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Girth welding technique on the oil and gas pipeline project of China

Yongli Sui

(National Engineering Laboratory for Pipeline Safety, China Petroleum

Pipeline Research Institute, Langfang 065000, China)

Abstract: This paper introduces the development of girth welding of the

oil and gas transmission pipeline in china, focusing on the research and

application of automatic welding equipment. Based on characteristics

revealed in the development of high strength line pipe, it points out a

few technically difficult factors in girth welding, including wide range

of actual line pipe various composition of alloy, refinement of grain and

increase of carbon equivalent. It also describes girth welding processing

and groove of China’s high strength line pipe, indicating that the

reliability, adaptability and stability of CPP automatic welding system

in China’s pipeline construction has been highly recognized. This paper

argues that self-shielded flux cored arc welding (FCAW-S) and low

hydrogen electrode manual welding (SMAW ) will still be a choice for oil

and gas pipeline construction, while automatic welding will be the major

method in the future.

Key words: Transmission Pipeline; Girth Welding; Welding Technology;

Welding Quality

DOI: 10.7512/ j.issn.1001-2303.2017.13.02

Dr. Yongli Sui Email: cnpcsuiyongli@sina.com

0 IntroductionSo far, the total length of oil and gas transmission pipeline in China

has accumulated to nearly 110 thousand kilometers, forming a pipeline

system which covers the whole China and connects overseas. Therefore,

it becomes an energy artery that promotes economic development

and benefits nearly 1 billion people. As the transmission capacity and

distance of the transmission pipeline continuously increase, more and

more line pipe of high pressure, large diameter, high strength and high

toughness are applied in construction. Among them, X70 pipe with a

design pressure of 10MPa and a diameter of 1016mm has reached a

length of about 14600 km, while X80 pipe with a design pressure of

12MPa and a diameter of 1219mm about 12200km. This brings new

challenges for girth welding in transmission pipeline and makes girth

welding on high grade line pipe become a bottleneck that restricts its

development.

1 Development of girth weldingChina has undergone several major changes in girth welding

technology. In the 1970s, the traditional welding method, namely low

hydrogen electrode arc welding with uphill, was mostly applied. It has

the advantage of easy and flexible operation and can adapt to various

construction environment. However, it also suffers from the larger

groove gap, thicker beads, and lower speed, efficiency and quality.

Therefore, it is now used mainly in welding small diameter pipeline and

repair welding.

Downhill arc welding was promoted in the 1980s, with the use of

cellulose and low hydrogen electrode. This method can be realized by

adopting a high current, multi-layer and rapid welding operation, not

only decreasing layers’ thickness and the groove gap, but also increasing

welding efficiency. Shielded metal arc welding (SMAW) enjoys high

flexibility, convenience and applicability. Its deposition efficiency and

mechanical properties still can meet the needs of pipeline construction

today due to the continuous improvement of processing property.

In the 1990s, self-shielded flux cored arc welding (FCAW-S) arose.

Since this method has an extremely strong ability of wind resistance,

no protective atmosphere is needed. A semi-automatic welding torch

Yongli Sui is a professor of engineering. Dr. Sui has been working in the area of pipeline welding technology for 25 years. As deputy chief engineer of the China petroleum pipeline research institute and chief welding technology expert of CPP, Dr. Sui participated and led a number of key research projects within the China Petroleum Pipeline Bureau (CPP). Dr Sui was responsible for the completions of more than 20 research projects and the welding procedure qualifications of more than 120 domestic and international pipeline projects. Dr. Sui has authored 11 articles in the drafting of industrial codes and standards and published paper more than 40.

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is used while the wire feeder continuously feeds electrodes. FCAW-S

applies much higher welding current and speed than SMAW, therefore

it improves welding efficiency and reduces joints, greatly raising the

qualification rate. It develops the most quickly in pipeline construction

in China since it has little investment in equipment, high utilization rate,

short payback period, and is cost-effective in quality, efficiency, material

consumption and energy-usage, conforming to the concept of low-cost

automatic welding.

Automatic gas metal arc welding (GMAW) has been gradually

applied since 2000, with the increase of strength grade, diameter

and thickness of pipe. The characteristics of this method are listed as

below: excellent joint performance, high requirement for construction

management, little affected by human factor, high welding efficiency,

low labor intensity, adaptability to adverse weather condition, and great

potential for the construction of large diameter and thick wall pipe.

As automatic welding platform gradually matures, automatic welding

will improve continuously in both quality and economic benefit, and

gradually become the main field welding method.

2 Development of automatic welding in ChinaIt is in 1999 that China recorded the earliest automatic welding

application in Hong Kong-Beijing gas pipeline and Zhengzhou-Yima

gas pipeline, where the total amount of pipeline girth welding by

automatic external welding machines reached 8.8km. The application

of automatic welding began to expand since the construction of the

West-East Gas Pipeline Project in 2001. Now the total length of pipeline

welded by automatic welder is about 2600 km.

Today, China has formed CPP pipeline automatic welding system,

consisting of beveling machine, internal welding machine, single-torch

and double-torches welding machine that are all made in China. The

beveling machine has been developed and designed since the 1990s,

and now the CPP900-FM beveling machine can process shape V, U, X

and compound grooves. This equipment enjoys easy operation, high

processing precision, good groove shape and can process a single

groove within 2min. In 2000, research and development of the internal

welding machine began, and the CPP900-IIW internal welding machine

today not only can precisely joint the ends of pipes, completing inner

circumferential welding on the roots within 90 seconds, but also can

achieve wireless remote welding and adjust the groove gap. The single-

torch external welding machine launched in 1995, while the double-

torches external welding machine in 2002. Today the CPP900-W1

and CPP900-W2 external welding machines combine functions of

traditional automatic welding machine and innovative achievements

on automatic control and mechanical transmission. They realize double-

axes automatic tracking on the X-Y weld seam with convenient

and easy installation, fixation and operation. After nearly 20 years of

application, the CPP system gains highly recognition in pipeline project

construction due to its reliability, stability and environmental adaptability.

Time to construct Project name Pipe type Diameter

/mmWall thickness

/mmWelding length

/km1999 Hong Kong-Beijing Gas Pipeline X65 711 7.1 1.3

1999 Zhengzhou-Yima Gas Pipeline 16Mn 426 7.8 7.5

2000 Zhangzhou Water Pipeline Q235 1220 14 1.0

2000 Sebei-Xining-Lanzhou Gas Pipeline X70 660 10.3 8.5

2001-2002 West-East Gas Pipeline Project X70 1016 14.6,21 704

2005 Jining Branch Pipeline of West-East Pipeline Project X80 1016 15.3 7.8

2005-2006 India East-West Gas Pipeline Project X70 1219 17.2,20.7 50

2007-2008 Eastern Siberia-Pacific Ocean (ESPO) Pipeline in Russia K60 1220 19,24 54.3

2008-2009 the Second West-East Gas Pipeline Project X80 1219 15.3,18.4,22 662

2008-2009 Branch A/B of China-Central Asia Gas Pipeline Project X70 1067 15.9,19.1 605

2012-2013 Branch C of China-Central Asia Gas Pipeline Project X80 1219 17.5 277

2013 The 3rd West-East Gas Pipeline Project X80 1219 16.5 239.4

Total length 2617.8

Fig.1 CPP900-FM beveling machine Fig.2 CPP900-IIW internal

welding machine

Fig.3 CPP900-W1 single-torch

external machine

Fig.4 CPP900-W2 double-torches

external machine

Table 1 Automatic welding in China’s pipeline projects

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Till now laser-arc hybrid automatic welding technology and

equipment have been in the research and development and design.

Since this technology has high penetration and efficiency, it is regarded

as an innovation in pipeline industry. This method has been successfully

implemented in the laboratory on all position root welding on line pipe

with 8mm root face, and is under research on field adaptability of laser

generator and cooling system. The technology and equipment need

continuous improvement during the process to meet the requirements

of field welding.

3 Major technical difficulties of high grade line pipe girth welding

The development of line pipe reveals that there are two ways to raise

intensity. One depends on the metallurgical composition design and

the smelting technology that precisely controls alloy composition. The

other relies on the rolling process that accurately controls the cooling

rate. These excellently resolve cold cracks and the brittleness of HAZ in

high intensity pipeline steel. However, there are some new technical

difficulties in welding.

(1) Joints strength and toughness matching due to increasing

strength and wide range of actual strength of the steel pipe

On the one hand, the weld metal is an as-cast structure formed by

heating, melting and solidification, making it more difficult to match

strength and toughness in this structure than in pipe that has done

TMCP treatment. The higher the strength grade of steel, the more

difficult to match weld strength and toughness. On the other hand,

the wide range of the actual strength of steel pipe makes it difficult

to realize a standard match of higher or equal strength. Although the

tensile strength of girth-welded joints is not lower than the minimum

specified tensile strength of the pipe, the girth-welded joint is matched

with an equal or lower strength compared to the actual strength of the

steel pipe, which requires a rigorous selection of welding materials and

process.

(2) Decreasing weldability of girth-welded joints due to the increase

content of alloying elements and the wide range of the actual alloy

(a) Pipe strength distribution in plant A

(b）Pipe strength distribution in plant B

Fig. 5 Strength distribution of pipes from different sources of supply in

an pipeline project using X80 pipe

It is necessary to add some alloying elements and few micro-alloying

elements to raise the strength of line pipe, as shown in Table 2. Actual

alloying elements vary greatly, affecting the fusion ratio of the base

metal in welding, therefore, they will influence the stability of welding

and integrated performance of HAZ and weld metal, such as strength,

toughness, hardness etc. Higher requirements are put forward for

selecting groove type, welding process parameters and welding

materials.

Table 2 Metallurgical composition and microstructure design of pipeline steel

Strength grade Base metal Additional alloying elements

Additional micro-alloying elements Final structure Remark

B and X42 C-Mn-Si — — Ferritic + Pearlitic —X52 to X70 C-Mn-Si Cu, Ni, Mo Nb < 0.06% Ferritic + Pearlitic —X65, X70 and X80

C-Mn-Si Cr, Mo Nb < 0.11% Ferrite / Acicular Ferrite low finishing temperatureC-Mn-Si Cu, Ni, Cr, Mo Nb < 0.07% Ferrite / Acicular Ferrite high finishing temperature

Table 3 Chemical composition of X80 pipe and semi-automatic FCAW-S welds %

A steel C Si Mn P S Cr Mo Ni Al Cu Nb Ti V

pipe 0.05 0.21 1.69 0.011 0.003 0.19 0.006 0.01 0.031 0.095 0.041 0.016 0.003

welding 0.05 0.12 1.49 0.012 0.004 0.06 0.007 1.80 0.891 0.036 0.013 0.006 0.002

B steel C Si Mn P S Cr Mo Ni Al Cu Nb Ti V

pipe 0.05 0.20 1.78 0.008 0.003 0.32 0.18 0.008 0.04 0.009 0.086 0.014 0.03

welding 0.05 0.12 1.52 0.011 0.004 0.084 0.047 1.86 0.973 0.014 0.022 0.006 0.001

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As shown in Table 3 and Figure 6, the base metal melted into the

weld metal will put different effects on the alloy composition and

microstructure of the weld metal when the actual alloy elements of the

pipeline changes widely. Meanwhile, the factors that affect the cooling

rate, such as the welding process parameters and the construction

environment temperature, will influence the mechanical properties of

the girth welds. This makes joints of FCAW-S, FCAW-G and SMAW

show a greater dispersion of the Charpy impact energy.

(a) Plant A

(b) Plant B

Fig. 6 CCT curve of FCW-S welds

(3) Significant softening of HAZ due to the refining of steel grain

The HAZ of high strength pipeline steel softens because of the lower

cooling speed after welding compared with that of rolling cooling, grain

growth and the dissolution of second phase particles formed by micro-

alloying elements. Therefore, tensile fractures mostly occur at HAZ,

especially for semi-automatic welding and SMAW which input large

amount of heat, as shown in Figure 7. The interpass temperature, the

number of passes and the welding sequence then need to be controlled

strictly to ensure a small amount of the welding heat input.

(4) Root welding technology - the key to control welding quality

and construction efficiency

The sensitivity of welding cold crack increases according to the

significant raise of carbon equivalent as the strength grade of pipeline

steel improves. In the field construction of high strength steel pipeline,

it is the most important to ensure that the root welding process can

effectively avoid the welding crack, and has good welding performance

and efficiency. Through the test and analysis of the cold crack sensitivity

of X80 steel pipe, it is considered that root welding with cellulose

electrode tends to leave a larger cold crack. In this connection, it is

recommended to use low hydrogen electrode manual welding or

semi-automatic welding with solid wire or metal powder cored wire

in X80 steel pipe root welding. This technology has been applied and

implemented in the construction of X80 steel pipeline in china.

4 Application of girth welding technique on high grade pipeline

Aiming at resolving above difficulties in girth welding on high

strength line pipe, below list several girth welding technologies that are

applied in oil and gas pipeline projects, of which automatic welding

will gradually become the major method in future construction of high

strength pipeline.

(1) Combined semi-automatic welding of “gas shielded solid

wire or metal powder cored wire+self--shielded flux cored wire”

(PGMAW+FCAW--S)

Semi-automatic welding methods using solid wire and metal

powder cored wire are root welding methods that adopt STT or RMD to

complete the melt-through bead welding. The self- shielded flux cored

semi-automatic welding is applied in filling and cover welding, whose

groove types are shown in Figure 8 (a) and (d). This combined process

has the advantages of flexible operation and strong environmental

adaptability, and is suitable for the construction of complex terrains,

such as hills, slopes, water networks and other areas that are not

conducive for large-scale equipment.

(2) Manual welding of low hydrogen electrode (SMAW)

In this process, low hydrogen electrodes which are suitable for

one side welding both sides formation are selected in vertical up root

welding. The high strength and toughness ones are chosen in filling and

cover welding, either vertical up or down. The groove types are shown

in Figure 8 (a) and (d). This method is mainly used for repair welding of

(a) (b) (c)

(a) SMAW (b) Semi-automatic FCW-S

(c) Gas shielded solid wire automatic welding

Fig.7 Transverse tensile samples of girth-welded joints with different

heat input

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welds, tie-in welding, and so on.

(3) Combined automatic welding of "internal welding machine+

single--torch external welding machine+double--torches welding

machine" (GMAW)

The automatic root welding using the inner welding machine and

the gas shielded solid wire are done from inside of the steel pipe to

realize DC constant voltage CO2 short-circuit transfer. From outside,

double-torches external welding machine and the gas shielded solid

wire are selected in hot, filling and cover welding to achieve DC pulsed

spray transfer (PGMAW). The groove type is shown in Figure 8 (c). This

combined process is the main method in China's oil and gas pipeline

construction, since it has high efficiency and good quality.

(4) Combined automatic welding of "internal welding machine+

single--torch external welding machine" (GMAW)

The automatic root welding using the inner welding machine and

the gas shielded solid wire are done from inside of the steel pipe to

realize DC constant voltage CO2 short-circuit transfer. From outside,

single-torch external welding machine and the gas shielded solid wire

are selected in hot, filling and cover welding to achieve DC constant

voltage CO2 short-circuit transfer or DC pulsed spray transfer (PGMAW).

The groove type is shown in Figure 8 (c). This combined process is the

main method in China's oil and gas pipeline construction, since it has

high efficiency and good quality.

(5) Combined welding of "gas shielded solid wire or metal powder

cored wire semi-automatic welding + single-torch machine automatic

welding machine with gas shielded flux cored wire " (GMAW)

The two semi-automatic welding methods using solid wire and

metal powder cored wire adopt STT or RMD to complete one side

welding both sides formation root welding. The single-torch external

welding machine and the gas shielded flux cored wire from outside

of the pipe are selected in hot, filling and cover welding to achieve

DC constant voltage CO2 short-circuit transfer. The groove types are

shown in Figure 8 (a), (b) and (d). This method is mainly used in the

construction in complex terrains and girth welding on complex butts.

5 Conclusion(1) China has undergone several major changes in pipeline girth

welding. With increases in steel strength grade, pipe’s diameter and

wall thickness, the welding method gradually transfers from vertical up

or down manual welding to semi-automatic and automatic welding.

However, semi-automatic and manual welding will remain to be the

choice, considering different topography and climatic conditions.

(2) CPP automatic welding system consists beveling machine,

internal welding machine, single-torch and double-torched external

welding machine. After nearly 20 years’ development, the CPP system

gains highly recognition in pipeline project construction due to its

reliability, stability and environmental adaptability.

(3) Improvements in high grade pipeline steel excellently resolve cold

cracks and the brittleness of HAZ while bring new technical difficulties in

welding. Therefore, the satisfied mechanical properties of girth-welded

joints are ensured by strictly selecting groove type, welding process

parameters and welding materials to guarantee a small amount of the

welding heat input.

(4) The welding technology and groove types of high grade steel

pipe have undergone strict welding evaluation and plentiful practical

application. For this reason, it conforms to the characteristics of long-

distance pipeline construction and the requirement of relevant design

documents and standards, performing good welding quality.

(a) V groove

(b) Double-V groove

(c) Compound groove for internal welding machine

(d) Groove for different wall thickness pipes

Fig.8 Groove types