Fatigue Life Study For CO₂Pipelines Considering Crack Closure Effect

Capturing and isolating CO2from the related burning source, and transporting it to the oil and gas field or submarine stratum by pipelines is called CCS technology, which could not only stop or dramatically reduce CO2emissions into the atmosphere, but also enhance oil recovery so as to produce considerable economic benefit. An efficient and safe long distance pipeline transportation technique for CO2is one of the indispensably important links. However, corrosion of in-service pipelines, containing external environmental erosion and internal medium corrosion, is serious. In addition, defects of pipelines are inevitable in the fabrication process of the pipelines. All of the birth or acquired defects and cracks will gradually extend under the cyclic stresses induced by environmental loads and working loads, which eventually causes disastrous pipeline leaks and even non-telegraphing fracture failure. Therefore, the CO2pipeline corrosion fatigue problems is worth our concerns. It is theoretical meaningful of making reasonable evaluation to the existing fatigue damage according to inspection results. In this paper, a research on the fatigue life prediction of CO2pipelines containing half elliptical surface crack is developed. The main contents of my paper include:(1) Based on the classic Paris formula describing the crack propagation in the fatigue fracture theory, considering the crack closure effect found by Elber, and applying the stress intensity factor formula of semi-elliptical surface crack along the radial direction presented by Zahoor, the specific form of the modified Paris formula describing the crack propagation is given. For the specific crack pipe, the obtained results from the above formulae is compared with the corresponding data in the literature. Hence, rationality of the proposed formula is verified. Meanwhile, according to the theory describing the path of semielliptical surface crack growth proposed by Newman-Raju, conclusions are drawn that it is reasonable that the full size pipeline fatigue life is predicted by the modified formula in terms of the small sample tests. The presented formula provides a theoretical basis for the residual fatigue life evaluation of CO2pipelines containing a semi-elliptical surface crack, and gives the certain reference value for detection of pipeline leak life.(2) By using the commercial finite element software-NSYS and considering the plastically induced crack closure effect in the calculation of the effective stress intensity factor, a two dimensional FE model for a pipeline with a localized crack is built. Specifically, the grid refinement treatment for the cracked region is done, then the opening stress is calculated under cyclic loads. In the calculation, crack growth is simulated through the gradual stress release of the crack tip nodes and monitoring for displacements and stresses of them. And hence, the real open and close stresses are obtained. The results show that the proceduce can better simulate the2d crack closure effect. By using the results calculated by the FE procedure and the improved Paris formula, the fatigue life of a CO2crack pipeline is calculated, and the life results of the pipeline containing a surface corrosion defect is compared with those with no corrosed surface defect.(3) Using the improved Paris formula, we discussed the influence to the fatigue life of CO2pipeline the pipe containg semielliptical surface crack by some paramenters such as wall thickness, stress ratio, corrosion defects size and position. The results show that:for the parameter of wall thickness, the fatigue life of the pipeline increase with the increasing pipe wall thickness; for the parameter of stress ratio, With the increasing of the stress ratio, the effective stress of crack reduce gradually, the fatigue life of pipeline increase gradually; for the parameter of corrosion defect size, along with the enlarging of the radius of the corrosion defects, the fatigue life of pipeline decrease gradually; for the parameter of corrosion defect position, the existence of the corrosion defects always reduce the fatigue life of pipeline.If the distance between corrosion defect and crack is refer to "d", and the corrosion radius is refer to "r", when d<5r, the infection is greater as the distance become further, the fatigue life of pipeline is increasing, when d≥5r, the influence is relatively small, and the fatigue life of pipeline eventually become stable.