Integrating Diverse Approaches to the Reliability of Engineering Structures: Applying Probabilistic ECA to Offshore Wind Turbines
TWI Industrial Member Report 1171-2023 [pdf / 1327 KB]
By Yin Jin Janin
Industrial Need
The probability of failure, POF (the converse of which is ‘reliability’) of engineering components and structures depends on numerous factors, including design, manufacture, inspection, operation and human factors. Reliability may be calculated on the basis of direct observation (what proportion of components fail prematurely?) or theoretical methods (what failure mechanisms are possible, and how likely is it that the conditions for failure will be present during operation?), or some mixture of the two. In this study, two methods are used to estimate probability of failure, using approaches drawn from a range of standards, and comparing the outcomes with situations related to other standards. Relevant documents include BS 7910 (BSI, 2019), R6 (EDF Energy, 2001), API 581 (API, 2016), PD 5500 (BSI, 2018) and EN 13445 (BSI, 2014), and ASME B31.3 (ASME, 2018):
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A ‘top-down’ method based on a mixture of failure statistics and engineering judgement.
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A ‘bottom-up’ method based on fracture mechanics/engineering critical assessment (ECA). Both ‘deterministic’ (using a single characteristic value of each variable) and probabilistic approaches were used, the latter allowing direct estimation of probability of failure.
Key Findings
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PFM calculations have been carried out to estimate probability of failure (POF) of a longitudinal electron-beam weld (EBW) and a conventional circumferential metal-arc weld in a grade S355 offshore wind turbine (OWT) monopile, operating at ‑10°C under ultimate limit state (ULS) and fatigue limit state (FLS) loading conditions.
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Values of (probability of failure with tensile properties as stochastic variables) were consistently lower for both longitudinal and circumferential welds than those of (probability of failure with fracture toughness as a stochastic variable). This highlights the significance of toughness properties and crack driving force in determining the overall failure probabilities.
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Reduction in POF by two to three orders of magnitude could be achieved if welds are stress-relieved. But this alone was not sufficient to justify the need for post-weld heat treatment given there were other significant factors (economic and logistic) to be considered.
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Probabilistic fracture mechanics (PFM) calculations performed as combined fracture and fatigue assessments estimated similar POF as those predicted using fracture only assessments. It was demonstrated that the same results and significantly shorter computational time could be achieved when fracture only PFM calculations were performed assuming a given grown size associated with a given interval as long as fatigue crack growth parameters were handled deterministically.
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PFM calculations treating Paris law parameters alone as a stochastic variable did not predict any failure. Fracture toughness as a variable in combined fracture and fatigue PFM calculations has demonstrated to have a greater effect on POF than other variables.