Designing a Cathodic Protection system to protect storage tanks

Cathodic protection systems are used extensively to prevent steel structures from corroding, especially when failure of the structure will have serious consequences, such as loss of life and/or injury, and damage to property and/or environment. When correctly designed and operated, these cathodic protection systems significantly reduce the rate of corrosion and thereby extend the useful life expectancy of the structure. When designing or operating a cathodic protection system, it is important to ensure that foreign structures are not damaged by the system.

In the past, predicting cathodic protection interference before a cathodic protection system was installed was difficult. Mitigating unwanted cathodic protection interference was a task of adopting very conservative designs combined with extensive post-commissioning programmes of measuring potential shifts on foreign structures. Moreover, defining a criterion for cathodic protection interference in terms of a potential shift is fraught with danger. In some cases a particular potential shift may pose a serious threat while in other circumstances the same shift may be insignificant.

Determining the response of the structure to the cathodic protection system is not trivial because it is a function of three things:

• the location of the structure with respect to electric fields generated by the cathodic protection system
• the magnitude of the electric fields
• the electrochemical response of the structure to the interference

Until the development of computer modelling techniques this has been an insurmountable problem, but now the necessary tools to accomplish these tasks are available.

The Problem

Designing a cathodic protection system to protect the external surface or floors on a number of large, above ground chemical storage tanks located in close proximity to each other offers a significant challenge if cathodic protection interference is to be minimised. What makes this task difficult is that, in some cases, the tank floors are uncoated.

Thus large cathodic protection currents are required to fully protect the steel. The effect of this large current, combined with the steel floor being located at ground level and immediately adjacent to buried pipelines and steel foundations, creates the ideal situation for cathodic protection interference.

Description of the CP System

The 56.5m diameter tanks are positioned 28m apart (84.5m from tank centre to tank centre) and aligned in a row. The external floors of all three tanks are to be cathodically protected using anode groundbeds, as shown in the picture. Each tank will be protected using its own DC power source.

The Challenge

In order to find an optimum solution the engineer needed to investigate the depth of the groundbeds and the current to be supplied. In this simulator BEASY has been used to simulate the system and predict the protection potentials on the tank floors. You can use the model to investigate the optimum design by varying the conductivity of the ground, and the depth of the anodes.