From the magazine, HDD

Big rig in NZ

Project CARE is an initiative to clean up New Zealand’s North Shore beaches by 2021. The second stage of the Birkdale construction program called for some innovative design solutions to allow for the installation of a wastewater pipeline under sports fields, bush reserves, driveways and a garage at a depth of 32 m.

To achieve this, the council contracted Smythe Contractors Ltd to implement the non-disruptive horizontal directional drilling (HDD) approach to install the 950 m long, 560 mm diameter sewer pipeline in one continuous operation.

Smythe, along with OPUS/SKM, worked to minimise consenting processes and disruption to the neighbouring residents by developing a trenchless design solution.

The right tool for the job

Smythe used the largest maxi rig to reach New Zealand shores, the American Augers DD440T, which has 200 tonnes of pullback force and 60,000ft lbs of torque. The rig is ideally suited for the New Zealand ground conditions as its configuration weights in at 50 tonnes and manoeuvres into position on a 25 tonne track frame.

Drill rigs this size regularly perform 1,000 m of 1 m diameter HDPE or steel pipe installations and are well suited for large pipe installations.

A computer-designed drill plan was undertaken to link the bases of three deep manholes spaced from 12 – 32 m along its path.

In addition, the route passed beneath a sports field, a freshwater stream, and between residential properties, and beneath a garage, before arriving at a reserve area 950 m from its origin.

The final pipe alignment included both horizontal and vertical curves which were drilled at depths of over 30 m through the predominant sandstone material found in the North Shore.

Combining new steering resources from Europe, the pilot hold successfully linked the five key manholes over the 950 m interval after some navigational challenges due to local magnetic interference, which caused deviation in the planned course.

Forming the tunnel

The geology of the area indicated the presence of East Coast Bays sandstone of variable hardness. The combination of the DD440T Drill rig and aggressive enlargement reamers were selected to grind through to create an 820 mm diameter tunnel from end to end.

This required large volumes of drilling fluids to lubricate and transport the cuttings to collection ponds. The expansive drill platform area in War Memorial Park allowed for room to construct processing ponds.

General Manager Simon Payne said “These ponds were used in conjunction with fluid recycling equipment to recover as much of the drilling fluids as possible during the drilling, reaming and pullback phases”

Pulling back the pipe

Normally the pipe pullback is a continuous 24 hour operation, with minimal delays or stoppages to reduce the risk of jamming the pipe.

However, the Birkdale contract lacked noise consent and required interruptions to butt weld the pipe strings together. This meant that the pullback hours were restricted from 7 am to 7 pm over a five-day period.

This raised concerns as stopping the pullback for twelve hours overnight increases loads due to settling of drilling fluids and clays gripping the pipe circumference. The delay could also increase the risk when restarting the pullback the next morning.

To mitigate this risk, much emphasis was placed on a drill fluid (mud design) that both retained the hole profile and reduced friction around the pipe circumference.

“We estimated that five days were required to complete the pipe pullback, and due to good hole preparation and mud design, the pullback pressures never rose above their overnight readings,” Mr Payne said.

The full 950 m was installed successfully without exceeding the allowable design loads of the product pipe and using only 30 per cent of the rig’s pullback capacity.

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