turns, i.e., the joint reliability is increased. The shortcoming of the known joint consists in the difficulty-to-make of its elements. Besides, such a thread is easily galled in case of casual handling. Restoration of worn-out threaded elements is possible only under the conditions of the specialized repair centers, which cannot be numerous due to complexity of equipment. Another shortcoming of the known threaded element is its irregularity. The oil-well tubings, as well as the elements for their jointing are the vital assemblies of the drilling technique and are widely employed at the vastest territories. Therefore, they should be strictly standardized and unified for ensuring both reliability and compatibility. Development of new kinds of joints with improved performance characteristics is obviously necessary, but their check and introduction require many years of work. Therefore, the technical concepts improving the service life of standardized threaded joints without deviation from the requirements of the standards are more relevant and called for.
• Known is the threaded joint for tubes comprising threaded sections and tightly fitted unthreaded sections that ensure air tightness when tightening the thread (RF Patent No. 2258171 according to class F16L15/00). Separation of the sections ensuring air tightness and those ensuring the required compacting force is the effective technique widely employed in vacuum engineering. However, with respect to the drilling equipment this technique does not solve the problem of increasing the thread service life as the tightening forces still remain huge and the joint does not withstand more than few cycles without repair. Besides, its air tightness greatly depends on the fineness of mated surfaces, which cannot be always ensured afield. Another shortcoming of the known joint is the fact that it requires higher accuracy of manufacturing, which in its turn aggravated by the location of one of the unthreaded sections ensuring air tightness deep inside the internally threaded element (coupling). One more shortcoming of the known joint is the fact that its geometry does not comply with the effective standards.
• Also known is the threaded joint for steel tubes comprising threaded sections and tightly fitted unthreaded sections that ensure air tightness when tightening the thread (RF Patent No. 2248495 according to class F16L15/04). Solid-film lubricant with a porous-zinc or zinc-alloy sublayer is applied onto at least one of the sections by blasting with particles consisting of iron cores and zinc shells. Such a joint ensures more reliable air tightness of the joint than the preceding analog, but this is attained through considerable complication of the manufacture, the more so that one of unthreaded sections ensuring air tightness is located deep inside the internally threaded element (coupling), which hampers its processing. The remaining shortcomings of the known joint are similar to those of the preceding analog.
• Also known are the threaded joints for steel tubes, in which solid-film lubricant consisting of a lubrication powder (for example, molybdenum disulfide) and a binding agent is applied onto the threaded surfaces of at least one of the sections for increasing their reliability (RF Patents Nos 2258170, 2258859, and 2262029 according to class F16L15/00). The shortcoming of the known joints consists in complicacy of preparation and application of solid-film lubricant, which requires special preparatory treatment of the threaded surface, including creation of the porous sublayer. Irrespective of all complicacy, the service life of these joints does not exceed 10 to 20 screwing/unscrewing cycles.
• Known is the method of manufacture of the threaded joint with a trapezoid profile of the tapered thread for the oil well tubing. This method consists in primarily setting seven basic parameters external diameter and thickness of the tube, dimensions of the tight spigot, etc.) and then in determining seven dependent parameters (length and taper of the seal, shoulder angle, flank angle, etc.) from the suggested formulas (RF application for invention No. 2003130748 according to class F16L15/04). The published patent claim does not contain the results of practical implementation of the suggested method, which would enable to judge its efficiency. Besides, the calculated parameters are always corrected in practice pursuant to the test results as the dimensions of the finished articles inevitably have deviations from those preset on the basis of computations. As the joint designed in accordance with the known method will not correspond to the effective standards, its mastering requires long time.
• Known is the tube of the tubing string comprising a joint coupling and reducing coupling, whose threaded surfaces are covered with a two-layer protecting coating consisting a zinc layer, 10 to 14 μm thick, applied by the thermal diffusion galvanizing method and a phosphatic film, 2 to 3 μm thick, applied onto the former (RF Utility Patent No. 38498 according to class F16L15/08). The suggested coating ensures anti-corrosion protection of the joint elements during storage and transportation, but the coating thickness is insufficient for reliable hermetic encapsulation of the joint and increase of its service life. The phosphatic coating is undurable and easily eroded. As in threading the surface finish is usually fixed at Rz 20, the declared thickness of 2 to 3 μm is several times smaller than unevennesses and by no means can considerably increase the thread service life. The technology of thermal diffusion galvanizing of the thread surfaces corresponding to the above surface finish does not ensure guaranteed continuity of the coating, less than 15 μm thick.
• The taper threaded joint of oil-well tubing or tubing string with the joint coupling featuring the inlet cylindrical counterbore and the mated externally threaded element (nipple), example, tube or reducing coupling, where thermal diffusion powder galvanizing is made on the threaded surfaces of the coupling and externally threaded element is the closest to the suggested one by the technical essence and attainable result. (RF Utility Patent No. 30913 according to class F16L15/08). The thickness of the coating on the threaded surfaces is selected experimentally so that the coating is entire and rather strong and at the same time does not peel in screwing. The aim of the coating application is to increase the joint service life rather than anti-corrosion protection and the coating is quite thicker than those usually employed. Therefore, the coating noticeably increases the joint tightness. Nearly half of the manufactured couplings satisfying the standard requirements prior to the coating application had the tightness in excess of the permissible limits after the coating application. This compels the user to sort out the manufactured couplings prior to the coating application so that only a thin phosphatic coating not ensuring the service life increase is applied on those, which will not satisfy the standard requirements after galvanizing. If the user ordered only galvanized couplings with the increased service life, the output of finished articles evaluated with respect to the tightness value will be nearly the half of the production which unacceptable for the manufacture, particularly in case of making the couplings of expensive steel with special properties.
• Under service conditions the joint aptness is evaluated with respect to the tightness value in the manually screwed joint. According to above RD 39-136-95 “ . . . if the nipple thread with a torque being smaller than the minimum one is screwed into the coupling to the last thread turn or if after screwing with the maximum torque two vacant turns have not entered into the coupling, both tubes shall be rejected . . . ”. The tightness is visually evaluated by workmen proceeding from the number of turns projecting over the coupling edge and the screwing torque of the hydraulic wrench. When new couplings with thermal diffusion galvanizing and an increased tightness are delivered to the wells, the workmen guided by the instructions and previous experience will either reject new couplings or try to attain the required tightness by exceeding the standard screwing torque, thus reducing the joint reliability. Therefore, the known solution, which is rather effective, as has been proved by the tests, turn out to be inapplicable in practice on a wide scale due to the necessity to change the instructions, retraining of the personnel, manufacture of new calibration tools, repair mountings and all these should be done at thousands of wells all over the world.
• The known coupling has one more shortcoming. According to the effective standards after forced screwing there should remain not more than two nipple turns, while in the known joint more than two thread turns remain outside the coupling in case of forced screwing. During benchmark tests for service life evaluated by the number of screwing/unscrewing cycles it is of no importance, but under service conditions the turns unprotected by the coupling counterbore are damaged by the well medium's mechanical effect and can be worn out or damaged even prior to the first elevation from the well. As far as they wear out, the tightness value is reduced, the turns go deeper into the coupling and the joint has to be rejected when the mutilated nipple turns reach the coupling turns though this joint has not worked out the service life determined by the benchmark tests. For ordinary joints withstanding 6-8 cycles the wear or damage of the turns projecting over is not so probable and high unlike in the case of the joints' coatings withstanding over 50 cycles. Therefore, the service life increase of the known joint cannot be guaranteed even if the above organizational restrictions can be overcome.
SUMMARY OF THE INVENTION
• The technical problem solved by the present invention proposal consists in increasing operational reliability of the threaded joint, whose coupling has a coating on the threaded part applied by the thermal diffusion galvanizing method by means of a proper standard tightness.
• The above problem is solved when in the known tubular joint coupling of the oil-well tubing or drill string comprising a threaded element with external taper triangular thread, for example, tube or reducing coupling, and a threaded element with internal thread, for example, coupling or reducing coupling, having the inlet cylindrical counterbore and the threaded part with a coating applied by the thermal diffusion galvanizing method the length of the inlet cylindrical counterbore of the coupling is increased by the value of the increment of thread tightness ΔA calculated from the formula: ΔA=kδ_min÷kδ_max where k is the empirical coefficient equal to 70 and δ_min and δ_max are the minimum and maximum coating thicknesses, respectively.
• The above problem is also solved when in the known tubular joint coupling of the oil-well tubing or drill string comprising a threaded element with external taper trapezoid thread, for example, tube or reducing coupling, and a threaded element with internal thread, for example, coupling or reducing coupling, having the inlet cylindrical counterbore and the threaded part with a coating applied by the thermal diffusion powder galvanizing method the length of the inlet cylindrical counterbore of the coupling is increased by the value of the increment of thread tightness ΔA calculated from the formula: ΔA=kδ_min÷kδ_max, where k is the empirical coefficient equal to 30 and δ_min and δ_max are the minimum and maximum coating thicknesses, respectively.
• The technical result of extension of the length of the inlet cylindrical counterbore of the internally threaded coupling by the tightness increment value caused by galvanizing, which is determined from the declared empirical formula consists in increasing the joint reliability under service conditions, as a possibility of overtightening the joint in the desire to reach the nominal tightness and a damage of end thread turns projecting from the coupling are ruled out in the course of operation. This result is attained without consumption of time and funds for re-training of the personnel, publication and delivery of new normative materials to each drilling site, manufacture of new calibrating tools and repair mountings. In its turn, this makes it possible to immediately begin supplying the suggested joints and increase the service life of standard threaded joints of oil-well tubings from 6÷8 to at least 50 screwing/unscrewing cycles.
• Besides, in case of forced screwing with a standard torque the joint reliability also increases because the threaded part of the externally threaded element is nearly completely hidden inside the counterbore due to the increased counterbore length and thus protected against the mechanical effect of the well medium.
• The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:
• FIG. 1 is a view of one manually screwed threaded joint, whose coupling has a coating on a threaded part.
• FIG. 2 is a view of a suggested manually screwed threaded joint.
• FIG. 3 is a view of a suggested threaded joint screwed with an established operating torque.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The suggested threaded joint (FIG. 1) consists of the externally threaded element 1, for example, a well tube or a reducing coupling, hereinafter for brevity referred to as the nipple, and the internally threaded element 2 mated with it, for example, a coupling or a reducing coupling, hereinafter for brevity referred to as the coupling. The entry section of the coupling 2 has the cylindrical counterbore 3. The counterbore diameter is somewhat greater than the diameter of the unthreaded part of the nipple 1. The coating 4 applied by the diffusion powder galvanizing method is found on the threaded part of the coupling. The thickness of the coating 4 is usually within 15 to 30 μm, but can be even thicker—up to 50 μm. An anti-corrosive phosphatic coating with a thickness of several microns can be applied over the galvanized coating. The threaded surface of the nipple 1 has no coating or the standard-corrosive phosphatic coating with a thickness of 2 to 3 μm is applied onto it.
• Depth G of the counterbore 3 measured from the face end 5 of the coupling 2 to the thread beginning is equal to the sum of standard depth g of the counterbore rated for joints without galvanized coating and the value of thread tightness increment ΔA, i.e., G=g+ΔA. At that the value of ΔA determined experimentally does not depend on the thread pitch or diameter and is calculated from the formula: ΔA=kδ_min÷kδ_max where k is the empirical coefficient equal to 70 for triangular thread and 30 for trapezoid thread for the coating thickness within 15 to 50 μm. In determining the formula for ΔA, it was taken into account that the well attending personnel determine the tightness by sight according to the number of turns projecting over the coupling edge. Accuracy of such determination is equal to approximately ¼÷⅓ of a turn and, accordingly, the suggested couplings, whose tightness is within the standard tolerance zone, are found usable under service conditions no matter whether the counterbore depth is increased by kδ_min or by kδ_max.
• The expression for ΔA is true for any standard taper threads irrespective of the thread diameter or pitch. The values of k are determined experimentally during the tests of the coated couplings. Setting the value of ΔA proceeding from mean coating thickness δ_med established due to technological considerations as optimal for the joint service life and coating strength is preferable. In this case the value of ΔA will be equal to kδ_med. The value of δ_med need not be the mean arithmetic value between δ_min and δ_max. Thus, it has been experimentally established that the increased service life of the joint is ensured at the coating thicknesses being within a range of 15 to 30 μm, but the best results are attained if the parameters of the engineering process of coating application are oriented towards obtaining δ_med=25 μm. However, if due to the engineering process deviations the mean coating thickness in one of the batches of couplings appears to be less than δ_med, but within the tolerance zone, it is expedient to calculate a new value of ΔA corresponding to this batch and accordingly reduce the counterbore length. As long as the coupling has threads and counterbores on both sides, the coupling length is increased by 2ΔA as a result of the use of the suggested solution. As for the reducing couplings having the internal thread only at one side, their length is increased by ΔA.
• Different values of coefficients k for triangular and trapezoid threads is explained by the fact that in the triangular thread the turns of the nipple and coupling contact along the thread centerline, i.e., along the flanks, while in the joints with the trapezoid thread the fit is done with respect to the internal or internal and external thread diameters.
• The suggested joint is manufactured and used as follows.
• The coupling blank is made with the counterbore depth increased by a value of ΔA. The increase of the externally threaded element by a value of ΔA for reducing couplings and by a value of 2 ΔA for couplings is preferable. In the coupling thread-cutting machine compensation by a value of ΔA is introduced so that the basic thread plane displace by the same distance towards the smaller diameter of the thread taper. After thread cutting in the coupling the axial tightness is checked by means of a standard thread gage (FIG. 2). At that, the couplings with the tightness equal to tightness A preset in the standards for threaded joints less the value of ΔA, i.e. the couplings with tightness (A−ΔA) are considered fit. Thus the whole tightness tolerance zone preset in the standards is implemented. As long as the departure from the standard inspection method takes place at the intermediate manufacture stage, no coordination or permits are required.
• The couplings that have passed the test are subjected to diffusion powder galvanizing. The thickness of the applied coating can amount to 15 to 50 μm. The manufactured couplings are subjected to outgoing inspection by means of standard gages in accordance with the standard procedure, i.e., the coupling tightness checked with the aid of a plug gage should amount to standard value A with allowance for limit deviations. In this case, the incoming inspection of couplings at the user's will indicate standard value A, i.e., any changes in the operating instructions will not be needed.
• During the first forced screwing of the suggested joint the tightness value is reduced by 0.5 to 0.8 mm. Further on the tightness of the galvanized coupling's thread is reduced at an average rate of 0.05 mm per screwing/unscrewing cycle.
• The experience of benchmark and field tests of oil-well tubings with couplings, whose thread is galvanized by the thermal diffusion method and the counterbore length is increased according to the present suggestion shows that these joints are assembled with the torques similar to ordinary oil-well tubings. Axial and diametral tightnesses of the joint also correspond to those of ordinary oil-well tubings. In assembling the joints use was made of greases usually employed in the oilmen's practice. Assembly of the suggested joints does not require any special equipment. There were no cases of rejection of fit joints by the well attending personnel.
• While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims (2)

1. A threaded joint for an oil-well tubing comprising:

a threaded element with external tapered triangular threading; and

a threaded element with internal threading including:

a cylindrical recess inlet, and

a threaded portion,

wherein the threaded portion has at least a coating applied by a thermal diffusion powder galvanizing method,

wherein the length of the cylindrical recess inlet is increased by an increment value of tightness caused by the coating, and

wherein the increment value ΔA is calculated in accordance with the following formula:

ΔA=kδ _min ÷kδ _max,

wherein k is an empirical coefficient equal to 70, and

δ_min and δ_max are minimum and maximum coating thicknesses, respectively.

2. A threaded joint for an oil-well tubing comprising:

a threaded element with external tapered trapezoid threading; and

a threaded element with internal threading including:

a cylindrical recess inlet, and

a threaded portion,

wherein the threaded portion has at least a coating applied by a thermal diffusion powder galvanizing method,

wherein the length of the cylindrical recess inlet is increased by an increment value of tightness caused by the coating, and

wherein the increment value ΔA is calculated in accordance with the following formula:

ΔA=kδ_min÷kδ_max,

wherein k is an empirical coefficient equal to 30, and

δ_min and δ_max are minimum and maximum coating thicknesses, respectively.

Images