Valves and valve actuation – Electrically actuated valve – Remote or follow-up control system for electrical actuator
Reexamination Certificate
2001-05-16
2003-07-22
Gartenberg, Ehud (Department: 3754)
Valves and valve actuation
Electrically actuated valve
Remote or follow-up control system for electrical actuator
Reexamination Certificate
active
06595487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a system for the control of valves in a subsea environment. More particularly, this invention relates to a system for actuating valves in oil and gas production wells and methods for controlling actuation of such valves.
2. The Prior Art
The control of subsea valves constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved as well as the risks associated with environmental and safety issues. As the exploration for oil and gas moves into ever deeper waters, the problems (and expense) associated with such activities gets ever larger.
Before describing the current state-of-the-art relative to such valve control systems and methods, a brief description will be made of the production system, per se, in need of control. Oil and gas that are produced in a well flows upward through the well tubing to a wellhead placed on the seabed. A so-called Christmas Tree is connected to the wellhead and contains a number of valves for regulating the produced fluids. These valves must be of a size to be able to contain the, often large, pressures in the well. In addition, a control module is usually provided, containing also a number of pilot valves, some for controlling the main Christmas Tree valves, but also valves for injection of chemical fluids into the well.
Known systems used for controlling the opening and closing of both the main gate valves in the Christmas Tree and others, for example injection valves, are either hydraulic, pneumatic or electro-hydraulic actuators, but the most common is the hydraulic type. These actuators generally utilize an externally controlled flow of hydraulic fluid, under pressure, to drive a piston within an actuator cylinder. This cylinder compresses a spring stack and moves the valve stem to open or close the valve depending on its configuration. In an emergency situation when the valve must be closed quickly, the spring force is used to achieve this. In a subsea environment, it is mandatory to provide the actuators with such fail-safe close configuration.
The actuators may be driven directly, that is being supplied with fluid through a line extending to the surface, or from an accumulator located on the seabed at or near the well. In the latter case, pilot valves are arranged to control the supply of fluid to the actuators. Pilot valves can be controlled hydraulically, that is with a secondary supply line from the surface, or more often electrically and is therefore called an electro-hydraulic system.
However, hydraulic actuators have a number of operational disadvantages that would make the actuator unusable for some subsea applications. For example, as the distance between the hydraulic actuator and the topside control center increases, the amount of hydraulic fluid needed to fill the lines and operate the valves also increases. Over long distances the fluid can lose pressure and therefore accumulators have to be installed close to the actuator to back up the pressure. Within some existing systems the time between a signal being sent from a control center and the actuators responding to the signal can be a matter of minutes, which in emergency situations can be too long. Furthermore, when the distance between the two sites reaches an optimum length, the hydraulic actuator will not be able to operate at all. An increase in water depth can also affect the actuator performance. The cost and size of the hydraulic lines and umbilicals also greatly increase as the operational depths and distances compound. It has therefore become apparent that a new design of actuator control is required that will not be affected by depth/distance and that will have a fast response time, and it has been decided that an all-electric valve actuator will meet these requirements.
U.S. Pat. No. 5,497,672 to Appleford et al describe a valve actuator comprising a stem extending through the actuator for coupling to the valve such that axial movement of the stem opens and closes the valve, an outer shaft arranged coaxially around the stem and in threaded engagement therewith and means coupling the shaft to an electric motor to cause rotational and thus axial movement of the shaft relative to the stem against a spring bias. The shaft is releasable retained in a predetermined position when the valve is open with the spring held in compressed state by a solenoid-actuated latching mechanism. De-activation of the solenoid mechanism, for example by a loss of electric power, releases the shaft to allow axial movement of both the shaft and stem, under action of the spring, in order to close the valve.
The above mentioned actuator needs the spring for providing the required fail safe characteristics. It is therefore not a true all electric valve actuator. Since the valves are designed so that the actuators must overcome the pressure differential between the pressure in the well and the ambient (hydrostatic) pressure, in order to open or close the valve, this spring is considered necessary to be able to close the valve at the loss of electric power. If the well pressure is higher than the ambient pressure, the return spring must be designed large enough to be able to keep the valve shut. Contraversive, if the ambient pressure is larger than the well pressure, the driving fluid pressure must be larger. At greater depths, the hydrostatic pressure will normally be several orders of magnitude higher than the well pressure and the driving fluid must not only be able to move the actuator against the return spring but also overcome the pressure caused by the water depth.
When actuators of this type has found little application for subsea use it is because of the many disadvantages found. The size of the return spring, which as mentioned above is dependent upon well pressure and ambient pressure, results in that the actuator being of the same magnitude of size as in a conventional hydraulic actuator and neither weight nor space is reduced. The latch mechanism is less operational safe and even short-time power losses will make the actuator close the valve.
U.S. Pat. No. 5,519,295 to Jatnieks describes an electric actuator comprising a stem extending through the actuator for coupling to a component such as a valve such that axial movement of the stem opens and closes the valve, an outer shaft in threaded engagement therewith via a gear train and means coupling the shaft to an electric motor to cause rotational and thus axial movement of the shaft relative to the stem. The actuator has a controller having input power connected across a capacitor and control terminals for controlling the power applied from the input power terminals to the motor which comprises the actuator's prime mover. Electric energy is stored in the capacitor and a sensor monitors the presence of power provided to the controller's input power terminals When failure of this power is sensed, the capacitor current flows to the controller, which drives the actuator to toward a the preselected (normally closing) position.
This design, however, is not very useful in a subsea environment, since a capacitor is not reliable in seawater and would need costly shielding to avoid short-circuiting, which means that it must be housed in a one atmosphere container.
EPO patent application no. 984,133 to Cooper Cameron Corp. describes a valve actuating module in a subsea environment having a plurality of valve actuators for operating respective valve elements. The module is arranged to receive primary power and control signals from an external source in order to operate the valves. The necessary failsafe condition is achieved by providing the motor with a local back-up power source which, in the event of loss of primary power, is sufficient to drive the motor to return the valve to a safe position, the back-up power source being a battery which is kept charged by the primary power source through a umbilical.
Note may also be made to the article <<Subsea Electric Valve Actuator>>, by Sigbjrn
Almedal Roy
Johansen John A.
Magnus Heyn Halfdan
Cartagena Melvin
Gartenberg Ehud
Kongsberg Offshore A/S
Young & Thompson
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