Patent Publication Number: US-6209638-B1

Title: Casing accessory equipment

Description:
BACKGROUND—FIELD OF THE INVENTION 
     The present invention relates generally to accessory equipment for oilfield tubular strings, especially casing strings used in casing earth boreholes drilled for oil and natural gas production. More specifically, the present invention relates to casing accessory equipment, including casing string centralizers and “float” equipment used in cementing the casing strings in place, which have bow spring centralizers mounted thereon such that the centralizers may collapse and permit the accessory equipment to readily pass through tight annular clearances (such as through wellheads and the like) no larger than the largest body diameter of the accessory equipment, then permit the centralizers to spring back out to properly centralize the casing string in open hole or a larger casing string. 
     BACKGROUND—RELATED ART 
     Earth boreholes drilled for oil and natural gas wells typically have one or more “casing strings” run and cemented in place during the course of the drilling program. A typical drilling sequence is to drill a length of open hole in the earth (perhaps several thousand feet in length), then lower a casing string having an outer diameter somewhat smaller than the diameter of the drilled hole to a position usually near the bottom of the open hole section. The casing string (which also may be several thousands of feet long) is usually comprised of a number of joints, each being on the order of forty feet long, connected to one another by threaded connections or other connection means. Cement is then pumped down through the inner bore of the casing, exits the bottom of the casing, and is displaced upwardly to a desired “cement top” depth in the annulus between the casing and the open hole. The cement supports and anchors the casing in place, and (ideally) forms a hydraulic bond between the casing string and the wall of the borehole. It is important, then, for the cement to be displaced to the required depth, and for the casing to be substantially centered in the borehole, so that a uniform cement sheath may be formed around the casing string (that is, if the casing were touching the borehole wall on one side, a cement bond could not be formed at that location). 
     Several types of casing accessory equipment assist in properly placing the cement so as to ensure a good hydraulic bond between the casing string and the wall of the borehole. “Float equipment” (which includes float shoes and float collars) includes usually relatively short (perhaps three to four feet long) sections of tubular member, with the bore of the tubular member having a casting therein, in which a check or one-way valve is seated. Usually, a “float shoe” is affixed to the very bottom end of the casing string; one or more joints of casing, referred to as “shoe joints” are then made up above the float shoe; then a “float collar” is made up, with the remainder of the casing string made up to complete the casing string. The one way valves in the float shoe and float collar permit cement (and other fluids) to be pumped down through the casing string, out the casing shoe and up around the casing string, but prevent any flow back into the casing string. The outer diameter of the tubular member used in float equipment is typically somewhat larger than the nominal diameter of the casing string to which it is attached, typically roughly equal to the diameter of the casing “collars” which comprise part of the threaded connections between casing joints. 
     “Centralizers” are often mounted on casing strings to center the casing string in the borehole and obtain a uniform thickness cement sheath around the casing string. The centralizers provide blades (of different possible configurations, as discussed herein) extending out from the casing wall and contacting the borehole, thereby holding the casing string off of direct contact with the borehole, and substantially centralizing the casing therein. To accomplish that goal, the centralizer blades typically form a total centralizer diameter roughly the diameter of the borehole in which the casing string is run. 
     Different types of centralizers have been used. One type comprises a solid central tubular body having a plurality of solid blades integral with the central body, the blades extending out to the desired diameter. Yet another type in use for many years are “bow spring” centralizers, which typically comprise a pair of spaced-apart bands which can be opened to encircle a casing string, then locked in place on the casing; and a number of outwardly bowed, resilient bow spring blades connecting the two bands, spaced around the circumference of the bands. The configuration of bow spring centralizers permits the bow spring blades to at least partially collapse as the casing string is run into the borehole and passes through any restricted diameter location, such as a piece of equipment having an inner diameter smaller than the at-rest bow spring diameter, then spring back out after passage through the reduced diameter equipment. One type of casing accessory equipment which comprises a bow spring centralizer is a “casing string centralizer”, which comprises a relatively short tubular member (on the order of three to four feet), which is made up into the casing string at selected locations over its length. However, even when collapsed, a conventional bow spring centralizer mounted on a casing string presents a diameter necessarily larger than the nominal casing diameter. 
     Centralization of a casing string near its bottom end, in particular around the float equipment, is usually considered especially important to securing a uniform cement sheath and consequently a hydraulic seal around the bottom end of the casing string. Toward that end, placement of centralizers very near or even on the float equipment is desirable. 
     The advent of drilling offshore wells in very deep water depths has given rise to arrangements of subsea wellheads, casing strings and the like with very tight clearances for passage of casing strings therethrough. Often, drilling devices such as underreamers and bi-center (or eccentric) drill bits are used to drill a borehole below a given casing string, in which the borehole has a diameter greater than the inner diameter of the casing string above the open hole section being drilled. Although it is desirable to centralize the succeeding casing string, including the casing float equipment, when the succeeding casing string is run, mounting a conventional bow spring centralizer on the casing and on conventional float equipment (which as described above, typically has an outer diameter somewhat larger than the nominal casing diameter) usually results in a diameter (even with the centralizer collapsed) too large to permit passage of such float equipment through restricted diameter passageways. In addition, such conventionally mounted bow spring centralizers do not shield the bands of the centralizers from scraping and catching on obstructions, ledges and the like, which pose further serious problems. 
     It is an object, then, of the present invention to provide improved casing accessory equipment especially adapted for use in situations where casing strings must be run through very close clearance passages. It is a further object of the present invention to provide casing accessory equipment, in particular casing string centralizers and float equipment, having a reduced diameter central section with a bow spring centralizer mounted in the reduced diameter central section, wherein the bow spring centralizer may be collapsed to a diameter no greater than that of the float equipment, and wherein the bow spring centralizers are “dragged” through tight clearance locations rather than being “pushed” therethrough. It is a further object of the present invention to provide casing accessory equipment in which the bands of the bow spring centralizers are shielded from catching on obstructions. Further objects of the present invention will become apparent through a reading of the ensuing description and the drawings. 
     SUMMERY OF THE PRESENT INVENTION 
     The casing accessory equipment of the present invention comprises an elongated, hollow tubular member having a central recessed section and an enlarged section on each end. A bow spring centralizer is mounted on the tubular member in the central recessed section. The bow spring centralizer comprises a pair of spaced apart bands which fit closely about and encircle the central recessed section of the tubular member, while still being movable on the tubular member. A plurality of outwardly-bowed, resilient bow spring blades connect the two bands, and are spaced around the circumference of the two bands. A plurality of lugs are fixed to the tubular member in the central recessed section. Windows in the bands fit over the lugs, thereby restricting the rotational and longitudinal movement of the bow spring centralizers within the central recessed section. The dimensions, spacing and configuration of the windows, lugs, and bow spring blades and bands are such that some longitudinal back-and-forth movement of the bow spring centralizer on the central body is permitted, even when the bow spring blades are fully collapsed. The diameters of the enlarged end sections are generally equal to or somewhat larger than the diameter of the bands, so that the bands are shielded from catching on obstructions when being run in the wellbore. When used as float equipment, the bore of the tubular member has an insert therein holding float one or more one way valves, for single-direction fluid flow therethrough. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of one embodiment of the casing accessory equipment of the present invention, in particular a float shoe. 
     FIG. 2 is a view of the float shoe of the present invention, without the bow spring centralizer in place. 
     FIG. 3 is a cross section view of the float shoe of the present invention, without the cast insert and the bow spring centralizer. 
     FIG. 3 a  is a cross section view of the float shoe without the bow spring centralizer, showing the insert and check valve in place. 
     FIG. 4 is a view of another embodiment of the casing accessory equipment of the present invention, in particular a float collar. 
     FIG. 5 is a view of the float collar of the present invention, without the bow spring centralizer in place. 
     FIG. 6 is a cross section view of the float collar of the present invention, without the cast insert and the bow spring centralizer. 
     FIG. 6 a  is a cross section view of the float collar, without the bow spring centralizer and showing the insert and check valve in place. 
     FIGS. 7 a-   7   d  are views of the float shoe of the present invention, in different modes of being pushed into or pulled out of a borehole. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     Although various embodiments of the present invention are contemplated, with reference to the drawings several presently preferred embodiments are herein described. 
     In the float shoe embodiment, with reference particularly to FIGS. 1 through 3 a , the casing accessory equipment of the present invention comprises a float shoe  10  having an elongated tubular member  20  having a central bore  30 . Typically, tubular member  20  is on the order of three to four feet long, but may be of any desired length. Tubular member  10  is typically of a metal alloy similar to that of the casing string to which it is attached; however, any variety of metals or metal alloys, well known in the art, may be used. 
     Upper end  10   a  of tubular member  20 , in the presently preferred embodiment, is adapted to be connected to a casing string via threaded connection  40 . However, it is understood that other types of connections, for example “snap collar” connections, or even means such as welding and the like, may be used. 
     Tubular member  20  has a central recessed section  25 , thereby forming enlarged diameter sections  26  and  27  at each end. Central recessed section  25  provides a location for the bow spring centralizer to be placed, as will be later described, while permitting the total diameter of the tool at the location of the centralizer to be no greater than that of the larger diameter end sections  26  and  27 . Central recessed section  25  may be formed by turning down the nominal diameter of tubular member  20 , or may be formed by forging, casting or other means well known in the art. End sections  26  and  27  typically have a diameter somewhat larger than the nominal diameter of the casing string to which float shoe  10  is attached, and typically roughly equal to the diameter of the collars which comprise part of the threaded connections between joints of the casing string, and generally equal to or somewhat larger than the diameter of the bands  55  (described below). In one embodiment, seen in FIGS. 1 through 3 a , angled holes  11  permit fluid flow in an angled upward direction, or “up-jet” action. 
     A plurality of lugs  40  are disposed about the circumference of central recessed section  25 , and in the presently preferred embodiment the lugs are arranged in two spaced-apart groups, so as to form two spaced apart sets of four lugs each, disposed about the circumference of central recessed section  25 . In particular, FIGS. 2,  3  and  3   a , which omit the bow spring centralizers (hereinafter described), show the presently preferred embodiment of the arrangement of lugs  40 . Although four lugs are depicted in each set, it is understood that a greater or lesser number of lugs may be used. Lugs  40  may be formed by leaving metal in place during the forming of central recessed section  25 ; or by fixing sections of metal on the outer surface of central recessed section  25  by welding, brazing, by the use of threaded connections or pins, or other means known in the art. 
     Bow spring centralizer  50  comprises a pair of spaced apart bands  55  connected by a plurality of outwardly-bowed bow spring blades  60 . Bands  55  are typically metal, with bow spring blades  60  of spring steel. Both ends of bow spring blades  60  are connected to bands  55  by welding or other like means. Each of bands  55  has a number of windows  70 , which number may be one or more to provide a sufficient number of windows to accommodate the number of lugs  40 , as will be described. Bow spring centralizer  50  may be mounted on central recessed section  25  in several manners. Bands  55  may be hinged to permit bands  55  (and consequently bow spring centralizer  50 ) to be opened up, placed around central recessed section  25 , then pinned closed to lock bow spring centralizer  50  thereon. Alternatively, bands  55  could be cut, bow spring centralizer  50  placed around central section  25 , then welded together again. By whatever method of attachment, bow spring centralizer  50  is mounted so that windows  70  fit over lugs  40 , as shown in FIG. 1, thereby restraining the rotational and longitudinal movement of bow spring centralizer  50  within central recessed section  25 . 
     FIG. 2 shows float shoe  10  without bow spring centralizer mounted thereon, for clarity. FIG. 3 is a cross-section of float shoe  10 , showing placement of lugs  40  and other elements of the apparatus. FIG. 3 a  is a partial cross section of float shoe  10 , including insert  15  and one way valve  16 . Insert  16  may be of cement or other suitable composite material, which may be cast in place within the bore  30  of float shoe  10 . 
     With reference to FIGS. 7 a  through  7   d , several key aspects of the present invention while in use are shown, in particular with the float shoe embodiment (it is understood that a similar sequence would apply to the float collar embodiment of the present invention, as well). In FIG. 7 a , float shoe  10  is moving downhole in the direction of arrow A. Bow spring centralizer  50  is “relaxed”, in that the diameter of the passageway is sufficient to accommodate its maximum diameter, while being moved downhole due to contact with lowermost set of lugs  40 . In FIG. 7 b , a restricted diameter passageway has been encountered. Bow spring centralizer  50  is “dragged” through the restriction by the lowermost set of lugs  40  engaging the lowermost set of windows  70 , while bow spring centralizer  50  at least partially collapses. As bow spring centralizer  50  collapses, the spacing between bands  55  increases, and the size, configuration, and spacing of lugs  40 , and bow spring centralizer  50 , including windows  70  and bands  55  permits bow spring centralizer  50  to fully collapse without uppermost windows  70  contacting uppermost lugs  40 , as can be seen in FIG. 7 b . This “dragging” action, as opposed to “pushing” the centralizer (as would happen if bow spring centralizer would be forced downhole by contact with uppermost set of lugs  40 ), eases passage of the bow springs through the restricted diameter passage. The size, configuration, and spacing of the different components of the invention, including lugs  40 , windows  70 , bands  55 , and bow spring blades  60  are such that even when bow spring centralizer  50  is substantially completely collapsed to accommodate a small diameter passageway, some amount of longitudinal movement of the bands  55  (and consequently bow spring centralizer  50 ) is still possible. It can be seen in FIGS. 7 a  and  7   b  that enlarged end sections  26  and  27 , having a diameter equal to or somewhat larger than the diameter of bands  55 , shield bands  55  from obstructions and the like. 
     FIGS. 7 c  and  7   d  show a similar sequence, with float shoe  10  now being pulled upward in the direction of arrow B. In FIG. 7 c , the passageway is of sufficient diameter that bow spring centralizer  50  is relaxed. When a restricted diameter passageway is encountered, as in FIG. 7 d , bow spring centralizer  50  is dragged rather than pushed through the restricted diameter section, this time by contacting the uppermost set of lugs  40 . 
     Another presently preferred embodiment of the present invention is shown in FIGS. 4,  5 ,  6 , and  6   a , in this embodiment a float collar  80 . In FIGS. 4,  5 ,  6 , and  6   a , where applicable, like parts have like part numbers with the float shoe embodiment. In general, float collar  80  has both upper and lower ends  80   a  and  80   b  adapted for connection to a casing string (via threaded connection or other means well known in the art), as opposed to float shoe  10 , which typically has only an upper end  10   a  adapted for connection to a casing string. Float collar  80  comprises central recessed section  85  and larger diameter end sections  86  and  87 . As seen particularly in FIGS. 5,  6  and  6   a , float collar  80  also comprises lugs  40 , and an insert  15  and one way valve  16 . 
     It is to be understood that the sequence of running float collar  80  through a various diameter passageways is substantially the same as the sequence described above for float shoe  10  in FIGS. 7 a  through  7   d.    
     The casing string centralizer of the present invention may be readily understood from the above description of float shoe  10  and float collar  80 . The casing string centralizer is essentially the configuration of float collar  80 , both ends of which are adapted for connection to a casing string, and without the insert and one way valve. 
     Although the above description contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, various materials could be used for the different parts of the invention; dimensions may be varied to suit given applications; different numbers of lugs and placement at different locations on the central recessed section is possible; and different numbers of bow spring blades may be used, etc. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.