Fatigue crack initiation in corroded offshore mooring chains
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Mooring chains on floating offshore structures suffer from combined degradation from corrosion and fatigue. Corrosion reduces cross-sectional areas of the chains and creates surface pits. During cyclic loading of the chains, these pits can raise stress amplitudes locally and act as initiation points for fatigue cracks. The current work is focused on fatigue initiation in R4-grade mooring chain steel from both naturally corroded surfaces, idealized geometries of corrosion pits and fatigue cracks blunted by corrosion. The full-field digital image correlation (DIC) technique was used to measure crack initiation on irregular, corroded surfaces from offshore mooring chain during fatigue testing in air. Finite element analysis (FEA) was paired with the DIC measurements to establish an initiation S-N curve for the surface. The first cracks initiated at 13-24% of the lifetimes of the specimens. Initiation lives were much shorter than in specimens with machined hemispherical notches, indicating that fatigue tests of machined specimens yield nonconservative initiation life estimates for mooring chains. Crack initiation was measured close to the mouths of the hemispherical notches. In a straincontrolled fatigue test, mean stress relaxation (MSR) was observed even at a stress amplitude below the cyclic yield stress. When taking cyclic softening and MSR into account in fatigue initiation predictions for the hemispherical notches, accuracy was significantly improved compared to calculations based on a monotonic material model. Other specimens were subjected to alternating phases of corrosion and fatigue to replicate in-service seasonal load variations. No significant difference in fatigue lifetimes was registered between specimens subjected to continuous fatigue and those periodically interrupted by phases of accelerated corrosion. Moreover, eight months of natural corrosion of specimens with fatigue cracks did not have a large effect on their remaining fatigue lifetimes. Thus, crack tip blunting from dissolution by corrosion during calm summer seasons is not expected to have a lifetime-extending effect on fatigue-cracked mooring chains. FEA stress calculations based on a dynamic load analysis of an offshore mooring system were performed. Results showed that the stress amplitudes in the fatigue tests performed in the current work represents well the highest stress amplitudes in the mooring chain links in service. It is suggested how the method for generating fatigue initiation S-N curves in the current work can be modified for fatigue tests in saltwater environments.
Består avPaper 1: Qvale, Paul; P. Zarandi, Ershad; Ås, Sigmund Kyrre; Skallerud, Bjørn Helge. Digital image correlation for continuous mapping of fatigue crack initiation sites on corroded surface from offshore mooring chain. International Journal of Fatigue 2021 ;Volum 151. s.
Paper 2: Qvale, Paul; P. Zarandi, Ershad; Arredondo, Alberto; Ås, Sigmund Kyrre; Skallerud, Bjørn Helge. Effect of cyclic softening and mean stress relaxation on fatigue crack initiation in a hemispherical notch. Fatigue & Fracture of Engineering Materials & Structures (FFEMS) 2022 s. -
Paper 3: Qvale, Paul; Nordhagen, Håkon Ottar; Ås, Sigmund Kyrre; Skallerud, Bjørn Helge. Effect of long periods of corrosion on the fatigue lifetime of offshore mooring chain steel. Marine Structures 2022 ;Volum 85. s. -