Measurement of pipeline potentials

The principle of a close interval potentials surveys (CIPS) is to record the potential profile of a pipeline over its entire length by taking potential readings at approximately 1-m intervals. A reference electrode is connected to the pipeline at a test post and positioned in the ground over the pipeline at regular intervals for the measurement of the potential difference between the reference electrode and the pipeline (Fig. 13.35). The CIPS technique provides a complete pipe to soil potential profile and the interpretation of results, including the identification of defects, is relatively straightforward. 

The variation in the on and off potentials or the potential difference along the pipeline will usually indicate faults that prevent the attainment of complete cathodic protection. The protection current requirement of the pipeline may be estimated from experience if the age and type of pipeline is known (see Fig. 5-3). Figure 3-20 shows the variation in the on and off potentials of a 9-km pipeline section DN 800 with 10-mm wall thickness. At the end of the pipeline, at 31.84 km, an insulating unit is built in. The cathodic protection station is situated at 22.99 km. Between this and the end of the pipeline there are four pipe current measuring points. The applied protection current densities and coating resistances of individual pipeline sections are calculated from Eqs. (3-40) and (3-41). In the upper diagram the values of... 

The principle of the measurement is described with the help of Fig. 2-7 [50]. Potential measurement is not appropriate in pipelines due to defective connections or too distant connections and low accuracy. Measurements of potential difference are more effective. Figure 3-24 contains information on the details in the neighborhood of a local anode the positions of the cathodes and reference electrodes (Fig. 3-24a), a schematic representation of the potential variation (Fig. 3-24b), and the derived values (Fig. 3-24c). Figure 2-8 should be referred to in case of possible difficulties in interpreting the potential distribution and sign. The electrical potentials of the pipeline and the reference electrodes are designated by... 

The determination of AU values as well as the potential is effected by simple measurements of the difference in potential between two reference electrodes over the pipeline. To reduce possible electrode errors, partial sums can also be measured for a larger span, e.g., (a - O ) = 300 m ... 

Switehing of proteetion installations to measure the off potential should always be on the de side, particularly if there are ae voltage effeets between anode and pipeline that are rectified in the rectifier and provide a proteetion current, or when it is a matter of foreed drainage where the drainage eurrent flows from the pipeline via the reetifier to the rails. If the reetifiers control potential, the eontrol eireuit must also be switehed off otherwise eurrent surges will oecur when it is switehed on. 

Cathodic protection with magnesium anodes can be just as economical as impressed current anode assemblies for pipelines only a few kilometers in length and with protection current densities below 10 xA m" e.g., in isolated stretches of new pipeline in old networks and steel distribution or service pipes. In this case, several anodes would be connected to the pipeline in a group at test points. The distance from the pipeline is about 1 to 3 m. The measurement of the off potential... 

The cathodic protection of pipelines is best monitored by an intensive measurement technique according to Section 3.7, by an off potential survey eveiy 3 years and by remote monitoring of pipe/soil potentials. After installation of parallel pipelines, it can be ascertained by intensive measurements whether new damage of the pipe coating has occurred. These measurements provide evidence of possible external actions that can cause mechanical damage. 

Cathodic protection of uncoated objects in the soil is technically possible however, the high current requirement, as well as measures for the necessary uniform current distribution and for //f-free potential measurement, result in high costs. In determining the costs of cathodic protection of pipelines, it has to be remembered that costs will increase with increases in the following factors ... 

Fig. 23-14 Close proximity of a PE-coated DN 600 gas pipeline and a 380-kV high-voltage line, (a) Plan view, (b) comparison between measured and calculated pipeline potentials without connected grounds, and (c) comparison between measured and calculated pipeline potentials with connected grounds.

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