Balance line control system with reset feature for floating piston

#The field of the invention is hydraulic control systems for borehole tools and more particularly systems that employ a control line and a balance line to the surface with a floating piston isolating a balance chamber. In the event leakage of tubing pressure prevents downward movement of the floating piston on safety valve closure, a pressure equalization enabled by applied pressure on the balance line allows reset of the floating piston to allow continued operation of the safety valve despite the tubing pressure leak.

#Subsurface safety valves are typically hydraulically controlled from a remote location using one or two control lines. An advantage of a two control line system is that hydrostatic pressure in each line is canceled out so that a closure spring for a flow tube does not need to resist hydrostatic pressure as is the case with single control line systems. In two line control systems pressure on top of an operating piston moves a flow tube against a flapper to open the valve. Removal of such pressure from the main control line allows a closure spring to reverse movement of the flow tube to allow the flapper to rotate 90 degrees to closed position of the safety valve. In the past operators have wanted or regulations required a barrier in the second or balance control line so that if tubing pressure leaks into the hydraulic system there would be a barrier to keep hydrocarbons from reaching a surface location through the balance line.

#The floating piston in the balance line served this purpose as a barrier. In normal valve operations pressure applied in the main control line to the top of a piston whose movement shifted the flow tube would result in hydraulic fluid displacement to the underside of the floating piston. Conversely, as pressure was removed from the main control line and the closure spring pushed up the flow tube hydraulic fluid would be drawn into the safety valve from under the floating piston to enable the safety valve to close. The floating piston would just move up when the safety valve open and reverse its motion when the safety valve closed, each time displacing an equal volume of hydraulic fluid as movement of the operating piston had displaced. The floating piston was sometimes biased toward the down position to put it in the ready position for safety valve opening.

#FIG. 1 is a schematic of the present invention showing the safety valve closed or pressure reduced in the balance chamber;

#FIG. 2 is the view of FIG. 1 with the safety valve open or the balance chamber gaining pressure;

#FIG. 3 shows pressure applied into the balance line opening the equalizing valve and allowing the bias on the floating piston to reposition the floating piston such that the safety valve can be opened.

#Referring to FIG. 1, the normal operation of the control system 10 will be described. An operating control line 12 extends from a remote location to a subsurface safety valve 14 located in a borehole or conduits associated with a borehole that are not shown. The safety valve 14 is a type well known in the art and generally has a hydraulic piston moving a flow tube to rotate a flapper to open the valve when pressure is applied to the operating control line 12. When pressure is removed from operating control line 12 a closure spring is able to push the flow tube away from the flapper to let the flapper rotate 90 degrees to a closed position against a flapper seat. Moving the flow tube requires delivery of hydraulic fluid against an operating piston 13 in the safety valve 14. Movement of such a piston displaces fluid out of the safety valve body to a balance chamber 16 that is directly below the floating piston 18. Floating piston 18 is biased by spring 20 pushing from support 22 against shoulder 24 on the floating piston 18. Taper 26 represents a lower travel stop for the floating piston 18. Support 22 surrounds the floating piston 18 and guides its movement up to upper stop 28. The piston 18 does not necessarily have to reach the stop 28 as its upper movement can be limited by fully compressing spring 20 between shoulder 24 and support 22 or limited by maximum fluid displacement from valve 14.

#Operating control line 12 branches into lines 30 and 32. Line 32 goes to the top of the operating piston inside the safety valve 14 and line 30 goes to the underside of equalizing valve 34 at inlet 36 below the valve member 38 that has a seal 40 to hold the pressure in the operating control line 12. Coming out of the safety valve 14 from below the operating piston of the safety valve 14 is line 44 that branches into lines 46 and 48. Line 46 goes into an annular space where spring 42 is located. Spring 42 pushes up on valve member 38 to hold head 50 against seat 52. Pressure in line 46 acts below head 50 also acts in the same direction as spring 42. Note that the seal area at seat 52 is larger than the seal 40 so that pressure in line 46 creates a net force on head 50 against seat 52. Stop 54 limits the movement of head 50 away from seat 52. Lines 56 and 58 join to become the balance line 60 that goes to a remote surface location. As previously stated the purpose of line 60 is to offset the hydrostatic pressure in operating control line 12 but it has another purpose as will be described.

#Valve member 38 does not move during normal operation of the safety valve 14. Floating piston 18 is in a lower position shown in FIG. 1 when the safety valve 14 is closed. To open the safety valve 14 the pressure in operating control line is raised. This opens the safety valve as described above and displaces hydraulic fluid into lines 44 and 48 causing the floating piston 18 to move up as shown in FIG. 2. Note that the volume of chamber 16 has increased in FIG. 2 as compared to FIG. 1. When this happens there is no flow in line 46 because the head 50 is against seat 52. Upward movement of the floating piston 18 displaces fluid into lines 58 and 60. There is no flow in line 56 as the path of least resistance is into the balance line 60. This is because when the pressure is raised in operating control line 12 it is also applied at 36 to push up on the equalizing valve member 38 and displaced fluid from valve 14 through lines 44 and 46 adds to the force to hold the head 50 against the seat 52.