К ОЦЕНКЕ ПРОДОЛЬНОЙ УСТОЙЧИВОСТИ МАГИСТРАЛЬНОГО ТРУБОПРОВОДА

Abstract

Background During operation of underground pipelines in soft soil the probability of loss of longitudinal stability due to changes of operating pressure and temperature is high. The loss of longitudinal stability increases the likelihood of occurrence of corrugation that subsequently leads to the accelerated development of cracks, and to failure. Aims and Objectives To calculate stability, building codes (SNiP) propose formula including S value representing equivalent longitudinal axial force in the section of the pipeline. This value must be less than the longitudinal critical force Ncr. The document proposes that S value be determined by the formula implicitly dependent on the internal pressure P and, explicitly, on the differential temperature Dt. Yet, if we consider a special case where there is no change in temperature, and express explicitly the dependence on the pressure, we’ll get incorrect result. The error lies in the fact that with increasing internal pressure P the S parameter increases proportionally, and at certain high value of pressure Pcr the condition of longitudinal stability is violated. This is despite the fact that the longitudinal stresses in the pipe wall are tensile. However, it is known that the loss of longitudinal stability can only occur during rod compression, and not tension. Methods The article provides the analysis of the experiment that became the basis for the formula of stability proposed in the SNiP 2.05.06-85 *. Here different boundary conditions and loading schemes are considered. The physical meaning of S value was studied on the base of analyzing different cases. It was shown that the «equivalent longitudinal axial force in a section of the pipeline» in the formula of stability in the document is none other than the longitudinal compressive force that must be applied to the pipe (the wall of the pipe with the product), so that its length would not change at the current product pressure and specified temperature difference. Hence it was concluded that the formula of stability in the SNiP is erroneous, and the proper condition is , where N is longitudinal axial force in the pipe itself (only in metallic part), and in order to determine the value of N the equilibrium condition only of the pipe proper shall be used, and effect of the product inside the pipe should be considered as an external force in relation to the pipe. Despite the conclusion is evident, the error appears in the documents for many years. Moreover, S value has «penetrated» into more important formulas describing the longitudinal and transverse bending of pipelines, involving longitudinal and transverse forces at the same time. This formula serves the basis for determination of technological parameters of operations in repair of existing pipelines. Erroneous original formula inevitably brings to erroneous results, and, as a consequence, to unforeseen events (incidents, accidents) during repair of pipelines. Results The article also refined the formula for longitudinal-transverse bending, which should involve only those forces that impact on the pipe proper. At the same time the internal pressure P and temperature differential Dt should be treated as an external action and original condition.