9 Option 3: Horizontal Directional Drill 9.1 Introduction...

SECTION 9 Option 3: Horizontal Directional Drill

J:\JOBS\42066678\6000 REPORTS\HARBOUR CROSSING OPTIONS REPORT\HARBOUR CROSSING OPTIONS REPORT NO26 REV J. DOC\5-OCT-06

9 Option 3: Horizontal Directional Drill

9.1 Introduction

Horizontal Directional Drilling (HDD) is a technique for installing pipelines at depth (usually over 20 metres below ground level) across otherwise difficult-to-access terrain. Although a relatively new technique, several wastewater pipes of the diameter and length required for the Southern Pipeline have been successfully installed. The main principals of HDD construction are summarised in this section.

9.2 Site Geology

The site’s geology determines the correct drilling rig choice, drilling fluid composition, tools and techniques, and is needed to identify conditions such as geological faults and gravel. The drill path is then engineered using computer modelling to suit the pipe material, geology and any underground obstacles. Section 3 of this report describes the geotechnical work undertaken to provide the background information required to assess the site geology. A specialist HDD contractor provided advice on the specific geotechnical and geological information to be obtained as part of the geotechnical investigations. Photos and diagrams from the HDD contractor are reproduced with their permission at the end of this section.

9.3 Proposed HDD Layout

Only one HDD harbour crossing option has been investigated in detail in this harbour crossing options report although the pipeline could be drilled to either First Avenue or The Strand and other options could be considered using the same principals as set out in this report. A layout of the proposed HDD configuration is given in Drawing 12300-G-601-010 at the end of this section.

It is proposed that the drilling rig and all support equipment will be sited at the end of Matapihi Road. From here a hole of approximately 150 mm diameter will be drilled down to around 70 metres below mean sea level across the harbour and then up into either First Avenue or The Strand. On the City side sufficient space will need to be available to “string” the pipeline. “Stringing” involves welding 12 metre length (typically) of pipe into longer lengths. Ideally the “longer length” should equal the total length of the crossing (1200 metres in this case) but usually there is insufficient space available for a full length, so segments are assembled. If First Avenue is used the segment lengths will be four lengths of 250 metres. If The Strand is used segment lengths are expected to be 280 metres long, or possibly longer, if more space can be used.

Total set up and drilling time is expected to be 6 to 9 months. The period to occupy either First Avenue or The Strand stringing areas is expected to be 3 months minimum and 6 months maximum. A layout of the construction area is shown on Traffic Design Groups plan 6 at the end of this section.



9.4 Proposed Construction Method

Figure 8- 1 attached is a schematic of the HDD Drilling Process. Plate 8-1 shows the type of drilling rig expected to be used.

9.4.1 Drilling the Pilot Hole

The drilling rig will be set up at Matapihi and accurately align to the drill along the proposed pipeline route. Drilling commences and the fluid (drilling mud) provides a water jet for drilling softer materials, or to provide power to a downhole motor which drives a drilling bit in hard material. The fluid (drilling mud) is normally a mixture of bentonite (a safe natural clay) and water. The drilling `mud’ also helps support the walls of the drillhole, acts as a lubricant and carries excavated material out of the hole. As the mud comes back up the drillhole, it is collected, filtered to separate out excavated material and recycled back down the hole in a closed loop system.

The downhole drilling assembly is guided by electronic instruments that transmit steering data, in real time, to a surface computer. This allows the drill operator to precisely locate the drill bit and guide it along the predetermined pipeline route (usually better than plus or minus one metre).

Complex three-dimensional paths can be followed to avoid underground obstacles.

9.4.2 Reaming the Pilot Hole

For the HDD alignment proposed, the geotechnical investigations have shown that steel casing pipe will be required to be driven or pushed through the upper layers on both sides of the harbour. The sequence of work will be;

• Set up the drilling rig on the Matapihi side;

• Drive a casing on the Matapihi side to approximately 50 metres depth;

• Drill a 150 millimetre diameter pilot hole across the harbour to First Avenue.

• Once the pilot hole has been completed attached a casing to the pilot drill bit and pull and drive a second casing from the First Avenue side down to approximately 50 metres below ground level.

• The drill bit is then replaced with a reamer which is pulled back, or pushed forward, to enlarge the pilot hole, through the casing holes to the size needed for the finished pipeline.

• Depending on the material and final size, several reaming passes may be needed.



9.4.3 Installing and Testing the Product Pipe

After the final reaming pass, the wastewater pipeline is drawn back behind the reamer. Pipes are generally made from high density polyethylene (HDPE) or steel. Once installed, the pipe is pressure tested to ensure its structural integrity and the coatings are tested to ensure there will be minimal corrosion.

9.5 Environmental Considerations

The following is a summary of the main environmental constraints for the HDD crossing option:-

Traffic:

• No permanent effects.

• Temporary effects relate to construction phase of 6 to 9 months. Due to the need to assemble the pipeline into long lengths prior to pulling the pipeline under the harbour, a working area of approximately 250 metre by 10 metre (as shown on the Traffic Design Group (TDG) Drawing No. 6) is required in First Avenue between Devonport Road and Cameron Road. In addition to the construction space shown some unloading of materials in the street may be required, although this could be scheduled for particular times of the day to minimize traffic delays.

• As shown on Drawing No. 6 one lane of traffic in each direction will be available the majority of the time. TDG have advised that although on-street parking will be reduced by 50% (note that not all parking spaces are in use during the day so the overall effect of the 50% reduction may not be significant).

• Footpath access will be maintained.

• Alternative car parking or alternative traffic management will be required at the Matapihi Road end for 6 to 9 months during the drilling operation.

Visual:

• No long term permanent effects.

• Temporary effects relate to construction phase of 6 to 9 months, in First Avenue and at the Matapihi Road end.

Noise:

• No long term permanent effects.

• Noise from the drilling rig at Matapihi will need to be managed and may require a dispensation from the District Plan noise levels as the operation, by necessity, will be 24 hours a day 7 days a week for up to 9 months. The drilling equipment and associated noisy pumping equipment will be housed in noise reduction buildings / covers. There will also be some noise in First Avenue from the day-to-day operation of excavation machinery.



• Driving or placing steel casing pipe into the harbour to allow the pipe to pass through unstable soils will produce noise but it is expected that this will be mitigated to comply with the District Plan.

Dust and Debris:

• No long term or short term effects are expected, because the majority of the HDD operation involves liquids which will suppress dust.

• Mud debris will be controlled as at any other construction site.

Odour:

• No short or long term odour is expected.

Dredging and Marine Aspects:

• No dredging is proposed and no effects on the boating or marine area are expected.

9 Option 3: Horizontal Directional Drill 9.1 Introduction...

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