Shtoyko I. Methods for residual lifetime assessment of gas and oil pipelines taking into account their corrosion-hydrogen degradation

The dissertation for competition of the scientific degree as a candidate of engineering sciences by 01.02.04 speciality – Mechanics of deformable solid. – Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Lviv, 2018. The work is dedicated to the development of models and methods for determining the pipes residual lifetime of the oil and gas pipelines, under the influence of stress and physico-chemical factors, namely, a non-stationary operating loading mode, hydraulic attacks, and hydrogenation effect from the natural gas, the simultaneous action of hydrogenation and corrosion, in-service steel pipes degradation. Thus, based on the energetic approach, the calculation model for determining the gas pipeline residual lifetime under cyclic change of natural gas pressure and hydrogenation influence was developed. Using this calculation model, the residual lifetime for X60 steel gas pipeline under cyclic loading and hydrogenation from natural gas was calculated. Using the finite element method, a distribution of hydrogen concentration in the wall of the gas pipelines and their welding joint, as well as in a compact specimen, was found. These data were used in constructing the mathematic model for determining the kinetics and the period of fatigue crack growth in a ring welding joint from steel X70 of the gas pipeline under hydrogenation and cyclic change of the natural gas pressure, under the corrosion potential and under cathodic protection. The methods of the residual lifetime determining of oil pipelines with internal and external cracks under hydraulic shocks, laminar and turbulent oil flows were developed. The methods of the residual lifetime determining of the gas and oil pipelines steel during their in-service degradation, under simultaneous action of constant loads and a corrosive-hydrogenating environment were developed. The crack initiation after a certain service time was considered, up to the essential degradation level. The pipeline steel degradation during their operation was shown to reduce the residual lifetime essentially. Keywords: pipes of gas and oil pipelines, calculation methods for the residual lifetime assessment, corrosion-hydrogen degradation, energy approach, hydrogen concentration, hydrogen embrittlement, stress corrosion cracking.