Acoustic Emission Health Monitoring For Cracked Tubular Joints Of Offshore Platform Based On Microscopic Damage Mechanism

Offshore platforms work as a huge truss structure to develop ocean oil and gasresources and being in complex marine environment for long-term. The reason of thehuge catastrophic accidents occurred on marine structures over the past decade issummarized in this paper. A lot of investigations show that the fatigue damage is themain factor of structure failure. The physical mechanism of the fatigue damage ismuch more complicated. There is no uniform theory. And there are still many basicproblems to be solved. Therefore, it is necessary to work out the fatigue damage lawunder the marine environment. Effective monitoring of offshore platform damageaccumulation process, repair and reinforcement for the structure in a timely mannerare both important to extend the life of equipment.K-type pipe joint is a key part of underwater structure and is also the weakestlink. The tubular joints’ structural failure is mainly caused by deformation and fatiguecrack propagation of the joint. This article introduced the study of evaluationmechanism of fatigue and damage based on fracture mechanics and thermodynamicsfirstly. Under microscopic scale, the microscopic mechanisms of fatigue damageevolution and fatigue damage law of evolution are revealed in both point defect anddislocation. If the entire crystal is under continuous action of the load, the internalatomic escape and result vacancy. The crack tip can also release a lot of dislocations.Vacancies and dislocations can change chemical potential. And they will cause surfacediffusion flow and resulting material fatigue fracture.Secondly, on the basis of theoretical research, from the experimental point ofview, the AE signals of various crack states of the K-joint are collected on the realtime. The concept and detection principles of acoustic emission are introduced. Themechanism of acoustic emission when fatigue crack initiate and propagate is analyzedby the fracture mechanics and mechanisms of dislocations generated. There is acommon introduce of signal analysis methods. Use a PXWAE full waveform acousticemission signal acquisition system to monitor crack growth and fracture in a simulation test. Then the acoustic emission signals during the whole process can begot and make preparation for the subsequent signal analysis.Finally, based on the overall empirical mode decomposition and Hilbert-Huangtransform theory, to achieve the fatigue crack acoustic emission signal decompositionand reconstruction, which reveals the time-frequency characteristics of acousticemission signals in various stages. This method overcame the mode mixingphenomenon. Construct Hilbert marginal spectrum and the energy spectrum, and thusexplore the fatigue crack propagation energy variation.The study results enriched further the theory of fatigue damage, explained thephenomenon of fatigue damage under the condition which is far away from thefatigue limit, validated the effectiveness and correctness of the theoretical model andprovided the theoretical basis for its engineering application. The use of acousticemission can promote the health monitoring on line for cracked tubular joints,improve the real-time property, economy and safety of monitoring process, andachieve the intelligent identification.