You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Provisional Application Ser. No. 61/227,310, filed Jul. 21, 2009, entitled “Dual Stripper Assembly for Slick Cable,” by the same inventor. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to the field of well head equipment used in oil and gas well services. More specifically, the present invention relates to an apparatus that provides a pressure seal around a slick cable used in oil field operations. 
         [0004]    2. Description of the Prior Art 
         [0005]    The use of so called “wireline” conveyance equipment and procedures are well known in the oil and gas industry for running a multitude of different types of well tools and other well equipment in oil and gas wells while the wells are under pressure. For example, in well logging operations, two basic types of wireline conveyance have traditionally been used, slickline and twisted or braided cable. The twisted or braided cable typically consists of a large-diameter cable that mechanically supports a hanging instrument. A wireline truck on the surface will usually be required for support. A data cable of this type supplies power and provides a communication connection down the well to the instrument in the well bore. Slickline, on the other hand, is typically a smaller diameter line in the form of a solid wire on the order of ⅛ inch or smaller, e.g., like piano-wire and does not typically provide real time data at the well surface. 
         [0006]    Slickline logging tools have been developed in recent years to enable data collection in deep oil and gas wells. The well casing is completed by setting pipe and grouting it in place with cement. The cement seals the annulus between the soil and the outside diameter of the pipe. The top of the pipe is threaded and a blow-out preventer is installed. Some type sealing device will typically be provided on the upper end of the upper tubing section which will permit the wireline to move into and out of a lubricator while the lubricator is under pressure form the well. A closing valve, such as a gate valve, and a second pipe provide a sealed enclosure above the well head. The sealed enclosure is long enough to accommodate a logging tool with a top sub attached to the slickline cable. The cable exits the lubricator through a sealing gland that enables the slick line to enter the sealed enclosure under pressure. When the gate valve is fully opened, the logging tool descends into the well casing, while the sealing device maintains a seal with the slick line as the hoist lowers the logging tool into the well bore. 
         [0007]    Conventional slickline logging tools are designed with internal recording memory to log data during descent and ascent in the hole. After returning from the well, recorded digital data is read out on the surface and chart recordings are used to display the data for analysis. Twisted or braided cable, on the other hand, may provide real time data communication to the well surface. The wireline or cable not only serves to support the tools and other equipment when running them into and withdrawing them from a well, but may also be used to apply forces when manipulating tools and other equipment present in the well. 
         [0008]    Depending upon the particular type of equipment present, the sealing device referred to above may take the form of a what is referred to as a stripper, stuffing box, sealing gland, pack-off heads, etc. In each case, the device performs the safety function of pressure containment during wireline operations. For example, wireline (or slick-line) pack off heads (oil savers) have been used by the oil field service industry for many years. A pack off head is designed to make a pressure seal around a wireline to contain the well pressure during trips in and out of the well. If during wireline operations a well kick were experienced, an unsafe condition would occur if the well head was not contained but instead left open to atmosphere. A typical pack off head includes a hard rubber insert with a passage where the wire line passes through the annulus. To seal around the wireline, the hard rubber insert is axially compressed, which reduces the cross sectional area of the passage. Reducing the cross sectional area of the passage causes the inner radius of the passage to fit snugly around the outer radius of the wire line, thus preventing fluid flow through the passage. The typical prior art pack off head only functioned to seal around a static line. This was due at least in part to the fact that the prior art braided cable would quickly destroy the seal if it moved though the energized seal element in use. 
         [0009]    Traditionally, pack off heads have been manual or hydraulic. A manual style pack off head is usually comprised of a threaded cap that compresses the rubber packing element as the cap is screwed down onto the head assembly. This operation is typically performed by hand. The hydraulic style pack off head has a hydraulic cylinder that is expanded via hydraulic pressure provided by a hand pump connected to the head by a hydraulic hose. The pack off head hydraulic cylinder moves as pressure is supplied to it, expansion of the pack off head hydraulic cylinder in turn compresses the pack off element to provide a seal around the wireline. 
         [0010]    Despite the advances which have been made in the wireline arts, there continues to exist a need for improvements in the area of cable conveyance pressure containment devices of the type discussed above. For example, while slick line or braided cable have been widely used in the past, new types of “slick cable” are now beginning to appear on the scene. The slick cable will have a smooth outer diameter in the same manner as traditional slick line, while allowing real time data communication with the downhole tool in the well bore in the manner of traditional braided cable. Preferably, the new slick cable will be capable of dynamic movement through the seal elements, even when the seal elements are energized to contain well pressure. The outer diameter of these types of slick cable will be much larger than traditional slick line, e.g., larger than ½ inch. New pressure containment devices are needed which have the ability to accept these new types of slick cable. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a solution to certain of the previously mentioned deficiencies noted in the prior art in the form of a dual stripper assembly which is used in conjunction with a wellhead situated on a hydrocarbon producing well bore, particularly where the assembly is used with a slick cable having a smooth or uniform outer diameter. The assembly allows dynamic stripping of the slick cable through the assembly while the well head is under pressure. The preferred assembly includes a dual isolation end element body having a hollow interior. An isolation sub is received within the hollow interior of the end element body, the isolation sub having an interior bore which communicates each of oppositely arranged upper and lower ends thereof. A cylindrically shaped upper hydraulic cylinder body is mounted on the upper end of the isolation sub and end cap, the upper hydraulic cylinder body having a cylindrically shaped internal piston chamber with a cylindrical upper piston slidably received therein. The upper piston also has a cylindrical interior bore. A cylindrical sleeve member is received within the cylindrical interior bore of the upper piston. 
         [0012]    A resilient wiper element is closely received within the cylindrical interior bore of the upper piston, the resilient wiper element having a central bore appropriately sized to receive and seal around a slick cable. The upper element has cylindrical sidewalls and a convex, cone shaped upper extent. 
         [0013]    A spring retainer guard including a retaining flange is attached to the upper hydraulic piston for movement upwardly and downwardly with the upper hydraulic piston. A cone shaped wiper retainer is received within the cylindrical sleeve member and has a lower concave extent which contacts and mates with the convex cone shaped upper extent of the resilient wiper element. The upper retainer also has an upper extent which is contained by the spring retainer guard. An externally mounted return spring is received about the exterior sidewalls of the upper hydraulic piston between the spring retainer guard at an upper extent and an upper portion of the hydraulic cylinder at an opposite lower extent. 
         [0014]    An upper hydraulic port is connected to a source of hydraulic fluid for communicating hydraulic fluid to an interior region of the upper hydraulic cylinder body for moving the upper hydraulic piston in a downward direction relative to the body. This action, in turn, compresses the return spring and causes the wiper retainer to compress the resilient wiper element, whereby the wiper element seals around a slick cable passing through the central bore thereof. 
         [0015]    The preferred assembly also has a second, lower hydraulic cylinder body, lower hydraulic piston and associated resilient wiper element similarly arranged in mirror image fashion on a lower end of the dual isolation end element body for use as a backup in case of failure of the upper wiper element assembly. 
         [0016]    In one preferred version of the dual stripper assembly of the invention, the slick cable has a uniform outer diameter, the outer diameter ranging from about ⅛ to about 15/32 inches. The resilient wiper element is made of an elastomer such as rubber and has an outer diameter which is greater than about 2 inches. The wiper element has an interior bore which is sized for the slick cable it will receive, allowing dynamic movement of the slick cable through an energized wiper element. With these dimensions, the assembly of the invention has been tested to hold greater than about 8000 psi pressure in the well bore. 
         [0017]    In one version of the assembly of the invention, the lower hydraulic cylinder body houses a velocity check valve which is actuated in the case of an unexpected event such as a cable breaking to isolate well bore pressure. The assembly is also preferably provided with an isolator lock retainer having a pair of internal spline elements which can be installed to lock the dual isolation end element body and the isolation sub to prevent relative vertical movement there between. Removal of the internal spline elements by pulling out a pair of external ears allows the isolation sub, wiper retainers and associated wiper elements to be removed from the assembly for maintenance. Removal of the isolation sub, wiper retainers and associated wiper elements from the dual isolation end element body allows a slick cable with an associated cable head to be pulled freely through the body of the assembly. 
         [0018]    Additional objects, features and advantages will be apparent in the written description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a simplified, perspective view of the dual stripper assembly of the invention in place on a well head. 
           [0020]      FIG. 2  is an exploded view of the dual stripper of  FIG. 1  showing the internal components thereof. 
           [0021]      FIG. 3  is a cross sectional view of the assembled dual stripper of the invention showing the operative components thereof. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processes and manufacturing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the invention herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the claimed invention. 
         [0023]      FIG. 1  is a simplified representation of the general environment of the dual stripper assembly of the invention when in use on a well head. A blowout preventer  11  rests atop a wellhead  13 . Usually at least two lubricator risers  15 ,  17  sit atop the blowout preventer  11 . The dual stripper assembly  19  of the invention sets atop the lubricator riser  17  and is threadedly connected thereto. 
         [0024]      FIG. 3  is a cross-sectional view of a preferred version of the dual stripper assembly  19  of the invention. The dual stripper assembly  19  includes a dual isolation end element body  21  having a hollow interior  23 . The dual isolation end element body  21  has a port  87  which is used to charge an internal reservoir  88  with lubricating grease. An isolation sub  25  is received within the hollow interior  23  of the end cap  21 . The isolation sub  25  has a stepped interior bore  27  which communicates with each of the oppositely arranged upper and lower ends  29 ,  31  thereof. The exterior of the isolation sub  25  is also stepped and decreases in external diameter between upper and lower generally cylindrical regions thereof. 
         [0025]    As perhaps best seen in  FIGS. 2 and 3 , an isolator lock retainer comprising a pair of internal spline elements  33 ,  35  can be installed to lock the dual isolation end element body  21  and the isolation sub  25  to prevent relative vertical movement there between. The spline elements  33 ,  35  are received in machined recesses provided between the element body  21  and sub  25  and are inserted and removed by means of external ears ( 32 ,  24  in  FIG. 2 ). 
         [0026]    As best seen in  FIG. 3 , a cylindrically shaped upper hydraulic cylinder body  37  is mounted on the upper end of the isolation sub  25  and end element body  21 . The upper hydraulic cylinder body has a cylindrically shaped internal piston chamber  39  with a cylindrical upper piston  41  slidably received therein. The upper piston  41  also has a cylindrical interior bore  43  and an interior threaded upper extent  44 . A cylindrical sleeve member  45  is received within the cylindrical interior bore  43  of the upper piston  41 . 
         [0027]    A resilient wiper element  47  is closely received within the cylindrical interior bore  43  of the upper piston  41 . The resilient wiper element  47  has cylindrical exterior sidewalls and a convex, cone shaped upper extent. The wiper element also has a central bore  49  which is appropriately sized to receive and seal around a slick cable. The upper element  47  will typically be formed of a suitable elastomeric material, such as a suitable rubber. To insure that the wiper element  47  will seal around the slick cable, it is important that the elastomeric material be sufficiently pliable to perform under extreme cold or hot conditions for which it will be used, and yet be of adequate resiliency to sustain the pressure applied to it to preclude leakage between the wiper element  47  and the slick cable. 
         [0028]    A spring retainer guard including a retaining flange  51  has an exterior threaded surface  52  which engages the threaded surface  44  of the upper hydraulic piston  41  for movement upwardly and downwardly with the upper hydraulic piston. A wiper retainer  53  is received within the cylindrical sleeve member  45  and has a lower concave extent  55  which contacts an upper mating surface of the resilient wiper element  47 . The retainer  53  also has an upper extent  57  which is received within an interior region of the spring retainer guard  50 . The retainer  53  has an internal bore  54  which communicates with the central bore  49  of the wiper element  47 . 
         [0029]    A top guide sub  59  having a central bore  60  is threadedly connected to a top region of the spring retainer  50  and has an interior recess or bore which retains a series of packing materials  61  in the form of disk shaped elements which form a lubrication seal at the upper end of the assembly. The packing elements  61  have central bores which communicate with the bore  49  of the wiper element so that they can receive the slick cable as it passes through the assembly to keep the cable clean as it leaves the assembly. 
         [0030]    An externally mounted return spring  63  is mounted about the exterior sidewalls of the upper hydraulic piston  41  between the flange  51  of the spring retainer guard  50  at an upper extent thereof and an upper portion of the hydraulic cylinder  37  at an opposite lower extent. An upper hydraulic port  67  is connected to a source of hydraulic fluid through a conventional fitting  68 . The source of hydraulic fluid will typically be a hydraulic hand pump (not shown) present on the rig floor. The port  67  communicates hydraulic fluid from the pressurized source to the upper hydraulic cylinder body  37  for moving the upper hydraulic piston  41  in a downward direction relative to the body, as viewed in  FIG. 3 . This action serves to compress the return spring  63  and causes the wiper retainer element  53  to compress the resilient wiper element  47  radially inward, whereby the wiper element seals around a slick cable passing through the central bore  49  thereof. 
         [0031]    The slick cable ( 89  in  FIG. 2 ) will generally have a uniform outer diameter which is about ⅛ inch or greater in diameter. Preferably, the slick cable  89  will have a uniform outer diameter which is in the range from about ⅛ to about 15/32 inches, unlike the prior art “piano wire” slick wirelines used in the past. The resilient wiper element  47  will have an outer diameter which is greater than about 2 inches. The wiper element  47  is designed to hold at last about 8000 psi pressure in the wellbore and has been tested to 10,000 psi. The slick cable which is capable of being dynamically run through the device will typically have internal communication lines for transmitting data from a tool at a subterranean location in the wellbore in the surface in real time, unlike traditional slick line used in the prior art. 
         [0032]    As shown in  FIG. 3 , the preferred device of the invention will also have a second, lower hydraulic cylinder body, lower hydraulic piston and associated resilient wiper element similarly arranged on a lower end of the dual isolation end cap for use as a backup in case of failure of the upper wiper element assembly. For example, with reference to  FIG. 3 , the lower hydraulic cylinder body  69  has an internally threaded surface  71  which matingly engages the externally threaded surface of a lower end cap  73 . A lower hydraulic piston  75  has a ring-shaped piston portion  77  which is received within the lower piston chamber  75 . A lower resilient wiper element  79  is received within the bore of a sleeve member  81 , as in the case of the upper wiper element. A wiper retainer  83  rests atop the resilient wiper element  79  and contacts the lower end  31  of the isolation sub  25 . 
         [0033]    The further details and construction of the lower wiper assembly are generally a mirror image of the upper assembly previously described. However, as shown in  FIG. 3 , a velocity check valve  85  is located within an internal region of the end cap  73  below the resilient wiper element  79 . The velocity check valve  85  has internal passages and a floating check ball which would be actuated in the case of a cable breaking to seal off the relevant internal passages and isolate the wellbore pressure. 
         [0034]    With reference now to  FIG. 2 , the internal components of the device are shown in exploded fashion. It will be appreciated that removal of the internal spline elements  33 ,  35  allows the isolation sub  25 , wiper retainers  53 ,  83  and associated wiper elements  47 ,  79  to be removed from the assembly for maintenance. Removal of the isolation sub, wiper retainers and associated wiper elements also allows a slick cable ( 89  in  FIG. 2 ) within an associated cable head  91  to be pulled freely through the body of the assembly. 
         [0035]    With reference again to  FIG. 3 , the operation of the device will be briefly explained with respect to the upper wiper assembly. In operation, pressure at port  67  forces piston  41  downwardly thereby compressing the wiper retainer and spring retainer to compress the spring  63 . The cone shaped wiper retainer  53  presses down on the wiper element  47  causing it to expand radially inwardly toward the slick cable which may be either static or moving dynamically through the wiper element central bore  49 . During wireline operations the wiper inner passage (bore  49 ) will experience some wear due to the dynamic movement of the slick cable  89  through the energized seal (wiper element  47 ). Although the wear will gradually result in material loss of the inner annulus of the wiper element  47 , wiper element will continue to seal against the slick cable because of the constant compressive force applied to it by the wiper retainer  53 . When the upper region of the piston  41  is no longer exposed to the hydraulic pressure through the port  67 , the spring  63  will return the piston  41  and spring retainer guard  50  to their original positions. The dimensions and characteristics of the spring  63  are determined based on the well parameters. When the spring  63  moves the piston  41  to its original position, the wiper retainer  53  will cease to apply force to the wiper element  47 , and therefore no longer compress it. 
         [0036]    An invention has been provided with several advantages. The dual stripper of the invention can accommodate slick cable of a larger diameter than the slick wireline used in the past. Unlike the pack off designs of the prior art which only sealed around a static wireline, the dual stripper design of the invention will seal around a dynamically moving slick cable. The slick cable causes less wear and tear on the internal wiper elements and yet allows communication with the downhole tool in real time, if desired. Because the assembly basically features a mirror image of the pack off construction, failure of the upper wiper element can be compensated by actuation of the lower wiper element as a backup measure. The externally mounted return spring and particular sizing of the wiper element and associated components of the assembly make it particularly suited for use with slick cable. 
         [0037]    While the invention has been shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.

Summary:
The dual stripper device of the invention has a hollow body formed for coaxial connection to the wellhead which provides for unobstructed passage of a slick cable, such as a data transmitting cable which transmits data in real time to the well surface. Disposed coaxially within the hollow body is a resilient wiper element with an internal bore formed along its longitudinal axis for passage of the slick cable. A hydraulically actuated piston applies a compressive force to the wiper element so that a pressure seal is formed between the wiper element and the cable when the wiper element is sufficiently compressed by the piston. The seal can be maintained during dynamic movement of the slick cable through the wiper element. A mirror image backup wiper assembly can be actuated in the case of failure of the first wiper assembly.