Selection of steel pipe wall thickness for high pressure and large diameter oil and gas pipelines

Selection of steel pipe wall thickness for high pressure and large diameter oil and gas pipelines

For oil and gas long-distance pipeline system, the reasonable selection of steel pipe, not only for the safe operation of the pipeline system is essential, but also can save the pipeline construction investment. Described the traditional principles of the relevant standards for the selection of steel pipe wall thickness and the selection of steel pipe wall thickness in high-grade areas. The feasibility of using a strength design factor of 0.8 to select the wall thickness of the pipeline in sparsely populated areas is analyzed; the practicality of reducing the wall thickness of gas transmission pipelines in high-grade areas and the fracture mechanics of the available thickness of the material is studied in view of the limitations of the production of thick-walled steel pipes. Through the analysis of the fracture mechanics of the pipeline limit, combined with engineering examples to calculate the validation, the reference value of the theoretical wall thickness of high pressure large diameter pipeline in the selection of different steel grades.

At present, for the selection of wall thickness of high-pressure long-distance oil (gas) pipelines, most of them adopt the third strength theory and select different strength design factors [1,2] to calculate the wall thickness of pipelines according to the current design codes. With the increasing mileage of pipeline construction in China, the geological features along the pipeline tend to diversify, and pipeline designers face new challenges, such as: in particularly sparsely populated areas, the use of 0.8 strength design factor to select the wall thickness of the pipeline is feasible; thick-walled steel pipe production has difficulty in limiting the wall thickness of steel pipe in high-grade areas, such as fracture mechanics. The following for the existing high pressure large diameter oil and gas pipeline wall thickness of steel pipe to explore, for the reference of designers.

Wall thickness of steel pipe selection principles

Traditional principles

ASME B31.8-2007 [3] stipulates that: the construction of pipelines in sparsely populated areas, allowing for a design factor of 0.8. In order to ensure pipeline safety and reduce the cost of pipeline construction, the impact of a variety of factors need to be considered. First, the pipe: 0.8 design factor corresponds to the steel pipe material performance differences, including API Spec5L X70 steel grade, including higher steel grade, the need to take into account the mechanical properties of the pipeline system; second is the inspection standard: the wall thickness of the steel pipe itself within the range of delamination, banding tissue segregation, weld hardness levels, such as strict limits, according to the application link to 80% yield stress corresponding to the steel pipe test pressure The level of standard qualification; three is the topography: the need to consider whether the range of height differences will cause the local retention of corrosive media in the pipeline, as well as landslides and ground subsidence; four is the construction supervision: in accordance with the requirements of integrity management, strict quality control of each link; five is the quality of welding: comprehensive consideration of welding preheating requirements, after the welding process on the former layer of the thermal effects of welded metal and other factors, the site ring weld welding should be able to Achieving multi-layer welding (3 layers or more than 3 layers) as the principle, taking into account the root welding and root welding after the grinding process, the residual height limit, the diameter of commonly used welding materials, etc., the objective minimum wall thickness of steel pipe needs to be controlled at about 5mm; sixth is the pipeline transmission medium: gas pipeline compared with oil pipeline, should be more ensure its corrosion resistance, need to focus on the entire service life of the upstream H₂S, dehydration,  CO₂ The reliability of the processing equipment and the reliability of the cathodic protection system; seven is the stability of the radial direction of the pipeline.

Selection principles for high-grade areas

Although according to the traditional principles, in accordance with GB 50251-2003 “gas pipeline engineering design specification” in the strength design factor can be calculated to select the wall thickness, but many high-grade areas due to its geological factors, the need to further explore the selection of steel wall thickness conditions, which is extremely important for guiding production practice.
For secondary, tertiary and quaternary areas of the choice of pipe wall thickness, there is no rigorous statistical information or test data to support. Therefore, in areas of high regional grade, especially four areas, when the calculated wall thickness of steel pipe in actual production is difficult to achieve, should be considered separately. GB 50028-2006 “Urban Gas Design Code” stipulates that for large pipeline projects, the wall thickness of steel pipes in high-grade areas can be selected through actual tests [4].

Fracture mechanics qualification

From the fracture mechanics point of view, the pipe can only be within a reasonable wall thickness to ensure that it is in a plane stress state during operation, i.e., to meet the requirements of the stress-based design concept; the most important limiting factor for this reasonable wall thickness is the technically achieved material fracture mechanics toughness Kc value, and is closely related to the yield strength of the material itself [5]. When the pipe is stressed, if the triaxial stress state, that is, the plane strain state, even if the material itself is good plasticity and toughness, but also prone to brittle damage, because this triaxial stress state severely limits the material itself toughness play, so that the steel damage behavior tends to brittle damage characteristics.
According to the theory of fracture mechanics, in order to ensure that the material is stressed in the plane stress state (two-axis stress state), the wall thickness of the steel pipe should meet:

In the formula:

• t for the wall thickness of the pipe, mm;
• Kc for the material itself fracture mechanical toughness, MPa / (mm) 3 / 2;
• Ys for the minimum yield strength of steel pipe, MPa.

According to the formula (2) calculate the different wall thickness of X65, X70, X80, X100, X120 steel grade pipe line steel drop hammer tear test (DWTT) to obtain the full plastic fracture required fracture mechanical toughness Kc value (Table 1).
Table.1 Fracture mechanical toughness values Kc required to obtain full plasticity fracture by DWTT for each steel grade pipeline steel

Steel pipe strength grade	Material fracture mechanics toughness value Kc/(MPa•mm–3/2）
	10mm	15mm	18.4mm	20mm	22mm	26mm	32mm	40mm	50mm
X65/L450	3567	4 369	4839	5 044	5 291	5752	6381	7 134	7 976
X70/L485	3844	4 708	5215	5437	5 702	6199	6 877	7689	8 596
X80/L555	4399	5388	5 967	6222	6 525	7094	7 870	8 799	9 837
X100/L690	5469	6 699	7 419	7 735	8 112	8819	9784	10 939	12 230
X120/L830	6579	8058	8924	9304	9 758	10 609	11769	13 158	14 711

At present, according to the international frontier technology level, high quality steel fracture mechanical toughness Kc is difficult to reach 6500MPa / (mm) 3 / 2, theoretically, X65, X70, X80, X100, X120 steel grade steel pipe reasonable wall thickness range should be 32mm, 26mm, 22mm, 15mm, 10mm or so. Japan’s high-performance pipeline steel committee (HLP) to carry out a series of tests on the X80 steel grade steel DWTT (Table 2). When the thickness of the steel pipe is greater than 25mm, the steel pipe is highly sensitive to steel defects when damaged by force, that is, the material has been in a triaxial stress state when stressed, the test to some extent to verify the conclusions in Table 1. At present, the above problem can only be identified from the nature of the problem, there are many issues that need to be studied in depth, such as the scope of application of the formula; steel available wall thickness and steel chemical composition, tissue state, grain size and other relevant mechanical properties of the relationship between factors, the mechanism and weight of the impact of each factor; steel pipe plasticity capacity of the limited degree and the impact of the boundaries of the use of steel pipe performance. At present, the wall thickness of steel pipe for the pipeline, should try to avoid using more than the theoretical range of wall thickness.
Table.2 X80 steel grade steel fracture toughness characteristics of different wall thicknesses

Specimen thickness / MM	DWTT 100% plastic fracture dynamic tearing fracture work /j	Charpy impact energy / J
10	5000	433
15	10000	490
20	15000	470
25	20000 – 23000	427 – 551
32	30000 – 36000	454 – 618

Table.3 different steel grade, pipe diameter, design pressure conditions in different areas of the grade steel pipe selection wall thickness

Steel pipe strength grade	Steel pipe wall thickness /mm
	Diameter 1219 mm, pressure 12 MPa				Diameter 1016 mm, pressure 10 MPa
	Primary area	Secondary area	Tertiary region	Level IV region	Primary area	Secondary area	Tertiary region	Level IV region
X65/L450	22.6	27.1	32.6	40.7 Not recommended	15.7	18.9	22.6	28.3
X70/L485	21	25.2	30.2	37.8 Not recommended	14.6	17.5	21	26.2
X80/L555	18.4	22	26.4 Not recommended	33.0 Not recommended	12.8	15.3	18.4	22.9
X 100/L690	14.8	17.7 Not recommended	21.2 Not recommended	26.5 Not recommended	10.3	12.3	14.8	18.5
X120/L830	12.3 Not recommended	14.7 Not recommended	17.7 Not recommended	22.1 Not recommended	8.6,10.3	10.2,10. 3	12.3 Not recommended	18.5 Not recommended

According to the above conclusions, in the diameter-thickness ratio (pipe outside diameter D and the ratio of pipe wall thickness t) is not greater than 100, X65, X70, X80, X100, X120 steel grade by diameter 1219mm, pressure 12MPa and diameter 1016mm, pressure 10MPa calculate the wall thickness of steel pipe selection (Table 3).
Comprehensive Table 1, Table 2 and Table 3 data, the following conclusions can be drawn: X120/L830 steel grade steel pipe is basically suitable for primary and secondary areas 1016mm diameter, pressure 10MPa pipeline, three areas, four areas should be used with caution; 1219mm diameter, pressure 12MPa pipeline should be used with caution. X100/L690 steel grade steel pipe is basically suitable for 1016mm diameter X80/L555 steel grade steel pipe is basically suitable for 1016mm diameter, pressure 10MPa pipeline in the first-class to fourth-class areas; 1219mm diameter, pressure 12MPa pipeline should be used with caution. 1219mm diameter, pressure 12MPa pipeline in the first-class areas and second-class areas, the third-class areas and fourth-class areas should be used with caution. X70/L485 and X65/L450 steel grade pipes are basically suitable for Class I to Class IV areas of 1016mm diameter and 10MPa pressure pipelines; 1219mm diameter and 12MPa pressure pipelines are suitable for Class I to Class III areas.
Authors: Xu Yanxin, Ma Xuehai, pangbaohua, Liao Xuan

Source:  China Oil and Gas Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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References:

• [1] Professional Standardization Committee for Oil and Gas Field and Pipeline Construction Design. GB50251-2003 Design specification for gas transmission pipelines [S]. Beijing: China Planning Press, 2003.
• [2] China National Petroleum Corporation. GB50253-2006 Design specification for oil transmission pipelines [S]. Beijing: China Planning Publishing House, 2007.
• [3] American Society of Mechanical Engineers. ASME B31.8-2007 Gas Transmission and Distribution Piping Systems [S]. Beijing: Weapon Industry Press, 2007.
• [4] Ministry of Construction of the People’s Republic of China. GB50028-2006 Design Code for Town Gas [S]. Beijing: Construction Industry Publishing House, 2007.
• [5] Wang Yikang. Modern high performance steel for gas pipelines [C]// Proceedings of the Pipeline Steel Forum . Beijing: 2009.