Vibration Analysis And Crack Identification For Cracked Pipe-like Structures

As important engineering structures,pipes are widely met in many applications,e.g. chemical plants,power plants,and gas and oil transportation,et al.Submarine pipelines play an important role in the process of oil and gas transportation,regarded as the lifeline of marine oil and gas fields.Therefore,the structural security and reliability of submarine pipelines have received more attention in practice.However,dent,crack and damages are inevitably occurred in structural components of submarine pipelines due to material aging, corrosion and environmental loads.The damage accumulation and performance deterioration would seriously reduce the resisting capacity of the structures against the disaster,even result in collapse.Thus,the safe operation and the prolongation of the service life urgently motivate the research on the structural vibration analysis,health diagnosis and safety assessment for submarine pipelines.The key issues of the above problems,i.e.crack modeling,structural condition recognition and crack identification et al,are stressed in the study.The main achievements of the dissertation are summarized in the following:(1) The equations of local flexibility for cracked pipes are theoretically proposed.The local flexibility coefficients for typical crack sections,due to the straight part-through,the circumferential part- through crack and the straight through crack were deduced by linear fracture mechanics and the theory of strain energy release rate.The local flexibility matrix was numerically by the adaptive Simpson algorithm.The results show that the presented local flexibility of cracked pipes can be used to simulate the local mechanics behavior precisely, which provides required parameters for dynamic analysis and crack identification for pipe-like structure with open crack.The presented method overcomes the shortcoming of the traditional crack model.(2) Based on the proposed local flexibility matrix,the analysis models for cracked pipes are presented by finite element and torsional spring approach.The scaling model was used to validate the proposed local flexibility and cracked pipe model by comparing the theoretical results to experimental data.The measured frequencies of the craked pipes show good agreements with the calculated results,which indicates that the proposed flexibility equations and crack models are reasonable.Moreover,the vibration characteristics analysis for cantilever cracked pipe and simple support cracked pipe are conducted respectively.The effects of crack location,crack depth on natural frequency are discussed in detail.Then,crack in the pipe-like structure was proposed to be identified by contour diagram of crack locations versus crack depth.(3) The effects of boundary constraints on the vibration characteristics is presented by taking consideration of the ambient loads,and an effective approach of structural condition identification for free span(CIFS) is proposed Firstly,an interaction model of the contact between the pipeline and seabed was established with respect to boundary constraint variations,which can be simulated by means of altering dynamic stiffness of soil spring. Based on the proposed model,the vibration characteristics of free spanning pipeline under different environmental factors,such as oil property,environmental load,axial force and span length,were studied.Secondly,a methodology to identify the operational conditions of free span based on nonlinear kernel discriminant analysis(KDA) was proposed.The sensitive features were extracted as condition vector for CIFS.Finally,the dynamic model test of free span was used to validate the feasibility.The numerical and experimental results indicate that the proposed method is suitable to capture the discriminative feature underlying in the condition feature vectors,and it is suitable for health diagnosis of submarine pipelines in practice.(4)The nonlinear dynamic behavior of cracked structure is studied and an approach to identify a fatigue crack is presented.A breathing crack model was introduced to describe the nonlinear dynamic behavior of fatigue crack,which simulate as a frictionless contact plane problem.The nonlinear dynamic characteristics of a cantilever beam with a fatigue crack subjected to harmonic load were studied.The effects of various factors,such as the excitation frequency,crack location and crack depth on nonlinear dynamic characteristics were discussed.Moreover,nonlinear dynamic analysis of free spanning pipeline was conducted and a sensitive-damage feature was extracted to identify the severity of a fatigue crack. Finally,Fatigue test with three-point bending was used to simulate actual fatigue crack in pipe specimens.And dynamic model tests of cracked free spanning pipelines both underwater and without water were used to validate the proposed approach.The results indicate that proposed feature efficiently assesses the crack severity even in the case of the presence of the observed noise.