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Control Equipment – Subsea

The production control system provides control of all functions of the subsea production system. The production control systems, as such, are only concerned with controlling production and safety valves and monitoring devices and are not used to provide control of subsea connector latching and unlatching or operation of vertical access valves, for example. Typically, subsea functions include operation/control of:

• a downhole safety valve (DHSV);
• subsea chokes;
• production valves mounted on the x-mas tree; and
• utility functions such as monitoring of fluid characteristics, pressure leakage and valve positions, etc. (see Figure 1).

Figure 1: Subsea Production Valves

Switching of the tree-mounted production valves or adjustment of chokes is by means of hydraulic, spring- return actuators. Typical fluid volumes associated with actuators of tree valves range from one to four litres, and, for DHSVs, only a few millilitres. A hydraulic control system controls production valves by supplying or venting valves’ actuators.

Typical operation pressures are 3,000psi (200bar) for x-mas tree functions, and 9,000psi to 12,000psi (600–800bar) for DHSV functions. Actuation pressures for tree valves and DHSVs vary widely as they are a function of water depth and process pressures. The maximum expected actuation pressures occur during opening of valves that are in closed, pressurised positions, typically as follows:

• tree valve open: 750psig to 2,200psig (50–140bar);
• tree valve closed: 600psig to 1,000psig (20–65bar);
• DHSV open – 2,500psig to 9,200psig (160–600bar); and
• DHSV closed – 500psig to 4,500psig (32–290bar).

Types of Hydraulic Control System

Direct Hydraulic

This is the simplest type of control system in which HPU and well control panels for each individual well to be controlled are located topside. Well control panels use solenoid-operated control valves. Hydraulic signals are transmitted via umbilicals to actuators of production control valves mounted on the subsea tree. Each actuator of a tree-mounted production valve has a separate supply line. The advantages of this type of control system (see Figure 2) are relative simplicity, high reliability, ease of service and minimisation of a number of subsea components. However, the umbilical is complex because it must contain all individual hydraulic lines for all controlled tree components.

Figure 2: Direct Hydraulic Control System

Direct hydraulic control systems are limited to applications where the distance between the production facility and the subsea tree is less than 3–4km. This limitation is due to low speed of response of the system caused by the necessity topressurise the fluid and transmit it between the production facility and the tree. The response of the control system is influenced by both physical properties of hydraulic fluid, i.e. bulk modulus and viscosity, the characteristics of the umbilical, i.e. its volume, and compliance and volume of actuators. A typical closing time for tree-mounted valves (actuator volume of three litres) when using a direct hydraulic control and with an offset distance of 10km is approximately eight minutes.

No feedback information on system subsea performance is provided by the system; however, some information about subsea operations can be obtained by monitoring pressure in control lines and by measuring fluid supply and returns.

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