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.
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.
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. All of the vital, essential, and nonessential loads can typically be divided into three duty categories:
- Continuous duty;
- Intermittent duty;
- Standby duty (those that are not out of service).
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), 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.
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. 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.
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.
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. The UPS supplies electrical power to the MCS, EPU, and HPU, which then combines the power and other data to the TUTU. 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. 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 Wup 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.
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;
- Magnetic bearings;
- 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.
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.
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-Velectrical 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.
Electrical Power Unit (EPU)
The EPU 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. 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:
- 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.
Electrical Power Distribution
The subsea electrical distribution system distributes electrical power and signals from the umbilical termination head to each well. 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. 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).
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