Abstract
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.
Highlights
Wind turbines are playing a key role in decarbonising world power production systems
The crack growth stress is taken as the fatigue load case which corresponds to an operating state under the normal turbulence model (NTM) and normal sea state (NSS) where wave height and cross zero periods are obtained from the joint probability function of the site, assuming no current; it corresponds to Design Load Case (DLC) 1.2 from the IEC standard (IEC, 2019) and is assumed to represent the entire fatigue state (Gentils et al, 2017)
The accelerated crack growth rate is reflected in fracture mechanics by changing the Paris law constants to those observed in the corrosive environment
Summary
Wind turbines are playing a key role in decarbonising world power production systems. There are two approaches for quantifying fatigue damage: the S–N (stress vs number of cycles) method and the fracture mechanics (FM) approach Standards such as IEC 61400-3 (IEC, 2009), DNVGLST-0126 (DNVGL, 2016a), DNVGL-ST-0437 (DNVGL, 2016b), and DNVGL-RP-C203 (DNV, 2010) are commonly used for the design of offshore wind turbines against fatigue failure. Quality by developing and implementing new welding technologies Those processes may inevitably have altered characteristics (defect rates, sizes, and geometry; residual stresses; material toughness; etc.), which affect fatigue failure of the joint. Considering these variables using S–N data requires the development of a bespoke fatigue test programme which is not always feasible (Lassen and Recho, 2013). The application of the developed methods to a monopile support structure is demonstrated
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