February 2013, Vol. 240 No. 2
Features
A Pig Tale in Two Parts
People usually laugh when I tell them I chase pigs for a living. But the truth is that it’s very serious business – one which I have been perfecting for more than 25 years. I’ve tracked pigs in all kinds of pipelines, all over the world, and over the years I have seen all kinds of crazy things happen with pigs in pipelines. Probably the most important thing I’ve learned would be to expect the unexpected.
This tenet was never truer than during a recent pig run where a surprise awaited us at the trap after what had appeared to be another perfect tracking job. We discovered after opening the pig trap door that only the drive section of an inline inspection tool was in the barrel. How could this be? We had listened to this pig for the last few days and no one could recall a change in sound that would indicate a change in the pig’s configuration; it had triggered the above-ground marker (AGMs) flawlessly the length of the run.
Backing up 72 hours earlier…Hunter McDonnell Pipeline Services (HM) was contracted to track and benchmark a cleaning pig and an inline inspection tool through a crude oil pipeline in western Canada. The ILI vendor was providing its own AGM boxes to be used at all the benchmark sites. As part of our own R&D field testing for HM’s Armadillo AGMs, we also brought Armadillo AGM boxes to deploy concurrently with the ILI vendor’s.
We launched the pig early in the day; it was my shift that tracked the pig out of the launch barrel and past the first few sites. It takes a few sites to determine what type of sound a pig is generally going to produce as you listen to it travel through the pipeline. The first three sites were valves. Nothing seemed to be out of line and both the vendor’s and the Armadillo AGM boxes were triggering like clockwork. In fact, we tracked the pig and recorded passages all the way to the trap without incident. Or so we thought.
Somewhere in the 200+ mile segment of pipe, the front section and the rest of the tool had parted ways. This particular pig was configured with a 22-Hz transmitter in its nose. That explained why it triggered AGMs all the way to the end. But it also eliminated any chance of finding the pig with standard locating methods. A rescue pig was one solution that could find the missing section; however, this could also seal off the line if the pig was stuck somehow. This was not a good option.
I polled the trackers to see if anyone could recall hearing anything abnormal or perhaps noticed a sound change during the run. No one could recall anything significant. Our initial inclination was that it must have happened near the end of the run, otherwise we would have noticed the change. I had the Armadillo AGM boxes collected as we downloaded the pig passage information from every site and looked for hints as to where the pig might have separated.
This is where the significance of the Armadillo AGM boxes comes in. Unlike the ILI vendor’s AGMs, or any other AGM for that matter, the Armadillo AGM boxes record a continuous log the entire time that they are on and deployed. That includes all the sensors, which is also unique with two axis of 22Hz, three axis of magnetic sensors, and the unique acoustic sensor. It was the acoustic sensor that we eventually used to determine where the pig had separated. Through analysis of the recorded geophone data we could identify sound events that even the most experienced tracker would – and did – miss during the pig run.
We started with the data recorded on my Armadillo box that was deployed at the launch to see if we could determine whether the whole pig had left the launcher. After carefully listening to the replay, we concluded that the entire pig had been launched as we could hear the sound of the wheels moving and the discs and cups hitting girth welds.
Our initial interpretation of the Armadillo sensor data identified that at the third valve there had been a normal passage; when we looked closer we could see some acoustic and magnetic activity long after the initial 22Hz. This could have been attributed to local interference so we continued to study the data recorded at this site.
I recalled tracking this location because there is an AGM site at a road crossing that is approximately 100 meters upstream of a valve and as the pig passed the valve I received a phone call. Not an unusual circumstance, but my time on the phone caused me to delay retrieval of the AGMs prior to moving downstream. Call it providence because all the while the Armadillo AGM was recording.
When we played back the passage we saw the 22Hz and heard the pig passage at the road at 14:11:16 followed by the valve hit at 14:12:03. This is exactly what I heard as the tracker at the site, and was what I had written down during the run. However, when we looked closer at the entire recording, we saw and heard another pig passage at the road after the documented 14:12:03 passage time. The Armadillo AGM recorded another pig, without a 22Hz transmitter, at 14:14:40 and a valve passage of 14:15:32.
Backing this up were the indications from the magnetic sensors (see image). This data confirmed that the pig was definitely separated at this point and, in fact, we confirmed that the drive section was already 3½ minutes ahead of the rest of the inspection pig. Closer examination of the two upstream Armadillo AGM boxes found that the tool actually separated prior to the first valve. This explained why we never heard a change in the pig configuration the entire run.
By calculating the difference in time between the road and the valve we were able calculate speeds to extrapolate that the rear section would probably not have been close enough to the front section to make it through the next pump station. During pig runs the pump stations must be taken off line and the mainline valve at the station opened to allow pig passages.
When the drive section was tracked through the station the order would have been given to close the mainline valve and resume operation of the pump station. This would have occurred long before the remainder of the inline inspection pig would have passed. As a result, we decided to go to that pump station to check for the missing part of the pig and confirmed it was there as soon as product was pumped past the station.
The pipeline operator then decided to allow the remaining ILI section to run the rest of the way down the pipeline. Once it was traveling again, we decided to deploy Armadillo AGMs, hoping the remaining section of the separated pig was still gathering data. We were able to track it up to the point that it became lodged in a valve. We then monitored the site until the line was shut down prior to the stuck pig being excavated and cut out.
Given that we deployed Armadillo AGM boxes at all the tracking sites, we were able to very quickly analyze the recorded data and react to an unexpected turn of events. Additionally, because the Armadillo AGMs were only deployed and recording at each site for as long as we occupied the site, we only recorded the separated part of the pig twice. When the ILI vendor and Armadillo AGMs triggered on the 22Hz transmitter in the drive section, due to the front section passing each tracking site, valve and pump station, we packed up and moved on, oblivious to the changed state of the tool. The acoustic change caused by the pig separating was also only determined from careful analysis of the recorded Armadillo data, not evident to the experienced ear of the trackers.
It’s not every day you think you’ve run a perfect tracking job – only to discover that most of the pig didn’t actually get to the trap. But…expect the unexpected! Thanks to the Armadillo AGM soundtrack, the data was replayed and the puzzle solved – with only one precise excavation and cut out required to remove a broken pig. So, this tale had a happy ending, at least for the trackers!
Author
Jim Hunter is the Operations Manager for Hunter McDonnell (HM) Pipeline Services.
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