The pipeline will supply gas from an off-take station on the Eastern Gas Pipeline to a new 400 MW gas-fired, combined cycle power station currently under construction by TRUenergy.

The alliance engaged UEA Pty Ltd to undertake HDD works including the boring and installation of 763 m of 273mm steel pipe, in rock rated up to 120MPa. UEA used its 136 tonne Vermeer D300x500 HDD machine as well as a DFE 700gal/min recycling system, and 700gal/min Weatherford mud pump.

Site specific problems included varying rock strengths and fractured rock; depth to allow for clearance of bridge piers on the F6 Freeway - the bore was required to be at least 28 m deep; dealing with the requirements of State Rail for the South Coast rail line crossing; and, tight tolerances imposed on the bend radius.

Pilot bore

To assist in working to the tolerances a surveyor was engaged to peg out the route and verify the profile. Once this was completed the information was entered into the bore planner software and a profile was created.

Due to the tight time constraints and prior work commitments for its maxi drill, UEA decided to utilise its 25 tonne Vermeer D50x100 HDD machine to commence the pilot bore. Using an in-house Digitrak SST steering tool the initial 500m was pilot-bored in nine days. Upon verification, the alignment was found to be incorrect due to a magnetised sonde housing that significantly affected the magnetometers within the steering tool. This pilot bore was then abandoned.

UEA then mobilised the Vermeer D300x500 and a new pilot bore commenced. This time a Paratrak 2 steering system was utilised to track the bore. The Paratrak system allowed very accurate monitoring of the bend radius throughout the pilot bore to ensure that the tolerances were not compromised. The geotechnical information stated that there was a likelihood of encountering clay approximately 80 m from the exit point. UEA stopped the pilot bore at this distance, pulled the rods out and set up to forward ream.

Throughout the pilot bore the ground conditions varied in consistency and a percentage appeared to be broken. With the varying ground conditions, Baroid’s experience was utilised to develop a successful mud plan to cope with the broken ground and deal with the large mud losses that were happening.

Forward ream

A hole-opener with 16 in. medium formation cutters were used initially to forward ream the 700 m of rock. Progress throughout the reaming process was slow using the selected cutters and some experimentation was required with the cutter selection to reach an acceptable progression rate.

To avoid large frac-outs in the clay section, the D50x100 was moved to the exit side, pilot bored down and intersected the initial 8 in. pilot bore created by the D300. An 8 in. sewer plug was placed in the hole to aid with the forward reaming process currently being undertaken by the D300. Once the rock section had been completed by the D300, it was remobilised to the exit side where it proceeded to forward ream the clay section. Once completed it was then remobilised to the entry side in preparation to pull the dummy pipe.

Dummy pipe pull

As part of the process for borehole approval, UEA was required to pull a section of dummy pipe approximately 36 m long. Once the pipe had been pulled in, it was inspected for damage and tested. The client was happy with the sample condition and the bore hole was approved for final installation of the product pipe.

Pullback of product pipe

The 763 m of gas pipeline had been placed on rollers in preparation for pullback to reduce friction during the process and to protect the Naprock parent coating on the pipe. The pulling head was attached to the drill rods the day before pullback. Pullback commenced at 6.30 am and the pipe was pulled in over an eight-hour period. Throughout the pullback process the pipe coating was tested to ensure its integrity prior to entering the hole. Once the pipe pullback was complete, the pipeline was tested again. After a few anxious moments, the pipeline passed the test parameters and the project was complete.

Outcome

This project provided UEA with some significant challenges in particular steering tool and hole-opener selection. The project was completed within a few days of the extended contract completion date. UEA has said that the Nacap/TRUenergy Alliance was an excellent client to work with from the top down, particularly in regards to the management’s proactive approach to any site issues.

UEA said it was happy to have this project as part of its résumé due to the length and the tight tolerances required for a steel pipeline. The project has provided the company with some excellent experience that will put it in good stead when called upon by future clients to undertake similar projects.