Operational modal identification of offshore wind turbine structure based on modified stochastic subspace identification method considering harmonic interference

Abstract

The structural modal parameter information can be generally obtained from different vibration responses triggered by unknown excitations using the classic modal identification methods which are based on the assumption that the input to the structure is exactly the same or close to the stationary random white noise. For the actual projects such as the offshore wind turbine structure, however, the imposed loads cannot be considered as the pure white noise excitation because the harmonic components emerge obviously in vibration responses due to periodic rotation excitations of the rotor. When the created harmonic components have greater energy and their frequencies are close to any natural modal frequency of the structure, the classic methods can no longer separate harmonic modes from actual structural operational modes under this strong harmonic disturbance, thus false modes may be generated and the identification accuracy may be badly affected. To identify the actual structural operational modes accurately under strong harmonic excitation, a modified stochastic subspace identification (SSI) method considering harmonic interference called the harmonic modification SSI (HM-SSI) method was proposed in this paper assuming the input harmonic frequencies are known and time-invariant. Then the effectiveness and accuracy of the HM-SSI method were verified through a simple numerical model of cantilever beam excited by various harmonic inputs superimposed on random loads. Besides, the superior robustness and better accuracy of identification results affected by the level of noise and the proportion of harmonic energy were reflected. Finally, the modal parameter information under different operational conditions was obtained and the safety assessment of an operational wind turbine was made based on the measured data from one offshore wind turbine test prototype at high running speed.

Health Monitoring and Safety Evaluation of the Offshore Wind Turbine Structure: A Review and Discussion of Future Development

“…Additionally, a new modal identification method called the harmonic modified stochastic subspace identification (HM-SSI) method was proposed on the basis of Mohanty's ideas, which introduced the strong harmonic interference information obtained from measurement data into the traditional SSI method to possess stronger anti-noise capabilities and better recognition stability. Subsequently, this new method was used to successfully achieve the operational modal identification and safety evaluation of the OWT structure [107]. The modified SSI technique considering harmonic influence was also used to achieve the rapid structural health assessment of the wind turbine structure by analyzing the relationship between identified modal frequencies and harmonics [108].…”

“…An occupational risk-scoring method was also proposed for the production stage of wind turbines and all processes [191]. Dong proposed an integral operational modal identification method of the OWT in the range of full power based on traditional and modified SSI techniques and illustrated the modal identification results and the relationship between modal parameters and environmental/operational factors [107,192]. Additionally, the modified SSI technique considering harmonic influence was also used to achieve the structural health assessment of the wind turbine structure by analyzing the relationship between identified modal frequencies and harmonics.…”

Dynamic Impedance of the Wide-Shallow Bucket Foundation for Offshore Wind Turbine Using Coupled Finite–Infinite Element Method

“…According to the slenderness ratio slenderness ratio (depth to diameter ratio), the bucket foundations can be divided into two types, namely, narrow deep and wide shallow [15]. In project engineering, the wide-shallow bucket foundations (WSBF) have been applied in offshore wind farms (OWF) including Frederikshavn, Denmark, 2002 [17], Qidong, China, 2010 [18], and Xiangshui, China, 2017 [19], respectively, as shown in Figure 1. It is widely recognized that the design of the OWT requires accurate evaluations of the natural frequency and dynamic response, which are dependent on the dynamic impedance of the foundation and soil-foundation interaction.…”

In-process modal parameter identification for spindle health monitoring

“…While hammer-strikes have been widely used for identifying dynamic changes in a rotating spindle tool-tip for improved chatter stability prediction [10], a non-intrusive method is desirable to Output-only modal analysis has been studied in the area of civil engineering [11][12][13][14][15][16][17] real operating conditions of the structure; and (4) the method is capable of identifying the same modal parameters as the classical modal analysis, except for not being able to separate the mass and stiffness effect of the structure. Of the various techniques that have been explored for output-only modal analysis, such as natural excitation [11,12], stochastic subspace identification (SSI) [18], frequency domain decomposition [19], and random decrement [20], SSI has attracted increasing attention, and its application has been extended from civil engineering [21,22] to various domains, such as power system [23], aerospace [24][25][26] and mechanical engineering [27][28][29][30][31][32][33]. As an example, a recursive adaptive SSI method was proposed for real-time monitoring low frequency electromechanical modes in power systems [23].…”

“…In another study, SSI performed on operational vibration data measured on a laser cutting machine has revealed an additional mode of vibration with direct influence on the cutting results, which wasn't identified by the classical modal analysis [27]. With a harmonic modification SSI approach, the modal frequency of the offshore wind turbine structure under the strong harmonic interference caused by rotating blades can be identified with increased accuracy [32].…”