Fitness-for-purpose assessment of cracked offshore wind turbine monopile |
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Affiliation: | 1. Fluid Structure Interactions Group, University of Southampton, Boldrewood Innovation Campus, Southampton, UK;2. Lloyd’s Register Global Technology Centre, Boldrewood Innovation Campus, Southampton, UK;1. Key Laboratory of High Performance Ship Technology of Ministry of Education, Wuhan University of Technology, Wuhan, 430063, Hubei, PR China;2. School of Naval Architecture, Ocean and Energy Power Engineering, Wuhan University of Technology, Wuhan, 430063, Hubei, PR China |
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Abstract: | In this paper, the procedure for flaw acceptability assessment is examined through a case study of a semi-elliptical surface crack in an offshore monopile as it grows till it forms a through thickness crack. Using the procedure prescribed in an industrial standard (BS 7910), the fracture ratio, Kr is shown to increase monotonically with increasing crack depth. The load ratio, Lr, is initially insensitive to the crack depth. However, there is a rapid increase in Lr when the crack depth to thickness ratio exceeds 80%. Lr values obtained from detailed 3D FE limit analysis using elastic-perfectly-plastic material behaviour do not exhibit the asymptotic behaviour predicted by BS 7910 as the flaw transitions from deep crack to through-thickness crack. Furthermore, Kr predicted by BS 7910 is shown to be an over-estimation for the typical dimensions of offshore monopiles. The findings suggest that a structure with a deep flaw may be identified as unacceptable based on BS 7910 when it may still possess a non-trivial amount of structural residual life. This is a concern for monopiles where crack growth as a large flaw forms a significant part of the total life. |
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Keywords: | Failure assessment diagram Fracture ratio Load ratio Stress intensity factor Reference stress Limit load Monopile |
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