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In this article we explore some of the most frequently asked questions and debunk common myths about horizontal directional drilling (HDD). We’ll discuss the benefits it provides when assessed against other pipe installation methods, and focus on the environmental upside, flexibility of design and how the technology supports construction in congested, urban, industrial and sensitive environments.

Do I need to dig exit and entry shafts to undertake HDD?

A: No. HDD has entry and exit sites; however, it doesn’t require deep shafts or large pits. While construction equipment is required within these sites, this equipment remains on the surface, and is often bunded and contained within a relatively small footprint that can minimise environmental impacts. For large diameter HDD, the onsite operational equipment generally includes:

Entry site

• Entry pit generally 3m x 3m x 1m
• Power units and generators
• Water and mud slurry pumps
• Mud mixing tanks and recyclers
• Drill pipe racks and transport
• The rig itself and support equipment such as:
• Vacuum truck
• Small excavator
• Tool storage & setdown
• Site amenities

Exit site

• Exit pit generally 3m x 2m x 1m
• Drill rod storage & set-down
• Rollers and pipeline handling equipment
• Pipe racks
• Room for Product pipe string during pullback

Is HDD installation limited by depth like other installation methods?

A: No, with HDD installation you can easily reach depths of more than 50m without the hassle and cost of digging and shoring trenches or using complicated support structures to support deep shafts. Going deeper can reduce the risk of hydrofracturing (frac-out), which is when drilling fluid or mud comprising non-toxic bentonite, soil and water escapes from the bore hole to the surface.

With more coverage the likelihood of fluid escaping to the surface is significantly reduced, further reducing environmental impact. The actual depth limit depends on the project geometry such as entry and exit locations, bore design length and ground conditions, but it’s the perfect method of pipeline installation when you want to keep the landscape intact and go under existing assets rather than through or over them.

What’s the difference between HDD and microtunnelling?

A: Microtunnelling is pit launched while HDD is delivered from the surface. Microtunnelling tends to be 30–50 per cent more expensive than HDD due to the cost of the jacking pipe, constructing the launch pits, the number of pits, and the depths of those pits. Accuracy is another factor often discussed when comparing the two methods. Microtunneling machines are very precise, boring to within 10mm of a target.

However, it is slower and involves more setup than HDD. HDD has an average accuracy of within 10cm to a target point, better for larger-scale projects that don’t require millimetre precision. And using state of the art guidance technology, skilled drillers now deliver gravity sewer grades of 1:150 using HDPE.

HDD also offers the advantage of speed and flexibility and can get in and out with minimal setup while still maintaining the high degree of accuracy required by asset owners. HDD can drill both vertical and horizontal radiuses to avoid obstacles, which is only achievable with very large diameter microtunnelling and tunnel boring machines. The pit-to-pit distance limitations for each microtunneling machine are not a problem for HDD, with maxi rigs now capable of drilling large diameter bores for more than 1km without surfacing.

How long does a typical HDD pipe installation take?

A: HDD installation time varies depending on the length, size and diameter of the pipeline. For very large pipes, this can take weeks or months. However, HDD is much faster than other alternatives.

What pipe installation technique should I choose?

A: Choosing a pipe installation technique is really a case of the right tool for the right job. HDD is a great mix between microtunneling and tunnel boring and is also often a lower cost option when factoring in all the elements and costs involved with pipe installation. HDD also offers many environmental benefits that make it ideal if environmental constraints are a factor.

About the expert, Anthony Doherty

Widely regarded as an industry expert, Anthony began drilling in 1994, and has overseen HDD installations since the technology first arrived in Australia. Of particular note, Anthony was the first person to undertake the HDD installation of polyethylene pipeline for the Australian water industry in 1995. With his extensive history and experience in the Australian HDD industry, Anthony has used his knowledge to develop several innovative patents for equipment that are still in use today. Anthony founded AHD Trenchless in 2009, and since that time he has built the company into an undisputed leader of HDD technology in Australia.

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