PIPELINE WELDING & ASSEMBLY (part 2)
PROCEDURE AND ORGANIZATION OF WELDING AND ASSEMBLY WORK
The number of welds made during construction of the line part of the oil pipeline was as follows:
- by manual arc welding (WPS-01 and -02 procedures) 4,730 joints of 30" pipes with wall thicknesses of 11.08 and 13.03 mm at an average daily rate of 0.9 to 1.2 km
- by CRC AW automatic welding (WPS-13 procedure) 4,194 joints of 30" pipes with a wall thickness of 11.08 mm at an average daily rate of 1.5 to 1.9 km
During seam overhead welding according to procedure WPS-01, the pipe joint was assembled on an internal self-propelled pneumatic lineup clamp with a clearance of 1.6-2 mm. Wedges were used to maintain the clearance during the joint assembly and welding process. The root layer of the joint was applied by two welders in approximately 9 minutes. Welding was done on straight polarity, which increased penetration of the pipe edges and welding speed. Electrode travel in the groove was downhill, resting the sleeve of the coating on the joint edges and forming a keyhole under the arc in the molten pool.
In the vertical part of the joint, when the molten metal and slag begin to flow under the arc, the welding current and travel speed were increased. In the overhead part of the joint, the welding current was reduced in order to reduce the weight of the molten pool and improve the formation of a reverse bead.
During the welding process, the welder also kept an eye out for any shift in the edges or change in the joint opening. If the edges shifted, the arc was directed at the farther edge and the electrode tilted toward the joint plane. At the same time, the welder watched out for any burnoff of the closer edge in order to prevent a lack of root penetration. The start-to-finish segments after each electrode were notched with a grinder and abrasive disk. The Indian welders, who were the most experienced, mostly did no notching, and after replacing the electrode continued welding. After the welding, mechanics flushed the joint perimeter, opening up the slag pockets.
Applying the second layer - the hot pass - is the most complex operation in welding with cellulosic electrodes. When applying the hot pass, Russian welders frequently use the wrong method - holding the electrode down without manipulating the end. Doing this requires that the root layer be thoroughly ground, which makes it thinner and thus results in the likelihood of burn-through of the root bead and an increase in the specified interval between the root pass and the hot pass of more than 5 minutes. This leads to reduced diffusion of atomic hydrogen out of the seam and the danger of cracks appearing. The correct way to do the hot pass is by a whipping motion of the electrode tip so that slag is swept out of the pockets. In addition, the welding current source should be set for a steeper volt-ampere curve while keeping the welding current at maximum (in accordance with WPS-01).
The fill layers of the weld were deposited using 4.0mm diameter Fox BVD 85 electrodes for vertical down welding. During the welding process, the arc length should be as short as possible due to the increased tendency to form pores, including start and stop pores. The electrode tip is weaved from side to side in a zig-zag motion without increasing the arc length where the direction changes at the edge of the joint. The missing fill in the groove on the vertical sections of the joint (10-8 and 2-4 hrs of the perimeter) is completed with a final (correcting) fill layer so that it is flush with the pipe edges. On the remaining sections of the joint perimeter just before the face layer is applied, the groove should be unfilled for approximately 0.5-1.0 mm to the pipe edges.
The face layer of the seam is deposited with the welding current set at 20-30 amp less than that used for the fill passes. The width of lateral oscillation of the electrode should not be more than twice its diameter. The width of the layer should be 3-4 mm greater than the width of the groove after the fill layers have been deposited. To avoid undercuts along the edges in the overhead position, the lateral oscillations of the electrode tip should preferably be U-shaped instead of zig-zag. With this welding technique the arc length must be kept as short as possible to avoid pores forming in the overhead position due to insufficient arc protection. For joining pipes with wall thicknesses greater than 15 mm, it is advisable to apply the face layer in two parallel beads.
The actual rate of seam overhead welding using the WPS-01 procedure was 15-20 minutes per joint. The overhead team consisted of 10 welders working simultaneously in 5 welding tents:
Tent 1 - root pass, average welding time 9 min
Tent 2 - hot pass, average welding time 4 min
Tent 3 - fill pass, average welding time 12 min
Tent 4 - fill pass, average welding time 11 min
Tent 5 - face pass, average welding time 13 min
The supply sources used were 2-station and 4-station Arcotrac and Liebherr welding tractors fitted with Lincoln DC-400 welding rectifiers.
For the CRC AW automatic welding (WPS-13 procedure) there were 11 welding operators: one for the internal welding heads and 10 for the external welding heads. They worked in five tents:
Tent 1 - hot pass
Tent 2 - 1st fill pass
Tent 3 - 2nd fill pass
Tent 4 - face pass
Tent 5 - face pass
In line with the procedure requirements, before being welded the pipe ends were cut with hydraulically-driven circular cross-saws to obtain a special, narrow-gap two-sided bevel. This operation was done by a team using two circular cross-saws with hydraulic stations on the pipelayers. The guide belts for moving the welding heads along the pipe joints were mounted by the following team.
The head team of the welding column assembled the pipe joints (with no clearance) on an internal pneumatic self-propelled lineup clamp combined with a welding machine that welded the root layer of the joint from inside the pipe using six welding heads. The remaining teams welded the outside layers inside the tents listed above using external welding heads and solid-section 0.9 mm diameter wire. The root and face layers were applied in a shield gas mixture of argon + carbon dioxide (75% + 25%), and the hot pass and fill layers in carbon dioxide (100%). All the layers were welded downhill. The system comprised one 4-station (Liebherr) and five 2-station (Arcotrac) welding tractors with hydraulic boom manipulators from which the welding tents were suspended.
Welding of lap joints, taper joints and line valve assemblies was done according to procedures WPS-02 and WPS-03 using external lineup clamps. The joints were assembled using rigid external lineup clamps manufactured by CRC Evans and break-over lineup clamps produced by Russian manufacturers. Experience showed that the CRC Evans clamps were better at eliminating the height difference of the pipe edges in the joint since they use a hydraulic jack. The drawbacks of these clamps are their considerable weight, the difficulty of depositing the root layer around 50% of the perimeter before the clamp is removed, and their high cost compared with Russian-made external lineup clamps.
The pipe joints were assembled with a clearance of 2.5-3 mm. For applying the root layer, 3.2 mm diameter electrodes were used. The welding direction was from bottom to top - uphill. Polarity was straight for welding with cellulosic electrodes and reverse when using basic-coating electrodes. Two welders working at the same time apply about 50% of the root layer perimeter on the external lineup clamp, after which the clamp is removed and welding of the remaining 50% is completed, making sure to notch the start-finish sections.
One of the important factors in welding these kinds of joints is to maintain the preheat temperature. The method traditionally used in pipeline construction whereby the joint is preheated until the external lineup clamp is fitted leads to a drop in the preheat temperature at the start of welding the root layer (to below 80-100 °C) and consequently to the likelihood of cracks appearing. To eliminate this drawback, the diameter of the collar burners was increased so that preheating could be done after assembly with the external lineup clamp in place on the pipe joint. The fill and face layers were welded downhill with basic-coating electrodes according to the WPS-01 procedure or uphill according to the traditional WPS-23 procedure.
Lap joints were removed by teams consisting of two pipelayer operators, a welding tractor operator, two welders, a gas cutter, foreman, and a rigger. Lightweight fan-type tents developed by Russian specialists from their experience of earlier projects were used to protect weld areas.
Installation of surge relief stations on the existing SHBAB-1 oil pipeline was carried out without halting the oil flow by means of hot taps - welding split tees to the oil pipeline and then tying in safety valve stations using TD Williamson equipment.
Welding of split tees for hot taps was done using the WPS -10, -11 and -12 procedures. The members of the split tee were mounted onto the operating pipeline at the tie-in point and held in place on the pipe by two break-over external lineup clamps. First, two horizontal seams were welded to join the two members of the tee into a single structure. Welding was done with beads using the step-back method. The clamps were allowed to be removed after 25% of the cross-section of the horizontal seams had been welded. After the horizontal seams were welded, circular fillet welds were made to join the tee to the pipeline. Welding was done from the bottom up with separate beads using the step-back method by two welders at the same time. The anchor flanges were welded in the same way.
Defects were removed using the WPS-03 and -28 procedures by repair teams consisting of an experienced welder and a welding tractor operator.
According to client specifications, a repeat repair was permitted one time.
Repairs were made both outside and inside the pipeline. The defects were marked out by the repair team using a measuring band (similar to the bands used by defect detector operators). The defects were ground with abrasive disks. For repairing a root layer from the outside, abrasive disks 2.2 mm thick were used for a section that was to be cut through, and for all other cases the thickness was 4 and 6 mm. Grinding of the defects was generally done by the welding tractor operator, but if a through cut was required, it was done by the repair welder with a hacksaw to obtain an even opening of 2.5 - 3 mm.
To reduce the likelihood of cracks appearing during repair of the root layer of lap joints, the following sequence of process operations was used in the project:
- preheat the joint to be repaired using a propane collar burner to 120-150 °C
- remove the defective section
- reheat the joint again, immediately before welding, to 120-150 °C
- weld the defective section while strictly maintaining the interpass temperature
- when welding is done, put a thermal wrap around the joint to reduce the cooling rate
When the root layer of lap joints was repaired in the above sequence, there were ultimately no cracks.
Of great practical interest here was the welding process monitoring procedure that was developed in line with the quality management system and used in the project.
At the preparatory stage it is verified that:
- the relevant (approved) welding procedure is available
- the pipes meet project requirements and the specifications, and that they are free of unacceptable defects
- welders and welding operators have the appropriate (unexpired) certification
- welders are properly equipped (coveralls, boots, leggings, mask, electrode holder, electrode case)
- the equipment and tools are available and in working condition (correct lineup clamp, clearance wedges, electric grinders, return lead clamp, welding protection tents, welding cable, welding current remote control, grounding at end of welded pipeline string, pipe rollers)
- the beveling of the pipe ends matches, including geometry
- the joint is correctly assembled, including fulfillment of requirements regarding the offset of factory-welded seams and their location
- the equipment for preheating the pipe ends is in good working order
- welding materials are prepared (baked) and that they have certificates
- the welding machines are in good working order
During the welding process the following are monitored:
- size of the root opening and the amount that it changes during the root pass
- use of no less than two welders simultaneously for welding pipes more than 12" in diameter
- removal (release) of the lineup clamp
- interval between completion of the root layer and start of the hot pass when using an internal lineup clamp
- the welding current
- quality of cleaning of each pass
- that welding is performed in line with the specified procedure
- absence of arc striking on the body of the pipes
- completion of the prescribed number of passes
- interpass temperature meets requirements
- use of thermal wraps
When the welding is completed it is checked for seam geometry and absence of visible unacceptable defects. The edge misalignment is measured and the weld is inspected to make sure that it has been cleaned of slag and molten metal spatter, and that it has been appropriately marked
The final operations at the end of the work shift are to cap off open sections of the pipeline, complete welding of the face layer on all the welded joints, verify the number of remaining electrodes, and clear away any foreign objects from the production area.
WELDING QUALITY CONTROL AND BASIC DEFECTS IN WELDED JOINTS
Welding quality control was subcontracted to Vetco, a local company that used gamma defect detectors and X-ray machines, including self-propelled pipeline crawlers for internal pipe inspection. The fillet and horizontal welds on the split tees were inspected using powder magnetography and dye penetrant examination. The average percentage of seam overhead joints inspected was 10% of the total number welded, but the overhead joints welded by the CRC AW automatic welding machine were 100% inspected using a Pipewizard automatic ultrasound computer system made in Canada. The inspections were performed by Stroytransgas engineers.
In pipeline construction practice outside Russia, welding quality is generally assessed based on the percentage of unacceptable defects out of the total number of joints:
- up to 5% is excellent quality
- up to 7% is good quality
- up to 9% is satisfactory
- more than 10% is unsatisfactory
At the facility that was built, repairs were made to 155 joints that had been welded on the CRC AW automatic welder, and to 410 manually welded joints. The total percentage of repair was 6.33%, including 8.6% for manual welding and 3.7% for automatic. Applying the international assessment criteria it can be stated that the quality of manual welding operations was good, and automatic welding excellent.
These levels were achieved thanks to constant monitoring by the office of the project's chief welding engineer, analysis of the causes of the weld joint defects, and determination of the methods to remedy them.
The results of this work are summed up in Table 1, which shows the typical defects encountered in manual arc welding on the project, their causes, and their remedies.
Rasil Varisovich LUGUMANOV,
Chief Welding Engineer of the project’s construction
department in the Kingdom of Saudi Arabia
Vladimir Petrovich YATSENKO,
Acting Deputy Chief of the Construction
Technologies Department, Chief Welding Engineer
of Stroytransgaz, PhD