PSI - Issue 57

Baran Yeter et al. / Procedia Structural Integrity 57 (2024) 133–143 Baran Yeter & Feargal Brennan/ Structural Integrity Procedia 00 (2023) 000 – 000

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4.2. Probabilistic Structural Integrity Assessment Probabilistic methods offer a more sophisticated way to measure structural safety against the failure mechanisms such as fatigue and fracture than the deterministic approach because the uncertainties inherent in these failure mechanisms are quite significant, thus adopting a conservative approach can result in overdesigning the structures (Yeter & Garbatov, 2022). The present study aims to contribute to understanding the probabilistic nature of fatigue crack growth (FCG) from a stochastic loading standpoint. It is expected to have a discrepancy between FCG under constant amplitude and variable amplitude loading. This is because the FCG accounts under variable amplitude loading (VAL), accounting for the effects stemming from the sequence of VAL as well as the interaction between the high-amplitude stress cycle followed by low-amplitude stress cycles. It is also worth noting that some of the stress cycles in time history are not sufficient to contribute to the crack growth, especially after plasticity occurred as a result of an overload. Based on the methodology presented in the previous sections, the probabilistic structural integrity assessment is performed through the Monte Carlo simulations using the Latin hypercube sampling to regenerate the time history representing the long-term structural response of the FOWT. Fig. 6 (left) demonstrates how uncertainty around crack growth increases over time. The result of the probabilistic assessment indicates that a crack already shows a fast growth phase after 15 years of service life; however, there is a significant variation regarding when the through thickness crack is reached. The uncertainty regarding the crack size at a given time during the service is presented in terms of the Coefficient of Variation (CoV) in Fig. 6 (right), where CoV can reach up to 10% towards the end the service life. Such a degree of uncertainty must be included in the fatigue reliability assessment needed for inspecting and maintaining the FOWT structure.

Fig. 6. Scatter of the crack propagation over the service life (left) and change in CoV over the years The variability in fatigue crack growth (FCG) can exacerbate when the overload-induced plasticity is included in the FCG analysis. This is particularly important for offshore wind turbine structures because a considerable portion of the stress ranges are present around the low end of the stress range distribution, which is different from other marine structures like offshore and ship structures whose stress range distribution is determined by the waves-induced loads. The impact is expected to matter more when the crack is deep enough. To illustrate this clearly, an FCG simulation under 1-year variable amplitude service loading is carried out for the tubular connection at the tower base with an initial crack depth of 15 mm. As shown in Fig. 7, the difference in the crack depth at a given time between FCG with and without the effect of overload-induced plasticity is quite significant; moreover, this difference increases with time. It can be argued that this difference should be much less during the early stages of service life; however, a larger difference should be expected for the later stages of service life. In addition, the estimated crack growth ranges from approximately 21 mm to 38 mm after a year of service loading, which infers that the FCG considering load sequence and retardation effects, is subjected to a higher degree of uncertainty.