Coatings continued pipelines

As production increased, new coating materials for pipe protection evolved and a variety of. synthetic materials became available. In 1950, continuous-laid pipelines in America and Italy were coated with polyvinyl chloride tapes applied... 

The most usual forms of buried metal structures are pipelines, piles, tanks and power and telephone cables. Power cables must usually have some metal protection, covered by expensive continuous factory-applied sheathings of considerable thickness. Since water, gas and petroleum pipelines provide the greatest area of metal surfaces to be protected below ground, a detailed discussion of the protection given to them would appear to be the best means of dealing with coatings for underground use. 

In the case of long continuously-welded steel pipelines the above pipecoating methods present the disadvantage that the joints have to be coated in the field after welding. To overcome this difficulty equipment which travels along the welded pipeline has been developed. 

As can be seen, pipelines suffer from both internal and external corrosion. Internal corrosion is due to presence of sulfur-bearing gases such as hydrogen sulfide, carbon dioxide, and moisture, which is entrapped as brine from the sea. The usual rule of thumb is that the internal coatings are applied only if the crude/gas is sour in nature (its hydrogen sulfide concentration is more than 500 ppm). Otherwise, the internal corrosion is usually tackled by addition of inhibitors, either continuously or in a batch process. 

On buried structures and pipelines that are initially wrapped either with or without the use of cathodic protection, the coating will eventually deteriorate. When it is inconvenient, uneconomical, or impossible to lift and rewrap, continued protection for the structure may be had by installing sufficient zinc anodes. Provided the zinc anodes are replaced when necessary, a relatively straightforward job, the useful life of the structure may be extended indefinitely. 

In recent years, continuous zinc ribbon anodes have been used in a variety of underground applications (Kurr, 1973 Peabody, 1976 O Connell, 1977). This type of product has broadened the applications for zinc anodes, for it provides small increments of current continuously along the entire length of a cathode. Its uses are generally considered to lie in specialty applications, where other methods of cathodic protection are either impractical or extremely costly (see later section on induced ac on pipelines). Bagnulo (1973, 1984) has developed a tape with an electrically conducting adhesive as described in the Mechanical Coatings part of Chapter 1. 

As regard the future trends of utilization of epoxy resins, high-performance coatings, followed by electrical/electronic ones and adhesives, flooring and paving applications, composites and molding products will continue to be their primary application worldwide. Further future developments of epoxy resins are related to their use as matrices of composites for dental applications and for trenchless rehabilitation of existing pipelines. 

Low resistivities and high chloride concentrations in the soil may lead to corrosion of buried steel pipehnes or structures. Cathodic protection should be considered for all buried steel pipehnes or structures. Where cathodic protection is not provided, corrosion monitoring equipment should be incorporated into the design to allow the operating staff to monitor the condition of the pipelines or structures. Nonwelded joints should be bonded for electrical continuity. In addition, coatings should also be considered. Coatings may be used alone or in con-jimction with cathodic protection. 

In addition to corrosion protection, many pipelines require thermal insulation to prevent hydrocarbons to produce waxes or hydrates. These heavier components can clog lines and require immediate attention. There is thus a continuous need of improvements in coating as oil and gas operations extend to unprecedented depths and temperatures [7]. As indicated in Fig. 10.4, pipelines have been coated with a variety of protective coatings with a wide performance range over the past 50 years. The advantages and disadvantages of the main coatings used for pipeline protection are summarized in Table 10.8. 

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