Chapter 22 Feasibility for Damage Identification in Offshore Wind Jacket Structures Through Numerical Modelling of Global Dynamics Mark Richmond, Ursula Smolka, and Athanasios Kolios Abstract Operational modal analysis is currently being researched to detect damages in offshore wind turbine structures. To make practical considerations, such as where to place sensors, it is valuable to know how different damages present themselves in the structural dynamics and what kind of damages can be detected given certain detectability limits. Through conducting natural frequency analysis, the modal response of a structure to damages are explored and resulting considerations for damage detection are discussed. A state of the art, detailed, numerical design model of the Wikinger jacket structure is used to investigate damage detection. Aspects of the model are changed to simulate damage, including member damage, scour, corrosion and more. The resulting modal parameters are calculated, these parameters are compared to those from an unaltered structure and metrics are calculated including frequency change, modal assurance criterion (MAC). It is found that when the frequencies of two modes intersect, there is a large change in the mode shape of the two modes from the undamaged case and the choice of which modes to compare between healthy and damaged can change the results significantly. The findings from each simulated damage are presented. Keywords Offshore wind jacket structure · Natural frequencies · Modal shapes · Damage identification · Finite element model 22.1 Introduction Data on failed structures is typically not available, the being able to identify failure when it occurs is of significant economic benefit. To overcome this lack of data, researchers have modeled structures to simulate damage. Some researchers aimed to detect damage in their model in the presence of measurement noise; for example, Malekzehtab et al. used mode shapes and natural frequency in an objective function, combined with a penalty term to avoid false positives [1]. Liu et al. found, even with 3% noise, modal flexibility could still effectively detect damage [2]. Wang et al. investigated their approach in the presence of temperature variation and used Modal Strain Energy Decomposition to detect damage [3] while Xu et al. used a residual strain energy-based approach [4]. This work differs from others in that a detailed design model is used which includes more complex aspects of the structure and allows for a more detailed investigation. This work includes the mass and moment of inertia for the nacelle and blades, an aspect not present in many other studies, but which is important in the modal properties of a wind turbine structure. Both element damage and scour are simulated allowing for direct comparison of their impacts on the same structure. There are four levels of damage identification: damage detection, damage localization, damage severity assessment and damage consequence/progression [5]. Detecting a damage is beneficial because while the turbine continues to operate, the remaining fatigue life is used much more rapidly. By detecting the damage and parking the turbine, the excess consumption of fatigue life can be reduced. By localizing the damage, time and resources can be saved in its repair. With the quantification and assessment of damage, decisions regarding the growth and operational impact can be made. A great deal of knowledge is required to achieve all four levels. The use of virtual sensing, along with pre-existing detailed design models, can a cost-effective way of improving the detection and assessment of damage. Operational modal analysis (OMA) can provide M. Richmond ( ) · A. Kolios Naval Architecture Ocean and Marine Engineering, University of Strathclyde, Glasgow, UK M. Richmond · U. Smolka Offshore Wind, New Services Department, Ramboll, Germany e-mail: mark.richmond@strath.ac.uk; ursula.smolka@ramboll.com © The Society for Experimental Mechanics, Inc. 2021 B. Dilworth (ed.), Topics in Modal Analysis & Testing, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-47717-2_22 221
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