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BACKGROUND 
   The present invention relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a system and method for cementing through a safety valve. 
   It is sometimes desirable to be able to flow cement through a tubing retrievable safety valve. In these circumstances it is unfortunately quite likely that cement will contact a closure mechanism of the safety valve and will subsequently prevent proper operation of the closure mechanism. 
   In some prior safety valves, the closure mechanism can be isolated from an internal flow passage of the safety valve. However, typically these safety valves require that pressure be applied to a control line connected to the safety valve to maintain the isolation of the closure mechanism, and/or an unreliable metal-to-metal seal is used to achieve the isolation. 
   It will be appreciated that improvements are needed in the art of safety valves for use in cementing operations. 
   It is an object of the present invention to provide such improvements. Other objects, benefits and unique aspects of the invention are described below. 
   SUMMARY 
   In carrying out the principles of the present invention, a safety valve system and associated method are provided which solve at least one problem in the art. One example is described below in which a safety valve includes a unique latching device for isolating a closure mechanism during cementing operations. Another example is described below in which the latching device is pressure operated to release an opening prong for operating the closure mechanism between open and closed positions. 
   In one aspect of the invention, a safety valve system is provided which includes a flow passage, a closure mechanism and an opening prong which is repeatedly displaceable to thereby repeatedly operate the closure mechanism between open and closed positions. A latching device initially maintains the opening prong in a position in which the opening prong isolates the closure mechanism from the flow passage. When the latching device is subsequently released the opening prong is permitted to displace to another position in which the closure mechanism is exposed to the flow passage. 
   In another aspect of the invention, a method of operating a safety valve is provided. The method includes the steps of: 
   1) latching an opening prong of the safety valve in a first position in which a closure mechanism is in an open position and the closure mechanism is isolated from a flow passage extending through the safety valve; 
   2) releasing the opening prong and thereby permitting the opening prong to displace to a second position in which the closure mechanism is in a closed position and the closure mechanism is exposed to the flow passage; and 
   3) displacing the opening prong to a third position in which the closure mechanism is in the open position and the closure mechanism is exposed to the flow passage. 
   These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings, in which similar elements are indicated in the various figures using the same reference numbers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic partially cross-sectional side view of a safety valve system embodying principles of the present invention; 
       FIG. 2  is a cross-sectional view of a safety valve which may be used in the system of  FIG. 1 , the safety valve being depicted in an initial run-in and cementing configuration; 
       FIG. 3  is a cross-sectional view of the safety valve in a closed configuration; and 
       FIG. 4  is a cross-sectional view of the safety valve in an open configuration. 
   

   DETAILED DESCRIPTION 
   It is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. The embodiments are described merely as examples of useful applications of the principles of the invention, which is not limited to any specific details of these embodiments. 
   In the following description of the representative embodiments of the invention, directional terms, such as “above”, “below”, “upper”, “lower”; etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth&#39;s surface along a wellbore, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth&#39;s surface along the wellbore. 
   Representatively illustrated in  FIG. 1  is a safety valve system  10  which embodies principles of the present invention. A tubular string  12  (such as a production tubing string) is installed in a wellbore  14 . A flow passage  16  extends through the tubular string  12 , for example, to produce fluids to the surface from a subterranean reservoir. 
   A safety valve  18  is interconnected as part of the tubular string  12 . The safety valve  18  is used to shut off flow through the passage  16  in emergency situations, such as to prevent uncontrolled discharge of fluids from the passage. 
   A line  20  may be connected to the safety valve  18  in order to permit operation of the safety valve from a remote location, such as the earth&#39;s surface or another location in the well. In the system  10 , the line  20  is a hydraulic control line, but in other embodiments the line could be an electrical line, a fiber optic line, or any other type of line. Furthermore, it is not necessary for a safety valve to be operated using any type of line at all in keeping with the principles of the invention. 
   At this point it should be noted that the invention is not limited to the specific details of the system  10  described herein. Many other types of systems and methods can be used, without departing from the principles of the invention. 
   Referring additionally now to  FIG. 2 , the safety valve  18  is representatively illustrated in an enlarged scale cross-sectional view. In this view it may be seen that the safety valve  18  includes unique features which make it particularly suitable for use in situations where it is desired to flow cement through the flow passage  16  of the tubular string  12  in completion operations. 
   The flow passage  16  extends longitudinally through the safety valve  18 . A closure mechanism  22  is used to selectively permit and prevent flow through the passage  16 . The closure mechanism  22  includes a flapper  24 , seat  26 , spring  28  and pivot  30 . 
   As depicted in  FIG. 2 , the closure mechanism  22  is in an open position in which flow through the passage  16  is permitted. In a closed position of the closure mechanism  22 , the spring  28  biases the flapper  24  to pivot upwardly about the pivot  30  and thereby sealingly engage the seat  26  and prevent flow through the passage  16 . 
   A tubular opening prong  32  holds the flapper  24  pivoted downward as shown in  FIG. 2  while the tubular string  12  is installed in the wellbore  14 . “Opening prong” is a term used in the safety valve art to describe a member which is displaced to cause operation of a closure assembly between its open and closed positions. Opening prongs are also sometimes referred to as flow tubes or operating mandrels, etc. 
   In one unique feature of the safety valve  18 , a seal  34  seals between a lower end of the opening prong  32  and an outer housing assembly  36 , so that the closure assembly  22  is isolated from the flow passage  16 . The seal  34  and additional seals  38 ,  40 , along with the opening prong  32  and a tubular mandrel  42  isolate a fluid chamber  44  from the flow passage  16 . 
   Prior to interconnecting the safety valve  18  in the tubular string  12  and installing it in the well, the fluid chamber  44  is filled with a fluid, such as water, salt water, water with cement inhibitor, hydraulic fluid, etc. The fluid is introduced into the fluid chamber  44  via a fill port  46 . 
   In order to allow air to escape from the chamber  44  when it is filled, the chamber is equipped with a relief valve  48 . Although the relief valve  48  is shown in  FIG. 2  as being at a lower end of the chamber  44 , the safety valve  18  would preferably be turned upside down during filling of the chamber so that the relief valve is positioned at an upper end of the chamber to thereby allow air in the chamber to escape when it is filled with fluid. 
   Preferably, some fluid is also allowed to escape through the relief valve  48  when the chamber  44  is filled, to ensure that the air is purged from the chamber. Other techniques, such as evacuating air from the chamber using a vacuum, etc., may be used in keeping with the principles of the invention. 
   The relief valve  48  opens when a predetermined pressure is reached in the chamber  44 . This not only allows air to escape when the chamber  44  is filled with fluid, but also allows the fluid to escape in order to prevent over-pressuring the chamber, for example, due to expansion of the fluid when the fluid is heated after the tubular string  12  is installed in the well. 
   A latching device  50  maintains the opening prong  32  in the position depicted in  FIG. 2  while the safety valve  18  is interconnected in the tubular string  12 , while the tubular string is installed in the well, and while cement is flowed through the flow passage  16 . In another unique feature of the safety valve  18 , this is accomplished without a need to apply pressure to the line  20  connected to the safety valve. 
   The latching device  50  includes a piston  52  which is biased to displace in response to a pressure differential between the flow passage  16  and an atmospheric or gas chamber  54 . The piston  52  is threaded to an outer sleeve  56 , and a lower end of an inner sleeve  58  is retained between the piston and the outer sleeve. 
   Shear pins  60  retain the outer sleeve  56 , piston  52  and inner sleeve  58  in the position depicted in  FIG. 2 . In this position, an upper end of the inner sleeve  58  is received within a lower end of the opening prong  32 . 
   Preferably, an interference fit or press fit exists between the opening prong  32  and the inner sleeve  58 . For example, at least a portion of an inner diameter of the lower end of the opening prong  32  may be smaller than at least a portion of an outer diameter of the upper end of the inner sleeve  58  prior to these elements being pressed together so that the outer diameter is received in the inner diameter. 
   To press the upper end of the inner sleeve  58  into the lower end of the opening prong  32 , a special assembly tool may be used to apply a downwardly directed force to the opening prong via a latching profile  62  formed in the opening prong. Once the interference fit between the opening prong  32  and the inner sleeve  58  is achieved, the opening prong will be maintained in its downwardly disposed latched position as depicted in  FIG. 2 , even though a spring  64  biases the opening prong in an upward direction. 
   Note that, in its latched position the opening prong  32  is also maintained in sealing engagement with the seal  34 , and the chamber  44  is isolated from the flow passage  16 . Thus, the chamber  44  can be filled with fluid after the opening prong  32  is maintained in its latched position due to the interference fit between the opening prong and the inner sleeve  58  of the latching device  50 . 
   To release the latching device  50  and permit upward displacement of the opening prong  32 , increased pressure is applied to the flow passage  16  (for example, at the end of the cementing operation), thereby causing an increased pressure differential from the flow passage to the gas chamber  54 . This increased pressure differential causes the shear pins  60  to shear, allowing the piston  52 , outer sleeve  56  and inner sleeve  58  to displace downward, thereby withdrawing the upper end of the inner sleeve from within the lower end of the opening prong  32  and releasing the latching device  50 . 
   Of course, other types of latching devices could be used in place of the latching device  50  in keeping with the principles of the invention. Latching devices including elements such as spring biased lugs or dogs, C-rings, etc., could be used to releasably maintain the opening prong  32  in its downwardly disposed latched position during installation and cementing operations, and then to release the opening prong for upward displacement once the cementing operation has been completed. 
   Referring additionally now to  FIG. 3 , the safety valve  18  is representatively illustrated after the latching device  50  has been released. Note that the piston  52 , outer sleeve  56  and inner sleeve  58  have been downwardly displaced due to the pressure differential between the flow passage  16  and the gas chamber  54 , and shearing of the shear pins  60 . 
   The opening prong  32  has been displaced upward by the biasing force exerted by the spring  64  after release of the interference fit between the opening prong and inner sleeve  58 . In this upwardly disposed position of the opening prong  32 , the closure mechanism  22  is closed, with the spring  28  pivoting the flapper  24  upward to sealingly engage the seat  26 . 
   Referring additionally now to  FIG. 4 , the safety valve  18  is representatively illustrated after increased pressure has been applied to a piston chamber  66  to thereby cause a rod piston  68  to displace downwardly. The piston chamber  66  is in communication with the line  20  via a port  70  when the safety valve  18  is installed as depicted in  FIG. 1 . 
   Downward displacement of the piston  68  causes the opening prong  32  to displace downwardly also, thereby opening the closure mechanism  22 . Note, however, that the opening prong  32  does not displace downwardly sufficiently far to sealingly engage the seal  34 . 
   Therefore, in this downwardly disposed position of the opening prong  32 , the chamber  44  is not isolated from the flow passage  16 . A spring-biased slip joint  74  between the opening prong  32  and the piston  68  allows the opening prong to be further downwardly disposed relative to the piston in the  FIG. 1  configuration (in which the latching device  50  maintains the opening prong downward in engagement with the seal  34 ) as compared to the  FIG. 4  configuration (in which the latching device is released). This unique feature of the safety valve  18  allows debris (such as sand, etc., which might accumulate in the chamber  44  while the closure mechanism is closed) to escape from the chamber while the closure mechanism is open. 
   However, the opening prong  32  could displace downwardly sufficiently far to engage the seal  34  and provide isolation between the chamber  44  and the flow passage  16 , if desired. For example, downward displacement of the piston  68  is limited by its engagement with a seat  72 , but the seat could be positioned lower in the housing assembly  36  to allow the piston to bias the opening prong  32  further downward to engage the seal  34  in other embodiments. 
   The safety valve  18  can be repeatedly cycled between its closed ( FIG. 3 ) and open ( FIG. 4 ) configurations as many times as desired by varying pressure in the line  20 . Increased pressure is applied to the line  20  to open the closure mechanism  22 , and pressure in the line is decreased to close the closure mechanism. Thus, after the cementing operation, and after the latching device  50  is released, the safety valve  18  operates similar to a conventional hydraulically operated safety valve. 
   Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are within the scope of the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.

Summary:
System and method for cementing through a safety valve. A system includes a flow passage, closure mechanism, opening prong repeatedly displaceable to thereby repeatedly operate the closure mechanism open and closed, and a latching device initially maintaining the opening prong positioned isolating the closure mechanism from the flow passage, and releasing the latching device permits the opening prong to open and close the closure mechanism while it is exposed to the flow passage. A method includes the steps of: latching an opening prong so that a closure mechanism is open and is isolated from a flow passage; releasing the opening prong so that the closure mechanism is closed and is exposed to the flow passage; and displacing the opening prong to a position in which the closure mechanism is open and is exposed to the flow passage.