The design of OWTs relies on integrated load analyses tools that simulate the response of the entire OWT (including the rotor-nacelle assembly, support structure and foundation) under combined aerodynamic and hydrodynamic loading. Despite all efforts to develop accurate integrated models, these often fail to reproduce the measured natural frequencies, partly due to the current foundation modelling. This paper presents a new foundation model for integrated analyses of monopile-based OWTs. The model follows the macro-element approach, where the response of a pile and the surrounding soil is condensed to a force-displacement relation at seabed. The model formulation uses multi-surface plasticity and it reproduces key characteristics in monopile foundation behaviour that are not accounted for in current industry practice. The basic features of the model are described and its limitations are discussed. The performance of the macro-element model is compared against field test measurements and results from FEA. The comparison indicates that the macro-element model can reproduce accurately the non-linear load-displacement response and hysteretic behaviour measured in field tests and computed in FEA. This confirms that the model can simulate the pile and soil behaviour with the same level of accuracy as FEA, but with a considerable reduction in computational effort.
This paper presents an approach for reliability analysis of engineering structures, referred to as Metamodel Line Sampling (MLS). The approach utilizes a metamodel of the performance function, within the framework of the Line Sampling method, to reduce computational demands associated with the reliability analysis of engineering structures. Given a metamodel of the performance function, the failure probability is estimated as a product of a metamodel-based failure probability and a correction coefficient. The correction coefficient accounts for the error in the metamodel estimate of failure probability introduced by the replacement of the performance function with a metamodel. Computational efficiency and accuracy of the MLS approach are evaluated with the Kriging metamodel on analytical reliability problems and a practical reliability problem of a monopile foundation for offshore wind turbine. The MLS approach demonstrated efficient performance in low to medium-dimensional reliability problems.
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