PSI - Issue 68

M. Álvarez et al. / Procedia Structural Integrity 68 (2025) 272–278

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M. Álvarez et al. / Structural Integrity Procedia 00 (2025) 000–000

a finite life and corrosion damage cannot be completely avoided during service life, leading to pitting corrosion on the steel surface. In early offshore wind turbines (OWT), monopile interior was designed as a closed compartment completely air and watertight, so low and uniform corrosion rates were expected. However, further inspections demonstrated water exchanges due to seals and airtight platform leakages, increasing corrosion rate and producing localized corrosion [Mathiesen et al. (2016)]. If the ingress of seawater is significant, tidal variations may occur directly inside the monopile and the water level may rise and fall with the tide inside the column as occurs in the outside, producing wet and immersed areas [Khodabux et al. (2020)]. This alternation of wet and dry stages is a very import factor to be considered, especially in the splash zone, where the external surface is exposed to atmospheric air and seawater during regular intervals defined by waves and tidal action. It has been noted that pitting corrosion rate increases significantly under wet–dry condition [Han et al.2014]. In addition to corrosion, monopile structure is subjected to a spectrum of structural loads, such as weight of the rotor and nacelle assembly, bending load from wind, wave loads, current loads, and vibrations due to rotor blades [Biswal et al. (2021)]. These loads lead to dynamic (monotonic or cyclic) stresses that increase corrosion pit growth kinetics [Jakubowski (2015)]. The coupling effect of fatigue degradation under cyclic loading conditions occurring in a corrosive environment must consider both the acceleration of stress on corrosion and the increase of corrosion over time [Liao et al. (2022)]. Some studies have considered precorroded samples with fatigue testing in air [Alvarez et al. (2022), Shojai et al. (2022), Shamir et al. (2023)], while other studies performed simultaneous CF tests with samples completely immersed [Morgantini 2020, Igwemezie et al. (2020)] or continuously wet by artificial seawater [Pargeter et al. (2008), Palin-Luc et al. (2010)]. This study aims to investigate the effect of wet-dry conditions in the remaining life of structural steel S355G10+M, during a combined corrosion-fatigue process in presence of localized corrosion defects. By means of a phenomenological approach, a comparison was established between experimental results and S-N curves in seawater environment constructed with theoretical models published in the literature. Additionally, experimental results and analytical predictions were compared against conservative design curves for S355 steel free corrosion in seawater. This information can be useful towards decision making in predictive O&M and the revision of designing rules in offshore monopiles with localized corrosion defects and subjected to free corrosion.

Nomenclature OWT Offshore wind turbine CF Corrosion-fatigue E Young’s modulus σ max Maximum stress value σ y Yield strength σ u Ultimate strength HV Vickers hardness R Load ratio SEM

Scanning Electron Microscope

Endurance limit

σ ∞

Endurance limit for corroded material

σ ∞,corr

2. Experimental conditions 2.1. Material

Structural steel S355G10+M was used in this study, with the following chemical composition (wt%) according to BS EN 10225 [British Standards Institution (2009)]: 0.12% C, 0.15-0.55% Si, 1.65%max Mn, 0.015% P, 0.005% S, 0.20% Cr, 0.08% Mo, 0.70% Ni, 0.015-0.055% Al, 0.30% Cu, 0.01% N, 0.03% Nb, 0.025% Ti, 0.06% V, 0.06% Nb+V, 0.08% Nb+V+Ti, and balance Fe. This material is commonly used to manufacture offshore wind turbines monopile foundations and many others offshore structures [Mehmanparast (2021)]. S355G10+M was characterized