Experimental hydropneum tests of pipelines made of monolithic and composite pipes in polygon (natural) conditions

The expediency of reducing the parallellaid the reads in one direction, the need to increase the economic indicators of gas transportation caused a rapid increase in the diameter of the used pipes and the value of the working pressure; the current diameter of the used pipes is 1420 mm at a pressure of about 7.5-8.0 MPa, as shown by the results of experimental studies, including field tests, close to the optimal. All this contributed to the fact that in recent years much attention has been paid to the development of new pipe designs, in particular the use of multilayer pipes in the manufacture of pipelines using current automated production technology. Multilayer pipes satisfactorily resist the spread of brittle and ductile fractures. The fractures topped in both tested sections along the length of one pipe. At the same time, the brittle nature of the fracture at the entrance to the multilayer pipe immediately changed to ductile. Due to the fact that the transverse stiffness of multilayer pipes is much lower than the stiffness of the same pipes with a monolithic wall, the force driving viscous fracture along the gas pipeline in the section of multilayer pipes is correspondingly lower than in the section of pipes with a monolithic wall. The reduction in the force of driving destruction occurs due to a sharp decrease in the area of the pipe sides, which perceive the gas pressure and load the metal at the crack tip. Therefore, a typical scheme for stopping fracture in a gas pipeline made of multilayer pipes is the transition of a longitudinally propagating crack in to an annular one with the stopping of fracture along the boundary of multilayer and monolithic pipes. During the experiments, the operability of the pipes was also checked in the case of depressurization of the outer layer of a multilayer pipe loaded with internal gas pressure, which simulated mechanical damage to the pipe. It was found that in a four-layer pipe, a complete rupture of the outer layer does not violate its operability even with a rupture length of 700-900 mm, which is 3 times greater than the critical crack length for similar pipes with a monolithic wall. Thus, while ensuring the tightness of the inner layer, multilayer pipes provides at is factory operability of gas pipelines and situations requiring a sudden stop of the gas pipeline in the event of a through crack will be practically excluded. It should be recommended that the total number of layers in multilayer pipes beat least three. A significant increase in the number of layers in a multilayer pipe and the associated reduction in layer thickness is not recommended due to a decrease in transverse stiffness, which complicates the process of pipeline construction due to the possibility of loss of stability or breakage of pipes during installation and laying work.