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FIELD OF THE INVENTION 
     The present invention relates to a blowout preventer having modified annular seal assemblies at the piston interface. 
     BACKGROUND OF THE INVENTION 
     Annular or spherical blowout preventers (BOPs) are large, hydraulically operated valves used in the oil and gas industry to control a wellbore at the ground level. Annular BOPs primarily function to seal or close the wellbore, when actuated, to thereby prevent a blowout. 
     Structurally, annular BOPS comprise a body formed by mating upper and lower housings that enclose a deformable, resilient packing element for sealing its central bore which connects with the wellbore. The body further encloses a hydraulically operated piston, which actuates the opening and closing of the resilient packing element. 
     Contamination of the BOP hydraulic fluid system, by fluid and/or debris from the wellbore, needs to be prevented in order for the BOP to function properly. Various types of pressure seals are used to provide a physical separation between the fluids in the hydraulic fluid chamber of the BOP and its central bore. The prior art pressure seals conventionally used in this service, however, have not satisfactorily withstood the damage and degradation that occurs over time as a result of exposure to contaminants in the wellbore fluid. In addition, such common pressure seals are of a size and shape that they can roll or twist in use, thereby releasing their sealed engagement and allowing leakage of fluid. Accordingly, frequent inspection, repair and replacement of the common pressure seals is required in order to prevent the leakage of wellbore fluid into the hydraulic fluid system of the BOP. 
     One attempt to solve this problem has been to utilize what is known as a single, dual-functioning wiper seal, which serves to provide:
         a) a pressure seal for preventing leakage of fluid from the wellbore into the hydraulic fluid system of the BOP, and   b) a wiper seal for preventing dirt and debris from the fluid in the wellbore from entering the hydraulic fluid system.       

     Dual-functioning seals, however, are not effective pressure seals and are still prone to degradation over time. Frequent repair and/or replacement of dual-functioning wiper seals continues to pose a problem for BOP manufacturers and operators in the oil and gas industry. 
     There is therefore a need for modified annular seal assembly that is designed to effectively seal the interface between the hydraulic fluid system of a BOP and the fluid within the wellbore, while at the same time resisting damage cause by wellbore fluid contaminants. 
     SUMMARY OF THE INVENTION 
     A hydraulically-operated annular blowout preventer, for controlling a wellbore, comprising modified annular seal assemblies at the piston interface, is described. The blowout preventer comprises:
         a U-shaped annular lower housing, having upstanding, spaced apart, inner and outer walls;   the housing defines a central bore for connecting with the wellbore;   a vertically movable piston; and   an adapter ring connected to the outer wall and extending toward the inner wall of the lower housing;
 
wherein the housing and the adapter ring form an annular hydraulic fluid chamber. The upper end of the inner wall and the adapter ring are spaced apart such as to define an annular gap. The lower end of the piston is positioned within the chamber and its upper end extends upwardly through the gap, thereby providing a physical separation between the hydraulic fluid chamber and the wellbore.
       

     The inner wall of the housing and the adapter ring each form an annular peripheral groove at the gap and each groove is associated with:
         a discrete wiper seal, for preventing fluid and contained debris and/or contaminants originating from the wellbore from entering the hydraulic fluid chamber,   a discrete pressure seal, positioned below and abutting the wiper seal, the pressure seal being upwardly oriented for containing pressurized wellbore fluid and isolating the hydraulic chamber at the gap, and   a retainer member for preventing both radial and axial movement of the wiper seal.       

     Various advantages and features of the present invention will become readily understood from the following detailed description taken in connection with the appended claims and the attached drawings, but omitting the main packing. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of an annular blowout preventer showing two piston seal assemblies as defined herein. 
         FIG. 2  is an amplified view of the piston seal assemblies as shown in  FIG. 1 . 
         FIG. 3  is a further amplified view of one piston seal assembly as shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A piston seal assembly will now be described with reference to  FIGS. 1 to 3 . 
     By way of background, a hydraulically operated blowout preventer (BOP), generally identified by  100 , comprises a body  1 , having an upper housing  2  and a lower housing  3  with a bore  4  extending therethrough. The bore  4  is an extension of, and communicates with, the wellbore of the well (not shown). 
     The lower housing  3  is annular and U-shaped, having a base wall  5  and inner and outer, radially spaced apart, upwardly projecting walls  6 ,  7 . The walls  5 ,  6  and  7  cooperate to form an annular hydraulic fluid chamber  8 . The inner wall  6  defines a section of the bore  4 , which forms part of the wellbore. 
     An adapter ring  10  is connected with the upper end of the outer wall  7  and projects inwardly to partly close the upper end of the hydraulic fluid chamber  8 . The upper end of the inner wall  6  and the adapter ring  10  define an annular gap  9  between them. 
     An annular piston  12  is slidably received by the chamber  8 . The annular piston  12  comprises a vertically extending wall  13 , an outwardly projecting lip  14  at the lower end of the piston wall  13  and an inwardly projecting lip  15  at the upper end of the wall  13 . The outwardly projecting lip  14  is slidably positioned within the chamber  8 , the vertically wall  13  extends through the gap  9 , and the inwardly projecting lip  15  corresponds with the bore  4 . 
     The vertical wall  13  of the piston  12  forms a first sealable interface  17  with the inner wall  6  of the lower housing  3 , and a second sealable interface  19  with the adapter ring  10 . First and second interfaces  17 , 19  seal the annular gap  9  and serve to isolate the fluid in the hydraulic fluid chamber  8  from the fluid and/or contaminants in the bore  4 . 
     Having regard to  FIG. 2 , a piston seal assembly, generally referred to as  20 , is provided at each of the first and second interfaces  17 ,  19 . The piston seal assembly  20  comprises a discrete wiper seal  22  and a discrete pressure seal  24  positioned within an annular peripheral groove  26  and below the wiper seal  22 , so that the wiper seal  22  shields and protects the pressure seal  24  from damage and degradation due to contaminants in the bore  4 . 
     The wiper seal  22  may be extruded nitrile rubber having a durometer hardness of at least 70 (Shore A scale). In one preferred embodiment, the durometer hardness or the wiper seal  22  is between 85-95 Shore A. In a more preferred embodiment, the durometer hardness of the wiper seal is about 95 Shore A. 
     The pressure seal  24  of the assembly provides a seal, upwardly oriented to prevent fluid and/or smaller contaminants that were not retained by the wiper seal  22  from leaking into the hydraulic fluid chamber  8 . The pressure seal  24  may comprise a Polypak™ seal (Parker Seals, Utah, U.S.A.). In one embodiment, the pressure seal  24  may be a rectangular Polypak™ seal (Parker Seals, Utah, U.S.A.) thereby reducing the rolling or twisting of the seal within the groove  26 . 
     Each piston seal assembly  20  further comprises a retainer member  28   a, b , which is specifically configured to retain the wiper seal  22  and contain the pressure seal  24  in place in the groove  26 . The retainer member  28  forms the upper end of the groove  26 . At the first interface  17 , the first retainer member  28   a  is formed by a lateral flange  30 , that is integral to and projects outwardly from the inner wall  6 , toward the inner surface of the piston&#39;s  12  vertical wall  13 . At the second interface  19 , a second retainer member  28   b  is formed by a lateral flange  30  is integral to and projects inwardly from the adapter ring  10  toward the outer surface of the piston&#39;s  12  vertical wall  13 . 
     In one embodiment, each retainer member  28   a, b  further comprises a downwardly depending lug  31  which projects from the lateral flange  30 , thereby forming a “hook-shaped” configuration (see  FIG. 3 ). Each retainer member  28   a, b  thus releasably receives or is embedded in the wiper seal  22  associated therewith in a “snap-fit” engagement and reduces both axial and radial movement of the wiper seal  22 . The retainer member  28   a, b  further retains the pressure seal  24  within the annular peripheral groove  26  by providing the lug  31 , which extends downwardly into the peripheral groove  26  such that the pressure seal  24  within the groove  26  is abutted by the wiper seal  22 . The wiper seal  22  has an upwardly projecting lip  22   a  which interlocks with the lug  31 , as illustrated in  FIG. 3 . 
     In use then, the wiper seal  22  is retained in a “shielding” position at the first interface  17 , by interference fit, between the retainer member  28   a  of the inner wall  6 , the inner surface of the piston&#39;s  12  vertical wall  13  and the pressure seal  24 . At the second interface  19 , the wiper seal  22  is retained in position, by interference fit, between the retainer member  28   b  of the adapter ring  10 , the outer surface of the piston&#39;s  12  vertical wall  13  and the pressure seal  24 . 
     In conclusions, the interlocking of the wiper seal  22  and retainer member  28   a, b  configuration is used to effectively reduce both the axial and radial movement of the wiper seal  22  and to anchor the wiper seal  22  in a shielding relation to the pressure seal  24  within an annular BOP. 
     Although preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications to the wiper seal assembly defined herein might be made without departing from the scope of the invention. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. 
     In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Summary:
A hydraulically-operated annular blowout preventer, for controlling a wellbore and comprising a wiper seal assembly, is described. The wiper seal assembly comprises a discrete wiper seal, for preventing debris and/or contaminants from the wellbore from entering the hydraulic fluid chamber of the blowout preventer, a discrete pressure seal, positioned below and abutting the wiper seal in the same seal groove, for isolating the hydraulic chamber, and a retainer member for preventing both radial and axial movement of the wiper seal.