GB2283061A - Actuators for operating below the surface of a liquid - Google Patents

Abstract

In actuators for operating valves in undersea equipment it is desirable that the actuator is able to move to a failed-safe position. In accordance with the invention, an actuator for a valve 13 includes a first space 16 communicating with the surrounding water and a second space 14, separated from the first by a diaphragm 17 and containing air at or near atmospheric pressure. The valve is actuated by a spindle 12 connected to a rack 11 iself engaging a pinion 5. The pinion is connected to a motor 3 via pinion 4, gear wheel 2, and dog clutch 6. The dog clutch is held in the clutched position by a lever 8 operated by a solenoid 9 so that normally the valve is held in the open position. When the electrical supply fails and the solenoid ceases to operate, the force acting on the spindle stops and the valve is moved to the closed position by the upward movement of the diaphragm under the imbalance of forces acting on it. <IMAGE>

Description

ACTUATORS FOR OPERATING BELOW THE SURFACE OF A LIQUID This invention relates to actuators used for operating equipment which is situated beneath the surface of a liquid and, in particular, to the operation of fluid valves in undersea equipment used for oil and gas drilling, exploration and recovery operations, the equipment normally being housed in pods at the operating level. Such actuators may be operated by electric motors, or by hydraulic or pneumatic means, in most cases the power supply being installed on the surface and the energy being conveyed to the actuator, along with the command and monitoring signals via electric cables and/or hoses conveying the operating oil or gas.

In many cases it is desirable to provide a fail-safe function such that, in the event of a failure of the normal power supply, the actuator will still be able to move the actuating member into a failed-safe state. Such an operation can be automatically triggered by the failure of the main energy supply or may be operated at will, using a small auxiliary energy supply and switch.

In other cases, where a mechanical or electrical failure of the actuator can result in the spindle of the valve being mechanically locked into a random position between the open and closed valve states, it is necessary to provide an over-ride mechanism, with a latch, such that stored energy can be released to extend, or contract, or rotate the valve spindle so that the valve can be brought into a failed-safe state.

The foregoing fail-safe operations are usually accomplished by the use of compression springs, so designed to urge the valve into it's safe condition: the spring can either be allowed to expand or contract under the normal operating condition, or can be latched into a compressed position providing an energy store, releasable when a failure of the actuator or it's normal energy supply has occured.

In deep undersea operations the hydrostatic pressure created by the surrounding water is considerable, requiring substantial walls on the equipment and minimum equipment size or, alternatively, the filling of the internal spaces of the equipment with a liquid which is in contact with the surrounding water via a pressure balancing diaphragm. As an example of the pressure rise on a body submerged in sea water, the pressure increases by about 1 bar (0.1 N/sp. mm.) for every 10 metres submerged below the free surface.

Existing designs of fail-safe actuators present problems in requiring substantial sizes of springs (or other equivalent energy storage devices) which take up large amounts of room in the undersea equipment pods, and in requiring special handling during equipment build and service in order to deal with the large amounts of stored energy in a safe manner.

An object of the invention is to provide a fail-safe actuator which overcomes these problems.

According to this invention an actuator for use under the surface of a body of liquid (e.g. a liquid such as water open to the atmosphere, or liquid in contact with a gas within a closed volume), is characterised in that said actuator defines a first space adapted to communicate with the surrounding liquid to produce a first pressure equal to the hydrostatic pressure of the surrounding liquid, and a second space sealed from said surrounding liquid and providing a second pressure different from the first pressure, in that actuating means are provided between the two spaces which are held against a force produced by the pressure differential of said spaces in a normal operating position, and in that release means are provided for releasing the actuating means and hence cause actuating movement of the latter.

Preferably the actuating means comprises a diaphragm, piston, or rotatable valve.

Thus, in accordance with this invention, the fail-safe springs normally provided are replaced, in whole or in part, by utilising the energy available in the surrounding liquid by virtue of the hydrostatic pressure exerted by the liquid on the actuating means.

In one preferred form of the invention, in which the actuator is an electric actuator having an output shaft connected to the actuating means, said first and second spaces surround the output shaft and the actuating means comprises a diaphragm or piston surrounding and attached to the output shaft having one side in communication with the second space (which may contain air or gas at atmospheric pressure), the other side being in contact with the surrounding liquid so that when the actuator is submerged a pressure difference is created across the diaphragm or piston. For any given submerged depth of the actuator this pressure difference will be substantially constant: there will, however, be a small reduction in the difference as the output shaft moves in the direction to allow the diaphragm or piston to compress the air or other gas in said second space.Thus, the diaphragm or piston will exert a permanent force on the output shaft due to the pressure difference acting over the diaphragm or piston area, this force replacing the fail-safe return spring normally installed in an actuator.

Such an electric actuator is particularly appropriate for use with an axially operating valve, in which case said output shaft would be connected to the valve spindle.

However, it will be appreciated that the actuator could also be used with a rotary valve in which case the output shaft could be connected to the rotary spindle via a rack and pinion, or a rod and lever in such a way that reciprocal movement of the output shaft is translated into rotary movement of the spindle, or the spindle may be provided with a vane and vane seals, the vane having air or another gas on one side and liquid on the other side, without departing from the scope of this invention.

It will also be appreciated that, particularly in the case where the actuator is to be used under the sea, it may be necessary to provide a second diaphragm and hydraulic buffer fluid between the sea water and the aforementioned operating diaphragm or piston in order to protect the working surfaces of the actuator from the corrosion and marine growth and debris contained in the sea water, again without departing from the scope of this invention.

In another preferred form of the invention, where the actuator is to be used in conjunction with a valve, said second space is normally maintained at, or near, atmospheric pressure and is provided with a small pilot valve, connecting said space to the surrounding liquid.

The output shaft of the actuator is telescopic and operation of the pilot valve allows liquid to enter the space thereby causing the output shaft to move the valve spindle to the fail-safe position. This provides a single fail-safe operation; for repeated fail-safe operations means may be provided for pumping out the liquid from said second space and re-closing the pilot valve. A spring may be provided in the telescopic section of the output shaft in order to urge the spindle towards one or other of the travel limits provided by the telescopic mechanism.

The use of hydrostatic pressure of the surrounding liquid to provide the fail-safe energy, in accordance with the invention, can also be applied to hydraulic and pneumatic actuators, in which case the said second space is filled with hydraulic fluid at an appropriate pressure, the said first space again being in communication with the surrounding liquid and the actuating means between said spaces comprises a piston. A pump and valve 6 arranged to either release the hydrostatic energy in order to operate the piston, or to allow the piston to return to a state opposite to the fail-safe state of the actuator, would also be provided.

The hydrostatic fluid, preferably of a specific gravity approximately the same as the surrounding liquid, may be conveyed to the actuator when in situ by a flexible hose, in which case a header tank may be provided at the surface end of the hose so that there is no significant pressure difference between the liquid in the hose and the surrounding liquid. This embodiment may be necessary in sea applications, where the sea water must not come into contact with any moving parts of the actuator or valve, and is an alternative to the aforementioned second diaphragm and buffer hydraulic fluid.The header tank may be an enclosed tank and gas pressure may be applied to the surface of the liquid in the tank for adjusting the pressure acting on the actuating means of the submerged actuator The preferred embodiments of the invention will now be described in more detail with reference to the accompanying drawings in which: Fig. 1 is a diagram of a first embodiment illustrating an electric actuator and valve assembly, Fig. 2 is a diagram illustrating a second embodiment of an electric actuator and valve assembly, Figs. 3A, B and C are diagrams illustrating three states of a third embodiment of a hydraulically operated valve actuator particularly for underseas applications, and Fig. 4 is a diagram illustrating a modified form of the actuator illustrated in Figs. 3A to C.

With reference to Fig. 1 the electric actuator comprises an electric motor 1 which drives a gear wheel 2 via a reduction gear box 3 and pinion 4. The gear wheel 2 drives a pinion 5 via a dog clutch 6, the pinion 5 being slidably mounted on a shaft 7 so that the clutch 6 may be engaged or disengaged by operation of a cranked lever 8 and solenoid 9. The lever 8 is urged towards the de-clutched position by a spring 10. The pinion 5 drives a rack 11 constituting the output shaft of the actuator, the lower end of which constitutes a spindle 12 for a valve 13.

A space 14 (said second space), containing the rack and pinion gear and clutch is separated from the surrounding water by a water-tight housing 15. The space 14 contains air at, or near, atmospheric pressure. A lower space 16 (said first space) is separated from the upper space 14 by a flexible diaphragm 17: the centre region of the diaphragm is attached to the valve spindle assembly 11 and 12. The lower space 16 is in contact with the surrounding sea water via a duct 18. Thus, the hydrostatic pressure of the sea water exerts a constant upward pressure on the diaphragm 17 which, when multiplied by the effective area of the diaphragm, becomes an upward force on the shaft/spindle assembly 11 and 12.

For normal operation of the valve 13 the actuator is held in the clutched mode, the force generated by the rack 11 being sufficient to overcome the opposing force produced by the pressure differential across the diaphragm 17. In the event of a failure of the electric supply to the actuator and the solenoid 9, the solenoid is de-energised, releasing the clutch 6 and so allowing the shaft/spindle assembly to rise and close the valve 13.

The solenoid 9 may also be provided with a separate switch so that the valve 13 may be rapidly closed under hydrostatic sea water pressure irrespective of the state of the actuator.

With reference to Fig. 2, this embodiment is similar to Fig. 1, but the output shaft 11 is additionally provided with a telescopic section 19 and 20 which is held in it's extended position by a spring 21. The lower space 16 is initially filled with air at or near atmospheric pressure, the surrounding sea water being excluded from this space by a valve 22 which is normally in the shut position.

In the event of a failure of the actuator, or it's energy supply, the valve 22 can be opened by remote means, or by means associated with a fault condition in the actuator, allowing water to enter the space 16. The diaphragm now has hydrostatic sea water pressure acting on it's lower side, the resulting pressure difference between the lower and upper sides times the effective diaphragm area producing a force sufficient to overcome the spring 21, thus closing the valve 13. This fail-safe operation can be performed irrespective of the position of the actuator shaft 11.

If more than a single fail-safe operation is required, the space 16 may be provided with a second valve and a scavenge pump so that the water may be pumped out of the space 16 until the pressure differential across the diaphragm 17 has fallen sufficiently to allow the spring 21 to extend to it's normal working position.

With reference to Fig. 3A to C, this embodiment is in the form of an undersea hydraulic valve actuator. The valve 13 and actuator 23 are mounted on a plate (not shown), the valve spindle 12 being attached to a rod 24 of an hydraulic piston 25. A spring 26 holds the rod and piston in the retracted position corresponding to the valve closed state. The actuator 23 defines an enclosed space 27 (said first space) sealed from the sea water by a diaphragm 28. A pilot valve 29 is provided which can be operated by remote means.

In the position shown in Fig. 3A the ports of this pilot valve connect two spaces 30 and 31 (said second space) so that no pressure difference exists across the piston 25. In this initial state of the actuator the spaces 30, 31 are filled with air or another gas at or near atmospheric pressure. The space 27 is filled with hydraulic oil which, by virtue of the diaphragm 28, will be at the same pressure as the surrounding hydrostatic water pressure. In this state the piston 25 is held against an inner travel stop 32 by the combined force of the spring 26 and the force produced by the hydrostatic water pressure acting on the annular area formed by the difference in diameter between the rod 24 and the spindle 12. An additional closing force may also be produced by the pressure of the fluid, inside the valve body, acting on the valve stem.

Fig. 3B shows the state of the actuator at the end of the first valve opening operation. The pilot valve 29 has been operated so that the ports now connect the spaces 27 and 30. In consequence oil has flowed into the space 30, forcing the piston 25 towards the valve open position.

At the end of the operation, the net valve opening force will be equal to the piston area times the hydrostatic water pressure less the aforementioned opposing forces.

Fig. 3C shows the condition of the actuator when the pilot valve 29 has been returned to the original state as shown in Fig. 3A. This operation of the pilot valve 29 isolates the space 27 and opens space 30 to the air or other gas in space 31. The pressure on either side of the piston 25 can now equalise causing the piston to return to the inner travel stop 32 due to the force exerted by the spring 26 and the force exerted by the hydrostatic pressure on the net area of rod 24. The final state, after this second operation, is to leave a volume of oil in a sump 34 equal to the full stroke piston displacement. A positive displacement pump 33 is provided to return this volume of oil to the space 27.

The actuator will now be in the state as shown in Fig. 3A.

The pump must be capable of pumping against the full hydrostatic pressure acting on the diaphragm 28 and must be of such a type that there is no back flow when the pump is stationary.

In hydraulic installations it is sometimes necessary to separate the oil from the air (or other gas) to prevent the air from becoming entrained in the oil and forming an emulsion. There is also a certain amount of air solution in the oil which is liable to come out of solution in the low pressure regions of the system. One preferred method of preventing this inter-mixing of oil and air is shown in Fig. 4, in which a "floppy" diaphragm 35 is provided in the sump 34 and both the discharge from the pilot valve 29 and the entry to the pump 33 are positioned on the hydraulic oil side of the floppy diaphragm 35.

It will be appreciated that, provided oil and sump volumes are generous compared to the piston displacement volume, several operations of the actuator may be completed before it will be necessary to pump back the oil to the space 27 being the diaphragm 28. For this reason and for infrequent operations of the actuator, the pumping capacity provided by the pump 33 can be relatively low.

It will also be appreciated that, as the oil volume in the sump 34 builds up from successive operations, the air pressure in the space 31 will rise causing a loss of net force available to open the valve. The relationship between pressure and volume of an enclosed gas follows the general law: P X Vn = Constant. where: "P" is the absolute pressure and "V" is the contained volume.

For relatively fast compression or expansion the value of the index "n" reaches the adiabatic value which, for air, varies between 1.4 and 1.8 depending on the initial pressure and temperature. However, for the slow operations such as are usual in valve actuators, the operation will be near isothermal (i.e. no change of temperature of the air or gas) and the value of the index "n" falls to approximately unity. The law then becomes; P X V = Constant.

In a particular example, for an actuator with actuating means in the form of a diaphragm working at an undersea depth of about 1800 metres, the hydrostatic pressure on the diaphragm will be approximately 185 bar.

Assuming that the gas space 31 is at 1 bar absolute pressure (i.e. approximately sea level atmospheric) before oil enters the sump, the pressure will only rise to 2 bar absolute when the oil in the sump has taken up half the avilable total volume of the spaces 31 and 34. Even if the 2 bar absolute pressure acted over the whole of the piston area (which is not possible due to the presence of the piston rod) the percentage loss in actuating force will only be about 0.54%.

It will also be appreciated that the spring 26 may not be necessary in deep water operations where the net force produced by the hydrostatic pressure acting on the rod 24 will be sufficient to close the valve.

Referring again to Fig. 4, it is possible and within the scope of this invention, to separate the air or other gas space 31 from the sump 34 so that oil is exhausted and returned to and from a chamber (not shown) separate from the rest of the actuator.

Claims (11)

1. An actuator for use beneath the surface of a body of liquid, characterised in that said actuator defines a first space adapted to communicate with the surrounding liquid to produce a first pressure equal to the hydrostatic pressure of the surrounding liquid, and a second space sealed from said surrounding liquid and providing a second pressure different from the first pressure, in that actuating means are provided between the two spaces which are held against a force produced by the pressure differential of said spaces in a normal operating position, and in that release means are provided for releasing the actuating means and hence cause actuating movement of the latter.

2. An actuator according to Claim 1, characterised in that said actuating means comprises a diaphragm, piston or rotatable vane.

3. An actuator according to Claim 1, or 2, said actuator being in the form of an electric actuator having an output shaft connected to the actuating means, characterised in that said first and second spaces surround the output shaft and the actuating means comprises a diaphragm or piston surrounding and attached to the output shaft having one side in communication with the second space, the other side being in contact with the surrounding liquid so that when the actuator is submerged a pressure difference is created across the diaphragm or piston.

4. An actuator according to Claim 3, for undersea applications, characterised in that a second diaphragm and hydraulic buffer fluid is provided in such a way that the working surfaces of the actuator are protected from the sea water.

5. An actuator according to Claim 3 or 4, characterised in that said second space is normally maintained at or near atmospheric pressure, in that a pilot valve connects said space with the surrounding liquid, and in that said output shaft is telescopic so that operation of the pilot valve will cause liquid to enter said space and move the output shaft to a fail-safe position.

6. An actuator according to Claim 5, characterised in that means are provided for pumping out liquid from said space and means are provided for reclosing the pilot valve to enable repeated fail-safe operations to be effected.

7. An actuator according to Claim 1, said actuator being in the form of an hydraulic or pneumatic actuator, characterised in that second space is filled with hydraulic fluid at an appropriate pressure, the said first space again being in communication with the surrounding liquid, and in that said actuating means between said spaces comprises a piston.

8. An actuator according to Claim 7, characterised in that a pump and valve is provided and arranged to either release hydrostatic energy in order to operate the piston, or to allow the piston to return to, a required state.

9. An actuator according to Claim 7, or 8, characterised in that hydrostatic fluid is conveyed to the actuator when in situ by a flexible hose, and in that a header tank is provided at the surface end of the hose so that there is no significant pressure difference between the liquid in the hose and the surrounding liquid.

10. An actuator according to Claim 9, characterised in that the header tank is an enclosed tank and gas pressure is applied to the surface of the liquid in the tank for adjusting the pressure acting on the actuating means of the submerged actuator.

11. An actuator constructed, arranged and adapted to operate substantially as hereinbefore described with reference to Figure 1, Figure 2, Figures 3A to C, or Figure 4 of the accompanying drawings.