From the magazine, HDD

Going deep with HDD

Just outside of Auckland, a strategic watermain laid through densely covered hills had become problematic for its owner. The cast iron watermain, laid during World War II, was left exposed in many locations, and ground movement and erosion in the region had led to a number of maintenance issues and poor performance.

A proposed new 1 km pipeline would follow a straight line and would be a siphon. The pipeline would have a height difference between ends of 40 m and a low point of 80 m below the high point.

It would cross under five streams, pass beneath bluffs up to 40 m deep, intersect conglomerate gravel beds and pass through sections of faulted ground. The above ground route was covered in dense bush with no vehicle access.

Horizontal directional drilling (HDD) was the proposed construction method, and upon completion, the proposed pipeline would offer a smooth, straight alignment with increased capacity, improved efficiency and security in an unstable environment.

The HDD component of the project was awarded to Universal Underground Limited (UUL). UUL proposed a non-standard pipe size to give the client the exact diameter they required, which provided the most cost effective tender price.

Going bush

A Universal 200×240 drill rig, and a mud motor with a Digitrak magnetic steering tool was chosen for the pilot hole drilling.

The pilot drilling progressed on plan from the low end unit, passing under the third stream near the 600 m mark. At this location drilling fluid started exiting in a wide area in and around the stream in this gully. This gully was later named “÷Tunnel Gully’ due to its close proximity to the tunnel used for the cast iron pipeline. Progressing the pilot hole under this gully led to the drill string becoming jammed.

A heli drill rig was dropped into the bush and two ground investigation bore holes were drilled, revealing substantially weak beds of barely cemented sands. It was suspected that sand falling into the pilot hole had caused the tool to jam. New downhole equipment was ordered from overseas and UUL went about salvaging as much of the drill string from the hole as possible. A company from the oil industry was employed to feed an explosive charge down the drill string and 350 m of rods were eventually recovered. Grout injection was undertaken in Tunnel Gully to help stabilise the formation.

When the new equipment arrived it was straight back into drilling the pilot hole. Pilot drilling progressed 900 m before a hard turn upwards under the last stream was required to meet the exit target 140 m from the end and up a 40 m vertical climb. However, the mud motor failed to achieve the targeted vertical gains and the exit location was going to be over shot.

A small drill rig was established on site and used to drill down the steep embankment and intersect the existing pilot hole nine metres below the stream and 40 m below the small drill rig platform. The difficulty of this manoeuvre was no small feat. The small drill had to land its drill string into a 260 mm diameter pilot hole – like landing an aeroplane. Simply drilling straight to the existing pilot hole would not work. The steep and densely vegetated embankment and depth of the drilling made the operation very tricky. The drill string had to achieve a depth of 18 m before arriving under the stream, on line and grade. Every longitudinal metre had a different grade to achieve as it bent around to eventually match the target grade. A survey and computer bore plan was undertaken for the shot and recalculated every 15 m.

Although over shooting the stream, skilled drill operator Tim Mitchell successfully landed the drill string into the existing pilot hole, creating a 1 km path through the job for the first time.

Reaming challenges

In order to keep the tunnel fluid pressure to a minimum, the drilling rig was relocated to the top platform. Fluid recycling was undertaken at the bottom site, before the cleaned fluid was pumped back to the drill rig at the top site and reused in the tunnel.

The pilot hole needed to be enlarged to over 1 m diameter and this was achieved with three reamer passes. As the reaming progressed, the 850 mm, 15 m long pipes were delivered to the water filter station, where they were welded into two 520 m strings. Each pipe string weighed almost 100 tonnes and manoeuvring the pipe up a windy metal track proved very difficult. Adding to the difficulties, there was a requirement to keep the track open to small vehicles in case emergency works were required at the dam head.

During reaming, the reamer came to a slow but complete stop under Tunnel Gully (coincidently right beside the jammed pilot tools). This did not make any sense at the time and many things were tried to get the reamer progressing but without success. The only remaining option was to trip the tool 500 m out and see what the problem was. The reamer was found completely packed with sand, however it was otherwise in good condition. The reamer was cleaned and tripped back to the face, this all taking about one week. Again, 50 m down the track the reamer again came to stop. Again the reamer was tripped back out the hole and found to be packed with sand. It was cleaned out and pulled back up to the face, costing another week. After clearing Tunnel Gully the reamer progressed without event.

The ground conditions under Tunnel Gully presented an unacceptable risk to the project so on completion of the reaming, six swabbing passes were undertaken.

Tunnel installation

The drill was then moved to the bottom platform in order to pull the pipe downhill. A 1.5 tonne nose cone was fitted to the pipe and connected to the drill string. One bulldozer winch applying 40 tonnes of push on a double purchase was set up at the top platform and two winch trucks assisted the pipe as it snaked down the track. When the pulling/pushing forces started to climb, water ballast was systematically added into the pipe to manage the pipe/tunnel roof friction. The water volume was carefully monitored because if the pipe was over filled, the water would become dead weight in the empty part of the tunnel. The pipe was installed over a five day period, which included stopping at the midway point to weld the two pipe strings together. Surprisingly the force required to install the pipeline didn’t exceed half of the drills capability.

This was an extremely tough project in a sensitive ecological environment. Millions of litres of fluid were used to remove nearly a thousand cubic metres of tunnel material and protecting the environment was always a high priority. This project required a high level of skill and commitment and although this project proved extremely tough, perseverance and determination won in the end.

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