The changes in the structural state of the weld material of gas pipes at different seam cross-sections are considered. The classification of external and internal weld defects caused by different types of factors carried out, the stress concentration in the transition zone of the weld to the pipe metal is the most significant factor. The analysis of atmospheric effects on the welded seams of pipelines, that initiate adsorption and electrochemical processes of the metal microstructure, which cause corrosion on the external surface of the weld, is given. Taking into account the non-stationary loading mode of pipes due to fluctuations in the internal gas pressure, corrosion and fatigue damages are also added to these defects, which cause the development of fatigue cracks, leading to a violation of the system’s tightness and even to the welds destruction. The method for increasing the bearing capacity of pipelines is proposed by applying a technological operation of the external surface plastic deformation of the weld and of the transition zone to the pipe metal, which creates residual compressive stresses that relieve working tensile stresses in the stress concentration zone.

The new method for a comprehensive assessment of the strength, durability and material capacity of the critical elements of the main gas pipelines, taking into account the combined impact of factors causing damage (environment, corrosion, random loading, geometry variation, material defects, etc.), which are changing the bearing capacity and material capacity of these structures, is proposed. As the main damaging factor, the process of corrosion fatigue is accepted, the qualitative and quantitative assessment of which are realized by applying a set of equations of comparable fatigue lines obtained by fatigue tests carried out in air and in corrosive environment. By the joint solution of these equations, the functions of the corrosion action coefficients are obtained in a wide range of cyclic strength and durability, which in standard calculation procedures are performed only for cyclic strength and only at the inflection point of the fatigue lines (NG 5  106 cycles).The issues of reducing the material consumption and ensuring the cost-effectiveness of structures, by using relatively cheap materials for pipelines - low-carbon and low-alloy structural steels subjected to surface hardening in stressed sections of pipes (edge welds) in order to significantly increase the physical and mechanical characteristics of the used steel grades are considered. In order to increase the corrosion resistance of these sections, contemporary polymer anticorrosive coatings are used.

The change in the structural state of the gas pipes’ weld material at different seam cross-sections have been considered. The classification of external and internal defects of the joint material caused by the influence of different types of factors, most significant of which are the material defects in the transition zone of the joint to the pipe base metal. The analysis of atmospheric effects on the pipeline welds materials initiating adsorption and electrochemical processes in the metal microstructure that cause corrosion on the seam outer surface is given. Taking into account the non-stationary mode of pipes loading due to fluctuations in internal gas pressure, corrosion-fatigue damage is also added to these material defects, which cause the fatigue cracks development in the material, leading to a violation of the system tightness and even to welds destruction. The structure of a mobile device is proposed for performing a complex of technological operations for assembling large-diameter ground gas pipeline and for ensuring installation accuracy and maintenance processing of the welded pipes’ edges, automatic welding along the seam perimeter, cleaning, grinding and hardening of the seam material outer surface, applying a protective coating against atmospheric influences. The device facilitates the work of installation staff, increases the reliability and bearing capacity of the gas transmission system installed in difficult terrain.