| The design coefficient is a key indicator to measure the delivery capacity and pressure capacity of the gas pipeline.The higher the design coefficient,the greater the pressure on the pipe wall,the greater the amount of natural gas.Although some countries abroad have already adopted a design coefficient of over 0.72 on gas pipelines in part of Class 1 area.However,due to the limited objective technical conditions,most of the gas pipelines in China’s Class 1 area still use the design coefficient of 0.72.In recent years,the domestic metallurgy technology,pipe making technology,construction technology,welding technology and quality control have made great progress.The physical quality of pipeline steel has reached or even exceeded the international advanced level.In addition,China’s pipeline risk management and integrity management has also been significantly improved.It has a good condition and foundation to improve the design coefficient of gas pipeline in our country.Therefore,based on the reliability theory,it has a wide range of engineering background and practical significance to study the design coefficient and influencing factors of gas pipeline in Class 1 area in China.Through the research of domestic and international relevant literature and engineering practice,this paper analyzes the feasibility of improving the design coefficient in Class 1 area of gas pipeline from the aspects of failure rate,risk management,stress corrosion cracking and pressure testing technology.This paper also analyze the influence of improving the design coefficient on the critical limit size,the piercing resistance of the pipeline and the fracture toughness of gas pipeline combined with engineering examples.Stress-strength interference theory and AFOSM Method are applied in the Eastern Sino-Russian Pipeline to justify the feasibility of improving the design coefficient of gas pipeline in Class 1 area to 0.8.The main conclusions of this paper are as follows:(1)The increase of design coefficient has some influence on the accident rate of pipeline,but the failure of pipeline is not decided by the design coefficient.The improvement of the pipeline risk management level is the root cause of the decrease of the pipeline failure rate.Therefore,it is feasible to improve the design coefficient of gas pipeline in Class 1 area.(2)Through the theoretical analysis of the actual yield stress and the investigation of yield strength of the steel pipeline,it shows that the hoop stress is greater than 110%SMYS when the pipeline yields.A pressure-volume hydraulic pressure test was carried out in the Yining-Horgos section of the West-East Gas Pipeline,and it was found that the pressure-volume curve did not show a downward deflection when the circumferential stress reached 100.5% SMYS,indicating that the pressure is safe.Through the investigation of the relevant data of the pipeline burst pressure,it is found that the stress of different grades of straight seam and spiral seam pipeline when it burst is higher than 120%SMYS.Therefore,consider improving the design factor by increasing the operating pressure.(3)When the design coefficient is improved from 0.72 to 0.8,and the critical limit size of the pipeline containing corrosion defects is reduced to a certain extent.When the operating pressure,steel grade,pipe diameter,impact length and impact width of the gas pipeline are the same,improving the design coefficient from 0.72 to 0.8 will reduce the piercing resistance of the pipeline.Taking Class 1 area of the 3rd West-East Gas Pipeline as an example,respectively use the Battle hyperbola method,BTC plus Leis correction formula and BTC plus correction coefficient method to calculate the crack arrest toughness of pipeline diameter 1219 mm and X80 under the design factor of 0.72 and 0.8.The results show that with the increase of the design coefficient,the requirement of the fracture toughness value of the gas pipeline is also increased to a certain extent.(4)The reliability of defect-free pipelines and pipelines with volumetric defects in Class 1 area under 0.72 design coefficient in Eastern Sino-Russian Pipeline is greater than the API579 recommended target reliability.On this basis,according to the API579 recommended target reliability,it’s calculated that the maximum allowable design factor of defect-free pipelines is 1.The DNV RP-F101 method is more conservative than the PCORRC method in the case of volumetric defects.In Class 1 area,according to the DNV RP-F101 method.The maximum allowable value of the design coefficient calculated by increasing the pressure is 0.808.The maximum allowable value of the design coefficient calculated by reducing the wall thickness is 0.796.In order to improve the design coefficient to 0.8,we can control single parameter to satisfy the safety requirement. |
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