These modernisations include high stress crack resistant polyethylene pipe (HSCR PE), the Magnetics Mag8 Walkover Tracking System and use of Time of Flight Diffraction Ultrasonic Testing technology, in addition to the trenchless method of horizontal directional drilling (HDD).
The Logan City Council says trenchless installation was chosen as the most practical solution considering it avoided the need for a major diversion and longer pipeline route. HDD was specifically used for the pressure main installation as the profile can be curved, avoiding the need for deep entry and exit pits – as opposed to alternative trenchless methods that were considered such as microtunnelling.
Project overview
Logan Water Infrastructure Alliance Manager Mark Vaughan and Downer Project Manager Graeme Nicholson said the project was completed in only eight weeks, including the time for setting up and demobilising the equipment.
“After setting up the drill rig and associated tanks and equipment for processing and recycling the drilling mud, a mud return line – 160 mm diameter – was strung out over land and suspended across the river,” said Mr Nicholson.
This process allows the drilling mud to be pumped back over the river for cleaning and recycling, which is a part of the drilling process. Following this, drilling commenced with installation of the pilot bore of 250 mm for the full length of the drill.
The drill head was steered according to a pre-designed drill profile up to 24 m beneath the river and the pilot bore was reamed up to size required for the product pipe in two stages of 450 mm and 900 mm. Simultaneously, the product pipe was welded together in a continuous pipe string of 480 m and made ready for pulling into place.
HDD technologies
HSCR PE is a recent innovation in materials technology, which has been specified in other HDD projects by the Logan City Council recently. Since trenchless technology applications can increase the risk of pipes being damaged during installation and subsequent cracks in the pipe propagating over time, HSCR PE – supplied by Iplex – was used as it contains a resin that reduces this tendency.
Additionally, Mr Nicholson says the Magnetics Mag8 Walkover Tracking System was first used in Australia on a wastewater pipeline project also in Logan, which is currently under construction. The Logan River HDD, however, is the first project to be completed using this technology.
The tracking system was used to monitor the location of the drilling head and ensure the drill followed the correct alignment in combination with a maxi drill rig. The tracking system can track drilling up to 90 m deep, which is much deeper than competing systems that can usually only track to 18 m.
Time of Flight Diffraction Ultrasonic Testing technology
The new technology was used to test every weld on the pipe, which added confidence in the integrity of each of the pipe welds. Sound waves were transmitted through the weld to reflect signals back to the operator that were then analysed to determine whether there are any hidden faults.
Destructive testing of selected welds will only indicate deficiencies in the pipe welding method and not specific defects in each in situ weld – as they must be destroyed to be tested in such a way – so it was not considered sufficient in isolation. Ultrasonic testing, however, is one of the only non-destructive test methods that can identify invisible defects.
Comprehensive testing regime
“A number of testing and quality assurance measures were implemented ‘over and above’ industry normal practice for horizontal directional drilling pipe installation to ‘raise the bar’ and provide added assurance against future pipe failure given the proximity of the sewer pipe to the Logan River,” says Mr Vaughan.
This included the ultrasonic testing of every pipe weld to ensure any defects or imperfections in the weld were identified, with the implementation of a ‘zero defect’ criteria to ensure rejection of any defects in the weld whatsoever. Additionally, a pre-installation pressure test on the welded pipe string was carried out where typically only a post-installation pressure test is undertaken.
Mr Vaughan says that an additional section of the pipe of approximately 10 m was pulled through the underground drill to inspect the pipe and confirm any damage caused to the pipe surface during the pulling process was within acceptable limits.
“As well as the [aforementioned], it’s fairly standard practice to carry out ‘destructive testing’ of pipe welds,” says Mr Vaughan.
“It was ensured a pilot test weld was sent to the lab and pulled apart to prove that the welding methodology and equipment was sound before pipe welding could start on the permanent works. A sample of further welds were also destructively tested.”
Additional consultations
Mr Vaughan and Mr Nicholson said a lot of effort went in to preparing and implementing the regimen for the high standard of quality assurance, in which documents were prepared by the HDD contractor in line with this specification. Third party consultants were also engaged, including Le Hunt Consulting Engineers and Trenchless Adviser.
Le Hunt Consulting Engineers was able to recommend procedures and testing that would ensure a high-quality outcome and additionally review the contractor’s quality assurance documents prior to approval of the works commencing.
Meanwhile, Trenchless Adviser was engaged to oversee the contractor’s work in the field and ensure the suitability of methods and equipment. Additionally, drill logs were reviewed and both drilling pressures and mud return quantities were monitored to ensure the risk of frac out was mitigated.
Project success
The Logan City Council’s Flagstone to Cedar Grove Wastewater Treatment Plant project was completed on time and under budget with no environmental incidents. The council attributes this mainly due to the use of efficient drilling methods and completing pipe reaming in two passes – as it reduced the risk of ‘frac outs’ by minimising drilling timeframes.
By choosing, managing and executing HDD to its full potential, the project achieved a lowest impact construction method for both the community and the environmentally sensitive area of the Logan River waterways.
For more information visit the Logan City Council website.
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