This paper is designed to outline the hydraulic aspects of subsea control system. To familiarize the reader, various system configurations and their particular applications are discussed. A detailed particular applications are discussed. A detailed discussion on the major features of a hydraulic control system is then followed by a description of a typical manifold system and actual characteristics of an existing system.
The paper outlines important aspects such as hydraulic fluid selection and maintenance, specifications for hydraulic power units and subsea control lines, selection of valve actuators and the interfacing of equipment both at the design and installation phases.
Subsea control systems are used to provide actuation of subsea control valves on equipment ranging from one or two functions on a production gas/oil line to BOPs, satellite trees and finally to complex manifold systems with over 300 valves. In addition status indication, such as production pressures and valves positions, may be required back at the control station. positions, may be required back at the control station. The system may also be required to detect adverse conditions and effect its own automatic shut-down.
Systems can differ in many ways however the format for each is always the same with a surface control unit, a control line and a subsea system. The surface unit may be a simple hydraulic skid with control valves or a complex micro processor and VDU system. The control line could be one or two simple hoses or a composite electric and hydraulic bundle. The subsea system could be one or two valve actuators or a manifold arrangement requiring electronic subsea units, robot intervention and built in hydraulic power units. Figure 1 shows a simple block diagram of a control system.
Whatever configuration is selected for a control system one if not all, of the following sub systems will be required:
As far as it is known electrical power for direct operation of valves has not been applied subsea and it is thus likely that hydraulic power will be used, except in instances where diver or remote vehicles are acceptable.
Hydraulic power can be provided directly from the surface station be it land based or a workover rig platform. In either case the fluid will be conducted platform. In either case the fluid will be conducted using control lines. These lines may be constructed from either metal tubing or elastomeric material, various factors such as the length, number of hoses, handling problems and cost usually define the selection. Fig 2 shows the factors which normally lead to the control line specification. In the end it will be a trade- off between reliability, initial cost and maintenance and costs to repair.
A satellite tree with only seven or eight valves, situated up to five kilometers from the surface unit could easily deploy a direct system where fluid is pumped directly through a control line to the valves, pumped directly through a control line to the valves, however compare this to a complex manifold system where two to three hundred valves require actuation and clearly the cost and installation difficulties of a system using direct hoses becomes questionable. Add problems of response time (valve actuation time) due to problems of response time (valve actuation time) due to hose length and it becomes necessary to consider further options. Table 1 lists some of the systems available and in existence today.
In cases where basic valve actuation is required with no status data monitoring subsea valves can each be fitted with a dedicated hydraulic line connected directly to the surface control panel. This method is ideal for simple systems within a 5 km radius of the surface unit however valve response times may be slow.