In order to remove the issues of stray current corrosion and to accommodate the requirements for earthing between structures many clients are adopting an inter-bonded approach to complex plants. In this approach all structures are made into an equi-potential belt such that conventional soil-based remote CP systems do not cause harm. The issues of design of multiple cathodic protection system types to address the various structures such as piles, pile-caps, pits, buildings, intakes, ponds, pipework, etc. are elaborated. In addition, the ability to accurately monitor and control such complex systems in a manner that allows true data to be determined without interference is demonstrated.
The use of a multi-system approach to cathodic protection is primarily driven by the increasing complexity of the types of structures, which are part of the common cathode in modern plants.
Traditionally cathodic protection systems and their designs were based on single cathode structures like cross country pipelines or individual tanks. As the technology matured CP was applied to a wider range of structures but usually the design philosophies adopted remained unchanged.
Perhaps the first use of complex cathode structures was protection of whole tank farms. Very early into this period it was realised that using a single deep well anode system did not provide an even current spread to match the theoretical current requirement.
Over time other structures began to be included in the complex cathode system and eventually this included reinforced concrete structures as well. At the current stage many client specifications require protection of all critical structures but even now it is rare for any specification to refer to the remaining structures other than as a "current drain allowance".
To avoid deleterious interaction in a complex structure, all metallic parts of the structure included in the cathodic protection scheme must be electrically continuous. Isolating joints in the incoming or outgoing pipelines are the only isolation required and these should be located outside the zone of influence of the cathodic protection systems in the plant.
At present only one international standard exists to assist in such CP systems, i.e. EN-14505 1. From a conceptual viewpoint this is given in ANNEX A of the standard.