Early detection of bolt loosening is a major concern in the oil and gas industry. In this study, a vibration-based health monitoring strategy has been developed for detecting the loosening of bolts in a pipeline's bolted flange joint. Both numerical and experimental studies are conducted to verify the integrity of our implementation as well as of an enhancement developed along with it. Several damage scenarios are simulated by the loosening of the bolts through varying the applied torque on each bolt. An electric impact hammer is used to vibrate (excite) the system in a consistent manner. The induced vibration signals are collected via piezoceramic sensors bonded onto the pipe and flange. These signals are transferred remotely by a wireless data acquisition module and then processed with a code developed in-house in the MATLAB environment. After normalization and filtering of the signals, the empirical mode decomposition is applied to establish an effective energy-based damage index. The assessment of the damage indices thus obtained for the various scenarios verifies the integrity of the proposed methodology for identifying the damage and its progression in bolted joints as well as the major enhancements applied onto the methodology.

Detection of damage in vital and high-cost infrastructures has been one of the major concerns of operators of such structures in the past two decades. The growing demands in oil and gas have forced extraction of such vital fuels in deep waters. Bolted joints are widely used in pipes transporting oil and gas in deep waters. However, they do develop inservice problems, such as loosened bolts, which if remained undetected, could cause significant environmental damage and economical loss, especially in the case of pipes used in deep-water oil extraction. In this study, the energy damage index (EDI) calculated based on a novel vibration-based damage detection methodology using the empirical mode decomposition (EMD) is used to establish the existence of damage due to loosened bolts in common industrial bolted joints. A complete finite element (FE) model is established to simulate the whole process using the implicit dynamic solver of the commercial software ABAQUS ß . Also, a comprehensive experimental program was designed and carried out to verify the FE results. Results show that the EDI based on the EMD method is a powerful tool for not only detecting the damage, but also the progression of the damage in bolted joints.

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This paper presents the application of a novel vibration-based technique for detecting fatigue cracks in structures. The method utilizes the empirical mode decomposition method (EMD) to establish an effective energy-based damage index. To investigate the feasibility of the method, fatigue cracks of different sizes were introduced in an aluminum beam subjected to a cyclic load under a three-point bending configuration. The vibration signals corresponding to the healthy and the damaged states of the beam were acquired via piezoceramic sensors. The signals were then processed by the proposed methodology to obtain the damage indices. In addition, for the sake of comparison, the frequency and damping analysis were performed on the test specimen. The results of this study concluded with two major observations. Firstly, the method was highly successful in not only predicting the presence of the fatigue crack, but also in quantifying its progression. Secondly, the proposed energy-based damage index was proved to be superior to the frequency-based methods in terms of sensitivity to the damage detection and quantification. As a result, this technique could be regarded as an efficient non-destructive tool, since it is simple, cost-effective and does not rely on analytical modeling of structures. In addition, the capability of the finite element method (FEM) in mimicking the experiments, and hence for consideration as an effective tool for conducting future parametric studies, was also investigated.