CATHODIC PROTECTION SYSTEM (CPS) DESIGN CONCEPT A. PreambleThe design of the cathodic protection system(CPS) shall be an integral part of the total pipeline design. The CPS is one of the methods employed to minimise the corrosion of the pipeline and maintain its integrity. The Cathodic protection is applied to the coated structure to provide corrosion control to areas where thecoating may be damaged.
Other methods include: Effective coating of the pipeline and Corrosion allowance in the thickness design
A combination of applying both a coating and cathodic protection will normally results in the most practical and economic overall protection system. The design, construction and commissioning of the CP scheme shall be in accordance with BS Code of Practice for Cathodic Protection (CP 1021), NACE Standards NACE RP0169; SP0572 AND SP0177 and other acceptable international codes. Before the definition of the cathodic protection design for the 110km pipeline system we shall undertake a site survey including: A soil resistivity measurement along the pipeline route to assess the apparent aggressiveness level and, at points either side of the route, for the installation of impressed current anodes or otherwise. The cathodic protection system shall be designed so that any external corrosion on the pipeline is eliminated and any adverse stray current effect on the pipeline or on foreign structures/pipelines is avoided. Additional Factors to be considered in the design of the corrosion prevention includes: i. Nature of the product to be transported. Working temperature or pressure of the piping system and the tendency of backfill to cause soil stress on the pipeline. ii. iii. iv. Location of the pipeline considering the population density of the route. Other installations around or along the same route. Economic factors such as installation cost and protection maintenance cost over the life span of the pipeline.
Since this route is known to have a number of existing CPS. A predesign natural potential survey shall be required to determine the additional potential requirement for the pipeline especially at the OB/OB end of the pipeline.
Cathodic protection can be achieved in two ways:
By the use of galvanic (sacrificial) anodes, or By impressed current. Galvanic anode systems employ reactive metals as auxiliary anodes that are directly
electrically connected to the steel to be protected. The difference in natural potentials between the anode and the steel, as indicated by their relative positions in the electro-chemical series, causes a positive current to flow in the electrolyte, from the anode to the steel. Thus, the whole surface of the steel becomes more negatively charged and becomes the cathode. The metals commonly used, as sacrificial anodes are aluminium, zinc and magnesium. These metals are alloyed to improve the long-term performance and dissolution characteristics.A galvanic system requires: i) ii) iii) Sacrificial anodes Direct welding to the structure or a conductor connecting the anode to the structure Secure and minimum resistance connections between conductor and structure, and between conductor and anode.
Impressed-current systems employ inert (zero or low dissolution) anodes and use an
external source of dc power(rectified ac) to impress a current from an external anode onto the cathode surface.An impressed-current system requires: i) ii) iii) iv) Inert anodes (clusters of which, connected together often in a backfill, are called the groundbed). A dc power source. Electrically well insulated, minimum resistance and secure conductors between anodes and power source. Secure and minimum resistance connections between power source and structure.
GENERAL ROUTE SOIL RESISTIVITY SURVEYSoil Resistivity In carrying out the route resistivity survey. The appropriate survey technique shall be applied that will predict reliably where and to what extent corrosion of buried pipeline will occur along the 110km route.
For any buried structure the most important preliminary investigation is the measurement of soil resistivity at various points, the main purpose being to locate suitable places for ground beds and establish the soil resistance along the pipeline route. The pipeline route survey shall include general information of the terrain along the pipeline route indicating type of terrain and vegetation, physical and environmental conditions such as farm land, forests, open fields, desert, swamps, rocks rivers crossings, railways, major and minor roads, overhead power lines etc..) SOIL RESISTIVITY MEASUREMENT TECHNIQUE /METHODS AND PROCEDURE Soil resistivity can be measured by different methods, depending on the location and the purpose. The most common method is the "Wenner" or 4-terminal method. This is an in-situ method using 4 pins driven into the ground. A known alternating current is passed through the ground and the resulting voltage indicates the soil resistance. Changing the electrode spacing can vary the influence of depth. The resistivity is calculated by means of appropriate formulae. A description of the 4-pin Wenner Method and soil box method is given in ASTM G 57. Soil resistivity, .cm Under 1500 1500 to 5000 Above 5000 In general, in very corrosive soils cathodic protection shall be applied. In moderately corrosive soils additional tests shall be done to determine the requirement for cathodic protection. In slightly corrosive soils no cathodic protection is required unless there is a known corrosion history of similar installations under comparable soil conditions. Some relatively high-resistivity soils may still be very corrosive, e.g. acidic peaty soils and anaerobic soils containing sulphate-reducing bacteria. Over the life of a facility conditions can change, e.g. changing water tables and changing climates. Chemical analyses of the soil samples shall collected to determine the concentrations of various salts and the ph values of soil samples taken at points along the route.
Soil CorrosivityVery corrosive Moderately corrosive Slightly corrosive
INVESTIGATION OF SOIL COMPOSITION The investigation shall include chemical analyses to determine the concentrations of various salts and the pH values of soil samples taken at various points at the possible groundbed site or along the pipeline route. These soil characteristics must be identified in order to establish the requirements for the protection. The presence or not of sulphides, sulphates, chloride, pH and other constituents will affect the level of current requirements necessary for the effective protection of the buried pipeline. Corrosion can occur in fairly high-resistivity soils if there is a considerable variation in composition and/or resistivity at different points at the construction site or along the route of a pipeline, causing concentration-cell effects. Differences in soil composition such as in the case of partial land fill or reclamation may also cause concentration cell effects, which may require the installation of interference protection system. For existing pipelines met along the pipeline route, soil variations can be detected by measuring the natural potential of the pipeline at regular intervals. Such analysis may indicate areas of high salt concentrations, bacterial activity and the presence of acidic waste. Generally, water pH less than 6.5 is associated with uniform corrosion, while pHs between 6.5 and 8.0 can be associated with pitting corrosion. Some studies have suggested that systems using only pH to control corrosion shall maintain a pH of at least 9.0 to reduce the availability of hydrogen ions as electron receptors. However, pH is not the only factor in the corrosion equation; carbonate and alkalinity levels affect corrosion as well. Generally, an increase in pH and alkalinity can decrease corrosion rates and help form a protective layer of scale on corrodible pipe material. Chemicals commonly used for pH and alkalinity adjustment are hydrated lime (CaOH2 or calcium hydroxide), caustic soda (NaOH or sodium hydroxide), soda ash (Na2CO3 or sodium carbonate), and sodium bicarbonate (NaHCO3, essentially baking soda). Care must be taken, however, to maintain pH at a level that will control corrosion but not conflict with optimum pH levels for disinfection and control of disinfection by-products. 4|Page
High salt concentrations in original soil are usually indicated by the soil resistivity measurements. Bacterial activity In soils and water, bacterial activity is a common phenomenon. Bacteria produce substances that may influence corrosivity. The absence of oxygen, particularly in waterlogged soils, may provide a corrosive environment for iron and steel through the growth of sulphate reducing bacteria (SRB), which generate hydrogen sulphide. These microorganisms can exist in active form only in the absence of free oxygen and obtain their energy from the reduction of sulphates into sulphides. Bacterial corrosion of iron and steel under damp anaerobic conditions is usually rapid and severe. This type of attack can often be recognised by the bright (unoxidized) appearance of the corroded surfaces and the emission of hydrogen sulphide. The presence of sulphate reducing bacteria creates special requirements for cathodic protection systems The resistivity of soil can vary greatly with its water content and with the electrolyte dissolved in the water. Thus the soil resistivity at a given location may vary with the season of the year and the rainfall. However, if the soil is well drained and washed free of electrolytes by frequent heavy rain it maintains a fairly high resistivity even when moist. A professional analysis of the soil chemical composition and redox potential is requires to determine the effective corrosion control method to achieve a