Computer systems: Supervisory, Control and Data Acquisition Systems (SCADA)
SCADA is not the name of a computer software package but a term applied to a system which collects data and uses it for monitoring, simple control and trend analysis. Acquired data can be passed to other systems. SCADA systems are employed throughout the oil industry but in relation to departmental interests they are principally encountered controlling loading gantries and cross country pipelines.
Features that can be expected in a SCADA pipeline system
The features that can be expected in a SCADA pipeline system are:
- computer monitors displaying schematic diagrams of the pipeline and the position of valves and the readings from pipeline sensors;
- the ability of operators to operate valves & pumps remotely;
- software modelling the pipeline to predict the arrival of interfaces, tracking the parcels in the pipeline and detecting leaks & estimating the location;
- automatic control of valves and pumps for operational and safety purposes; and
- controlling and monitoring gas oil marker injection.
Control considerations - technical
The departmental interest in a SCADA system is confined to obtaining assurance that volumes of product moving are correctly identified at the correct time and correct volumes and that controls are in place to ensure all product movements are passed to revenue reporting systems.
The revenue concerns of a SCADA system will be several of the following:
Identification of a movement having taken place.
A SCADA system may have no completeness controls and retain no record of a completed movement. The point at which meter tickets are produced and the controls over sequential numbering must be established. The primary control over completeness is the daily reconciliation of stock balances to reported movements.
Identification of the product being monitored.
The system may be configured so that a feeder valve is linked to a product and as such the opening of the specified valve causes the system to assume, for example, unmarked gas oil is being moved. The contents of source tanks may not be under the control or ownership of SCADA system operator and the potential exists for changes of product storage tanks not to be notified to the operator.
Correct measurement of volume.
Meters may be remote from the operator site and the risks of interference and fallback procedures in the event of meter or meter transmission failure should be evaluated.
Correct STA conversions.
The STA conversions can be expected to be performed within the system. The sources of the required data (detailed in the section on flow computers above) should be established and the accuracy evaluated.
Identification of source & destination of the product.
Correct date & time of movement events.
Actions taken in response to alarms.