Research On Fatigue Life And Reliability Of Oil Well Tube Material With Variable Amplitude Corrosion

In the oil and gas industry,the service conditions of oil well pipes are complicated and harsh due to the influence of various factors such as load,environmental corrosion,temperature,and air pressure.There are various failure modes of oil well pipes,and corrosion fatigue damage is one of the main failure modes.Therefore,it is of great engineering significance to accurately estimate the corrosion fatigue life of oil well pipe materials in terms of reducing economic losses in the oil and gas industry and ensuring safety in production.In the prediction research of the corrosion fatigue life of oil well pipes,using probability theory and statistics to show the dispersion and reliability of the corrosion fatigue life of oil well pipes is a more reasonable and effective method.In this paper,TP140 high-strength pipe string steel and C110 high-strength acid-resistant casing steel,which are commonly used materials in oil and gas industry,are selected as the research objects.Based on the results of small-sample constant-amplitude corrosion fatigue life test results,arbitrary specified loads of oil-well tubular components are studied.A method of amplitude-varying corrosion fatigue life with arbitrary given reliability under the spectrum.For two kinds of oil well pipe materials,through multi-sample equal-amplitude corrosion fatigue life experiments at the same stress level,the probability distribution characteristics of fatigue life are analyzed,and the corrosion fatigue life estimates with specified reliability are obtained under different stress levels,and then arbitrary P-S-N curve expression specifying the reliability.The amplitude spectrum of variable amplitude corrosion fatigue load block is set.The load block spectrum is composed of alternating loads with different cycles.Using the set load block spectrum and P-S-N curve expressions with different specified reliability levels,the variable amplitude corrosion fatigue life represented by the load history when the cumulative damage degree reaches 1.0 is calculated without considering the load sequence effect,and a set of Life prediction of variable amplitude corrosion fatigue characterized by load history with different reliability.A set of amplitude-varying corrosion fatigue life experiments was performed using the same load block spectrum,and a set of results of amplitude-corrosion fatigue life experiments characterized by load history were obtained.The probability distribution function of variable amplitude corrosion fatigue life is estimated using theprinciple of probability statistics,and the variable amplitude corrosion fatigue life with the same reliability as that specified in the previous step is calculated,compared with the predicted life,and discuss the feasibility of the life prediction method selected in this paper.The main results of the study are as follows: The constant-amplitude corrosion fatigue life of the oil well pipe materials TP140 and C110 steel follow a log-normal distribution at the same equivalent stress level;the corrosion fatigue life curves of the oil well pipe materials TP140 and C110 steel are obtained,respectively.And PSN curves;the TP140 steel and C110 steel variable-amplitude corrosion fatigue life data points of oil well pipe materials are distributed around their predicted life data points,and they are in good agreement.Therefore,the fatigue life prediction model selected in this paper can be used to predict the variable amplitude corrosion fatigue life of oil well pipes TP140 and C110 steel,and it is reasonable and effective.