Chapter 8
Subsea Power Supply
Contents
8.1. Introduction
8.2.1. Design Codes, Standards, and Specifications
8.2.2. Electrical Load Calculation
8.2.3. Power Supply Selection
8.2.3.1. Power Supply from Topside UPS
8.2.3.2. Power Supply from a Subsea UPS
8.2.3.3. Power Supply from Subsea Generators
8.2.4. Electrical Power Unit (EPU)
8.2.5. Electrical Power Distribution
8.3.1. Hydraulic Power Unit (HPU)
8.3.1.1. Accumulators
8.3.1.2. Pumps
8.3.1.3. Reservoir
8.3.1.4. Control and Monitoring
8.1 Introduction
The power supply for a subsea production system is designed according to the subsea control system (Chapter 7). Different control system types (direct hydraulic, electrohydraulic, all-electric, etc.) require different power system designs. However, basically two types of power systems are used: an electrical power system or a hydraulic power system.
The power system supplies either electrical or hydraulic power to the subsea equipment: valves and actuators on subsea trees/manifolds, transducers and sensors, SCM, SEM, pumps, motors, etc.. The power sources can come from either an onshore factory (in a subsea-to-beach field layout), as shown in Figure 8-1, or from the site (platform or subsea generators).
Figure 8-1 Power Supply from Onshore to Subsea
(Courtesy of Vetco Gray)
Figure 8-2 illustrates the subsea power distribution when the power sources are coming from a surface vessel.
Figure 8-2 Typical Subsea Power Distribution [1]
8.2 Electrical Power System
The electrical power system in a typical subsea production system that provides power generation, power distribution, power transmission, and electricity from electric motors. The power is either generated on site (from a platform) or onshore (in a subsea-to-beach filed layout). To ensure continuous production from a subsea field, it is of utmost importance that the subsea system’s associated electrical power system be designed adequately. Figure 8-3 shows the design process for an electrical power system.
Figure 8-3 Electrical Power System Design Process
8.2.1 Design Codes, Standards, and Specifications
Various organizations have developed many electrical codes and standards that are accepted by industries throughout the world. These codes and standards specify the rules and guidelines for the design and installation of electrical systems. Tables 8-1 to 8-4 list some of the major international codes and standards used for subsea field development.
Table 8-1. American Petroleum Institute
| API RP 14F | Recommended Practice for Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Division 1 and Division 2 Locations |
| API RP 17A | Recommended Practice for Design and Operation of Subsea Production Systems |
| API RP 17H Draft | ROV Interfaces with Subsea Equipment |
| API RP 500 | Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2 |
| API SPEC 17D | Specification for Subsea Wellhead and Christmas Tree Equipment |
| API SPEC 17E | Specification for Subsea Production Control Umbilicals |
Table 8-2. International Electrotechnical Commission
| IEC 50 (426) | International Electrotechnical Vocabulary (IEV)-Chapter 426- Electrical Apparatus for Explosive Atmosphere |
Table 8-3. Institute of Electrical and Electronics Engineers
| Std. 100 | Standard Dictionary of Electrical and Electronics Terms |
| Std. 141 | Electrical Power Distribution or Industry Plants |
| Std. 399 | Recommended Practice for Power Systems Analysis |
Table 8-4. International Standards Organization
| ISO 13628-5 | Petroleum and Natural Gas Industries—Design and Operation of Subsea Production Systems—Part 5: Subsea Control Umbilicals |
| ISO 13628-6 | Petroleum and Natural Gas Industries—Design and Operation of Subsea Production Systems—Part 6: Subsea Production Control Systems |
8.2.2 Electrical Load Calculation
Electrical load calculation is one of the earliest tasks during electrical power system design. Engineers should estimate the required electrical load of all of the subsea elements that will consume the electricity so that they can select an adequate power supply.
Each local load may be classified into several different categories, for example, vital, essential, and nonessential. Individual oil companies often use their own terminology and terms such as “emergency” and “normal” are frequently encountered. In general terms, there are three ways of considering a load or group of loads and these may be cast in the form of questions as shown in Table 8-5.
Table 8-5. Typical Electrical Load Categories [2]
| Load Categories | Classification Questions |
| Vital | Will the loss of power jeopardize safety of personnel or cause serious damage within the platform/vessel? (YES) |
| Essential | Will the loss of power cause a degradation or loss of the oil/gas production? (YES) |
| Nonessential | Does the loss have no effect on safety or production? (YES) |
All of the vital, essential, and nonessential loads can typically be divided into three duty categories [2]:
Hence, each particular switchboard (e.g., from the EPU) will usually cover all three of these categories. We will call these C for continuous duty, I for intermittent duty, and S for standby duty. Let the total amount of each at this particular switchboard be Csum , Isum , and Ssum . Each of these totals will consist of the active power and the corresponding reactive power.
To estimate the total consumption for this particular switchboard, it is necessary to assign a diversity factor to each total amount. Let these factors be D. The total load can be considered in two forms, the total plant running load (TPRL) and the total plant peak load (TPPL), thus:
(8-1)
(8-2)
where
Dc : diversity factor for sum of continuous duty (Csum );
Oil companies that use this approach have different values for their diversity factors, largely based on experience gained over many years of designing plants. Besides, different types of host facilities may warrant different diversity factors [2]. Typically,
The continuous loads are associated with power consumption that remains constant during the lifetime of the system regardless of the operation taking place at any one time. Such consumers would include the subsea production communication unit (SPCU, located on the platform) and the monitoring sensors.
Intermittent loads are considered the loads that depend on the operational state of the system. A typical example would be a load due to valve actuation or HPU system activation. For the duration of each operation, the power requirement for the system increases to accommodate the operation. For the definition of the momentary loads, apart from the corresponding power requirement, it is essential to identify the duration and frequency of operations as well as a statistical description of operating occurrences in a specified time period.
Note that at no point during its lifetime should the subsea power system run idle (without load), except for the case of a temporary production shutdown. Tables 8-6 and 8-7 present typical values for continuous and intermittent loads during the operation of electrohydraulic and all-electric production systems, respectively. The data are presented in terms of electrical loads. Note that the use of a choke valve can be either continuous or intermittent, depending on field requirements.
Table 8-6. Load Schedule for an Electrohydraulic Control System [3]
Table 8-7. Load Schedule for an All-Electric Control System [3]
8.2.3 Power Supply Selection
After the load has been carefully estimated, the ratings for the power supply sources must be selected. For the electrical system applied in offshore oil/gas fields, the power transmission can be from onshore or offshore. Offshore power transmission can occur on the surface or subsea.
8.2.3.1 Power Supply from Topside UPS
Typically, the electrical power supply for a subsea production system is from the UPS, which has its own rechargeable batteries.
Figure 8-4 shows that for an electrohydraulic control system type, the UPS supplies electrical power to the MCS, EPU, and HPU, which then combines the power and other data to the TUTU.
Figure 8-4 Electrical Power Supply from UPS for Subsea Production System
Figure 8-5 shows a picture of a UPS and battery rack. The UPS protects the system from electrical power surges and blackouts. Electric power should be supplied from the host platform main supply. The UPS typically operates by rectifying and smoothing the incoming supply, converting it to DC, which can then be used to charge associated batteries. The output from the batteries is then converted back to AC and is ready for use to power the subsea system. In the case of failure of the main incoming supply, the output from the batteries is quickly switched to power the DC-to-AC converter, thus ensuring a constant supply.
Figure 8-5 UPS and Battery Rack
UPS systems are well known in industry, offices, and today even at home in situations where a power-consuming device must not lose its power supply. UPSs are available in small versions which are able to provide power from about 100 W up to several hundred kilowatts for from a few minutes up to many hours. During this time span the critical equipment supplied with power has either to be transferred into a power-off tolerable state or external power has to be resupplied; that is, grid power has to return or alternative power has to be provided. Usually the UPS is purchased from a specialist manufacture of such devices and is not built by a subsea control system supplier.
8.2.3.2 Power Supply from a Subsea UPS
A UPS is always located as close as possible to the power-consuming device to avoid as many fault sources as possible and is usually under control of the responsible operator of the power-consuming device.
By installing a subsea UPS system, costs may be reduced because fewer cables are required compared to having the UPS located topside because the UPS can be fed from the subsea main power supply. The short-circuit level of a UPS is low and the challenge of having enough short-circuit power available in a subsea installation to achieve the correct relay protection and discrimination philosophy can be solved by having the UPS subsea close to the power consumers.
In general, a subsea UPS can be used in all applications where distribution of low voltage (typically 400 V) is required subsea. The following are typical consumers of low-voltage subsea power supplied by a UPS:
• Several control systems located in a geographically small area;
• Electric actuators for valves;
• Switchgear monitoring and control;
• Measuring devices for current and voltage in switchgear, transformers, motors, and other electrical installations.
A conventional UPS comprises an energy storage means and two power converters. A control and monitoring system is also a part of a UPS. Because power conversion involves losses resulting in heat, UPS systems may need cooling systems to transfer the heat to a heat sink [4].
The UPS should be designed to operate safely in the sited environment. The UPS is designed to enable the system to ride through short (seconds to minutes) power losses and to permit sufficient time for a graceful shutdown if necessary.
8.2.3.3 Power Supply from Subsea Generators
Electrical power can also come from subsea generators. Several types of subsea generators have been used in subsea field developments.
Autonomous systems consist of an electrical power source, which is typically seawater batteries or thermoelectric couplers. The power source utilizes the difference in temperature between the well stream and ambient seawater. The seawater battery solution requires a DC-to-AC converter to transform the voltage from, for example, 1 to 24 V (e.g., an SEM requires a 24-V electrical power supply). A seawater battery should have the capacity to operate the system for 5 years or more. The thermocoupler solution requires an accumulator to be able to operate the system when the well is not producing.
8.2.4 Electrical Power Unit (EPU)
The EPU shown in Figure 8-6 supplies dual, isolated, single-phase power for the subsea system through the composite service umbilical, together with power supply modules for the MCS and HPU.
Figure 8-6 Electrical Power Unit (EPU)
The EPU supplies electrical power at the desired voltage and frequency to subsea users. Power transmission is performed via the electrical umbilical and the subsea electrical distribution system.
The EPU should be designed to operate safely in the sited environment and allow for individual pair connection/disconnection and easy access to individual power systems for maintenance and repair. The EPU should contain redundant communication modems and filters to allow user definition of system monitoring, operation, and reconfiguration unless those modems reside in the MCS. The EPU prevents the potential damage (to subsea control modules and MCS) caused by voltage spikes and fluctuations and receives input voltage from a UPS.
The EPU usually has two outputs: a DC busbar and an AC line. The energy storage units are tapped to the DC busbar, whereas the AC output is connected to the SEM.
The typical features of the EPU are as follows [5]:
• Fully enclosed, proprietary powder-coated steel enclosure, incorporated into the MCS suite, with front and rear access;
• Standard design suitable for safe area, that is, a nonhazardous gases area and air-conditioned environment;
• Dedicated dual-channel power supplies, including fault detection to the subsea electronics module;
• Modems and signal isolation to effect the “communications on power” transmission system;
• Control and monitoring to the master control station;
• Electrical power backup input terminal in the event of a power supply outage to both the MCS and EPU.
8.2.5 Electrical Power Distribution
The subsea electrical distribution system distributes electrical power and signals from the umbilical termination head to each well. As introduced in Chapter 3, electrical power (as well as hydraulic pressure, chemical supply, and communications) is provided to a subsea system through an electrohydraulic umbilical.
The SUTA is the main distribution point for the electrical supplies (also hydraulic and chemical) to various components of a subsea production system. The SUTA is permanently attached to the umbilical. Hydraulic and chemical tubes from the umbilical can have dedicated destinations or may be shared between multiple subsea trees, manifolds, or flowline sleds.
Electrical cables from the umbilical can also have dedicated destinations to electrical components of a subsea production system or may be shared by multiple SCMs or other devices. The electrical connection is made through electrical connectors on electrical flying leads (EFLs). The number of electrical connectors in series should be kept to a minimum. Redundant routing should, if possible, follow different paths. To minimize electrical stresses on conductive connectors, voltage levels should be kept as low as practical. Figure 8-7 shows the electrical as well as hydraulic power distribution in subsea production system.
Figure 8-7 Electrical Distribution in Subsea Production System
Connection of electrical distribution cabling and electrical jumpers should be made by ROV or diver using simple tools, with minimum implications on rig/vessel time. Manifold electrical distribution cabling and jumper cables from the umbilical termination to the SCM should be repairable or reconfigurable by the ROV or diver.
The subsea electrical power distribution system differs from a topside system by being a point-to-point system with limited routing alternatives. The number of components shall be kept to a minimum, without losing required flexibility. Detailed electrical calculations and simulations are mandatory to ensure operation/transmission of the high-voltage distribution network under all load conditions (full load, no load, rapid change in load, short circuits).
8.3 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.
Figure 8-8 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.
8.3.1 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.
Figure 8-9 Hydraulic Power Unit (HPU) (Courtesy of Oceaneering)
The HPU normally consists of the following components:
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.
Figure 8-10 Typical Hydraulic Power Unit Schematic
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.
8.3.1.1 Accumulators
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.
8.3.1.2 Pumps
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.
8.3.1.3 Reservoir
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 [6]. 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 oil-based 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 [7].
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:
8.3.1.4 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:
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.
REFERENCES
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3. Stavropoulos M, Shepheard B, Dixon M, Jackson D. Subsea Electrical Power Generation for Localized Subsea Applications. Houston: OTC 15366, Offshore Technology Conference; 2003.
4. G. Aalvik, Subsea Uninterruptible Power Supply System and Arrangement, International Application No. PCT/NO2006/000405, 2007.
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6. NORSOK Standards, Subsea Production Control Systems, NORSOK, U-CR-005, Rev. 1. (1995).
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