Corrosion Simulation Newsletter
An interesting article was published in the September 2014 issue of the NACE Materials Performance magazine. The article presents design considerations and some of the challenges a corrosion engineer encounters while designing a cathodic protection (CP) system for offshore wind turbine foundations. The authors were Sameer Ayyar, Jacob Jansson and Ruth Sorenson from COWI A/S Copenhagen, Denmark.
The study was supported by computer modelling where a number of options were evaluated. In the figure shown here the model results for an anode cage attached to a monopile structure at a level of -5 to -10m can be seen. The immersed surface of the monopile is bare. Corrosion protection is extended to all the external immersed surfaces and 5m below the mudline.
Project: London Array Offshore Wind Farm. Image source. COWI.
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Once a structure is commissioned information on the condition of the structure and the performance of the CP system is obtained by surveying the structure to obtain data on the potentials and in some cases field gradients. The number of locations where data is collected and the frequency of such surveys are very much dependent upon the operator and can vary significantly. Data is typically provided in tabular form and has to be assessed by the CP engineer along with other integrity data to identify anomalies (i.e. where the potential is outside the expected range) and to identify trends which may indicate that the design is not performing as expected.
Physical scale modeling (PSM) has been used for many years, in particular for naval applications to validate the corrosion control measures such as cathodic protection systems installed on ships. Tom Curtin based in BEASY's US Office has recently used PSM as part of a project where the aim was to investigate the CP provided to ballast tanks to verify that the computer modeling results were providing accurate predictions.
The design verification of CP systems applied to underground facilities including storage tanks, buried piping, equipment bases, grounding systems, reinforced concrete and other underground structures is complex challenge. Space limitations may restrict installation of anodes in certain areas and shielding effects from buried structures may affect the distribution of the protective current. Modelling provides a tool to ensure adequate protection by evaluating ground bed design options and locations to mitigate the effect of interference and shielding.
A recent study investigated the impact of interference between nearby storage tanks and pipelines on the performance of the CP system and the potentials achieved.
The modelling study aimed to verify the protection provided including the interference between the CP systems for the underground infrastructure comprising three tank bases, pipelines with different quality coatings, ICCP Anodes, grounding rods, tank bottom ICCP Grid and sacrificial anodes.
BEASY attended the recent NACE Corrosion Conference in Orlando where we made contributions to the conference program, technical meetings and exhibition hall, showing our corrosion modelling services and software.
We highlight two presentations on the benefits of modelling for corrosion control in structures and ballast tanks.