Patent Application
20080066921
To aid in focus on the invention the subsurface safety valve will be shown schematically since the focus of the invention is on the control system that operates the valve. What is shown in
The control system's purpose is to operate the flapper 10 between its closed position shown and the open position using some of the previously described stock components to do so. A control line 20 extends from the schematically illustrated surface 22. Line 20 branches into segments 24 and 26. Piston housings 28 and 30 are preferably aligned. Segment 26 extends into inlet 32 on housing 28. Segment 24 extends into inlet 34 on housing 30.
Piston 36 in housing 28 has an upper control chamber seal 38 and a connecting rod 40 that passes through opening 42 and has an upper tubing seal 44. Piston 36 divides its bore into chambers 46 and 48. Chamber 46, the higher pressure chamber, is in fluid communication with inlet 32 while chamber 48, the lower pressure chamber, is in communication with port 50.
Housing 30 has a piston 52 that has a lower control chamber seal 54 and a connecting rod 56. Rod 56 exits housing 30 through opening 58 that is sealed with a lower tubing pressure seal 60. Piston 52 divides housing 30 into chambers 62, the lower pressure chamber, and 64, the higher pressure chamber. Line segment 24 enters chamber 64 through inlet 34. Chamber 62 has a port 66.
Insensitivity to tubing pressure or pressure balance in the context of the combined dimension of the rod 40 and its seal 44 on one hand and the combined dimension of the rod 56 and its seal 60 on the other hand is defined as closeness in their areas that can include an area disparity of as much as 10%.
Ports 50 and 66 are connected by line 68 which further comprises a larger volume reservoir 70. Line 68 and reservoir 70 are preferably filled with a compressible fluid such as air or nitrogen, for example, at the surface, when the components are assembled. Other fluids or fluid types can also be used.
While a coupler 72 could be used, it is not required. Coupler 72 allows easy assembly of rods 40 and 56 to each other. One way to do this is to put a T-shaped end on coupler 72 that can slide into a mating receptacle at the end of rod 56. The other end of the coupler 72 can be threaded or pinned or otherwise secured to rod 40, other examples are but not limited to, ball/socket or u-joint configurations. This feature permits a certain amount of misalignment of rods 40 and 56 consistent with preferred manufacturing tolerances. A more pronounced offset can also be accommodated in rods 40 or 56 or in coupler 72.
In the preferred embodiment, pistons 36 and 52 are rod pistons that are aligned axially to facilitate coupling the rods 40 and 56 to each other. The diameter of piston 36 is larger than the diameter of piston 52 for a reason that will be explained when reviewing the operating procedure and the various failure modes. While rod pistons are preferred, other types of pistons can be used such as annularly shaped pistons, for example. Because the piston diameters are unequal a given movement of the pistons toward the flapper 10 reduces the volume of chamber 48 while the volume of chamber 62 increases. This could result in pressure buildup in these chambers as the compressible fluid in the jumper line 68 has its pressure increased due to volume reduction when the pistons move in a direction toward flapper 10. The addition of the reservoir 70 minimizes this pressure spike that could impede the normal operation of the control system. With the reservoir 70 the volume reduction from piston movement has a negligible pressure buildup in chambers 48 and 62.
Despite the fact that a single control line 20 comes down from the surface 22, the effect of control line hydrostatic pressure is reduced as the same hydrostatic pressure acts downwardly on piston 36 in chamber 46 and upwardly on piston 52 in chamber 64. The required control pressure to open the valve is further reduced since the tubing pressure is balanced given that seals 44 and 60 are of equal size. Thus, it is not necessary for the control pressure to overcome tubing pressure prior to compressing the spring to open the valve. Since pistons 36 and 52 are of different diameters, the net force on them is the hydrostatic pressure acting on the difference of their areas, which difference is quite small, by design. Yet it is this difference in area of the pistons that accounts for the net force when the pressure is elevated in line 20 to shift the pistons toward the flapper 10 so as to open the valve by engaging shoulder 74 on tab 16 and overcoming the force of spring 18. Spring 18 is designed to overcome the hydrostatic net force as explained above and friction in the piston and connecting rod seals as well as the weight of the pistons and their connecting rods and a little more for a safety factor.
Accordingly, to open the flapper 10 a pressure buildup in line 20 overcomes the resistance of spring 18 and shoulder 74 pushes down tab 16 driving the flow tube 14 against the flapper 10 and rotating it 90 degrees and away from its seat (not shown) to a position behind the shifted flow tube 14. To normally close the flapper 10 the pressure in line 20 is reduced to allow the spring 18 to overcome the net force from hydrostatic, friction and weight forces described above so as to drive the flow tube 14 back up which allows the flapper spring (not shown) to rotate the flapper 90 degrees to get to its closed position against its seat (not shown).
Failure modes can happen in one of four ways depending on which of the four seals 38, 44, 60 or 54 starts leaking. If seal 38 leaks pressure in chamber 46 which is control line pressure in line 20, communicates to chamber 48 from chamber 46, putting piston 36 in pressure balance. Chamber 48 also communicates to chamber 62 through jumper line 68. This puts the pressure from branch 26 into chamber 62 and the same pressure from branch 24 into chamber 64. Now piston 52 is in pressure balance. With both pistons in pressure balance, spring 18 closes flapper 10 by shifting up the flow tube 14.
If seal 54 fails the pressure from the control line 20 through branch 24 gets into both chambers 64 and 62 putting piston 52 in pressure balance. Because of jumper line 68 the pressure in chamber 62 is the same as chamber 48. Thus the pressure from branch 24 gets all the way to chamber 48 while the same pressure that is in branch 24 gets to chamber 46 through branch 26. Again, both pistons are in pressure balance and the spring 18 shifts the flow tube 14 upwardly allowing the flapper 10 to rotate 90 degrees to its closed position shown in
If seal 44 fails tubing pressure will enter chamber 48 and through jumper 68 will also enter chamber 62. If the leak is large enough, even with pressure applied in line 20 a net unbalanced force will be created from having tubing pressure in chambers 48 and 62 until at some point the combination of that unbalanced pressure caused by the size difference in the pistons 36 and 52 will shift the piston upward to the closed position in combination with spring 18 which will cause the flow tube 14 to be moved up to allow the flapper 10 to rotate 90 degrees to its closed position.
If seal 60 fails, tubing pressure will enter both chambers 62 directly and 48 through the jumper line 68. The same result obtains as when seal 44 fails, as described above.
Those skilled in the art will now appreciate that the system provides for failsafe operation in a very simple design. A single control line that splits and connects into high pressure chambers which are isolated from tubing pressure and comprised of opposed pistons of different sizes, allow only a very small net force from control line hydrostatic pressure to exist. This pressure can be simply offset with proper sizing of the return spring 18 that need not be sized to offset full control line hydrostatic pressure and without the need to compensate for the tubing pressure at the valve since the design eliminates this need by balancing the tubing pressure at inner seals 44 and 60. By the same token, the difference in piston sizes allows for opening the flapper with applied pressure in the control line to the point where the unbalanced force on the two pistons is great enough to overcome the force of the return spring 18. The jumper line 68 connects the low pressure chambers 48 and 62 to facilitate tandem movement of pistons 36 and 52 as well as serving as a conduit to equalize pressure across the pistons if seals 38 or 54 fail. If either seal 44 or 60 fails, tubing pressure gets into both low pressure chambers 48 and 62 and by virtue of piston 36 being larger than piston 52 forces both pistons up due to a net unbalanced force acting in that direction and the flapper 10 can close. The reservoir 70 eliminates significant pressure buildup due to a net volume reduction between chambers 48 and 62 as the pistons move to open flapper 10. The large volume of reservoir 70 relative to line 68 and the amount of volume reduction experienced during the flapper opening operation prevents pressure buildup, which, if it occurred, would fight the opening of the valve for the same reason as a leak in seals 44 or 60 would tend to move the control system to the flapper closed position.
While one pair of rod pistons is illustrated, multiple pairs can be used. Wholly or partially annular piston shapes can be used or be combined with rod pistons. Optionally, the tab 16 can be connected directly to rods 40 or 56 for movement of the flow tube in opposed directions.
While the control system is described in context of a subsurface safety valve, it can be used for other downhole tools where the final controlled element differs from a flow tube driven flapper, which is simply a specific execution of the invention. The pistons can move a sleeve or set slips or a packer element, for examples of some final controlled elements.
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.
