1 The status quo of China's oil and gas pipeline construction Pipeline transportation is the main means of energy transmission, and pipelines occupy a very important position in modern production and life. It has been widely used in the transportation of water, oil, gas, coal and communications, power supply, transportation, transportation and drainage, and is the lifeline of society. In recent years, with the rapid development of the national economy, the increase in energy demand and the adjustment of the energy structure, the amount of long-distance oil and gas pipelines in China has been increasing, and the technology of pipeline construction has also continued to develop.

Since the completion of the Xinjiang Karamay to Dushanzi oil pipeline in 1959, with the successive development and construction of oil and gas fields such as Daqing, Shengli, Sichuan, North China, Central Plains, Qinghai, Tarim and Tuha, the construction of oil and gas pipelines has made remarkable achievements . The oil and gas transportation pattern of "North Oil to South Transportation", "West Oil to East", "West to East Gas Transmission", and "Sea-to-Earth Landing" have been initially formed. The Harbin-China Crude Oil Pipeline Project, West-East Gas Pipeline Project, Shaanxi Key pipeline projects such as the Beijing Second Line Gas Pipeline Project, Western Crude Oil, Product Oil Pipeline Project, Ningbo-Shanghai-Nanjing Crude Oil Pipeline Project.

Since the middle and late of the last century, China's pipeline welding construction technology has been continuously improved, and various types of oil and gas pipeline construction have made great progress. Pipeline welding construction technology has been continuously improved and developed, and the production bureaus of developed countries have continued to shrink. In some technologies, it has even reached the world level.

With the continuous growth of the national economy's demand for energy, China has gradually established long-distance energy pipelines and regional oil pipeline networks. Jointly tackled technical problems such as large drop differential, static pressure control in complex terrain, pipeline dissatisfaction flow, and mixed conveyance interface monitoring for sequential transportation. This has made a new leap in China's oil and gas pipelines in transportation technology, automation technology, construction technology and other aspects. After further improvement of the regional oil and gas pipeline network, it will have a more positive impact on environmental protection and the improvement of people's quality of life.

The energy structure is also more reasonable and more conducive to the country's sustainable development.

2 Defect cracks in pipeline welding construction are the most harmful defects in welding, which mainly include crystal cracks, liquefaction cracks, delayed cracks and other forms. Due to its extensibility, the crack will continue to extend in the presence of internal stress in the weld bead until the weld bead is destroyed. Therefore, in the construction of long-distance pipelines, crack defects are not allowed to exist, and usually are not allowed to be repaired, and must be cut and re-welded.

Unmelted refers to the part between the weld bead and the base metal or between the weld bead and the weld bead that has not been completely melted and joined. It mainly occurs at the joint position of the pipe clock at 1 o'clock and 11 o'clock and at the bottom of the pipe at 6 o'clock. Unfused can be divided into three types: root unfused, layer unfused, and groove unfused. The root is not fused mainly because the weld metal and the base metal and the weld joint are not fused during the priming process, and the interlayer is not fused mainly because the weld metal between the layers is not fused during the welding process. The groove unmelted is the unmelted between the weld metal and the base metal groove, and the root unmelted is more likely to occur. Unfused is a planar defect, which is easy to cause stress concentration and is only second to welding cracks. Therefore, unfused defects are not allowed in the weld.

The so-called undercut is the so-called weld bead bite, mainly because the deposited metal fails to cover the groove of the base material during the welding process, leaving a gap below the base material at the edge of the weld bead. Shallow undercuts may not be treated, but undercuts will have a serious impact on the mechanical properties of the weld bead. Undercuts will reduce the effective cross-sectional area of ​​the base metal, causing stress concentration at the undercuts and reducing joint strength, especially It is the welding of low-alloy high-strength steel. In addition, the edge structure of the undercut is hardened, which is easy to cause cracks. This has seriously affected the safety of pipeline transportation.

Welding stress is caused by the restriction of shrinkage after welding. The more severe the restriction, the greater the internal stress. It is inevitable to leave a certain residual stress after welding, but it can be given a certain degree of control and adjustment through appropriate process measures.

In the pipeline construction, the cracks are basically caused by the inadequate execution of the process regulations and the excessive external stress. Therefore, in the construction, steel pipes with good crack resistance should be selected to make steel pipes, welding materials and drying should be selected reasonably, and construction should be carried out in strict accordance with the requirements of welding process evaluation to ensure the plasticity and toughness of the weld structure; Insulation measures should be taken during winter construction, if necessary, heat treatment or post-weld heating; strict control of the stress on the group, and try not to use external pairing devices to force pairing, especially when the pipeline is connected to the dead end, jacks, Pipelayer lifts up the excavator, presses down, etc. to adjust the gap between the strong group construction method.

3.2 Pipeline non-fusion prevention measures In the pipeline construction, bevel processing must be carried out in strict accordance with the specifications, the angle should not be too small, the gap width should be ensured, the blunt edge should not be too thick, and the interlayer cleaning should not be excessive, so as not to cause the groove to be widened and formed The operation methods such as grooves should be stable, the current should be controlled, the input of line energy should be too small, and the phenomenon of unfusion in pipeline construction should be prevented.

Preventive measures for undercuts During the construction process, the welding parameters should be adjusted to avoid excessive current, excessive arc length, and inadequate arc force, which may result in inadequate welding of the molten pool. At the same time, it is necessary to adjust the inclination angle of the welding rod or welding wire to prevent the occurrence of uneven blowing. The operation method should be stable, the swing of the transport bar should be in place, and the undercut should be strictly controlled.

The preheating method, synchronous shrinking method and other measures are used to control the welding stress.

In pipeline welding, the greater the welding temperature difference, the greater the residual stress. At the same time, from the organizational transformation, the faster the cooling, the greater the organizational stress. Preheating can reduce the temperature difference and slow down the cooling rate, thereby reducing welding stress. At the same time, the shrinkage of the welding seam is pinned by the cold metal next to it to form tensile stress. The colder metal next to the effective section does not allow it to shrink and form a larger stress. Take appropriate technological measures to allow it or part When it shrinks, it can relieve or partially relieve residual stress. Synchronous contraction method is based on the technological measures taken by this principle. In the construction, a reasonable welding sequence is adopted to try to make the welding seam shrink more freely, especially those welding seams with larger shrinkage and larger residual stress can also achieve the purpose of controlling welding stress.

Pipeline construction is of great significance to ensure the safety of China's energy lifeline and provide a strong guarantee for social and economic construction. At present, there are still many shortcomings in the construction of oil and gas pipelines in China. Only by constantly exploring new measures in practice and strengthening the innovation of pipeline welding technology can we ensure the safety of the energy lifeline and provide energy security for China's modernization.

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