Marine Structures and Offshore Pipelines

In the tidal zone and the spray zone (known as the splash zone), cathodic protection is generally not very effective. Here thick coatings or sheathing with corrosion-resistance materials (e.g., based on NiCu) are necessary to prevent corrosion attack [4]. The coatings are severely mechanically stressed and must be so formed that repair is possible even under spray conditions. Their stability against cathodic polarization (see Section 17.2), marine growths, UV rays and seawater must be ensured [4,5]. 

In contrast to pipelines and harbor installations, platforms are dynamically loaded. Therefore in the choice of steels, in addition to strength and types of machinability, the risk of corrosion fatigue and strain-induced stress corrosion must be taken into account in combination with cathodic protection (see Sections 2.3.3 to 2.3.5). 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be 

Current output Limited, self-regulating Controlled (manual or automatic) 

Current distribution With many anodes — good Less good with few anodes 

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Marine Structures and Offshore Pipelines 387 16.7.2 Harbor Structures... 

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. 

External corrosion problems in oil and gas production normally are similar to those found in the pipeline industry, but as the lines are shorter and smaller in diameter, their economic impact on the total cost of production is limited. Atmospheric corrosion of structures and vessels is a problem for offshore fields and those operating near marine environments. 

The structural and manufacturing aspects of steel were described in Chapter 6. The use of steel in marine vessels, tmdersea pipelines, oil rigs and offshore wind turbines carries with it the problem of corrosion. Seawater contains dissolved ionic salts and is able to act as an electrolyte. Typically, it is equivalent to a 3.5% aqueous solution of NaCl and also contains ions such as K, Mg, Ca, Br , [804] and [HCOs] . The concentration of dissolved O2 in the world s oceans and seas varies with depth. The combination of water, O2 and an electrolyte leads to oxidation of the iron in submerged steel or steel that is in the splash zone of the waves. At pH 7, the relevant half-equations and reduction potentials... 

Marine Structures (0951-8339). This journal aims to provide a medium for presentation and discussion of the latest developments in research, design, fabrication, and in-service experience relating to marine structures, i.e., all structures of steel, concrete, light alloy or composite construction having an interface with the sea, including ships, fixed and mobile offshore platforms, snhmarine and submersibles, pipelines, subsea systans for shallow and deep ocean operations, and coastal structures such as piers. 

Submerged marine structures. Cathodic protection of submerged marine structures such as steel jackets of offshore oil and gas platforms and pipelines is widely provided by sacrificial anode systems. A... 

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