Article by Patricia Dauxerre, Polysoude
___
The field of offshore applications can be particularly challenging for welders. The flow lines and export lines used to transport crude, processed oil or gas fluids are subject to tremendously adverse forces, both external and internal. During the laying process, water pressure, strong currents and extremes of temperature must be contended with, whilst internally, piping can suffer from chemical attacks caused by aggressive production fluids.
Predictable precision
Pipes can be welded manually or by means of mechanized or automated welding. If no equipment for automated welding is available, the root pass and the hot pass can be produced by manual TIG welding with a filler material in the form of rods. However, the welding of the root pass is extremely delicate, as deviations from the acceptable heat input can crucially alter the corrosion resistance of the pipe material.
Hence, there are definite disadvantages to manual welding: the dependence on the skills and performance of the welders, a lack of sustained reproducibility, limited quality control and low productivity. However mechanized or automated TIG welding equipment can produce any desired quantity of welds, with each individual joint exceeding the requirements of the strictest production objectives – the ‘Zero Risk/Zero Defects’ approach. No longer dependent on the skills of the staff entrusted with the operation of equipment, results are excellent, sustainable and quality is predetermined.
Mechanized or automated TIG welding ensures that the procedure and all related parameters are optimized and approved separately in advance. Results are finally documented as welding instructions and the related programs are implemented into the machines in the workshop or on site.
Shah Deniz 2 project
Polysoude’s TIG technology was implemented in the Shah Deniz 2 project in Azerbaijan. A contractor needed to weld a number of 16” CRA line pipes, flanges and bends. The carrier pipes with a wall thickness of 41.9 mm were made of API 5L X65 with a 3.0 mm internal ERNICrMo-3 clad. In the workshop, the 12 m length pipes had to be joined into 24 m sections. The company decided to use manual TIG welding of the root and hot pass in the 5G Up position and Submerged Arc Welding (SMAW) for the filler passes.
However, during approval they discovered that the filler welds did not comply with the technical requirements. Problems were caused at the start and stop zone of a welding pass, as well as lack of fusion and related repair work. Furthermore, the important heat input of the SMAW prevented the welds from reaching certain weld metal properties, such as toughness and yield strength. Polysoude facilitated the change to automated hot wire TIG welding of the filler passes. A Hot Wire GTAW station (pipe rotating 1G) was ordered. As the pipe ends were already machined for manual welding with a 30° V-preparation, root and hot passes continued to be produced by manual TIG welding.
In the process of automated TIG welding, the filler passes are laid with the pipes in the 1G position. Adjustable supports allow fast and exact positioning and aligning of the CRA pipes, rotation of the pipes is ensured by a head stock, and the welding set is fixed in an optimized position at the end of a boom. At the beginning of a weld cycle, before the ignition of the arc, the torch can be moved smoothly towards the workpiece. When the electrode touches the base of the groove preparation, it is retracted until the programmed distance to the workpiece is reached.
The related device is called Arc Voltage Control (AVC). Once the arc is struck, it is used to keep the arc length constant, so that multipasswelding can be carried out without the need for further adjustments between passes. Another useful and innovative feature of the installation is called Torch Oscillation Control (OSC), which allows the torch to move transversally to the direction of welding. The desired width of a welding pass is achieved by programmed periodical movements of the torch to both sides of the groove. The filler wire comes from a spool ingeniously fitted inside the motorized wire feeder; this means that wire feeding can be started or stopped at any moment and, if necessary, the wire end can be retracted. Wire feeding speeds and pulsed wire feeding are programmed and managed by the power source.
Before a welding sequence of mechanized or automated TIG welding can begin, the operator must ensure that the workpieces are correctly positioned. However, after the welding cycle has started, the equipment is completely controlled and monitored by the uniquely designed power source. Unlike GMAW processes, TIG welds do not require any machining or grinding operations either at their start or end, or between the passes.
The melting rates of cold wire TIG welding are quite moderate when compared with competing processes. The filler wire entering the weld pool is cold, and the energy to melt it is delivered entirely by the electric arc. As a result, the melting rate is slower, which consequently affects the weld speed. Hot wire TIG welding, on the other hand, substantially increases both the melting rate and welding speed.
In the Shah Deniz 2 project, by using automated hot wire TIG welding equipment, the time needed for filling and capping of a girth weld of the 16in. line pipe was 7hr 30min. The resulting sound, defect free joint brought about an immense increase in productivity, as time-consuming repair work was no longer necessary and the controlled heat input of the process guaranteed that the required mechanical properties of the welds were achieved, without additional attention.
Further proof of the proficiency of TIG welding technology, has come from the Khazzan project in Oman, where a different approach has been adopted. The company needed to execute approximately 19,000 welds on 12in. and 16in. 22% duplex stainless steel pipes. It was the intention to utilize the advantages of automated TIG welding to its limits. J-preparation for orbital GTAW of root and hot pass of the 12in. pipes and orbital GTAW for the cap pass ensured increased productivity. As an additional measure for the 16in. pipes, a narrow groove preparation was executed. All welds were executed successfully within the scheduled period of time.
Narrow groove preparation of pipe ends is an efficient option, to improve overall productivity of the joining operations of line pipes. The mechanical characteristics of the pipe material and behavior in terms of welding shrinkage are considered in order to determine the slim profile of the weld groove (the angle of the weld groove is kept as small as possible). This preparation of the pipe ends requires the removal of less material, so that machining becomes easier and faster. As less material is removed, less material is required to be replaced by the weld: welding time becomes shorter, and filler material consumption decreases.
An example of a macrographic section of a joint between CRA coated workpieces shows the perfect geometry of the narrow groove TIG weld. Line pipes are usually produced in length of 6 or 12 m and often welded together to 12 or 24 m long sections. As the pipes can be rotated during this procedure, automated welding equipment, as shown, can be used. However, during the laying of a pipeline, either from a barge or as landline, the pipes cannot be rotated. In these cases orbital welding equipment is required.
