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.

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).

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).

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).

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.

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 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.

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.

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.

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.

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.

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.

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.

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