The paper deals with the scientific and applied problems of determining and standardizing the strength of main oil pipelines. These problems can be solved using basic and verification strength calculations for all major stages of pipeline life cycle. At the design stage, the basic strength calculation is carried out using a deterministic method based on comparing operating stresses with permissible ones determined by safety factors (foreign standards) or limit states and limit resistances (Russian standards). The purpose of the basic calculation is to determine the minimum required pipe wall thickness for specified pipe pressures and diameters and selected pipe steels. During the construction, testing, and operation phases, a system of strength verification calculation is used, taking into account time factors with changes in mechanical properties (due to aging and degradation) and wall thickness (due to corrosion and defect growth). During the prolonged operation of a pipeline, changes accumulate, which are related to the conditions of its operation: transportation process parameters, rheological properties of pumped products, and composition of equipment of pump stations change. From time to time, there is a need to change the transportation pattern, while replacing pipeline sections during major repairs and overhaul makes pipeline length change. These changes (taking into account the effect accumulated over a long period of operation) may result in a change in the operating pressures on the pipeline route, established in the original project. These processes affect the values of safety factor, which also become a function of time. Based on the calculations, taking into account the actual safety factors, the decision is made on further operation, decommissioning or repair and renewal of a pipeline section.
The factors that need to be taken into account when substantiating the strength of a main pipeline that has been operated for a long time are provided; the main ones include: use of actual mechanical characteristics of pipes, accounting of accumulated damage, susceptibility of a pipe steel to aging and degradation, as well as data on actual loading of a pipeline due to internal pressure. The paper underlines the main methods of obtaining the information necessary for strength calculation, including intra-pipe diagnostics and mechanical tests of pipe and metal samples. The areas of change of safety factors when deterministic, statistical, and probabilistic design characteristics are introduced are analyzed. The problems for which statistical and probabilistic verification calculations are used are identified. Basic calculation expressions for basic and verification calculations for corresponding stages of pipeline life cycle are proposed.
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