Abstract
The monitoring of the condition of the offshore wind turbine during its operational states offers the possibility of performing accurate assessments of the remaining life-time as well as supporting maintenance decisions during its entire life. The efficacy of structural monitoring in the case of the offshore wind turbine, though, is undermined by the practical limitations connected to the measurement system in terms of cost, weight and feasibility of sensor mounting (e.g. at muddline level 30m below the water level). This limitation is overcome by reconstructing the full-field response of the structure based on the limited number of measured accelerations and a calibrated Finite Element Model of the system. A modal decomposition and expansion approach is used for reconstructing the responses at all degrees of freedom of the finite element model. The paper will demonstrate the possibility to predict dynamic strains from acceleration measurements based on the aforementioned methodology. These virtual dynamic strains will then be evaluated and validated based on actual strain measurements obtained from a monitoring campaign on an offshore Vestas V90 3 MW wind turbine on a monopile foundation.
Highlights
Many large-scale offshore wind farm projects use monopile foundation to obtain a cost effective design
The qualitative good match indicated in the figures is quantified through the quality indicators which prove the high correlation in terms of amplitude, temporal evolution and frequency content between the measured and predicted signals
The observed differences can be attributed to the non-optimally tuned Finite element model resulting in a mismatch between the experimentally obtained modal characteristics and the corresponding numerical used for the analysis
Summary
Many large-scale offshore wind farm projects use monopile foundation to obtain a cost effective design. The iterative algorithm makes these approaches time consuming and computationally expensive Another approach includes response estimation based on the concept of modal expansion. These methods intelligently extrapolate measured data to generate virtual strain measurements. This paper deals with real scale offshore wind turbine instrumented with a limited number of sensors (accelerometers, strain gauges) and demonstrates the possibility to predict dynamic strains from acceleration measurements. The relation between the modal coordinate and the acceleration/strain in an arbitrary point is established by making use of the corresponding numerically obtained mode shapes [9, 10]. The proposed algorithm and the virtual dynamic strains will be evaluated and validated based on actual measurements obtained from a monitoring campaign on an offshore Vestas V90 3 MW wind turbine on a monopile foundation. A schematic representation of the modal decomposition and expansion approach for the case of the offshore wind turbine is presented in figure 1
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