Cathodic Protection Is Moving To The Numerical Simulation Era

Operators are facing increased direct current (DC) interference issues due to denser pipeline networks. Furthermore, alternating current (AC) corrosion effects are augmented within more and more complex utilities corridors.

Cathodic protection (CP) systems for pipeline networks and storage tanks are evolving toward increasing complexity while new norms are calling for the usage of modeling techniques in the design process (e.g. NPR2760 for AC mitigation in The Netherlands).

With the development of external corrosion direct assessment (ECDA) techniques and standardization of integrity management databases, asset managers have been looking to develop tools to help prioritize surveys and maintenance schemes.

Evolution of numerical techniques (steered by mechanical industries such as automotive and aerospace) today allows modeling of the electro-chemical behavior of a CP system under complex interference conditions.

Applications
As for any engineering application, simulation models speed the process of optimizing designs and evaluating alternatives. Techniques are constantly evolving and benefits are maximized when simulation models are tightly linked with the process of field data gathering, leading to optimized integrity management activities. Examples of applications in pipelines and storage tank industries are:

• At the design stage, use of simulation models will depend on the complexity of the configuration and norms in application. Selection of the model approach will be linked to the type and quality of available data. Only advanced models (BEM and FEM, see hereafter) are able to optimize a CP system design through determining the IR-free potential.
• For mitigation, simulation models allow the detailed investigation of interferences of neighboring elements. Both DC and AC interferences can be modeled and optimal mitigation applied.
• For field surveys, sub-networks models are in use at the pre-assessment stage of DCVG campaigns. The model allows the virtual planning of the field survey and can help, for instance, to determine the optimal interruption scheme of rectifiers.
• For integrity management, complete networks can be modeled and fed with ECDA data. The model can then be utilized as the link between direct assessment information and continuous monitoring of “On” potentials. As the monitored values evolve, the simulation model allows determination of root causes and their effect on pipeline integrity, providing a powerful tool for prioritization of maintenance schemes.

Figure 2: Simulation of the CP system of a buried pipeline network.

Figure 3: AC mitigation effect on a buried pipeline.

Challenges
In its most general form a CP system consists of a cathode (structure to be protected), an anode (that protects the cathode), an electrolyte (the soil) and the metallic connection between cathode and anode. For pipeline networks, these parameters vary along the developed length.

When designing a CP system for a pipeline network, the aim is to obtain a pipe-to-soil potential - or IR-free (IR = ohmic drop) potential on the entire structure - that is more negative than a certain minimum protection level.