Options 2, 4 and 5: Submarine Pipelines - Taurangaecontent.tauranga.govt.nz/data/projects/files/pipeline/rc13559... · On bottom stability weight ... The pipeline would be assembled

SECTION 8 Options 2, 4 and 5: Submarine Pipelines

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

8 Options 2, 4 and 5: Submarine Pipelines

8.1 Introduction

Submarine pipelines are a common means of conveying fluids through a water barrier such as an estuary crossing, or across the sea from offshore oil wells to an onshore location. As the majority of the installation of submarine pipeline is in the oil and gas industry, the design and construction methodology were mainly developed by that industry. However, it is fully applicable to other pipelines; including sewers across tidal estuaries, such as the Tauranga Harbour.

The following factors have been taken into account in the consideration of the submarine options:-

Physical Constraints

– Type and intensity of shipping and the pressure of anchoring zones;

– Presence of fishing grounds and other sensitive areas;

– Presence of other pipelines and services;

– Presence of wrecks or other obstructions; and

– Presence of regular dredged and dumping areas.

Hydrodynamic and Construction Constraints

– Sediment transport and scouring patterns;

– Seabed instability;

– Seabed currents;

– Installation limitations;

– Tides, valves and coastal erosion; and

– Topography and geology.

Three preliminary submarine pipeline options across the Tauranga Harbour have been considered and are shown in plan on Drawing 12300-G-601-007 page 8-7. Drawing 12300-G-601-008 has also been included as this shows in some detail the expected topography of the proposed submarine pipelines as they follow the level of the seabed across the harbour.

8.2 Hydrodynamic Constraints

ASR has modelled and assessed the hydrodynamic behaviour of the harbour in the vicinity of the crossing options and has carried out a bathymetry survey (seabed surface profile survey). The ASR report covers sediment transport and scour patterns, seabed current, tides and waves. The ASR preliminary report on the hydrodynamic assessment is attached in Appendix F, Volume 2 of this report.

The summary of the ASR report findings relevant to the submarine pipeline options are:



i. The expected wave conditions are small (less than 0.15m in height for 95% of the time with periods less than 2 seconds), the wave induced force acting on the pipeline would therefore be negligible during construction.

ii. The preliminary geotechnical assessment of the seabed along the route of Option 2 suggests a 2 metre layer of sand with an average particle size (d50) of between 0.33 to 0.44mm. The corresponding critical velocity is between 1.3cm/s and 1.4cm/s. Whilst there would be further bed movement due to turbulence at the bed level, the scour patterns indicated in the ASR report (Appendix F) appear to be stable. Therefore if the pipeline were permanently installed either on, or under, the bed along pipeline Option 2, the bed will stabilise after installation.

iii. A stability analysis was carried out based on DNV (Det Norske Veritas) guidelines which are the internationally recognised standards for submarine pipeline design. The design parameters were based on ASR modelled results for seabed currents and bathymetry. The analysis shows that the proposed submarine pipeline route Option 2 is viable and the required pipe details would be as follows:

Pipe size = DN 900, ID 914.2 wall thickness 9.5mm

Pipe material = Epoxy coated mild steel or specially formulated polyethylene pipe.

Pipe external concrete coating = 50mm thick

Corrosion allowance = 3 mm to 5.0 mm

Dry weight = 700kg/m

Submerged weight = 680kg/m

On bottom stability weight = 190kg/m

Approximate Pull Force (working load) = 50 Tonne (which is acceptable).

A plan of the topography of the harbour channel (bathymetry) is included in the ASR report, reproduced in Appendix F, Volume 2 of this report, as well as an indicative long section of the proposed pipeline route (Drawing 12300-G-601-008).

8.3 Hydrodynamic Modelling

ASR was commissioned to model the hydrodynamics of the estuary seabed and in particular the impact of the two of the submarine pipe routes as described in Section 2.0 on estuary bed scour and stability. The details of the model study are attached in Appendix F. The results indicate that the impact of an submarine pipe on the surface for Option 2 on the main channel is negligible and very localised. It is also highlighted that the pipe as the pipeline is proposed to be buried; the impact on the channel would be even smaller.



However there are some concerns with Option 5: Submarine Memorial to Matapihi as there appears to be a potential for mud to build up in the zone between the pipe and the railway bridge should the pipe be laid on the surface (or re-surface after laying due to estuary bed movement). This mud build-up may impact adversely on the inter-tidal zone and alter the biota and has the potential to increase pollutant contamination. As such, Option 2: Submarine and Embankment is considered the better alternative if a submarine pipeline is the preferred option.

8.4 Proposed Installation Technique for Submarine Pipeline

8.4.1 Submarine Pipeline Options

There are generally two categories of construction techniques employed in laying submarine pipelines across an estuary,

a) The pipe is assembled on one bank and pulled into place from the other bank. The pipe pull can be either “bottom” tow or “mid level” tow (namely partially suspended in the water body).

b) The pipe is lowered from a barge (the “float and sink” method).

Due to the cross current velocity of approximately 1.3m/s at the water surface, the “float and sink” method of pipe installation is not recommended.

For this project, the installation would comprise assembling the pipe on one bank (Matapihi side) on a widened railway embankment (the proposed embankment extension is shown on Drawing G-601-003) and pulling from the opposite bank (at First Avenue) using a winch. Using a bottom tow method minimises the risk to the pipeline from surface wind and higher surface currents.

The existing rail embankment on the Matapihi side would need to be widened to provide a suitable area to assemble the pipe and also to minimise the length of the submarine component of the harbour crossing.

8.4.2 Installation Method

The installation methodology is summarised below, with the methodology described in detail in Appendix C.

Two “transition pits” will be constructed in advance of the pipe pulling to enable the transition connection between the land-based pipeline and the submarine pipeline to be constructed. One would be located at chainage 0 metres (First Avenue) and the other at the seaward end of the embankment, at chainage 600 m. These structures would consist of temporary sheet piled cofferdams installed from a barge. These pits would be approximately 4 metres wide and 20 metres to 30 metres long in plan and would be excavated to the proposed pipe invert, using equipment operating from a barge.



The pipeline would be assembled on the causeway on temporary railway lines with trolleys / bogeys to carry the assembled pipe. For the 600 metres pipeline, 12 metres lengths will be welded together to form 340 metres and 220 metres lengths of pipeline prior to pulling across the harbour.

The assembled pipe would then be pulled along the rail into the water to reach the other side of the bank. Suitable rollers or rails are used to assist in the pulling operations. The pulling force is usually provided by a winch located on the opposite bank, attached to steel cables, while a bulldozer may assist in pushing on the assembly side (i.e. Matapihi embankment).

Some preliminary dredging may be required prior to the pipeline installation, depending on the final pipeline design and route location. The dredging will be undertaken using a combination of barge mounted long reach excavator, sand pump and suction dredger.

The pipe is then pulled along the bottom of the seabed. With the pipeline placed in the trench on the sea bed. Some additional excavation will be required to get the pipeline to its design level. This excavation would be undertaken using a combination of barge mounted long reach excavator, jetting equipment, sand pump and suction dredger, in conjunction with the pipeline installation.

It is not proposed to make any special provision for backfilling the pipeline trench as preliminary investigations by ASR indicate that backfilling would happen naturally and relatively quickly by the scouring and deposition action of the flow in the channel.

There is no special requirement for cover depth in terms of scour as even partial burial of the pipeline would ensure that local scouring would be minimised, however a depth of one metre of cover above the pipe to the seabed level is recommended to prevent accidental damage to the pipe (i.e. from boats).

8.4.3 Working Space and Access

At the Matapihi side, access to the pipe stringing area would be along the proposed widened railway embankment. It is envisaged that the pipeline would be assembled in this location. At First Avenue, there is a working area of about 13 metres by 11 metres at the beach level and access is either through First Avenue, which would require temporary construction of a ramp because of a steep slope, or by barge along the waterfront. The winch and related pulling system would be located at this area (Drawing 12300-G-601-007 at the end of this section of the report). A barge may need to be moored in this area for the duration of the construction period (approximately 6 to 9 months) to provide additional working space.

8.5 Constructability Check

A constructability check was undertaken by Brian Perry Ltd (BPL) for the submarine pipeline methodology as outlined in the previous sections. Brian Perry Ltd was commissioned to review the Options 2 and 5 and develop a practical construction methodology, their report is included in Appendix C.



The summary of Brian Perry Ltd’s findings is that the submarine pipeline proposal is viable subject to the risks identified in their report. Many of these risks have already been mitigated by the geotechnical and hydrodynamic investigations carried out as part of the investigations.

8.6 Environmental Considerations

The following is a summary of the main environmental considerations for the submarine pipeline options 2, 4 and 5:-

Traffic:

• No permanent road traffic effects.

• Temporary effects relate to construction phase of 6 to 9 months. There will be a significant increase in traffic along Matapihi Road to supply construction materials for the embankment and pipeline assembly areas.

• Access to the western side of the harbour (at First Avenue where a winch will be sited for Option 2) will require part of the end of First Avenue to be closed off.

Visual:

• Permanent effect will be an embankment in the harbour for Option 2.

• For Options 4 and 5 there is a potential permanent effect of the trench line being visible at low tide.

• Temporary effects relate to construction phase of 6 to 9 months, with a barge moored at the foreshore in First Avenue, a winch in First Avenue and the construction of the embankment parallel to the existing railway embankment.

Noise:

• No long term permanent effects.

• Noise from the construction operation for 4 to 6 months (mostly during daylight hours).

• Noise from the winch and other construction work at First Avenue for Option 2. Driving or placing steel sheet piles at First Avenue and on the embankment side at Matapihi for about six weeks will produce noise but it is expected that this can be mitigated to comply with the District Plan requirements (for Option 2).

• For Options 4 and 5 driving of support piles for trestles will be noisy but occur only during the day.

Dust and Debris:

• No long term effects expected.

• Earthworks in First Avenue will create dust for Option 2. Dust control likely to be required at Matapihi Road.



• Sediment debris will be generated by jetting or trenching the pipeline into the seabed. For Option 5 this debris will be difficult to control.

• Ongoing debris /sediment issues may continue for Options 4 and 5 in the longer term.

Odour:

• No short or long term odour is expected.

Dredging and Marine Aspects:

• No dredging proposed and no effects on boating / marine area expected. There is the embankment construction (Matapihi end) in marine area for Option 2.

• There is a boat ramp 600m from the bridge, but it is rarely used. Although there are no restrictions for recreational boats to anchor, boats are unlikely to anchor, especially in the middle of the estuary where the proposed pipeline will be located. The pipeline traverses an area identified as Moorings Zone in the Regional Coastal Plan.

• There is very little commercial marine traffic in the area where the proposed submarine Option 2 is located. Traffic is limited to recreational boats and kayaks. The speed limit is 5 knots.

• For options 4 and 5 excavation would be required to lay the pipe and trestles would need to be installed. These would to required to comply with navigational regulations.

Documents

Options 2, 4 and 5: Submarine Pipelines - Taurangaecontent.tauranga.govt.nz/data/projects/files/pipeline/rc13559... · On bottom stability weight ... The pipeline would be assembled