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Structural Integrity of S355 TMCP Steels in Offshore Applications: Insights from Fatigue Testing
The steel industry is actively working to develop high-quality steel grades. These grades are often specified by European standards like EN10025 S355 (or EN10225 S355).
Table A outlines common structural grades used in offshore designs, including wind turbine support structures. Table B shows advanced thermomechanically controlled processed (TMCP) steels used for XL wind turbine support structures.
Studies show that S355 TMCP steels generally offer better fatigue damage tolerance compared to standard S355 steels. See papers: 1. Materials selection for XL wind turbine support structures: A corrosion-fatigue perspective, 2. Critical assessment of fatigue crack growth rate sensitivity in ferrite-pearlite steels.
However, when these steel grades are welded to form components, any claimed superiority among S355 subgrades becomes less relevant. This is because the overall structural integrity shifts to the welded joints. The structural integrity of the component is further reduced if the welded joint is exposed to fluctuating stress environments.
In thick plates, cracks start in a small zone within the heat-affected zone (HAZ) near the weld toe. Picture C illustrates this tendency. The cracks propagate into the parent metal rather than through the HAZ. If a crack propagates through the HAZ, the grade of the base material becomes irrelevant. If it propagates through the base metal, the fatigue-resistant properties of the baseplate become critical.
In thinner materials, crack is more likely to propagate through the HAZ. In thicker plates, crack tend to propagate through the base metal. Hence, material selection should be tailored to these different conditions.
– Studies carried out at Structural Integrity Lab, Cranfield University.
