Study On Corrosion Damage Identification Of Pipeline Based On Vibration Modal Analysis

Pipeline which is one of the five leading transportation tools plays an important role in transiting liquid and gas. Transportation by pipeline can never be replaced in petro-chemistry and natural gas industry. Due to tremendous growth of pipeline accidents, damage detection in pipeline is an important issue from the point of view of safety and functionality. It is essential to carry out periodical inspection in pipelines to detect any pipeline damage, which may require major or minor repair for safety and serviceability of the structures. The cost of repair is obviously lesser than that required for the reconstruction of the whole pipeline system. Nondestructive technique such as leak magnetic field methods, ultrasonic testing, altemating current magnetic field methods, eddy-current methods, radiograph, acousticemission, etc., may be used to detect damage in the industry pipelines and offshore platform pipelines. However, most of these nondestructive techniques used to evaluate the damage in pipeline require much time and money to be applied. Therefore, the development of damage identification methods which are cheaper and faster to perform is very important. The problems can be avoided through the Use of vibration-monitoring such as modal analysis. Much of pipeline system can be quickly detected using modal analysis detection method which is new tool applied for pipeline damage detection recently. The main task of this research work focuses on how to determine the location and magnitude of damage in a pipeline structure. The major contents are summarized as follows:In section 1, the history of the physical basis for the vibration-based nondestructive damage detection techniques is introduced and the main work of this dissertation is drawing out.In section 2, vibration models are constructed and the fundamentals of modal theory and pipeline modal test are introduced.In section 3, according to damage index which is proposed by Kim and Stubbs, numerical examples and experimental studies are carried out to verify the feasibility of the eigenparameter method and stiffness sentivity ratio method in pipeline structure. Numerical examples have been studied to show this method can only indicate the location of the one damaged element region of the beam-like pipeline. For multiple damage scenarios the parameter is not able to locate clearly the damaged zones. Apart from numerical examples, experimental studies are also carried out to verify the feasibility of these methodologies in real pipeline structure. The results show that the eigenparameter method is not effective for real pipeline structure and stiffness sentivity ratio method can only detect the large damage.In section 4, based on structural modal test theory and finite element method, strain sensitivity ratio method of detection for the corrosion damage in pressure pipeline is presented. First, the damage-locatization criterions to locate damage through displacement mode and strain mode are established. Secondly, the numerical example verifies that the result using the first three order modal shape is basically consistent with the practical damage. Finally, all kinds of pipelines with different damages are prepared and detected by this damage detection method. The result shows that the damage location predicted by this method is the same as the practical one. These results proved that strain sensitivity ratio method locating damage in pressure pipeline only required measurements of few of the pipeline's natural frequencies and the lower displacement mode under both the undamaged and damaged states. And this method is shown to provide good predictions of damage location. For thelower-order modal shape can be measured easily, this method has many advantages at practical engineering applications.In section 5, the orthogonality conditions sensitivities method is presented for damage identification of pipeline structures. Numerical examples demonstrate that the proposed methods are effective and reliable for the simulated pipeline vibration model by using few of the pipeline's natural frequencies and the lower displacement mode. Experimental studies are also carried out to verify the feasibility of the methodology in real pipeline structures applications. This method is shown to provide good predictions of pipeline damage location through numerical examples and experimental studies. This method has many advantages at practical engineering applications.In section 6, a practical method estimate size of corrosion pipeline damage using changes in natural frequencies of a pipeline structure is presented, which is based on fracture mechanics. Numerical examples and experimental studies show that the size of pipeline damage can be estimated with a relatively small size error. This method is effective and feasible for the safety assessment of pipelines.In the last of this dissertation, the research is summarized and the future extensions of the relevant study are discussed.