Marine structures are frequently monitored or surveyed as part of an ongoing integrity management program to obtain data on corrosion potentials, anode consumption and in some cases field gradients. One objective is to determine how effective the CP system is at providing protection to the structure and another is to verify that sacrificial anodes are being consumed at a rate consistent with design assumptions. However it is not economic or in many cases even feasible to measure potentials on a complete structure or to collect data from all anodes, so the CP engineer is required to infer the condition of the overall structure from the available data.

In particular for structures with complex geometry, the accuracy of data collected may not be reliable, as for example potential readings obtained with a measurement probe can be affected by nearby metal surfaces, leading to significant error in the data collected particularly for field gradients. Similarly access problems can result in error if the probe cannot be oriented or placed on the structure or anode in the required location. These issues require the CP engineer to use his judgment to determine which data to use and which to discard.

Computer modeling has been used in the design of CP systems firstly to optimize the design, and secondly to provide assurance to the operator that requirements for anode life are met, and that the system will sustain the potentials required to protect the structure. In such a case the structure geometry and the anode design are supplied as data to the model and the model predicts the potentials and the anode consumption rates over the life of the structure.

However another application of modeling is to use it to convert information we can measure into the information we want to know. In particular modeling can be used to expand limited survey data into a prediction of potentials on all structural surfaces, and to provide data on the performance of all anodes.

A case study is presented in which computer modeling is used to evaluate data from a single survey, then to enhance the data to provide a view of potentials over the complete structure and to provide a view of consumption of all anodes.

Keywords: CP, Cathodic Protection, Modeling, CP Survey

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