Hrabovskyy R. Scientific fundamentals of safe operation and durability assessment of long-term exploited oil-and-gas pipelines based on parameters of their defectiveness

The dissertation focuses on problems of development of criteria for assessment of safe operation and forecasting of strength and residual lifetime of the long-term exploited oil-and-gas pipelines with defects for making of the expert conclusions on pos-sibility of their further safe work or decommissioning. Based on studies of surface corrosion fatigue cracking of steels for oil-and-gas pipelines, the criterion relation between surface crack length, characteristic value of applied cyclic stress and parame-ters of the electrochemical dissolution of metal was derived. On this ground the formula for calculation of the period of nuclea-tion of surface corrosion fatigue crack has been proposed and verified. The new experimental data on the corrosion fatigue crack growth behaviour of pipeline steels were received and analysed depending on the operation time and the presence of cor-rosion environments. The calculation method and algorithm of its numerical realisation for assessment of the specificity of fatigue crack growth in the wall of pipe was developed and on this base the kinetics of crack propagation was determined for defects of different shape under fatigue and corrosion fatigue conditions. The criteria of safe operation and for assessment of residual lifetime were proposed on the basis of concept of threshold defect and conditions of potential catastrophic failure of defective pipe. The method for determination of the failure pressure in the defected oil-and-gas pipelines was developed. The developed methods were used for fracture risk assessment and residual lifetime evaluation of some linear sections of the long-term exploited oil-and-gas pipelines with defects.