Patent Application
20080314599
The field of this invention is control systems for downhole tools and more specifically control systems that operate through a control line running parallel to a tubular string where the downhole tool is a subsurface safety valve.
Subsurface safety valves (SSV) are employed in most wells to prevent uncontrolled flow to the surface. They generally comprise a valve member, known as a flapper, which pivots under the assistance of a torsion spring mounted to the pivot shaft for the flapper. The flapper is urged into the open position by the movement of a hollow tube, known as a flow tube, against it. The downward flow tube movement rotates the flapper against the torsion spring and turns the flapper 90 degrees as the flow tube advances downhole in front of the flapper. In this position, the SSV is open and flow toward the surface is enabled. Normally, the flow tube is moved against the bias of a closure spring to get the flapper to the open position. A control line from the surface that runs parallel to the string and in the surrounding annular space is the source of hydraulic pressure to an operating piston which is operatively engaged to the flow tube. When pressure in the control line is applied the operating piston is urged to move and takes the flow tube with it against the bias of the closure spring as the flapper is pushed into the open position behind the flow tube. If the control line pressure is released the stored energy in the closure spring biases the flow tube away from the flapper to allow the pivot torsion spring on the flapper to move it against a seat to close off flow from downhole. This is the basic flapper valve operation.
Over the years, various developments in control systems have been developed. Some are focused on making sure the flapper closes in the event there is a seal failure in one or more of the system seals or even ruptures of the tubular string and the associated control line. One example of this is U.S. Pat. No. 6,866,101 taking
Other designs have communicated the back side of an operating piston to the annulus as shown in U.S. Pat. No. 6,988,556. However, this feature was in context of isolating the operating piston totally from tubing pressure and transmitting force to a flow tube from an operating piston so isolated by magnetic force. Doing this required two closure springs to account for issues that could arise unique to the use of magnetic force to drive a flow tube from an operating piston.
What is needed and provided by the present invention is a control system featuring an operating piston that is exposed to tubing pressure and in pressure balance from the tubing pressure. The operating piston has a reference pressure on the other side of the operating piston to the annular space preferably through a port communicating the annulus to a chamber in the tool housing in fluid communication with the back side of the operating piston. These and other aspects 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 drawings while recognizing that the full scope of the invention is defined by the appended claims.
A control system for a subsurface safety valve features a control line from the surface leading to one side of an operating piston. The operating piston is exposed to tubing pressure and is in pressure balance from tubing pressure. The opposite end of the operating piston is exposed to a chamber in the housing of the subsurface safety valve and that chamber is open through a port to the surrounding annulus. The operating piston is coupled to the flow tube and a power spring so that pressure in the control line shifts the operating piston and the flow tube against the power spring to rotate the flapper open. Provisions for fail closed mode if certain seals fail are also made through a passage extending from between seals near the upper end of the operating piston to the chamber open to the annulus.
A control line 1 is connected to the housing 12 of a subsurface safety valve S shown in half section. As is typical for subsurface safety valves (SSV) there is a flow tube 10 whose downward movement opens a flapper 8. Downward movement of the flow tube 10 compresses a power spring 9. As long as there is pressure in chamber 2 from control line 1 the operating piston 14 will be displaced down from the position shown in
Seals 4 and 6 surround operating piston 14 and are preferably of the same size or nearly the same size so as to put the piston 14 in pressure balance from tubing pressure found in zone 16 within the valve housing 12. Seal 6 separates annulus pressure in chamber 7 from tubing pressure in zone 16. Chamber 7 has a port 18 that is open to the surrounding annulus 20. Seal 3 separates pressure in control line 1 from the annulus 20 pressure found between seals 3 and 4. Annulus 20 pressure gets between seals 3 and 4 through passage 22 that extends from chamber 7 through the piston 14 and out the side of piston 14 between seals 3 and 4.
If seal 3 fails, pressure in control line 1 will dissipate into the annulus 20 through passage 22, chamber 7 and port 18 depending on the size of port 18. If the port 18 is fairly small and the leak past seal 3 is fairly significant, then the pressure in control line 1 will equalize with chamber 7 on the other side of the piston 14 and that will allow the power spring 9 to push up the flow tube 10 to let the flapper 8 go to the closed position. If the control line 1 pressure falls, because of a leak in seal 3, to the point that it close to matches the pressure in annulus 20, then the power spring will push the flow tube 10 up to let the valve S close because there would not be any net force on the operating piston 14. If annulus pressure 20 is higher than control line pressure 2, failure of seal 3 will cause pressures in chambers 7 and 2 to equalize and valve will close.
If seal 6 fails, the tubing pressure at 16 will communicate with the annulus 20. If this happens, there will be pressure balance between seals 4 and 6 due to passage 22. From the tubing side there will be an upward force on piston 14 at seal 3 to force piston 14 up to close the flapper 8. However, depending on the size of the leak and the size of port 18 and the pressure difference between tubing pressure at 16 and annulus pressure 20 it is possible to pressurize chamber 7 with tubing pressure leaking into it to push up the piston 14 so that the flapper 8 closes. The surface signal of this condition could be an increasing annulus pressure 20 pressure over time and the control pressure to hold the valve open will increase. On the other hand, if the pressure in the annulus 20 is higher than tubing pressure at 16 when seal 6 fails, the flapper 8 will stay open as long as pressure is maintained in the control line 1. Over time a pressure loss in the annulus 20 may serve as an indicator that seal 6 has failed causing the ESD system to remove pressure from the safety valve thus closing the flapper 8.
If seal 4 fails then seals 3 and 6 will be in pressure balance from the tubing side. Similarly if seal 6 fails, seals 3 and 4 will be in pressure balance from the tubing side due to line 22. Depending on the size of port 18 and whether the pressure in the annulus 20 is higher or lower than tubing pressure at 16, there could be different outcomes. If tubing pressure at 16 exceeds annulus pressure at 20 and port 18 acts as a restriction, then pressure will build up in chamber 7 and create a net upward force on piston 14 to close the valve S. If port 18 is not restrictive under the previous scenario, tubing pressure will escape to the annulus 20 and slowly build its pressure to a point where chamber 7 will rise in pressure and piston 14 will be pushed up with that pressure and power spring 9 upon removal of control pressure. On the other hand, if annulus 20 pressure exceeds the tubing pressure at 16 when seal 4 fails and the valve S is open, the valve will stay open and the annulus 20 will depressurize at some rate into the valve S through line 22 until that condition is sensed within the ESD system. At that point, the ESD system will release control pressure thereby closing the flapper 8.
Those skilled in the art will appreciate the simplicity of the design which allows elimination of a balance line to the surface but still allows taking advantage of the annulus pressure to act in tandem with power spring 9 to urge the piston 14 to a closed position. Depending on the application it may still be advantageous to size the spring 9 to handle the full hydrostatic pressure in control line 1 in the event that annulus pressure 20 gets reduced by high pressure gas within the tubing 16 leaking into the annulus 20 through seals 4 or 6 and temporarily decreasing annulus pressure 20 (by replacing annulus liquid with gas, for example) that may be needed to move the piston 14 along with the force from spring 9. Alternatively, if annulus pressure change is noticed at the surface, the pressure in control line 1 can be removed to let the valve S close even under a leak condition as explained above. Accordingly, the spring 9 can also be sized to offset the net difference in control line 1 hydrostatic and annulus 20 hydrostatic and made a bit stronger to account for seal friction acting on piston 14. Taking annulus pressure into account, it is possible to design the spring 9 to move the piston 14 to the closed position of valve S with a force approximating the hydrostatic pressure less the expected annulus pressure.
This design presents a piston 14 that operates in pressure balance from tubing pressure at 16 while eliminating the expense and risk of a second balance control line. In some applications, there may not be room in the wellbore for such a second control line.
Redundant systems can be provided that involve the described control system where the pistons are each linked to a flow tube with each system, when placed in service operating as described above. The redundant system not in use can be isolated to shield the hydrostatic pressure in that control line from adding to the force the closure spring needs to overcome to get the flow tube to move up.
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.
