Subsea Control System 

A subsea well requires a Xmas tree at the wellhead, housing valves that control the flow of fluids from the well and chemical injection fluids to the well. These valves are operated by actuators, which themselves require a source of hydraulic pressure to operate.

The primary objective of a subsea control system is to operate these hydraulic actuators which are attached to gate valves on a subsea Xmas tree. In its simplest form, such as for single wells, the actuator is connected directly via an umbilical hose core to a source of hydraulic fluid from the Topsides facility and is operated by supplying fluid to, or venting fluid from the actuator (see Section 2.1, “Direct Hydraulic”)

When a field development requires two or more wells, there may be a requirement for more complex control, such as subsea choking of wells, commingling into a flowline, and a more sophisticated control system type may be required. The various types of Control System are described in Chapter 2, Control System Types.

The surface location of the subsea wells and the number of wells often dictates the system architecture, and conversely if the control system architecture is studied at the conceptual phase in a development, the well location may be influenced by the control system capability.

The time taken for a subsea valve to respond to a control signal from the Topsides facility ("valve response time") may not be important for a single low energy well, or conversely can be a safety critical item in emergency shutdown system applications (ESD systems), or a potential environmental hazard if a High Integrity Pipeline Protection System (HIPPS) is used to protect a pipeline which is not fully rated to the shut in pressure of a well.

Data may is always required for production or injection well. This can be for reservoir management, well monitoring, flow assurance, or diagnostics. The means for provision of data may also dictate the control system type.

As field development complexity increases and technology advances, the capability of control systems has had to improve from the original basic on/off control requirement. Depending on the development, there may be the need for such items as controlling subsea booster pumps or submersible lift pumps, subsea separator control, or multi-phase metering. Due to the changing technology, this list of possibilities is not exhaustive.

This chapter is an introduction to the types of subsea control systems, their architecture, and some of the basic pointers for the selection of each type of system. The actual selection can only be made with knowledge of the full field architecture, the economics of the control system and umbilicals, and with consideration for all other costs such as platform or vessel access, and installation and hook up costs.

A subsea control system consists of many component parts and therefore has many interfaces. A key component and critical interface is the umbilical itself, which is usually from a different supplier than the subsea control system. This section should be read in conjunction with the umbilical chapter.

Offshore vessel time either during installation, hook-up and final commissioning, or later for field intervention is at a premium cost especially in remote locations where suitable vessels are not readily available. It is important therefore to focus on system detail and reliability including physical interfaces and material compatibility.

Tip Click these links below for access to 3D resources: 3D Interactive Model of Umbilical Structures 3D Pictures of umbilicals

Reference photographs courtesy of OneSusbea and TechnipFMC.