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High temperature and pressure conditions lead to expanding the pipelines used to transport different types of fluids. A compressive axial force arises if this expansion is prevented. The present work aims to analyze pipelines’ instability due to internal pressure influence by making a simulation using ANSYS - Mechanical APDL 2019 R3. The study presents comparisons between increasing internal pressure from (0 to 10 MPa) on some pipe parameters like a natural frequency and instability for two pined-pined pipe thicknesses, which are 0.5 and 1 mm. It is concluded that the increase in internal pressure will decrease the natural frequency and stability of the vibrated system but will increase the modal damping ratio and total deformation of the tested pipe.

During the pigging operation of a certain oil depot mixed transmission pipeline, there was a π-type compensator under significant stress, resulting in apparent displacement of the bent pipe. For this problem, the OLGA software was used to simulate the pigging process of the mixed pipeline, and the impact of the fluid on the π-compensator bend pipe during the occurrence of slug flow inside the pipeline was calculated and analysed. The calculation results showed that when made a pigging for the liquid-phase fluid pipe, the pipeline pressure declined along the pipeline; and for the gas-liquid two-phase flow pipe, there would be gas accumulation at the bottom of the compensator riser, and then the slug flow was formed. Pipeline pressure at the slug flow increased suddenly, causing pressure fluctuations in the pipeline to increase, pressure drop along the pipeline decreased sharply, and pressure loss occurred at downstream; pressure fluctuations caused by slug flow generated strong fluid shock, resulting in pipeline's jump and relative displacement, which destroyed the pipe system. The research results are of guiding significance for solving the problem of pigging operation in the multiphase pipeline and provide reference for other similar pigging operations.