Hydraulic Power System
The hydraulic power system for a subsea production system provides a stable and clean supply of hydraulic fluid to the remotely operated subsea valves. The fluid is supplied via the umbilical to the subsea hydraulic distribution system, and to the SCM to operate subsea valve actuators. Figure 8-8 illustrates a typical hydraulic power system. Hydraulic systems for control of subsea production systems can be categorized in two groups:
- Open circuits where return fluid from the control module is exhausted to the sea;
- Closed circuits where return fluid is routed back to the HPU through a return line.
Open circuits utilize simple umbilicals, but do need equipment to prevent a vacuum in the return side of the system during operation. Without this equipment, a vacuum will occur due to a check valve in the exhaust line, mounted to prevent seawater ingress in the system. To avoid creating a vacuum, a bladder is included in the return line to pressure compensate the return line to the outside water. The hydraulic system comprises two different supply circuits with different pressure levels. The LP supply will typically have a 21.0-MPa differential pressure. The HP supply will typically be in the range of 34.5 to 69.0 Mpa (5000 to 10,000 psi) differential pressure. The LP circuits are used for subsea tree and manifold functions, whereas the HP circuit is for the surface-controlled subsurface safety valve (SCSSV). The control valves used in a hydraulic control system will typically be three-way, two-position valves that reset to the closed position on loss of hydraulic supply pressure (fail-safe closed). The valves will typically be pilot operated with solenoid-operated pilot stages to actuate the main selector valve. To reduce power consumption and solenoid size, but increase reliability, it is common practice to operate the pilot stages for the HP valves on the lower pressure supply.
Hydraulic Power Unit (HPU)
The HPU is a skid-mounted unit designed to supply water-based biodegradable or mineral oil hydraulic fluid to control the subsea facilities that control the subsea valves. Figure 8-9 shows a typical HPU. The HPU normally consists of the following components:
- Pressure-compensated reservoir;
- Electrical motors;
- Hydraulic pumps;
- Control valves;
- Equipment to control start and stop of pumps.
Figure 8-10 shows a typical hydraulic power unit schematic. As introduced before, the hydraulic power unit includes two separate fluid reservoirs. One reservoir is used for filling of new fluid, return fluid from subsea (if implemented), and return fluid from depressurization of the system. The other reservoir is used for supplying clean fluid to the subsea system. The HPU also provides LP and HP hydraulic supplies to the subsea system. Self-contained and totally enclosed, the HPU includes duty and backup electrically driven hydraulic pumps, accumulators, dual redundant filters, and instrumentation for each LP and HP hydraulic circuit. The unit operates autonomously under the control of its dedicated programmable logic controller (PLC), which provides interlocks, pump motor control, and an interface with the MCS. Dual hydraulic supplies are provided at both high pressure (SCSSV supply) and low pressure (all other functions). LP supplies are fed to the internal SCM headers via a directional control changeover valve. The changeover valve should be independently operated from the HMI, such that the header can be connected to either supply. Supply pressure measurement should be displayed. HP supplies should be controlled and monitored in a similar manner. The hydraulic discharge pressure of each function is monitored and displayed on the HMI.
Accumulators on the HPU should provide pump pressure damping capabilities. They should have sufficient capacity for the operation of all valves on one subsea tree with the HPU pumps disabled. Accumulators would also be of sufficient capacity to accommodate system cycle rate and recharging of the pumps. If all electric power to pumps was lost, the accumulator would have sufficient capacity to supply certain redundancies.
All pumps should be operational when initially charging the accumulators or initially filling the system on start-up. The pump (and accumulator) sizes should be optimized to avoid excessive pump cycling and premature failure. The quantity of pumps (and other components) per supply circuit should be determined through a reliability analysis. Pump sizing is determined by hydraulic analysis. Both analyses are performed prior to starting the detailed design for the HPU. Pulsation dampeners are provided immediately downstream of the pumps, if required, for proper operation of the HPU. All pumps should be electrically driven, supplied from the platform electrical power system. Pumps should have the capacity to quickly regain operating pressures after a hydraulic depressurization of all systems. There are different types of pumps, but the most common type uses accumulators that are charged by fixed pumps. These pumps, which start and stop at various preprogrammed pressures, are controlled by a PLC.
The HPU has a low-pressure fluid storage reservoir to store control fluid and a high-pressure (3000-psi) storage reservoir. One of the two separate fluid reservoirs is used for filling of new fluid, return fluid from subsea (if implemented), and return fluid from depressurization of the system. The other reservoir is used for supplying clean fluid to the subsea system. Fluid reservoirs should be made from stainless steel and equipped with circulating pumps and filters. Sample points should be made at the lowest point of the reservoir and at pump outlets . The hydraulic fluid reservoirs should also be equipped with visual level indicators. Calibration of level transmitters should be possible without draining of tanks. The HPU reservoir should contain level transmitters, level gauges, drain ports, filters, air vents, and an opening suitable for cleanout. The supply and return reservoirs may share a common tank structure utilizing a baffle for separation of clean and dirty fluid. The baffle should not extend to the top of the reservoir so that fluid from overfilling or ESD venting can spill over into the opposite reservoir.
Level Sensor The reservoir level-sensing system should meet the following requirements:
- The low-level switch should be at a level sufficient to provide a minimum of 5 min of pump operating time.
- The low-level switch should be located at a level above the drain port to prevent the pumps from ingesting air into the suctions.
- The high-level switch should be located at a level equal to 90% of the reservoir capacity.
Control Fluid The control fluids are oil-based or water-based liquids that are used to convey control and/or hydraulic power from the surface HPU or local storage to the SCM and subsea valve actuators. Both water-based and oilbased fluids are used in hydraulic systems. The use of synthetic hydrocarbon control fluids has been infrequent in recent years, and their use is usually confined to electrohydraulic control systems. Water-based hydraulic fluids are used most extensively. The characteristics of high water content–based control fluids depend on the ethylene glycol content (typically 10% to 40%), and viscosity varies with temperature (typically 2� to 10� C). Because government regulations do not allow venting of mineral-based oil into the sea, if the system uses this type of fluid, it must be a closed-loop system, which adds an extra conduit in the umbilical, making it more complex. Required fluid cleanliness for control systems is Class 6 of National Aerospace Standard (NAS) 1638 . The water-based hydraulic fluid should be an aqueous solution. The oil-based hydraulic fluid should be a homogeneous miscible solution. The fluid should retain its properties and remain a homogeneous solution, within the temperature range, from manufacture through field-life operation. The first synthetic hydrocarbon control fluid was utilized on Shell’s Cormorant Underwater Manifold Centre in the early 1980s. This type of control fluid has low viscosity, great stability, and excellent materials compatibility, and is tolerant of seawater contamination. This fluid requires the control system to incorporate return lines and an oil purification system (filter, vacuum dehydration to remove water). The cost of synthetic hydrocarbon control fluids is approximately four times that of mineral hydraulic oils. The first water-based control fluid was utilized on Statoil’s Gullfaks development in the early 1980s. This type of control fluid has a very low viscosity and is discharged to the sea after use. This fluid requires the control system to incorporate higher specification metals, plastics, and elastomers. The cost of water-based control fluids is approximately twice that of mineral hydraulic oils. Control fluid performance influences control system safety, reliability, and cost of ownership. Control fluids also affect the environment. The control fluid performances are as follows:
- The control fluid must be capable of tolerating all conditions and be compatible with all materials encountered throughout the control system.
- The control fluid is a primary interface between components and between subsystems. It is also an interface between different but connected systems.
- To maintain control system performance, system components must continue to function within their performance limits for the life of the system and that includes the control fluid.
- Any reduction in control fluid performance can have an adverse effect throughout the control system. The factors that can reduce the performance of a control fluid in use are as follows:
- Conditions exceeding the operating parameters of the control fluid;
- Poor product stability, resulting in a reduction in control fluid performance over time;
- Contaminants interfering with the ability of the control fluid to function.
Control and Monitoring
The HPU is typically supplied with an electronic control panel, including a small PLC with a digital display, control buttons, and status lamps. The electronics interface with other system modules, for remote monitoring and control. The HPU parameters monitored from the safety automation system should typically be:
- Nonregulated supply pressure;
- Regulated supply pressure;
- Fluid levels;
- Pump status;
- Return flow (if applicable).
The control panel may be a stand-alone or an integral part of the HPU. It utilizes a series of valves to direct the hydraulic and/or electric signals or power to the appropriate functions. Displays should be required to indicate hydraulic power connections from the HPU to the topside umbilical termination (or distribution) units, riser umbilicals, and subsea distribution to the individual hydraulic supplies to the SCMs. Links should be provided to individual hydraulic circuit displays.
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