Assessment of pig run times

The client runs cleaning pigs through an oil pipeline for flow and corrosion control purposes. Flow velocities are small and run times are therefore quite long. Ongoing refurbishment of the pigs required that new cups were attached to some of them, and anecdotal evidence suggested that the pigs with the new cups were taking much longer to travel through the pipeline. This was causing problems as the a pig could not be launched until the previous one had cleared the pipeline, and so pigging frequency was effectively being reduced. The client had an in-house calculation for predicting run times that had worked reasonably well in the past and also wanted this to be updated.

Jee examined pig run times and process data to determine which pigs had arrived late and to see if there was any correlation between them. This showed that the most important factor was indeed the new cups, and subsequent measurement of both old and new cups confirmed that the new cups were considerably thicker and stiffer and would have greater friction at the pipe wall. This, in conjunction with the low flow velocity was causing the long run times.

Examination of the client’s pig run time calculation showed an error that had the effect of overestimating the pig velocity, and so pigs would tend to arrive later than predicted. This would get worse at lower velocities. Examination of the process data and actual pig run times allowed a more accurate empirical equation to be developed to predict run times for the pigs.

Benefits to the client included

•  More accurate predictions of pig run time, enabling better planning of pig runs
•  Confirmation of cause of late running pigs