Technologie Marine - Marine Technology

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The protection of marine and submarine structures against corrosion is often assured by cathodic protection (CP) systems. These can use sacrificial anodes (galvanic coupling of a sacrificial material with the structure) or impressed current (use of a non-consumable anode with a DC source).

The design of such systems requires the geometry and the lifetime of the structure, the physical and electrical parameters of the environment and construction materials to be taken into account.

The PROCOR (cathodic PROtection against CORrosion) software has been developed in collaboration between the CETIM (CEntre Technique des Industries Mécaniques), Elf Aquitaine and IFREMER (Institut Français de Recherche pour l'Exploitation de la MER). It can be applied to any problem related to the design or the verification of CP systems of underwater or underground structures.

PROCOR allows the optimisation of the CP system in terms of the number, location and type of anodes by evaluating the potential and the current density at every point of the structure on a time basis. It can be used to minimise or prevent under and/or overprotection of a part or the whole of a structure.

PROCOR is based on the Boundary Element Method (BEM). This numerical approach is very economical and only requires the modelling of the surfaces to be protected (the cathode), the anodic surfaces and the boundaries of the electrolyte (when necessary). Other numerical methods, for instance finite element, would require a complete modelling of the electrolyte.

PROCOR can be applied to external problems (underwater structures) or internal problems (tanks). Moreover, the BEM allows to extend the electrolyte external boundary to infinity (i.e. to study an immersed structure in a very large medium without the need of modelling the external boundaries). So it is possible to study complex structures with very simple surface models without affecting accuracy of the results.

The basic elements used in PROCOR are linear or quadratic surface elements (3 or 6 node triangles and 4 or 8 node quadrangles). These elements can model anodic or cathodic surfaces, a boundary between two sub-regions (different electrolytes or different structural parts) or an external boundary of the problem. In each sub-region the conductivity is constant.

For the study of long structures (e.g. pipelines, legs/tubulars of offshore platforms, etc...), PROCOR includes a specific 3 node tubular element formulation. This element allows the cost of modelling to be reduced. In the case of very long structures, the current circulation inside the structure due to the resistivity of the construction material cannot be neglected. This phenomenon is particularly significant when modelling underground pipelines. The tubular element can take it into account.

The Nisancioglu's algorithm, which automatically generates for every chosen element in the model a non-linear polarisation curve, has been implemented. It depends on the electrical and electro-chemical characteristics of the electrolyte and materials, and also on the potential history for the element considered. Thus, PROCOR is able to model the effect of the onset and the growth of calcareous deposit on the polarisation of the structure.

The PROCOR results are given in the form of the potential and the current density at every point of the cathodic and anodic surfaces, and for a given time step when using the Nisancioglu's algorithm. The potential can also be calculated at any chosen point in the electrolyte and results mapped out.

In order to achieve good modelling results, it is essential to use the most representative data from the actual location. For this reason and to validate the PROCOR model, a CP monitoring system was installed on a platform operated by ELF Congo in the Tchendo field. The monitors recorded the steel potentials and current densities at selected locations on the structure and current outputs of anodes

The data had been recorded from the launching of the steel jacket and used for the PROCOR model validation. By iterating the data, we obtained good agreement between actual measurements and the model. Figure 1 illustrates the comparison of measured and calculated potentials over 200 days at one location of the structure. Similar comparisons were obtained for cathodic current densities and anode current outputs.

Practical applications of PROCOR so far include :

• the verification of distribution of anodes in complex areas of structures,
• the effect of a temporary electrical connection of a poorly protected drilling platform to a jacket,
• the effect of potential levels on steel embedded in concrete and related risks of hydrogen embrittlement of prestressed cable anchors,
• the effect of CP levels on bare girth weld areas on an offshore pipeline and the possibility of boosting the CP levels on this pipeline using impressed current at its opposite ends.

Another interesting use of the model is to minimise the number of measurements thus reducing the cost of underwater inspection offshore.

Studies for engineering companies which want to use numerical models to improve the design of their CP systems can be performed with PROCOR by CETIM.