 Welcome to the cathodic protection engineering lectures. As you have seen that assessing the corrosion condition of buried structures is an important part of cathodic protection engineering. In the last lecture, we started the part 1 of how to assess the pipeline condition. Today, we will be looking at the part 2. In the last lecture, the two important techniques, the pipeline pipe to soil potential. In the last lecture, two important techniques we discussed, one the soil resistivity, the other pipe to soil potential. In this talk, we will look at the following. One is the line current survey, followed by that we look at how to identify hard scars in the pipelines. Then we will go on to discuss drain current measurements, coating resistivity, coating defects and then we will end this lecture on how to then we will end, then we will end this lecture on how to prioritize the techniques. Let us start with discussing the line current. The line current is very important with respect to cathodically protected pipelines as well as unprotected pipelines. The current distribution in a cathodically protected pipeline decides how effectively the cathodic protection is done for a particular pipeline. In addition to this, the stray current corrosion we have seen earlier is an important problem in pipelines. Line current measurement could enable us to identify the location where the stray current corrosion occurs. In the unprotected pipelines, there are specific areas where the pipeline suffers severe corrosion, they are called as hard spots. Line current measurements is based on a simple principle of Ohm's law. If one measures the potential across a known segment and if the resistance of the pipeline is known, then it is possible to measure the current flowing in that particular segment. Typically, about under feet length of the pipeline is selected for measuring the pipeline current. Line current survey, there are two types of techniques. We shall look at the line current survey as two techniques. We shall look at these two techniques in detail. The first technique is 2 point probe, it is relatively simple. What is shown is the circuitry to measure the line current in a pipeline. It involves measuring the potential across a known segment of the pipeline length you see here and normally one uses a millivolt meter in order to measure the potential drop that occurs on the pipeline because of the flow of current. If the resistivity is known, then it is possible to know what is the current flowing in the pipeline. However, the resistivity of the pipeline is not known, then you add an additional circuitry wherein initially a known amount of current is passed through these two points using a DC source and at the same time one measures the voltage. When the voltage is measured and for a known current, it is possible to estimate the resistance offered by the pipeline in this segment. Generally, the pipeline resistivity lies in the range of 15 to 23 ohms centimeter. As a consequence, the voltage drop in the pipeline is considered to be significantly less. So, the voltmeter must have a resolution in terms of microvolt. In the 2 point probe technique, there is a problem. The problem is of contact resistance. These contacts can significantly add to the resistance measured in the measurement. So, that is one of the disadvantage of the 2 probe technique. In order to overcome this, the 4 probe technique is used. The 4 probe technique we have 4 probes you can see here right and through the external probes the current is passed, a known current is passed and the through the internal probes the potential is measured. Since we know the current, since you know the voltage, the resistance of the pipeline is measured accurately. In this case, there is no need to have to know the resistivity of the pipeline. So, this technique is advantages as compared to the 2 point probe method. So, far we looked at 3 important techniques involved in surveys. One is the pipe to soil resistivity, pipe to soil potentials and the pipeline current. We shall see how these surveys can be utilized to assess the pipeline condition. First, let us take the example of how you apply this technique to odd spots in a pipeline. You can use these techniques to locate an odd spot in a pipeline. Before we talk about how we employ these techniques to detect the odd spots, let me first describe what is mean by odd spots. In a long pipelines over a small regions, the pipelines is suspected to corrode at a higher rate as compared to remaining areas. As a consequence of say difference in soil resistivity are some other reasons. So, the highly corroding areas are rendered as the anodes, the other areas are rendered as a cathode. The current flows between the anode on the cathode. The current leaves this anode here and then goes to the cathode and then current flows through the pipelines. So, these are parade pipelines, they are not cathodically protected. So, let us see how you can locate this segment of the pipeline that is experiencing severe corrosion. We can use three techniques pipe to soil potential, resistivity and the line current measurements. And we also know that the pipe to soil potential there are two types of pipe to soil potential measurements. One is the pipe to soil potential close, other is pipe to soil potential remote. What I am going to show you is a schematic of the measurements made over a pipeline segment. The data corresponding to pipe to soil potential remote. What you see here? How it is changing? And the pipe to soil potential close, you can see how the pipe to soil potential close is varying in a very irregular manner. You can see this here. And the line current, the line current varies in a very systematic manner. You can see there is a rise in the current for a distance and then the current really falls. As opposed to again line current, the resistivity varies in a very irregular manner. It is expected the local soil may have different soil chemistry and as a consequence the soil resistivity really changes. But overall you will notice that at this location the pipe to soil potential is relatively positive. I mean the pipe to soil potential remote is is positive. And we also see that at this location the current leaves the pipeline, the current is higher it leaves the pipeline here. So, this location corresponds to the arts parts. So, the pipeline at this location will experience a more severe corrosion in relation to the remaining segment of the pipeline. So, it is possible to identify arts parts using the three parameters pipe to soil potential remote, pipe to soil potential close, soil resistivity and the line current measurements. All these parameters can be used to look at where the pipeline will undergo severe corrosion. The advantage of this technique is these structures are buried. So, all these measurements are made above the ground. So, that is a real advantage of using these techniques. So, how to control this? One way to control is install a sacrificial anode at the location of this arts part and you make a electrical connection to the pipeline. With this it is possible to protect the pipeline from severe corrosion by just spending 15 percent of the overall cost of cathodic protection of the pipeline. So, that is a real advantage of using multiple surveys. Let me move on to the next technique the drain current measurement technique. This technique is primarily used to determine the performance of the anodes. We all know that the anodes are the source of current the anodes they disperse the current in the soil from the anode the current goes into the soil and through the soil the current enters the pipeline the cathodic protection of the pipeline is happening. Now, how do you know how do you determine whether the anodes are really functioning or not is to determine the current that is flowing from the anode to the pipeline and this current is called as the drain current. Notably if the anodes pass high current especially their sacrificial anodes lower the life of these anodes. So, it is necessary to periodically determine the drain current measurements. The drain current measurements become very important when one notices that the pipeline is not adequately protected cathodically. If the potential drops below minus 0.85 volts one of the reason could be that the drain current in the anodes are not properly are not adequate the anodes may not properly function. Either the resistance of the anodes have gone up or the anodes have broken in either case that anodes lose the capacity to dispense current in the soil. The technique used to measure the drain current is relatively simple to insert an ammeter in series in the circuit as you note here there is a pipeline this is a station you can see it is a rectifier in the case of impressed current system. If it is a sacrificial anode systems you do not have a rectifier you need to connect the ammeter in series between the pipeline and the anodes and the current is so measured. So, the current so measured gives indication about the performance of the anodes or if the anodes have increased its resistivity. Let us move on to the next important topic of determining the quality of protective coatings in a buried pipelines or buried structures. If you look at typically the cathodically protected structures they are most all of them are painted. Typical cathodic protection structures are coated with high performance paint coatings because these coatings when they apply can bring down the current required for cathodic protection from a few hundred amperes down to a few tens of micro amperes. However, the protection current depends upon the quality of the coatings. In addition to reducing the current required for cathodic protection coating also reduces the current attenuation causing a wider spread of current along the pipelines requiring a few stations. This is a typical setup that is used to determine the current this is a typical setup used to determine coating resistance. If one has to measure the coating resistivity across a known segment of the pipeline let us say about 4 to 9 kilometers one requires a temporary ground bed to pass the current in the DC current source and they are connected to the pipeline through an an interrupter. The current that is passing along the pipeline at these two locations are measured using potential drop using a milli volt meters. The pipe to soil potential at these locations are measured using a copper copper sulphate electrode. This is the typical arrangement required to measure the resistivity of the coating in a buried structures. The principle involved in measuring the coating resistivity is very simple. One measures the current flowing in the known segment of the pipeline of say known area of A for example, the current is measured when the cathodic protection is turned on and when it is turned off. The difference gives is the current that is passing due to cathodic protection. Similarly, at these locations the pipe to soil potential was measured in the on condition on the off conditions. So, once we know the driving voltage required pass this current the resistance offered by the coating is obtained using the Ohm's law. The resistivity of the coating is obtained by simply multiplying the resistance with the area of the pipeline. The current flowing in a pipeline is measured as seen before using a 2 point probe method wherein you need to know the pipeline resistivity and the potential drop occurs along the given line line span. Let me now explain to you how one would arrive at the resistivity of the coating. The following steps are involved. One turns on the DC source that is that is the power source here and then measures the pipe to soil potentials at the test point 1 and the test point 2. So, it is given as E on 1 and E on 2 these two are the pipe to soil potential measured at these two test stations test points. When it is turned on the current that is flowing at the test point 2 and the test point 1 are measured that is measured by knowing the current flowing at the test point 1 and the test point 2 are calculated based on the potential drop that are measured at these two locations. Then you turn off the DC power source we make a similar measurements of pipe to soil remote at the test point 1 and test point 2. We also calculate the current that is flowing at the test point 2 and test point 1. Once it is done then one can calculate the current that passes through the point 2 and point 1 by subtracting the current flowing in the on condition on the off conditions. That gives you the current that is entering the pipeline because of the cathodic protection of the pipeline. Now the current that is the current that is entering this this segment of the pipeline how much current that is entered can be determined by subtracting the current that is flowing at the test point 1 and the current that is flowing at the test point 2. So, that gives you the actual current that is entering this segment of the pipeline ok. Now the driving force for the current to enter the pipeline can be calculated again based on the pipe to soil remote potential measured and the on condition and the off conditions. The difference gives you the driving voltage that is that is applied to get the desired current flowing on the pipelines. So, the overall driving force required to pass the current in this segment of the pipeline can be considered as the mean driving force that happened at the segment the test point 2 on the test point 1. So, that you can able to calculate using this equation. Now the coating resistance can be obtained by using the ratio that is the potential that is average that we measure here upon the current that is entering the pipeline in this segment of the pipeline. That gives the overall coating resistance and the coating resistivity is obtained by multiplying the coating resistance with the area of the pipeline. So, by this method it is possible to determine the resistivity of the coating and once you know the resistivity of the coating then you will able to know the protective capacity of the coating. The paint coatings applied on the buried structures have a limited life or a time period they disintegrate. The coating also get damaged whenever some maintenance work being carried out on the structures. When these coatings are damaged the current required for cathodic protection significantly increases and in fact at these locations the pipelines may not be adequately protected cathodically. So, it is very essential that these defects are identified and they are repaired with a fresh paint. And this is done using an above ground survey techniques. The two techniques are used in order to detect these coating defects. One is AC voltage gradient abbreviated as ACVG. The other case is direct current voltage gradient technique. The principle employed in detecting these defects are as follows. When you have a defect in the coating and the pipeline which is defect free there exists a potential gradient between these two locations because the resistance offered by the coating increases the voltage and so, this voltage gradient is you is measured is an indication of the presence of the defect. Let us look at the ACVG technique and ACVG technique is famously called as person already detected named after the person who invented in 1941. In 1941 J. M. Person introduced this technique to determine the location where the coatings have failed in buried structures. In the ACVG technique usually a low amplitude of AC signal is applied with the frequency ranging between 175 hertz around this range. The detection of these defects are done by two persons who walk across along the pipelines. These two persons they wear metal chelate fastened to their shoes and they walk with a distance of about 68 meter because they are wearing the metal chelates they pick up the current and so, if a person is very closely defect as compared to the other one who is away from the defect the voltage gradient is measured and this converted into odd signal and thereby the location is identified. But this technique suffers from the drawbacks the drawbacks are it is not applicable for deep water crossings and also inaccessible areas are sometimes we may have a crowded pipelines it is very difficult to detect or apply and when the defects are smaller size it is very difficult to determine these defect areas. The DCVG technique is a relatively newer technique and the disc techniques is used not only to detect the coating defect location but also the size of the coating defects. In this technique one uses two reference electrodes same again copper copper sulphate electrodes to measure the voltage gradients in the soil above the cathodically protected pipelines. The significant this technique is very distinct the distinctive feature of this technique is that even a small defects can be located very accurately this technique has a ability to detect even the smaller defects of the order of 10 centimeters. What I shown here is how one measures the pipe to soil potential measurements to locate the defect. This is a pipeline what is presented here and you have a defect and this buried in the soil the two reference electrodes are kept over the soil actually and they are connected through a voltmeter and the voltmeter measures the potential gradient between the two reference electrodes when the these two electrodes are moved along the pipeline with a fixed distance of about a couple of meters. Whenever one of the electrodes encounter the defect a potential gradient establishes between the two electrodes and thus identify the defect present in the coating. There are situations where this problem can arise this technique cannot detect the defects it so happens that the two reference electrodes are kept exactly over two defects of similar nature then the voltmeter cannot detect the potential between these two because there does not exist any potential difference between these two defects so in which case it is not possible for this technique to detect. It is also possible that the spacing between the two electrodes are very symmetrical across the defect which means the electrodes cannot detect the defect at all because the voltage gradient does not exist between these two locations. So, there are some disadvantages in using this technique. There are other advantages in using DCVG technique. In fact, this technique can be used in a complex network of pipelines. The person already detected technique cannot be used in a very highly congested areas or pipelines in a in a in a plant in a chemical in chemical plants. The other advantage is eye accuracy in locating the defects. No trailing wires are involved it is a very small segment small wire that is connecting the two reference electrodes. It can be used in combination with other techniques and unlike your person detected technique in this technique requires only one single operator. We have seen several different techniques for assessing the corrosion condition of the pipelines. It is very important that we use them very judiciously so that the cost of survey becomes very less. So, what I listed here is four levels in which you can use these techniques. The first level is the most simple one if the cathode protection that so whenever you see that the pipeline is under protected you will use the first pass that is checking the CP stations ok. And see that the CP station is properly functioning in terms of cables or the fuses in the rectifier. These are fairly simple procedures it can be adopted if anything is a problem it can be easily set out it can be easily set right. If you do not find any problem over here in the CP station we go to the next level of surveys which is closed interval potential survey. This also we call hot spot surveys. Potential measurements is still a relatively simpler technique as compared to the other technique. If this see the closed interval potential survey does not indicate the problem then one move moves on to DCVG to identify any coating damages in the pipelines. You can also use pipeline current mapping and soil resistivity surveys. And with this it is possible to identify a location where there is a problem then we go for excavation that is called bellow examination. You can also use ultrasonic and magnetic flux measurements and microbiological testing so that you confirm the problems experienced by the pipelines. NASRP0169 provides various information about the surveys in terms of determining the CP and related issues a coating maintenance, current shows, anode when you use sacrificial cathodic productions, test point surveys, there are case crossings and foreign line crossings. The more practical problems can be seen using this particular standard. Before I close I would like to give you a glimpse of how to address the troubleshooting in a typical cathodic production problems. If there is no power to rectifier ok, you can see that it is easy look at the causes, circuit breakers off, imputes use blown, insulation breakdown. If there is low current, low protection current then it can be any of these reasons cable snapped, the anode resistance has increased. You have seen that earlier the anode resistance is increased, the drain current measurements is made in order to identify the anodes of the problems insufficient number of anodes rectifier output fuse blown. Protection current is too high then anode resistance decreased, insulating flange shorted short to foreign structures insufficient protection potentials, the flanges insulation has been shorted, they shorted to foreign structures, the anode resistance again is increased, the resistance of the electrode reference electrode got contaminated, the reference electrode got contaminated. Protection potentials uncontrollable, there is very likelihood of stray current pickup and break in lead to electrode or the structures. To summarize what you have seen so far, the surveys are can be used as a diagnostic tools to assess corrosion of buried pipeline structures. It is used for effective insulation of cathodic protection systems. For example, soil resistivity surveys, it talks about how severe the corrosion of the pipeline will be. You can use the surveys to locate the coating defects and their conditions, the performance of the anodes be it sacrificial or impress current anodes. The existence of striker and corrosion, hard spots and cost effective cathodic protections. I hope this two lectures will give you a brief overview about how to assess the corrosion of the buried structures. Thank you.