CP system design work in the marine environment makes use of structure surface area, required current density, and anode resistance formulae to select the number and mass of anodes required to protect a structure.

The formulae cannot take account of restrictions to current flow caused by close proximity of structural surfaces to each other, and consequently CP systems which have been designed in the recommended way may sometimes not protect parts of a structure.

In some situations it may be judged that cathodic protection is not required on some surfaces, but recommended practice is that current drain to such surfaces needs to be taken into account in the design.

This paper first investigates the general effects caused when cavities and narrow channels/annuli are present, and attempts to identify patterns of behaviour. A series of computer-based parameter studies is performed in which, for example, separation between two structural surfaces is varied, and the extent of the protected area is measured. Secondly this paper investigates effects of geometry on current flowing through a hole in a plate. This situation frequently occurs in design of mud-mats – when the interest is to determine current drain to buried surfaces, and sometime occurs when free flooding cavities need to be protected. Again computer-based studies are used, and for example the size of the hole is varied.

Key words: IR drop, constricted flow, CP design

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