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: 
FIELD OF THE INVENTION 
   The field of the invention is control systems for subsurface safety valves (SSV) and more particularly a device that allows changeover to a redundant system while isolating a closure spring from the hydrostatic pressure effects of one of the control lines from the surface to the SSV. 
   BACKGROUND OF THE INVENTION 
   SSVs are used in production strings to control the well. They are mounted in the string and are hydraulically controlled from the surface. Typically a control line runs parallel to the production string and is connected to the SSV housing. Applying pressure moves a piston that is connected to a flow tube. The flow tube is pushed against a closure spring by the piston. The flow tube also engages a flapper to rotate it 90 degrees so that the flow tube can advance as the open flapper is now outside the flow tube. The housing has a seat and the flapper is biased by a torsion spring against the seat. The movement of the piston to urge the flow tube to move winds the torsion spring and compresses the closure spring at the same time. When pressure is removed or lost from the control line, the closure spring pushes the flow tube and interconnected operating piston against the hydrostatic pressure in the control line so that as the flow tube rises the torsion spring is enabled to rotate the flapper into contact with the seat. 
   If a problem occurs within the SSV it usually means that it has to be pulled with the production string. Variations involving balance control lines or pressurized chambers in the SSV housing have been developed to allow offsetting of hydrostatic pressure since the hydrostatic pressure in the main control line is offset and that allows a smaller closure spring to close the valve without having to also overcome the hydrostatic pressure in the control line. 
   Problems could occur in the hydraulic actuation system such as a control line leak or an operating piston seal leak, for example. Dual operating control systems have been developed so that one operates the SSV while the other system is isolated until needed. In these systems, each control system had its own control line and operating piston where both operating pistons were engaged to the flow tube. In order not to burden the single closure spring with the added hydrostatic pressure from two parallel control lines the system that is offline is isolated with a rupture disc so that the hydrostatic pressure above the disc is not felt by the closure spring until the disc is broken, generally by raising tubing pressure. 
   However, in subsea systems the delivered pressures are controlled and can&#39;t be arbitrarily raised to affect a switch to the backup control system by raising the pressure in the system above the normal operating range. This condition in subsea systems has been addressed by the present invention. There are the two control lines each going to a discrete independent operating piston. Each piston is coupled to a rod and the two rods interact. The rod associated with the piston where control line pressure is applied is free to move to operate the SSV in the normal manner. The movement of the first piston and its associated rod results in support for the other rod in a variety of ways explained below. The result is that the rod associated with the non-pressurized system has the hydrostatic pressure in its control line isolated from the closure spring. Removing applied pressure from the control lines lets the system go back to neutral so that either of the two redundant systems can be thereafter activated. Those skilled in the art will gain a better understanding of the invention from the description of the preferred embodiment with the associated drawings that appear below with the understanding that the claims define the full scope of the invention. 
   SUMMARY OF THE INVENTION 
   A system is provided for switching between redundant control systems for a subsurface safety valve (SSV) while being able to isolate the closure spring from hydrostatic pressure in the control line of the system that is not being used. There are two control lines that connect to discrete operating pistons that are both coupled to the flow tube. Each operating piston is connected to a control rod with the control rods terminating near opposed ends of a pivoting member. Pushing down on one rod pushes up on the other rod so that the other rod is held supported and the hydrostatic pressure in its associated control line doesn&#39;t materially affect the force needed by the closure spring to close the SSV. Releasing control line pressure puts the system in neutral to allow either of the systems to be reselected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section view of one embodiment of the system in a neutral position; 
       FIG. 2  is the view of  FIG. 1  with one system activated and the other having its hydrostatic pressure isolated; 
       FIG. 3  is an alternative embodiment in the neutral position; 
       FIG. 4  is the view of  FIG. 3  with one system actuated and the other having its hydrostatic pressure isolated; 
       FIG. 5  shows how the isolated system is released from isolation; 
       FIG. 6  shows how the isolated system is held in isolation; 
       FIG. 7  is a perspective view of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   For clarity, most of the common components of SSVs are omitted from the FIGS. Instead the focus is on showing the flow tube and operating pistons that are attached to it. Those skilled in the art will know that a closure spring is below the flow tube and is compressed when the flow tube is forced down by the operating piston. In turn, pressure in a control line is delivered to an operating piston that can be of an annular or rod shape and is sealed in a bore in the SSV housing. Of course, the flow tube rotates the flapper when moved down and the torsion spring on the flapper pivot rotates the flapper to its seat when the closure spring pushes up the flow tube. 
   With all that as an introduction to typical components in a SSV, the drawings will show how those systems interact when redundant systems are provided and there is a need to be able to switch between them as well as to isolate hydrostatic pressure from the control line associated with the system that is not in use. 
     FIGS. 1 and 2  are illustrative of one embodiment. Arrows  10  and  12  schematically illustrate control lines from the surface to a SSV housing  14 . The housing  14  is shown cut away so that the flow tube  16  within can be seen. Line  12  leads to operating piston  18  and line  10  leads to operating piston  20 . As is well known in the art the operating pistons  18  and  20  have seals in a bore in the housing  14  so that applied pressure in their respective control lines  12  and  10  results in movement of the respective piston. Piston  18  has a clamp or similar device  22  attached to it while piston  20  has a similar device  24 . Devices  22  and  24  are designed to move in tandem with their respective piston. Flow tube  16  has a radial surface  26  that is designed to be engaged by clamps  22  or  24  when either one is moved from the  FIG. 1  position by pressure applied in control lines  10  or  12 . As is well known in the art, the flow tube  16  has its downward motion resisted by a closure spring. Additionally, downward movement of the flow tube  16  rotates a flapper 90 degrees and away from its seat and behind the flow tube  16  to define the valve open position. The closure spring acting on the flow tube  16  returns it to the valve closed position shown in  FIG. 1 . 
   Mounted within the housing  14  is a pivoting member  28  on which rests the lower ends  30  and  32  of rods  34  and  36  respectively. Rod  34  is clamped to piston  18  and rod  36  is clamped to piston  20  respectively by clamps  22  and  24  for tandem movement. Shown illustratively on rod  36  but also useful on rod  34  is a wear pad  38  that gives lateral support to the rod  36  when pivoting member  28  is rotated against it, as shown in  FIG. 2 . As also shown in  FIG. 2 , the pivoting member  28  is underneath lower end  30  so as to support rod  34 . Since rod  34  is attached to piston  18  through clamp  22 , the hydrostatic pressure in control line  12  is supported in the  FIG. 2  position from pivot pin  40 . 
     FIG. 2  shows control line pressure applied to control line  10  while no external pressure is applied to control line  12 . Piston  20  with attached clamp  24  has been pushed down. Clamp  24  has engaged surface  26  so that the flow tube  16  moves in tandem with clamp  24 . That very movement brings down rod  36 , which causes pivoting member  28  to rotate clockwise about pivot pin  40  until pivoting member  28  is pushing laterally on wear pad  38 . At the same time, another portion of pivoting member  28  has gotten under lower end  30  because of the frusto-conical shape of member  28 . In the  FIG. 2  position, rod  34  and piston  18  clamped to it are fully supported from member  28  so that the hydrostatic pressure from line  12 , which at this time has no applied pressure, is transmitted through rod  34  and pivot pin  40  laterally into wear pad  38 . When pressure is removed from line  10 , the closure spring that acts on the flow tube  16  pushes it up to allow the components to return from the  FIG. 2  position back to the  FIG. 1  position. Subsequently, applying pressure to line  12  simply makes the member  28  rotate counterclockwise as clamp  22  lands on shoulder  26  to push the flow tube  16  down to open the SSV. 
   What is illustrated in  FIGS. 1 and 2  is a SSV with a redundant control system where the control system that is off line has its hydrostatic pressure in its respective control line isolated from having any force applied to the flow tube  16  so that the closure spring shown schematically as  29  can be sized for the hydrostatic pressure from a single control line when there are redundant control systems in place, particularly in a situation where pressures higher than the normal operating pressures to open the SSV cannot be applied, such as in subsea systems. It should be noted that unlike a backup system that is isolated with a rupture disc, this system continues to isolate hydrostatic pressure from the control line of a dual system that is not in active use regardless of how many times cycling has gone on between the redundant systems. In a system where the redundant system is isolated with a rupture disc, once the disc is broken, the hydrostatic pressure in the associated control line will no longer be isolated. 
     FIG. 3  shows a preferred embodiment that is similar in operation to  FIGS. 1 and 2  except in the manner the hydrostatic pressure in the off line system is isolated from the flow tube  16 ′. Instead of transmitting the hydrostatic force through pivoting member  28  and its pin  40  into a lateral load on a wear pad such as  38  on the rod that has been pushed down by the control system that has had pressure applied to it, the preferred system of  FIG. 3  employs a series of collets  42  that have a support surface  44 . Collets  42  are sprung radially outwardly but do not move longitudinally. As shown in  FIG. 4  the collar  22 ′ gets pushed up in the manner previously described until it goes higher than support surface  44 . From that point piston  18 ′ is supported and the hydrostatic pressure in line  12 ′ is effectively isolated from flow tube  16 ′ and from the closure spring that eventually has to push it up when applied pressure is removed from control line  10 ′. Clamp  22 ′ resists all the hydrostatic, when landed on support surface  44 , so that little if any lateral force is transmitted through pivoting member  28 ′ to rod  36 ′ after clockwise rotation of member  28 ′. Just as before for moving down the flow tube  16 ′ there is a shoulder  26 ′ for either clamp  22 ′ or  24 ′ to engage to push down the flow tube  16 ′. The difference is how a clamp such as  22 ′ once resting on support surface  44  is enabled to move down beyond it. This can better be understood by looking at the section views of  FIGS. 5 and 6 . In  FIG. 6 , clamp  22 ′ is shown supported from surface  44  of collets  42 . Shoulder  26 ′ is also illustrated in the pushed down position that has resulted from clamp  24 ′ pushing it down. When applied pressure in control line  10 ′ is removed the closure spring abutting flow tube  16 ′ will push it up relative to surface  44  that is stationary but sprung radially outwardly. As the flow tube  16 ′ comes up with rod  36 ′ shoulder  26 ′ is also moving up and bringing circumferential channel  46  close to the ends  48  of collets  42 . The conclusion of this movement is shown in  FIG. 5  where the ends  48  have been pulled inwardly by landing in channel  46 . As soon as that happens, the hydrostatic pressure in line  12 ′ can push down rod  34 ′ and the pivoting member  28 ′ rotates counterclockwise from the  FIG. 4  position back to the  FIG. 3  position.  FIG. 7  is simply a perspective view of  FIG. 6 . 
   While motion of the components in one direction and a return to the neutral position has been described, those skilled in the art will appreciate that with a redundant system available, either one can be actuated first and the difference is simply the pivot direction of member  28  or  28 ′. Thus, the advantage of isolating hydrostatic pressure from one of the surface control lines from the flow tube is simply accomplished in either embodiment particularly in a situation where the hydraulic system is regulated not to exceed the normal range of operating pressures. Additionally, the illustrated systems offer an advantage over rupture disc isolation in that they are cycle independent as compared to a rupture disc system which works once and is disabled. Further, the use of a rupture disc for an isolator carries additional risks of fragments breaking off the disc when it is deliberately broken and causing the piston below to jam or its seals to leak. Either event will normally require pulling a string with the SSV at significant cost. While a variety of solutions to a changeover from one redundant system to another have been illustrated, those skilled in the art will appreciate that the invention encompasses redundant systems that allow for changeover any number of times while isolating the closure spring from hydrostatic of any redundant line(s). While one backup system has been illustrated, more than one backup system can be integrated into a SSV. 
   While clamped rods have been illustrated in conjunction with pivoting member  28 , those skilled in the art will appreciate that such rods can be eliminated for protruding structures directly from a piston. In  FIG. 1  for example, clamp  24  can still engage surface  26  but rod  36  can be replaced with a tab coming out of piston  20  and positioned to engage pivoting member  28  to rotate it clockwise. In the same manner, rod  34  can also be replaced. 
   The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below.

Summary:
A system is provided for switching between redundant control systems for a subsurface safety valve (SSV) while being able to isolate the closure spring from hydrostatic pressure in the control line of the system that is not being used. There are two control lines that connect to discrete operating pistons that are both coupled to the flow tube. Each operating piston is connected to a control rod with the control rods terminating near opposed ends of a pivoting member. Pushing down on one rod pushes up on the other rod so that the other rod is held supported and the hydrostatic pressure in its associated control line doesn&#39;t affect the force needed by the closure spring to close the SSV. Releasing control line pressure puts the system in neutral to allow either of the systems to be reselected.