Installation and downward welding process of long-distance natural gas pipeline

In recent years, the development and utilization of natural gas resources have achieved great economic and social benefits. It has reduced air pollution, improved the living environment and improved the quality of life of urban and rural residents. The power plant uses natural gas to generate electricity, so the installation and welding of long-distance pipeline is put on the construction schedule. In this paper, the installation project of special gas transmission pipeline for Jiangsu Guoxin Yixing gas turbine cogeneration project is comprehensively discussed in terms of directly buried pipe construction, crossing construction, splicing construction and welding construction.

Project overview

The project is a bid section II project for the installation and construction of special gas transmission pipeline for Jiangsu Guoxin Yixing gas turbine cogeneration project. The engineering design pressure is 6.3MPa, X60 straight seam double-sided submerged arc welded steel pipe d508 × 11.1 with a total length of 10km (mileage 2 500-6 400; mileage 12 860-18 600).
According to the actual situation of the project and on-site construction conditions, the construction shall be carried out in accordance with the characteristics and principles of reliability, rationality and feasibility, so as to achieve the rapid, high-quality, safe and efficient construction management objectives, construction drawings and various requirements of the owner. And implement the current national construction and acceptance specifications and standards.

Directly buried pipe construction

Pipeline construction process

  • 1) The construction process of long-distance pipeline is generally as follows: Measurement and setting out – cleaning of construction operation zone, construction and transportation pipeline – transportation of anti-corrosion pipe – processing groove – re laying of pipe – nozzle preparation, nozzle team – pipe assembly welding – crater inspection – heat shrinkable sleeve patching – nozzle excavation – pipe trenching – nozzle team, pipe welding – crater inspection – cleaning and pressure – pipe trench backfilling – landform restoration.
  • 2) The construction process of buried pipeline is generally as follows: handling the earth moving procedures – measuring and setting out according to the drawing – piling – digging pipe trench – treatment of trench bottom cushion – retesting the elevation – pipeline prefabrication and anti-corrosion – pipe laying and alignment – pipe orifice connection – partial soil backfilling – inspection before pressure test – sectional system experiment – inspection before concealment – backfill – Final hydraulic test of the system.

Construction steps

The construction of pipelines will be carried out in process sequence and in accordance with corresponding specifications.

Assembly, welding and inspection

The project is to lay high-pressure natural gas pipeline, using x60m d508 × 11.1 straight seam double-sided submerged arc welded steel pipe, with a length of 8.95km and a design pressure of 6.3MPa.

Pipe welding


Welders must be assessed according to the requirements of assessment rules for welding operators of special equipment and obtain the certificate issued by the quality and technical supervision department before they can undertake the welding work within the scope of corresponding projects within the period of validity. The welding position of the certified welder shall be marked with the welder’s steel seal to make the weld traceable.

Welding materials

According to the engineering design requirements: manual downward welding is adopted, and E6010 cellulose electrode is selected for root welding. Semi automatic downward gas shielded welding is adopted for thermal welding, filler welding and cover welding, and e71t8-k6 welding wire is selected. The selected electrode shall comply with AWS A5.1 and A5.5, and the welding wire shall comply with AWS a5.29.

Welding equipment

According to the requirements of welding materials and construction conditions, DC arc welding machine or semi-automatic welding machine shall be used for welding equipment.

Construction process flow

Downward welding process

(1) Downward welding technology is adopted for back sealing welding of natural gas pipeline. During construction, manual downward welding is used to complete the backing welding and all welding layers, and gas shielded welding is used for thermal welding, filling welding and cover welding.
(2) The process flow of downward welding is as follows: pipe laying → pigging and grinding → assembly → welding.
(3) Before pipe laying, sandbags shall be assembled in the operation zone and pipe piers shall be driven. The support height shall be 0.6m, and there shall be 2 support piers for each pipe. On site, the pipes shall be placed in zigzag shape along the operation zone, connected from head to tail, and the adjacent pipe orifices shall be staggered by 0.2 ~ 0.3m for assembly and welding.
(4) Before pigging and polishing the pipe assembly, remove the sundries in the pipe and the oil stain, rust and other impurities on the surfaces at both ends of the groove. Polish within the range of > 10mm on both sides of the pipe orifice to expose the metallic luster. Pay attention to protecting the groove and use it when cleaning.
(5) Assembly and tack welding pipe assembly and tack welding are the key to ensure the welding quality of downward welding and the good forming of the back of pipe joint. Tack welding is a part of the formal weld. It shall be welded on one side and formed on both sides, and attention shall be paid to ensuring the welding quality.
(6) Pipe assembly shall not be forcibly assembled in any way. After the root welding is completed by using the internal aligning device, and then removing the aligning device, the root weld bead can be completed at one time; Using the external alignment device, tack weld 6 ~ 7 places evenly on the whole circumference of the welded junction. The cumulative length of tack weld shall not be less than 50% of the circumference of the pipe before removing the external alignment device. The assembly size shall be qualified according to the welding procedure.
(7) When connecting the end of the pipe, the external alignment device shall be used for assembly; The groove shall be machined or gas cut. The groove surface shall be polished by gas cutting to remove the cutting trace. The vertical deviation between the notch surface and the pipe centerline shall be less than 1.5mm. The groove size is shown in Figure .
20211103094748 22153 - Installation and downward welding process of long-distance natural gas pipeline
(8) When butt joint with equal wall thickness, the misalignment shall meet the size specified in the process evaluation; The following requirements shall be implemented for butt joint of unequal wall pipes with the same diameter: the misalignment of the interface shall not exceed 10% of the thin wall thickness and shall not be greater than 1mm; Wall thickness difference ≤ 1mm, can be directly butt welded; The wall thickness difference is ≤ 30% of the thin wall thickness. The pipe end of the thick wall pipe shall be processed to ensure that the misalignment of the interface does not exceed 10% of the thin wall thickness.
(9) The pipe welds of connecting pipes shall be staggered by more than 100mm.
(10) The radius of curvature of pipeline elastic laying shall be ≥ 1000D.

Welding sequence

The downward welding of the pipeline should adopt flow operation, which is welded by four welders at the same time, one group of two welders for symmetrical welding, and the whole team is arranged in a row along the pipeline. In order to shorten the root welding time and ensure the interlayer temperature. In addition, opening welding from different positions can effectively ensure that the butt joint gap will not change.


In pipe welding, the welded joint is prone to defects. When joint welding is required, careful operation shall be carried out to avoid defects. During the joint, the slag at the arc pit shall be removed first, and the arc shall be ignited at the top of the arc pit. After igniting the arc, pause slightly and swing gently until the arc pit is filled, and then weld at the normal welding speed.

Welding speed

In the welding process, the correct welding speed shall be maintained in order to obtain good quality weld beads. The specific welding speed shall be determined according to the pipe diameter, wall thickness and other conditions. If the welding speed is low, the molten pool will flow downward, resulting in pores and slag inclusion; Excessive welding speed will cause insufficient weld bead filling.

Welding of filler bead

When welding and filling the weld bead, the general method of strip transportation is straight pulling. When the pipe wall thickness is large, row welding can be used to fill the wider groove. The direct strip transportation method is not only easy to operate, but also can obtain good welding quality and mechanical properties. In addition, in order to make the edge of the weld bead fuse well, it can also swing slightly during strip transportation.

Welding of cover weld bead

During cover welding, the welded joint shall be staggered from the welded joint during filling welding. Before cover welding, the height between the weld bead of filler welding and the pipe surface shall be less than 1.5mm.

Technical measures

The welding shall follow the principle of thin layer and multiple passes. Impurities such as slag and spatter shall be carefully removed between layers, and the next layer shall be welded after it is confirmed that there are no defects. It shall be preheated to about 100 ℃ before welding.
The arc starting or arc ending points between each layer of weld beads shall be staggered by 20 ~ 30mm. The arc starting and arc ending points of root welding shall ensure penetration. After root welding, the excessively convex deposited metal in the weld bead shall be polished off to avoid slag inclusion, and the welding rod shall be replaced quickly.
After the root pass is welded, the hot pass welding shall be carried out as soon as possible, and the welding interval between the root pass and the hot pass shall not exceed 5 minutes.
In case of abnormal phenomena such as easy adhesion during welding, stop welding, find out the causes and solve them in time.
Technical measures to prevent undercut: during welding, the arc length shall be properly controlled, and the welding speed shall be controlled by flat and oval molten pool, so as to make the uniform transition between molten metal and base metal, so as to achieve beautiful appearance.
A welding protective shed shall be erected on the welding site. Since the construction of the project may encounter rainy season, effective protective measures must be taken before welding.
After the end of the welding day, the pipe orifice shall be blocked with a pipe cover to prevent sundries from entering the pipeline.
After the weld junction is welded, the welder number, weld junction number and pile number shall be written with paint or marker pen on the downstream side of the air flow 5cm away from the weld junction.

Weld inspection and repair measures

Appearance inspection

After the weld is welded, the welding slag and spatter on the weld surface shall be removed in time, and the surface quality shall be carefully checked according to the corresponding specifications. The surface shall be free of cracks, incomplete fusion, pores, slag inclusion and other defects.

Nondestructive testing

NDT personnel shall be undertaken by personnel with corresponding qualification for NDT of special equipment.
(1) X-ray flaw detection, ultrasonic flaw detection
The acceptance standard of X-ray flaw detection shall be in accordance with the current nondestructive testing of pressure equipment (JB / T4730-2005), and grade II is qualified;
The acceptance standard of ultrasonic flaw detection shall be in accordance with the current nondestructive testing of pressure equipment (JB / T4730-2005), and grade I is qualified.
(2) Flaw detection proportion
All welds shall be subject to 100% X-ray flaw detection and ultrasonic flaw detection.

Weld repair

(1) Inform the welder to repair the welds that fail to pass the appearance inspection in time;
(2) The welds that fail to pass the NDT inspection shall be repaired in time, and must be inspected again after repair. Repair shall not be more than twice, such as cutting and re welding more than twice;
(3) Repair process procedure
The technician shall formulate the repair process according to the NDT report;
The NDT personnel shall cooperate to determine the defect location;
The quality inspection personnel shall appoint welders to repair them and supervise the repair process;
Remove welding defects with a grinder and weld with the established repair process;
After repair, clean the surface, mark the welder’s mark, and conduct NDT after passing the appearance inspection.
The anode connecting cable and anode steel core are connected by brazing or tin welding, and the length of bilateral weld is ≥ 50mm.
After the cable is welded with the anode steel core, necessary protective measures shall be taken to prevent the joint from being damaged.
The anode cable and pipeline are connected by welding copper nose on the reinforcing plate (the material is the same as that of the pipe). The anti-corrosion coating on the surface of the pipeline welded with the reinforcing plate shall be stripped off.
The reinforcing plate and the pipe shall be fillet welded all around, and the weld length shall be ≥ 100mm. The cable and the pipe reinforcing plate are connected by copper nose tin welding or copper welding. After welding, the joint must be subject to anti-corrosion and insulation treatment again, and its material and grade shall be consistent with the original anti-corrosion coating.


Through ultrasonic flaw detection and X-ray photography, the welds of pipe section meet the quantity and level required by the design;
Before the pullback of the pullback pipeline, the traction head shall be welded at the front end of the pullback pipeline. The welding shall comply with the provisions of relevant standards and must be fully welded. After the welding, the X-ray photo flaw detection shall be carried out. The pullback of the pipeline can be carried out only after it is qualified.

Inlay construction

The so-called inlay construction is the connection of pipe joints at both ends such as directional crossing and pipe jacking crossing.

Preparation for splicing

The pipe laying and equipment installation of the project have been completed, including the directional crossing project;
The welds of the whole section of pipeline meet the quality and level required by the design through NDT;
The strength and tightness tests and clearing of sections such as directional crossing and pipe jacking have been completed.
Personnel, equipment and materials are fully equipped.

Splicing sequence

Determine the length of short pipe and conduct welding operation according to the site construction conditions;
NDT shall be carried out for the embedded welds, and external anti-corrosion shall be carried out for the welded junction after it is qualified.

Technical measures for safety and quality

The inlay quality management will strictly implement the design drawings, main specifications and standards; When implementing management, it shall be based on the company’s quality management system documents.
The acceptance of welds in the process of inlaying is the same as that in welding and inspection construction.
The construction personnel participating in the splicing shall have certain construction experience in the construction of gas pipeline, and the special types of work (crane, electrician, welder, etc.) shall have work certificates.
All kinds of mechanical equipment for splicing shall be maintained before use to ensure normal operation during splicing.
Before construction, the construction personnel shall be carefully familiar with the drawings and technical requirements, and the construction shall be carried out in strict accordance with the drawings.
The connecting pipe shall be provided with the manufacturer’s warranty and relevant quality inspection data, and its inspection indicators shall comply with the current national or ministerial technical standards.
During the inlay construction, relevant personnel shall focus on the inspection and review of the inlay quality, and make the quality supervision record of the pipe inlay joint.
The inlaid work area shall be enclosed, and irrelevant personnel and vehicles shall not enter the work area.

Construction quality control of winter, rainy season and windproof measures

Technical measures for winter construction

Technical measures for welding construction

Effective measures shall be taken for welding construction in winter to ensure welding quality. It mainly includes measures such as increasing preheating temperature, post welding insulation and reasonable arrangement of welding construction time.
Increase preheating temperature before welding
In order to reduce the heat loss due to low temperature environment, the preheating range of pipe orifice is increased from 75mm on both sides of the groove required by the specification to 120mm on both sides of the groove, and the preheating temperature is 110 ~ 120 ℃. Digital display infrared thermometer is used for temperature measurement, and it is measured 100 ~ 120mm away from the pipe diameter. It is necessary to measure 8 points on the uniform circumference. The root pass shall be welded immediately after preheating.
Measures to ensure interlayer temperature
Specification requirements: cellulose electrode interlayer temperature ≥ 80 ℃; Interlayer temperature of other weld passes ≥ 60 ℃; In order to ensure the interpass temperature, the welding of each layer must be compact. After the previous process is completed, the next process shall be carried out immediately. Before welding each layer of weld bead, 8 points on the uniform circumference shall be measured. When the interlayer temperature is lower than the specification requirements, the flame heater shall be used for re preheating, and ensure that the interlayer temperature of cellulose electrode is ≥ 80 ℃ and that of other weld beads is ≥ 60 ℃. Increase the collection of preheating and interpass temperature. The preheating of each weld junction and the interpass temperature of each weld pass before welding must be measured and recorded.
Post weld insulation
When the ambient temperature is lower than 5 ℃, measures shall be taken to cover the weld bead with thermal insulation quilt after welding to prevent sudden cooling of the weld bead. The thermal insulation quilt is 3.2m × 1m × 50mm asbestos quilt. The specific method is as follows: bake the asbestos quilt with a blowtorch to above 80 ℃, then immediately wrap the completed welded junction while hot, cover it with felt, and bind it tightly with rubber band. The insulation time is 30 ~ 40min, and the insulation diagram is shown in Figure 6.
Fig. 6 schematic diagram of thermal insulation
Reasonably arrange the welding time
Stop welding when the ambient temperature is lower than – 5 ℃. The welding time shall be arranged when the temperature is higher than – 5 ℃ every day, and the measures required in Articles 1, 2 and 3 shall be taken.

Technical measures for anti-corrosion construction

Firstly, the construction shall be carried out in strict accordance with the code for construction and acceptance of anti-corrosion joint coating. On this basis, the following winter prevention measures shall be taken.
Since the temperature is low in the morning and evening after winter, it is difficult to ensure the anti-corrosion quality, the construction in the morning and evening shall be avoided, and the construction shall be arranged after the temperature rises as much as possible.
Increase preheating temperature
In order to counteract the heat loss effect under low temperature environment, the upper limit of preheating temperature shall be reached when preheating the butt weld. The temperature control shall be measured by infrared thermometer, and 8 points shall be measured along the circumference.
Shorten preparation time before coating
In order to reduce the temperature drop caused by heat loss, the preparation time from preheating to covering the shrink band should be minimized. Prepare the primer and shrink band in advance. After the weld joint is preheated, immediately carry out coating preparations such as painting primer, baking PE layer and roughening, so as to shorten the preparation time before coating.

Technical measures for construction in rainy season

Welding in rainy days shall be avoided as far as possible. If required by the project, the welding part must be provided with a rainproof shed, and drying measures shall be taken in the rainproof shed to make the welding environment meet the requirements. Welding is not allowed in rainy days without rain proof measures. Welding is not allowed when the humidity exceeds the standard.
The management of welding materials shall be strengthened in rainy season. The drying materials must be dried according to the requirements of welding process. The dried welding rods shall be placed in the insulation cylinder on site to prevent moisture and can be taken at any time.
The anti-corrosion joint coating operation in rainy days shall be avoided as far as possible. If necessary, rain proof measures must be taken, and the surface of the steel pipe shall be dry. After rust removal, baking shall be carried out, and then rust removal shall be carried out again to ensure the quality of anti-corrosion joint coating.
Welding windproof measures: the welding shall be carried out in the way of windproof shed. When the wind speed is greater than 8m / s, the welding operation shall be stopped.

Welding quality control

  • Welding shall be carried out in strict accordance with the requirements of welding procedure qualification and process specification;
  • Welders work with certificates;
  • The electric welding machine used shall be in good condition;
  • Environmental requirements: humidity < 90% RH, wind speed < 5m / s, temperature higher than – 5 ℃. If the above conditions cannot be met, protective measures shall be taken or welding operation shall be stopped;
  • The welding rod (welding wire) must be provided with quality certificate. The storage and use of welding rod and welding wire shall comply with relevant regulations and shall be taken at any time. In principle, the welding rod shall be used on the same day when the tank is opened, and the welding rod shall be placed in the insulation cylinder after necessary baking on the next day;
  • During commencement, the daily welding quantity shall be appropriately reduced, and the daily welding quantity can be increased only after the welding process is fully mastered;
  • When the air humidity is high, all welding work is carried out in the protective shed. The protective shed is equipped with a base to isolate the ground moisture. The shed is also equipped with a dehumidifier to reduce the relative humidity in the air. The welding operation can be started after the welding requirements are met;
  • The welding rod shall be inspected before use, and it shall not be used if the coating cracks and falls off are found;
  • After work every day, gather the wire feeder equipped with welding wire and place it in the room with dehumidifier to prevent the welding wire from being damp;
  • For the unfinished welded junction, wrap and fix it with non sticky polyethylene tape before the end of each day to isolate it from the humid air outside. Before welding every day, start the dehumidifier for dehumidification, remove the polyethylene tape, and heat it to the temperature required by the specification with a heater before welding.


The project adopts strict management and careful construction, and is based on the commitment of laying a pipeline, ensuring investment, creating a high-quality project and ensuring a long-term plan. The one-time acceptance rate of project quality is 100%; The first pass rate of on-site welding is more than 98%; Meet the requirements of the owner and drawings, the project quality meets the requirements of acceptance specifications, and the project is qualified in one-time acceptance.

Source: China Natural Gas Pipeline Provider:



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