4 Open/Closed Hydraulic Systems
The terminology "open/closed hydraulic system" defines whether the used control fluid is dumped to sea locally or returned to the surface after valve actuation.
A simple system such as a direct hydraulic system can be defined as a "closed" system, because after actuating the hand valve in the control panel, the control fluid flows along the umbilical hose into the subsea valve actuator to open it. On closure the reverse happens as when the actuating hand valve in the control panel is put into the closed position, the open actuator volume is forced by the valve return spring back along the supply umbilical hose to the HPU reservoir.
A sequenced hydraulic system behaves in a similar manner to the direct hydraulic system, and is therefore also a "closed system".
For a piloted hydraulic system the pilot line behaves in a similar manner to the direct hydraulic system on operating the pilot of the three way pilot valve which in turn provides hydraulic fluid to the subsea valve actuator.
However the benefit of the piloted hydraulic system is that the hydraulic supply hose to the umbilical is continually pressurised to avoid the delay in pressurising and venting the hose on valve actuation. When the pilot command is executed open, the pilot valve switches the hydraulic supply into the subsea valve actuator to open the subsea valve.
When the pilot valve is actuated closed the pilot valve spindle blocks the hydraulic supply port and connects the actuator to sea. The subsea valve then forces the actuator volume to sea as it closes under the return spring energy. The venting of the actuator volume improves the valve response time by not having to return the actuator volume of fluid back up to topside reservoir.
This dumping of fluid to sea local to the subsea valve actuator is what is defined as an "open hydraulic system" (or "open-loop").
An electro-hydraulic Multiplexed system has its actuation solenoids in the SCM local to the subsea valves to be actuated. However uniquely, the electro-hydraulic Multiplexed control system can be used either in an open or closed hydraulic system.
Generally mineral oil based EHM hydraulic systems return the spent fluid back along a return line in the umbilical to the tank, as it is not environmentally friendly to dump mineral oil to sea. This type of system is called a "closed" hydraulic system.
"Water-Based" (water + additives) Electro-Hydraulic Multiplexed hydraulic systems which use totally biodegradable hydraulic fluid can dump the hydraulic fluid to sea locally at the SCM in the open hydraulic system mode. However water based hydraulic fluid is also used for closed hydraulic systems.
The selection of an open or closed hydraulic system may be a user preference but is mainly an economic decision where the economics is a trade-off based on the volume of fluid to be used for valve actuation over the field life cycle, against the incremental umbilical cost due to the addition of a suitably sized return line in the umbilical.
There is also an increasing tendency to minimise environmental pollution in seawater, albeit by using a fully biodegradable hydraulic fluid. Current legislation permits the use of these fluids (which contain biocides and lubricants) up to a certain tonnage per year (many fluids are graded Class E, allowing up to 1000 tonnes per year according to UK legislation "UK Notification Scheme for the Selection of Chemicals for Use Offshore, OCNS"), although future legislation changes may possibly further restrict their use.
The density of control fluid is a consideration, particularly for deep water applications. Water-based fluid has a Specific Gravity (SG) of > 1 whereas mineral oil has an SG of <1.
Selection of the control fluid depends on a number of factors:
System type (open or closed loop)
Suitability for the required operating temperature & pressures.
Adequate (low) viscosity at lowest expected ambient temperatures.
Provides an acceptable level of protection against biological/fungal growth (may require initial system cleansing using proprietary sterilisation fluid.).
Provides acceptable lubrication.
Available at the required cleanliness level (typically AS 4059 Class 6 B-F or better)
Fully compatible with the control system equipment and umbilicals (depends on selected equipment Suppliers having tested their equipment with it).
Acceptable toxicity level and/or permitted discharge quantities at Country of use.
Available at the required geographical location within acceptable delivery times and volumes.
Cost
Not producing corrosion or Hydrogen Induced Stress Cracking (HISC) issue.
A particular factor in the selection of fluid is that of the downhole temperature. Care must be taken to select a fluid suitable for the expected temperature. "Ordinary" water-based fluids have an upper temperature of typically 85 degrees and alternative formulations are available for higher temperatures to about 150 degrees.
