Browsing by Author "Müller, Kolja"

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Open Access
Application of a Monte Carlo procedure for probabilistic fatigue design of floating offshore wind turbines
(2018) Müller, Kolja; Cheng, Po Wen
Fatigue load assessment of floating offshore wind turbines poses new challenges on the feasibility of numerical procedures. Due to the increased sensitivity of the considered system with respect to the environmental conditions from wind and ocean, the application of common procedures used for fixed-bottom structures results in either inaccurate simulation results or hard-to-quantify conservatism in the system design. Monte Carlo-based sampling procedures provide a more realistic approach to deal with the large variation in the environmental conditions, although basic randomization has shown slow convergence. Specialized sampling methods allow efficient coverage of the complete design space, resulting in faster convergence and hence a reduced number of required simulations. In this study, a quasi-random sampling approach based on Sobol sequences is applied to select representative events for the determination of the lifetime damage. This is calculated applying Monte Carlo integration, using subsets of a resulting total of 16 200 coupled time-domain simulations performed with the simulation code FAST. The considered system is the Danmarks Tekniske Universitet (DTU) 10 MW reference turbine installed on the LIFES50+ OO-Star Wind Floater Semi 10 MW floating platform. Statistical properties of the considered environmental parameters (i.e., wind speed, wave height and wave period) are determined based on the measurement data from the Gulf of Maine, USA. Convergence analyses show that it is sufficient to perform around 200 simulations in order to reach less than 10 % uncertainty of lifetime fatigue damage-equivalent loading. Complementary in-depth investigation is performed, focusing on the load sensitivity and the impact of outliers (i.e., values far away from the mean). Recommendations for the implementation of the proposed methodology in the design process are also provided.
Open Access
Comparison of measured and simulated structural loads of an offshore wind turbine at Alpha Ventus
(2016) Müller, Kolja; Reiber, Mario; Cheng, Po Wen
A comparison of fatigue and extreme loads from simulations with full-scale measurements collected over a period of ten months in the offshore test field, Alpha Ventus, is presented in this paper. There are two goals of this study: (1) to check if the measured range of fatigue and extreme loads can be captured correctly by simulations when the variations of relevant environmental parameters are taken into account; and (2) to investigate if measured extreme loads can be reproduced by simulations when ten-minute averages of the environmental parameters are used. The results show a good overall match of loads when the variation of environmental parameters is considered but an insufficient match when the events of maximum load occurrence are compared.
Open Access
Improved tank test procedures for scaled floating offshore wind turbines
(2014) Müller, Kolja; Sandner, Frank; Bredmose, Henrik; Azcona, José; Manjock, Andreas; Pereira, Ricardo
This study collects issues from previous tank test campaigns of scaled Floating Offshore Wind Turbines (FOWT), compares the different scaling methodologies, points out critical aspects and shows possible alternatives and recommendations for future tests depending on the specific objective. Furthermore, it gives practical recommendations for the modeling and construction of scaled rotors. The presented scaling procedure will be applied in tank tests within the EU Seventh Framework Program InnWind (ENERGY.2012.2.3.1 "Innovative wind conversion systems (10-20MW) for offshore applications").
Open Access
Load sensitivity analysis for a floating wind turbine on a steel semi-submersible substructure
(2018) Müller, Kolja; Faerron Guzman, Ricardo; Cheng, Po Wen; Galván, Josean; Sánchez Lara, Miren; Rodríguez Arías, Raúl; Manjock, Andreas
The site-specific load verification for floating offshore wind turbines requires the consideration of the complex interaction of the different system components and their environment. Sensitivity analyses help reducing the simulation amount for both fatigue and ultimate load analysis significantly by highlighting relevant load parameters and increase the understanding for the system behavior in its real environment. Aligned with work in the H2020 project LIFES50+, this study investigates different approaches for global sensitivity analysis using quasi-random sampling for the independent variables. Two different load case groups are analyzed: (1) fatigue loads during power production, (2) ultimate loads during power production and severe sea state. The considered system is the public DTU 10MW turbine’s rotor-nacelle assembly, installed on the public NAUTILUS-10 floating structure. Load simulations are performed by using FAST v8. Simulations are set up based on the LIFES50+ Site B (medium severity). A comparison is made to a similar study with a different platform (Olav Olsen semi-submersible) in order to observe if similar conclusions can be reached for the different floater types.
Open Access
Optimization of floating offshore wind turbine platforms with a self-tuning controller
(2017) Lemmer, Frank; Müller, Kolja; Yu, Wei; Schlipf, David; Cheng, Po Wen
The dynamic response of floating offshore wind turbines is complex and requires numerous design iterations in order to converge at a cost-efficient hull shape with reduced responses to wind and waves. In this article, a framework is presented, which allows the optimization of design parameters with respect to user-defined criteria such as load reduction and material costs. The optimization uses a simplified nonlinear model of the floating wind turbine and a self-tuning model-based controller. The results are shown for a concrete three-column semi-submersible and a 10MW wind turbine, for which a reduction of the fluctuating wind and wave loads is possible through the optimization. However, this happens at increased material costs for the platform due to voluminous heave plates or increased column spacing.
Open Access
A surrogate modeling approach for fatigue damage assessment of floating wind turbines
(2018) Müller, Kolja; Cheng, Po Wen
Fatigue analysis for floating wind turbines poses a novel challenge to calculation workflows if a probabilistic load environment is to be considered. The increased complexity of the structure itself as well as its interaction with the environment require a coupled and more detailed analysis with respect to resolution of environmental conditions compared to fixed bottom systems. Different approaches to address the computing challenge for floating turbines are possible to support engineering judgement and have been investigated in the past, with conservative binning on the one end of the accuracy scale and computation intensive Monte Carlo simulations on the other end. This study investigates the feasibility of regression based surrogate models based on radial basis functions. The investigation performed here is aligned with work performed in the H2020 project LIFES50+. Consequently, the considered system is the DTU 10MW Reference Wind Turbine installed on the LIFES50+ OO-Star Wind Floater Semi 10MW. The site under investigation is the LIFES50+ Site B (Gulf of Maine) medium severity representative site. Results show a similar convergence of lifetime fatigue load prediction as with Monte Carlo simulations indicating that this technique may be an alternative if a response model of the considered system is of interest. This may be interesting if damage loading is to be calculated at a different site and if a classification of met-ocean conditions is available.