The Colongra Lateral Pipeline not only acts as a carriage way for the gas, but also a storage facility. The dual purpose of the pipeline created unique welding issues for the infrastructure project as it needs to cycle through a wide range of pressure conditions, and consequently temperature variations, on a regular basis.
Jemena Engineering Manager Andrew O’Neill said “Any welding flaws could grow due to fatigue (with the regular pressure changes). The welding needs to meet the demanding fatigue requirements.”
The high-strength X70 grade pipe with 24 and 30 mm thick walls specified for the project needed to be welded with exceptionally stringent quality control, meeting American National Standards Institute Class 900 requirements. “This project was very different in terms of welding requirements. We knew that a traditional pipeline welding solution (stove pipe welding) was not going to be viable on the Colongra Pipeline,” said Jemena Site Manager and Commissioning Co-ordinator Paul Grace.
“With traditional methods and the pipe composition and wall thickness there would have been a high level risk of cracking. It had to be low hydrogen and the weld toe profile needed to suit the onerous fatigue requirements. We needed to find a very different solution,” continued Mr Grace.
The pipe is also unique in that it loops back on itself to increase the length of the pipeline by three kilometres, and hence storage capacity. Jemena had a specialised pipe bending machine built in the United States for the project and this looping of the pipe added to the precision welding requirements.
An extensive testing period to develop a welding solution for the unique pipeline was instigated by Jemena and Diona. The key criteria in the testing of equipment and consumables was:
* The production of high integrity/sound welds that meet stricter quality than standard pipeline projects;
* Developing a process that had a good production rate;
* A solution that was efficient in the size of product – not large equipment that got in the way all the time; and,
* Equipment that could stand the challenge of working in the field.
Extensive trials were conducted with controlled wave form processes, including surface tension transfer (STT). To develop the best solution tests were also conducted using flux-cored arc welding gas shielded, STT and submerged arc welding, and manual metal arc welding low hydrogen processes.
“We evaluated several welding equipment waveform technologies as part of this project. Lincoln Electric’s semi-auto/STTÂ® with mechanised flux-cored welding was selected due to the consistency of the weld quality,” said Mr Grace. “The productivity was also good considering the thickness of the pipe being welded and the stringent defect assessment criteria.”
All STT root runs were completed using a semi-automatic process, followed with G70M semi-automatic hot pass before fillout. The Lincoln Electric STT or Surface Tension Transfer process ultilises sophisticated software to precisely control the current throughout the welding cycle instantaneously.
Diona Project Manager Hugh Daly said “While there are some greater logistics requirements in using this process, the productivity rewards are great.”
Darren Barrington, one of the welding contractors working on the Jemena project said “On this job, we were doing two joints a day comfortably.”
The dual wire feeder capabilities of the Power Wave/STT power source also enabled efficient switching between processes.
“For this project it is was extremely important to have good toughness of the weld joints,” said Mr Grace. “And the Lincoln Electric G70M had good strength matching with Charpy Impact Testing delivering toughness averages of
100 joules at -10 degrees. Such good toughness assisted with stress analysis and improved allowable discontinuity size.”
“The G70M is a beautiful wire to run- it washes in nicely – solidifies nicely – no undercut,” said Mr Barrington. “I had not done much with wire before, but I am definitely a convert now. The flux-cored wire also carried quite a bit more metal making fill-out quicker.”
“The repair rate on this project has been good given the extremely stringent defect acceptance criteria and the challenges,” said Mr Grace. “The defect rate has been low – lower than anticipated especially when you consider the time of flight diffraction (TOFD) ultrasonic testing that was used.
“Unlike typical pipe welding projects where welds are inspected by radiography, because of the stringent fatigue/stress analysis for this project, TOFD ultrasonic testing was used as the inspection methods. This has a much higher defect detectability rate than radiography,” he said.
Phenomenal efficiency in welding station set up and placement by Jemena’s contractor, Diona, in readiness for the welding operators, has been key to the productivity. The solidly constructed weld hut, rather than tent like structures, features the Lincoln Electric wire feeders, guns and other equipment permanently mounted on the wall. Once the sections of pipe were laid with precision, the hut is placed over each weld section, the truck with generator and STT/power waves moved into position and connected – all done within a 20 minute time frame.
“The efficiency is extremely high and the project was delivered quicker than expected,” said Mr Daly. “I don’t think there is any way we could improve on how this project has proceeded. A lot of money was invested early on researching welding methods and I am certain that we picked the most efficient method available.”
Mr Grace said “We also had the issue that all this was very new to all of us”” welding operators, inspectors, contractors. At the start it was a steep learning curve for all involved.
“There was not a lot of experience in STT for field pipeline construction in this country and we did find that the majority of welding done with STT is roly poly welding where the pipe spins – this field work was totally different where you could not rotate the pipe. The biggest challenge for us was welding in 12 and
6 o’clock position.”
Lincoln Electric provided technical support and some initial training at their Padstow manufacturing site.
“There were a couple of tricks to learn, such as the 12 and 6 o’clock positions,” said Mr Barrington. “But once you get used to this process it is user-friendly. Now I would pick the STT/flux-cored process any day of the week. It is going to be the way of the future for large diameter, heavy wall pipe.”
Mr Barrington also said that the Lincoln Electric STT Power Wave machines performed well in the field.
“This project has been unique,” said Mr Grace. “While the diameter of the pipe is reasonably common in Europe and North America, the fatigue factor is not. To our knowledge, it has not been done before and it is an important step in developing greener power generation.”