1. Field of the Invention
The invention relates generally to control valve systems and, more particularly, it concerns a control valve system for opening and closing blowout preventers.
2. Description of the Prior Art
Two major classes of blowout preventers--spherical and ram-type--are currently utilized to shut off uncontrolled flow of pressurized fluid in oil and gas wells. In operation, a spherical blowout preventer uses a working fluid which is injected on the closing side of a built-in piston to force the piston against a deformable closure element, thereby expanding the element into the flow path of the line fluid to cut off flow. The ram-type blowout preventer includes a hydraulic cylinder having a rod attached to its piston and a ram connected to the rod which acts as the closure element to close the passage of pressurized fluid. For simplicity, this application will focus on ram-type blowout preventers. It should be apparent to those skilled in the art, however, that this discussion could equally apply to spherical blowout preventers in its advantageous features.
Operator cylinders having a closed cylinder with a piston slidably mounted therein, and a rod secured to the piston and extending out of one end of the cylinder, have long been utilized as operators for blowout preventers. These types of preventers may be characterized as having a blind side of the piston and a rod side of the piston as designated by the location of the rod.
Typically, such an operator cylinder functions by injecting fluid into the cylinder on the blind side in order to move the rod to an extended position such that the rod moves the blowout preventer closure means to close off flow from the well. During the movement of the piston and rod, fluid contained on the rod side is discharged to a reservoir.
This filling of one side with new fluid and emptying the second side of old fluid demands great volumes of fluid to move the piston and rod from a fully opened position to a fully closed position. Horsepower requirements to close the piston are also significant due to the volumetric relationship between flow through the pump and the pressure of the lines. That is, the greater the volume of flow at a given pressure, the greater the installed horsepower requirements.
Many attempts have been made to reduce the horsepower and fluid requirements of an operator cylinder. In U.S. Pat. No. 3,036,807 to Lucky, a valve apparatus is provided which utilizes the downhole pressure of the line fluid created by the blowout to aid in closing the blowout preventer. The use of line fluid, however, can increase both corrosion and contamination of the valve parts, thereby shortening the useful life of the valve.
In U.S. Pat. No. 3,299,957 to O'Neil, a system is shown in FIG. 18 comprising an accumulator cylinder utilized in conjunction with a pump means. The pump means is continuously operated to effectively raise the piston and pressurize the accumulator cylinder. The system, however, allows the use of a lower horsepower input pump rather than minimizing overall horsepower requirements and fluid requirements. As in many of the other prior art devices, fluid expelled from the pistons during the lowering motion of the pistons is discharged to a reservoir each time the pistons are lowered.
Other attempts are believed to have been made to reduce the overall horsepower requirements, but these are believed to have involved costly modifications to the blowout preventer structure.
Hence, to provide an improved control valve system, it is desirable to provide a system requiring less horsepower to operate the system, while also decreasing hydraulic fluid requirements of the system.