A fracture mechanics framework for optimising design and inspection of offshore wind turbine support structures against fatigue failure

Amirafshari, Peyman and Brennan, Feargal and Kolios, Athanasios (2021) A fracture mechanics framework for optimising design and inspection of offshore wind turbine support structures against fatigue failure. Wind Energy Science, 6 (3). pp. 677-699. ISSN 2366-7451 (https://doi.org/10.5194/wes-6-677-2021)

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Abstract

Offshore wind turbine (OWT) support structures need to be designed against fatigue failure under cyclic aerodynamic and wave loading. The fatigue failure can be accelerated in a corrosive sea environment. Traditionally, a stress-life approach called the S-N (stress-number of cycles) curve method has been used for the design of structures against fatigue failure. There are a number of limitations in the S-N approach related to welded structures which can be addressed by the fracture mechanics approach. In this paper the limitations of the S-N approach related to OWT support structure are addressed and a fatigue design framework based on fracture mechanics is developed. The application of the framework to a monopile OWT support structure is demonstrated and optimisation of in-service inspection of the structure is studied. It was found that both the design of the weld joint and non-destructive testing (NDT) techniques can be optimised to reduce in-service inspection frequency. Furthermore, probabilistic fracture mechanics as a form of risk-based design is outlined and its application to the monopile support structure is studied. The probabilistic model showed a better capability to account for NDT reliability over a range of possible crack sizes as well as to provide a risk associated with the chosen inspection time which can be used in inspection cost-benefit analysis. There are a number of areas for future research, including a better estimate of fatigue stress with a time-history analysis, the application of the framework to other types of support structures such as jackets and tripods, and integration of risk-based optimisation with a cost-benefit analysis.

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