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

Transcription

1 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 , 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: / j.issn Dr. Yongli Sui cnpcsuiyongli@sina.com 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 Introduction So 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 1016 mm has reached a length of about 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 welding China 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 73

2 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. 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 singletorch external welding machine launched in 1995, while the doubletorches external welding machine in 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 doubleaxes 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. 2 Development of automatic welding in China It 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 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 Fig.1 CPP900-FM beveling machine Fig.3 CPP900-W1 single-torch external machine Fig.2 CPP900-IIW internal welding machine Fig.4 CPP900-W2 double-torches external machine Time to construct Table 1 Automatic welding in China s pipeline projects Project name Pipe type Diameter /mm Wall thickness /mm Welding length /km 1999 Hong Kong-Beijing Gas Pipeline X Zhengzhou-Yima Gas Pipeline 16Mn Zhangzhou Water Pipeline Q Sebei-Xining-Lanzhou Gas Pipeline X West-East Gas Pipeline Project X , Jining Branch Pipeline of West-East Pipeline Project X India East-West Gas Pipeline Project X , Eastern Siberia-Pacific Ocean (ESPO) Pipeline in Russia K , the Second West-East Gas Pipeline Project X ,18.4, Branch A/B of China-Central Asia Gas Pipeline Project X , Branch C of China-Central Asia Gas Pipeline Project X The 3rd West-East Gas Pipeline Project X Total length

3 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. Strength grade Base metal Table 2 Metallurgical composition and microstructure design of pipeline steel Additional alloying elements Additional micro-alloying elements Final structure B and X42 C-Mn-Si Ferritic + Pearlitic Remark X52 to X70 C-Mn-Si Cu, Ni, Mo Nb < 0.06% Ferritic + Pearlitic C-Mn-Si Cr, Mo Nb < 0.11% Ferrite / Acicular Ferrite low finishing temperature X65, X70 and X80 C-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 welding B steel C Si Mn P S Cr Mo Ni Al Cu Nb Ti V pipe welding

4 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 ) (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 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 (a) Plant A 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. (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 microalloying 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, 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 76

5 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 (a) V groove (b) Double-V groove realize DC constant voltage CO 2 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 CO 2 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 CO 2 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. (c) Compound groove for internal welding machine (d) Groove for different wall thickness pipes Fig.8 Groove types 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 CO 2 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 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 longdistance pipeline construction and the requirement of relevant design documents and standards, performing good welding quality. 77