With the development of ocean oil and gas industry, as well as Marine oil and gas exploration and development to the deep water area, subsea production system has become an important means of deep-water exploration mode. In the early days, the main form of underwater valve is gate valve. But with the development of deep-water oil and gas fields and the increase of demand for large aperture and high pressure valve, ball valve is more and more widely used in deep sea environment. This article introduces an underwater ball valve with underwater vehicle (ROV) interface of single-acting hydraulic actuators. 

Operating Principle 
Under normal conditions, the valve is closed by the actuators that driven by hydraulic cylinder, opened by reset spring. When debugging hydraulic source fault, it is driven by the ROV interface. Underwater ball valve actuator is comprised of hydraulic drive mechanism, spring reset mechanism, ROV drive mechanism, pressure compensator and position indicator (figure 1).

 
 

Each actuator cavity has the pressure compensator. Compensator uses metal diaphragm box structure. One side of metal diaphragm is connected with the sea; the other side is connected actuators each cavity. Due to actuator cavity is filled with the medium, when there is water pressure, metal diaphragm will occur elastic deformation. The actuator cavity pressure in the body will be equal to water pressure, which can adopts thinner shell structure, but also can simplify the structure of seal. 

In order to satisfy the demand of the ROV operation, the actuator is equipped with a visual position indicator, through the ROV underwater cameras to observe mechanism position for corresponding operation. Ball valve position indicator is set at the top of the actuator, directly connected to the output shaft, displaying output shaft rotation Angle. 

Underwater ball valve actuators adopts single-acting hydraulic cylinders, relying on the fluid pressure and spring force driving reciprocating movement respectively, to realize the opening/closing function. Actuator has two kinds of work modes: hydraulic control and ROV control. 

When hydraulic operated valve is open, the hydraulic oil through tubing into the oil chamber of the actuator, forces the pistons to move to the left, the driving rod connected with piston and the rack moves follow them. Rack rotates ball valve stem by the gears on the stem, driving the ball valve closed. In the course of the piston moving left, spring is compressed, limiting the biggest range of the actuator by the left screw of hydraulic cylinder. When need to open the valve, removing the oil hydraulic oil pressure in the cavity, the elastic of spring of can overcome opening force of the valve and friction force between piston and cylinder wall. Then, it makes the piston move to the right, until the spring restores the initial state, the valve is fully open. 

When ROV operated valve is closed, first of all, the ROV mechanical arm is connected with underwater ROV actuator interface. Under the action of the ROV mechanical arm clockwise torque, the ROV joint through the gear and shaft sleeve drives screw rotation. Under the effect of driving nut, screw moves to left and promote rack moves to the left. Rack drives valve stem through gears on the ball valve stem, driving the ball valve closed. When the ROV operated valve is open, the ROV mechanical arm rotates counterclockwise. Then, screw moves to the right. Under the circumstance of the restoring force of spring, rack drives the valve stem to rotate, implementing valve open. 

Conclusion 
The design focuses on the calculation of hydraulic control end and the ROV control end, determining the main design parameters and checking on the screw intensity and the transmission ratio. It is proved to be a feasible solution of underwater ball valve with hydraulic actuators.