Electromagnetic Pig Tracking - What is the detection envelope and what can I achieve?
When we are asked “what range can I get?” in relation to an Electromagnetic (EM) Pig Tracking and locating system, we are really talking about the detection envelope. The detection envelope is the standoff distance from the pipeline combined with the distance upstream and downstream of the EM transmitter location.
Pig Tracking operations
When “tracking” pigs using an EM transmitter and receiver system, there are three typical operations with varying levels of complexity:
1. Confirm a stationary pig is or is not at the expected location
This is a very common operation made simpler by the fact that the operator knows the expected location of a pig within a launcher or receiver.
2. Locate a stationary pig at an unknown location (a lost pig)
If a pig stalls in the pipeline, the time taken to locate it could lead to cost implications particularly for subsea ROV operations. The normal procedure to locate a stalled pig is to step methodically along the line stopping at fixed intervals to try and detect a signal.
If dealing with a long section of pipe, it may be tempting to take large steps, but this could result in the pig being missed. In contrast, taking very small steps could result in more steps and time taken than is necessary.
The optimum step size is normally about 50% of the detectable envelope width. This size should guarantee that you do not miss the pig and you find it in a relatively short timeframe.
3. Confirm passage of a moving pig at a particular location(s)
The most common method of tracking a moving pig is “leapfrogging” using a team of operators. Upon detection of the pig, an operator “leapfrogs” ahead of the rest of the team to the next location, ready to detect the pig as it passes again. Tracking pig in this way allows you to narrow down the search area in the event the pig was to stall. In this instance, understanding of the detection envelope will ensure that operators are separated by an appropriate distance.
There are several project specific factors that can affect detectability including pipeline diameter, wall thickness, pig design and if the pipeline is buried. In addition, the configuration of the transmitter itself can have an impact – this is something that is often overlooked.
There are a few basic transmitter settings that can be used to improve detectability.
Increasing output power will expand the detection envelope in all directions but this will reduce battery life. EM transmitters should be configured with the maximum possible output power which still achieves the maximum battery lifetime required for the project, a compromise must be made between the two.
Pulse Rate and Pulse Length
Understanding pulse rate is critical particularly if tracking a moving pig. In this situation, the operator is normally stationary with the receiver placed on the pipe wall. If the transmitter is in pulsing mode, it may not actually be transmitting at the point it passes – there may be “dead spots” between pulses.
Increasing the pulse rate and/or pulse length will increase the size of the envelope but at the expense of battery life. There is also the option of setting a transmitter in continuous mode particularly for very fast-moving pigs, but this will result in an even larger reduction in battery life.
All OEL EMTx transmitters can be configured for transmission between 10Hz and 30Hz. Typically, lower frequencies will expand the detection in all directions. The improvement is relatively small when dealing with thin wall pipelines but becomes significant when dealing with larger wall thicknesses.
The below chart demonstrates the impact of different frequencies and the ability to identify individual pigs.
How do I know what detection envelope my transmitter will achieve?
Armed with the knowledge of how to improve detectability, the next obvious question is what is the size of my detection envelope? As we now know, there are number of project considerations and transmitter configurations that will influence this.
At present, the most reliable method of determining this envelope is to carry out representative testing with the assistance of the experienced engineers at Online Electronics under simulated project conditions. Ultimately, we plan to provide Output Power, Pulse Rate and Frequency data points to operators through our EMTx Config application. Over time we will be able to improve the accuracy of these data points with testing and/or by matching them to an FEA model.
Please contact us to discuss your specific projects and transmitter configurations.