Abstract
Abstract. Decoupled load simulations are a computationally efficient method to perform a dynamic analysis of an offshore wind turbine. Modelling the dynamic interactions between rotor and support structure, especially the damping caused by the rotating rotor, is of importance, since it influences the structural response significantly and has a major impact on estimating fatigue lifetime. Linear damping is usually used for this purpose, but experimentally and analytically derived formulas to calculate an aerodynamic damping ratio often show discrepancies to measurement and simulation data. In this study decoupled simulation methods with reduced and full rotor loads are compared to an integrated simulation. The accuracy of decoupled methods is evaluated and an optimization is performed to obtain aerodynamic damping ratios for different wind speeds that provide the best results with respect to variance and equivalent fatigue loads at distinct output locations. Results show that aerodynamic damping is not linear, but it is possible to match desired output using decoupled models. Moreover, damping ratios obtained from the empirical study suggest that aerodynamic damping increases for higher wind speeds.
Highlights
The simulation of an offshore wind turbine (OWT) in time domain under combined aerodynamic and hydrodynamic loading is currently considered as the most accurate method to analyse the support structure dynamics and forms the basis for interpretation of the characteristic combine load effect (DNVGL-ST, 2016)
A stochastic search method is used to minimize the objective function. This does not guarantee that the global minimum is found, but it is sufficient for this study, since this paper aims not to provide a methodology to obtain aerodynamic damping coefficients but to evaluate the accuracy and limitations of using decoupled simulations and linear dampers to represent the interaction between support structure and wind turbine rather than determining generally valid damping coefficients
The decoupled models differ in number of rotor load time series applied on tower top and the number of viscous dampers representing the dynamic interaction between support structure and rotor
Summary
The simulation of an offshore wind turbine (OWT) in time domain under combined aerodynamic and hydrodynamic loading is currently considered as the most accurate method to analyse the support structure dynamics and forms the basis for interpretation of the characteristic combine load effect (DNVGL-ST, 2016) It is still computationally demanding and requires special simulation software. Conceptual or parameter studies during the design process (Arany et al, 2017) or for research purpose (Cheng et al, 2012) usually necessitate to perform numerous simulations, thereby leading to a time-consuming task These simulations are, often carried out with different analysis methods, such as frequency-domain calculations (van der Tempel and de Vries, 2005; Ziegler et al, 2015), substructuring techniques (van der Valk and Rixen, 2012), and/or simplified or reduced models (Muskulus and Schafhirt, 2015). Harnessing the power of graphics processing units and performing the computation in parallel results in an additional 12 times faster execution time, thereby accelerating the dynamic analysis by a factor of 4722 compared to a time-domain simulation, including aeroelastic calculations (Schafhirt et al, 2015)
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