To build a windfarm – first prepare the site

Of two windfarms being constructed in the North Island, one will be the largest while the other will have the tallest turbines in the country. Richard Silcock reports on the massive civil works these projects require.

 Mercury Energy is developing a large windfarm along a 14-kilometre ridge at Turitea on the northern extremity of the Tararua Range, south-east of Palmerston North.

At a cost of $450 million it will be the largest windfarm in the country, with a total of 60 wind turbines. It is being constructed in two stages, with the northern section made up of 33 wind turbines and the southern section 27 turbines.

This development is close to two existing windfarms, one on Tararua and the other at the southern end of the Ruahine Range.

Mercury has contracted Vestas Asia Pacific (a subsidiary of Vestas Wind Systems) to supply and oversee construction and maintenance of the development.

“The windfarm will have 3.6 MW and 3.8 MW Vesta turbines,” says Pierre Pretorius, Vestas head of construction for Australasia.

“Once complete and fully operational, Turitea will have the capacity to generate up to 222MW, enough to supply the needs of 118,000 homes.

“The wind resource at this location is regarded as exceptional due to the high average consistent wind speed and it has been rated as being in the top five percent of windfarm sites worldwide.”

Prior to the erection of the turbine towers significant ground preparation work and clearance of vegetation has taken place and considerable earthworks have been required for the tower foundations and the formation of haul roads to transport the towers and turbines to site.

This preparatory work started last summer when Downer was contracted to facilitate and oversee the civil and electrical work.

“Executing the balance of plant work covers a wide range of tasks including design, vegetation clearance, environmental control, cutting and forming haul roads, filling gullies, preparing hard stands for construction cranes, and carrying out the excavation and concrete work required for the turbine tower foundations,” says Marc Papke, Downer’s project director for the development.

Stringfellows Contracting has been sub-contracted by Downers for most of the earthworks and they have nearly completed the northern section having shifted around 480,000 cubic metres of earth and excavated tower foundation footings using GPS controlled hydraulic excavators to precisely gauge location and depth.

In addition, around 32 kilometres of haul roads are being constructed and metalled.

“This was a colossal challenge, especially over the wet winter period,” says Marc.

“The foundation excavations are up to six metres deep depending on the depth to base rock. Once dug a reinforced steel cylindrical cage is lowered into position and filled with 330 cubic metres of concrete and 40 tonnes of reinforcing steel before being backfilled with earth.

“Each concrete foundation is topped with a 15 metre-wide octagon shaped structure on which a tower will be bolted.”

Blackley Construction of Palmerston North, and one other sub-contractor, is carrying out 22 kilometres of trenching and cabling work required for the northern section, with the cabling linking each turbine to a substation that is being constructed by Downer.

Each tower will be 69 metres high, with the overall height from the base of the tower to the tip of a vertical turbine blade reaching 125 metres.

The rotor blades, which are being manufactured in Italy, are 112 metres in circumference when assembled. The towers and turbines, which were constructed at the Vestas facility in China, have been shipped to Port Taranaki and are being hauled to the site by Machine Movers, a division of Tranzcarr.

The 27 turbines for the southern section will have an output at 3.8MW per turbine compared to the 3.6MW turbines that are being used for the northern section.

Pierre Pretorius says this 0.2 MW per turbine increase in capacity helps compensate for the slightly lower average wind speed these turbines will encounter as they will be positioned slightly lower on the hill side.

Mercury and Downer will work with Electrix to connect the power generated from the windfarm to the national grid via a transmission facility which is also currently under construction.

It is expected this wind farm will be completed and commissioned by mid next year.

Taranaki project

The other windfarm under construction is located near Patea in South Taranaki. It will have the tallest turbine towers erected in the country.

The $277 million Waipipi Wind Farm development is being constructed by Waverley Wind Farms, a subsidiary of Tilt Renewables NZ with the groundwork contracted to Higgins Contractors.

The site covers 980 hectares of coastal farmland and will, when completed have 31, 4.3MW Seimens Gamesa turbines generating just over 133MW (enough to power 65,000 homes). Each tower will be 160 metres high and support a turbine and the rotor blades that are 130 metres in diameter.

As these turbine generators, hubs and rotor blades are larger than those used at Turitea a lessor number will be needed to generate the required power.

Pre-construction earthworks started in September last year and Higgins project manager, Jarrod Franklin-Browne, says the company has made significant progress with the earthworks despite the lockdown in March/April.

“We have completed the perimeter fencing, upgraded and widened 4.5 kilometres of road from SH3 to the site, and over 199,000 cubic metres of earthworks, which includes excavations for the tower foundations and 21 kilometres of haul roads,” he says.

“An unusual feature of this wind farm is that it is being constructed on flat coastal land near the sea on what was formerly an iron-sands mine prior to farming, and this has meant paying extra attention to the foundations.

“Due to the nature of the sandy ground each foundation requires dynamic compaction of the surrounding ground or rammed aggregate piles driven into the ground to densify it. This is to ensure there is no movement or liquefaction in the event of an earthquake and we have engaged Contract Landscapes (CLL) to do this specialist ground improvement work.

“In some locations, due to the high-water table we’ve also had to construct structural fill compacted earth hillocks in order to raise the ground level. In some places the groundwater is only half a metre down.

“We have poured all of the concrete foundations and expect to have our part of the contract completed this month [November 2020].”

Higgins had a large team working on the project with a range of machinery including seven CAT excavators, a number of Moxy’s and truck and trailer units. There are also two concrete batching plants on site and all the cut to waste material was used to fill a large pond on the site.

Some 11 kilometres of overhead transmission lines to the Transpower substation at Waverley have been completed by ElectroNet and many of the turbines have also been assembled.

A special crane was brought from Australia with a boom of just over 115 metres to lift each tower in three sections, and the generator and the rotor blades. This was achieved in seven lifts.

This windfarm development is expected to be commissioned by March next year.


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