Technical Presentations at the January 2015 Meeting

1.1  Corrosion Control of Offshore Wind Turbine Structures - specification, application and performance of coating systems’, Thomas Walsh, RWE npower

After a brief outline of typical offshore wind farm assets, the presentation gave a description of the foundation configuration of monopile based Wind Turbine Generator (WTG) structures. The corrosion protection requirements for the primary and secondary steelwork of the Transition Piece (TP) both internally and externally was discussed with reference to the design life and corrosivity classifications contained within EN ISO 12944-2.

The development of a specification for the protection of the foundation structure was discussed in terms of the durability requirements and corrosivity of the exposure environment. The logistics and challenges of coating multiple items to the required standard was also discussed with a particular emphasis placed on the detail required for inclusion in the corrosion protection specification.

Examples of factory based corrosion protection treatments were presented. These included both hot dip galvanising (HDG) treatment of secondary steel work and organic coating application to the primary steelwork of the TP.

The presentation concluded with some examples of the field performance of coating systems together with a brief discussion on the key technical requirements for the warranty of such coating systems.

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1.2 ‘Cathodic Protection System Selection for Corrosion Mitigation inside Offshore Wind Turbine Monopiles’, Robin Jacob, CP Consultancy

Monopiles are the most commonly used foundation for offshore wind farms. These large diameter steel piles are flooded with seawater when installed and the internal surfaces are usually uncoated. To date, corrosion mitigation has most commonly relied on producing an air tight environment by sealing the internal decks and j-tube penetrations for cables, thus preventing seawater and oxygen ingress. As a result, it is assumed that corrosion of the internal steel surfaces will effectively cease once the oxygen originally present is consumed. In practice, the sealing arrangements often become ineffective, resulting in corrosion of the internal surfaces due to the replenishment of oxygen, either by air ingress above the waterline, or by tidal replenishment of seawater.

The presentation discusses the applicability of internally installed cathodic protection to overcome the corrosion experienced in the event of oxygen ingress. In particular the changes in chemistry produced by cathodic protection in enclosed spaces are highlighted, as these lead to markedly different design requirements compared to the more normal open seawater condition. These changes impose risks which require specific consideration, in particular the generation of hydrogen, and its accumulation in the air space in the monopile, and the development of acid conditions in the seawater. Conclusions are reached in respect of cathodic protection system selection and design, and comment made on supply and installation considerations, particularly in existing monopiles.

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4.1    Understanding Corrosion Protection by Formation of Iron Carbonate Films in CO2 Conditions’, Danny Burkle, University of Leeds

In the oil and gas industry, internal corrosion of carbon steel pipelines is commonly encountered during production and transportation. Iron carbonate is the main corrosion product layer in a CO2 corrosion environment. The formation of an iron carbonate layer can protect the steel from further corrosion by acting as a diffusion barrier and also by covering portions of the steel surface. Partial removal of the protective iron carbonate layer can lead to severe localised corrosion by exposing the bare steel substrate inducing a galvanic effect established between layer-covered and layer-free areas. Understanding the mechanisms of protective iron carbonate layer is extremely important.

The focus of this project is to investigate and understand the factors governing the rate of precipitation, formation and morphology of the iron carbonate layer on the corroding surface after changing key influential parameters under flowing conditions. This is an important step in order to take advantage of the positive attributes of iron carbonate film formation and reduce the occurrence of localised corrosion attack. This is attempted through the development of a novel flow cell design to characterise the iron carbonate layer in-situ. Both static and hydrodynamic experiments will be carried and in-situ XRD analysis of carbon steel surface in a CO2 corrosive environment which will provide real-time kinetics on the different phases forming on the corroding surface. It is intended that this research will provide new insights into the early stages of kinetics and formation of the iron carbonate layer and how it develops over time.

A number of questions are still being asked in the area of formation of iron carbonate, its removal and the level of protection offered in different environments relating to the gaps. The research conducted aims to answer such questions and has the potential to enable oil and gas companies to improve integrity management through identifying conditions which may result in localised corrosion as a result of iron carbonate removal.

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4.2   CP and Structural Assessments of Aging Jetties - What Laid Beneath’, Chris Lynch, Corrpro Companies Europe

This presentation looked in detail at a survey done on an aging jetty system. Problems found were discussed and solutions recommended.

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