Pipes conveying fluid is frequently applied in the oil & gas industry. Excessive flow velocity in the pipe will give rise to instability of the pipeline structure. It is highly significant to determine the critical velocity for structural stability design and safety evaluation of those pipes. In practical engineering, pipes conveying fluid is often affected by thermal load, such as heating crude oil pipeline and heating pipeline. The natural vibration frequency and critical flow velocity of pipes conveying fluid under thermal load are different from those of the ordinary flow transmission pipeline. Based on Hamilton's principle, the vibration partial differential equation of pipes conveying fluid supported at both ends under thermal load is derived. By separating variables, the equation is simplified into a univariate quartic homogeneous ordinary differential equation, the general solution is obtained according to the critical velocity conditions, and the analytical expression of critical velocity of pipes conveying fluid suitable for different boundary conditions is given. Based on numerical examples, the effects of linear thermal stress and nonlinear thermal stress on the critical velocity of pipes conveying fluid under different boundary conditions are analyzed, and compared with the results of differential quadrature calculation method to verify the accuracy of the analytical calculation method. The research demonstrated that compared with the differential quadrature method, the proposed analytical method is simpler and more accurate, and can more conveniently obtain the critical velocity of pipes conveying fluid system, which is conducive to guiding the engineering practice. Under the action of linear thermal stress and nonlinear thermal stress, the critical flow rate of the pipes conveying fluid system decreases with the increase of thermal load, and the decreasing speed is faster and faster. In the same case, the critical velocity under nonlinear thermal stress is greater than that under linear thermal stress, and the gap between them gradually increases with the increase of thermal load. Compared with the boundary conditions, it is found that the fixed boundary conditions can bear the largest thermal load. Therefore, the application of fixed boundary conditions to pipes conveying fluid system under thermal load is conducive to improve the stability of the system. The analytical method of critical velocity of pipes conveying fluid under thermal load proposed in this paper can easily and quickly obtain the accurate critical velocity in the engineering field, which provides a reference basis for the design and safety evaluation of pipes conveying fluid system under thermal load.

The scale of onshore natural gas pipelines in China is huge, and the pipeline routes have complex and diverse terrain, landforms, and climate characteristics. Faced with the complexity of pipelines and the environment in which they operate, improving the intrinsic safety level of oil and gas pipelines is a common demand of the industry and the public. Ensuring the safe and reliable operation of onshore natural gas pipelines has become an increasingly important focus of industry attention. To achieve this goal, it is possible to improve the design method of onshore natural gas pipelines from the source. At present, traditional stress based design methods are widely used in natural gas pipeline design. Due to the use of a single safety factor, it is difficult to consider the safety margin of oil and gas pipelines. To address this issue, Reliability Based Design and Assessment (RBDA) is becoming a trend in modern onshore natural gas pipeline design. This method can quantify the risks throughout the entire life cycle of the pipeline and avoid using unreasonable or overly conservative design standards. Therefore, this study integrates the reliability design method of the CSA Z662 "Oil and Gas Pipeline System" standard specification for oil and gas pipelines, and combines it with the reliability research revised by some domestic institutions according to China's national conditions. Using the RBDA method process, the main causes of pipeline failure (mainly corrosion and third-party damage) are determined based on a certain section of actual natural gas pipeline. The limit state of the pipeline is designed during its life cycle, and the corresponding limit state equation type is selected based on the limit state. Multiple data statistical analysis methods and software are used to determine the distribution types of various random variable parameters involved in the equation. Then, the Monte Carlo method is used to calculate the failure probability of the natural gas pipeline under the design wall thickness condition, and the weights of other failure causes are considered comprehensively to obtain the natural gas pipeline failure probability. The reliability of the pipeline, Compare the reliability data with the target reliability determined by domestic and foreign standard specifications to verify whether it meets the reliability target.Thus, the wall thickness design of the large-diameter natural gas pipeline section was completed, realizing the overall process of using RBDA method for the design of 1016mm large-diameter natural gas pipelines.