| Under the condition of service,the oil well pipe will not only be eroded by corrosive media,but also suffer from the fatigue damage caused by alternating loads such as internal pressure,external pressure,bending and vibration.Corrosion fatigue phenomenon of oil well pipe is produced under the combined action of the two factors,which eventually leads to accelerated fracture of pipe body and affects the safe production of oilfield.Thus,it is of great significance to accurately evaluate the corrosion fatigue life of oil well pipe to ensure the safe and stable production of oilfield.However,the corrosion fatigue life of oil well pipe is very sensitive to material factors,sample surface factors and environmental factors,leading to a certain degree of dispersion of corrosion fatigue life of oil well pipe,so it is very difficult to accurately evaluate.Therefore,it is of more engineering significance to give reliability evaluation while evaluating corrosion fatigue life of oil well pipe.In view of the above problems,this paper adopts an uncertain method to predict the corrosion fatigue life of oil well pipes.The difference between this method and the traditional deterministic method is that former considers various uncertain factors as random variables and input parameter during the oil well pipe under service conditions.Combining the experimental method and the analytical method as the specific implementation method,and using the concepts and methods of statistics and probability theory to analysis the dispersion of the corrosion fatigue life of the oil well pipe,and finally obtain a kind of prediction model,which can predict the corrosion fatigue life of the oil well pipe with any specified reliability under the action of any given load spectrum,and the prediction result is verifiable.TP140 high strength casing steel commonly used in oilfields was selected as the research object of this paper.Firstly,the equal-amplitude corrosion fatigue experiment of 4 groups of stress levels and multiple samples in each stress level were carried out,and analyzing the dispersion of equal-amplitude corrosion fatigue life of materials.Based on Manson-Coffin stress-life model,the S-N curves with reliability of 0.1%,5%,50%,95% and 99.9% and their mathematical expressions were gained by calculating and analyzing.Then the arbitrary load spectrum for verification is designed.Combining S-N mathematical model and Miner linear cumulative damage theory,corrosion fatigue life with corresponding reliability is predicted under the variable-amplitude.Finally,under the action of the same load spectrum,multi-sample and variable-amplitude corrosion fatigue experiments were carried out to obtain the experimental life of the material,then comparing the results with the predicted life,and drawing the following conclusions.?The equal-amplitude corrosion fatigue life of TP140 steel under the same stress level obeys the lognormal distribution.?P-S-N curves and its mathematical expressions of TP140 steels were obtained by analyzing and calculating.?Both the predicted life and the experimental life of variable-amplitude corrosion fatigue of TP140 steel obey lognormal distribution.?The predicted life of variable-amplitude corrosion fatigue of TP140 steel is basically the same as the experimental life of variable-amplitude corrosion fatigue.The above conclusions show that the oil well pipe corrosion fatigue prediction method proposed in this paper is effective and economical and practical.It can improve and supplement the theory of corrosion fatigue.It provides a theoretical guidance for the safety of oil and gas wells and the prevention of corrosion fatigue fracture of oil well pipes in the process of oil and gas development,and has important engineering application value. |
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