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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to plugs for use in subterranean wells. More particularly, the invention relates to plugs used to block the flow of production fluid through the flow bore of a tubing string in a well. 
     2. Background of the Related Art 
     In well completion, it is often necessary to block the flow of liquids in the flow bore of a tubing string in order to isolate the upper portion of the tubing string from production fluids. Some tubing plugs are retrievable and are typically run into the tubing on coil tubing or cable and are then removed in the same way. Other tubing plugs are installed between adjacent pieces of tubing and lowered into the wellbore with the tubing string. Removal of these plugs requires either that the entire tubing string be pulled from the well or that the plugs be remotely opened when fluid flow through the flow bore is desired. 
     One type of plug installed between pieces of tubing string includes a central frangible element that can be either pierced or smashed by mechanical means. An example includes a one-piece frangible ceramic sealing element which, after use, is shattered by impacting with a tooth-faced, blind box hammer under force of gravity. In each of these cases, the remaining pieces of the seal must be washed out of the wellbore with completion fluid or the like making these designs unsuitable for many customers. Additionally, some designs which use a mechanical impact means to destroy the flow blocking element require an additional tool run on wire line or coil tubing to lower and then remove the impact means. 
     Other plugs installed between pieces of tubing are opened remotely through precise pulses of pressure which either destroy the seal element or actuate some valve located on the plug, thereby opening the sealing surface to flow. In still other instances, the plugs are destroyed with an explosive detonation also leaving bits of debris in the well which must be removed. 
     Also known in the art are temporary plugs made with a compressed mixture of salt and sand. These plugs may be rapidly dispersed, essentially in their entirety, by exposure of the salt and sand mixture to a wellbore fluid. However, these systems generally have been configured to block pressurized fluid from only one direction, usually downward, from the earth&#39;s surface and are therefore useful only in one direction. Another known plug assembly includes the plug member which has a frangible or dome-shaped portion shaped in a arcurate fashion, whereby one side of the plug presents a convex surface and another side presents a concave surface. The dome configuration of these plugs typically causes the plug member to be significantly more resistant to pressure from its convex side than its concave side. Consequently, these plugs are also practically capable of blocking fluid pressure from only a single direction. 
     From the foregoing it can be seen that it would be desirable to provide a plug which can be installed between pieces of tubing and which can be remotely opened without leaving debris in the wellbore. Additionally, it would be desirable to provide a plug which can be opened remotely without the use of special tools either at the earth&#39;s surface or lowered into the wellbore to the plug. Additionally, it would be desirable to have a plug which can be opened without the use of explosives or complicated pulses of pressure from the earth&#39;s surface. Finally, it would be desirable to have a dome-shaped plug which effectively withstands pressure from two directions and does not present a threat of destruction if significant fluid pressure is placed on its concave side. 
     SUMMARY OF THE INVENTION 
     A bi-directional plug is provided which blocks the flow of fluid through a flowbore of a tubing string and can be opened by simple mechanical means, requiring no external tools. In one aspect of the invention, a piston is slidably mounted in a piston housing and fixed in a first position with a plurality of shear screws. A chamber formed within the piston housing creates a pressure differential causing the shear screws to break when a, certain pressure is applied to the surface area of the piston. When the shear screws fail, the piston accelerates towards an extended position and strikes a scored, dome-shaped plug. The piston penetrates the plug, permanently pressing and housing pieces of the plug against the wall of a plug housing and opening the flow bore of the plug to fluid in either direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1A is a side view of a first end the tubing plug of the present invention, partially in section with the piston in a retracted position; 
     FIG. 1B is a side view, partially in section of the second end of the tubing plug; 
     FIG. 2 is an end view of the dome portion plug; 
     FIG. 3 is an end view of the piston; 
     FIG. 4 is a side view, partially in section of the tubing plug of the present invention with the piston in initial contact with the dome-shaped portion of the plug; 
     FIG. 5 is a side view, partially in section of the tubing plug with the piston having pierced the dome-shaped portion of the plug; and 
     FIG. 6 is a view, partially in section of a wellbore including the tubing plug of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is best described by reference to the drawings. FIGS. 1A &amp; 1B are side views, partially in section of the assembled tubing plug  100 . For the purposes herein, the terms casing, liner and tubing are used interchangeably. The assembly is divided into two drawings to better illustrate the various parts thereof. The assembly includes a piston housing  105 ; an actuating member or annular piston  110  slidably mounted in the piston housing; a plug housing  120  disposed at one end of the piston housing; a chamber  250  formed between the piston  110  and the piston housing  105 ; a plug  130  disposed within the plug housing; and a bearing ring  123  mounted in the interior of the piston housing. The piston housing  105  has internal threaded connection means  135  at a first end  136  for connection to an adjacent length of tubing (not shown) and an internal threaded area  140  at a second end  137  for connection to the plug body  120 . Formed adjacent to the second end  137  of piston housing  105  are set screw holes  139 . Set screw holes  139  are constructed and arranged to house threaded set screws (not shown) that extend through piston housing  105  and contact a first end  131  of plug  130 . The purpose of the set screws is to rotationally, and axially fix piston housing  105  to plug  130  to facilitate access to shear screws  198  as will be described herein. Also formed at the second end  137  of piston housing  105  is an area of enlarged inside diameter D 3 . 
     In the preferred embodiment, piston  110  is slidably mounted in the interior of piston housing  105 . The piston includes a first end  141  with an outwardly extending shoulder  145 , the shoulder having a groove  150  formed therein to house an O-ring  155 . O-ring  155  seals the annular area between the shoulder  145  and the interior surface  260  of piston housing  105 . A second end  111  of the piston  110 , most clearly visible in FIG. 3, terminates in an edge  160  having a flat surface  162  and a tapered, grooved surface  165  formed thereon. Piston edge  160  with its flat surface  162  and tapered, grooved surface  165  is designed to strike and open a scored dome  205  as will be described herein. Formed on the outside surface of piston  110  adjacent edge  160  are a plurality of countersunk or spot faces  170  constructed and arranged to form seats for shear screws  198  as will be described herein. 
     The plug housing  120 , includes externally formed threads  175  at a first end  176  for threaded connection to the piston housing  105 . A second end  177  of the plug housing  120  has an external threaded area  186  for connection to an adjacent piece of tubing (not shown). A plurality of access apertures  180  are equally spaced around the perimeter of the first end  176  of the plug housing  120 . First and second grooves  185   a,b  are formed in the interior of the plug housing for retaining o-ring seals  190   a,b.  O-ring seals  190   a,b  seal the annular area between the interior surface of the plug housing  120  and the exterior surface of plug  130 . Also formed in plug housing  120  is an enlarged interior diameter D 2  at a second end thereof. 
     A separate shoulder ring  123  is housed at the first end of the plug  131  and provides a landing for the piston shoulder  145  as described below. In the preferred embodiment, the shoulder ring is separate and can be made of high strength steel while the piston housing is constructed of alloy steel. 
     Plug  130  includes at a first end  131 , an externally formed groove  191  a for retention of an O-ring  195   a  and an internally formed groove  191   b  for retention of an O-ring  195   b.  O-rings  195   a,b  seal the annular area between the plug  130  and the piston housing  105  and the annular area between the plug  130  and the/piston  110 , respectively. A plurality of apertures  200  are equally spaced around the perimeter of the plug  130 . The apertures  200  are constructed and arranged to align with the spot faces  170  formed in the outer surface  255  of piston  110  as shown in FIG.  1 B. 
     In the preferred embodiment, the plug assembly provides a means of accessing and adjusting the shear screws  198  located in the apertures  200  formed in the plug  130 . Because the piston housing  105  is fixed to the plug  130  by set screws and because the piston  110  is fixed to the plug  130  by shear screws  198 , all three of the components can be rotated together with respect to the plug housing  120  as the piston housing  105  is unthreaded from the plug housing  120 . As the housings  105 ,  120  separate, the shear screws  198  become visible and accessible through the access holes  180 . In this manner, the shear screws  198  can be adjusted or replaced to meet the needs of a particular customer or the characteristics of a particular well. The housings can then be threaded back together, covering the shear screws  198 . The back angle formed at the second end  137  of the piston housing  105  and the back angle of the first end  176  of the plug housing  120  allow for a torsion lock when recommended tubing make-up torque is applied when tightening the piston/plug housing together. These mating surfaces  182  are shown in FIG.  1 B. 
     At a second end  132 , the plug includes a dome portion  205  which is visible in section in FIG.  1 B. The dome portion  205  is also visible in FIG. 2, an end view of the second end  132  of the plug  130 . The exterior or convex portion of the dome portion  205  includes scores  210  formed therein and extending from the top to the bottom or side of the dome. Each score  210  originates at an apex  215  of the dome and, in the preferred embodiment, becomes narrower and shallower as it approaches the side of the dome. The scores  210  are specifically formed to allow the dome portion to be broken outward along the scores as will be described below. In the preferred embodiment, the dome portion is made of an aluminum alloy and the scores are created by rotating a convex mill about a center point located on the centerline of the dome portion  205 . The plunge depth of the convex mill into the dome and the radius of the cutting path thereupon determines a variable or constant depth of the score  210  as it is machined into the dome. The characteristics of the dome portion regarding its resistance to pressure and likelihood of breaking open along the scores  210 , is determined by the depth and number of the scores along the dome. 
     Describing the parts of the plug assembly  100  and their relationship to one another, FIG. 1 depicts the assembly with the piston  110  in a fully retracted position, wherein the shoulder  145  of the piston  110  is in contact with shoulder  146  formed in the interior of the piston housing  105 . Fluid is prevented from entering the plug from the well surface by O-ring seal  155  located between the piston  110  and the piston housing  105 . Fluid is prevented from entering the downhole end of the plug by O-ring seal  190   a  located between the plug  130  and the plug body  120 . Annular chamber  250  is formed between the exterior surface  255  of the piston  110  and the interior surface  260  of the piston housing  105 . Because the tubing plug  100  is assembled at the earth&#39;s surface, the chamber  250  contains air at one atmosphere and, because it is sealed at each end by O-rings  155 ,  195   a  and  195   b,  the chamber will remain at one atmosphere regardless of its location in the well. The pressure in chamber  250  is preferably atmospheric, but can be a different pressure up to the pressure present in the tubing. 
     The piston is held in its retracted position by the shear screws  198  located in the threaded apertures  200  formed in the plug  130  which extend through the plug and seat in the aligned, counter sunk spaces  170  formed in the outer surface  255  of the piston  110 . The alignment of the apertures  200  with the countersunk spaces of the piston ensures that the edge  160  of the piston  110  is in the preferred alignment with the scores  210  in the dome portion  205 . 
     The plug of the present invention is designed with a differential between the well tubing pressure and the pressure in the atmospheric chamber  250  of the plug  100 . The differential ensures that when pressure is applied to the piston from the surface of the well, the piston is urged downward towards the dome portion  205  of the plug. The surface area of the piston, or that area acted upon by pressure from above, can be calculated. Assuming, for example, a piston having an outside diameter of 3.14″ and an inside diameter of 2.715″, the piston area is calculated as follows: 
     The plug of the present invention is designed with a differential between the well tubing pressure and the pressure in the atmospheric chamber  250  of the plug  100 . The differential ensures that when pressure is applied to the piston from the surface of the well, the piston is urged downward towards the dome portion  205  of the plug The surface area of the piston, or that area acted upon by pressure from above, can be calculated. Assuming, for example, a piston having an outside diameter of 3.14″ and an inside diameter of 2.715″, the piston area is calculated as follows: 
     
       
         Piston Area ( Ap )={ pi ×(3.14/2) 2 −( pi× (2.715/2) 2 }=1.954 in. 2   
       
     
     With a known piston area and a known pressure applied to the piston area, the force applied to the piston, or piston force Fp can also be calculated as follows: 
     
       
         Piston Force ( Fp )=1500 psi×1.954 in. 2 =2,933 lbs. 
       
     
     The force applied to the piston to cause the shear screws to break is that force needed to overcome the resisting force of the shear screws and the shear screws can be selected to break at a desired force. In the present example, the shear screws would be designed to fail at a force of no more than 2,933 lbs. This force can be brought to bear by the pressure of fluid in the tubing string above the plug and by additional pressure applied to fluid in the tubing string at the surface of the well. When the piston force exceeds the resistance force of the shear screws, the shear screws fail. Since the hydrostatic pressure acting on the piston area in the wellbore exceeds the opposing pressure extended on that portion of the piston within the chamber  250 , the piston will accelerate forward towards the dome portion of the plug. 
     The dome portion of the plug, with its equally spaced scores, is designed to break open when a certain force is applied thereto. With a breaking force established, the plug can be designed with the required acceleration of the piston and corresponding length of the piston stroke necessary to ensure the dome portion breaks open upon contact with the piston. For example, a dome portion having a certain score design thereupon and constructed of an aluminum material requires an energy of 15,000 lbs. in. to break open along the scores  210 . The energy applied by the piston to this particular dome portion therefore, would necessarily have to exceed 15,000 lbs. in order to fully break open the plug. Determining the kinetic energy (Ek) of the piston requires the following calculations: 
     
       
           Ek= ½ ×M×v   2   (1) 
       
     
     where v=velocity at location of piston where it contacts the dome portion 
     where M=mass of the piston 
     where 
     
       
           v   2 =2 ×a×d   (2) 
       
     
     (assuming initial velocity of the piston is 0). 
     where d= 
     acceleration length (distance traveled from rest to point 
     where piston contacts the dome portion of the plug); and 
     where a=acceleration of piston prior to contact between the piston and the plug. 
     and 
     
       
           F=P×A=M×a   (3) 
       
     
     where F=Force applied to piston. 
     where P=Differential Pressure acting on the Piston. 
     where A=Annular area between surface  260  and  255 . 
     rearranging equation (3) gives: 
     
       
           a= ( P×A ) /M   
       
     
     substituting a=(P×A)/M into equation (2) gives: 
       v   2 =2×{( P×A )/ M}×d   
     substituting v 2 =[2×{(P×A)/M}×d 9   into equation (1) gives:              Ek   =                  1   /   2     ×   M   ×     [     2   ×     {       (     P   ×   A     )     /   M     }     ×   d     ]                   =                  1   /   2     ×   M   ×   2   ×   P   ×     A   /   M     ×   d                 =                P   ×   A   ×   d                 Ek   =                1   ,   500                 psi   ×   1.954                   in   2     ×   6.099                   in   .                   =                17   ,   876                 Lb                   in   .                                    
     According the example above, the piston will have 17,876 lb in. of kinetic energy as it contacts the dome portion of the plug. Because the dome portion requires only 15,000 lb. in of energy in order to break open along the scores formed therein, the plug will open if a pressure of 1500 psi is applied to the piston area and the shear screws fail, allowing the piston to accelerate forward. 
     FIG. 4 shows the tubing plug  100  with the piston  110  in its initial contact position with the dome portion  205  of the plug  130 . The second end of the piston  111 , with its edge  160  is constructed and arranged to contact the concave side of the dome portion causing the dome portion  205  to break open along the scores  210  formed in its outside, convexed face. After the dome portion  205  breaks along the scores, the second end of the piston moves through the space previously occupied by the dome portion and pushes the attached pieces or petals  206  of the dome into the expanded inner diameter D 2  of the plug body, where they are permanently held by the outer surface  255  piston  110 . The hydrostatic differential between tubing pressure and the atmospheric pressure in the atmospheric chamber ensures the piston remains in its fully extended position. Additionally, the wedging action of the piston against the deformed petals  206  of the dome portion helps retain the piston in the extended position. FIG. 5 shows the tubing plug with the piston in its fully extended position. The bore of the plug is now completely open and will allow the passage of fluid or equipment in either direction. 
     FIG. 6 is a side view depicting the tubing plug  100  of the present invention installed in a well  600 . The well consists of a bore  602  which, in FIG. 6 includes a horizontal portion  605 . However, it will be appreciated by those skilled in the art that the plug of the present invention could be used in any well, horizontal or vertical. As the well is completed and intermediate casing  608  has been installed along the walls of the wellbore, a string of liner  610  with the tubing plug  100  of the present invention installed between two subsequent pieces of liner, is inserted into the wellbore. In the example shown in FIG. 6, the intermediate casing in the wellbore terminates at point “A” and thereafter the bore will be lined with the string of liner  610  including tubing plug  100 . From the bottom of the wellbore, the liner string  610  includes a sand screen  500  having slots  510  to accept production fluid, the tubing plug  100  of the present invention installed adjacent the sand screen and, an inflatable packer  550 . A liner top packer  560  is installed with running tool  620  at the top of the casing liner  610 . 
     As the liner string  610  with the plug  100  is installed, the finer bore  615  above the plug is isolated from production fluid or drilling mud. The inflatable packer  550  located above the tubing plug  100  is inflated with pressure from the well surface and the annulus  618  between the liner string  610  and the intermediate casing  608  above the packer  550  can be isolated from fluid. Additionally, the annular area between the liner and the intermediate casing wall is further isolated by the liner top packer  560 . 
     When the well is ready to produce and there is no longer a need to isolate the liner from well fluid, pressure is applied to the tubing plug  100  in the form of hydrostatic pressure in the liner above the plug and additional surface pressure applied from the well surface. As the combined pressure exceeds the resistance force of the shear screws (2,933 lbs. in the example herein), the shear screws  198  fail and the piston  110  accelerates forward, causing the dome portion  205  to break open and opening the liner bore to fluid flow in both directions. 
     Using a plug having mechanical features of the type described herein including a pressure differential between an atmospheric chamber and tubing pressure, the plug of the present invention can be designed to fit a variety of needs based on a customer&#39;s desires. 
     While foregoing is directed to the preferred embodiment of the present invention other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. For example, the scores  210  formed on the dome potion  205  of the plug are designed to open when contacted by the edge  160  of the piston  110  with its grooved, tapered surface. Various arrangements are possible between the dome and the piston so long as the weakening formations of the dome are matched with formations on the edge of the piston. These choices of design are fully within the scope of the invention.

Summary:
A bi-directional plug blocks the flow of fluid through a flow bore of a tubing string and can be opened by simple mechanical means requiring no external tools. A piston is slidably mounted in a piston housing and fixed in a first position with a plurality of shear screws. An atmospheric chamber formed within the piston housing creates a pressure differential causing the shear screws to fail when a certain pressure is applied to the surface area of the piston. When the shear screws break, the piston accelerates and strikes a scored, dome-shaped plug. The piston penetrates the plug, permanently pressing and housing pieces of the plug against the wall of a plug housing and opening the flow bore of the plug to fluid.