## 2 Research on the Configuration of Failsafe Closing and Low-Power Holding Mechanism  
Based on the function of the all-electric subsea gate valve actuator, a configuration of the all-electric subsea gate valve actuator is proposed in this paper, as shown in Figure 1, wherein the structure mainly includes the subsea gate valve assembly, failsafe closing and low-power holding mechanism, subsea ambient pressure compensating mechanism, screw drive mechanism, multi-motor redundant driving mechanism, valve position indicating mechanism and ROV interface. During the valve opening process, multi-motor redundant driving mechanism or ROV drives the valve stem and gate downward to open the subsea gate valve through the screw drive mechanism. After the subsea gate valve is fully opened, the low-power holding mechanism locks the failsafe closing mechanism to ensure that the valve remains open with the minimum power consumption. When the all-electric subsea production control system breaks down, the low-power holding mechanism and multi-motor redundant driving mechanism are power-off, the failsafe closing mechanism drives the valve stem and gate upward to realize the emergency failure close. Failsafe closing mechanisms of the subsea valve include the mechanical failsafe closing mechanism and Uninterrupted Power System (UPS) failsafe closing mechanism [29; 30]. As the service life cycle of the subsea Xmas tree or manifold is usually around 20 years, long term continuous power supply makes the UPS failsafe closing mechanism unpractical. The mechanical failsafe closing mechanism is most commonly used for this purpose. In the application of the mechanical failsafe closing mechanism for a subsea valve actuator, the spring-type configuration is reliable and widely used [4; 25]. However, the force of the closing spring needs to be big enough to oppose the high-pressure fluid in the valve  
Figure 1: Schematic diagram of the all-electric subsea gate valve actuator. when a failure close occurs. So when the valve is opened, the power system needs to resist the large spring forces, which consume a lot of energy [25]. Thus, the low-power holding mechanism also needed to be designed in the actuator of this paper. In addition, a single Electric Subsea Control Module (ESCM) is claimed to control up to 32 functional groups [6]. However, a single ESCM can control up to 16 functional groups including sensors in an engineering application [12], which puts forward requirements for the number of the actuator's control cables. Based on the above analysis, the low-power holding mechanism is proposed in this paper, as shown in Figure 2, in which a spiral groove is machined inside the locking ring sleeve; a thread and four cam-like notches are machined outside the locking ring and inside the bottom, respectively. Four locking tabs are adopted due to the higher symmetry and stability of this configuration, compared with the design of two locking tabs. Furthermore, the requirements of this configuration for the size of the locking ring are moderate, because each locking tab needs a cam-like notch of the locking ring. The higher the number of locking tabs, the higher the number of cam-like notches necessary, which leads the size of the locking ring being easily unable to meet the requirements. Thus, the configuration of four locking tabs has advantages in realizing product function, improving product performance and meeting design constraints. The locking and holding process of the low-power holding mechanism is shown in Figure 2. The driving mechanism pushes the spring support cylinder against the closing spring until the valve is fully opened, at which point the closing spring is at the limit, and the electromagnetics are energized and push the locking ring sleeve upward. The locking ring sleeve drives the locking ring to rotate forward through the spiral groove. Meanwhile, the locking ring drives locking tabs to move inward through the cam-like notch.