• Elahi, Seyed Ahmad
• Mehri Sofiani, Farid
• Waele, Wim De
• Chaudhuri, Somsubhro

Abstract

The lifetime of steel support structures for offshore wind turbines is affected by fatigue and corrosion damage. Due to the time-variant uncertainties associated with environmental conditions and mechanical stresses, numerical models that can accurately predict corrosion and fatigue deterioration are needed for a reliable assessment of structural integrity. One of the main ambitions of the project MAXWind is to develop a numerical framework for corrosion-fatigue assessment, which consists of models for pitting corrosion, initiation and propagation of short cracks from pits, and long crack propagation. Integrating these models allows to develop so-called “smart” S-N curve. A microstructure-based model is used to simulate the initiation and propagation of short cracks from corrosion pits. Using data of pit shape evolution versus time of exposure to sea water as input to the short crack model, the degraded fatigue strength of S355 steel due to pitting corrosion representative for the North Sea is determined (Figure 1). Figure 2 illustrates the concept of the so-called smart S-N curve and virtual load spectra used to calculate fatigue damage accumulation. The black curve is the S-N curve in air, i.e., without exposure to sea water, of which the knee-point is arbitrarily taken at 10^7 cycles. The fatigue strength degradation shown in Figure 1 corresponds to the orange line (fatigue strength line) in Figure 2. The fatigue damage accumulation corresponding to the load spectrum for a specific year can be computed using Miner’s rule. The load bins which are below the fatigue strength line are assumed not to contribute to damage accumulation. Since pitting corrosion causes the fatigue strength line to have a decreasing behavior, more load bins with lower stress values will contribute to damage accumulation as time passes. Using representative load spectra based on monitoring campaigns and characterizing the time-dependent fatigue strength of the material, will allow to assess the lifetime of an OWT structure in the design stage. In future work, the corrosion fatigue model will be extended with algorithms for long crack growth simulation.This will allow to calculate the remaining useful life of OWT structures and can support decision-making activities, viz. definition of optimized inspection and maintenance plans, and finally have an impact on reducing energy production O&M expenses.

Topics

• impedance spectroscopy
• microstructure
• simulation
• crack
• strength
• pitting corrosion
• steel
• fatigue