On October 21 and 22 in Midrand, Lincoln Electric South Africa hosted a series of Pipeline seminars by global pipeline welding specialist, James Lamond. African Fusion attends and reports.
Lincoln Electric welding specialist, Josef Henning, demonstrates the STT (surface tension transfer) welding process for root pass pipe welding.
Setting the tone for the day, Lamond says new pipeline welding equipment, processes and materials can have a major impact on the success and profitability of pipeline construction projects. “But to succeed, contractors have to know exactly what they are dealing with, particularly with respect to materials of construction and the strategy chosen by the client. Poor understanding and organisation can have serious and expensive consequences,” he warns adding that, through real pipeline successes and strategy decisions, he aims to challenge current thinking and to introduce available welding options that can significantly improve productivity and profitability for pipeline contractors.
Showing some real footage from 2013 of a pipeline under construction in Poland, he says that Lincoln Electric offers the full range of processes and consumables and is a trusted construction partner for pipelines all over the world. “This X70 pipeline is being constructed at temperatures of -20 °C in snow and ice. With the higher strength steels, a new modern challenge has emerged, in that the consumable has to overmatch the strength of the X70 pipe material, which demands that the exact material properties of the pipe of are known,” he says.
Explaining further, he says that this overmatching strategy is being adopted where seismic movement of the ground may cause excessive strain on an in-service pipeline. “The challenge that arises is that the tensile and yield strength specifications for the different pipe material grades overlap significantly. An X65 material, for example, is required to have a tensile strength of between 450 and 600 MPa, while an X80’s tensile strength must be between 555 and 705 MPa. A steel specified to X65 with a strength of 565 MPa, therefore, can have a real tensile strength that also complies to X80 specifications,” Lamond explains. “So to overmatch the consumable, the contractor has to know exactly what material he is dealing with. The materials specification, alone, is not enough,” he adds.
Turning attention back to the pipeline video, he says: “Successful pipeline welding is done as a well-organised procession. All the equipment is set up in portable work station tents, first for root welding and followed by filler passes,” Lamond says. The video shows pipes being aligned using and internal clamping system – “to achieve high/low bore matching of within 2,0 mm” – and a crane immediately lowers a tent over the new pipe joint. “The root pass on this pipeline was inserted by two welders using Lincoln’s solid wire STT (surface tension transfer) welding process. 90% of those who go bankrupt fail at this stage. Root jointing of pipe sections should take three minutes. If, due to fit up issues, bad organisation or excessive grinding, you have too may bad days and not enough good days on a pipeline project, you are inviting ‘le catastrophe’, as they say in France,” he warns.
Showing two welders completing a root pass in real time – 2 min 43 sec – Lamond adds that each welder usually makes only one stop/start on his half of the root, at around the 4:00 position where he repositions to complete the overhead section. “This is to reduce the amount of grinding required. Grinding takes time and time is money. On a good day, only the stop/starts need grinding and if any more grinding needs to be done, then warning signs should be flashing,” he suggests. “If just two minutes per root weld is added for grinding, three fewer pipe sections will be added to the pipeline on that day,” he says.
Once completed, the root welding team picks up the tent and moves to the next joint and a second tent is placed to fill the joint. On this pipeline in Poland, two Bug-O Piper Plus mechanised welding systems were used with Outershield 91K2-HSR flux-cored welding wire. “This is production welding. Two Bug-O’s are being used at the same time on the same pipe ring. Note the torch angle, which is set at a 5.0° lag, so that in all positions, the welding arc pushes the slag to the front of the weld pool, preventing entrapment,” Lamond points out.
Returning to his presentation, he reveals that, in Africa, some 22 000 km of pipeline – in Mozambique, Tanzania, Nigeria, Namibia, the Congo and more – are currently being planned. “As in all parts of the world, Africa faces shortages of skills; rising labour, material, and energy costs; a decrease in the world oil price and intense competition. In addition, more stringent quality demands are being applied for the higher strength pipe steels, which demand that old construction practices be updated,” he says.
”Conventional stick welding is still OK for some applications, and these processes have remained unchanged for over 40 years – but well-established practices must be followed and complacency has to be avoided!” he exclaims.
Stronger pipeline materials, such as the X70 and X80 grades are driving the industry away from the use of cellulosic stick electrodes, which are associated with high hydrogen content. “When we reach X70 properties, in terms of real strengths, then the family of low-hydrogen stick electrodes, called Pipeliner LHD should be considered due to their low hydrogen levels (less than 5.0 ml) – and these are also specified when wall thickness is greater than 12.7 mm.
“Low productivity, however, associated with the use of stick electrodes, poses serious risks and challenges, which is driving a trend towards the use of semi-automatic and mechanised welding,” says Lamond.
“Productivity on a pipeline project is all about operating factors, the ratio of arc time to non-arc time. When stick welding, along with stop/starts and stub losses, a significant amount of grinding is required between passes.
“A cellulosic root pass has a concave top surface that must be ground flat before applying the hot pass. This can result in a 1.6-2.0 mm fusion layer, and in worst-case high/low mismatch of more than 2.0 mm, grinding can remove the root – and the welder has no way of knowing when this will happen. These are the productivity risks that tend to drive the adoption of the more mechanised processes,” he suggests, adding that the use of modern rot welding processes such as STT is twice as fast as cellulosic welding, because neither grinding nor a hot pass are required.
STT + flux-cored wire
By adopting more modern technology, such as the use of Lincoln’s STT process followed by mechanised flux-cored welding, operating factors on pipelines can be increased to between 60 and 70 %. “You need to respect the welders to achieve these results, but high productivity levels are no longer exceptional and do not require massive levels of investment,” Lamond assures.
Both of these are gas-shielded processes, so they do not like windy conditions – hence the tent. Being low hydrogen processes, gas shielded methods can tolerate lower levels of pre-heat and the deposit generally has very good mechanical properties. “While good skills levels are required for STT root welding, fewer welders are needed and welders can easily be trained to use the process effectively,” he adds.
Benefits of STT, according to Lamond, include:
- Reduced training time: It is difficult to find experienced highly skilled welders, especially for the critical root pass. STT offers ease-of-use, resulting in shorter training times compared to other welding processes.
- Lower repair rates: STT minimises the most common defects.
- Low smoke and spatter: STT uses high frequency inverter technology resulting in high quality welds with less spatter and fume generation.
- The process makes it easier to perform open root welding on pipe with better back bead profiles and edge fusion: The STT process is designed to allow surface tension to ‘suck’ the weld bead into an open root gap of around 3.0 mm. The resulting weld bead has a uniform flat profile with a higher alignment measurement (4.0 mm) than standard cellulose electrodes (1.6 to 2.0 mm). The flat STT profile removes the need for grinding and no hot pass is required. This makes it possible for the root welders to move on immediately after completing a singe pass.
- The thicker nugget also reduces the risk of bead shrinkage or ‘suck back’.
“Semi-automatic processes place the welding controls into the welding system instead of in the hands of the welder. But skilled and knowledgeable people are still needed, to insert the root welds and to tend the mechanised processes, for example. On a pipeline, every single welding joint is different. Welders are under-rated and, because of their ability to react the variations they see, they keep many engineers out of trouble,” Lamond says.