Delivering the desalination pipeline across the bay

McConnell Dowell is working as part of the Water Delivery Alliance with Sydney Water to deliver the Sydney Desalination Project distribution network, creating a crucial link between the new desalination plant and Sydney’s existing water grid.

The 18 km pipeline and 250 ML/d pump station will initially carry water from the new desalination plant to the City Water Tunnel at Erskineville. The pipeline traverses Kurnell Bay and Botany Bay, and passes through the inner urban areas of Kyeemagh, Tempe, St Peters, Alexandria and Erskineville.

The pipeline includes 7.1 km of twin 1,400 mm diameter steel pipelines across Botany Bay and 6.4 km of 1,800 mm diameter mild steel cement lined (MSCL) pipe slipped inside 2,100 mm diameter concrete pipes, installed by microtunnelling.

The balance of the pipeline is approximately 4.5 km of 1,800 mm diameter MSCL pipeline installed by dig and lay conventional trenching methods with sheet-piled and trench box shoring. The size of the pipeline has been designed to have a capacity suitable for the desalination plant, with a maximum output of up to 500 ML/d.

Meeting a bay of challenges

The Water Delivery Alliance (WDA) is well placed to deliver the project ahead of schedule with construction advanced on all four disciplines including trenching, pump station, tunnelling and the Botany Bay crossing.

McConnell Dowell, lead by Adam Adamczewski (Botany Bay Crossing) and Steve Norman (Tunnelling), is completing approximately 70 per cent – or 14 km of the 18 km length – of works for the Alliance. The pipeline trenching component is progressing well with 60 per cent in the ground. The final 1.3 km of trenching will be completed in the fourth quarter of 2009.

Since the commencement of construction in January 2008, 200 direct employees and 140 subcontractors have worked on the project. The project is expected to grow and exceed 400 employees as it nears its peak manpower rates in the coming months.

The Silver Beach Compound, on the west side of the bay, is the origin of the tunnelling operation for the crossing of the twin 1,400 mm diameter pipelines across Botany Bay. The 7 km pipelay across the bay commenced at the end of April with a 230 m land pull for the connection to the tunnel.

The scope of work is relatively straightforward with the lay-down of the twin steel pipes across Botany Bay and tie-in via a subaqueous bifurcate to the predriven 1,800 mm diameter tunnel extending 800 m into the bay. The 17.5 mm wall thickness pipes are tri-laminated, cement mortar lined and concrete weight coated. Arguably, this project will become the largest tandem pipelay project ever attempted in the world, making it another first for the McConnell Dowell team.

The purpose built 100 m x 30 m Nebula pipelay barge is fitted with:

  • An eight point mooring system incorporating 60 tonne winches with 7 tonne delta flipper anchors;
  • Two 75 tonne tensioners, one on each production line, which keep the pipe under tension as the pipes roll from the back of the barge;
  • Two 88 tonne abandonment and recovery winches;
  • Two 250 tonne crawlers; and,
  • Various purpose built equipment required for the project.

Each 12 m pipe joint weighs over 30 tonnes and is delivered to the Nebula via a 33 m or 55 m transport barge. Once transported to the Nebula, a hydraulic pipe handling system elevates the pipe joints into the firing line where they pass through two weld stations, an ultrasonic test station, internal cement mortar infill station, heat shrink wrap and foam infill station. The pipe progresses to the 32 m long stinger on the stern and into a pre-dredged trench in Botany Bay.

With an output production rate of 12 m every 2 hours, the project team of 28 staff and supervisors will ensure the process runs smoothly and efficiently, and meets the safety and environmental requirements that McConnell Dowell applies on all of its projects.

The final component of construction – the pump station – is situated on the desalination plant site. With the construction of the pump station and electrical buildings well underway, delivery of the surge vessels and pump motors are expected to arrive in June 2009 from the Alliance’s supporting overseas manufacturers ready for installation and commissioning.

With over 30 sites operating in tandem at any one time, McConnell Dowell’s ongoing commitment to safety is outstanding with all sites recording excellent safety results. The company’s cross-pollination process in all areas makes for good working relationships between team members as well as clean, well organised work sites.

Tunnelling crews utilised “˜Dora the Borer’ (one of three tunnel boring machines) in the tunnelling activities, which received the thumbs up from all involved before the commencement of work.

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