A New Model for Fatigue Load Sequence Effects in Offshore Wind Turbine Substructures and Its Implications for Design Life
- R. C. Dragt (Department of Structural Dynamics) | S. T. Hengeveld (Netherlands Institute for Applied Scientific Research (TNO)) | J. Maljaars (Eindhoven University of Technology)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- underloads, overloads, Offshore Wind Turbines, load sequence effects, crack growth, fatigue
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Measurements at Offshore Wind Turbine (OWT) substructures show that typical loading patterns include overloads, underloads, and shifts in mean stress cycles, due to the everchanging wind direction. These events follow each other in time and are further expressed as load sequences. Load sequences may affect the fatigue life. A novel model consisting of a set of explicit equations is developed to estimate the fatigue crack growth including effects of load sequences. The model is based on both experiments and finite element simulations. The model allows for (combinations of) over- and underloads and mean changes. With this model, an extrapolation is made towards realistic loading on OWT substructures to gain insight into the expected effect of load sequences on the fatigue life.
The fatigue damage (or life) estimations for Offshore Wind Turbine (OWT) substructures are currently made using the Palmer-Miner rule of linear cumulative damage, which allows to consider the contribution to damage of large and small stress ranges. However, this rule does not consider the experimentally observed possibility of acceleration or retardation of crack growth, e.g. when many small stress cycles are preceded by a large stress cycle. To illustrate, Fig. 1 shows the outcome of an experiment where a large stress peak, called an overload (OL), results in retardation of the crack growth rate. This retardation effect is experimentally shown for various materials, for example aluminium (e.g. Yisheng and Schijve, 1995) and steel (e.g. Maljaars et al., 2015).
In realistic OWT structures, only considering Constant Amplitude (CA) loading is not sufficient to describe the complex loading behaviour. Moving towards a Variable Amplitude (VA) loading means that load sequence effects, such as retardation and acceleration, may have to be considered when estimating the fatigue crack growth.
The joint industry project FeLoSeFI (Fatigue life Load Sequence effects and Failure probability driven Inspection) investigated the effect of load sequences on the fatigue life of OWT substructures and developed models and tools to estimate the fatigue damage and optimize maintenance intervals (Courage et al., 2017; Dragt et al., 2017). This paper describes the use of fracture mechanics methods to estimate the crack growth while including load sequences, based on the load cycles applied and the current state of the crack.
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