| dc.description.abstract | The cost of offshore wind turbines is greatly affected by the environmental load and geological conditions, and offshore geological surveys are difficult and costly. Thus it is difficult to have complete and sufficient geological and geotechnical information. Therefore, geological uncertainty has a significant impact on the construction cost of offshore wind turbine foundations. This highlights the importance of research on the influence of geological uncertainty on the foundation design of offshore wind turbines. This study is based on a small number of exploration holes in the feasibility stage of the Changbin offshore wind farm. The geological uncertainty is quantified by the total entropy of the artificial geological model of the wind farm produced by the Markov random field. Furthermore, the accuracy of the artificial model produced by given different numbers of exploration hole data is discussed. The results show that for a simple stratum distribution, an accurate stratum model can be obtained by planning 9 evenly distributed exploration holes. Using the artificial stratum model above, the influence of geological uncertainty on the vertical bearing capacity of the monopile foundation of offshore wind turbines at a specific location is investigated. It is also found that the design pile length obtained by using 9 uniformly distributed exploration holes is almost the same as the design pile length of the actual stratum. Combining the above results with the Monte Carlo simulation method can clearly quantify the influence of stratum and geotechnical parameter uncertainty on the failure probability of the vertical bearing capacity of monopile foundation separately or as a whole. Using a similar method, this study can also quantify the failure probability of the design natural frequency of the monopile foundation of offshore wind turbines.
Since the natural frequency of the support structure of offshore wind turbine is an important inspection item for the serviceability limit state, this study focuses on the influence of geological uncertainty on the natural frequency. First, the sensitivity analysis of the geological and structural parameters to the natural frequency is carried out in order to grasp the degree of influence of various parameters on the natural frequency, and the differences between the evaluated results of various simplified methods and finite element method are also investigated. The research results show that for the structural parameters, the order of influence from large to small is: tower height, diameter of tower column and monopile, concentrated mass, pile length and wall thickness, and for geotechnical parameters, the shear wave velocity is the most important factor, and the relative density has a slight influence. The critical length-to- diameter ratio of the monopile foundation that stabilizes the natural frequency of the system is in the range of 1.5 to 1.7, which reveals that the shear wave velocity of the shallow soil within 2 times the pile diameter under the seabed has a greater influence on the natural frequency than the deep soil, so the survey accuracy of soil shear wave velocity in the shallow soil is very important. The equivalent pile head stiffness method and the equivalent cantilever beam method proposed in this study can greatly simplify the numerical analysis of pile-soil interaction, and have sufficient accuracy (the maximum error is within 2%). It is recommended to be applied to engineering practice.
Finally, this study uses the exploration data of an actual wind farm to study the influence of wave velocity profiles constructed in different ways (geotechnical uncertainty) and strata differences in different hole (stratum uncertainty) on the natural frequency of the wind turbine system. The accurate distributed spring method by finite element is used to evaluate the natural frequency and the analysis software uses SAP2000. The results show that the natural frequency calculated by the wave velocity profile of PS Logging and its regression model at each drilling position is significantly higher than that calculated by the wave velocity profile of CPT-based and its regression model. The CPT-based wave velocity profile of each wind turbine location can better reflect the influence of geological uncertainty on the natural frequency of each wind turbine location. | en_US |