The existing pipe was originally trenched through the peat soils for approximately 200 m before crossing the Anglesea River. In 1983, after the Ash Wednesday fires ravaged the area, man-made wetlands were created to extinguish underground peat fires. The public now uses the wetlands extensively – especially school groups, who paddle through the weaving canals and explore the ‘islands’ linked by boardwalks to learn about the fragile estuarine ecosystem.
In August 2011, a leak was discovered in the pipeline that delivers recycled water from the Aireys Inlet water reclamation plant and sewage from west Anglesea to the Anglesea water reclamation plant. The pipe, which was carrying about 85 per cent Class C recycled water and 15 per cent sewage, was shut down immediately.
After a stainless steel sleeve installed by divers failed to stop the leak, other means were then investigated to repair the pipeline. Barwon Water decided that the pipeline needed to be replaced and the best way to do this with minimal disruption to residents and disturbance to the environmentally sensitive area would be by utilising horizontal directional drilling (HDD).
An above ground temporary bypass line of 180 mm high density polyethylene (HDPE) was established to carry sewage over the river. Timing was crucial for the pipeline, which had to be commissioned before Christmas when the population and demand on the system would grow four-fold.Article continues below…
Secure sites were set up on either side of the river, one on Coogoorah Reserve on the southern side of the river, and one on Wray Street to the north. Both sites were in high pedestrian traffic areas bordered by residences and holiday homes – making safety, aesthetics and noise considerations paramount.
Prior to the commencement of construction, Dunstans designed the proposed HDD methodology taking into account all known parameters that would affect the project.
Working in an estuary
The Australian Mud Company (AMC) developed a potassium chloride/polymer drilling fluid using geo-data provided from GHD. The drilling fluid had to be inhibitive so that the chloride levels would not impact on any environmental conditions in the estuary. However, it still needed to be able to maintain hole integrity, whilst performing the usual requirements for hole cleaning and solids removal, and also being 100 per cent biodegradable and environmentally friendly.
Assisted by Transco Engineering and Highside Drilling Services, the pilot hole methodology comprised a 4 ¾ inch bottom home assembly (BHA) that housed the Parratrack 2 wireline tool supplied by Highside Drilling Services, along with a Transco MFG 1.5 degree bent sub and 6 ¾ milled tooth Tricone.
Because most of the bore path was either underwater or in the sensitive wetlands, Dunstans was unable to use a ground wire, therefore the AC beacon was the preferred method for magnetic corrections. The profile started with a 12 degree entry angle followed by a 250 m radius, taking the profile to horizontal at 15 m deep from surface and 10 m below the lowest part of the bottom of the river. Because of the constraints of the urban area, the exit angle was designed at 18 degrees.
The jetting assembly achieved excellent penetration rates for the first 240 m, encountering varied geologies. However 100 m prior to exit, the 1.5 degree jetting assembly was failing to achieve inclination due to a layer of stiff clay above the tooling. With a small allowable window for exit, it was decided to trip out and run back in hole with a mud motor set at 1.83 degrees. Once back on face, the desired build was achieved and the stiffer formation carried all the way through to exiting on target. The pilot hole BHA was then broken down to commence hole opening.
Whilst the pilot hole was being constructed, the Dunstans welding crew were fabricating and installing the 160 mm HDPE mud return line, following the above ground path of the temporary bypass line. The pipe welding crew then constructed the product pipe utilising the McElroy Tracstar 500. The new pipeline consisted of 340 m of 315 mm PN20 HDPE, which was welded and strung out along the river before being pulled back along Wray Street just prior to being installed in the borehole. This involved blocking a number of streets for around six hours, with Altus Traffic given the task of redirecting traffic and maintaining a safe work area.
The hole opening began utilising a Transco MFG 20 inch bidirectional fly cutter, followed by a tail string to aid in tripping out of hole when required. The first 75 m were back reamed with excellent penetration rates. When coming to the stiff section that was encountered on the pilot hole, penetration slowed. It was decided to trip out of hole and run back in with a Transco 14 inch hole opener piggy backed with a 20 inch hole opener. Both hole openers were fitted with milled tooth Transco MFG cutters. The 45 m section was revealed to be a stiff coal seam, which was successfully reamed. The hole openers were then tripped out and replaced by the 20 inch fly cutter which completed the remainder of the crossing with excellent penetration rates.
The pipe pull utilised a 16 inch Transco MFG barrel reamer in front of the 50 tonne swivel. The pipe was installed over a three hour period with installation loads reaching a maximum 6.5 tonnes — reflecting exactly the stress analysis provided to the Barwon Water Alliance prior to construction.
The project was successfully commissioned and reinstated on 22 December, just before the Christmas deadline. Dunstans and Barwon Water have also contributed to a fund to improve Coogoorah Reserve by laying turf over the area, leaving it in better condition than before the project began.