The fiber-optic distributed acoustic sensing (DAS) technique has increasingly become more attractive for structural health monitoring (SHM) and non-destructive evaluation (NDE) purposes. When it comes to traditional acoustic NDE methods, the presence of weldings can present a significant challenge as it can heavily scatter waves resulting in complex data analysis and interpretation. The present work aims to develop an improved understanding and interpretation framework in cases where welds play an important role in the signal with an emphasis on the steel shell of a canister, typically used for Dry Cask Storage Systems (DCSSs) that house spent nuclear waste fuel rods. We also introduce a promising approach in the use of guided ultrasonic waves along with fiber optic sensors that seeks to overcome the challenges that emerge when using traditional acoustic sensing based NDE techniques in welded structures. The study is conducted in a simulation theoretical manner, using a canister model constructed from a representative stainless-steel plate, with different configurations of weldings typically present for DCSS structure. Progressively increasing complexity of the weld physical representation is considered to fully incorporate in physics-based analysis. Furthermore, the acoustic response of these models is obtained from the simulations as a response of an assumed DAS or quasi-distributed acoustic sensing Q-DAS system network. The features originated from the welds are extracted and analyzed, and additional features associated with structure integrity associated with corrosion defects, etc. will also be explored for NDE inspection as in a traditional acoustic NDE approach.
Fiber-optic distributed acoustic sensing (DAS) is becoming an increasingly important tool for real-time monitoring of energy and civil infrastructure structural health such as pipelines. We present a systematic theoretical study of the potential for DAS to be directly coupled with guided ultrasonic waves typically used in conventional acoustic non-destructive evaluation (NDE) methods for real-time pipeline health monitoring. We are referring to this innovative new NDE technique as ultrasonic guided wave and optical fiber sensor fusion. In the practical application of DAS coupled with guided ultrasonic waves, the structural design of (1) the specific guided waves excited, (2) the physical installation of the acoustic transducers and the fiber optic sensors, and (3) the functional performance specifications (gauge length, sensitivity, Etc.) of fiber optic DAS have an important influence on overall capabilities of the monitoring system. Meanwhile, physics-based analysis of acoustic waves is still a challenge due to the complex nature of the Lamb wave when it propagates, scatters, and disperses in the presence of structural defects. In this work, we simulate carbon steel pipes relevant for oil and gas pipeline applications with diameters of approximately 6-12” and wall thickness of 0.5” as the objects to be monitored. By establishing and implementing these capabilities, we seek to pursue an in-depth study on structural parameter optimization of DAS network, measurement range, and signal processing with an ultimate goal of increasing the sensitivity and efficacy of DAS to defect identification for various modes of corrosion expected in practice. To study the characteristics of scattered acoustic waves and performance of DAS for defect identification, we simulated the response of DAS for multiple pipe structures, defect types, and DAS sensor network configuration using finite element software Ansys, then the properties of signal response are extracted to construct defect-sensitive features. The raw data simulated, and the associated features extracted can ultimately be utilized as annotated training data to benchmark various designs for DAS applications, guided acoustic excitation sources, and learning model parameters to enhance early detection of potentially problematic defects.
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