Modularized control of installation and quality of stainless steel pipelines abroad

In order to improve the safety and economy of LNG pipeline system operation, the process of modular control of foreign ICHTHYS module for stainless steel pipeline’s excellent corrosion resistance and low temperature performance characteristics is analyzed and discussed. Combined with the ICHTHYS module in the construction practice, from the stainless steel pipeline stacking and finished product protection, material, grouping, welding, argon, deformation control, deformation adjustment, and other seven aspects of quality control on the elaboration, to improve the stability of the quality of pipeline construction and the overall quality of the project construction has a certain reference value.

ICHTHYS module is an onshore LNG project with a module length of about 360m, a width of 48m, and a total of 25 pipeline systems. The total length of the construction pipeline is 93491m, among which the stainless steel pipeline is 17536m, accounting for 18.76%. This study combines the construction practice of the ICHTHYS module, the study of stainless steel pipeline in the field construction of stacking and finished product protection, material, grouping, welding, argon, deformation control, deformation regulation of 7 aspects of technical management, and quality control.

1. Material stacking and finished product protection

In order to avoid stainless steel pipe due to carbon steel pollution and damage to its corrosion resistance, stainless steel pipe should be stacked and protected according to the requirements of the following three rules:

• ① use a special warehouse for storing, welding, argon filling, deformation control, deformation control;
• ② as shown in Figure 1, Figure 2, the stainless steel pipe spraying treatment, out of the workshop wrapped in tin foil for further protection;
• ③ in the transportation process, the stainless steel pipe is not used in the transportation process, but in the transportation process, the stainless steel pipe is not used.
• ④ in the process of transportation, the prefabricated pipe is stored in the cage to avoid damage caused by repeated inverted transportation.

Figure.1 Spray treatment

Figure.2 Tin foil protection

2. Stainless steel piping materialization

Stainless steel pipe materialization involves pipe cutting and marking transplantation, and the following measures can be taken to control its processing quality:

• ① marking the material after leaving the warehouse or transplanting the furnace lot number by color marking to ensure the correct use of the material.
• ② using portable spectral analysis to analyze the quality of stainless steel pipes and ensure the material is used correctly.
• ③ check the drawings to confirm that the length, poundage, and material of the pipes to be discharged meet the requirements.
• ④ use plasma or cold cutting method to intercept the stainless steel pipe, use special cutting tools to ensure that the port is free of defects or burrs, and mark with a special marker for low-chlorine stainless steel.

3. Pipe alignment

Pipe alignment affects the welding quality and appearance of the weld. To ensure the quality of material alignment, one should do the following:

• ① to ensure adequate size of the spot welding to meet the alignment of the joints, spot welding thickness of 3.2-4.8mm, spot welding length of 12.5-25.4mm.
• ② under normal conditions, when the pipe diameter ≤ 101.6mm, take the number of equidistant spot welds at least 3; pipe diameter > 101.6mm, take the number of equidistant spot welds at least 4. Under special conditions, the designated person determines the appropriate number of spot welds after inspection.
• ③ in the bevel for group spot welding, welding is completed before removing all spot welding.
• ④ adopt the backward method of arc initiation in the bevel during welding, and do not initiate arc on the base material.
• ⑤ according to the Welding Process Procedure (WPS), the bevel gap, bevel angle, and blunt edge adjustment, as shown in Figure 3. The bevel size is too large to improve construction costs and produces excessive stress in the weld, causing deformation, cracks, and other problems; the bevel size is too small and is prone to welding incomplete and slag and other defects.
• ⑥ stainless steel pipe pairing using adjustable bracket, the top of the bracket using stainless steel plate.
• ⑦ the implementation of regional prefabrication to avoid mixing with carbon steel, low alloy steel, or other metal materials, as shown in Figure 4.

Figure.3 Bevel size for different shapes of butt welding

Figure.4 Stainless steel pipe fittings prefabrication of zones

4. Pipe welding

Pipe welding is an important link to determine the project’s quality. Therefore, stainless steel pipe construction for welding control has strict requirements.
(1) Welding engineers to prepare welding process guidebook, technical briefing.
(2) Welders must be licensed, and the project assessment and approval only after qualifying for duty.
(3) Welder on duty should be clear about welding process requirements and check whether the lead welding consumables and process requirements are consistent to avoid misuse.
(4) Before welding, each node is marked with weld identification and inspection information (Figure 5), including weld number, welding process, welder, welding consumables, grouping, appearance, and NDT testing.

Figure.5 Inspection information box
(5) The longitudinal welds of adjacent welded pipes are staggered from each other, and openings and external parts and the longitudinal welds of adjacent welded pipes are staggered at least 45° or 150mm apart to avoid longitudinal welds appearing to clash with pipe butt weld channel at the bottom.
(6) Pipe adjacent to the two butt ring weld distance and the distance between the subsidiary parts of the fillet weld to meet the requirements, see Table 1.
Table.1 Weld distance

Distance from weld toe to weld toe d	Two butt welds	Butt welds and fillet welds
5T or 50 mm	Take the maximum value of d
5T<d<50 mm		ok
2T<d<5T		Butt weld RT, fillet weld PT
<2T	Resection	Resection

(7) Accessories (pipe supports, guards, punching pipe, etc.) must add process holes to accelerate heat dispersion and improve welding quality. Process holes can be used to test the pressure test process to check whether the accessory weld through the base material. After the completion of the test pressure, the available DOW732 silicone sealing process hole.
(8) Welding in bad weather, the welding area should be adequately protected and tested (such as tents, baking handles, and temperature gun/pen) to ensure the welding quality. When the wind speed is large, set up a windproof tent; humidity is large, with the grill to dry preheating; preheating should be throughout the thickness, preheating range of at least 25mm on both sides of the weld. Preheating after welding, welding at any time during the process of temperature measurement gun for temperature detection. Arc extinguished, cannot immediately lift the torch, need to pay attention to the protective effect of lagging gas on the weld.
(9) Equipped with the appropriate auxiliary tools (heat preservation bucket, knock slag hammer, welding inspection ruler, etc.), drying the welding rod in the heat preservation bucket, heat preservation bucket on-site heating to ensure that the temperature inside the bucket is not less than 80 ℃, welding inspection ruler to detect the amount of the maximum allowable amount of misalignment for 3mm.
(10) Welding end inspection, with the help of endoscopy for internal oxidation detection. If the color is unqualified, with a diameter of 600mm or more, use chromic acid series of pickling paste for artificial pickling passivation. Before using the surface of the debris clean and then the pickling paste coated on the surface, the thickness of 1-2mm, according to the thickness of the surface oxidation and the temperature to determine the length of the coating time, to achieve clean can be clean. For a diameter of 600mm or less, the online cycle pickling method is used, whereby hoses connect the installed pipes to form a circuit for cleaning. In addition, the percentage of ferrite content testing and its content requirements should be > 5%.

5. Pipes filled with argon

Stainless steel pipe welding is easily oxidized to reduce the degree of oxidation; the requirements of welding in the back of the punch argon, argon content is different, and the oxidation of different colors, as shown in Figure 6.

Figure.6 Oxidation color comparison chart
ICHTHYS module on-site welding evaluation requirements oxygen content <0.05%. The following three cases meet the argon-filling conditions.

• ① butt joint of the root weld should be used with backing gas TIG welding process while backing gas should be retained at least until the completion of the second layer of welding and monitoring; ① in the pipe sheathing welding process, the gas should be used in the pipe sheathing welding process, the gas should be used in the pipe sheathing welding process.
• ② in the pipe guard welding process, when the thickness of the pipe wall < 6.35mm requires the use of backing gas for protection;
• ③ 25.4mm (1in) pipe below the back of the need to maintain a continuous argon filling.
• Different diameters of the argon filling method are different.

(1) Small diameter using the whole pipe flushing argon, pipe length is longer when using segmented group welding, a small number of intermediate joints can be used to weld soluble paper on both sides of the blocking, generally from the welding mouth on both sides of the 200-300mm. Whole pipe argon filling method of operation: one end of the pipe with water-soluble paper or sponge blocking, argon from the other end of the pipe, with an oxygen analyzer to monitor the readings to meet the weld requirements, can be initiated.
(2) Large diameter can be used for local argon filling, argon before the punch; the first will be placed on the plate from the weld 100-150mm position, from one end of the tube inflatable, and the placement of the other end of the center of the plate hole out of gas. To reduce the tube argon from the gap in the loss of counterpart, before welding can be passed along the gap in the weld on the water-soluble paper, welding is welded while the paper is removed. To supplement the argon leakage, the whole welding process should be constantly to the pipe filled with the appropriate argon flow. Special attention should be paid to the inflatable air chamber should be filled with air before proceeding.
(3) Large diameter pipe can also be used to seal the inflatable capsule. Using inflatable capsules can solve the elbow at the weld sealing difficulties. The use of high-quality rubber made of pipe blocking airbag, through the inflatable method so that its expansion and pressure up to the prescribed requirements, the airbag will fill the entire pipeline section, at this time, in the pipeline blocking airbag wall and pipeline friction to achieve the purpose of blocking, to achieve the goal of the pipe section of the rapid blocking and closure. Specific operations are:

• ① determine the matching inflatable capsule according to the pipe diameter size;
• ② the inflatable capsule placed in the stainless steel pipe, drag the inflatable capsule so that the light-emitting central band is located in the weld to ensure that the two ends of the inflatable dam are in the weld on both sides of the equidistant, as shown in Figure 7;
• ③ argon gas through the main into the gas hose into the inflatable dam, inflatable dam gradually inflatable expansion until the stainless steel pipe and close contact;
• ④ inflatable dam and stainless steel pipe close contact, continue to inflate until the preset pressure of the inflatable capsule is more than the preset pressure, then through the internal pressure reducing valve to start exhausting, argon from the inflatable dam into the stainless steel pipe, the use of the principle of replacement of the stainless steel pipe in the air exhaust until the entire stainless steel pipe is full of argon, and the replacement of the complete can be used to detect the oxygen content detector to detect when the oxygen content of the welding requirements can be welded; only after welding requirements before welding;
• ⑤ inflatable in two stages, inflatable speed depends on the flow control valve to control to effectively ensure that the argon gas input is in line with the preset provisions. First, high-pressure gas input requires high flow rate gas to achieve the local space of oxygen replacement; Secondly, when the local space on both sides of the weld is to achieve the replacement, switch to low-pressure gas input. Continuing to replace oxygen at a low flow rate ensures the quality of the weld and does not exert excessive pressure on the weld molding. Furthermore, if additional gas flow adjustment is required, it can be realized by operating the secondary gas hose;
• ⑥ the inflatable capsule system should not be deflated immediately after the stainless steel pipe is welded but only after the weld has cooled down.

Figure.7 Inflatable dam placement location schematic diagram

6. Welding deformation of the pipe

Stainless steel pipe welding deformation causes and carbon steel welding deformation causes similar, compared with carbon steel, stainless steel welding deformation is greater. Stainless steel welding deformation of the treatment method and carbon steel welding deformation of the same treatment method.

• (1) The use of small-diameter welding consumables and smaller current, control of reasonable welding line energy, and the specific welding parameters are shown in Table 2.
• (2) The left-right-middle welding sequence is used for welding, and one line should not be welded to the end to prevent warping. The large diameter pipe can be welded symmetrically in the same direction by two people simultaneously. When the wall thickness of the base metal pipe is more than 8 mm, the weld bead should be welded in multiple layers, and the maximum temperature between layers should be less than 150 °C.
• (3) Welding fixture will be fixed before welding to avoid warping.
• (4) Reduce the weld gap and the number of grooving.
• (5) Maintain proper weld size.
• (6) Implement anti-deformation measures after welding to prevent deformation.

Table.2 Welding parameters

Welding Layer	Welding method	Filler metal diameter/mm	Current/A	Voltage/V	Welding speed/(mm/min)	Gas flow rate/(L/min)	Maximum heat input/(kJ/mm)
Root	GTAW	1.6	DCSP 50–80	45181	50–80	15–25	1.3
Root	GTAW	2	DCSP 60–90	45212	50–80	15–25	1.3
Root	GTAW	2.4	DCSP 80–120	45212	50–80	15–25	1.6
Filling	GTAW	1.6	DCSP 60–120	45212	80–120	15–25	1.2
Filling	GTAW	2	DCSP 90–130	45212	70–120	15–25	1.2
Filling	GTAW	2.4	DCSP 100–200	45215	70–140	15–25	1.6
Filling	GTAW	3.2	DCSP 160–220	14–16	90–150	15–25	1.6
Filling	FCAW-G	1.2	DCRP 130–180	22–24	130–165	15–25	1.3
Filling	SAW	2.4	DCRP 300–400	27–32	350–500		1.5
Filling	SAW	3.2	DCRP 350–420	28–33	400–500		1.6

7. Pipe deformation adjustment

Pipes can be used to correct the straightness of the pipe by using the weld shrinkage generated by partial or complete rewelding of the circumferential weld.

• (1) For pipe diameters >50.8 mm (2 in), groove the weld by grinding. The grooving area should be 2/3 of the circumferential weld, and the grooving depth should be 1/3-1/2 of the pipe wall.
• (2) The same joint correction is allowed to use only two times. The joint must be cut off and rewelded if the two corrections still need to be qualified.
• (3) If the pipe straightness deviation is large, the joint must be removed and re-welded.

8. Conclusion

With the widespread use of stainless steel pipe, its quality control measures need to be strictly scientific and targeted, strengthen the site quality control, and the overall pipeline project construction is critical. Stainless steel pipe construction process, material stacking requirements partition and neat; finished product protection needs to be effectively combined with physical protection and chemical protection; under the material link requirements to drawings as a guide to sequence control; group requirements for regional prefabrication, and prefabrication process of spot welding, beveling process parameters for standard control; welding process requirements for specialization, welders, equipment, methods, welding materials, construction environment for strict requirements; argon filled pipe is designed to protect, and the quality control measures need to be strict with scientific and targeted, strengthen the site quality control, is crucial to the overall pipeline project construction. A pipeline full of argon is designed to protect, according to the pipe diameter size method; welding deformation and deformation adjustment belong to the quality control of the back-end assessment and correction. In summary, stainless steel pipe construction quality controls all aspects of the steps and requirements to effectively ensure the quality of construction to meet the overall project requirements.
Author: Ma Zhijie