Design principle of strength of gas transmission pipeline trunk line
The selection of wall thickness of large-diameter and high-pressure natural gas pipelines not only affects the safe operation of pipelines, but also is directly related to the investment in pipeline construction. The design concept, scope of application and technical advantages of each standard are analyzed by comparing the relevant contents of domestic and foreign standards for the strength design of straight sections of gas pipeline trunk lines such as GB 50251-2015, ASME B31.8-2007, ГОСТР 55989-2014, ISO 13623-2009 and CSAZ 662-11-2011, In particular, the differences in the design results of pipeline wall thickness between the standards are compared and analyzed with respect to the characteristics of increasing flexural strength ratio of pipeline steel with increasing grade. With the development of large diameter and high pressure pipeline technology, the Russian standard based on the design principle of material tensile limit wall thickness can better reflect the changes in the characteristics of high steel grade pipeline steel materials.
Commonly used codes for gas transmission pipeline engineering design at home and abroad include GB 50251-2015 “Code for Gas Transmission Pipeline Engineering Design”, ASME B31.8-2007 “Gas Transmission and Distribution Piping Systems”, ISO 13623-2009 “Oil and Gas Industry – Pipeline Transmission Systems”, CSAZ 662 -11-2011 “Oil and Gas Pipeline Systems“, ГОСТР 55989-2014 “Mainline Gas Transmission Pipelines – Design Standards for Pressure above 10 MPa: Basic Requirements”. For the design of straight pipe section wall thickness, the calculation methods of each standard are different . Especially in recent years, with the development of large diameter, high steel grade and high pressure pipelines, the differences in calculation results of various standards have led to an increasing difference in engineering investment . Therefore, the design concept and technical advantages of each standard, especially for pipeline steel as the grade increases the characteristics of the flexural strength ratio increases , a comparative analysis of the differences in the design results of each standard on the wall thickness of the pipeline, can provide an important reference for the design of new pipelines in China.
Comparison of wall thickness design of straight section of main line pipeline
Strength design methods of domestic and foreign standards
In the Russian standard ГОСТР 55989-2014, the calculation formula for the design wall thickness of the pipeline is:
In the formula:
- td is the design wall thickness of the pipe, mm;
- tu is the thickness of the pipe wall determined according to the strength limit, mm;
- ty is the thickness of the pipe wall determined according to the yield limit, mm; p is the working pressure, taken as 12 MPa;
- γfp is the load (internal pressure) reliability factor, taken as 1.1;
- D is the outer diameter of the pipe, taken as 1422 mm;
- Ru is the design value of the strength of the pipe, MPa;
- Ry is the pipe yield strength design value, MPa;
- γdu for the calculation of the strength of the pipe working conditions coefficient;
- γdy for the calculation of the yield point of the pipe working conditions coefficient (Table 1), the type of ordinary straight pipe section for H;
- γmu for the calculation of the strength of the pipe material reliability coefficient (Table 2), according to the high-grade pipeline steel manufacturing process, generally take 1.34;
- γmy for the calculation of the yield point of the pipe material reliability coefficient coefficient, take 1.15;
- γn for the calculation of the yield point of the pipe material reliability responsibility factor, take 1.10;
- σu for the pipe and welding connection part of the material tensile strength limit value;
- σy for the pipe and welding connection part of the material yield strength limit value.
Table.1 Calculation of the yield point of the pipe working conditions coefficient
|Pipe section type||Coefficient of pipeline working conditions|
|H (normal level)||0.88||0.91|
|C (medium level)||0.74||0.76|
|B (advanced level)||0.59||0.63|
Table.2 Calculation of the strength of the pipe material reliability coefficient
|1.34||Welded steel pipes made of controlled rolled steel and heat strengthened pipes, controlled rolled steel and heat strengthened pipes adopt continuous process weld flux and are manufactured by double-sided arc welding (negative tolerance of pipe wall thickness does not exceed 5%, and 100% of metal base metal and welded connection parts pass the compactness inspection by nondestructive method).|
|1.4||Welded steel pipes made of normalized steel, heat strengthened steel and controlled rolling steel are made of continuous process weld flux and double-sided arc welding (100% of the metal base metal and welded connection parts pass the compactness inspection by nondestructive method). 100% of seamless steel pipes pass the metal compactness inspection by nondestructive method.|
|1.47||Welded steel pipes with continuous process weld flux, manufactured by double-sided arc welding, and 100% of the welded connection parts pass the compactness inspection by nondestructive method. The welded joints of steel pipes manufactured by electric welding and touch welding through high-frequency welding points shall be subject to heat treatment and 100% shall pass the inspection by nondestructive method.|
|1.55||Other seamless steel pipes and welded steel pipes.|
GB 50251-2015, the calculation formula for the wall thickness of the pipe is:
- σs for the minimum yield strength of steel pipe, MPa;
- K for the strength design factor; φ for the weld coefficient;
- T for the temperature reduction factor (when the temperature difference is less than 120 ℃, take 1.0).
ASME B31.8-2007, the calculation formula for the wall thickness of the pipe is:
- S is the minimum yield strength, MPa;
- F for the design parameters; E for the weld coefficient.
ISO13623-2009, the calculation formula for the wall thickness of the pipe is:
- σhp for the fluid pressure caused by the circumferential stress, MPa;
- pid for the design pressure, MPa; pod for the minimum external hydrostatic pressure, MPa;
- Fh is the design coefficient of circumferential stress;
- σD is the design strength, MPa.
The standard does not consider the effect of temperature changes on the pipe.
CSAZ 662-2011, the calculation formula for the wall thickness of the pipe is:
In the formula:
- L for the regional parameters;
- J is the weld coefficient.
Comparison of wall thickness design in the first level of China-Russia East Line
The design of the wall thickness of the first-class area pipeline of the East China Line with a pipe diameter of 1422 mm, design pressure of 12 MPa and material X80 is taken as an example, and the above-mentioned standards are used for calculation (Table 3). The results show that the wall thickness calculated using Russian standard ГОСТР 55989-2014 is the largest, and the wall thickness calculated using GB 50251-2015 and ASME B31.8-2007 is the same, and slightly larger than the calculated values using ISO13623-2009 and CSAZ 662-2011.
Table.3 Calculation results of wall thickness of straight pipe section in the first level of China-Russia East Line using different standards
|ГОСТР 55989-2014||GB 50251-2015||ASME B31.8-2007||ISO 13623-2009||CSA Z662-2011|
Analysis of results
The design concepts of GB 50251-2015, ASME B31.8-2007, ISO13623-2009, and CSAZ 662-2011 are basically the same, and all adopt deterministic methods based on the material The wall thickness is determined based on the yield strength of the material, but the calculation results of each code differ slightly due to the different selection of design factors.
In Russia, the population distribution along the pipeline route is not considered due to the sparsely populated area, and only special sections are considered, such as crossing rivers and highways, etc. ГОСТР 55989-2014 adopts the concept of reliability, which is based on engineering experience and takes into account the uncertainty of load, material properties, pipeline use, pipeline working environment and construction environment, which is different from the design concept of China and North American standards. For the strength design, the tensile limit is used to design the pipe wall thickness for the pipe pressure of 10~25MPa and the material of X65 and above steel grade (the yield strength ratio is more than 0.83).
Scope of application
ГОСТР 55989-2014 clearly stipulates that it is applicable to pipes with pipe diameter below 1400mm (including 1400mm) and conveying pressure from 10 to 25MPa, while GB 50251-2015 does not clearly stipulate the applicable pipe diameter and pressure grade range. With the development of large diameter, high pressure and high steel grade pipelines, the consequences of accidental pipeline rupture are becoming more and more serious, the limitations of the standard will gradually appear, and should be revised in a timely manner.
The China-Russia East Line is a 1422mm pipe diameter, 12MPa pressure, material X80 pipeline, passing through the alpine permafrost area, the design is extremely difficult, and there is no mature experience to draw on. At present, the second line of West-East gas transmission has laid 4800km of X80 steel pipe , but the commissioning time is relatively short, and it has not been tested by complex climate, geological disasters and other sudden environmental changes. In view of the current situation of the operation of the Mida pipeline , the construction and welding quality of the pipeline in the alpine permafrost zone in China need to be further improved, and the reliability of the pipeline operation needs to be further confirmed because there is no actual data on the operation of the pipeline in extreme environments. In this case, the large-scale construction of the larger diameter of the East China-Russia line is of great political and economic significance, and the choice of the wall thickness design method is particularly important.
Design method based on tensile limit
As the steel grade increases, the yield strength ratio (the ratio of yield strength to tensile strength) increases and the elongation decreases  (Figure 1). The lower the yield-to-tensile ratio, the greater the deformation capacity of the material from the beginning of plastic deformation to the final fracture, which can effectively relieve the stress concentration due to overload, but too low a yield-to-tensile ratio will cause the loss of strength of the material, resulting in material waste. On the contrary, the higher the yield-to-tensile ratio, the smaller the deformation capacity from the initial plastic deformation to the final fracture of the material. Therefore, using the traditional yield strength design criteria, the safety margin gradually decreases as the steel grade of pipeline steel increases. It has been suggested that the application of high-strength pipeline steel and the design of pipelines under complex working conditions require new design criteria, such as ultimate load design methods and reliability design methods, to improve the safety level of pipelines [8,9,10,11,12,13,14].
Fig.1 Nominal stress-strain curves for several different materials
GB 50251-2015, ASME B31.8-2007, ISO13623-2009, CSAZ 662-2011 are based on the yield strength of the pipe for pipe strength design, while ГОСТР 55989-2014 is based on the tensile limit design method for high strength steels with pipe materials of steel grade X65 and above. According to the provisions of APISpec5L-2012 “Steel pipes for pipeline transmission systems in oil and gas industry” on yield strength of steel (Table 4), the calculated wall thicknesses of straight pipe sections in the primary area of the Russian-Chinese Eastern route, for example, were compared with GB 50251-2015 and Russian standards under the conditions of operating pressure 12 MPa and outer diameter 1422 mm using different grades of pipeline steel (Figure 2). The lower the steel grade, the larger the margin from yielding to fracture, the smaller the wall thickness calculated by ГОСТР 55989-2014 using the tensile ultimate load design method compared with GB 50251-2015; as the steel grade increases, the yield strength ratio of the material increases, the margin from yielding to fracture becomes smaller, and the wall thickness calculated by ГОСТР 55989-2014 is larger compared with GB 50251-2015. The wall thickness calculated by ГОСТР 55989-2014 is larger compared to GB 50251-2015. It can be seen that the safety margin of the design of ГОСТР 55989-2014 reflects the changes in the material properties due to the adoption of the tensile limit design criterion.
Table.4 API Spec5L-2012 performance requirements for welded steel pipes
|Steel pipe grade||Minimum yield strength/MPa||Minimum tensile strength/MPa|
Figure.2 Calculation results of pipe wall thicknesses corresponding to GB 50251-2015 and ГОСТР 55989-2014 for different grades of pipeline steel
- (1) Taking the design of the wall thickness of the straight pipe section in the primary area of the China-Russia East Line as an example, the wall thickness calculated by ГОСТР 55989-2014 is the largest, 25.15 mm; the calculation methods of GB 50251-2015 and ASME B31.8-2007 are the same, 21.35 mm, which is slightly greater than that of ISO13623-2009, CSAZ 662-2011 The calculation result is slightly larger than that of ISO13623-2009 and CSAZ 662-2011.
- (2) The design concepts of Chinese and North American standards are different from those of Russian standards: the former takes into account the population distribution along the pipeline, adopts different design coefficients, and uses deterministic methods to calculate the strength of the pipeline; the latter does not take into account the population distribution along the pipeline, but is based on the reliability method and takes into account the load, material characteristics, pipeline use, pipeline working environment and construction environment based on engineering experience. The latter does not take into account the population distribution along the pipeline route; instead, it is based on the reliability method and takes into account the uncertainty of load, material characteristics, pipeline usage, pipeline working environment and construction environment in combination with engineering experience.
- (3) The strength design principles of the Chinese and North American standards are different from those of the Russian standard: the former is based on the yield strength of the material for wall thickness design; for high steel grade pipes, the latter is based on the tensile limit of the material for wall thickness design. With the development of high steel grade, large diameter and high pressure pipes, the wall thickness design principle of Russian standard can reflect the change of material properties.
- (4) GB 50251-2015 does not specify the applicable range of pipe diameters and pressure levels for gas transmission pipelines, and its technical limitations will gradually emerge with the continued development of large-diameter, high-pressure, high-steel-grade pipelines. If the standard is adopted for the design and construction of the China-Russia East Line, sufficient data are needed to prove its reliability, otherwise a certain safety margin should be considered.
Authors: Yan Feng, Ouyang Xin, Wang Pengyu, Diao Yu, Nie Chaofei, Wang Yubin, Zhang Jingnan, Gao Kun
Source: China Gas Transmission Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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