Study for West-East gas pipeline shows safety benefits of overmatched girth welds

March 10, 2003
Welding tests in advance of construction of China's West-East Pipeline indicated that an overmatch between a girth weld and line pipe metal would improve limit load properties and strengthen fracture resistance of the line pipe.

Welding tests in advance of construction of China's West-East Pipeline indicated that an overmatch between a girth weld and line pipe metal would improve limit load properties and strengthen fracture resistance of the line pipe.

But the tests also indicated the danger that the increased weld strength might undermine its resistance to hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC), making the weld operation more difficult.

If the ratio between weld and pipe metal strengths is not too large, however, the tests showed that an overmatched weld provided desirable qualities.

The tests were performed by the Welding Technology Center of the Pipeline Bureau of China National Petroleum Corp.

The West-East Pipeline is the first high pressure, high-grade steel (X-70), large-diameter (1,016 mm; 40-in.) gas transmission pipeline in China. Previous study had shown that the strength mismatch between girth weld and pipe material greatly affects the integrity of the pipeline.

Overmatch has been preferred for low-grade pipeline (X-60 or less). For higher strength steel pipe, undermatching has been recommended to reduce the sensitivity to cold cracking, HIC, and SCC.

This article is based on mechanical tests for the effect of different mismatched girth welds used on the West-East Pipeline.

The study indicated that mismatched girth welds affect such properties of the line pipe as limit load, fracture toughness, and threshold defect sizes.

Based on the research results, some advice for design, maintenance, and repair of the pipeline is presented.

Girth weld

Analyses on pipeline failures have shown that the girth weld can be the weakest point in a pipeline. It's very important to study how to improve the quality of the weld.

An overmatched girth weld is preferred for oil and gas transmission pipelines.1 2 The increase of steel grade, however, makes holding that overmatch more difficult.

To begin with, the weldability of high-grade pipeline steel is weakened, which will decrease the efficiency of weld operation.

Secondly, a high-strength weld will lead to an increase in the sensitivity to HIC and SCC, and weld toughness will likely be undermined, as well.

In Canada, to avoid these problems, an undermatched girth weld is adopted in some high strength (X-80) pipelines.3

Traditionally, weld mismatch means ratio of yield strength of the weld to the base metal. A strength mismatch ratio (MS) equal to the value of yield strength of weld compared with base metal; that is, MS = sSW/sSB.

An overmatched weld is one whose yield strength exceeds that of base metal (MS > 1); an undermatched weld is one whose yield strength is less than than that of base metal (MS < 1). At present, there are several new mismatch concepts such as ultimate strength mismatch and toughness mismatch.4 5

Effect on limit load

Pipe and pipeline data used in this article are from test results on the West-East Pipeline of China. The yield strength (sy) of the base metal of the line pipe is 500 MPa, the ultimate strength (su) is 600 MPa, and the elongation ratio (d) is 33%.

The program MPC-PREFIS was used to analyze limit load and safety of a pipe free of brittle fracture. This program is based on the rules of Kr-Lr twain-criteria; its core technology is Failure Assessment Diagram (FAD).6 Fig. 1 shows a map of FAD technology.

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Equation 1 (see accompanying box) expresses the failure-assessment curves in Fig. 1.

Assuming that there is no defect defined in API Specfication 5L in the girth weld of the pipeline, the maximum allowable imperfection dimensions for an undercut are:

Maximum depth 0.8 mm.

Maximum length 50% t (t = pipe wall thickness).

In this article, an undercut is treated as a surface crack, a conservative assumption.

Residual stress is also considered here, based on residual stress test results of a large number of pipe joints. Typically, the outside of the pipe has residual tension stress of 250 MPa, while the inside of the pipe has residual compression stress of 200 MPa.

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Analyzing with the PREFIS program reveals the limit load of pipes with different strength mismatch ratios, as shown in Fig. 2.

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As shown there, assuming the same base-metal strength, pipe with overmatched or evenmatched weld has a higher limit load than that with undermatched weld. Fig. 3 describes the limit-load changes with strength mismatch ratio.

Under the condition of MS < 1, the limit load drops quickly as MS decreases, whereas under the condition of MS > 1, the limit load changes little as MS increases.

Effect on weld toughness

The shape and magnitude of the plastic zone at the tip of the crack is a dimensional toughness index, which means that the larger the plastic zone, the higher the toughness of the weld structure. Deformation of the larger plastic zone occurs more easily and uniformly, the stress in it could be released to a greater extent, and the degree of tri-axial stress at the tip of the crack is much lower. All of this indicates much higher toughness.

Finite-element analysis indicates that an undermatched weld can deform more easily than an overmatched weld at the beginning.7 But during successive deformation, the undermatched weld seam is inhibited by both sides of high-strength base metal, and the plastic zone is restricted in the weld seam and can hardly grow into base metal.

As to the overmatched weld, because the strength of the base metal is lower than that of the weld metal, the plastic zone of the weld seam can easily grow into the base metal and then rapidly propagate into it.

In comparison with the overmatched weld, when loading is applied, the undermatched weld has less plastic zone, plastic deformation is mainly undertaken by the weld metal, and stress concentration at the tip of crack is more serious, so that the crack will grow at a lower crack driving force.

For an overmatched weld, plastic deformation occurs on both base and weld metal, and a crack will grow at a higher driving force. An overmatched weld, therefore, has better fracture resistance than an undermatched one.

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The fracture toughness requirement of West-East Pipeline with different mismatched girth weld was analyzed by theoretical calculation as follows:

According to the Strip Tension Model,8 fracture toughness crack-tip opening displacement (CTOD) can be calculated by Equation 2.

According to FAD method, there is dr = d/dc and Sr = s/ss. Then Equation 2 can change to Equation 3.

Assuming Sr = 0.9, then dr = 0.54 (see Equation 4).

As to the weld, it is governed by Equations 5 and 6.

Equation 7 shows the expression for fracture toughness CTOD.

Pipe for the West-East Pipeline is X-70, 1,016-mm OD, and 14.7-mm WT.

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According to Leak-Before-Burst Rules,9 assuming there is an outside surface crack with depth of t/4 (t is the nominal WT of pipe), and maximum allowed stress of the pipeline equals 72% of yield strength of the base metal, Equation 7 yields results of the fracture toughness requirements for different mismatched weld. Table 1 shows the calculation results.

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Fig. 4 shows the relationship between fracture toughness requirements and weld mismatch ratios. The fracture toughness requirement (dmat) for undermatched weld is bigger than that for evenly matched and overmatched welds. This indicates, therefore, that an overmatched weld has much higher fracture resistance than an undermatched weld.

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To study the effect of strength mismatch on critical defect dimensions, a single parameter analysis of a surface crack depth (a)/length (2c) was performed with MPC-PREFIS. Under West-East Pipeline technical requirements, with a = 0.75mm and 2c equal to a series of values, Fig. 5 shows the analysis results of different mismatched welds.

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In the same way, with 2c = t/2 and crack depth (a) equal to a series of values, Fig. 6 shows the analysis results of different mismatched welds.

As shown in Figs. 5 and 6, strength mismatch has great effect on critical defect magnitude. Table 2 lists the final analysis results.

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Because the actual service of the West-East Pipeline is more complicated, a safety factor (f = 1/1.4) is taken into account for decisions of the critical defect magnitude of the pipeline.

Acknowledgment

The authors are grateful to the Key Laboratory for Mechanical and Environmental Behavior of Tubular Goods of CNPC for its financial assistance and permission to publish this article.

References

1. Ocak M., "Position on State-of-the-Art Document on Weld Mismatch," International Institute of Welding, Vol. X (1994), p. 1292.

2. Burkin, F. M., and Kocak, M., "Draft Definitive Statement on the Significance of Mismatch of Strength in Weld," International Institute of Welding, Vol. X (1993), p. 1282.

3. Minami, F., and Ohata, M., "The Effect of Weld Metal Yield Strength on the Fracture Behavior of Girth Welds in Grade 550 Pipe," Pipeline Technology, Vol. 1 (1995), pp. 441-461.

4. Defourny, J., and Dhaeyer, R., "Mismatch in High Strength Steel Welds–Highlights of European Workshop," International Institute of Welding, Vol. IX (1994), p. 1768.

5. Zhang, J.X., and Shi, Y.W., "The Study on Ductile Fracture of the over-matched Weldment with Mechanical heterogeneity," International Journal of Pressure Vessels and Piping, Vo. 75 (1998), pp. 773-776.

6. Draft API RP 579: API Recommended Practice For Fitness-For-Service; No. 8 (1997).

7. Tang, W., and Shi, Y.W., "Effect of crack depth and strength mismatch on fracture toughness of weld," Welding Journal of China, Vol. 16 (1995), No. 1, pp. 50-56.

8. Deng, Z.J., and Zhou, J.E., "Fracture and Fatigue of Mechanical Engineering Materials," Beijing: Mechanical Industry Press, 1994; p. 22.

9. ASME, Boiler and Pressure Vessel Code; New York: American Society of Mechanical Engineers, 1995.

Based on a presentation to the International Pipeline Conference 2002, Sept. 29-Oct. 3, 2002, Calgary.

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The author
C. J. Zhuang (zhuangcj@ tgrc.org) has been an engineer in the Tubular Goods Research Center of CNPC for 7 years and holds a Master of Science and Technology (1996) from Northwestern Polytechnic University in Aprail in Xi'an, Shannxi Province, China.