Abstract: In order to adjust the distance between compressor stations or increase the transporation quantity of gas trunk pipeline, it is often necessary to lay a certain length of parallel pipe and variable diameter pipe in nhe design of gas trunk pipeline. From the point of view of the lowest investment, the paper has dealt with the functional relations between the investment in parallel pipe and variable diameter pipe and the pipe diameter, and established the corresponding mathematical model, according to which it has given the formula and method of calculating the optimum technical parameters-diameter,thickness,and length-of parallel pipe and variable diameter pipe in gas truck pipeline.

Key words: Gas Trunk Pipeline, Investment, Parallel Pipe, Variable Diameter Pipe, Optimum Technical Parameters