Soil resistivity, pipeline

Fig. 4.10 Typical application of zinc anode grounding cells. Zinc cell capacity to be installed at each location is based on information concerning maximum fault current and fault duration obtained from operators of electric transmission line. Necessity for grounding cells between pipeline and electric transmission line to be based on a study of the length of exposure, electric transmission line voltage, spacing between pipeline and electric line, soil resistivity, pipeline coating condition, and so on (Downing, 1964).

The Wenner method is chiefly used to determine the grounding resistance along the pipeline track and the installation positions for cathodic structures. Local limited soil resistivity is most clearly determined from the grounding resistance of an inserted Shephard rod (see Fig. 3-18). Soil stratification can be recognized from the apparent specific soil resistivity, p, by the Wenner method, if a is varied. 

Soil resistivity measurements can be affected by uncoated metal objects in the soil. Values that are too low are occasionally obtained in built-up urban areas and in streets. Measurements parallel to a well-coated pipeline or to plastic-coated cables give no noticeable differences. With measurements in towns it is recommended, if... 

The data in Table 4-1 show the considerable influence of the electrical resistivity of soil. This is particularly so in categories 2, 7, and 10. From a profile of the soil resistance along the course of a pipeline with welded connections or with electrically conducting thrust couplings, one can readily recognize anodic areas, and... 

The current output of galvanic anodes depends on the specific soil resistivity in the installation area and can only be used in low-resistivity soils for pipelines with a low protection current requirement because of the low driving voltage. Impressed current anode installations can be used in soils with higher specific soil resistivities and where large protection currents are needed because of their variable output voltage. 

Additional individual anodes must be installed at points on the protected object where a sufficiently negative pipe/soil potential cannot be achieved. Since usually only the voltage cone is of interest, the place of installation does not depend on the specific soil resistivity. Coke backfill is not necessary, and the place of installation is determined by the local circumstances. Individual horizontal anodes are conveniently installed parallel to the pipeline at the depth of the pipe axis. The voltage, length and distance of the anodes from the protected object are chosen according to Section 9.1 so that criterion No. 6 or No. 7 in Table 3-3 is fulfilled. 

Usually in calculating ac interference it is assumed that/= 50 Hz, soil resistivity p = 50 O m and with/= 16% Hz due to the greater depth of penetration, p = 30 m. The following distances from the pipeline on both sides of the center of the right-of-way rail are those in which the extent of the interference should be determined ... 

This analysis is far from exact since it assumes a remote groundbed, uniform soil resistivity and uniform defect density in the coating. At best it demonstrates that attenuation is likely to follow an exponential decay and that it will be less severe for larger diameter pipes than for smaller. The problem is more difficult to solve for more complex structures (e.g. congested pipeline networks) and especially so for marine installations where the development of the calcareous deposit introduces the possibility of temporal variations in attenuation. 

Buried pipelines are subject to external corrosion from ground water and highly conductive soils. The corrosiveness of soils is often estimated based on soil resistivity measurement. The measurement is made with the Wenner four-pin method, which is used in conjunction with a Vibroground(1 and a Miller U 10-pin conductor set to determine the average electrical resistivities. A general relationship between corrosion and soil resistivity is as follows ... 

Successful application of cathodic protection depends upon the selection, design, installation, and maintenance of the system. Before designing the cathodic protection systems, adequate field data must be collected, analyzed, and evaluated. Nature and conditions of the soil are reflected by field measurements like soil resistance, hydrogen ion activity (pH), and the redox potential. To understand the nature of the pipeline, potential measurements, coating resistance, and meaningful design current requirement tests must be conducted. 

This method gives an average in situ resistance at a depth equal to the pin spacing. Foreign buried structures may interfere with the measurements when using the Wenner method. Therefore, it is important to position the pins perpendicular to the underground pipeline [44]. If the pins are positioned parallel to the pipeline, it would result in lower resistance values than the actual resistance. If the specific soil resistance varies vertically... 

It is essential to make an important decision on the selection of the CP system to be instaUed. The choice of the CP system is based on the environment in which the pipeline is operated. The merits of each system are iUustrated in Section 15.2.3. Sacrificial protection is generaUy employed in environments with low soil resistance (10,000 Q cm) with a relatively low protective current requirement because of the low driving voltage. WeU-coated and isolated pipeUnes favor the instaUation of a sacrificial system in high resistive soils. On the other hand, ICS are usuaUy instaUed in conditions where the current requirements are higher and for pipelines that are bare or poorly coated. In aU cases, the final decision is based on the total instaUation and annual maintenance cost of the systems. 

A. Deb, D. Sarma, M.K. Banegee, Significance of logarithmic averaging of soil resistivity data for underground pipelines using impressed current cathodic protection system, J. Met. Mater. Sci. 47 (2005) 169-176. 

A long pipeline of 8-in. diameter is buried 6 ft underground. The potential difference between two Cu—CUSO4 reference electrodes located on the soil surface over the pipe and a point at right angles 60 ft distant, is 1.25 V. The electrode over the pipe is negative to the other, and soil resistivity measures 3000 Q-cm. 

The principles of sacrificial anode cathodic protection were discovered by Sir Humphrey Davy in 1824. His results were used over the next century or so to protect the submerged metallic parts of ships from corrosion. In the early decades of the 20th century the technology was applied to underground pipelines. When it was found that the soil resistance was too high, impressed current cathodic protection was developed. 

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