![\"Fig.1:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-1_TIG-welding-300x278.jpg\")
\nFig.1: TIG Welding<\/p>\n
\nFig.1: TIG Welding<\/p>\n
From the very beginning, gas tungsten arc welding (GTAW) or, as it is commonly named, tungsten inert gas welding (TIG) has been considered as a particularly complicated joining process. The necessary equipment was expensive and subject to significant investments, shielding gas of the required purity was hard to get, and, above all, only especially trained individuals with specific skills were able to complete the complicated procedures.<\/p>\n
Nowadays, after several decades of development and progress, customer-focused solutions for mechanised or automated TIG welding can be obtained on the market. To respond to the ambitious demands of the oil and gas industry, contractors are driven to provide excellent quality at competitive prices within the shortest deadlines, i.e. they are obliged to make continuous efforts to increase their productivity.<\/p>\n
In the field of offshore applications, flowlines and export lines are used for the transport of cruel or processed oil or gas fluids. Due to strong external forces exerted on the pipes during the laying process — high pressure and\/or temperature and chemical attacks by aggressive production fluids from the inside and water pressure and current forces from the outside — the pipelines must withstand severe mechanical stress. They must also absorb high dynamic loads and they must provide corrosion resistance.<\/p>\n
The joining of the pipes can be carried out manually or by means of mechanised or automated welding. As the welding of the root pass is extremely delicate, deviations from the allowable heat input can crucially alter the corrosion resistance of the pipe material. Anyhow, if no equipment for mechanised welding is available, the root pass and the hot pass are still produced by manual TIG welding with a filler material addition by means of rods. It must not be understated that the disadvantages of the process are still the same: low productivity, extreme dependence on the skills and performance of the welders, lack of reproducibility, limited quality control, etc. With the help of mechanised or automated TIG welding equipment, any desired quantity of welds can be produced; with each individual joint exceeding the requirements of the strictest production objectives, also known as \u2018Zero Risk\/Zero Defects\u2019 approach. The obtainable results do not depend any longer on the manual welding skills of the staff entrusted with the operation of the equipment.<\/p>\n
Mechanised or automated TIG welding means that the procedure and the whole bundle of related parameters are optimized and approved separately in advance. The results are finally documented as welding instructions, and the related programs are implemented into the machines in the workshop or on site. Manual welding skills are no longer required to run automated TIG welding equipment, as proficient operators replace the manual welder.<\/p>\n
For the Shah Deniz 2 Project, a contractor needed to weld an important number of 16in. 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 pipes (with a length of 12 m each) had to be joined together to 24 m sections. Due to available equipment the company decided \u2013as it is still usually the case for onshore girth welds\u2013 to use a 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 at all with the technical requirements. Problems were caused by 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 the attended weld metal properties, such as toughness and yield strength.<\/p>\n
Nevertheless, to meet the project milestones the responsible manager felt compelled to change to mechanised hot wire TIG welding of the filler passes, and ordered a Hot Wire GTAW station (pipe rotating 1G). As the pipe ends were already machined for manual welding with a 30\u00b0 V-preparation, root and hot pass were further produced by manual TIG welding.<\/p>\n
By means of the mechanised TIG welding equipment, 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 optimized position at the end of a boom.<\/p>\n
At the beginning of a weld cycle, before the ignition of the arc, the torch is moved smoothly towards the workpiece. When the electrode touches the ground of the groove, it is retracted until the programmed distance to the workpiece is reached.<\/p>\n
The related device is called Arc Voltage Control (AVC). Once the arc is struck, it is used to keep the arc length constant, so multipass-welding can be carried out without need for further adjustments between the passes.<\/p>\n
![\"Fig.2](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-2_Welding_configuration_Fill-in-300x218.jpg\")
\nFig.2: Welding set on a boom end<\/p>\n
Another useful 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.<\/p>\n
![\"Oscillated](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-3_Oscillated-passes-appearance-225x300.jpg\")
\nFig.3: Oscillated passes appearance
\n![\"Fig.4:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-4_WPS_2-1024x480.jpg\")
\nFig.4: WPS<\/p>\n
The filler wire comes from a spool inside the motorised wire feeder; 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.<\/p>\n
Before mechanised or automated TIG welding takes place, the particular procedure has to be developed and approved. All functions and movements of the installation are initiated and controlled by the power source. Programming can be carried out by touchpad with an easy-to-understand, intuitive graphical user interface (GUI). The virtual synoptic of the GUI is presented on a tactile screen; it allows not only the complete weld data management but offers numerous auxiliary functions to support the development and finish of any TIG welding sequence. Some of the main features are:<\/p>\n
![\"Fig.5:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-5_P6HW-221x300.jpg\")
\nFig.5: Polysoude Power Source P6-HW<\/p>\n
\u2022 A complete documentation of the workpiece data.
\n\u2022 The creation of chained weld cycles to carry out a complete multi-pass welding sequence.
\n\u2022 A detailed description of non-programmable parameters (i.e. mechanical adjustments of the devices, type and characteristics of used gases, electrodes, filler wire etc.) appended by instructions and comments.
\n\u2022 A data base with a search function to deal with more than 8 parameters at the same time.
\n\u2022 Development wizards for computer-aided optimisation of welding parameters.
\n\u2022 Simulation of a programmed welding cycle and real-time monitoring of welding parameters and progress of the sequence in process.<\/p>\n
![\"Fig.6:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-6_Ecran_soudage_P6-HW-1-300x217.jpg\")
\nFig.6: Intuitive graphic user interface GUI<\/p>\n
Finalised and approved customer-specific welding sequences and instructions can be stored and transferred to the designated welding equipment by means of a PC or a USB flash drive.
\nBefore a welding sequence of mechanised or automated TIG welding can begin, the operator has to care for correct positioning of the workpieces. After the welding cycle has started, the equipment is completely controlled and monitored by the power source. Differently to GMAW processes, TIG welds do not require any machining or grinding operations neither at their start or end, nor between the passes.<\/p>\n
![\"Fig.7:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-7_Graphique_Taux-depot_CW-HW_EN_HD-278x300.jpg\")
\nFig.7: Hot wire process for increased productivity<\/p>\n
Compared to competing processes, melting rates of cold wire TIG welding must be considered as quite modest. The energy to melt the added filler wire is delivered entirely by the electric arc, so welding speed and wire feeding rest limited to low values.<\/p>\n
Hot wire TIG welding is a method to increase melting rate and welding speed substantially. The hot wire current is supplied by an additional, separate power source and transferred to the wire via a particular nozzle in the wire guide. Between the nozzle and the weld puddle, the wire heats up, so less energy is necessary for its final melting. As shown in Fig. 5, hot wire application allows the welding time of carbon steel pipes to be reduced to less than half of the time needed if the cold wire process is used. And, an important fact to be mentioned, the hot wire TIG process does not reduce the achievable quality of the welds in any way.<\/p>\n
With the mechanised 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 caused an immense gain of productivity, time-consuming repair work was not necessary any longer, and the controlled heat input of the process guaranteed to reach the required mechanical properties of the welds without special attention.<\/p>\n
Finally, the project has been finished within the intended time, but it is already clear that in the future the technical possibilities of the mechanised TIG welding equipment will be exploited entirely: volume-reducing J-preparation or even narrow groove welding will be introduced, so root pass and hot pass can also be produced by mechanised TIG welding.<\/p>\n
An experienced contractor of the Khazzan project in Oman has adopted a different approach. The company must execute approximately 19,000 welds on 12in. and 16in. 22% Duplex Stainless Steel pipes. From the beginning, it was the declared goal to attempt the advantages of automated TIG welding up to the possible 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.<\/p>\n
An efficient option to improve overall productivity of the joining operations of line pipes is a narrow groove preparation of their ends. The mechanical characteristics of the pipe material and their behavior in terms of welding shrinkage are considered to determine the slim profile of the weld groove (the angle of the weld groove is kept as small as possible). The preparation of the pipe ends requires less material to be removed, machining becomes easier and faster. Less material removed, less material to be replaced by the weld: welding time becomes shorter, filler material consumption decreases.<\/p>\n
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, mechanised welding equipment as shown can be used. During the laying of a pipeline, e.g. from a barge or as landline, the pipes cannot be rotated any longer, in these cases orbital welding equipment is required.<\/p>\n
![\"Fig.8:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-8_PH_CLAD_Macro_Narrow-Gap-Hot-Wire-150x150.jpg\")
\nFig.8: Macrographic section of a narrow groove TIG weld
\n![\"Fig.9:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-9_Welding-torch-for-narrow-groove.jpg\")
\nFig.9: Welding torch for narrow groove joint
\n![\"Fig.10:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-10_MU-510-300x246.jpg\")
\nFig.10: Hot-Wire (HW) automatic welding head<\/p>\n
Up to pipe ODs of 20in. dia. (510 mm), Polysoude\u2019s open orbital \u201chot wire\u201d welding heads (e.g. the type MU 510 HW) might be the tool of the choice. Fitting a certain range of different pipe diameters, the welding head is clamped on the pipe and offers the whole choice of the described state-of-the-art features: AVC and OSC, 2 integrated wire feeders, clutch for quick rewinding, hot wire welding etc.<\/p>\n
If the line pipe OD exceeds 20in., Polysoude open carriage welding heads of the POLYCAR type are perfect to do the job. A guide ring fitting the particular OD is mounted on the pipe and allows the POLYCAR to move precisely and safely around. The modular design of the POLYCAR allows cold and hot wire TIG welding, the rugged construction resists difficult conditions in the workshop or even on site.<\/p>\n
![\"Fig.11:](\"https:\/\/www.polysoude.com\/wp-content\/uploads\/2018\/03\/Fig-11_Orbital_carriage-type-welding-head-262x300.jpg\")
\nFig.11: Orbital carriage-type welding head<\/p>\n
To sum up, the only reason for industrial application of TIG welding is the resulting unrivalled joint quality. Different options are available to increase the productivity of the process, reaching at least rates comparable with those of the fiercest competing process, which is the modified Gas Metal Arc Welding (GMAW) process. But, even at similar melting rates, mechanised or automated TIG welding remains wrongly unpopular. On the other hand, striving for the \u2018Zero Risk\/Zero Defects\u2019 approach in the joining technology remains a serious incentive to employ mechanised or automated TIG welding.<\/p>\n
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Fig.1: TIG Welding From the very beginning, gas tungsten arc welding (GTAW) or, as it is commonly named, tungsten inert gas welding (TIG) has been considered as a particularly complicated joining process. The necessary equipment was expensive and subject to significant investments, shielding gas of the required purity was hard to get, and, above all, […]<\/p>\n","protected":false},"author":3,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"page-cta.php","meta":{"_acf_changed":false,"footnotes":""},"class_list":["post-19361","page","type-page","status-publish","hentry"],"acf":[],"yoast_head":"\n