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: 
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a differential pressure-activated valve in a sub to be placed in a tubular of a well before the tubular is placed in the well. More particularly, a barrier valve in a sub is provided to seal pressure in both directions, to open in response to a pressure applied uphole, to provide a full opening diameter of the tubular and to be locked in the open position. 
         [0003]    2. Description of Related Art 
         [0004]    Valves are used in diverse applications in the tubulars of wells. (“Tubulars” includes casing, liners and tubing.) For example, safety valves are placed in tubing that are designed to close if flow upward through the tubing is otherwise uncontrolled. Sliding sleeves to form valves are placed in casing to be opened or shut by devices placed inside the casing, and valves are placed in casing or tubing of complex (or “smart”) wells to control flow rate from different laterals of the well. Examples of valves for wells are in U.S. Pat No. 8,622,336 and in U.S. Pat No. 8,757,268. Further examples of valves to be inserted in tubulars are provided in U.S. Pub. No. 2009/0272539, disclosing a valve in a tubular that may be mechanically closed, and U.S. Pub. No. 2009/0229829, disclosing a valve having a valve element on trunnions that move along a track. 
         [0005]    One type of valve used in casing is a “float valve,” which is used at the shoe (bottom) or distal end of every casing that is cemented in a wellbore to prevent flowback (or U-tubing) of more dense cement slurry when pressure is released at the surface after displacing the cement slurry with water. A float valve is normally a simple ball check valve. The float valve may also be used in the process of “floating” casing into a well. “Floating” casing is used to allow casing to be placed in horizontal wells with less weight of the casing and less frictional resistance as the casing is placed in a horizontal segment of a well. Floating casing is accomplished by placing nitrogen or air inside the casing to decrease the weight of the casing. This facilitates inserting the casing over longer horizontal sections. One operator&#39;s experience with “floating” a tubular into a well is described in the paper “Statoil uses flotation of 10¾-in, liner to reach beyond 10 km in Gullfaks Field,”  Drilling Contractor , May/June 2007, pp. 66-74. 
         [0006]    There are risks associated with the process of floating casing or a liner into a well. Leaks in the tubular may occur that allow liquid to enter the tubular and result in the casing or liner becoming stuck in the well before it is properly placed. For this and other operations in drilling and running tubulars into wells, a valve that can be placed at selected locations along a tubular string and opened to the full diameter of the tubular by a pressure increase at the surface of the tubular is needed. A series of valves, each of which may be called a “cascade barrier valve,” may be preferred. This valve, when open, should allow movement of downhole tools through the tubular without restriction. When closed, cascade barrier valves at selected locations along the casing may be used to prevent fluid leaking in and filling a long interval of the casing while it is being floated into a horizontal well. 
         [0007]    In drilling or working on vertical, directional or horizontal wells, a plurality of pressure barriers is needed to decrease the risk of uncontrolled flow from a well. Valves in tubulars in wells that form a pressure barrier until opened by a surface operation and then are locked open to provide full inside diameter also offer wide opportunities for increasing well safety. 
         [0008]    What is needed is a valve in a sub that will seal pressure in both directions, open in response to a selected pressure applied uphole, provide a full opening diameter of the tubular when open and be locked in the open position. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    A full-opening valve in a sub for placement in a string of casing, liner or tubing that is opened by a selected pressure applied from the surface is provided. A shear ring or pin is selected to shear at a selected differential pressure in response to a pressure increase at the surface and allows a lower flow tube having the inside diameter of the tubular to move axially. This allows an upper flow tube having the same inside diameter to move axially, pushing open a flapper having dual sealing surfaces, which seal on the adjacent ends of the upper and lower flow tubes. Movement of the upper flow tube may push a pin supporting the flapper to move through a groove to a position where the flapper can move to the open position, where it conforms to the shape of the inside of the tubular. The flapper is locked in the open position for the life of the valve by operation of the upper flow tube and a snap ring, which locks the upper flow tube in position over the open flapper. The valve may be used to provide a pressure barrier in the casing during floating of the casing into a horizontal well or after the casing is in place or it may be used in tubing to prevent flow in the tubing in either direction until a selected pressure is applied at the surface. Valves may be adapted to open at a differential pressure across the valve which varies over a broad range of differential pressures within the operating pressure of the valve. 
         [0010]    The valve may be closed during deployment and once activated is locked in the open position. 
         [0011]    The isolation valve may be used by itself to provide a barrier in either the casing or the tubing, or may be used in conjunction with additional valves to form chambers in the tubular string. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0012]      FIG. 1  is a cross-section view of valve sub  10  in the closed or run-in condition. 
           [0013]      FIG. 2  is a cross-section view of valve sub  10  after shearing of a disk or pin and before opening of the flapper. 
           [0014]      FIG. 3  is a cross-section view of valve sub  10  in the open position. 
           [0015]      FIG. 4  is a cross-sectional view of the valve sub  10  at cross-section “ 4 ” in  FIG. 1 . 
           [0016]      FIG. 5  is an isometric view of the flapper in closed position with outer parts removed from the drawing. 
           [0017]      FIG. 6  is an isometric view of the flapper in the partially-open position with outer parts removed from the drawing, identifying upper and lower sealing surfaces on the flapper. 
           [0018]      FIG. 7  is an isometric view of the flapper in open position with outer parts removed from the drawing. 
           [0019]      FIG. 8  is a perspective view of the flapper. 
           [0020]      FIG. 9  is a side view of the flapper positioned between the upper and lower flow tube. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Referring to  FIG. 1 , isolation valve  10  has lock housing  12 , which is adapted to be joined in a tubular string, normally by pipe treads (not shown) and which preferably has the same inside diameter as the tubular string. Preferably, the outside diameter of the isolating valve is not more than the outside diameter of couplings in the tubular string. Seal  13  provides a barrier between lock housing  12  and upper spring housing  15 . Upper flow tube  18  preferably has the same inside diameter as the minimum inside diameter of lock housing  12  and is adapted to slide within upper spring housing  15 . Upper flow tube  18  may have internal shifting profile  11 . Shifting profile  11  may be used for optional manual shifting to open flapper  24  and allow intervention from uphole by jarring or other mechanical force if isolation valve  10  is not operating properly. Shifting profile  11  a may be the well-known “B-style” shifting profile, for example. Bearing  14  contacts upper coil spring  16  and allows low-resistance rotation of the end of the spring as it is compressed or expands. Snap ring  17  is compressed in the radial direction in the position shown in  FIG. 1  and is adapted to slide within upper spring housing  15  and once it finds the spring housing  15  lock ring profile  17   a , the snap ring  17  will lock the upper flow tube  18  open. Seal  21  perfects a seal between housing adapter  20  and lower spring housing  25 . 
         [0022]    Flapper  24  is supported by flapper pin  22  locked between the upper and lower flow tubes and flapper  24  is free to move along the axis of lower flow tube  29  as pressure is applied uphole (from the left side of  FIG. 1 ) to apply a known force to shear ring or pin  30 . Only a small displacement is required for shearing the ring or pin, so the ring or pin can be sheared even if a normal volume of liquid is on the low-pressure side of flapper  24 . 
         [0023]    Flapper  24  is supported within a flapper housing  35  best shown in  FIG. 5 . Key  34  maintains upper flow tube  19  and flapper housing  35  in axial alignment and key  44  maintains lower flow tube  29  in axial alignment with the flapper housing  33 . A shock absorbing element  33  is secured to flapper housing  35  by screws  42 . 
         [0024]    Referring to  FIG. 2 , shear ring or pin  30  is shown after shearing in the axial direction, with separation into two parts, the two parts being axially displaced, allowing flapper pin  22  and flapper  24  to move into position for opening before it has opened. The axial force of compressed shifting spring  26 , through bearing  28  (which reduces resistance to rotation of the shifting spring) then quickly moves lower flow tube  29  downhole to make a space for opening of flapper  24 . Flapper  24 , supported by pin  22 , is opened by the force of spring  16  acting on upper flow tube  18 . Lower flow tube  29  must move rapidly enough to allow flapper  24  to fully open without interference from lower flow tube  29 . The force of spring  26  is selected to be great enough to meet this requirement. After flapper  24  is open it is then covered in the open position by upper flow tube  18  as shown in  FIG. 3 . 
         [0025]      FIG. 3  shows flapper  24  in the open position. Note that flapper  24  has the same center of the radius of curvature in the radial plane when open as the upper flow tube  18  and lower spring housing  25 . This allows open flapper  24  to be located between upper flow tube  18  and lower spring housing  25 . Lower spring  26  has expanded in the axial direction, moving lower flow tube  29 . Snap ring  17  has moved in the axial direction such that radial compression of the snap ring has caused it to move radially outward into snap ring receptor  17   a . This causes upper flow tube  18  to be permanently locked in position, covering flapper  24 . Torque stop plug  31  may be located at the distal end of lower spring housing  25  to prevent radial movement between lower sub  32  and lower spring housing  25 . Lower spring housing  25  is joined to lower sub  32 , which may be adapted to be joined to a tubular (not shown). 
         [0026]      FIG. 4  shows cross-section  4  identified in  FIG. 1 . Lock housing  12  is shown behind the cross-section. Lower spring housing  25  concentrically encloses closed flapper  24  and lower spring  26 . Flapper pin  22  supports flapper  24 , which is shown in the open position in  FIG. 7 . 
         [0027]      FIG. 5  is an isometric view of flapper  24  in a closed position with parts not shown that block the view of the flapper.  FIG. 6  is an isometric view of the flapper in a partially open position. This view also identifies sealing surfaces  240  and  241  on the flapper shown in  FIG. 8 . When the flapper is closed, these surfaces mate with surfaces on upper flow tube  18  and lower flow tube  29  to form a hydraulic seal. Normally the sealing surfaces are covered with an elastomer or other type of sealing material.  FIG. 7  is an isometric view of flapper  24  in the open position with parts not shown that block the view of the flapper. 
         [0028]      FIG. 8  is a perspective view of the flapper  24 . Sealing surface  240  engages an end of upper flow tube  18  and sealing surface  241  engages an end of lower flow tube  29  as shown in  FIG. 9 . 
       MODE OF OPERATION 
       [0029]    The mode of operation is as follows. With the flapper closed, the formation is isolated and the flapper is sandwiched between the upper and lower flow tubes, effecting a bi-directional seal above and below the flapper. 
         [0030]    When hydrostatic pressure is applied from above, the flapper shears a shear ring or pin or any other destructible retention mechanism via the lower flow tube which then moves axially downward. When the destructible element  30  releases, the lower flow tube  29  moves axially downwardly by virtue of biased spring  26 , thereby allowing the flapper to freely rotate to the open position. 
         [0031]    The upper flow tube  18 , biased by a second spring  16  which is weaker than spring  26 , pushes the flapper to the fully open position shown in  FIG. 3 . 
         [0032]    Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.

Summary:
A valve to act as a barrier to fluid movement in a tubular is provided. A flapper in the valve may be opened by application of a selected pressure differential across the flapper. The flapper opens to allow a cylinder to shift and cover the open flapper. A method of placing a tubular string within a well using the valve as an isolation valve to form gas filled chambers for floating the tubular string into the well is also disclosed.