Abstract The dynamic characteristics of offshore wind turbines are heavily affected by environmental loads from wave and wind action and nonlinear soil behaviour. In the design of the monopile structures, the fatigue load due to wind and wave loading is one of the most important problems to consider. Since the fatigue damage is sensitive to the foundation stiffness and damping, increasing the accuracy of analysis tools used in the design and optimization process can improve the reliability of the structure and reduce conservatism, thereby leading to a more cost-efficient design. In this context, analysis of field data is important for calibrating and verifying purposes. This paper presents analysis of measured accelerations and strains from a wind farm in the North Sea with monopile foundations. Field data during idling conditions, collected over long periods of operation, are analysed and the natural frequencies are determined, and damping is estimated. The measured natural frequencies are compared to calculated values using an aero-servo-hydro-elastic code, showing a good agreement in the frequency range below 2 Hz. Variation of the natural frequencies with intensity of loading may indicate effect of soil nonlinearity on the overall OWT response. Since the first natural bending modes have the largest potential to mobilize soil reactions, they are of primary interest in this context. The effect of load (wave, wind and dynamic bending moment) on the first natural frequency is investigated using different analysis techniques in the frequency domain and time domain. A clear correlation between load level and first natural frequency is demonstrated. A simple nonlinear SSI model of the tower/soil system is employed to numerically investigate the observed changes in the measured first natural frequency with the level of loading and increased overall damping. The simulated results reproduce the general trends in the observed reduction in the first natural frequency and increased damping ratio with the load level. However, the effect of the load level is less than that observed in the measurements, indicating contribution also from other factors than soil nonlinearity.

中文翻译：

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使用来自海上风力涡轮机的动态场数据估算固有频率和阻尼

摘要 海上风力涡轮机的动态特性受到波浪和风作用以及非线性土壤行为的环境载荷的严重影响。在单桩结构的设计中，风浪载荷引起的疲劳载荷是需要考虑的最重要的问题之一。由于疲劳损伤对基础刚度和阻尼很敏感，因此在设计和优化过程中提高分析工具的准确性可以提高结构的可靠性并降低保守性，从而实现更具成本效益的设计。在这种情况下，现场数据分析对于校准和验证目的很重要。本文介绍了对北海风电场单桩基础测得的加速度和应变的分析。怠速条件下的现场数据，在长期运行中收集、分析和确定固有频率，并估计阻尼。将测量的自然频率与使用气动伺服水弹性代码的计算值进行比较，在低于 2 Hz 的频率范围内显示出良好的一致性。自然频率随载荷强度的变化可能表明土壤非线性对整体 OWT 响应的影响。由于第一个自然弯曲模式具有最大的调动土壤反应的潜力，因此它们在这方面具有主要意义。使用频域和时域中的不同分析技术研究载荷（波浪、风和动态弯矩）对第一固有频率的影响。证明了负载水平和第一固有频率之间的明显相关性。塔/土系统的一个简单的非线性 SSI 模型被用来数值研究测量的第一固有频率随负载水平和增加的整体阻尼的变化。模拟结果再现了观察到的第一固有频率降低和阻尼比随负载水平增加的总体趋势。然而，载荷水平的影响小于在测量中观察到的影响，这表明除土壤非线性之外的其他因素也有贡献。模拟结果再现了观察到的第一固有频率降低和阻尼比随负载水平增加的总体趋势。然而，载荷水平的影响小于在测量中观察到的影响，这表明除土壤非线性之外的其他因素也有贡献。模拟结果再现了观察到的第一固有频率降低和阻尼比随负载水平增加的总体趋势。然而，载荷水平的影响小于在测量中观察到的影响，这表明除土壤非线性之外的其他因素也有贡献。