Patent Publication Number: US-8118088-B2

Title: Shear activated safety valve system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 11/253,766 filed on Oct. 19, 2005. The entire disclosure of this prior application is incorporated herein by this reference. 
    
    
     BACKGROUND 
     The present invention relates generally to operations performed and equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a shear activated safety valve system. 
     In offshore well testing operations, it is common practice to use two safety valves connected to each other via a shear joint and ramlock sub. The shear joint is typically positioned in the shear rams, and the ramlock sub is typically positioned in the sealing rams of a subsea wellhead. The sealing rams seal about the ramlock sub. 
     In the event of an emergency, the shear rams can shear the shear joint, allowing an upper portion of the test string to be quickly retrieved either before or after the emergency has passed, and leaving a lower portion of the test string in the well below the wellhead. The lower safety valve prevents fluid from escaping from the well via the lower portion of the test string. 
     In the past, the safety valves have been generally operated using a control line or umbilical extending to a platform or rig at the surface of the water. It will be appreciated by those skilled in the art that it is quite expensive and time-consuming to install and pressure test this control line. 
     Safety valves have been developed which use a highly pressurized nitrogen chamber to produce a biasing force in an actuator of the valve. However, it will be appreciated that safety concerns need to be addressed when charging and handling such highly pressurized chambers at the surface. 
     Therefore, it may be seen that improvements are needed in the art of safety valve systems. The present invention provides such improvements. These improvements are not necessarily limited to the issues raised by the foregoing background information. 
     SUMMARY 
     In carrying out the principles of the present invention, a safety valve system is provided which solves at least one problem in the art. One example is described below in which hydrostatic pressure is used to produce biasing forces in actuator(s) of one or more safety valves. Another example is described below in which the actuator is connected to a line containing fluid pressurized above hydrostatic pressure, so that opening of the line permits the actuator to close the safety valve. 
     In one aspect of the invention, a safety valve system for use in a subterranean well includes at least one safety valve having an actuator. A line is connected to the actuator. The safety valve is operable by opening the line in the well, with the line being free of any connection to a surface control system. 
     In another aspect of the invention, a safety valve system is provided which includes multiple safety valves. An actuator of each safety valve is connected to an actuator of another safety valve via a line. A biasing force in each of the actuators is operative to close the respective one of the safety valves in response to opening of the line. The biasing force is produced at least in part by hydrostatic pressure in a well. 
     In yet another aspect of the invention, a method of operating a safety valve system in a subterranean well is provided. The method includes the steps of: installing in the well at least one safety valve having an actuator; and the installing step including connecting a line to the actuator without connecting the line to a surface control system. 
     In a further aspect of the invention, a method of operating a safety valve system is provided. The method includes the steps of: connecting actuators of multiple safety valves to each other with a line, and installing the safety valves in a well. Hydrostatic pressure in the well produces a biasing force in each of the actuators, so that opening of the line in the well is operative to permit the biasing forces to close the respective safety valves. 
     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 elevational view of a shear activated safety valve system embodying principles of the present invention; 
         FIG. 2  is an enlarged scale schematic cross-sectional view of a safety valve usable in the system of  FIG. 1 ; 
         FIG. 3  is a schematic elevational view of an alternate construction of the shear activated safety valve system; 
         FIG. 4  is a schematic cross-sectional view of another safety valve usable in the system of  FIG. 1 ; and 
         FIG. 5  is a schematic cross-sectional view of yet another safety valve usable in the system of  FIG. 1 . 
     
    
    
     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. The system  10  is depicted in  FIG. 1  as being used in conjunction with a formation testing operation on a subsea well, but it should be clearly understood that the invention is not limited to any of the details of this example. For example, the invention could be used in other types of operations (such as completion or intervention operations, etc.) and on other types of wells. 
     The system  10  includes a test string  12  installed in a subsea wellhead  14 . The test string  12  includes an upper safety valve  16 , a lower safety valve  18 , a shear joint  20  and a ramlock  22 . The safety valves  16 ,  18  are used to close off the test string  12  in the event of an emergency (such as an imminent safety hazard). 
     The shear joint  20  is positioned within shear rams  24  of the wellhead  14 . The shear rams  24  will close and shear the shear joint  20  if it is necessary to sever the test string  12 , for example, if the upper portion of the test string must be retrieved as quickly as possible in an emergency. 
     The ramlock  22  is positioned within sealing rams  26  of the wellhead  14 . The sealing rams  26  seal against an outer surface of the ramlock  22 , providing pressure isolation in an annulus  28  surrounding the test string  12 . Note that in some embodiments of the invention a ramlock may not be used. 
     A line  30  is connected between the upper safety valve  16  and the lower safety valve  18 . As described more fully below, the line  30  provides fluid communication between actuators of the safety valves  16 ,  18 . 
     In addition, when the shear rams  24  are operated to sever the shear joint  20 , the line  30  is also severed or otherwise opened, thereby causing both of the safety valves  16 ,  18  to close. By closing both of the safety valves  16 ,  18 , the test string  12  is isolated above and below the shear rams  24 . With the sealing rams  26  also sealed against the ramlock  22 , the well below the wellhead  14  is thereby completely isolated in an emergency. 
     Note that the line  30  is depicted in  FIG. 1  as being external to the shear joint  20  and internal to the ramlock  22 . By positioning the line  30  external to the shear joint  20  and constructing the line of a collapse resistant rubber composition in this area, the line is more reliably severed and will remain open after being severed. 
     By positioning the line  30  internal to the ramlock  22  (e.g., machined or otherwise formed in a sidewall of the tubular ramlock, integrally formed with the ramlock, etc.), the sealing rams  26  can more reliably seal against the exterior of the ramlock. However, it should be clearly understood that it is not necessary for the line  30  to be positioned as depicted in  FIG. 1 , and the line can be made of any type of material, or otherwise positioned, in keeping with the principles of the invention. 
     Referring additionally now to  FIG. 2 , an enlarged scale schematic cross-sectional view of the lower safety valve  18  is representatively illustrated, apart from the remainder of the test string  12 . The upper safety valve  16  is not shown in cross-section, but it is similar in most respects to the lower safety valve  18 . 
     In  FIG. 2  it may be seen that the safety valve  18  includes a ball closure mechanism  32 . Preferably, this mechanism  32  is of the type which includes a cutting device  82  (e.g., a ball of the closure mechanism) capable of shearing obstructions (such as coiled tubing, wireline, etc.—see obstruction  92  shown in  FIGS. 4 &amp; 5 ) in an internal flow passage  34  of the safety valve  18  when the mechanism is closed to seal off the passage. However, other types of closure mechanisms (such as those using flappers, sliding closures, etc.) could be used in keeping with the principles of the invention. 
     The closure mechanism  32  is operated by axial displacement of a generally tubular mandrel  36  of the safety valve  18 . In this example, upward displacement of the mandrel  36  is used to shift the closure mechanism  32  to a closed position, and downward displacement of the mandrel is used to shift the closure mechanism to an open position. Other types of displacements (such as rotational displacement, etc.) and combinations of displacements may be used to operate a closure mechanism in keeping with the principles of the invention. 
     The safety valve  18  includes an actuator  38  which is used to displace the mandrel  36 . The actuator  38  includes internal chambers  40 ,  42 ,  44 , pistons  46 ,  48  and seals  50 ,  52 ,  54 ,  56  for applying biasing forces to the mandrel  36  due to pressure differentials between the chambers. 
     The actuator  38  also includes a compression spring  58  for upwardly biasing the mandrel  36  (i.e., in a direction to close the closure mechanism  32 ), so that the safety valve  18  will “fail closed.” That is, in the absence of pressure differentials in the chambers  40 ,  42 ,  44  to properly operate the safety valve  18  (such as, in the event of failure of one or more of the seals  50 ,  52 ,  54 ,  56 ), the spring  58  will bias the mandrel  36  upward to close the closure mechanism  32 . 
     The upper chamber  40  is connected to the line  30 . In use, a similar chamber in the upper safety valve  16  would also be connected to the line  30 . In this manner, the actuators  38  of the safety valves  16 ,  18  are connected and in fluid communication via the line  30 . 
     Preferably, the line  30  and chambers  40  of the upper and lower safety valves  16 ,  18  are filled with liquid, such as hydraulic fluid. Due to thermal expansion and contraction of such liquids and a desire to prevent such expansion and contraction from inadvertently causing the mandrel  36  to displace and operate the closure mechanism  32 , the actuator  38  can include an accumulator  60  connected to the line  30 , for example, with a floating piston  62  and pressurized gas chamber  64 . The accumulator  60  may also be used to compensate for thermal expansion/contraction of the line  30  and components of the safety valves  16 ,  18  (such as chambers  40 ,  42 , etc.). 
     The accumulator  60  is depicted in  FIG. 2  as being an integral part of the safety valve  18 , but the accumulator could instead be a separate element of the test string  12  (as illustrated in  FIG. 3 ). Furthermore, it should be understood that the accumulator  60  is not necessary to compensate for thermal expansion or contraction of fluid in the line  30 , or thermal expansion/contraction of the line and components of the safety valves  16 ,  18 . 
     For example, a relatively compressible fluid, such as a silicone-based fluid, could be used in the line  30  and chambers  40  to provide compensation for thermal expansion and contraction, or another fluid with a relatively low coefficient of thermal expansion could be used, etc. In addition, the closure mechanism  32  could be designed so that relatively small displacements of the mandrel  36  due to expansion/contraction of the fluid in the line  30  and chamber  40  will not cause undesirable opening or closing of the closure mechanism. 
     The chamber  42  preferably contains air or an inert gas and has relatively low (for example, atmospheric) pressure therein when the safety valve  18  is installed. Of course, pressure in the chamber  42  will fluctuate somewhat with changing temperature in the well environment when the safety valve  18  is installed, and the pressure in the chamber will also change somewhat when the mandrel  36  is displaced (due to expansion and contraction of the chamber volume), but preferably the pressure in the chamber will remain substantially at a relatively low pressure. If desired, other pressures may be used in the chamber  42  in keeping with the principles of the invention. 
     The chamber  44  is preferably connected to the annulus  28  surrounding the safety valve  18  via openings  66 . When the safety valve  18  is installed in the well, hydrostatic pressure in the annulus  28  can be used to bias the piston  48  upwardly. 
     Note that the chamber  44  could instead be connected to the flow passage  34  via optional openings  68 , so that hydrostatic pressure in the passage could be used to bias the piston  48  upwardly. If the openings  68  are used, then the openings  66  would not be present in the safety valve  18 . 
     As described above, hydrostatic pressure in the chamber  44  biases the piston  48  upwardly. Relatively low pressure in the chamber  42  biases the piston  48  downwardly, but since the hydrostatic pressure is far greater than the pressure in the chamber  42  when the safety valve  18  is installed, and since the piston area of the piston exposed to the chamber  44  is greater than the piston area of the piston exposed to the chamber  42 , the net biasing force produced by this pressure differential across the piston  48  is directed upward. 
     In order to displace the mandrel  36  downward, the upward net biasing force produced by the pressure differential across the piston  48  and the upward biasing force exerted by the spring  58  is exceeded by a downwardly directed biasing force produced by pressure in the chamber  40  acting on a piston area of the piston  46  exposed to the chamber. Since the piston area of the piston  46  exposed to the chamber  40  is less than the piston area of the piston  48  exposed to the chamber  44 , it will be readily appreciated by those skilled in the art that pressure in the chamber  40  will be greater than hydrostatic pressure in order to displace the mandrel  36  downward, or to maintain the mandrel in its downward position as depicted in  FIG. 2 . 
     In a preferred method of installing the safety valves  16 ,  18 , the safety valves are assembled and interconnected in the test string  12  with the shear joint  20  and ramlock  22  therebetween. The actuators  38  of the safety valves  16 ,  18  are connected via the line  30 . 
     The line  30  and chambers  40  of the actuators  38  are filled with fluid. At this point, the spring  58  will be maintaining the mandrel  36  in an upward position and the safety valves  16 ,  18  will thus be closed. If the accumulator  60  is used, a gas (such as nitrogen) may be used to pressurize the chamber  64 , for example, via a filler valve  70 . 
     The safety valves  16 ,  18  are preferably opened prior to completely installing the test string  12  in the well. The closure mechanisms  32  are opened by applying sufficient pressure to the line  30  to overcome the upward biasing force exerted by the springs  58  and thereby displace the mandrels  36  downward. 
     Once the mandrels  36  have been displaced downward a sufficient distance to open the closure mechanisms  32 , additional pressure may be applied to the line  30  to somewhat compress the gas in the chamber  64 , so that the piston  62  will be able to displace after installation to adequately compensate for thermal expansion/contraction of the fluid in the line  30  and chambers  40 , and thermal expansion/contraction of the line and safety valves  16 ,  18 . 
     The test string  12  is then installed as depicted in  FIG. 1 . As the test string  12  is lowered into the well, hydrostatic pressure in the annulus  28  about the safety valves  16 ,  18  increases. Of course, if the test string  12  is filled with fluid as it is installed, then hydrostatic pressure in the passage  34  will also increase as the test string is installed. 
     This hydrostatic pressure (from the annulus  28  or passage  34 ) is communicated to the chambers  44  and thereby applies an increasing upward biasing force to the mandrels  36  due to the pressure differential across the pistons  48  as described above. This increased upward biasing force is countered by increased pressure in the line  30  and chambers  40 . 
     Fluid in the line  30  and chambers  40  is preferably a relatively incompressible fluid, so that as the upwardly biasing force due to the pressure differential across the pistons  48  increases, pressure in the line  30  and chambers  40  also increases, thereby preventing the volume of the chambers  40  from decreasing significantly, and thereby preventing the mandrels  36  from displacing upward significantly. As mentioned above, pressure in the line  30  and chambers  40  will be greater than hydrostatic pressure to maintain the mandrels  36  in their downwardly displaced positions and to maintain the closure mechanisms  32  in their open positions. 
     If it becomes necessary to close the safety valves  16 ,  18 , the shear rams  24  will be closed, thereby severing the shear joint  20  and line  30 , and thereby opening the line so that it communicates with the annulus  28 . In this manner, the line  30  and chambers  40  are exposed to hydrostatic pressure and the fluid in the line and chambers can no longer maintain the mandrels  36  in their downwardly displaced position. 
     At this point, the upward biasing force produced by the pressure differential across the pistons  48  and the biasing force exerted by the springs  58  will displace the mandrels  36  upward, thereby closing the closure mechanisms  32 . Upward displacement of the mandrels  36  is no longer prevented by the fluid in the chambers  40 , since the chambers can have no greater than hydrostatic pressure therein (due to opening of the line  30  to the annulus  28 ). Hydrostatic pressure in the chambers  40  cannot prevent upward displacement of the mandrels  36 , since the piston area of the piston  46  exposed to the chamber  40  is less than the piston area of the chamber  44  exposed to the piston  48 . When the closure mechanisms  32  close, any obstruction (such as obstruction  92  shown in  FIGS. 4 &amp; 5 ) in the passage  34  will be severed by the cutting devices  82 . 
     Note that other configurations of actuators could be used in the safety valves  16 ,  18  without departing from the principles of the invention. For example, the chambers  40 ,  42 ,  44  and pistons  46 ,  48  could be differently arranged, different numbers and types of chambers and pistons could be used, etc. Use of the spring  58  is not necessary, and other types of biasing devices (such as gas springs) could be used instead. 
     In a preferred embodiment, the upper safety valve  16  is constructed and installed so that it is inverted vertically (upside-down) as compared to the lower safety valve  18  as depicted in  FIG. 2 . In accordance with conventional “pump through” ball-type safety valve designs, this allows fluid to be circulated downward through the upper safety valve  16  after it has been closed, for example, to kill the well. 
     In one alternate configuration, the chambers  40 ,  42  could be reversed, so that the chamber  40  has relatively low pressure therein and the chamber  42  is connected to the line  30 . Many other configurations are possible, and it should be clearly understood that the actuator  38  is described herein as merely one example of a wide variety of actuators that could be used in keeping with the principles of the invention. 
     In another alternate configuration of the system  10 , only a single safety valve could be used. Thus, it is not necessary in keeping with the principles of the invention for multiple safety valves to be used. If only a single safety valve is used (for example, the lower safety valve  18 ), then a distal end of the line  30  could be closed off or connected to the separate accumulator  60  described below. The line  30  would still extend external to the shear joint  20 , and would be severed when the shear rams  24  are operated, thereby causing the safety valve  18  to close. 
     Referring additionally now to  FIG. 3 , the system  10  is representatively and schematically illustrated in an alternate configuration which permits upper and lower portions of the test string  12  to be separated without actuating the shear rams  24  to sever the shear joint  20  and line  30 . Note that in  FIG. 3  various details of the well, including the wellhead  14 , etc., are not shown for clarity. 
     In certain circumstances it may be desired to separate the upper portion of the test string  12  from the lower portion temporarily, for example, to accommodate a short term emergency or safety situation. Thus, in these circumstances it would be desirable to be able to reconnect the upper and lower portions of the test string  12  to permit continuation of the testing operation after the emergency or other safety situation has been dealt with. 
     In  FIG. 3 , the system  10  is depicted after the upper portion of the test string  12  has been disconnected from the lower portion of the test string using a latch assembly  72 . The latch assembly  72  includes an upper latch  74  connected at a lower end of the shear joint  20 , and a lower latch  76  connected at an upper end of the ramlock  22 . 
     The upper and lower latches  74 ,  76  may be disconnected from and reconnected to each other in the well after the test string  12  has been installed. For example, the latches  74 ,  76  could be connected to each other via J-slots, ratchet mechanisms (such as a RATCH-LATCH™ ratchet mechanism available from Halliburton Energy Services, Inc. of Houston, Tex.) which permit one or more sequence of disconnecting and reconnecting. 
     Preferably, at least the lower safety valve  18  will close when the upper portion of the test string  12  is disconnected from the lower portion of the test string. For this purpose, the lower latch  76  is provided with a check valve  78  which permits fluid in the line  30  to bleed off when the latches  74 ,  76  are disconnected from each other. 
     When the latches  74 ,  76  are reconnected, the check valve  78  can be opened and maintained open by a prong, stinger or other device (not shown) on the upper latch, so that the line  30  is open for flow in both directions between the safety valves  16 ,  18 . The upper latch  74  can include a valve  80  which is also opened when the latches  74 ,  76  are reconnected. 
     Prior to reconnecting the upper and lower portions of the test string  12 , the accumulator  60  can be charged at the surface with sufficient pressure, so that the lower safety valve  18  can be reopened when the latches  74 ,  76  are reconnected. In  FIG. 3 , the accumulator  60  is depicted as a separate element of the test string  12  connected above the upper safety valve  16 . 
     In this manner, the accumulator  60  can be conveniently provided with sufficient volume to displace a large enough quantity of fluid through the line  30  to open the lower safety valve  18  when the latches  74 ,  76  are reconnected. The accumulator  60  can be interconnected in the test string  12  at the surface after the upper and lower portions of the test string have been disconnected and the upper portion has been retrieved to the surface, or the accumulator can be included in the test string when initially installed in the well. 
     If the upper safety valve  16  is not used in the system  10  as depicted in the embodiment of  FIG. 3 , then the line  30  from the lower safety valve  18  could be connected to the accumulator  60  without also being connected to the upper safety valve. 
     Either of the safety valves  90 ,  130  described below and depicted in  FIGS. 4 &amp; 5  could be substituted for either of the safety valves  16 ,  18  in the embodiment of the system  10  shown in  FIG. 3 . 
     Note that in the system  10  described above, the line  30  does not extend to a surface rig or any other remote location. Thus, the time and expense of installing and pressure testing such long control line umbilicals is eliminated in the system  10 . Indeed, the line  30  in the system  10  is isolated from any surface control systems. 
     As used herein, the term “surface control system” is used to indicate a control system installed at the surface of the earth, at a sea floor or mudline, or on a rig or platform at the surface of a body of water. In conventional safety valve systems, a surface control system is remotely connected to a safety valve via a line, and the surface control system is thereby used to remotely supply pressure to the line and release pressure from the line to operate the safety valve. 
     Another advantage of the system  10  is that, in certain embodiments, it is not necessary to use highly pressurized nitrogen chambers. However, in some embodiments of the system  10  it may be advantageous to include the accumulator  60  or other chamber containing pressurized gas. Thus, the system  10  provides flexibility in determining whether or not in a particular situation a pressurized gas chamber should be used. 
     Referring additionally now to  FIG. 4 , another safety valve  90  which may be used in the system  10  is representatively illustrated. The safety valve  90  could be used in place of either of the upper and lower safety valves  16 ,  18 . Of course, the safety valve  90  could be used in systems other than the system  10 , without departing from the principles of the invention. 
     The ball closure mechanisms  32  of the safety valves  16 ,  18  described above are preferably designed so that an obstruction (such as a wireline, slickline, coiled tubing, etc.) in the passage  34  will be severed by the closure mechanism when the safety valve is closed. However, it may be desired to separate the functions of severing an obstruction and sealing against flow through the passage  34 , so that these functions can be performed independently. The safety valve  90  accomplishes this objective, as well as other objectives of the invention. 
     As depicted in  FIG. 4 , an obstruction  92  is positioned in an internal flow passage  94  formed through the safety valve  90 . The obstruction  92  will prevent a conventional flapper closure mechanism  96  from closing if the obstruction is not removed from within the closure mechanism. 
     To remove the obstruction  92 , the safety valve  90  includes an explosive cutting device  98  in the form of a circular shaped charge. Similar conventional explosive cutters are used to cut through damaged casing or to retrieve upper portions of stuck drill pipe, etc. In the safety valve  90 , the explosive cutting device  98  is directed inward to cut through the obstruction  92  positioned within the cutting device. 
     It will be appreciated that other types of cutting devices could be used in place of the cutting device  98 . For example, a fast-acting chemical, mechanical or other type of cutter could be used. 
     To detonate the cutting device  98 , a firing pin  100  is driven to impact a detonator or initiator  102 . A detonating cord  104  extends between the initiator  102  and the cutting device  98 . Thus, when the firing pin  100  impacts the initiator  102 , the initiator detonates and the cord  104  transfers the detonation to the cutting device  98 , which detonates and severs the obstruction  92  in the passage  94 . 
     To drive the firing pin  100  to impact the initiator  102 , a line  106  is connected to a chamber  108  above the firing pin. A chamber  110  below the firing pin  100  contains a relatively low pressure (such as atmospheric pressure). 
     The chamber  108  also initially contains a relatively low pressure. However, when the line  106  is severed or otherwise opened in the well, hydrostatic pressure is allowed to enter the chamber  108  and drive the firing pin  100  downward to impact the initiator  102 . 
     In practice, the line  106  would be positioned within the shear rams  24 , similar to the manner in which the line  30  is positioned within the shear rams in the system  10 . Thus, the line  106  could extend external to the shear joint  20  and internal to the ramlock  22  as described above. 
     When the shear rams  24  are operated to sever the test string  12 , the line  106  is also severed, thereby causing the obstruction  92  to be severed. Since tension would typically be present in the obstruction  92 , this severing of the obstruction will also cause the obstruction to be removed from within the closure mechanism  96 . 
     In the embodiment depicted in  FIG. 4 , the closure mechanism  96  includes a flapper  112  which is pivotably mounted relative to a seat  114 . A spring (not shown) biases the flapper  112  to pivot upwardly toward the seat  114  to seal off the passage  94 . 
     An actuator  126  for the closure mechanism  96  includes a tubular mandrel  116 . An upper portion of the mandrel  116  prevents the flapper  112  from pivoting upward, thereby maintaining the closure mechanism  96  in an open configuration. 
     A piston  118  on the mandrel  116  separates two chambers  120 ,  122 . Initially, when the safety valve  90  is installed in the well, each of the chambers  120 ,  122  contains a relatively low pressure, such as atmospheric pressure, and the piston  118  is balanced. 
     A line  124  is connected to the upper chamber  120 . The line  124  is severed when the shear rams  24  are operated, thereby permitting hydrostatic pressure to enter the upper chamber  120 . This causes a pressure differential across the piston  118 , biasing the mandrel  116  to displace downward, and permitting the flapper  112  to pivot upward and seal against the seat  114 , thereby preventing flow through the passage  94 . 
     In practice, the line  124  would be positioned within the shear rams  24 , similar to the manner in which the line  30  is positioned within the shear rams in the system  10 , and similar to the manner in which the line  106  is positioned. Thus, the line  124  could extend external to the shear joint  20  and internal to the ramlock  22  as described above. 
     If multiple safety valves  90  are used, then the line  106  could be connected between the chambers  108  in the safety valves, and the line  124  could be connected between the chambers  120  in the safety valves. In this manner, the obstruction  92  could be severed in each of the safety valves  90  when the line  106  is severed, and the closure mechanism  96  could be closed in each of the safety valves when the line  124  is severed. 
     However, it may be preferable to sever the obstruction  92  in only one of the safety valves  90  (to prevent a severed portion of the obstruction from becoming lodged in one of the closure mechanisms  96 ), so the cutting device  98  may only be used in one safety valve. If only one cutting device  98  is used, then a distal end of the line  106  could be closed off. If only one safety valve  90  is used, then distal ends of both of the lines  106 ,  124  could be closed off. 
     Referring additionally now to  FIG. 5 , another safety valve  130  which may be used in the system  10  is representatively illustrated. The safety valve  130  could be used in place of either of the upper and lower safety valves  16 ,  18 . Of course, the safety valve  130  could be used in systems other than the system  10 , without departing from the principles of the invention. 
     The safety valve  130  is similar in some respects to the safety valve  90  described above. The safety valve  130  is used to sever the obstruction  92  in the passage  94  in order to remove the obstruction from within the flapper closure mechanism  96 . In addition, the obstruction severing and passage sealing functions of the safety valve  130  are substantially independent of each other. 
     However, instead of the explosive cutting device  98 , the safety valve  130  includes a mechanical cutting device  132 . The cutting device  132  includes a blade  134 , an actuator  136  and an inclined ramp  138 . To sever the obstruction  92 , a tubular mandrel  140  of the actuator  136  is displaced upward, thereby displacing the blade  134  along the ramp  138 , causing the blade to displace laterally across the passage  94  and cut through the obstruction  92 . 
     The actuator  136  includes two chambers  142 ,  144 . The lower chamber  144  preferably contains a relatively low pressure, such as atmospheric pressure. It will be readily appreciated by those skilled in the art that when the safety valve  130  is installed in the well hydrostatic pressure acting on the mandrel  140  will cause the mandrel to be biased upwardly due to a differential between the hydrostatic pressure and the relatively low pressure in the chamber  144 . 
     Upward displacement of the mandrel  140  is prevented by fluid (such as a relatively incompressible liquid) contained in the upper chamber  142 . Release of this fluid from the chamber  142  will permit the mandrel  140  to displace upward, thereby displacing the blade  134  to cut through the obstruction  92 . 
     An actuator  146  for the closure mechanism  96  includes a similar set of chambers  148 ,  150  and a mandrel  152 . Relatively low pressure is contained in the lower chamber  150 . When the safety valve  130  is installed in the well, the mandrel  152  will be biased upwardly due to a pressure differential across the mandrel between hydrostatic pressure in the passage  94  and relatively low pressure in the chamber  150 . A fluid (such as a relatively incompressible liquid) is contained in the upper chamber  148  to prevent the mandrel  152  from displacing upward until the fluid in the upper chamber is released. 
     A lower portion of the mandrel  152  prevents the flapper  112  from pivoting upward toward the seat  114 . However, when the mandrel  152  displaces upward, the flapper  112  will be permitted to pivot upward to seal against the seat  114  and prevent flow through the passage  94 . 
     A line  154  is connected to each of the chambers  142 ,  148 . It will be readily appreciated that when hydrostatic pressure is applied to the passage  94  upon installation of the safety valve  130  in the well, pressure in the chambers  142 ,  148  and in the line  154  will be greater than hydrostatic, due to the differential pressure applied to the mandrels  140 ,  152 . 
     If the line  154  is severed or otherwise opened in the well, the fluid will be allowed to escape from the line and the chambers  142 ,  148 , and the mandrels  140 ,  152  will be permitted to displace upwardly. This will result in the obstruction  92  being severed and the closure mechanism  96  being closed. 
     In practice, the line  154  would be positioned within the shear rams  24 , similar to the manner in which the line  30  is positioned within the shear rams in the system  10 . Thus, the line  154  could extend external to the shear joint  20  and internal to the ramlock  22  as described above. 
     If multiple safety valves  130  are used, then the line  154  could be connected between the chambers  142 ,  148  in the safety valves. In this manner, the obstruction  92  could be severed and the closure mechanism  96  could be closed in each of the safety valves  130  when the line  154  is severed. 
     However, it may be preferable to sever the obstruction  92  in only one of the safety valves  130  (to prevent a severed portion of the obstruction from becoming lodged in one of the closure mechanisms  96 ), so the cutting device  132  may only be used in one safety valve. If only one safety valve  130  is used, then a distal end of the line  154  could be closed off. 
     The line  154  could be connected to an accumulator (such as the accumulator  60  described above, either internal to or external to the safety valve  130 ). The accumulator  60  could maintain pressure in the chambers  142 ,  148  regardless of thermal expansion/contraction of the chambers, line  154  and fluid therein. 
     Note that, similar to the safety valves  16 ,  18  described above, neither of the safety valves  90 ,  130  requires a line to extend to a surface control system, and neither of the safety valves  90 ,  130  requires that pressure be remotely applied to the safety valve to maintain it in an open configuration during installation. In certain preferred embodiments, the safety valves  90 ,  130  also do not require use of highly pressurized gas chambers. 
     Although the safety valves  16 ,  18  are described above as using the ball closure mechanism  32  and the safety valves  90 ,  130  are described as using the flapper closure mechanism  96 , any closure mechanism (including other types of closure mechanisms) may be used in any of these safety valves. Although the safety valves  16 ,  18  are described as using the ball closure mechanism  32  to sever the obstruction  92 , the safety valve  90  is described as using the explosive cutting device  98 , and the safety valve  130  is described as using the mechanical cutting device  132 , any cutting device (including other types of cutting devices) may be used in any of these safety valves. Furthermore, any of the safety valves  16 ,  18 ,  90 ,  130  described above may use any of the actuators  38 ,  126 ,  136 ,  146 , or any other types of actuators. 
     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.