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Gilbert Stream Realignment Project

P&P is proud to have supported Firstgas with the Gilbert Stream Maui Pipeline Realignment project. Our involvement with the work to date has included engineering and associated detailed design work. In addition we supported the sourcing and procurement of materials, managed geotechnical engineering, field/laboratory testing contracts, and provided construction contractor support.

Firstgas’ Maui pipeline (400 line) is 750mm in diameter and runs from the Oaonui Production Station (South Taranaki) to the Huntly Power Station (299km), in the North Island of New Zealand.

A section of the pipeline at Pukearuhe (North Taranaki) is threatened by ongoing marine erosion of the 50m high “White Cliffs”. At its closest approach the 400 line runs within 9m of the erosion points near Gilbert Stream.

The realignment project is replacing approximately 400m of the 400 line between tie-ins either side of Gilbert Stream, increasing the separation from the top of the cliffs to at least 25m. The realignment works included:

  • installing a 2m diameter HDPE culvert, approximately 50m long, to a bend of the deeply incised (35m deep) Gilbert Stream gully;

  • constructing a cut/fill embankment with a 15m wide crest right of way across the gully at the culvert;

  • installing the realigned section of pipeline in a trench over its full length; and,

  • constructing bell holes, test headers and temporary works to allow the tie-ins to be completed within the pipeline outage window.

The project involved many challenges and impacts including:

  • the criticality of the 400 line as NZ's largest capacity transmission pipeline, supplying gas to thousands of industrial, commercial and domestic users throughout the North Island including the Huntly Power Station;

  • the weather conditions that provide a very limited window for construction works in any year;

  • the site soil structure, as it has limited bearing capacity;

  • construction resourcing;

  • minimising the impact on the environment and the local wildlife; and,

  • minimising the disruption to the community.

Firstgas has successfully completed the culvert construction, installed the realigned pipeline section, carried out the Hydro-Testing, and prepared the tie-in and commissioning of the new section. The tie-in and commissioning is scheduled for Auckland Anniversary Weekend, 28-30 January 2022, with a projected outage window of ~55 hours.

We are delighted to have been part of this successful project and will update with further news as the project concludes.


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