Working HDD in the Otway Ranges

In 2011 Dunstans Construction Group, in conjunction with the Barwon Water Alliance, completed two horizontally directionally drilled crossings in the environmentally fragile Otway Ranges in order to replace multiple sections of the deteriorating Colac Pipeline.

The 28 km 419 mm PN16-grade Colac Pipeline, which connects the West Gellibrand and Olangolah reservoirs to Colac, was constructed in 1910. This upgrade, which involved the replacement of five sections of the pipeline totalling 6.2 km, is part of a staged replacement plan, with the first sections of the program completed from 2002-07.

HDD in the Olangolah

A third-generation construction company, Dunstans has embraced the horizontal directional drilling (HDD) technique – especially in areas where conventional pipelaying methods via trenching are unacceptable. The installation of the 500 mm diameter high-density polyethylene (HDPE) pipeline was to be completed using a Vermeer D100x120 drilling rig in conjunction with a DCS 450 cleaning system, developed and manufactured in-house, to process and recycle the drill fluid.

With the aid of Highside Drilling Services, the Dunstans HDD team designed the two crossings. The first crossing was 292 m long with a complex compound curve and a constant turning radius from entry to exit, which began at the foot of the weir wall beside the bed of the original Olangolah Creek. The second bore began approximately 1 km from the weir, beginning 10 m above the Olangolah Creek and dropping sharply under the creek through a dividing hill and under the Eel Creek, before rising quickly to its exit point 330 m later.

Treading carefully

Many adversities had to be overcome by the Dunstans crew in order to complete the pipeline upgrade.

The spring-fed Olangolah Reservoir is one of Colac’s main water storage facilities, with the clean and clear water being collected in the remote valley between Forrest and Apollo Bay. The valley is home to a range of rare fauna, including the rare tiger quoll (a native cat), platypus, wedge-tailed eagles, native rats and wallabies. It is also home to some of the rarest trees and plants in Australia, including the Beech Myrtle tree and the Mountain Ash – the tallest flowering plant in the world – making the protection of the environment especially important.

Although the Otway Ranges are only two hours west of Melbourne, the area is incredibly dense, making access for the drill spread and the delivery of materials extraordinarily difficult.

The project began in May to coincide with the winter months – when the demand on the system would be at its lowest – to allow for the temporary closure of the pipeline. Because of this, the 5 km
logging track to the reservoir was unpassable at the beginning of the project, and after its initial upgrading works it had to be constantly maintained by the Barwon Water Alliance crew to allow safe access to the sites and delivery of gear and materials. Heavy rain affected the first four months of the project, making this a very demanding task.

Because the area is so sensitive, all aspects of the operation were analysed to ensure that they were as environmentally compliant as possible. This began with the use of the HDD method to install the pipeline in areas that would have previously been trenched, such as the bank of the Olangolah Creek.

It was also reflected in the selection of the “˜pose little or no risk’ (PLONOR) approach to the environment-certified drilling fluid products, which were used to firstly minimise the risk of a frac-out and also to ensure that if a frac-out occurred, there would be little-to-no impact on the delicate ecosystem. Australian Mud Company worked closely with Dunstans to engineer suitable drilling fluids.

A stringent drilling program was initiated, which included constant monitoring of drill fluid pressures as well as the calculation of the volume of fluid in the system to make sure there was no frac-out or fluid loss into the ground. Fluid viscosity, density and the amount of cuttings returned were constantly measured to ensure that the borehole was clean and the fluid was working efficiently, which would enable trouble-free installation of the product pipe.

Working with tight conditions

The second crossing offered even more challenges than the first. The entry point was on a narrow track, which limited the amount of space to set up the spread. This is where the DCS450 cleaning system came into its own as a self-contained unit with generator, fluid mixing, and storage and cleaning shakers all in the one unit, and with a minimal footprint.

The bore length was over 350 m, with the exit point 2 m above the entry. This was compounded by the two creek crossings that had to be negotiated at roughly the beginning and the halfway point of the bore, requiring a steeper than usual entry and exit angle to ensure the appropriate ground coverage.

The return line from the exit pit to the cleaning system was almost 800 m long, and followed the access track as the direct path along the bore line was inaccessible. The return pump required pushing 15,000 L of drill fluid, and cuttings contained in the return line had to be tracked into position by a 30 tonne excavator, as a truck could not access the exit.

The 30 lengths of product pipe, weighing almost 1,000 kg each, had to be carried in three at a time to the welding set-up, past the exit pit by a four wheel drive truck, and stored along the track until they were ready for low-pressure butt welding. Again, because of space restrictions, the pipe string had to be welded in two sections of over 150 m, requiring a weld to be completed at the halfway point of the installation.

Despite these numerous challenges – including the geology of the second crossing which was also found to be of a much harder consistency, and therefore increased the amount of time spent on each pass – both crossings were completed successfully, with the product pipe installed, tested and connected to the existing line.

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