Mechanical characterisation of protective coatings for offshore wind turbine towers and transition pieces

Lopez Rodriguez, Isbelis Coromoto (2018) Mechanical characterisation of protective coatings for offshore wind turbine towers and transition pieces. Doctoral thesis, Northumbria University.

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Abstract

The wind energy industry is increasingly moving to offshore areas and increasing the wind turbines’ size and number. These are unmanned structures with limited access, costs of repairs are significantly higher than for their onshore counterparts, and higher reliability of the corrosion protection system is required. In recent years research has directed interest towards developing robust coating materials capable of tolerating hostile marine climates with minimum maintenance. When epoxy-based solvent-borne paints are applied onto metallic substrates and cured, intrinsic residual stresses originate within the coating due to film formation processes (cross-linking and solvent loss processes) and adhesion to the substrate. Excessive tensile residual stresses have an adverse effect on adhesion and may compromise the integrity of a complex system constituted by a substrate and coating scheme. Additionally, they reduce the coating’s capacity to undergo further tensile stresses during service.

In this study, three epoxy-based paint systems, used on turbine towers and transition pieces, were airless-sprayed onto thin steel substrate and cured to produce bi-layer samples. Additionally, free films were produced. Mechanical tests on supported and unsupported coatings were performed using dynamic mechanical analysis. A mathematical model based on composite beam stress analysis was applied to determine the Young’s modulus and residual stresses of the coatings. The mechanical and physical properties of the marine coatings were found to be dependent on coating thickness. The Young’s modulus of the coatings decreases with an increase in coating thickness. This was found to be due to increasing solvent retention, porosity and stiffness associated with an increase in coating thickness. Retained solvent acts as a plasticiser reducing the tensile strength of the material and the glass transition temperature. The rate of evaporation of the solvent also has a relevant effect on the mechanical response of the material; fast solvent loss rates were found to be associated with lower internal stress development. In the present study, it has been found that it is possible to design more reliable and predictable coating systems by a judicious manipulation of the paint formulation, and control of the dry film thickness and postcure schedule. A proper understanding of the stress development of organic coatings and the influencing factors is invaluable for not only offshore wind farms but also any structure where long-term corrosion protection (over 25 years) is required with minimal maintenance.

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