Analysis of the causes of fracture of the main gas pipeline

Abstract

A complex of research has been conducted to establish the causes of the accident of the main gas pipeline made of 1420x15.7 mm X70 steel after 20 years of operation. It is found that pipe destruction was initiated by the longitudinal crack with a length of 470 mm and a maximum depth of 6.8 mm, oriented in the direction of the pipe and perpendicular to the outer surface of the pipe identified as stress-corrosion cracking. The crack initiation is due to the disboundment of the tape protection coating, the high corrosion activity of the soil, and the complex stress-strain state caused by deviations from the design objectives during construction. The chemical composition the base metal of all investigated pipes corresponds to the requirements of the technical conditions for steel pipes in category X70; pipes welding are performed with the typical materials used in pipe welding plants. Despite the differences, the structural and mechanical characteristics (yield strength, ultimate strength, and impact strength) of the steel and weld metal of the studied pipes are identical. The relative elongation (d5) of the operated pipes is below the normalized values, but, it is necessary to take into account the possibility of deviations in these characteristics of steel in the initial state. Significant reserves of toughness indirectly indicate low damage of metal and, accordingly, satisfactory resistance to destruction of the metal of the studied pipes.

Keywords

• main pipeline
• h70 steel
• microstructure
• mechanical properties
• stress-corrosion cracking
• cathodic protection