MS#05.2 Corrosion Fatigue of Offshore Renewable Energy Structures

S. SHOJAI¹, A. MEHMANPARAST², E. GHAFOORI³, W. DE WAELE⁴
¹ DLR - Institute of Maritime Energy Systems|² University of Strathclyde|³ Institute for Steel Construction Leibniz University Hannover|⁴ University of Gent

Structures, structural integrity, materials

Offshore renewable energy structures are highly susceptible to corrosion under mari-time environmental conditions. Especially for wind turbines, which are additionally ex-posed to cyclic loading, this aspect becomes important. Despite corrosion protection systems, corrosion cannot be completely avoided during the lifetime of the structures.
Corrosion on offshore steel structures can occur as uniform corrosion, which causes thickness reduction, or pitting corrosion, which leads to local stress concentrations on the steel surface. Additionally, corrosion affects the mechanical material properties. As a result of the acidic conditions under the rust layer, hydrogen can be generated and diffuse into the steel surface. This leads to embrittlement of the material, which lead to faster crack initiation and propagation.
The fatigue design based on DNV considers both physical phenomena by reducing available Stress-life (SN) curves with a so-called environment reduction factor (ERF) for free corrosion (FC) and cathodic protection (environment).
However, this is only a rough estimate with scientific and mechanistic lacks, as the cor-rosion process and fatigue physics were convoluted in these tests. With this method, a consideration of stress concentrations caused by pitting or the embrittlement of material is not possible. Neither a differentiation is made between welded and non-welded, nor between moderately and strongly notched structural components, which can act differ-ently to corrosive environment.
Based on these research gaps, the following research objectives are identified and should be addressed in the Mini-Symposium:

• SN based fatigue
• Crack growth
• Numerical modelling
• Corrosion development
• Free corrosion/cathodic protection
• Different environmental impact zones (splash zone/submerged etc.)
• Microbiogical induced corrosion
• Accelerated corrosion fatigue testing