Offshore operations begin for Rosedale outfall

Large vessels are starting to appear off Mairangi Bay to begin dredging and drilling operations for North Shore City’s Rosedale tunnel and outfall project.

The dredge vessel Kamihia has arrived onsite and is working offshore through next week. It is using a machine to remove sand and silt from the seabed in preparation for connecting the tunnel pipe with the outfall pipe next year.

The large pipelay barge Seatow 80 will arrive on the site next week and begin drilling casing into a 2m diameter pipe sticking 6-8m out of the water. The barge will erect the drilling machine to connect the tunnel pipe to the outfall pipe.

Once the hole is drilled a riser will be installed using the barge. When the tunnel reaches the Mairangi Bay shore next year, the tunnellers will connect to the bottom of the riser. The riser takes treated effluent from the tunnel pipe to the outfall pipe, where it will be widely dispersed 2.8km offshore.

After installing the riser in late November and early December, Seatow 80 is due to begin its pipelay operations and towing in early January, and continue pipelaying until May 2009. The barge will lay five separate pipes, each 420m long.

Currently 600m offshore, Kamihia departs the site later this month and returns to the area in December. Working alongside the barge, Kamihia will begin dredging from that point to 2.8km offshore for about three months.

Once the drilling and pipelay begin, other tugboats and small vessels will arrive onsite to service the barge.

Coast Guard is fully aware of the operations and are given daily or bi-daily updates on the project. The barge crew will monitor radio channel 16 for any marine-related emergencies.

A Notice to Mariners (HO32-09) was issued in early October indicating the marine operations that will be done off Mairangi Bay until March. The Notice has been posted to all surf clubs, boating clubs and marine-based associations and organisations in the area.

The exclusion zone is marked by corner buoys and will soon have flashing lights installed on them. All mariners are requested to remain well clear of the area during operations.

The largest public project ever undertaken in North Shore, the outfall will discharge high-quality treated effluent from the Rosedale Wastewater Treatment Plant, 2.6km to the end of Mairangi Bay, then 2.8km out to sea into the Rangitoto Channel.

Tunnelling is due to begin in late November and the project is scheduled to be completed by June 2010.