Types Of Control Systems
The fundamental purpose of a control system is to open and close valves. However, other properties, such as instrumentation, provide chock control and important diagnostics. The five types of fundamental control systems are:
- Direct hydraulic;
- Piloted hydraulic;
- Sequenced hydraulic;
- Multiplex electrohydraulic;
Since the 1960s, the evolution of control system technology has proceeded from direct hydraulic to piloted and sequenced systems to provide improved response time and allow for long-distance tie-backs. Today, most subsea developments use the multiplex electrohydraulic control system. This is essentially a subsea computer /communication system consisting of hydraulic directional control valves. These electrically actuated valves allow stored pressure within subsea accumulators to be routed to individual hydraulic lines and onward to actuated gate valves and chokes on subsea production equipment. All-electric control systems are an attractive addition and an alternative to existing electrohydraulic systems. The all-electric subsea electric controls will reduce the cost of topside power generation and subsea umbilicals.
Direct Hydraulic Control System
The simplest remotely operated system for control and monitoring of a subsea system is the direct hydraulic control system. In this system each valve actuator is controlled through its own hydraulic line. This system is typically used for workover applications and small systems, and is especially common in single-satellite oil/gas fields of distances less than 15 km (9.3 miles). When the operator sets the control valve to open, the direct hydraulic pressure control fluid flows to the actuator. To close the valve, the operator sets the wellhead control panel valve to the closed position, venting hydraulic fluid from the actuator back to the reservoir. The major components of the system are as follows:
- Hydraulic power unit (HPU): The HPU supplies filtered and regulated hydraulic fluid to the subsea installations; it also applies the pressure that
drives motors, cylinders, and other complementary parts of a hydraulic system. Unlike standard pumps, these power units use multistage pressurization networks to move fluid, and they often incorporate temperature control devices.
- Wellhead control panel: The control panel is arranged for initiating predetermined commands to the wellheads. A logic circuit is connected to
provide an output control signal designated by the control panel.
- Control umbilical: A control umbilical connects topside equipment to subsea equipment. The control umbilical can transfer high- and lowpressure
fluid supplies, chemical injection fluids, annulus fluids, and electrical power/signals.
- Subsea tree: The subsea tree is the primary well control module. It provides a mechanism for flow control and well entry. The prominent features, advantages, and disadvantages of direct hydraulic control systems are summarized.
Piloted Hydraulic Control System
The piloted hydraulic control system has a dedicated hydraulic pilot supply (hose) for each subsea function and a hydraulic supply line to a simple subsea control module (SCM). At the SCM, a hydraulic accumulator provides a reserve of hydraulic energy to speed up the tree valve opening response time. This system is typically used in a single satellite of short to medium distances (4–25 km). The difference of the system from the direct hydraulic system is that the umbilical does not include large bore hoses to achieve the performance requirements. The pilot system uses a small bore hose for the pilot line and a larger bore hose for the supply line. The hydraulic pilot volume to actuate the pilot valve is very small and there is very little volume flow required to energize the pilot valve and to open the subsea valve. As a result, the valve actuation time is improved. The features, advantages and limitations of the pilot control system are summarized.
Sequenced Hydraulic Control System
The sequenced hydraulic control system consists of several sequence valves and accumulators. Various complicated program actions are designed by series-parallel for sequence valves. Valves in this system are opened in pre-determined sequences, depending on the magnitude of the signal from the topside. The system works by adjusting the regulator up to the opening pilot pressure of the first valve to open it. In order to keep sufficient differential between valve actuations to prevent spurious opening, and to keep the system hydraulic working pressure within the normal system design working pressures, the number of valve actuations in a sequence is limited.
Multiplexed Electrohydraulic Control System
The master control station (MCS) is implemented by a computer, and communicates with the microprocessor in the subsea electronics module (SEM), which is the communication link with the MCS and performs MCS commanded functions. The multiplexed electrohydraulic system allows many SCMs to be connected to the same communications, electrical, and hydraulic supply lines. The result is that many wells can be controlled via one simple umbilical, which is terminated at a subsea distribution unit (SDU). From the SDU, the connections to the individual wells and SCMs are made with jumper assemblies. The cost of a multiplexed electrohydraulic system is high due to the electronics within the SEM, the addition of the computer topside, and the required computer software. These costs, however, are balanced against smaller and less complex umbilicals and advancing technology, reducing the cost of the electronics. The system is typically used in complex fields of long distances (more than 5 km). When a digital signal is sent to the SEM, it excites the selected solenoid valve, thereby directing hydraulic fluid from the supply umbilical to the associated actuator. The multiplex electrohydraulic control system is capable of monitoring pressure, temperature, and valve positions by means of electrical signals, without further complicating the electrical connections through the umbilical.
All Electrical Control System
The all-electric control system is an all-electric–based system without the conventional hydraulic control of subsea components. The elimination of hydraulics means that any control system commands are sent in rapid succession without the usual retardation time required for accumulators to charge. This system is typically used in complex fields and marginal fields of long distances (usually greater than 5 km) and for high-pressure and high-temperature wells. For the subsea Xmas tree, the gate valves and the insert-retrievable choke are fitted with electric actuators. The tree contains dual, all-electric subsea control modules (SCM), which supply power and signal to individual actuators. The SCMs are normally subsea-retrievable. The main feature of this system is that the operation of the electric motors in valve actuators is performed by locally stored power from rechargeable Li-Ion batteries. The total power consumption of the system is quite low since only electronic supply power and the battery charging power is transferred from topside to subsea. The benefits of using all-electric control systems are very clear. An allelectric control system is simpler compared to a conventional electrohydraulic control system. It is favorable to use when developing marginal fields at long distances from a processing facility because of the lower cost of umbilicals, and it also provides solutions to the problems associated with high-pressure and high-temperature wells because there is no need for hydraulic fluid. In addition, it provides a higher degree of flexibility when expanding an existing system and when introducing new equipment into the system. Finally, the removal of the hydraulic system erases environmental and economic problems related to the leakage of hydraulic control fluids and the complexity of working with hydraulics.
 Society for Underwater Technology, Subsea Production Control, SUT, Subsea Awareness Course, 2008.
 B. Laurent, P.S. Jean, L. Robert, First Application of the All-Electric Subsea Production System Implementation of a New Technology, OTC 18819, Offshore Technology Conference, Houston, 2006.
 C.P. William, Subsea Control Module, U.S. Patent 6,161,618, 2000.
 International Electro-technical Commission, Functional safety of electrical/electronic/ programmable electronic safety-related systems, IEC 61508, 2010.
 J. Davalath, H.B. Skeels, S. Corneliussen, Current State of the Art in the Design of Subsea HIPPS Systems, OTC 14183, Offshore Technology Conference, Houston, 2002.
 International Organization of Standards, Petroleum and natural gas industries - Design and operation of subsea production Systems - Part 6: Subsea production control systems, ISO 13628, 2000.