Patent Application
20100116502
The field of this invention is control systems for downhole valves and, more particularly, for subsurface safety valves where the system is tubing pressure insensitive.
Subsurface safety valves are used in wells to close them off in the event of an uncontrolled condition to ensure the safety of surface personnel and prevent property damage and pollution. Typically these valves comprise a flapper, which is the closure element and is pivotally mounted to rotate 90 degrees between an open and a closed position. A hollow tube called a flow tube is actuated downwardly against the flapper to rotate it to a position behind the tube and off its seat. This is described as the open position. When the flow tube is retracted the flapper is urged by a spring mounted to its pivot rod to rotate to the closed position against a similarly shaped seat.
The flow tube is operated by a hydraulic control system that includes a control line from the surface to one side of a piston. Increasing pressure in the control line moves the piston in one direction and shifts the flow tube with it. This movement occurs against a closure spring that is generally sized to offset the hydrostatic pressure in the control line, friction losses on the piston seals and the weight of the components to be moved in an opposite direction to shift the flow tube up and away from the flapper so that the flapper can swing shut.
Normally, it is desirable to have the flapper go to a closed position in the event of failure modes in the hydraulic control system and during normal operation on loss or removal of control line pressure. The need to meet normal and failure mode requirements in a tubing pressure insensitive control system, particularly in a deep set safety valve application, has presented a challenge in the past. The results represent a variety of approaches that have added complexity to the design by including features to ensure the fail safe position is obtained regardless of which seals or connections fail. Some of these systems have overlays of pilot pistons and several pressurized gas reservoirs while others require multiple control lines from the surface in part to offset the pressure from control line hydrostatic pressure. Some recent examples of these efforts can be seen in U.S. Pat. Nos. 6,427,778 and 6,109,351.
Despite these efforts a tubing pressure insensitive control system for deep set safety valves that had greater simplicity, enhanced reliability and lower production cost remained a goal to be accomplished. The present invention provides a solution for this concern by isolating the control system from tubing pressure by sealing the internal passage of the valve around the flow tube. The seals are designed to be as nearly equal in dimension as possible so that internal tubing pressure provides a minimal or no net measurable force on the flow tube for the full range of expected tubing pressures. As an alternative, the operating piston of the control system can also have a portion exposed to tubing pressure with seals of equal or nearly equal diameters to get the same result of insensitivity to tubing pressure. Those skilled in the art will more readily understand the invention from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is measured by the attached claims.
A system isolates a control system for a downhole tool from the internal pressures in the tubing in which the tool is mounted. Opposed seals are used on a moving component in the tool so as to offset pressure induced forces regardless of the internal operating pressure of the tool. In a particular application to a subsurface safety valve the control system can be isolated from tubing pressure by offset seals between the passage and the flow tube or around exposed portions of the operating piston for the flow tube.
To isolate the piston 20 from pressure in passage 14 an upper seal 28 is shown in
Those skilled in the art will further appreciate that the body 30 can be configured to allow a closed chamber 34 where the spring 24 is now shown so that a two control line system can be used to offset control line hydrostatic pressure to allow using an even smaller spring 24 than can be used by isolation of the control system piston 20 from control line pressure using seals 28 and 32. Alternatively, a pressurized chamber in housing 30 can be used to offset control line hydrostatic pressure and elimination of the spring 24 in a single or dual control line system. It should be noted that chamber 34 can be at atmospheric pressure on tool assembly at the surface and that the movement of piston 20 changes the volume of chamber 34 with a slight pressure buildup that is not significant in aiding the closure spring 24 in closing the valve by moving the flow tube 16. Alternatively, chamber 34 can be initially charged with a high enough pressure on assembly that will offset hydrostatic pressure in the control line at the expected depth of use of the safety valve. Another option to offset hydrostatic on the back end of the piston 20 is to run a second control line which will offset the hydrostatic pressure in the control line going to connection 18.
While the preferred application is a subsurface safety valve other tools that have a control line system to actuate a piston to in turn move a component in a downhole tool can also benefit from sealing around the component to be ultimately operated by the piston of the control system that is in turn operated by applied control line pressure. Some examples can be other types of valves such as a ported sleeve actuated by a sliding sleeve or a ball type valve triggered remotely by surface applied hydraulic pressure, for some examples.
The above description is illustrative of the preferred embodiment and various alternatives and is not intended to embody the broadest scope of the invention, which is determined from the claims appended below, and properly given their full scope literally and equivalently.
