HDD a possibility for Tasmanian pulp mill project

Federal Government Environment Minister Tony Burke has given final approval for the Gunns Bell Bay pulp mill environmental impact management plan, including the construction of a 19 km effluent pipeline involving a shore crossing.

Mr Burke’s authorisation of three remaining modules completes the necessary environmental approvals under federal law for the mill.

As part of the project a 19 km effluent pipeline will be constructed to run from the north of the pulp mill site to the shore crossing at Four Mile Beach, with an additional 3 km offshore ocean outfall.

The shore crossing commences at Aerodrome Road and extends through the dunes, the beach and the surf zone.

There are two construction options for the section of shore crossing from the foredune to beyond the surf zone, with the preferred option being horizontal directional drilling (HDD) to a distance approximately 850 m offshore.

HDD of this distance requires a reduction in the diameter of the pipe which in turn requires engineering changes, including an increase in pumping energy and costs. The outside diameter of the pipe is reduced to 710 mm in the HDD section which reduces the HDD borehole required to 950 mm diameter. The pumping head is increased due to the increased friction loss with the smaller pipe diameter.

A pilot HDD hole from behind the dunes to beyond the surf zone will be attempted to obtain detailed geotechnical information and, if the pilot hole is successful, to evaluate the likely success of being able to back ream the pilot hole out to the full 950 mm diameter.

A final decision on the method will be made based on achieving the required vertical and horizontal directional tolerance and the stability and integrity of the drill hole.

Gunns Managing Director Greg L’Estrange said the approval was confirmation that the mill’s design and operation must meet stringent environmental standards.

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