Pipelines cathodic protection, design

The problem of corrosion of steel in concrete was first ascribed to stray current flows from trams and DC railway systems Hime, 1994). Once chloride, in the form of deicing salt, was identified as the major culprit (when trams disappeared but corrosion increased), an enterprising engineer in the California Department of Transportation (Caltrans) took a standard pipeline cathodic protection design and flattened it out on a bridge deck. 

Evans, T. E., Mechanisms of Cathodic Protection in Seawater . In Cathodic Protection Theory and Practice, 2nd International Conference, Stratford-upon-Avon, June (1989) Choate, D. L., Kochanezyk, R. W. and Lunden, K. C., Developments in Cathodic Protection Design and Maintenance for Marine Struaures and Pipelines , NACE Conference on Engineering Solutions for Corrosion in Oil and Gas Applications, Milan, Italy, November (1989) not included in Proceedings... 

L. E. Carlson, J.H. Prrzgerald III, F.R.D. Webster, Cathodic protection design for 1,900 miles (3,050 km) of high-pressure natural gas pipeline. Mater. Performance 40 (2001) 28—32. 

R.M. Degerstedt, K.J. KenneUey, M.E. Orazem,J.M. Estehan, Computer modeling aids. Traditional cathodic protection. Design methods for coated pipelines, Mater. Performance 35 (1996) 16—20. 

In addition, anodes are classified as sacrificial anodes and impressed-current anodes. The former must be anodic to the stmcture and must dissolve at a low rate, providing electrons to the cathode. On the other hand, the latter must have low consumption rates in cathodic protection designs. Specifically, sacrificial magnesium Mg) anodes are widely used in buried pipelines and domestic or industrial water heater applications. For instance, a Mg anode may protect as much as 8 Km of a coated pipeline buried in the seal [3]. 

A common cathodic protection design is the use of bracelet anodes that originally were zinc but now, with improved alloys, are usually aluminum. Alternately, high-silicon cast iron anodes mounted on sleds, buried in the sea bed 250 feet from a given pipeline and midway between shore and the spar buoy ship connection have performed well. Anode return cables can be a maintenance problem unless properly secured to the pipelines and buried at least 5 feet into the sea bed, between the pipelines and the anode sled. Anode beds can also be installed in the beach itself, but they must be deep enough to be in the saltwater intrusion area. 

The current needed for cathodic protection by impressed current is supplied from rectifier units. In Germany, the public electricity supply grid is so extensive that the CP transformer-rectifier (T-R) can be connected to it in most cases. Solar cells, thermogenerators or, for low protection currents, batteries, are only used as a source of current in exceptional cases (e.g., in sparsely populated areas) where there is no public electricity supply. Figure 8-1 shows the construction of a cathodic impressed current protection station for a pipeline. Housing, design and circuitry of the rectifier are described in this chapter. Chapter 7 gives information on impressed current anodes. 

An ER probe specifically designed for assessing the effectiveness of cathodic protection is shown in Fig. 19.55. The elements for this probe can be machined from the actual pipeline. 

Corrosion. Anticorrosion measures have become standard in pipeline design, construction, and maintenance in the oil and gas industries the principal measures are application of corrosion-preventive coatings and cathodic protection for exterior protection and chemical additives for interior protection. Pipe for pipelines may be bought with a variety of coatings, such as tar, fiber glass, felt and heavy paper, epoxy, polyethylene, etc, either pre-applied or coated and wrapped on the job with special machines as the pipe is lowered into the trench. An electric detector is used to determine if a coating gap (holiday) exists bare spots are coated before the pipe is laid (see Corrosion and corrosion control). 

Figure 6.14 Localized corrosion of (a) unprotected buried pipeline and (b) an improved design including cathodic protection of both tank and pipeline(Hanson)5...

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

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. 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

K. Bethime, W.H. Hartt, Applicability of the slope parameter method to the design of cathodic protection systems for marine pipelines. Corrosion 57 (2001) 78—83. 

By this means, this design helps to overcome most of technical and economical obstacles facing TEGs where used in cathodic protection stations under arctic conditions. They are therejfore an important contribution to effective cathodic protection of natural gas pipelines and hence to the security of gas supply for RAO GAZPROM and its customers. 

For the application of cathodic protection to structures to be protected, the initial considerations are best made at the early design and preconstruction phase of the structure. For underground structures, it may be necessary to visit the proposed site, or for pipelines the proposed route, to obtain additional information on low-resistivity areas, availability of electric power, and the existence of stray dc current or other possible interactions. Other considerations will include fundamental design decisions to select the type of system and the most suitable type of anode appropriate to that system. In addition, it will be a requirement to determine the size and number of the power sources or sacrificial anodes and their distribution on the structure. Other factors that must be considered to ensure that cathodic protection is applied in the most economic and reliable manner are given as follows. 

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