Abstract:
A subsurface safety valve is operable to close a fluid flow path by virtue of an axially movable flow sleeve. The valve includes a recockable actuator and a latch mechanism so that the valve can be moved to a closed position without overcoming the pressure head and frictional forces currently encountered in conventional safety valves. The latch mechanism includes one or more micro pistons.

Description:
[0001]    This non-provisional application claims priority to the U.S. Provisional Application No. 61/673,513 filed on Jul. 19, 2012. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This application is directed to a subsurface safety valve system for use in drilling oil or gas wells. Such valves are commonly used to prevent flow of oil or gas from the well to the surface when certain conditions occur. 
         [0004]    2. Description of Related Art 
         [0005]    Currently such safety valves are held in an open position by virtue of pressure in a control line from the surface acting on a piston in the valve which is operatively connected to a flow sleeve which moves axially to open a valve member. Movement of the sleeve also compresses a spring surrounding the flow sleeve. 
         [0006]    Upon the occurrence of an unfavorable event, the pressure is relieved via the control line so that the spring will move the flow sleeve upwardly so as to allow the valve, which may be a flapper valve to close. In so doing, the spring must overcome the pressure head caused by the hydraulic fluid and the flow resistance due to the small diameter of the control line. 
         [0007]    Some control lines in deep water subsea wells may be up to two miles or more in length and may extend a vertical distance of more than a mile. 
         [0008]    Consequently the pressure head and resistance to flow is quite high which can delay the response time for the valve and may in some cases result in failure. 
         [0009]    FSSD—or fail safe setting depth is a term known to all skilled in the art of Surface Controlled Subsurface Safety Valves (SCSSVs) and is discussed in detail in API-14A, the primary document controlling certification of all such valves. 
         [0010]    Simply put, the FSSD is the depth at which a SCSSV may not be set below because the force caused by the pressure head of a column of fluid in the control line from the surface acting on the valve&#39;s actuating piston is greater than the force of spring acting to close the valve. 
         [0011]    In deep set valves, it is impossible to employ a spring large enough to close the valve so a gas charge, normally nitrogen, is commonly used to offset a portion of the force of the pressure head, thereby allowing the valve to operate somewhat normally. In the nitrogen chamber, often a low lubricity oil is positioned between the piston seals and the nitrogen to protect the piston seals, and to reduce the effects of wear as the piston cycles repeatedly between open and closed. The term “somewhat” is used here due to the compressible nature of gasses. 
         [0012]    Pressure Charged SCSSVs actually have a “Fail Safe Setting Window” which is not absolute because of the changing nature of the downhole environment and its own particular wear characteristics. Normally deep set SCSSV&#39;s are utilized in deep ocean environments where temperatures are near freezing—33-40 degrees F. (or 1-3 degrees C.). SCSSVs are typically set 100 meters below the mud line of the ocean floor and are influenced by these temperatures. The temperature of the producing formation can be 300-400 Fahrenheit, meaning the SCSSV can warm to these temperatures during production of the well. However, if the well is shut-in the temperature can rapidly cool to that of the ocean floor. 
         [0013]    The result is that in a constant volume chamber, the pressure changes dramatically with temperature in application of Boyle&#39;s and Charles&#39; Law: P1/T1=P2/T2. Therefore, in a gas charged SCSSV, as the nitrogen chamber warms to, for example, 350 deg F., the nitrogen is able to offset a greater pressure head than when it has cooled to 33 Deg F. during shut in. Over time, repeated open and closing cycles cause minute longitudinal scratches in the piston bore and on the seals thereby allowing small amounts of oil to leak past the seals. With enough cycles, the seals can fail causing the nitrogen to leak off, triggering a highly complex valving system to auto-execute to prevent failure of the valve in the open position—if it works properly—and the above described “non-fail safe” scenario has not happened. This is a characteristic and risk assessment associated with all prior art deep set SCSSVs. 
         [0014]    What is known to all SCSSV designers is that reducing the piston area increases FSSD. Obviously, the opening force exerted by the control line fluid is equal to the pressure head times the piston area. As piston area approaches zero, FSSD approaches infinity. 
         [0015]    However, until the present invention there has always been a practical limitation of piston diameter. When the valve is closed the operating piston exists happily completely enclosed in the piston bore. However, as the valve opens, the piston strokes out of the bore and extends itself as a cantilevered beam until the valve is open. The length of the cantilevered piston is always greater than the flapper diameter, as it must push the flow tube to fully open the flapper. 
         [0016]    The cantilevered piston has two possible loading conditions; the first as a column, as the power spring places compressive force on the unsupported piston; the second in bending, as the repeated cyclic compression of the spring places a radial load on the cantilevered piston, AND the combination of both of these loads. The piston resists these forces by the yield strength of the material and its Moment of Inertia. Designers already use the strongest, most noble materials known. The problem is reducing Moment of Inertia by reducing diameter. If the piston gets too long and skinny, it will fail due to elastic instability, bending, or both. 
         [0017]    For this reason, most pistons have a practical diameter of ½ or ⅜ of an inch. In small tubing sized valves, pistons have been known to be ¼ inch. 
         [0018]    The short length of the micro piston in accordance with the instant invention allows practical diameters below ¼ of an inch and practically can be used at diameters of 0.100 inches or even 0.050 inches. The stroke of the micro-piston to release the flow tube is very small as well, as an example less than 1 inch. This means the micro-piston has much lower wear characteristics, may be used at depths of 15,000 feet or even deeper without a gas charge, and is virtually unaffected by gas accumulation in the annulus. The micro-piston, because of its short stroke, may also cycle 20,000 or 30,000 times before predicted failure. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    The above mentioned design defects are overcome by the current invention. A recockable actuator is located within the valve body that is not subject to the pressure head or flow line resistance to move the flow sleeve to close the valve. When the flow sleeve is moved to a position which opens the valve, a latching mechanism which includes a micro piston engages the flow sleeve to hold it in place and the actuator is disengaged from the flow sleeve. To close the valve, the latch mechanism is disengaged and the flow sleeve will move upwardly by virtue of the compressed spring without having to overcome the pressure head or fictional forces. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0020]      FIG. 1  is a cross-sectional view of an embodiment of the safety valve in the closed position. 
           [0021]      FIG. 2  is a cross-sectional view of the micro piston latching mechanism according to an embodiment of the invention. 
           [0022]      FIG. 3  is a cross-sectional view of an embodiment of the safety valve in the open position. 
           [0023]      FIG. 4  is a cross-sectional view of the latching mechanism shown with the safety valve in the open position. 
           [0024]      FIG. 5  is an enlarged view of the latching mechanism. 
           [0025]      FIG. 6  is a cross-sectional view of the safety valve of  FIG. 1  in an open, balanced piston condition. 
           [0026]      FIG. 7  is a cross-sectional view of the latching mechanism when the safety valve is in the position shown in  FIG. 6 . 
           [0027]      FIG. 8  is a cross-sectional view of the safety valve of  FIG. 1  with the flow tube moved back to the close position. 
           [0028]      FIG. 9  is a cross-sectional view of the latching mechanism when the valve is in the closed position shown in  FIG. 8 . 
           [0029]      FIG. 10  illustrates the factors that are used in calculating the failsafe setting depth. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 1  illustrates an embodiment of a deep set surface controlled subsurface safety valve according to the invention. Such valves are typically positioned below the sea floor at a depth that is limited by the design characteristics of the valve. As shown in  FIG. 1 , safety valve  10  includes a main housing or body which includes three sections  11 ,  71  and  72  suitably connected to each other. Safety valve  10  has an inlet  12  for connection to a tubular for example, production tubing and an outlet  13  for connection to a tubular which may be production tubing. Safety valve  10  includes a flow sleeve  20 , coil spring  25 , flapper valve  26  biased to a closed position and an axially movable piston  14 , located with housing portion  11 . Uphole portion  21  of the flow sleeve includes an annular groove  61  formed between two radially projecting flanges. A latching mechanism  50  shown in detail in  FIG. 2  surrounds the uphole portion  21  of the flow sleeve and is secured within housing portion  11 . 
         [0031]    Latching mechanism  50  includes an annular body  51  having an interior annular chamber within which is located an annular ring  52  and a coil spring  53 . Annular ring has an annular groove  63  shown in  FIG. 5  therein with a beveled surface  64  shown in  FIG. 5 . 
         [0032]    One or more micro pistons  55  are located within body  51  such that one end of the micro piston is exposed to a control line  54  for pressurized fluid and the other end of the piston is in contact with annular ring  52 . 
         [0033]    An annular collet  66  is positioned in an interior surface of body  51  and includes a plurality of flexible resilient fingers  56 . Fingers  56  have a rounded inwardly extending tab  62  that is adapted to be captured by groove  61  in the uphole portion of flow sleeve  21 . Fingers  56  also each have an outwardly extending sloping surface that terminates with an edge  57  that is adapted to be positioned within an annular, complimentary shaped groove  63  in the ring member  52 . Downward movement of ring member  52  as shown in  FIG. 2  is resisted by the coil spring  53 . 
         [0034]    In the position shown in  FIG. 1 , the flapper valve  26  is in the closed position against valve seat  27  and consequently there is no flow through the valve. In order to open the valve, fluid under pressure is conveyed to inlet  15  via a control line  81  that extends to the surface. The fluid pressure against the uphole surface of piston  14  will cause it to move downwardly looking at  FIG. 1 . As it moves a shoulder  18  on the piston engages an outwardly extending flange  29  on the flow sleeve and moves the flow sleeve downwardly thus pushing flapper valve  26  to an open position shown in  FIG. 3  and compressing spring  25 . 
         [0035]    At this point annular groove  61  formed on the outer surface of flow sleeve  21  comes into registry with the rounded tabs  62  on the flexible fingers  56  of the latching mechanism. As fluid pressure is applied to the upper end of micro piston or pistons  55  via inlet  54  and control line  82  which extends to the surface, one or more micro pistons push on ring member  52 . Due to the beveled surfaces in groove  63  and fingers  56 , downward movement of the ring will cause rounded tabs  62  to be moved radially inward and captured by ring  61  shown in  FIG. 4  thus locking flow sleeve and flapper valve  26  in an open position as shown in  FIG. 3 . Downward movement of the ring  52  also compresses spring  53 . 
         [0036]    Piston  14  includes a longitudinally extending small diameter bore  41  that will allow the pressure to eventually equalize on both ends of the piston so that piston  14  will move upwardly as shown in  FIG. 6  after a predetermined period of time. A slot  91  is provided in the lower portion of piston  14  so that it does not engage shoulder  18  as it moves upwardly. 
         [0037]    Should circumstances occur which require that the valve be in the closed position, pressure within control line  82  is relieved thus relieving the pressure on the uphole surface of micro piston(s)  55 . 
         [0038]    With the fluid pressure relieved, compressed coil spring  53  will move ring  52  upwards as shown in  FIG. 9 . Flexible, resilient fingers  56  will now return to their neutral position and in so doing tabs  62  will move out of annular ring  61  thereby releasing the uphole portion  21  of the flow sleeve. Coil spring  25  which was compressed during the opening of the valve will now move flow sleeve  20  in an upward direction by acting on shoulder  22  on the flow sleeve. This movement will allow flapper valve  26  to close on valve seat  27  and the valve will be in the closed position as shown in  FIG. 8 . As the flow sleeve is moved upward there are minimal forces that must be overcome as the piston  14  has previously moved to the position shown in  FIG. 6 . 
         [0039]      FIG. 10  illustrates an example of a surface controlled subsurface safety valve. The installation includes a rotary Kelly bushing  83 , tubing hanger  84 , water level  86 , mudline  88  and subsurface valve  10 . Distance  92  is the elevation, distance  94  is the air gap, distance  96  is the water depth and distance  96  is the valve depth. 
         [0040]    The failsafe setting depth (FSSD) is equal to 0.85 Pc/MHFG wherein:
       Pc=minimum closing pressure, psi   MHFG=maximum hydraulic fluid gradient, psi per foot (psi per foot=ppg×0.052)   For example, if the completion fluid is CaCl 2 , ppg max is 9 ppg. Assuming a minimum closing pressure of 800 psi,       
 
         [0000]    
       
         
           
             MHFG 
             = 
             
               
                 9.0 
                 × 
                 0.052 
               
               = 
               
                 0.468 
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                  
                 psi 
                  
                 
                     
                 
                  
                 per 
                  
                 
                     
                 
                  
                 foot 
               
             
           
         
       
       
         
           
             FSSD 
             = 
             
               
                 
                   0.85 
                   × 
                   800 
                 
                 0.468 
               
               = 
               
                 1 
                  
                 
                   , 
                 
                  
                 453 
                  
                 
                     
                 
                  
                 
                   ft 
                   . 
                 
               
             
           
         
       
     
         [0044]    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.