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Home / Technical Articles / Typical electrical distribution scheme and SCADA system of an oil/gas production plant

The power generation system

Power can be provided from mains power or from local gas turbines or diesel generator sets. Large facilities have high power demands, from 30 MW and upwards to several hundred MW. There is a tendency to generate electric power centrally and use electric drives for large equipment rather than multiple gas turbines, as this decreases maintenance and increases uptime.

Typical electrical distribution scheme and SCADA system of an oil/gas production plant
Typical electrical distribution scheme and SCADA system of an oil/gas production plant (photo credit: Trend Micro Blog)

The power generation system on a large facility is usually several gas turbines diving electric generators, 20-40 MW each. If exhaust heat is not needed in the main process, it can be used to drive exhaust steam turbines (dual cycle) for additional efficiency.

Voltage levels for high, medium and low voltage distribution switchboards are 13-130 kV, 2-8 kV and 300-600 V respectively. Power is generated and exchanged with mains or other facilities on the HV distribution board.

Relays are used for various protection functions (generator, motor, transformer, capacitor…).

High voltage is transformed to medium voltage switchboards to which large consumers are connected. LV switchboards feed a mix of normal consumers, Motor Control Centers (MCCs) and variable speed drives for motors up to a few hundred KW (Not necessarily separate as shown in the figure).

Typical electrical scheme of an gas/oil production facility
Figure 1 – Typical electrical scheme of an gas/oil production facility

A separate emergency power switchboard provides power for critical equipment. It can be powered from a local emergency generator if main power is lost. Computer systems are fed from an Uninterruptible Power System (UPS) with batteries, connected to the main or emergency switchboard.

A power management system is used for control of electrical switchgear and equipment. Its function is to optimize electricity generation and usage and to prevent major disturbances and plant outages (blackouts).

The power management system includes HV, MV and LV low voltage switchgear plus Motor Control Centers (MCC) and emergency generator sets. Functions include prioritization of loads, emergency load shedding (closing down of non-essential equipment) and prestart of generator sets (e.g. when additional power to start a big crude pump is required).

Large rotating equipment and generators are driven by gas turbines or large drives. Gas turbines for oil and gas production are generally modified aviation turbines in the 10-25 MW range.

These require quite extensive maintenance and have a relatively low overall efficiency (20-27% depending on application).

Siemens SGT-A65 gas turbine
Figure 2 – Siemens SGT-A65 gas turbine

Also, while a turbine is relatively small and light, it will usually require large and heavy support equipment such as large gears, air coolers/ffilters, exhaust units, and sound damping and lubrication units.

Therefore use of large variable speed drives is becoming more common. For pumps on subsea facilities this is the only option. For use on remote facilities, High Voltage DC transmission and HV motors can be used, from a main facility or power from shore.

This will also avoid local power generation at each facility and contribute to low manning or remote operation.


SCADA Measurement and Flow Control

Supervisory Control and Data Acquisition (SCADA) is normally associated with telemetry and wide area communications, for data gathering and control over large production sites, pipelines, or corporate data from multiple facilities.

With telemetry, the bandwidth is often quite low and based on telephone or local radio systems. SCADA systems are often optimized for efficient use of the available bandwidth. Wide area communication operates with wideband services, such as optical fibers and broadband Internet.

Remote Terminal Units (RTU) or local controls systems on wells, wellhead platforms, compressor and pump stations, and are connected to the SCADA system by means of the available communication media. SCADA systems have many of the same functions as the control system, and the difference mainly comes down to data architecture and use of communications.

A typical SCADA system collects data from, and supervises control of, third-party programmable logic controllers at pumping stations, mainline valves, and other areas where monitoring of critical conditions takes place.

Along the entire length of the pipeline, block valves are remotely monitored and controlled using advanced real-time SCADA processors designed to support complex remote applications.

The communications for the system is typically over the Ethernet and fiber optic lines as the backbone, backed up by public switched telephone networks.

SCADA structure in an oil and gas production plant
Figure 3 – SCADA structure in an oil and gas production plant

SCADA system designs vary widely, but there are elements common to all. Operational data for production plant must be gathered from locations that could be distributed widely across large geographical areas. Measurement transducers are polled frequently.

To efficiently perform basic functions, data must be accessible by operations personnel located in the field and at a central control center. Operations are monitored and controlled using SCADA systems that provide thousands of data signals to various controllers and operators.

Some data are provided at intervals of a few seconds, other data are provided at intervals of a few minutes, and still others on an hourly or daily basis. As data are updated, the superseded older data are normally stored for a period of time to support system audits, identify trends (both good and bad), and establish a historical operating record.

SCADA systems are configured with a variety of instrumentation. Electrical signals from measurement devices are typically converted to engineering units in computers, referred to as remote terminal units (RTUs), which are located at measurement sites.

Communication links are provided by radio, cell phone, private microwave, leased line, or satellite. Polling frequencies can be predetermined or on-demand.

Data from a given area of operations are often concentrated in computers at field offices, which are distributed throughout the production plant. SCADA software running on these field computers provides operational data and control to local operations personnel.

Central computers located at a control center, in turn, poll field computers. SCADA software runs on the central computers to provide controllers with displays of operational data and remote control capabilities.

With so much data available at such high frequency, the effectiveness of the SCADA system hinges on appropriate data presentation, analysis, and alarming.

A variety of data presentations are used to transform basic data into information. Trends, schematics, and other graphics are used to convey large amounts of data, which vary over time, in a concise and informative format. Often operational data is superimposed on facility and other schematics, permitting presentation of the data in an operational context.

Alarms are used to indicate that operating conditions are approaching or have exceeded prescribed tolerances. Attention can then be focused on problem diagnosis and appropriate actions.

RTU at the wellhead
Figure 4 – RTU at the wellhead (photo credit: intechww.com)

In addition to data collection and display, SCADA systems also often include data validation programs that seek to validate each piece of data before using it to support a calculation or represent a condition. Frequent and, in some cases, continuous data validation has been shown to greatly increase the sensitivity of the system while reducing incidents of false alarms.

SCADA systems at remote control centers provide operators with complete operational information about the pipeline system in one location.

Typical information includes:


1. Pipeline mimic/displays

The complete pipeline can be mimicked to provide the operator with instantaneous visual feedback on the status of any portion of the pipeline, including pumps, valves, tanks, etc.

These visual schematics include overviews of the entire pipeline system or systems and drill-down screens that take the viewer to an individual location or piece of equipment.

HMI SCADA of Real Oil Depot System
Figure 5 – HMI SCADA of Real Oil Depot System

2. Pump, compressor, and other equipment status

Equipment operation can be displayed with status (on/off) and other critical parameters associated with a piece of equipment such as flow, discharge pressure, vibration, case temperature, etc.


3. Valve status

Valve information can be displayed with valve positions (open/throttle/closed) depicted.


4. Alarms and alerts

Alarms and other operational indications are immediately available for operator response where complete system status is known and, in many cases, can be displayed. These can alert the controller to an unusual or abnormal operating situation or remind the controller about upcoming operating changes that need to be initiated.

Often, system configurations allow the operator to intervene to validate the alarm or to take the necessary corrective actions.

When operator intervention does not occur with a prescribed time frame, the system will automatically initiate actions that have been predetermined as being appropriate, given the circumstances.

Oil and gas production SCADA screen with alarms and notifications
Figure 6 – Oil and gas production SCADA screen with alarms and notifications

5. Analytical tools

Trending history and other analytical tools and graphical aids are available to assist personnel in their decision making under routine, abnormal, and emergency conditions.

The SCADA system is the central feature of a remote control center. Because the flow of product in the plant or a pipeline is typically a 24-hour-a-day, 7-day-a-week operation, the remote control centers are staffed continuously in order to monitor and maintain this round-the-clock operation.

Due to the data being transmitted from potentially many miles away, the operator oftentimes must respond to the alarm and direct a corresponding response from the remote control center based on the information depicted on the display provided by the SCADA system.

However, in other cases, decisions are made in conjunction with personnel located in the field at the affected location(s).

Sources:

  1. Oil and gas production handbook – ABB
  2. Overview of the Design, Construction, and Operation of Interstate Liquid Petroleum Pipelines – Argonne National laboratory

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

4 Comments


  1. aditya patanker
    Jun 13, 2019

    I m also Electrical and instrumentation Eng. I was designed scada for my company in wincc flexible 2008
    And connected with Existing PLC s7 300.
    I have 10 year experience in this field in steel and port indestry. If any opportunity in Ur indestry soo pls tell me. I am happy to join Ur company.
    Thanks and regards
    Aditya patanker
    7381097639


    • Nabil
      Jun 15, 2019

      Hello Aditya patanker,
      I’m not a recruiter, just I want awake you about the landline number of your country in your phone number.
      Regards
      Nabil


  2. Virender Singh
    Jun 13, 2019

    This is an excellent portal to learn new skills and nourish the existing ones.


  3. Uneeb Ur Rehman Ali
    Jun 12, 2019

    We can more improve this design by connecting our generator panel with large drive separately and LV Drives, MCC, LV Switchgear with emergency generator separately. And then interconnect all those panels with each other. In this way we can collect more efficient data and get better performance of system.

    Thanks.
    Regards:
    Uneeb Ur Rehman Ali
    Electrical Engineer

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