Nonlinear modal parameter identification in monopile-soil interaction by means of experimental data

In the design of offshore wind turbines, the dynamic interaction between a monopile foundation and the surrounding soil can be considered as one of the most difficult mechanisms to model. The existing methods for quantifying the stiffness provided by piles in soil are based on overly conservative methods from the petroleum industry. Also, the current modeled damping is not based on physically motivated damping principles. In the strive for wind energy to become more cost efficient, conservatisms need to be reduced. A more accurate model for both the stiffness and damping caused by the monopile-soil interaction is required. To this end, full scale dynamic shaker tests have been performed on an offshore monopile at the Westermeerwind wind farm in the Netherlands. The test data is used to asses the behavior of the proposed model for the soil-structure interaction. The soil-structure is known to have nonlinear softening stiffness characteristics. Due to the inherently hysteretic behavior of soil, a physically motivated damping model based on this softening behavior is studied as a model for the soil-structure behavior. The Bouc-Wen model of hysteresis is proposed to represent the soil-structure interaction at the monopile. This model is capable of describing both the softening behavior of the soil-structure, as well as the physically based damping induced by hysteresis loops. The nonlinear hysteresis study is performed on the basis of a single degree of freedom assumption for two reasons. First of all, because the most insightful models should be tested on validity before more complexity is added. Secondly, the single degree of freedom approach allows for an efficient and automated optimization scheme for finding linear and nonlinear parameters. The algorithm used in this research is alternating frequency/time harmonic balancing combined with the Levenberg-Marquardt nonlinear least squares algorithm. In this thesis, identification is performed on experimental dynamic shaker data using the single degree of freedom Bouc-Wen model with the goal to identify linear and nonlinear modal parameters. The identification of the modal parameters are divided in two types. The first type of parameters are assumed to be a priori known, based on industry standard preliminary ground research known as p − y curves. The second type of parameters can be determined using the proposed nonlinear identification algorithm and are related to the distribution of linear and nonlinear stiffness, the linear damping and the nonlinear hysteretic damping. Linear and nonlinear parameters for soil-structure interaction are successfully determined for the low frequency range until 5.5 Hz for a the single degree of freedom Bouc-Wen model. This provides valuable information for the offshore wind industry, since the design frequency of a full offshore wind turbine is in the low frequency spectrum range. These parameters are determined by manual fitting methods. The proposed automated algorithm for nonlinear parameter identification could not be used on the experimental data, which is due to a lack of sufficient normalization data and the missing phase information.