Subsea actuator assemblies and methods for extending the water depth capabilities of subsea actuator assemblies

A hydraulic pressure compensation system for valve actuator assemblies is described having particular application for subsea wellhead installations. The compensation system includes at least one valve actuator assembly having a housing that retains a reciprocable piston therewithin. The piston is spring biased into its fail safe configuration. The valve actuator assembly is hydraulically associated with an accumulator reservoir that defines a closed fluid reservoir and an open fluid reservoir that is exposed to ambient pressures. The two chambers are separated by a membrane. The valve actuator assembly is also operationally associated with a fluid pressure intensifier that boosts the ambient pressure of the accumulator so that an increased fluid pressure may be transmitted to the actuator assembly to bias the actuated valve toward its fail safe configuration. In a described embodiment, the fluid pressure intensifier comprises a housing that defines a chamber having a fluid inlet and fluid outlet. A dual-headed piston is moveably retained within the housing. The piston has an enlarged piston face and a reduced size piston face. Fluid pressure entering the fluid inlet is exerted upon the enlarged piston face, and due to the difference of piston face sizes, an increased pressure is transmitted out of the fluid outlet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to actuator assemblies for the selective actuation of valves. In particular aspects, the invention relates to improved hydraulic pressure arrangements and fail safe systems for use in such assemblies.

2. Description of the Related Art

Gate valves and other sliding stem-type valves operate by selectively inserting a reciprocable stem into the flow of fluid to stop the flow when desired. Such valves assemblies are often used with subsea wellheads in order to control the flow of oil or gas from the wellhead. Conventional subsea actuator assemblies are used to selectively open and close valves in subsea Christmas trees, manifolds and other assemblies. Examples of such actuator assemblies are described in U.S. Pat. Nos. 4,311,297 and 4,650,151.

Subsea environments create special problems for the operation of such valves. In deep water production systems it is essential that the valves be made insensitive to ambient hydrostatic pressures. In other words, the operation of the valves should not be affected appreciably by the surrounding water pressure. Additionally, it is important that the valves incorporate a fail-safe feature that is intended to maintain the valve in a closed (or, if appropriate, open) position in the event of a loss of control pressure. In conventional designs, mechanical springs are used to bias the stem into the desired closed (or open) configuration. Such designs are often quite effective at shallow depths. However, difficulties arise when they are used at greater depths. Special problems are created by placement of wellheads in deep waters. The greater the water depth, the greater the spring force required to counteract the effects of hydrostatic head pressure on an unbalanced stem area. American Standard API 17D requires that this factor be taken into consideration when specifying the unit depth rating for a valve assembly. Other constraints, particularly those relating to the size and weight of subsea assemblies make it increasingly problematic to simply increase the mechanical spring force for greater depths.

Use of actuator assemblies that are totally sealed, i.e., the stem is sealed from hydraulic pressure, solves the problems of insensitivity and providing an adequate bias force upon the stem. However, the existence of such assemblies is not a complete solution. Completely sealed assemblies create problems when requirements for an independent rotary or linear override mechanisms are specified for the wellhead. In addition, completely sealed assemblies make provision for position indication difficult.

Improvements to the systems of the prior art would be desirable.

SUMMARY OF THE INVENTION

The invention provides an improved hydraulic pressure compensation system for valve actuator assemblies. The system of the present invention has particular application for subsea wellhead installations. The improved compensation system includes at least one valve actuator assembly having a housing that retains a reciprocable piston therewithin. The piston is spring biased into its fail safe configuration. The valve actuator assembly is hydraulically associated with an accumulator reservoir that defines a closed fluid reservoir and an open fluid reservoir that is exposed to ambient pressures. The two chambers are separated by a membrane. The valve actuator assembly is also operationally associated with a fluid pressure intensifier that boosts the ambient pressure of the accumulator so that an increased fluid pressure may be transmitted to the actuator assembly to bias the actuated valve toward its fail safe configuration. In a described embodiment, the fluid pressure intensifier comprises a housing that defines a chamber having a fluid inlet and fluid outlet. A dual-headed piston is moveably retained within the housing. The piston has an enlarged piston face and a reduced size piston face. Fluid pressure entering the fluid inlet is exerted upon the enlarged piston face, and due to the difference of piston face sizes, an increased pressure is transmitted out of the fluid outlet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1depicts, in schematic fashion, an exemplary hydraulic pressure compensation system10for a plurality of subsea actuator assemblies12,14and16. The assemblies12,14and16each include an outer, generally cylindrical housing18with a piston20that is moveably disposed therein. A single exemplary actuator assembly12is shown in side cross-section inFIG. 3. The piston20features a piston head22with a stem24that, when moved axially, actuates a valve (not shown). A compressible spring28is used to bias each of the pistons20into a “fail-safe” closed (or open) position within its housing18. Although not shown inFIG. 1, an opposite end of stem24is exposed to sea water. This exposure creates a force equal to the hydrostatic pressure of the sea water times the pressure area of the stem. The force on stem24is opposite to the force exerted by spring28. AsFIG. 1shows, dedicated hydraulic power is provided to each of the actuator assemblies12,14and16and, when used to energize the actuator assemblies so as to compress the spring28, will open or close the valve associated with the energizer. The bias of the springs28upon the pistons20toward a closed position ensures that during a loss of hydraulic power from the dedicated power sources the valves will move to a fail safe position.

The system10includes a transfer barrier accumulator reservoir29that is interconnected in parallel via hydraulic piping, or conduits30to each of the actuator assemblies12,14,16. The reservoir29encloses a flexible membrane32that defines a closed fluid chamber34within the reservoir29. An open fluid chamber36is defined within the reservoir29and has a filtered opening38to the sea. The opening38allows the fluid chambers34,36to be exposed to ambient pressure. The fluid in the closed fluid chamber34is generally either hydrocarbon-based or a water glycol with corrosion inhibitors, depending upon the fluid used in the power side of the actuators12,14, and16. The membrane32transfers the hydrostatic head pressure present in the open fluid chamber36to the pressure compensation system10. The filling of the compensation system10with fluid is such that, as the actuators12,14,16are powered forward, there is sufficient volume for fluid displaced from the piston chambers to enter the transfer barrier accumulator.

The hydraulic piping arrangement30includes a fill point isolation valve40with a blanking plug42. These components are used to fill the compensation system10with an appropriate fluid during assembly of the system and prior to its deployment on the sea floor. A relief fitting44is also incorporated into the piping arrangement30. The relief fitting44is a relief valve that is biased into a closed position by a spring. Excessive fluid pressure, of the type that might damage the piping arrangement30is bled out through the relief fitting44.

A fluid pressure intensifier46is disposed within the piping assembly30between the reservoir29and the actuator assemblies12,14,16. The structure of an exemplary pressure intensifier46is illustrated inFIG. 2. As seen there, the intensifier46includes an outer, fluid tight housing48having a fluid inlet50at one end and a fluid outlet52at the opposite end. The fluid inlet50extends from the accumulator29to the intensifier46. The fluid outlet52leads toward the actuator assemblies12,14,16. The housing48has an enlarged diameter chamber section54and a reduced diameter chamber section56, each being filled with hydraulic fluid. A dual-headed piston58is moveably retained within the housing48so that an enlarged piston face60is presented within the enlarged chamber section54and a reduced-size piston face62is presented within the reduced diameter chamber section56. The ratio of sizes of area as between the enlarged piston face60and the reduced size piston face62may be tailored to the applicable water depth requirements for the system10taking due cognizance of any structural limitations (should the system be employed on existing hardware.)

The intensifier46receives fluid pressure from the fluid inlet50and transmits an increased fluid pressure into fluid outlet52via the difference in piston head area between the enlarged piston face60and the smaller face62. As a result, the ambient pressure of the accumulator29is boosted via the intensifier46so that a higher amount of pressure acting on the actuator piston area creates an additional load to augment the available spring load. The force of the spring and the boosted pressure cause the assemblies to move to their fail safe closed positions when the hydraulic pressure holding the piston in the opposite direction is removed. Thus, the assemblies12,14,16and system10are usable at greater depths than previous systems.

Referring toFIG. 3, a sleeve64depends from and moves with piston20, sleeve64having a passage to which a stem24(not shown inFIG. 3) is rigidly secured. Piston20has a seal65that slidingly engages a stationary tube66. Fluid from a port68flows into a chamber70on a first side of piston head22to move piston20in a first direction, which is downward as shown inFIG. 3. Actuator12is bolted to a valve (not shown) and has a sealed chamber72enclosing spring28and piston20. A port74applies and exhausts fluid from chamber72. Conduit52(FIG. 1) is in fluid communication with chamber72. The increased pressure over hydrostatic pressure in chamber72acts against the second side of piston head22, tending to move it upward (as shown inFIG. 3) relative to fixed tube66. Stem24(FIG. 1) is exposed to hydrostatic pressure through the open upper end of actuator12, the hydrostatic pressure applying a downward force on piston20. The increased pressure in chamber72counters this downward force, so that if the pressure in chamber70drops to ambient hydrostatic pressure, spring28is assisted by the higher pressure in chamber72to move the valve against the stem force to a fail safe portion.

The systems and methods of the present invention are advantageous since they allow for the retention of standard override and position indicator mechanisms. Additionally, they provide for reliable failsafe closure for actuated valves.

Those of skill in the art will recognize that many modifications and alterations of the described embodiment may be made. It is, therefore, intended that all equivalent modifications and variations fall within the spirit and scope of the present invention as claimed.