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Hard-wired Electrohydraulic

The hard-wired electrohydraulic system is similar to a piloted system but uses solenoid-operated hydraulic valves instead of hydraulically piloted valves (see Figure 5). Like in a piloted system, hydraulic power to operate tree functions is supplied by a subsea accumulator connected, via an umbilical, to a topside hydraulic power unit. A multiconductor electrical cable carries control signals from the topside to subsea solenoid valves. The control pod containing the solenoid operating valves and accumulator(s) is located on the tree, and electrical connections are included in the control umbilical termination and between the pod receiver plate and the pod.

Figure 5: Hardwired Electrohydraulic Control System

The advantages of hard-wired systems over the previous three systems are:

• a theoretically unlimited distance between production facility and subsea tree;
• independent control of selected functions;
• automatic sequencing if required;
• faster valve response than in previous systems;
• provision for data feedback from subsea functions for monitoring purposes; and
• a small control umbilical due to reduced number of hydraulic lines in the umbilical, although this is, to some extent, offset by the necessity of providing a multiconductor electrical cable.

The electrohydraulic controls tend to be more costly and less field serviceable than the other three control systems. Hard-wired systems are the most widely used today; however, for new installations, multiplexed electrohydraulic is the preferred system.

Multiplexed Electrohydraulic

This system is similar to the hard-wired electrohydraulic system but it takes advantage of multiplex technology to reduce the number of electrical lines and the complexity of subsea electrical connections (see Figure 6). Electronic coding and decoding logic is required at the surface and subsea, and a common cable supplies control signals (multiplexed digital data). This hydraulic power unit is mounted topside and supplies hydraulic power to a tree-mounted accumulator. The control valves used in these systems are normally latching types with pulse-energised solenoids, so the valves will stay in the last commanded position when an electric control signal is removed. To switch the valve, an electrical control signal of a few seconds is required.

Figure 6: Multiplexed Electrohydraulic Control System

Two types of hydraulic control valves are used: direct acting and pilot operated. The direct-acting, solenoid-actuated valves require a higher voltage control signal than the pilot-operated valves. When pilot-operated valves are used, the control signals actuate small pilot valves that direct the hydraulic fluid to either sides of the spools of the main hydraulic valves, thus shifting the spools in the desired direction and allowing hydraulic fluid to enter or leave actuators of the production valves.

The electronic module is built into the control pod together with the hydraulic control valves, and inductive couplers are used to make and break circuits. Full monitoring can be integrated with control functions without additional power and signal transmission equipment. The system has good system performance for long distances and is well suited for multi-well installations because the same umbilical can serve many wells without reducing overall performance. The simple umbilical allows redundancy to be built in without compromising the umbilical capacity, but the drawback is increased complexity subsea.

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