Backfill Pipelines

On subterranean pipeline, look for graphitic corrosion on the very bottom of the line where it rests on the backfill. When graphitic corrosion occurs under these conditions, the affected region may be a narrow zone running along the pipe bottom over some distance (Case History 17.1). 

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. 

A comprehensive study of the soil and microbial situation in the backfilled zone of pipe-line ditches has shown a number of significant facts ". The results of over a thousand bell-hole studies along operating oil and gas pipelines in widely separated geographical areas of the United States has led to the conclusion that the pipe-line ditch represents a marked disturbance of the... 

These anodes, like platinised Ti may be supplied in different forms e.g. rod, tube, mesh, wire, etc. They may be used for the cathodic protection of offshore structures, heat exchangers, or even pipelines as they can be installed in the soil surrounded by carbonaceous backfill, and are comparable in cost to platinised titanium. ... 

The anode may be installed in conventional groundbeds or be laid in close proximity to the cathode, e.g. parallel to a pipeline route. The anode may be buried either directly in soil or in carbonaceous backfill. The major applications for this material are tank protection, internal protection, mitigation of poor current distribution and hot spot protection, i.e. to supplement conventional cathodic protection systems and provide increased levels of cathodic protection in areas that exhibit low levels of protection. 

In pipelines, especially those that are highly stressed from internal pressure, uniform and adequate support of the pipe in the trench is essential. Unequal settlements may produce added bending stresses in the pipe. Lateral thrusts at branch connections may greatly increase the stresses in the branch connection itself, unless the fill is thoroughly consolidated or other provisions are made to resist the thrust. Rock shield shall not be draped over the pipe unless suitable backfill and padding are placed in the ditch to provide a continuous and adequate support of the pipe in the trench. 

A land-based arrangement for CP in soil is shown in Figure 10.18. A backfill of a carbonaceous material such as coke breeze or graphite particles is used around the anode. The backfill is electron-conducting, which makes the effective size of the anode larger. This in turn reduces the resistance between the anode and the structure, and it reduces the consumption of anode material. If the resistivity of the soil is very high, buried pipelines may be protected by means of a continuous anode along the pipeline at a suitable distance from it. 

The first anodes, developed by Stratfull, were silicon iron primary anodes in contact with a conductive coke breeze asphalt overlay (the secondary anode). This design was based upon CP designs for pipelines where a silicon iron anode is embedded in a carbon coke breeze backfill to give a large contact area and low resistance. The anode is then linked, via the transformer rectifier, to the pipeline to be protected. A modified form of the conductive... 

The system was straightforward. One of the popular impressed current pipeline cathodic protection anodes of that time was made of a corrosion resistant silicon iron, surrounded by a carbon cokebreeze backfill. A well was dug near the pipeline, the anode put in surrounded by the backfill and the system connected to a DC power supply, with the negative terminal connected to the pipeline to make a cathodic protection system. Richard Stratfull look pancake silicon iron anodes, fixed them on a bridge deck and applied a carbon cokebreeze asphalt overlay (Stratfull, 1974). The systems installed in 1973 and 1974 were reviewed in 1989 and were still working (Broomfield and Tinnea, 1992),... 

The resistance in the lead and the anode itself, which usually is so small that it can be neglected with extended cable connections, anodes or pipelines, the voltage drop in the metal must, of course, be taken into account. The transition resistance between the surface of the metal and the electrolyte with uncoated iron anodes in coke backfill, the transition resistance is usually low. With metals in soil, it can be increased by films of grease, paint, rust or deposits. It contains in addition an electrochemical polarization resistance that depends on the current [see Eq. (2-35)]. 

Carbon and low-alloy steels are probably the most commonly used materials for pipes handling water, petroleum products, and some chemicals. Reference 1 provides a summaiy of the different specifications used for pipelines. Steel tends to corrode by both pitting and uniform surface deterioration [2]. Steel must usually be protected from corrosion both on the inside from the material being carried and on the outside from corrosion by the atmosphere, soil, or water that surrounds the pipe. External corrosion protection is provided by material selection, selective backfill, barrier coatings, stray current control, and cathodic protection. Internal corrosion protection can be provided by inhibitors, coatings, design process control, and materials selection. 

Steward (1996) conducted an empirical study of vertical and horizontal pipelines. He demonstrated how backfill consisting of fine and coarse material could be classified... 

Coetzee (1990) determined that one-third of the loss of pipeline wall thickness associ ated with pumping mine water is due to corrosion because mine water is often acidic. Since corrosion is an important contributor to wear of backfilled pipes, it became evident that lining the pipes was necessary. To compare piping materials, tests were conducted by Steward (1996) and indicated that a polyurethane rubber at a Shore hardness 55 Shore A provided the best pipeline protection in a test with slurry pumped at a speed of 3 m/s. By comparison, ASTM steel 106 grade B wore seven times faster than polyurethane 82 Shore A, or high-density polyethylene. 

Steward, N. B. 1996. An Empirical Evaluation of the Wear of Backfill Transport Pipelines. Working paper, BHRA Group, Hydrotransport 13, Cranfield, England. 

Figure 8-3. Schematic diagram of an impressed current cathodic protection system 1 - structure to be protected, 2 - rectifier, 3 - anode in backfill, 4 -connection to pipeline, 5 - cathodic cable, 6 - anodic cable,...

Polymeric anodes produced from 1975, also called cable and composite anodes, ensure a beneficial linear distribution of current and potential on the protected structure in cathodic protection systems. They have found applications for the cathodic protection of underground pipelines, tanks, and reinforced concrete structures in the vicinity of factories and municipal areas. Cable anodes, similar to graphite and ferrosilicon anodes, are placed in a conducting backfill enabling the transfer of some electrochemical reactions to the carbon backfill/environment phase interface. The applied anodic current density usually ranges from 0.5 to 1.5 A m . Conduction of current in polymeric anodes can take place in two ways ... 

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