The Eastern Pipeline Alliance – comprising of AJ Lucas, Transfield Services, GHD and SunWater – have used HDD to bore 20 m below the bed of the Brisbane River for a distance of almost one kilometre, to install two pipelines as part of the largest water recycling project in the Southern Hemisphere. The first of the two pipelines was installed in a five-hour operation and the second pipeline was completed in May.

Project background

Queensland’s WCRW Project was developed in response to an unprecedented drought, with demand for water growing at an unsustainable level. The $2.4 billion project will have the capacity to deliver up to 232 megalitres of purified recycled water per day through a 200 km network of large diameter underground pipelines and is planned for completion by December this year.

The underground network will carry purified recycled water to power stations, industry and agriculture as well as to the region’s main reservoir, Wivenhoe Dam, to supplement drinking supplies. More than 150 km has been laid with the second stage of the Bundamba Advanced Water Treatment Plant near Ipswich all but complete and the advanced water treatment plants at Luggage Point and Gibson Island both at the half way point.

The challenges

The 320 km long Brisbane River played an important role in the urbanisation of Brisbane and is regarded as the largest commercial river in Australia. Flowing through Brisbane to the mouth at Moreton Bay, it contributes to the enjoyment of an outdoor lifestyle and provides a habitat to dugongs, turtles, dolphins and a wide variety of marine life.

Crossings of two sections beneath Brisbane River tributaries – Bulimba Creek and Aquarium Passage – have been successfully completed. After several weeks of meticulous preparation on the geologically difficult site, crossing under Bulimba Creek ended in the early hours of March 9, while the Aquarium Passage crossing was completed on March 24.

Bulimba Creek and Aquarium Passage are on the southern side of the river, close to the river mouth. Gibson Island is the location of one of the three new advanced water treatment plants being constructed by the WCRW Project.

Prior to the Bulimba Creek crossing, about 800 m of special steel pipeline was welded together and strung out on rollers on the southern side of Bulimba Creek. Meanwhile, north of the creek the HDD drive reamed out the bore to the required 107 cm through successive passes of the reamer.

The most delicate phase was the pullback of the 800 m pipe, an operation that lasted about five hours. As the pipe passed under the Port of Brisbane Motorway and Queensland Rail line, the pull-back was slowed to minimise pressure below this infrastructure.

HDD: protecting the environment

HDD allowed the pipeline to be installed beneath the ecologically sensitive Doboy Wetlands area without any impact to the surface.

The Doboy Wetlands, a habitat for a range of fish, crabs, birds, reptiles, amphibians and other animals, lies at the edge of the tidal and freshwater areas of the Brisbane River mouth. Dedicated volunteer groups work to protect the natural integrity of the environment and the complex web of life it supports.

The decision to use HDD protected sensitive environmental areas in the water crossings and also had advantages in limiting impact on established and operating infrastructure such as roads and railways. For the Bulimba Creek and Aquarium Passage pipeline sections, HDD minimised disruption to traffic on the busy motorways and ensured surrounding streets were not dug up as would have been the case had traditional trenching methods been used.

Western Corridor Recycled Water Pty Ltd CEO Keith Davies said the project continually sought better ways to build a product that was reliable, effective and functioned in the long term.

“Safety and environmental issues have been project priorities from day one.”

Mr Davies said “The use of HDD is a positive one for the project. Apart from the environmental and added safety benefits, HDD significantly reduced the cost for this section of the WCRW Project. It is a great example of the WCRW Project using innovative technology to reduce construction times, costs and environmental impacts.”

Through the HDD operations, some 3km of steel piping connected the WCRW Project pipeline between Pinkenba – located 12 km north east of Brisbane CBD near the Port of Brisbane – and Gibson Island, then from Gibson Island under Lytton Road at suburban Hemmant and through to Murarrie, located 8 km east of the CBD.

Crossing the river using HDD avoided the alternative of constructing 45 m deep shafts at either side of the river and building large platforms to support construction and storage, as well as related safety issues. The result is a reduction in construction costs, enhanced workplace safety, utilisation of less space and benefits to the environment.

The drilling

Project manager Shane Gaudin said HDD was far more complicated than simple trenching and laying pipe in a trench, and involved extensive skills and experience to be successful.

He said HDD Superintendents, Gary Hunter and Stephen Booth, were two of the world’s leading authorities with more than 50 years’ combined global experience.

“They have a wealth of ‘down-hole’ drilling, formation and tooling knowledge and are the ‘engine room’ of the whole operation,” Mr Gaudin said.

“To achieve the crossing took weeks of thorough planning on the challenging site, the right machinery and teams who understood the importance of the operation.”

Before drilling started, a series of vertical bores were dug to verify the nature of the formation to be drilled. The geotechnical data was incorporated into a borehole profile design to select the most appropriate formation or layer through which to drill and to also avoid any other obstacles such as power or gas lines.

The results of the formation testing determined the design depth for the crossings at 20 m below the bed of the river. On each of the boreholes, the entry angle is between 12 and 15 degrees and the exit angle is set at 6 degrees so that the profile generally consists of two bend sections and one ‘straight’ section at design depth.

Mr Gaudin said precision in HDD was paramount and a predetermined process, based on testing and experience, was used for the river crossing.

“Likewise, the quality of communication between the HDD superintendent and the driller is essential.

“The drill cab from where the drill rig is controlled is the centre of the operation and effectively drives all other tasks. In this type of formation, where speed and accuracy are mitigation measures in themselves, there is little room for error in execution.

“Timing is critical. The pilot hole took about two weeks to complete but the pullback operation had to be completed within six hours. This is because the material the team was drilling through is soft and there was a very real risk the hole could collapse, or the pipe could get stuck in the pull-back operation,” he said.

The pilot hole of 31.115 cm – the only ‘steered’ hole – was the first hole to be drilled. The profile was replicated by having special downhole steering tools which register electromagnetic signals calibrated to known positions on the surface to ensure precision.

At all times, the driller and steerer know the exact position, inclination, depth or length of the pilot hole. Once the pilot hole has been drilled, the reaming passes use downhole tools to follow the pilot hole while progressively opening up the hole to the desired borehole diameter.

The hole was gradually enlarged by pulling increasingly large reamers backwards. The diameter of the tunnel increased with each backward pull and when the desired diameter of the tunnel was achieved, a string of pipes was attached to the reamer and pulled through the tunnel to the other side of the waterway.

The crossing used three sections of pipe – one 666 mm wide section and two 760 mm in diameter sections. The variation in the diameters was calculated to allow for the different flows and pressures to which the underground pipeline will be subjected.

To power the drilling, lubricate the drilling head and consolidate the walls of the tunnel, a drilling fluid was pumped down the hole. This fluid was a mix of bentonite – an environmentally safe natural clay – and water. It also carried mud out of the hole, which was captured, filtered and recycled.

The entire drilling process was guided by a computer from the surface. The computer allowed the driller to precisely locate the drill bit and guide it along the predetermined path. The subterranean pipeline travelled almost 1 km across the river before the driller engineer steered the bore head to within 20 cm of the target on the other side of the river.

Various types of formation, including clays, sand, gravel and weathered rock sections, were encountered during the drilling. The swelling nature of the clays presented considerable challenges during the course of the drilling, but a sound drilling program and correct tooling meant that any potential issues were overcome.

Four boreholes of approximately 800m each – Aquarium Passage, Bulimba Creek and twin lines under the Brisbane River – were required for the crossings. Pipeline, the fully length of each borehole, plus 12 m extra was laid out for up to 1 km across vacant land. The full length was then welded progressively, internally lined, externally coated and hydro tested prior to the commencement of drilling.

The full pipeline was welded in a single continuous length in order to mitigate against the borehole being left ‘open’ in soft, unstable formation drilling. Fully welding the pipe string enabled the fastest installation to be achieved by pulling through the pipe in a continuous manner. On the surface, the welded string of pipes is pulled onto rollers ‘training’ the pipeline into a curve.

To give the pipeline the best opportunity to achieve its design life, the joints between the pipes were welded and sealed, the internal lining and external pipe coated and cathodic protection applied to protect against corrosion.

While the welded pipeline is not flexible in the real sense of the word, it has a minimum bending radius, which enables the pipeline to be pulled into the borehole. The engineering team designed the borehole and the stringing temporary structures to no more than 80 per cent of the yield stress on the pipe. This ensured the pipeline was not damaged during installation and good profile design meant no residual stresses affected the pipeline.

Mr Gaudin said a critical phase of the operation was when the fabricated pipeline was pulled into the borehole through a ‘break over’ structure.

“The break over is a series of cranes with roller cradles that train the pipe into a suitable radius as it is pulled into the borehole. Its purpose is to ensure the pipeline maximum yield stress is not exceeded so that the pipeline does not sustain any permanent deformation,” he said.

The Bulimba Creek stringing/right-of-way area was a 1,400 m radius, which the pipeline was fabricated along. This was considered a gentle radius. The situation for Aquarium Passage and the Brisbane River right-of-way areas were considerably more restricted and required special rollers and temporary structures to fabricate the pipelines around a 760 m radius. Regarded as a very tight radius, it correlates to about 80 per cent yield stress.

Lack of available open space meant that a straight section behind the borehole was not available and innovative alternatives were sought. One section was put together in curved sections to obtain the required length and, for the Brisbane River section the team built a small tunnel under a detour road to ensure there was enough space to weld the string in a continuous section.

Mr Davies said a huge effort went into the planning and execution of the HDD operation.

“Its success is a tribute to the dedication and skill of our workforce and the many different specialists who have contributed to the river crossing since we first considered the benefits of using this form of drilling for this part of the project,” he said.