Protection with Galvanic Anodes

Chemical analyses should be provided for all anodes used in the offshore and harbor area, together with results for current content in A h kg and current output in amperes [2,3]. The geometric shape and the number of anodes required is determined by these parameters. Expensive calculations for design based on grounding resistances are made only in exceptional cases because in practice there are too many uncertainties and the number and mass of the anodes have to be quoted with a corresponding safety factor. 

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This value can be considerably smaller. It corresponds in Fig. 6-1 to the ordinate of the intersection of the resistance graph of slope cCq with a 7(f/-r) curve that deviates markedly to the left of that plotted. The maximum current density is an important quantity for the setting up of cathodic protection with galvanic anodes and is dependent on the anode geometry and conductivity of the medium. 

No damaging interference to foreign objects has to be considered in cathodic protection with galvanic anodes because of the small current densities in soil and the lower anode voltages. 

The difficulties of such operations on the research platform Nordsee are described in Ref. 9. The Murchison platform was provided with a combination of impressed current protection and galvanic anodes because there was a limit to the load to be transported [12]. The anodes for platforms are installed and provided with cables at the yard. They are installed with redundancy and excess capacity so that no repairs are necessary if there is a breakdown. The lower part of the platform up to the splash zone is usually placed in position in the designated location at least 1 year before the erection of the deck structure so that impressed current protection cannot initially be put in operation. This requires cathodic protection with galvanic anodes for this period. This also means that the impressed current protection is more expensive than the galvanic anodes. 

Shafts and screws should also be included in cathodic protection with galvanic anodes and shaft slip rings. The transmission voltages should be below 40 mV (see Section 17.3.3.3). 

The following economic considerations apply particularly to the cathodic protection of pipelines. The total cost of protection with galvanic anodes should be less than the costs of an impressed current installation K q. 

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