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Cathodic protection (CP) systems are used extensively to prevent 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 environments. When correctly designed and operated these CP systems significantly reduce the rate of corrosion and thereby extend the useful life expectancy of the structure.
Nevertheless, the use of this technique can interfere with other structures. CP interference can occur when neighboring steel structures (i.e. foreign structures), are located within the electric fields of a CP system For example, when cathodically protected pipelines cross over other pipelines. Ships dock at cathodically protected jetties or pipelines approach cathodically protected tanks etc. In these situations the CP systems can accelerate the rate of corrosion on the foreign structure (Figure 1).
When designing or operating a CP system, it is important to ensure that foreign structures are not damaged by the system. In the past, predicting CP interference before a CP system was installed was difficult. Mitigating unwanted CP 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 CP 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.
It is notable that the assessment of CP interference is often left to the professional judgement of the CP engineer. Clearly, such subjective assessment is unsatisfactory. This article suggests a more scientific approach to the problem.
Ideally, the best approach to assessing CP interference is to determine the change in current density on the foreign structure induced by the CP system Current density is a more reliable parameter with which to assess CP interference because it is directly proportional to metal loss. However, in the past, the lack of a technique for determining how the CP affects the distribution of current on the foreign structure has prevented this approach from being adopted.
Determining the response of the foreign structure to the CP system is not trivial because it is a function of three things:
Until the development of high-speed computers and new computer modelling techniques this has been an insurmountable problem. However, the development of boundary element methods has finally provided the necessary tools to accomplish these tasks.
In this study, the BEASY Corrosion and CP software was used to predict levels of CP interference on buried structures. This information was used in an effort to optimise the CP design and thereby minimise CP interference.
Computer Simulation as an aid to CP System Design and Interference Predictions, Robert Adey and John Baynham, BEASY