Abstract:
A rotary slip for supporting a drill string having a plurality of slip segments connected to define an opening for insertion of the drill string, wherein each slip segment comprises a head region, a toe region, and an inner radial surface axially extending between the head and toe regions, and wherein the inner radial surface of each slip segment comprises a circumferential groove. A plurality of axially aligned drill string gripping inserts are attached to each slip segment between the head region and the circumferential groove, wherein each insert comprises a gripping surface for contacting the drill string. A load ring is disposed within the circumferential groove of each slip element, the load ring comprising at least one securing element which is engaged by one of the plurality of axially aligned inserts to secure the load ring within the circumferential groove.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Serial No. 60/345,226, filed on Jan. 4, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to an improved pipe-gripping structure and method of manufacturing a pipe-gripping structure, and more particularly, to a method of installing load rings within a slip assembly to provide a pipe-gripping structure having improved load lift properties.  
         BACKGROUND  
         [0003]    In the oilfield, when drilling for oil or gas, a platform is used to support a circular rotary table. Rotational energy is supplied to the rotary table through motors or the like, moving the rotary table in a circular fashion. The rotary table comprises a central kelly bushing which provides a central opening or bore through which a drill string passes. The kelly bushing typically provides four “pin holes” receptive of pins on a master bushing which when interlocked with the kelly bushing, drive a kelly held therein. The rotary table, kelly, master bushing and kelly bushing are art terms referring to the various parts of the drilling rig which actually impart the needed rotational force to the drill string to effect drilling. Such well drilling equipment is known in the art.  
           [0004]    When adding or removing a joint of pipe from the drill string, wedges called “slips” are inserted into a bowl, called a slip bowl, in the central opening of the rotary table. The slips hold the drill pipe to prevent it from falling into the well bore. The placement of the slips may be manual, in which case the slips are provided with handles for gripping and lifting by well personnel, often referred to as “roughnecks.” In other cases the slips may be moved into position using a powered mechanical or hydraulic system. Once the pipe is securely held by the slips, additional sections of pipe can be added to/or removed from the drill string.  
           [0005]    In some instances, slips comprise two arcuate slip segments hinged on either side of a center arcuate slip segment to form an orifice through which the drill string extends. Each slip segment has an inner surface comprising a plurality of axially milled grooves for receiving a series of vertically stacked gripping elements or inserts. The inserts have roughened surfaces which extend towards and grip the drill string when the slip is engaged with the pipe.  
           [0006]    In most slips, the axial grooves are of dovetail cross-section and are machined from the top down to a lower toe area of the slip by a dovetail cutter. The dovetail cutter is circular in shape and as the cutter is milled down to the bottom of the casting, the cutter leaves a radius at the bottom of the dovetail groove. Such a radius experiences high stress concentrations as the axial or “hook” loads of the pipe are transferred through the inserts to the terminal ends of the dovetail grooves. These high stress concentrations often result in deformation or failure of the bottom toe area of the slip segments.  
           [0007]    One solution to the high stress caused by the radius at the bottom of the dovetail groove, is to provide a circumferential relief groove for the cutter to pass through at the bottom shoulder of the slip segments such that the radius is eliminated. Half-moon inserts or load supporting rings are then inserted into the relief grooves to provide the dovetail groove a squared terminal end and a flat support surface for the inserts installed along the bottom shoulder. However, because of the large axial loads transferred through the inserts or load rings to the bottom shoulder, many of these inserts or load rings are either pushed out or must be hardened and welded in place to become a more permanent part of the bottom casting.  
           [0008]    Although these permanent load supporting devices may improve the performance of the slip, damage to the load supporting devices may require replacement of the entire slip segment. Damage to these load supporting devices may occur due to a variety of reasons. For example, if a slip is used to hold a drilling string large enough to create axial loads close to the slip&#39;s rated limit, any additional force caused by the movement of the rig will cause the inserts to jam and overload the load ring. In such instances, the load ring needs to be replaced. If the load ring is permanently welded to the bottom casing, then the entire slip would need to be replaced. Accordingly, it is important that the load rings be removable because they wear and can be overloaded.  
           [0009]    In response to the foregoing problems, removable load rings have been developed, such as those manufactured and sold by Varco International, Inc., Orange, Calif. 92868. Specifically, these load rings have been used with slip segments (Part No. 70102-1) for Varco&#39;s 1,000 ton elevator spider (Part No. 70100). These load rings are generally semi-circular and installed in relief grooves centrally disposed along the axial dovetail grooves and along the slip&#39;s bottom shoulder. These load rings are typically fastened in place by bolts.  
           [0010]    Other removable load rings include the type described in U.S. Pat. No. 6,264,395 (the &#39;395 patent). In an attempt to improve then existing slip assemblies, the &#39;395 patent discloses a slip assembly having slip segments with circumferential grooves cut at reverse angles. The circumferential grooves are adapted to receive complementary shaped surfaces of a load ring to prevent upward slippage of the load ring during loading. The load ring is secured within the grooves by bolts disposed at spaced intervals along the load ring.  
           [0011]    While existing removable load rings have been helpful in addressing the problems associated with permanently coupled inserts, the fasteners used to secure these load rings, such as threaded bolts or cotter pins, may provide additional problems. For example, the aforementioned fasteners may become loosened or fail under extreme axial loads and fall into the well bore.  
           [0012]    Accordingly, there is a need for a load ring that is removable and easy to install. It is desirable that such a load ring not be secured by fasteners or other means that might loosen and potentially fall into the well bore.  
         SUMMARY OF THE INVENTION  
         [0013]    An exemplary embodiment of the present invention includes a rotary slip for supporting a drill string comprising a plurality of slip segments connected to define an opening for insertion of the drill string, wherein each slip segment comprises a head region, a toe region, and an inner radial surface axially extending between the head and toe regions, and wherein the inner radial surface of each slip segment comprises a circumferential groove. A plurality of axially aligned drill string gripping inserts are attached to each slip segment between the head region and the circumferential groove, wherein each insert comprises a gripping surface for contacting the drill string. A load ring is disposed within the circumferential groove of each slip element, the load ring comprising at least one securing element which is engaged by one of the plurality of axially aligned inserts to secure the load ring within the circumferential groove.  
           [0014]    In another embodiment of the present invention, the inner radial surface of each slip segment of the above described rotary slip comprises at least one axial groove extending from the head region to the circumferential groove, such that each axial groove extends into the circumferential groove.  
           [0015]    In another embodiment of the present invention, the circumferential groove comprises a upper, lower and inner surfaces and the load ring comprises inner, outer, top and lower surfaces, such that the lower, outer and top surfaces of the load ring fit, respectively, within the lower, inner and upper surfaces of the circumferential groove. In addition, at least one tab protrudes from the top surface of the load ring, wherein each tab comprises a front surface and a back surface, such that the front surface of each tab is engaged by one of the plurality of axially aligned inserts to secure the load ring within the circumferential groove. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:  
         [0017]    [0017]FIG. 1 is a partial schematic cross-sectional view of a manual slip system in accordance with the present invention mounted onto a rotary table;  
         [0018]    [0018]FIG. 2 is an exploded perspective view of the slip system of FIG. 1;  
         [0019]    [0019]FIG. 3A is a partial horizontal cross-sectional taken along the line  3 - 3  of FIG. 1, in combination with a drill pipe shown in outline form;  
         [0020]    [0020]FIGS. 3B and 3C are partial cross-sectional views of a slip segment of the present invention having an insert disposed therein;  
         [0021]    [0021]FIG. 4 is a partial perspective view of a toe area of a slip segment in accordance with the present invention;  
         [0022]    [0022]FIG. 5A is a partial vertical sectional view taken along the line  5 - 5  of FIG. 4;  
         [0023]    [0023]FIG. 5B is the partial vertical sectional view of FIG. 5A having a load ring and an insert adjacent to a slig segment;  
         [0024]    [0024]FIG. 6A is a top view of a set of load rings in accordance with the present invention;  
         [0025]    [0025]FIG. 6B is a back view of a load ring in accordance with the present invention;  
         [0026]    [0026]FIG. 6C is a cross-sectional view taken along the line  6 C- 6 C of FIG. 6;  
         [0027]    [0027]FIG. 7A is a top view of a manual rotary slip in accordance with the present invention, wherein the slip is in a partially opened position;  
         [0028]    [0028]FIG. 7B is a cross-sectional view of the manual rotary slip of FIG. 7A; and  
         [0029]    [0029]FIG. 8 is a front elevational view of a power slip segment the load ring in accordance with one embodiment of the present invention installed in an insert carrier of a power slip system. 
     
    
     DETAILED DESCRIPTION  
       [0030]    [0030]FIG. 1 illustrates a conventional rotary table  12  for suspending a pipe or drill string  14  directly above a well bore and for rotating the drill string about a vertical axis  16 . The table  12  includes a manual slip system  10  according to the present invention. The system includes a slip bowl  18 , which is mounted within a central opening  19  of the master bushing  101  and a rotary slip assembly  20 , which is rotatably disposed within the slip bowl  18 . The slip bowl  18  is defined by a cylindrical outer wall  22  that extends axially between an upper “head” region  24  and a lower “toe” region  26 , and a tapered inner wall  28  having a reduced diameter at the toe region.  
         [0031]    The slip assembly  20  generally comprises a plurality of slip segments having tapered outer walls that are adapted to engage the tapered inner wall  28  of the bowl  18  to retain the slip assembly  20  from lateral, but not rotational movement within the bowl  18 . Each slip segment carries along its inner surface a series of inserts  60  which grip the drill string  14  to prevent the drill string  14  from falling into the well bore, and at least one circumferential groove  70 . In one embodiment, the circumferential groove  70  is disposed within the toe region  26  of each slip segment. In the present invention, as shown in FIG. 7, a load ring  90  is adapted to be received by the circumferential groove  70  to absorb the axial or “hook” loads imposed on the inserts  60  during operation. Although one embodiment of a slip is shown in the above referenced figures, it should be understood that the number of slip assemblies, slip segments, and inserts may vary.  
         [0032]    Referring to FIG. 2, in the depicted embodiment, the slip assembly  20  comprises a generally annular body  30  formed by a center slip segment  32 , a left hand segment slip  34  and a right hand slip segment  36 . The slip segments are symmetrically disposed about the vertical axis  16  and form an orifice  38 , as shown in FIG. 1, for receiving the drill string  14 . Although the embodiment shown in FIG. 2 depicts a slip assembly comprising three slip segments, it should be understood that the number of slip segments in each slip assembly may vary.  
         [0033]    The left and right hand slip segments  34  and  36  are hinged on opposite sides of the center slip segment  32  by a pair of hinge pins  40 . Each slip segment also includes a manual handle  42  coupled to the head of the segments to allow the operators to lift or hoist the slip assembly  20  out of engagement with the slip bowl  18 .  
         [0034]    Each slip segment has an arcuate body shape defined by an interior surface  50  and a downwardly tapered outer wall  52 . In one embodiment, the slip segments are cast from CMS 02 grade 150-135 steel, or CMS 01 steel. In an exemplary embodiment, a series of axial grooves  54  are milled lengthwise along the interior surface  50  of the slip segments. The axial grooves  54  extend from the head region  24  of the slip segments and terminate at the toe region  26  of the slip segments at the top of the circumferential groove  70  (as shown in detail in FIG. 4).  
         [0035]    As shown in FIG. 3, the axial grooves  54  comprise an inner surface  57  and spaced apart sidewalls  55 , which combine to form a cross-section that is adapted to receive and interlockingly engage a series of the inserts  60 . Any cross-section suitable for interlockingly engaging the inserts  60  to retain the inserts  60  within the grooves  54  may be utilized, such as, for example, a T-shaped cross-section  31  (as shown in FIG. 3B), a partial trapezoidal cross-section  33  (as shown in FIG. 3C) or a dove tailed cross-section. In one embodiment, the sidewalls  55  of the axial grooves  54  are angled or tapered to form the partial trapezoidal cross-section  33  and the inserts  60  are trapezoidal in shape, such that when the inserts  60  are placed within the axial grooves  54 , the angled side surfaces of the inserts  60  are interlockingly engaged with the angled sidewalls  55  of the axial grooves  54 .  
         [0036]    As is also shown in FIG. 3, a series of the inserts  60  is received by the axial grooves  54 . Each insert  60  includes contact surfaces  62  that form a cross-section corresponding to the cross-section of the groove  54 . Each insert  60  also includes a gripping surface  64 . In one embodiment, the contact surfaces  62  are retained within the axial grooves  54  and the gripping surfaces  64  extend out of the axial grooves  54  and into the orifice  38 . The gripping surfaces  64  comprise gripping elements  66  (as shown in FIG. 2), which effectively grip and support the drill string  14  when the drill string  14  is engaged by the slip. In one embodiment, the inserts  60  are vertically stacked within the axial grooves  54  in sets of five, but the number of inserts  60  stacked within the axial grooves  54  may vary based on considerations such as the outer diameter, the wall thickness, and the material strength of the drill string  14  that is being supported. In one embodiment, for example, the inserts  60  are formed from carburized 8620 low alloy steel.  
         [0037]    With reference to FIGS. 4 and 5A- 5 B, the circumferential groove  70  is formed by milling or otherwise cutting into the interior surface  50  of the slip segments at the toe region  26 . The circumferential groove  70  receives the load ring  90 , described below. The circumferential groove  70  is defined by an upper surface  72  that forms the terminal end of the axial grooves  54 , an inner surface  74 , and a lower surface  76 , which forms a shoulder  78  with the interior surface  50 . Oblong notches  80  are distributed along the upper surface  72  of the circumferential groove  70  to receive securing elements  96  (as shown in FIG. 6B) that are coupled to the load ring  90 . The notches  80  are disposed about the upper surface  72  at locations corresponding with the axial grooves  54 . Each notch  80  is positioned about the upper surface  72  such that a top portion  82  of the notch  80  is recessed into a corresponding axial groove  54  and a lower portion of the notch  84  is recessed into the inner surface  74  of the circumferential groove  70 .  
         [0038]    As shown in FIGS. 6A to  6 C, each load ring  90  comprises a substantially 120° arcuate segment having dimensions such that each load ring  90  fits securely within the circumferential groove  70 . The load ring  90  is defined by a lower surface  91  that engages the shoulder  78 , an outer surface  92  that engages the inner wall  74  of the groove, a top surface  93  that engages the upper surface  72  of the groove, and an inner surface  94  mounted flush to the interior surface  50  of the slip segment. In one embodiment, the load ring  90  is machined from a wrought metal, such as 40 series steel, 4141 or 4340, and hardened through a heat treatment process to a tensile strength of about 170 kips to about 175 kips.  
         [0039]    Extending upwardly from the top surface  93  and outwardly from the outer surface  92  are the securing elements or tabs  96  disposed at locations along the load ring  90  that correspond to the notches  80  in the slip segment. The tabs  96  are formed to a shape corresponding with the notches  80  such that the tabs  96  fully engage the notches  80  when the load ring  90  is installed within the circumferential groove  70 . Each tab  96  is appropriately formed such that when a back face  98  of the tab  96  is received within the notch  80 , a front face  97  of the tab  96  is flush with the an inner surface  57  of the axial groove  54 . Thus, the inserts  60  are able to slide within the axial grooves  54 , over the front face  97  of the tabs  96  to engage a top surface  93  of the load ring  90 , such that when one of the inserts  60  engages the load ring  90 , it engages the front face  97  of the tabs  96  and the top surface  93  of the load ring  90  to retain the load ring  90  within the circumferential groove  70 . In one embodiment, the tabs  96  and the corresponding notches  80  are “tightly toleranced” to allow the tabs  96  to “snugly” fit within the notches  80 . In one embodiment, the tabs  96  and notches  80  have curved edges.  
         [0040]    The present invention provides a removable load ring  90  which is advantageous over inserts or rings of the prior art. The load ring  90  of the present invention is not required to be hardened and welded in place during installation. It is important that the load rings be removable because they wear and can be overloaded during operation. Further, the load ring  90  of the present invention does not require any threaded bolts to secure the load ring  90  within the circumferential groove  70 . This is advantageous because it alleviates the possibility of bolts “backing out” or disengaging during operation and falling down the well bore.  
         [0041]    The load ring  90  is installed into each slip segment by first placing it within the circumferential groove  70  such that the load ring tabs  96  are fully engaged with the slip segment notches  80 . Next, the inserts  60  are vertically stacked within the slip segment axial grooves  54 . The first of the vertically stacked inserts  60  engages the load ring  90  to secure the load ring  90  within the circumferential groove  70 . Once the inserts  60  are stacked within the axial grooves  54 , a retainer ring  100  (FIG. 2), which sits within a shoulder located at the head of the slip segment, is used to retain the stacked inserts in place. The retainer ring is secured to the head region of the slip segments by threaded bolts.  
         [0042]    During operation, the axial or hook loads exerted from the drill string  14  to the inserts  60  act to further engage secure the inserts  60  against the load ring  90 . The load ring  90  functions to absorb the axial and hook loads and distribute them uniformly about the shoulder  78  of the circumferential groove  70 . Thus, the axial and hook loads are uniformly distributed about the shoulder  78  of the circumferential groove  70  and are not concentrated at the terminal ends of the grooves  54 . This uniform distribution of the load reduces the chance of deformation or failure about the toe region of the slip segments due to excessive axial or hook loads.  
         [0043]    While only one load ring  90  per slip segment is described in the embodiments above, any number of load rings may be used to change the distribution of the load carried by the inserts  60 . For example, as shown in FIGS. 7A and 7B, a second load ring  90 ′ may be used in a central region of each slip segment. Such a configuration may change the axial or hook load distribution along the length of the slip such that about 60% of the compressive load is carried by the load ring  90  at the toe region and about 40% of the compressive load is carried by the second load ring  90 ′ at the central region. In one embodiment, slips rated at about 350 tons to about 500 tons may utilize one load ring  90  and slips rated at about 750 tons or higher may utilize at least the load ring  90  and the second load ring  90 ′.  
         [0044]    In alternative embodiments, additional fasteners may be used to secure the load ring  90  within the circumferential groove  70 , such as cotter pins or threaded bolts, as shown in FIGS. 6A to  6 C. For example, each slip segment may comprise one or more openings  102  for receiving cotter pins or threaded bolts.  
         [0045]    The load ring of the present invention may not only be used in manual slip assemblies  10 , as shown in FIGS. 7A and 7B, but may also be used in insert carriers of power slip assemblies  10 ′, as shown in FIG. 8.  
         [0046]    It should be understood that the embodiments described and illustrated herein are illustrative only, and are not to be considered as limitations upon the scope of the present invention. Variations and modifications may be made in accordance with the spirit and scope of the present invention. Therefore, the invention is intended to be defined not by the specific features of the preferred embodiments as disclosed, but by the scope of the following claims.