Vibration Analysis For Cracked Pipe-like Structures

As one of the important structure forms, pipelines are commonly used in many project fields, e.g. chemical, oil and gas transportation, et al. Nowadays, with the vigorous exploitation of marine oil and gas resources, submarine pipelines as the most important way to gather and transport offshore hydrate link the upstream and downstream of exploitation, known as the lifeline of marine oil and gas field project. However, the harsh marine environment makes submarine pipes inevitably produce dent, crack and other damage. The accumulation and expansion of damage would seriously reduce the resisting capacity of the structures against the disaster, even resulting in collapse. This would cause not only the huge economic losses to the state, but also serious pollution of the marine environment, which destructs the local ecological balance and results in adverse social impacts. Thus, the safe operation and prolongation of the service life urgently motivate the research on the structural vibration analysis, health diagnosis and safe assessment for submarine pipelines. In this paper, a theoretical study on the vibration characteristics of crack pipes is carried out. The main contents include:(1) Based on linear fracture mechanics and strain energy release principle, the equations of local flexibility for crack pipes, due to the straight part-through and through two typical cracks at different directions under bending, were deduced by cutting up crack micro unit, and the local flexibility theorem combined axial force and bending moment was also discussed. To obtain the local flexibility coefficient, adaptive Simpson integration method was used to write numerical integration program codes. The experiment results by Naniwadekar et al. or the finite element method results are given to verify the validity of the local flexibility equations of a crack pipe. The study extends the local flexibility theorem of the crack pipe and provides a theoretical basis for the establishment of crack model.(2) Using the local flexibility coefficient, the finite element model and the spring hinge model of pipe with a crack at any direction are discussed. The effects of crack direction on the natural frequency by a FEM of pipe with a crack at any direction with the ANSYS software are considered. As a result, the 1st frequency of the crack pipe decreases while the direction angle increases. The direction of crack, which is an effective crack parameter, would reflect the changes of the natural vibration characteristics of the crack pipe.(3) The finite element dynamic contact model of the fatigue crack pipe is established. In this model, the singular and no friction contact elements are used. The fatigue crack under harmonic loading causes breathing behavior due to the contact activity on the crack surface. The effects of crack direction on nonlinear dynamic characteristics are discussed in detail. The results show that:while the external excitation frequency is half of the crack pipe natural frequency, the breathing crack pipes produce obvious super-harmonic resonance. With the direction angle increases, the amplitude of the 2nd harmonic continuously decreases. The energy ratio (PER) characterize the energy change of the resonant harmonic components of system.