Simulation Assisted Design of Storage Tank Base ICCP

This work is focused on the simulation assisted design of anode grid based ICCP systems applied to exterior surfaces of tank bases.

The simulation software allows representation of the anode grid and the intersecting grid of distribution bars, together with power supply and return cables, including resistances of each of these components and of connections between them.

Solution of the electrical circuit equations provides current flow and electrical potential throughout the grid of anode ribbons and distribution bars. Current flowing into the surrounding electrolyte from the anode ribbons, and into the surface of the tank base from the electrolyte, is described using polarisation curves, which define the non-linear relationship between current density and potential difference across the metal / adjacent electrolyte junction. Current flowing through the electrolyte is determined by solving the Laplacian equation, using the boundary element method. The entire process is non-linear, and is solved iteratively.

The results of the mathematical modelling include current density and protection potentials on all parts of the tank base, as well as power loss, current and potential throughout the circuit.

Performance of a CP system design is assessed by making the simulation software automatically adjust the ICCP output to satisfy some criterion, such as a required potential at a reference electrode. Bounds can be defined within which the adjustment must stay, as a result of which it may not always be possible to satisfy the design criteria.

Robustness of the CP Design can be determined by performing what-if studies investigating different damage scenarios, for example failure of one or more welds, or of a cable connection. Effects of adding resistances to some of the power supply cables can be determined, so that use of this method as a possible remedial technique (or some other form of power splitting) can be investigated.

The paper describes and discusses all aspects of the methodology, which it then applies to several different ICCP designs.