Abstract:
A corrosion-resistant alloy tube is formed and bonded to a pre-machined steel alloy rod to form a riser tensioner cylinder rod. During assembly, an epoxy is injected into an annular space between the tube and rod and then cured. The bonded tube is ground to a desired surface finish prior to installation and utilizes a double seal arrangement that prevents external pressure or corrosive fluids from entering the cured epoxy in the annular space.

Description:
[0001]    This application is a continuation-in-part of and claims priority to and the benefit of U.S. patent application Ser. No. 11/226,573 filed Sep. 14, 2005, entitled System, Method, and Apparatus for a Corrosion-Resistant Sleeve for Riser Tensioner Cylinder Rod, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates in general to offshore drilling rig riser tensioners and, in particular, to an improved system, method, and apparatus for corrosion-resistant riser tensioner cylinder rods having an outer sleeve retained with an annular layer of epoxy. 
         [0004]    2. Description of the Related Art 
         [0005]    Some types of offshore drilling rigs utilize “push-up” or “pull-up” type riser tensioners. The riser tensioner incorporates cylinder rods to maintain tension on the riser. The cylinder rods are subjected to a very corrosive environment caused by exposure to drilling muds, completion fluids, and general offshore environments. As a result, the rods currently being used are made from either a solid nickel-based alloy or a laser-clad cobalt-based layer that is applied to a steel alloy rod. Both of these current rod options are expensive and, in the case of cladding, result in long lead times with multiple process requirements in geographically remote locations. Consequently, there is a higher probability for damaged parts and scrap or scrappage. Thus, an improved design for riser tensioner cylinder rods would be desirable. 
       SUMMARY OF THE INVENTION 
       [0006]    One embodiment of a system, method, and apparatus for improving the cylinder rods for riser tensioners. The present invention overcomes the shortcomings of the prior art by placing a thin tube or pipe over a pre-machined steel alloy rod. The tube is formed from a corrosion-resistant alloy and is bonded to the rod with, e.g., a thin layer of epoxy. This design results in a much lower manufacturing cost (approximately one-third less than current technology) and shorter manufacturing lead times. The manufacturing process for installing the sleeve requires injection and curing of the epoxy between the pipe and rod. 
         [0007]    In one embodiment, the rod is machined with threaded end connections that serve to ultimately connect the rod assembly to the piston and rod extension of the cylinder assembly. The tubing is slid over the outer diameter of the rod and temporarily connected with two end connectors that center the tubing on the rod. The connectors also act as ports for injecting the epoxy which is pumped into the annular space on one end. The excess epoxy exits the opposite end and the retained epoxy is cured. The end connectors are then removed and the assembled part is ground to a final outer diameter before installation. The piston is connected and the rod clevis is made up to the cylinder rod and utilizes a double seal arrangement that prevents external pressure or corrosive fluids from entering the cured epoxy in the annular space. Advantageously, this process eliminates straightness and warping issues that commonly occur with prior art cladding operations. 
         [0008]    The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the features and advantages of the invention, as well as others which will become apparent are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
           [0010]      FIG. 1  is a partial sectional view of one type of floating platform with a riser supported by a tensioning mechanism constructed in accordance with the invention; 
           [0011]      FIG. 2  is a partially sectioned side view of one embodiment of a piston rod for a riser tensioning mechanism and is constructed in accordance with the invention; 
           [0012]      FIG. 3  is a sectional side view of one embodiment of a piston rod and end connectors for manufacturing thereof and is constructed in accordance with the invention; and 
           [0013]      FIG. 4  is an enlarged sectional side view of one embodiment of a portion of the piston rod and one of the end connectors of  FIG. 3  in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring to  FIG. 1 , one type of riser tensioning mechanism  10  is depicted. Although mechanism  10  is depicted as a “pull-up” type, one skilled in the art will recognize that the present invention is equally suitable for “push-up” type and other types of tensioning mechanisms. 
         [0015]    A riser  12  extends downwardly from a platform  14  to a subsea wellhead (not shown). Riser  12  has a longitudinal axis  16  and is surrounded by a plurality of hydraulic cylinders  18 . Each hydraulic cylinder  18  has a cylinder housing  24  having a chamber (not shown). A piston rod  26  has a rod end  28  that extends downward from each cylinder housing  24  and hydraulic cylinder  18 . The piston ends of rods  26  opposite rod ends  28  are disposed within the respective chambers (not shown) of cylinder housings  24 . Hydraulic fluid (not shown) is contained within the housing  24  for pulling piston rods  26  upward. Each hydraulic cylinder  18  also has accumulator  30  for accumulating hydraulic fluid from hydraulic cylinder  18  and for maintaining high pressure on the hydraulic fluid. A riser collar  32  rigidly connects to riser  12 . The piston rods  26  attach to riser collar  32  at the rod ends  28 . Cylinder shackles  34  rigidly connect cylinder housings  24  to platform  14 . 
         [0016]    In operation, the riser tensioning mechanism  10  pulls upward on riser  12  to maintain tension therein. Riser collar  32  connects to riser  12  and engages riser  12  below platform  14  and cylinder receiver  18 . Hydraulic fluid pressure is applied to hydraulic cylinders  18  so that riser  12  is maintained in constant tension. Riser collar  32  supports the weight of riser  12  in order to create a tensional force in riser  12 . Hydraulic cylinders  18  automatically adjust to changes in platform  14  position to allow for relative movement between riser  12  and platform  14 . In the event of a failure in one of the four hydraulic cylinders  18 , the remaining hydraulic cylinders  18  will continue to support riser  12  in tension without excessive bending moments being applied to the hydraulic cylinders  18 . 
         [0017]    Referring now to  FIG. 2 , one embodiment of a piston rod  26  constructed in accordance with the present invention is shown. Piston rod  26  is the structural or load carrying member of the rod assembly, which includes a covering  74  and adhesive  75  that are shown greatly exaggerated in size for ease of understanding. Covering  74  serves as a barrier to protect the structural steel inner member from the outside corrosive fluids and atmospheric conditions typically found in offshore platforms. 
         [0018]    As described above, piston rod  26  has axis  20  and includes a threaded rod end  28  for coupling with riser collar  32 , and a piston end  70  that locates in and moves axially relative to cylinder housing  24 . Piston rod  26  also comprises a solid shank  72  that extends and is located between ends  28 ,  70 . Piston rod  26  is formed from a pre-machined steel alloy, such as commonly available inexpensive steel alloys that are not corrosion resistant. 
         [0019]    In one embodiment, the outer surface of shank  72  is enveloped by and protected with a thin, corrosion-resistant material covering  74 . In one embodiment, it is only shank  72  that is covered by covering  74 . Covering  74  may have a radial thickness  76  in a range on the order of 0.005 to 1.0 inches. The covering  74  itself may comprise many different forms including a tube, pipe, coating, or still other suitable coverings for protecting piston rod  26  from corrosion. 
         [0020]    A layer of adhesive  75  is located between covering  74  and shank  72 . Adhesive  75 , which may comprise epoxy or other bonding agents has a radial thickness  77  in a range on the order of approximately 0.0025 to 0.5 inches. The layer of epoxy serves to bond the sleeve to the outer diameter of the rod, and also to support or “back up” the thin sleeve from collapse due to external pressure while the rod translates in and out of the cylinder assembly under pressure. 
         [0021]    One embodiment of a method for joining covering  74  to piston rod  26  is depicted in  FIGS. 3 and 4 . In this embodiment, the covering  74  is formed from a thin tube  74  of corrosion-resistant alloy, such as nickel or cobalt-based alloys. Tube  74  may be joined to piston rod  26  via a series of operations. In one embodiment, a pre-cut length of tubing  74  is placed around the outer surface of shank  26 . Tubing  74  closely receives the outer surface of shank  26 , but forms a thin annular recess there between. 
         [0022]    A set of end connectors  81 ,  83  are threadingly secured to the ends  28 ,  70  of piston rod  26 . The annulus between tube  74  and shank  72  is sealed by end connectors  81 ,  83  at each end of piston rod  26 . The end connectors  81 ,  83  serve to center the tube  74  relative to rod  26  and are provided with inlet and exit ports  85 ,  87 , respectively. The inlet and exit ports  85 ,  87  are axially aligned with exterior tapers  89  formed between shank  72  and ends  28 ,  70  to provide fluid communication with the annulus. 
         [0023]    In one embodiment, the annulus is pressurized via inlet port  85  with adhesive  75  which is pumped through the annulus before being released at exit port  87 . The annulus is pressurized and/or metered with adhesive  75  to completely fill the annulus volume and remove all air pockets. 
         [0024]    Alternatively, a vacuum may be formed between ports  85 ,  87  to evacuate the annulus and pull the adhesive through the annulus. The adhesive  75  is cured after annulus has been filled, and the end connectors  81 ,  83  are removed. Any necessary trimming of tube  74  is performed and the exterior surface of tube  74  is ground to a desired surface finish and outer diameter. The part may be ground between centers located at each end of the structural steel rod and following this operation is ready to be assembled into the cylinder. The piston is connected and the rod clevis is made up to the cylinder rod and utilizes a double seal arrangement that prevents external pressure or corrosive fluids from entering the cured epoxy in the annular space. 
         [0025]    While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, although this embodiment is described with tubing only covering the shank, other embodiments may require greater or lesser surface coverage of the structural steel member.