Welding knowledge of oil and gas pipelines

With the development of the petroleum industry, pipeline transportation of oil and gas has developed rapidly due to its safety, economy, specialization, and efficiency. Long distance, large diameter, and high pressure are becoming the development direction of onshore oil and gas transmission pipelines. Pipeline steel X56-X70 series high-strength steel has been widely used in pipeline construction. An X80 pipeline with a diameter of 1200mm and a length of 126km was built in 2014. In 1994, Canada trial-built a 1200mm\33km X80 pipeline. Due to the rapid construction of oil and gas pipelines, pipeline welding processes, welding equipment, and welding materials have also developed greatly. Many manufacturers have participated in market competition. There are many mature welding equipment and welding materials for pipeline construction at home and abroad. The following is a survey of pipeline welding equipment and materials produced by famous foreign manufacturers for reference.
 
1. Overview of commonly used welding processes for oil and gas pipelines at home and abroad
In the 1970s and 1980s, pipeline welding was mainly manual welding with downward cellulose electrodes and semi-automatic CO2 welding. Since these methods were manual operations, their efficiency was low, and the welding quality was also restricted by the level of manual skills. In the mid-1980s, due to the continuous development of power electronics technology and computer technology, the control technology of welding equipment entered the intelligent era, thus creating conditions for the successful implementation of new pipeline welding automation equipment and new processes, which has greatly improved the welding efficiency and welding quality of pipelines. Great improvements, such as the STT (The Surface Tension Transfer) CO2 gas-shielded welding power source technology and equipment developed by Lincoln Company, have attracted the attention of the world with its soft arc, minimal spatter, and excellent bottom welding quality, becoming a pipeline Welding, especially primer welding, is one of the preferred methods. Another example is the full-position automatic welding equipment for pipelines produced by MAGNATECH, which applies adaptive control technology, which not only overcomes the horizontal constraints of manual operation but also greatly improves welding efficiency and quality.

To summarize, the current construction techniques for pipeline welding mainly include the following:

1.1 Use cellulose downward welding rods for manual welding. When pipelines are seriously corroded by hydrogen sulfide or pipelines run in cold environments, use low hydrogen vertical downward welding rods for welding.
Due to the flexibility of manual welding and the low requirements for welding equipment, manual arc welding still accounts for 40-50% of the workload of outdoor pipeline welding. For example, in recent years, the pipeline project from Shaanxi to Beijing in my country was purchased from Bole Company There are more than 1,000 tons of various cellulose welding rods. It is predicted that the annual demand for welding rods for my country's oil and gas pipelines in the next few years will be 3-5,000 tons, and there is an increasing trend.
 
1.2 Weld the bottom of the cellulose electrode vertically and use CO2 gas-shielded welding to fill the surface.
Due to the high productivity and low cost of CO2 welding, this method has been continuously promoted and applied in recent years. However, for oil and gas pipeline welding, to achieve full-position welding, it must be completed within a smaller current range using the short-circuit transition method. The short-circuit transition method is used Because primer welding is prone to defects such as incomplete penetration, vertical downward cellulose welding rods are used to achieve single-sided welding, the back side is formed, and then the highly efficient CO2 gas-shielded welding is used to fill the surface. This process is more commonly used.
 
1.3 Semi-automatic welding with self-shielded flux cored wire
Self-shielded flux-cored wire semi-automatic welding is especially suitable for outdoor windy occasions. It does not use CO2 and relies on the gas protection generated by the flux core. It has good wind resistance and can be used for all-position welding of pipelines with high deposition rates. Currently, Lincoln Company The self-shielding flux-cored welding wire produced by the company is recognized by various countries. Its brands include NR-207, NR-204-H, NR-208-H, etc., which can be used for vertical downward welding of X70, X80, and other pipelines. However, this method also has the defect that the welding root is prone to not being fused during bottom welding.
 
1.4 CO2 gas-shielded semi-automatic or fully automatic welding with a high-performance welding machine.
As a result of in-depth research on CO2 gas-shielded welding short-circuit transition process control technology, high-performance power supplies that can timely control the welding current and voltage waveforms or control the output characteristics of electric energy have been produced abroad. The aforementioned STT surface of the American Lincoln Company Tension transfer welding technology belongs to the category of waveform control. Based on the improvement of the performance of welding equipment, semi-automatic and fully automatic CO2 gas-shielded welding of pipelines can be realized, which greatly improves the welding efficiency and welding quality. In addition, automatic TIG welding is also used for pipe welding in factories. This method has good quality but low production efficiency.

2 Welding materials for pipeline welding
1. Cellulose vertical downward welding rod
2. Solid core welding wire and flux cored welding wire