Magnetic Pig Signalling - has my pig really passed?
Non-intrusive magnetic pig signallers are widely regarded as the most reliable method of detecting a pig passage. These units are also very easy to install with minimum step up required making them a popular choice for detecting the passage of magnetic pigs during pre-commissioning or routine pipeline maintenance and cleaning.
However, under certain operational circumstances there may still be an element of doubt associated with magnetic pig signalling. Signallers will detect and signal a pig passage when the pre-set threshold level, is reached before reverting to listening mode, waiting for the next pig. But what if this threshold level is reached as the pig approaches and the pig stalls before it passes the pig signaler location? How would you know and what would be the implications of not knowing? Online Electronics have upgraded the detection mode on their topside and subsea magnetic signallers to a 2-stage detection algorithm which provides added assurance that a pig has safely passed the location. Once the threshold has been reached the unit displays PIG APPROACH to show the pig is approaching or within detection range. The unit only signals PIG PASSED when the second detection criteria is met, ie when the flux value drops by a set amount.
If the pig is seen (approaching) but does not meet the second detection criteria after a defined period of time, it is considered to be stalled and therefore no pig PASSAGE is signalled, the event is logged and the signaller reverts to listening mode. The user can review all logged events and differentiate between the pig safely passing the signaller and the pig potentially stalled by reviewing the accompanying graphical data.
This 2-stage detection mode provides assurance that a pig has or has not passed a given point. This can be critical depending on the operation, could prevent a valve being closed on a pig during receipt/launch operations, prevent lost time in identifying where a pig is and ultimately gives the operator knowledge it can act on.
Another new feature is “SURVEY" mode - this can be very useful for determining the magnetic signature of a pig during signaller commissioning. A graph is used to show a history of the measured magnetic flux signal from a moving pig within the pipeline.
When in survey mode, the screen initially displays about twelve seconds of data but can be adjusted by the user to 200 seconds. The graph scrolls left when full and updates until you exit survey mode. No pig detection will be signalled but the user can identify that the magnetic flux has changed during pig passage.
This information can be used to set the signaller threshold parameters and/or adjust pig magnetic strength to ensure that the unit is set up optimally to detect and signal subsequent pigs.
These upgrades provide operators with the added assurance that their pigs have in fact passed the pig signaller and are continuing along the pipeline or have safely arrived at their destination.