Abstract:
A flapper in a subsurface safety valve has at least one magnet that comes in close proximity with another magnet mounted in a fixed position on the valve body. There is a fixed magnet on the body adjacent to the fully open and the fully closed positions of the flapper. In each case like poles on the flapper magnet and the housing magnet come in close proximity as the flapper reaches its fully open and fully closed positions. The orientation of like poles adjacent each other creates a repelling force that damps or eliminates shock loading.

Description:
FIELD OF THE INVENTION 
     The field of this invention is tools used in a subterranean formation that have a movable component that is subjected to shock loading and the use of a field to cushion impact loads and more particularly using a magnetic field to control shock loading on a flapper of a subsurface safety valve. 
     BACKGROUND OF THE INVENTION 
     Magnets have been used to act as dampeners such as in the context of exercise equipment as illustrated in U.S. Pat. No. 5,752,879. Magnets have been used in fluid flow systems to hold a position of a moving component such as for example in an open or a closed position. Illustrative of a gas line and a medical device application are U.S. Pat. No. 5,209,454 and U.S. Pat. No. 5,970,801. In a similar vein is U.S. Pat. No. 7,527,069. The use of magnets to control the fixation of a movable member in a level control application is seen in U.S. Pat. No. 4,436,109. These disparate applications seek to use the force of a magnetic field for fixation to a given position. Some of them release the component when the magnetic field is deactivated. 
     In downhole applications and most particularly in valves where large pressure differentials can build in an instant as a valve member such as a flapper moves against a seat, there can be serious damage from the impact force that can be severe enough to deform the valve member or the mating seat. In the case of subsurface safety valve flappers, when opened but more so when allowed to close, there is a risk of flapper or seat damage or damage to both from a severe impact loading. Accordingly the present invention seeks to cushion or even eliminate the shock contact while still allowing the movable member to reach its intended ultimate position. In the context of a flapper, the preferred embodiment locates at least one magnet on the flapper and magnets in the housing adjacent the location of the flapper when it reaches its ultimate open or closed position. In this manner the application of a magnetic field to the pivoting flapper damps any impact with the seat in the closed position and any travel stop for the open position. These and other features of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated FIGS. while recognizing that the full scope of the invention is given by the appended claims. 
     SUMMARY OF THE INVENTION 
     A flapper in a subsurface safety valve has at least one magnet that comes in close proximity with another magnet mounted in a fixed position on the valve body. There is a fixed magnet on the body adjacent to the fully open and the fully closed positions of the flapper. In each case like poles on the flapper magnet and the housing magnet come in close proximity as the flapper reaches its fully open and fully closed positions. The orientation of like poles adjacent each other creates a repelling force that damps or eliminates shock loading. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of a flapper in a safety valve just about to close; 
         FIG. 2  is the view of  FIG. 1  with the flapper in the fully open position; and 
         FIG. 3  is the view of  FIG. 2  with the flapper in the fully closed position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The basic structure of a downhole subsurface safety valve is known to those skilled in the art. Basically, a hydraulic control line runs from the surface to the valve to operate a piston that is biased against the applied pressure in the control line. Pressurizing the control line moves the piston which is linked for tandem movement with a flow tube  10 . The flow tube  10  rides inside seat assembly  12  the lower end of which has a seat  14 . A flapper  16  is pivoted at  18  and the pivot shaft can have a spring to bias the flapper  16  into the closed position of  FIG. 3  when the pressure on the control line is removed and a closure spring pushes the piston in an opposed direction which has the effect of retracting the flow tube  10  at which point the spring on the pivot  18  initiates movement of the flapper  16  toward seat  14 . The flow trying to come uphole as represented by arrow  20  helps to get the flapper  16  moving toward its seat  14 . The seat  14  and the corresponding portion of the flapper  16  that lands on seat  14  are complex contoured shapes that are expensive to produce in computer controlled milling machines. It is very undesirable to get any deformation in the seat  14  or in the mating portion of the flapper  16 . 
     Those skilled in the art will see that as the flow tube  10  is retracted and the flapper starts movement from the  FIG. 2  to the  FIG. 1  to the  FIG. 3  positions, the velocity of the fluid represented by arrow  20  can result in slamming the conforming shapes of the seat  14  and the flapper  16  against each other. In the preferred embodiment, the use of a force of a magnetic field is designed to reduce the velocity of the rotating flapper  16  as it reaches the fully closed  FIG. 3  position and the fully open  FIG. 2  position. 
     The way the dampening is accomplished in the preferred embodiment is to fixedly mount a permanent magnet  22  and  24  in the housing  26  and a magnet  27  to the flapper  16  on an extending tab  28 . Tab  28  is preferably diametrically opposed from the location of the pivot connection  18 . The opposing surfaces of magnets  24  and  27  are of the same polarity so that they repel each other as they get closer together. The same can be said for magnets  22  and  27  as they approach each other when the flapper  16  goes toward the open position of  FIG. 2 . The end tab  28  is used to allow the magnets  24  and  27  to be away from the specially machined complementary surfaces that engage when the flapper  16  engages the seat  14 . It is cheaper to do it this way than to drill blind bores in the flapper and seat sealing surfaces although to do so can be an alternative way to use the magnets  24  and  27  to provide a dampening of the velocity and the resulting momentum force as the flapper  16  goes to the closed position of  FIG. 3 . As shown in  FIG. 3  magnet  24  is on a longer radius from pivot  18  than magnet  27  which still allows taking advantage of like poles repelling each other. The orientation can also be changed to position magnet  27  on the same arc as magnet  24  to create the dampening effect of magnets repelling each other. However, the offset orientation allows taking advantage of the repelling force when magnets  24  and  27  get close enough to each other, as shown in  FIG. 1 , and then deliberately reducing or eliminating the repelling force having already slowed the flapper  16  when the magnets  24  and  27  go side by side as shown in  FIG. 3 . In this configuration the flapper can seat within 5 seconds as required in Standard 14A of the American Petroleum Institute (API). The relative positions can be varied to take into account ease of assembly, cost, power of the magnets to repel each other and the size and weight of the flapper  16 . The overarching concept is the use of a field to reduce the velocity of a moving component in a downhole tool. From there the focus can get more specific to the use of a magnetic field and on down to permanent magnets and their relative positions in the open position of  FIG. 2  and the closed position of  FIG. 3 . 
     It should also be noted that introducing high pressure and high velocity gas in a downhole direction which is the reverse of arrow  20  from above a closed flapper  16  can accelerate the flapper  16  to the open position of  FIG. 2  with enough force to also cause potential damage. Clearly there is greater risk of damage in the flapper  16  going to the closed position of  FIG. 3 . However, magnet pair  22  and  27  serves to slow down the flapper  16  as it starts to slam to the fully open position. Again with this magnet pair there can be an axial offset between them in the direction of arrow  20  or the arc of magnet  27  can coincide with the location of magnet  22 . 
     Magnet pair  22  and  27  also prevent another problem. Sometimes when the flow tube  10  is raised by the control system (not shown) high velocity gas gets behind the flapper  16  in the open position and creates a low pressure zone behind the flapper  16  that in extreme cases holds the flapper in the open position where it needs to go to the closed position. The magnet pair  22  and  27  can provide a repelling force to drive the flapper  16  toward the closed position. To do this the preferred orientation of this pair of magnets is alignment. The flow tube  10  will push the flapper out of the way when going to the open position so alignment of this magnet pair is not an issue even if the repelling force does not diminish since the force behind the moving flow tube will overcome the repelling force in any event. The magnet  22  can optionally be eliminated. 
     While more complicated, one or more of the magnets can be powered electromagnets that can be selectively powered or turned off from a location removed from the valve. Other electrical fields are contemplated that can create a repelling force. It should be noted that the flapper momentum by definition overcomes the repelling force while it is being decelerated with the repelling force diminishing or going to zero when the magnets  24  and  27  get toward a radially aligned position shown in  FIG. 3 , so that the force of pressure on the flapper  16  in the closed position will tightly hold the closed position of  FIG. 3 . It is even possible to have the magnets attract in the  FIG. 3  position by having opposite poles close enough to each other to aid in holding flapper  16  in the closed position. In the open position the flow tube  10  holds back the flapper  16  and overcomes any repelling force as magnets  22  and  27  get close to each other. 
     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: