The offshore wind power (OWP) industry is developing towards deep sea areas, and thus the offshore wind turbine (OWT) will face more severe environmental conditions and excitations such as storms, typhoons and huge waves. The safety evaluation and early warning of OWT structures has become the key issue to ensure the stable operation of offshore wind farm (OWF) and the sustainable development of the OWP industry. In this research, one structural safety evaluation approach of OWT was proposed based on projection pursuit (PP) method and extension cloud model (ECM), and the evaluation results were discussed by considering two actual OWTs as the research objects. Firstly, the multivariate evaluation index system was established based on the safe threshold of different indexes calculated from numerical models of two OWT in the perspectives of time and frequency domain. Secondly, an index weight optimization method was proposed based on PP and particle swarm optimization (PSO) algorithm to accurately determine the importance of each indicator for safety evaluation. The ECM, which can leverage the advantages of normal cloud theory and extension model, was also utilized to establish a multi-level and multi-index safety evaluation method for OWT. The feasibility of the novel approach was further verified through the both randomly generated samples and typical measured data. Especially, one set of measured data, which shows the occurrence of excessive vibration for OWT, can be evaluated as a sub-healthy status. This result can effectively demonstrate the engineering applicability of the proposed method. Lastly, the influence of wind speed, rotation speed and power on the health status of OWT and its change rule was discussed considering the long-term monitoring data for one year. It is expected that the proposed approach can provide necessary assistance for the safe operation and intelligent maintenance of OWFs.

At present, the offshore wind power gradually shows a development trend towards the profound ocean with characteristic of larger capacity, higher tower and longer blades. Therefore, the offshore wind turbine (OWT) is more vulnerable to extreme loads and may lead to the negative influence of structural vibration safety. In this research, a new semi-active eddy current tuned mass damper (SEC-TMD) was proposed for the vibration control of OWT structures and its vibration reduction effect under multiple kinds of dynamic loads was also discussed. Firstly, taking one actual 3.0MW OWT supported by bucket foundation as object, the SEC-TMD was designed based on the linear quadratic optimal control (LQR) algorithm and bounded Hrovat optimal control algorithm in order to obtain the optimal control force of structural vibration systems. The relationship between damping coefficient and magnetic conductivity spacing was further fitted to achieve variable damping control of SEC-TMD. Secondly, the theoretical model of 3.0MW OWT structure installed with SEC-TMD was established and the effectiveness of the established theoretical model was verified through the comparison on modal analysis between the theoretical calculations and measured data identification. Finally, the vibration reduction effect of SEC-TMD on the measured OWT structure under wind-wave, seismic and vibration amplification conditions was analyzed and compared with the results calculated from the other passive dampers including EC-TMD and TLCD. It is shown that the maximum reduction percentage of peak value for vibration displacement and acceleration on the tower top can respectively reach 31.93%, 44.11%, 30.20% and 54.57%, 33.94%, 31.75% under three above conditions. Comparing with EC-TMD and TLCD, the better vibration reduction effect indicates that the proposed SEC-TMD has good engineering application value for the vibration suppression of OWT.

A CFD-based 3D scour model was built to study local scour characteristics of offshore wind power foundations under unidirectional flow. Firstly, a numerical model was built to calculate local scour depth and eddy current around offshore wind power barrel foundation on a scale of 1:20, which were consistent with the physical model experiments under same scale. Furthermore, the numerical simulation of local scour was carried out subject to original size, which led to the time curves of scour depth and foundation bed contours based on the survey points around the polygonal barrel foundation under the condition of scour equilibrium. The above results serve as technical reference and engineering experience to protect similar offshore wind power foundations from scouring.

The aerodynamic damping of the offshore wind turbine structure (OWT), which can be divided into along-wind aerodynamic damping and across-wind aerodynamic damping, is complex, changeable and closely related to the operation status of the wind turbine, the adopted control strategy and the wind speed. Further, the along-wind aerodynamic damping always plays a significant role in an operational OWT, which can effectively mitigate the dynamic response of the OWT structure. In this research, a new theoretical calculation method of the along-wind aerodynamic damping was firstly derived considering the contributions resulted from the changes in rotor speed with respect to wind speed. Then, this new proposed theoretical calculation process was verified by the simulation method in open-source software FAST and the parameters, which affect the along-wind aerodynamic damping such as wind speed, rotor speed and pitch angle, were also investigated. It is shown that the along-wind aerodynamic damping increases as the value of above influencing factors increases until the rotor speed approaches the rated rotor speed and the pitch angle is greater than the critical angle. Finally, the effect of aerodynamic damping on the performances of tuned mass damper (TMD) for an 5 MW OWT under operational condition was studied. The results demonstrate that the effectiveness of TMD is weakened with the increase of aerodynamic damping. Specifically, when the along-wind aerodynamic damping of the 5 MW OWT increases from 4.97% to 6.87%, the additional damping of the TMD decreases from 7.301% to 6.386%.

针对某筒型基础海上风电结构,基于实测振动响应数据,采用随机子空间法对所测海上风电结构的阻尼进行识别,研究海上风电筒型基础结构在停机及运行状态下阻尼的变化规律.结果表明:在停机状态下,海上风电筒型基础结构的顺风向阻尼和横风向阻尼均随外界风速的增大呈增长趋势,两者的平均值分别为0.952％和0.973％;当风电机组处于运行状态时,顺风向运行阻尼随外界风速的增大呈增长趋势,阻尼平均值在3.87％～5.28％之间变化,而对于横风向运行阻尼,其变化趋势受外界因素的影响较小,阻尼平均值在整个风速范围内在1.74％～3.37％之间变化,横风向运行阻尼小于顺风向运行阻尼.

筒型基础在我国海上风电工程中已推广应用,探究其在实际工程中的表现,是保证风机安全的关键.本文针对筒型基础风机结构的振动位移进行原型监测,并以单桩基础风机作为对照.首先基于风机实测自振频率,采用遗传算法拟合基础刚度,然后统计并对比两种基础风机的振动水平.结果表明:两种基础的水平刚度相差7％,筒型基础的旋转刚度是单桩的10倍;风速小于6 m/s时,筒型基础风机位移大,大于6 m/s时,单桩基础位移大;3P共振导致小风速下筒型基础位移较大,通过优化运行策略,减小振动位移最大达54.4％.筒型基础在抗倾稳定性和振动响应控制方面具有优势.

基于横风向气动力阻尼理论计算模型,以NREL-5 MW海上风电机组为例,对其运行过程中横风向气动力阻尼进行计算,并采用FAST软件对计算结果进行验证.之后,研究转速、叶片桨距角和运行方式对横风向气动力阻尼的影响.研究结果表明:NREL-5 MW海上风电机组结构运行状态下的横风向气动力阻尼在0％～0.8％范围内变化,其随风电机组运行转速及叶片桨距角的增大而增大;此外,海上风电机组不同运行方式对其横风向气动力阻尼也会产生较大影响.

The damping characteristic has an important influence on reliably predicting the dynamic response and fatigue life of the support structure of an offshore wind turbine (OWT). Generally, the damping sources of an OWT in parked conditions always include hydrodynamic damping, structural damping, and soil damping, ignoring aerodynamic damping. However, the difficulties in identifying the total damping of the OWT have created uncertainty and variation in the value of damping recommended in the design. In this research, an estimation approach for total damping was proposed for an OWT supported by a wide-shallow bucket foundation (WSBF), which is considered a novel bucket foundation, and its damping characteristics are still unclear. The hydrodynamic damping calculated based on Morison's equation of 0.0128% for the OWT supported by WSBF is smaller than the value of the monopile foundation. The soil damping is approximately 0.120%, obtained using the proposed theoretical method. Finally, the computational damping of the OWT in parked conditions was verified by the measured total damping of two parked OWTs at two different offshore wind farms (OWFs) of China. The results show that the damping obtained by these two methods was in good agreement with the average deviation of 11.20%.

Composite bucket foundations, which have been successfully transported, installed, and operated at the Qidong, Xiangshui, and Dafeng offshore wind farms in China, are economically advantageous due to the relatively simple transportation and installation process. The innovative one-step transportation and installation technology of foundation-tower-nacelle is the key phase in saving costs. In this paper, a "foundation lift ship" overall transport mode is proposed and introduced for the first time. Prototype data measurement, preliminary numerical simulation, and theoretical calculations were conducted to investigate whether the foundation-ship integrity, tower hoop stability, and various indexes of the nacelle met the requirements under the influences of various environmental factors. The multi-system coupling motion mechanism and analysis method of this new structure and transportation mode were expounded. Through the prototype observation data of the one-step overall transportation, the ship-foundation system reliability of the structure in the case of large wind and wave was confirmed. Furthermore, it was found that in the one-step overall transportation, the importance of factors to nacelle acceleration decreased in the order of wave height, current speed, and wind speed by the time and frequency domain analysis and data statistics.

The offshore wind turbine (OWT) supported by bucket foundations can be installed in the integrated transportation process by a dedicated vessel. During the integrated transportation process, the wind turbine is considered as a coupling system with the transport ship, which is easily influenced by waves and storms. In view of the motion response and influential factors, the heave and rock stiffness of the entire floating system was proposed, and then the analytical dynamic motion model of the coupling system was established based on the movement mechanism of the traditional floating body in the wave in this paper. Subsequently, the rationality of the proposed motion model was verified based on the field observation data, with the maximum deviation of the motion responses less than 14%. Further, the influence on the heave and pitch motion of the coupling system considering different factors (vessel speed, wave height, wind speed and wave angle) and the factor sensitivity were discussed by the novel analytical model. It is explained that the heave and pitch motion responses rise with the increase of the wave height and wave angle. Simultaneously, the responses decrease as the vessel speed increases considering sailing along the waves. On the contrary, the responses show an obvious increasing trend with the increase of vessel speed in the case of the top wave sailing. In addition, it is also illustrated that the wave height has the greatest influence on the heave and pitch motion responses, followed by the vessel speed. The wave angle has the lowest sensitivity when the heave and pitch motion are far away from its harmonic resonance region.

Offshore wind turbine (OWT) structures are highly sensitive to complex ambient excitations, especially extreme winds. To mitigate the vibrations of OWT structures under windstorm or typhoon conditions, a new eddy current with tuned mass damper (EC-TMD) system that combines the advantages of the eddy current damper and the tuned mass damper is proposed to install at the top of them. In the present study, the electromagnetic theory is applied to estimate the damping feature of the eddy current within the EC-TMD system. Then, the effectiveness of the EC-TMD system for vibration mitigation is demonstrated by small-scale tests. Furthermore, the EC-TMD system is used to alleviate structural vibrations of the OWT supported by composite bucket foundations (CBF) under extreme winds at the Xiangshui Wind Farm of China. It is found that the damping of the EC-TMD system can be ideally treated as having linear viscous damping characteristics, which are influenced by the gaps between the permanent magnets and the conductive materials as well as the permanent magnet layouts. Meanwhile, the RMS values of displacements of the OWT structure can be mitigated by 16% to 28%, and the acceleration can also be reduced significantly. Therefore, the excellent vibration-reducing performance of the EC-TMD system is confirmed, which provides meaningful guidance for application in the practical engineering of OWTs.