Drilling to take the pipeline under the harbour from Memorial Park to Matapihi was one of two crossing options once it had been established that attaching the pipeline to the Matapihi railway bridge was prohibitively expensive.
A specialist workforce from 14 countries is racing against the clock on a project to stop sewage polluting Tauranga Harbour in big storms.
The trickiest bit of the $99 million Southern Pipeline project involved 52 people from every corner of the globe harnessing their skills to finish a project essential to the development of the city's booming southern suburbs.
Brian Perry Civil, a division of Fletcher Group, won a $21m contract to drill and then winch a 916mm diameter steel outer pipe to a peak depth of 35m under Tauranga Harbour.
The 1.5km harbour crossing was the most technically challenging part of the project and was due to finish at the end of the year - seven years after construction of the pipeline began at Greerton in 2010.
It would mark the end of waste bubbling out manholes at the lowest point of central Tauranga's sewerage system when the Chapel St treatment works was unable to handle huge volumes of stormwater-swollen sewage.
Canadian Spencer Dryden operates the horizontal directional drilling rig controlling the harbour crossing leg of the Southern Pipeline. Photo/Andrew Warner
Drilling to take the pipeline under the harbour from Memorial Park to Matapihi was one of two crossing options once it had been established that attaching the pipeline to the Matapihi railway bridge was prohibitively expensive. The bridge would have needed rebuilding.
Southern Pipeline project director Steve Wiggill said the council called for expressions of interest for the harbour crossing and then invited three companies to submit a design and build contracts.
It was left to the contractors to decide whether they chose the drilling option under the harbour or the trenching option that buried the pipeline to a depth of three or four metres.
''They all chose the directional [drilling] option,'' he said.
Brian Perry Civil subcontracted the drilling to Lucas Drilling, who in turn contracted a Dutchman now living in Indonesia, Boris de Koning, to head the drilling operation.
''There are some fine pedigrees inside here,'' Mr de Koning said about the team inside the drilling control cab, or dog house. ''They are well-travelled human beings.''
Site view of the works. Photo/Andrew Warner
The driller Spencer Dryden was Canadian while the steering engineer and surveyor Steve Wilson was an Australian.
Mr de Koning said working conditions had improved hugely on 20 or 30 years ago when he would have been standing out in the rain doing the directional drilling.
''On a miserable day like today, the driller and steering engineer had the most envious jobs on the project.''
Other workers were from New Zealand, Scotland, England, India, Italy, South Africa, Malaysia, Zimbabwe, Ireland, Fiji and the Philippines.
The man, whose company was called Borizontal, summed up his philosophy towards his work: ''Good boring is boring boring.''
Drilling the main hole to take the steel pipeline began Monday from the Matapihi side, with the month-long pull through of the pipe expected to begin in about a month's time.
Once the steel outer casing pipe was in place, the much quicker job could begin of pulling through the 710mm diameter inner polyethylene pipe that carried the wastewater.
''It is a double containment system,'' Mr Wiggill said.
If a hole developed in the polyethylene, the steel pipe would catch the sewage in the short time it took for sensors to shut off flows from entering the harbour crossing.
The steel was protected with bonded epoxy on the outside plus cathodic protection by an electric charge to stop the steel corroding. It had a 100-year design life and would stand up to the severe earthquakes.
A lot of work had led up to the commencement of drilling yesterday for the main pipeline at a gentle 14-degree angle down to the target 35m depth before it started going uphill to resurface at Memorial Park.
It included the difficult task of drilling through 15m of very soft silts in order to install a sacrificial steel shell to the depth where the silts firmed up.
Construction of Southern Pipeline at Memorial Park. Photo/Andrew Warner
Mr Wiggill said horizontal directional drilling was simple but very effective and was a tried and true method used all over the world involving drilling the pilot hole and then using a reamer to make the hole bigger and bigger.
''We need the pipeline to keep up with growth and the shape the city is growing. The Lakes and Pyes Pa West did not exist 10 years ago and now it is creating waste.''
The Chapel St wastewater treatment plant had reached the limit of its capacity and they had to get these new flows going to Te Maunga. He said the Southern Pipeline would also give the council more flexibility to manage the overall wastewater system, including the link to the Judea pumping station.
Key tasks August to October of harbour crossing phase of Southern Pipeline - Horizontal directional drilling and welding steel and polyethylene pipes into strings - Pipeline installation 24/7 to keep momentum of steel pipe moving through sediments - Install the connection from Memorial Park pump station to the end of Jordan Field
