Effect Of Pressure Fluctuation And Oil Temperature Alternation On Security Of Pipelines

During the operating life of the pipeline, the pipelines, due to the long-term pressure fluctuation and oil temperature alternation, are always in the state of c ycle alternating stress, which leads to promote crack growth, and it would likely cause the pipeline fatigue damage and destruction accidents. This paper took research on effect of pressure fluctuation and oil temperature alternation on security of pipelines, based on the analysis of pipe stress factors, taking use of semi-empirical formula evaluatio n method, finite element method, and the probability of fracture mecha nics methods. It would provide guidance to long-term operation security and stability of pipelines.For I-type crack, the stress intensity factor increases with the crack length increases, indicating that an increase in crack length resulted in a significant increase in the crack tip stress and overall stress level improved with cracks sunk in increasingly dangerous state of fracture. For the axial semi-elliptical surface crack, stress levels were even higher in crack region and had plastic deformation. For elongated pipes crack, when crack depth increases, the pipeline were more easily plastically deformed under a small cross section,and reached the yield stress state.The strain was large, the crack opening angle increases, and the curvature direction is toggled, which is under greater risk of fracture.Take analysis on the effect of pressure fluctuations on pipeline safety. Each order harmonic response analysis differs in vibration modes for pipeline with both ends fixed.When the pipeline span increases, the natural frequency decreases. That means the more likely the pipeline vibrates in a smaller frequency. For the pipe containing crack defects, the greater the initial crack depth, the shorter its remaining life. Residual stress ratio of 0.1 for the remaining life was worse than an order of magnitude of 0.6. The greater the pressure fluctuations(stress is relatively small case), the more likely the crack pipe extended.The results with the analytical results of the pipeline life and finite element analysis method was similar.For batch pipelining of crude oils with different temperatures, considering the pressure and temperature on the remaining life of the pipeline, the stress intensity factor ranged maximum. Compared with the analytical method for pipeline, fatigue life values was small, indicating that changes in temperature cause large cha nge in stress intensity factor, making the crack growth rate increases, and indicating that fewer cycles could cause fracture than that under pressure alone.Take the use of reliability theory to analyze the fatigue life of pipeline. When the initial crack depth ratio is the same, the theoretical calculation of fatigue life was greater than the calculated value by reliability analysis. With initial crack depth increases, both fatigue lifes are declining. Meanwhile, the analytical solution will introduce a lot of uncertainty, while reliability analysis is more fit for the actual situation.