Banakhevych Y. Prediction of the remaining strength and lifetime of the defect-containing fragments of oil and gas transmission pipelines under service conditions

The dissertation is dedicated to development of theoretical models and relevant criteria and methods for evaluation of the remaining strength and lifetime of the defect-containing fragments of oil and gas transmission pipelines under cyclic and static stress and working environment. The criteria for cyclic strength of the pipeline with cracks have been developed. This allows determining the parameters of cyclic loading, which provide certain lifetime for the considered pipeline. The numerical model has been developed for determining the initiation time of a fatigue crack near the flattened stress concentrator. Using prediction for stress corrosion crack growth in the pipelines made for highly plastic and high strength materials the influence of aggressive corrosive and hydrogen-containing environment on strength and lifetime of gas pipelines has been determined. Employing the presented mathematic models a method for residual lifetime calculations is proposed for a defect-containing gas pipeline that works under the influence of wet hydrogen sulfide environment. For the first time closed expressions for calculating the assurance factors for pipelines with cracks in welded joints subject to residual stresses that vary along the length and across the section of a pipeline. The influence of these stresses is evaluated concerning static fracture toughness of a pipeline containing surface cracks in a welded joint and the corresponding category of danger according to the regulatory documentation is established.