Enzo Marino

Simulation of Nonlinear Waves on Offshore Wind Turbines and Associated Fatigue Load Assessment

By using a global simulation framework that employs a domain-decomposition strategy for computational efficiency, this study investigates the effects of fully nonlinear (FNL) waves on the fatigue loads exerted on the support structure (monopile) of a fixed-bottom offshore wind turbine. A comparison is made with more conventional linear wave hydrodynamics. The FNL numerical wave solver is invoked only on specific sub-domains where nonlinearities are detected; thus, only locally in space and time, a linear wave solution is replaced by the FNL results as input to the Morison equation used for the hydrodynamic loads. The accuracy and efficiency of this strategy allows long timedomain simulations where strongly nonlinear free-surface phenomena, like imminent breaking waves, are accounted for in the prediction of structural loads. The unsteady nonlinear free-surface problem governing the propagation of gravity waves is formulated using potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. The FNL solver is employed and associated hydrodynamic loads are predicted in conjunction with aerodynamic loads on the rotor of a 5-MW wind turbine using the NREL open-source software, FAST. We assess fatigue loads by means of both time- and frequency-domain methods. This study shows that the use of linear theory-based hydrodynamics can lead to significant underestimation of fatigue loads and damage.© 2014 ASME

Irregular Nonlinear Wave Simulation and Associated Loads on Offshore Wind Turbines

The accuracy of predicted loads on offshore wind turbines depends on the mathematical models employed to describe the combined action of the wind and waves. Using a global simulation framework that employs a domain-decomposition strategy for computational efficiency, this study investigates the effects of nonlinear waves on computed loads on the support structure (monopile) and the rotor-nacelle assembly of a bottom-supported offshore wind turbine. The fully nonlinear (FNL) numerical wave solver is invoked only on sub-domains where nonlinearities are detected; thus, only locally in space and time, a linear solution (and associated Morison hydrodynamics) is replaced by the FNL one. An efficient carefully tuned linear-nonlinear transition scheme makes it possible to run long simulations such that effects from weakly nonlinear up to fully nonlinear events, such as imminent breaking waves, can be accounted for. The unsteady nonlinear free-surface problem governing the propagation of gravity waves is formulated using potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. The FNL solver is employed and associated hydrodynamic loads are simulated in conjunction with aerodynamic loads on the rotor of a 5-MW wind turbine using the NREL open-source software, FAST. We assess load statistics associated with a single severe sea state. Such load statistics are needed in evaluating relevant load cases specified in offshore wind turbine design guidelines; in this context, the influence of nonlinear wave modeling and its selection over alternative linear or linearized wave modeling is compared. Ultimately, a study such as this one will seek to evaluate long-term loads using the FNL solver in computations directed towards reliability-based design of offshore wind turbines where a range of sea states will need to be evaluated.Copyright

Large wind turbines in earthquake areas: structural analyses, design/construction & in-situ testing

The Wind Energy Roadmap for the European Union was published by the European Commission on Oct. 7th, 2009, in the framework of its Communication of Financing Low Carbon Technologies. Following its publication, the roadmap was officially presented and discussed at the Strategic Energy Technology Plan (SET-Plan) workshop, held in Stockholm on October 21st and 22nd 2009, and organised by the European Commission and the Swedish Energy Agency.

Reliability Analysis of Offshore Wind Turbines : A BEM Model for Nonlinear Water Waves

This paper introduces the first part of a wider project in progress aiming at integrating numerical simulations into the general framework of Structural Reliability Analysis of Offshore Wind Turbines. Offshore systems today mainly require novel design procedures (e.g. structural optimization) which assure the minimization of the costs under the constraint of a fixed Structural Reliability (SR). Since the SR is never an absolute value it considerably depends on many factors such as the accuracy level of the idealized structural model in this first part the attention is most paid to restrict as much as possible the sources of uncertainties solely to those parameters which are intrinsically random by providing a tool which does not assume any a priori restriction on the water waves model and, at the same time, permits to compute the hydrodynamic forces with an adequate computational cost to perform Reliability Analyses. After a short presentation of the whole project, this contribution focuses on a Boundary Element model aiming at simulating fully nonlinear water waves associated to extreme events. While the rotor aerodynamics and a full-model simulation will be addressed in further developments to finally implement the most suitable method to perform Structural Reliability Analyses (SRA). Contact person: E. Marino, Department of Civil and Environmental Engineering, University of Florence, Via di Santa Marta, 3 – 50139 Firenze. Tel./fax 0039 (0)55 4796306. E-mail enzo.marino@dicea.unifi.it

Strength of Materials

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