Too isolated for large populations, and with ground conditions too hard for agriculture, the Northern Beaches’ natural beauty has remained largely intact over the years. With many locals living among the secluded beaches and national park areas and the advent of modern demands, the rollout of the National Broadband Network (NBN) to the area via the Mona Vale Western Foreshore NBN Outfalls was inevitable.
The challenge was finding a suitable solution to install this vital infrastructure in such an inaccessible and environmentally sensitive location with difficult rock conditions.
Strict environmental requirements
From the start, the project was bound by rigorous environmental conditions considering much of its location was covered by the Ku-ring-gai Chase National Park. A trenchless solution became a non-negotiable in order to minimise the environmental impact – with horizontal directional drilling (HDD) being the preferred method.
To undertake the job, Central Coast-based HDD specialist Superior HDD was engaged by Downer to complete the four HDPE HDD ocean outfall connections. Superior HDD Managing Director and Operations Manager James Burch recognised the project’s challenges immediately.
“This is a very sensitive national park area, with many parts only accessible by water,” he says.
“HDD was the only way to do it, and we knew with good planning, design, and risk mitigation, we would get it done right.”
Minimising risk
Mr Burch says his priority was first to fully understand the project’s risks in detail.
“We know the ground conditions in this area very well, and while the consistent Hawkesbury sandstone provides hard but predictable drilling conditions, being such a sensitive area, hydrofracture was not an option,” he says.
“So, we had our HDD design specialists complete a detailed profile and alignment and hydrofracture assessment, coming up with an ultraconservative, deep profile to minimise frac-out from that perspective.”
Mr Burch explains that from there, drilling fluid was the next step in the process.
“We needed bentonite to ensure optimal borehole performance and cuttings removal, but we couldn’t risk release of this fluid into the surrounding waters,” he says.
“So, we devised a detailed drilling fluid plan which used a bentonite program for most of the initial pilot, then a complete displacement of the hole, replacing all fluid with an environmentally low-impact xanthan gum mix.
“This allowed us to punch out using the biodegradable xanthan gum fluid, as well as to responsibly plan, monitor and analyse our returns for the entirety of the bores.”
Practical implications
During the job, Superior HDD found mobilising its Ditch Witch JT60 All Terrain midi rig and support equipment to the isolated sites also required many practical adaptations.
“Many of the project areas are accessible only by water, and being ocean outfalls, much of our support equipment was transported and set up on barges for the duration of the project,” says Mr Burch.
Along with limited accessibility, the steep and rocky environment added additional challenges.
“For more than one crossing, there was a 50 m elevation difference between the drill site and the mud recycling system on the barge.
“We rose to this challenge by supplying top of the range, powerful transfer pumps, as well as extensive auxiliary equipment to minimise possible interruptions.”
Community engagement
Mr Burch says the project required involvement of a range of stakeholders, with Superior HDD seizing the opportunity to demonstrate the strengths of HDD.
“We had eyes on us for the whole project. In such a pristine environment, the local community was very environmentally conscious,” says Mr Burch.
“We consulted closely and regularly with stakeholders to be fully aware of and compliant with all environmental requirements, and to put concerned locals’ minds at ease.
“We also don’t rely on subcontractors for any of our HDD equipment, so we were able to personally consult with major stakeholders like National Parks, and then directly supply all gear in compliance with their strict requirements, including tracked machinery only.”
Successful connections
Mr Burch says the final of the four connections was completed in late September, on budget and within required timelines.
“We were proud to showcase the accuracy and environmental benefits of HDD with this project, proving that with good preparation and risk mitigation, it’s a safe, responsible method in even the most pristine environments.”
This article was featured in the December 2020 edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet or mobile device, click here.
For more information visit the Superior HDD website.
If you have news you would like featured in Trenchless Australasia contact Journalist Sophie Venz at svenz@gs-press.com.au