So I’m sitting at my desk, trying to figure out how to make Excel do what I want it to, but all I am getting is “#VALUE!.” What does that even mean? Then my phone rings, and I’m truly grateful for a legitimate excuse to leave the problem at hand.
The call is from our construction supervisor who is overseeing the repair digs on a recently completed 200-mile coating survey on a buried pipeline. He says they are doing the digs on the largest reported signals and asks, can our survey method identify anomalies under concrete coating or swamp weights?
“Well…,” I hesitate because I had no idea that they had any concrete coating or weights on the pipe that we surveyed. I answer truthfully: “Yes, the survey methods can identify any bare steel that has an electric path to the soil.” Then I explain that from experience it is more likely that there is uncoated reinforcing wire in the concrete that is very close to, but not touching the pipe, and that this can and does create a response indistinguishable from a coating fault on the survey methodology that we used. I explain that I have seen this happen on concrete-coated pipe at river crossings and also at swamp and river weights.
“Just where was this dig?” I ask, and he replies “under that paved street in that luxury-gated community.”
Ouch…this was surely an expensive dig for what very well may turn out to be an unnecessary excavation, costing thousands of dollars. Worse yet, this surely caused a significant disruption for the retired population of the neighborhood, undoubtedly mostly made up of retired lawyers, judges and senators judging from the size and beauty of the homes!
My associate then asks; “So why didn’t we know this was concrete-coated pipe before now?” Now he knows better – when you ask me a question you need to be prepared for the answer, and I’ve been told that sometimes that can be a little like drinking from a fire hose.
But I start gently, “Well, for starters, the pipe material and route sheet data did not indicate there was a change in coating type, and technically there wasn’t.” You see, the concrete coating and swamp or river weights are typically applied over the standard coating, therefore, when you ask a computer for a list of coating transitions you don’t always get the concrete stuff. I scratch down a note to add this to our GIS query for our pre-coating survey data review.
We didn’t think to check for concrete because this location happens to be in the middle of a flat and dry community, and concrete coatings and weights are normally only put on the pipe to reduce buoyancy in wet areas. Think of how an oil tanker bobs like a cork on the ocean even though it’s made entirely of steel and filled to the top with crude oil. Like the tanker, the pipeline volume displaces water and soil with a greater density than the pipe and its product, making the pipe buoyant, so without the extra weight it could actually float up to the surface in wet soils.
“So why didn’t our coating survey detect the difference in the coating?” Well, now that is a more complex question, and requires some fundamental understanding of the scientific theory of the surveys. This particular survey was conducted using the Spectrum XLI system to perform the AC Voltage Gradient (ACVG) and AC Current Attenuation (ACCA) indirect inspection techniques (IIT). When I first start using all these abbreviations I usually got a glassy faraway look in the eye of my listener, and I’m immediately reminded of the word’s of Mike Myers playing Austin Powers: “Whoopty-doo Basil, but what does it all mean?” My associate is unfortunately on the other end of the phone so I don’t receive his visual cue, and I launch into my favorite topic of discussion.
Let’s start with the basics of the AC coating surveys. We start with the AC line illuminator or signal generator, which are just fancy names for a device that can produce an AC current at a specific and consistent frequency and power. Think of the AC line illuminator as a fancy inverter like the one you use to run AC devices in a vehicle; this one just has some extra buttons so that you can switch the output voltage, current and frequency. We then make metallic connections from the output terminals of the AC line illuminator; one to the pipeline, and the other terminal to a good ground rod planted as deep as possible into the earth’s crust. A metal tent peg or spike is usually too small, but a steel fence post is ideal.
Now you need to use your imagination and visualize that the AC current is going to flow like water in between the pipe coating and the pipe, where the AC signal generator is the pump, sucking from the earth and pushing AC out onto the pipeline. The pipeline coating is our output hose and anywhere that we have a hole or a crack in our coating, the AC is going to squirt, or gush out into the soil and be sucked back to the ground rod.
When we visualize the AC current flowing on the outside of the pipe and being kept in by the coating-like water being kept in the hose, we are actually visualizing the AC Skin Effect. AC current does flow on the surface of a metallic conductor vs. DC which flows throughout the metal itself. When we consider the analogy of AC current and water, we can associate the volume of water as the amount of current, measured in amps, and the pressure of the water flow is the potential measured in volts.
Thus, we can have large or small volumes of current flowing on our pipeline (equivalent to large or small volumes of water through our hose), and that current can be at a high or low voltage (equivalent to high or low pressure of the water in the hose). To increase the current flowing in our hose, we also automatically increase the pressure or voltage, this is Ohm’s law.
Measuring the volume and pressure of water in the hose would require us to attach a pressure gauge and flow meter. We could then find leaks in our hose by starting the pump and then walking along the hose taking pressure and volume measurements along its length. Where we measure a sudden drop in flow volume and pressure, we would assume we just passed a leak.
This would not work well for a pipeline as they are mostly buried, but we can take advantage of another phenomena associated with AC current that allows us to measure the AC current loss and associated voltage without contact. This is the AC magnetic field created by the current which surrounds the pipe and radiates outward. More importantly is that the strength of the magnetic field is directly proportionate to the amount of current flowing on the conductor. As the amount of AC current that is flowing in the conductor is increased, there is a corresponding increase in the magnitude of the magnetic field, and vice versa, so as less current flows, the magnetic field gets weaker.
An inductive coil placed in the magnetic field over the line can measure the strength of the magnetic field, and an array of coils can be used to calculate the distance to the center of the magnetic field and the amount of current flowing to create the magnetic field. The standard electromagnetic pipe locator is just such a device, and has been used for decades to very accurately measure the distance to buried pipes (depth of cover) and the amount of current flowing on the pipe.
Thus, if current flows off the pipe through a coating holiday, the amount of current left traveling past the defect is reduced, and so is the strength of the magnetic field. This reduction of the magnetic field is the fundamental principle of the ACCA survey; an electromagnetic pipe locator is moved along the pipe measuring the strength of the magnetic field and calculating the corresponding amount of current on the pipeline. Where we see drops in the amount of current, we associate this with leaks or alternate paths for our AC current (the current will also flow down tees and into other features).
The ACCA survey, in its simplest form, involves walking along the pipe and looking for any locations where a sudden drop in the current occurs, although we’re not directly measuring the current but are instead measuring the magnetic field. There is a catch, however, and that is that the magnetic field can also induce itself on any other ferromagnetic objects near the pipe. Just as a magnet held close to a box of steel pins will pick up as many pins as the strength of the field will permit, even causing the pins on the top of the pile to jump across the air gap to magically stick to the magnet. But the magnet does not lift all the pins to the magnet – its attractive forcers are induced into each pin that it attracts and reduced to a neutral state with a given number of pins.
This “induction” of the magnetic field onto other objects presents itself as a problem for the ACCA survey when there are steel or iron objects near the pipe. Now we are back to the issue we were discussing on our phone call previously, but I can see interest has now replaced the far off gaze in your eye, so I will go on. I can, after all, talk about this stuff for hours!
So, the reinforcing wire and rebar in the concrete weights and coatings put on pipelines gets induced with our magnetic field, just like stick pins being forced into service for the magnet. This issue goes further than you might think, as even metal construction scraps, metal fence posts, anchors and rebar in surface structures such as concrete roads and curbs can interfere with the readings by absorbing the magnetic field and artificially reducing the strength measured by the locator. These metal objects skew the locator readings, resulting in inaccurate depth of cover and current readings from the locator, and must be considered during the collection and analysis of ACCA survey data, and depth of cover for that matter!
What about the pressure gauge on the hose and how that works for finding coating faults? We can’t make metallic measurements to the pipe if it has coating and is buried, so we again need to use a secondary or indirect means to measure the pressure or voltage as we call it for AC current. Recall that in our water pump analogy, the AC current was pumped onto the pipe and then sucked back to the signal generator (our pump). If the pump is boosting pressure onto the pipe and there is a leak in the coating where the current squirts or gushes out into the soil, we would then be increasing the pressure in that soil next to the leak.
We can detect this increased pressure in the soil by placing one probe of an AC volt meter in the soil near the pipe, and one a distance away from the pipe (two feet is enough, but more is better to increase the readings). The volt meter works just like a pressure gauge and measures the pressure difference between the high-pressure leak area and the lower pressure remote soil. This is the principle and method used for ACVG surveys; two soil-contact probes (metal or half cells can be used) are placed on the ground making soil contact and measuring for pressure spikes (increased voltage) along the length of the pipe.
So, let’s review. Where there are leaks or holes in the pipe coating, the AC current flows out into the soil and the locator measures the current loss from measurements taken before and after the defect, while the ACVG survey measures the resulting pressure increase in the soil next to the pipe-coating defect. When foreign metal in proximity to the pipe gets induced with the AC magnetic field, it absorbs some of the AC current through induction. This results in a small amount of AC current on the object, and that current creates its own little voltage gradient. This can, and does, create what looks just like a coating fault. The amount of induction depends on the distance the object is from the pipe, and the strength of the magnetic field, but we have seen this effect on objects more than 10 feet from the pipe.
If, however, the metal object is a sufficient distance from the pipe, we can pinpoint the maximum signal and confirm that it is not on the pipe by location. This is as simple as moving the ACVG probes around to pinpoint the highest pressure or greatest voltage, and if it is not directly over the pipe, we can safely rule it as a foreign metal object or interference for the survey. This is not easily done if the object is in the same ditch as the pipe – such as the weights and concrete coating!
Remember that our AC current does behave very similarly to water, so now you want to know where does the current that left the pipe and entered the soil go? I’m glad you asked, because again, the AC current behaves just like the water. The current takes the path of least resistance, downhill, toward a lower pressure location. For the AC current, that is the ground source of the signal generator. And yes, it does take the path of least resistance, so if there are metal objects paralleling our pipeline, they act like drainage ditches and the current will travel through the soil, onto that foreign metal object and back to the ground pin at the line illuminator.
Do you think that enough current escaping from the coating defects ever accumulates on another conductor to create another magnetic field? Yes! Sure does, and this is another issue for the AC coating survey.
This is why these surveys are considered “indirect inspections” vs. direct inspections which interrogate the pipe by physical means. Our AC coating survey measures the magnetic field and voltage gradient which is subject to numerous possible sources of interference.
Are we improving on that interpretation? Yes, we sure are! Although five years ago we could only do a decibel ranking of the magnitude of the voltage gradient and current loss, we have now completed new research that can normalize the results for changes in the depth of cover, amount of current on the pipe, and by careful measurement of the voltage gradient, we can even determine the orientation around the pipe of where the coating defect is located!
So, we end our phone call with a plan to correlate the survey data to our alignment sheet construction records, and to check all the future digs for pipeline weights and concrete coating before proceeding with the direct excavations. We also make a creed to ensure industry learns more about these matters to prevent unnecessary digs that drain budget dollars from pipeline integrity programs. I turn back to my PC and my fingers go to work on the keyboard again, this time writing down the discussion we just had, and my wish to be distracted from my Excel problem is granted!
The contributions of Mark McMinn, Alliance Pipeline, to this article are gratefully acknowledged.
Shamus McDonnell is CEO and a co-founder of Hunter McDonnell Pipeline Service, Edmonton, Alberta.