Patent Application
20090032237
The field of this invention is subsurface safety valves for downhole use and more particularly operating systems for the flapper that employ one or more closure springs made of a shape memory alloy.
Subsurface safety valves are emergency devices that shut in a well. They are typically an integrated portion of a production string and are actuated through one or more control lines that run parallel to the production conduit in the surrounding annular space. Typically, these valves require pressure in the control line to hold the valve open and the valve closes on loss of or removal of control line pressure. These valves have a hinged valve member called a flapper that can pivot from being on a seat to define the valve closed position to being rotated off the seat to define the valve open position. Typically the control lines lead to an operating piston in the valve housing and that operating piston is linked to a flow tube that is biased by a closure spring. Applied control line pressure pushes the operating piston and takes the flow tube with it against the force of the closure spring. When the flow tube is forced down, it contacts the flapper that is then on the seat and rotates the flapper 90 degrees as it moves in front of the flapper so that flow can occur through the bore in the flow tube. The hinge for the flapper is biased by a torsion spring. When control line pressure is removed or lost, the closure spring releases its stored energy and pushes up the flow tube allowing the torsion spring to rotate the flow tube back to its seat for the valve closed position.
Springs to operate valves downhole that were made from shape memory alloys (SMA) have been suggested in U.S. Pat. Nos. 4,619,320; 5,199,497 and 6,433,991. In each instance a source of heat was provided to bring the SMA beyond the critical temperature to get it to change dimension and move a valve actuating member. These auxiliary heat sources were wire heaters, an exothermic chemical reaction or infrared or microwave energy. In each instance, the dimension change from the heating was used to have the SMA revert to a previous dimension that is usually longer to push another member.
What was not provided by the prior art and is addressed by the present invention is using a SMA for a closure spring where the material selected is such that the expected temperature of the well fluids keeps the SMA spring above its critical temperature and lets it operate in its normal fashion.
Prior to raising the SMA spring above its critical temperature, it does not operate as a conventional spring. It has no memory and produces zero spring force. Once the critical temperature is exceeded, the SMA material changes state and gains memory, producing a spring force; at which time a finite temperature range exists in which the spring force remains near constant along a finite stroke of the spring. For example, as the spring is compressed the force produced by the spring remains near constant.
This can be advantageous for the following reason: downhole tools are often designed to have a predetermined amount of spring force in its more uncompressed position. When moving parts force the spring into a more compressed position the force increases according to the spring constant. The biasing force that works against the spring to force the tool into the desired position must overcome this increased spring force. Since an SMA spring whose critical temperature has been exceeded produces a similar force at different compression lengths the force required to hold the downhole tool in a position in which the spring is further compressed relative to its initial length does not increase as the spring is compressed. This results in less force being required to hold a tool in a position in which a spring is compressed when using an SMA spring in comparison to a conventional spring, resulting in a lower opening/shifting pressure/force.
The SMA spring can be used by itself or in multiple quantities or mixed with traditional spring designs such as those made of steel. An individual spring can have a mix of materials such as SMA and steel. Auxiliary energy inputs to get above the critical temperature are not used. The well fluid temperature is high enough with the properly selected SMA to come to an equilibrium temperature above the critical SMA temperature. These and other features of the present invention will be more readily understood by those skilled in the art from a review of the description of the preferred embodiment and the associated drawing while recognizing that the claims are the full measure of the invention.
A shape memory material is used for a power spring in a downhole tool an example of which is a subsurface safety valve. The spring is sized for the force it will deliver when it goes past its transition temperature and reverts to an original shape. Opening pressures are lower because the spring force in the compressed position is lower for an SMA spring than a non-SMA spring. The trigger to cross the transition temperature comes from the expected temperature of well fluids at the mounting position of the downhole tool.
The preferred material for the power spring 26 is a shape memory alloy whose transition temperature will be exceeded by the expected well fluid temperature. The power spring 26 is reformed from its original shape before being mounted in position against the flow tube 24. When the valve S is lowered into position in the wellbore, the well fluids raise the temperature of the spring 26 above its transition temperature. That causes it to seek its original dimension and shape. At this time the SMA spring gains spring force which translates against the flow tube 24 to resist control line hydrostatic. As the valve opens the spring force remains near constant due to the properties of the SMA material. Opening pressures are lower because the spring force in the open position is lower for an SMA spring than a non-SMA spring. Since the temperature of well fluids at the expected depth is generally known within a narrow range ahead of the placement of the valve S, an appropriate material such as a shape memory alloy can be selected so that the expected well temperatures will elevate its temperature to above its transition temperature before the valve S is placed in service. In this manner, there doesn't need to be any artificial stimulus built into the tool that can malfunction. The temperature of well fluids is a heat source that is ever present and needs no equipment to enable using it. In this manner the cost of a downhole tool can be reduced and its reliability enhanced. The spring or other force storing component can be made smaller for the same ultimate output force as a steel counterpart.
While the preferred embodiment is a subsurface safety valve, other types of downhole tools are envisioned that use a force storing member to create movement in a downhole tool. Even in the context of a subsurface safety valve, the torsion spring that is part of the pivot assembly 22 can be made of a shape memory alloy that will cross its transition temperature when exposed to well fluids at its installed location. Some subsurface safety valves employ flapper equalizer valves that are spring loaded valve members in the flapper designed to be contacted by a flow tube before a flapper is pushed out of contact with its seat so as to equalize pressure across the closed flapper before an attempt is made to open it. The spring in a flapper equalizer valve can be made from a shape memory alloy and take advantage of the temperature of well fluid to release additional force so that it can be designed to be even smaller than if it were made with traditional materials such as steel. Other downhole tool applications are contemplated. Other materials that respond to thermal energy of well fluid for a boost in output force are also contemplated.
Other applications are envisioned particularly those that allow flow to reach the spring during normal operation so as to gain a greater certainty of expected well temperatures being at or above the critical temperature. Closing off flow can change well temperatures as can surface initiated operations such as injection operations. Applications such as sliding sleeves would leave a spring exposed to flowing well fluids despite the position of the valve, for example. Alternatively, a backup source of energy can be provided either independently of the SMA spring or integral to its design to ensure the availability of enough force for expected operations even if well temperatures take an excursion below the critical temperature. One example can be an auxiliary heater that can be automatically actuated on sensing of a low well fluid temperature.
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.
