All in a days work

The new 90MW Kawerau geothermal power station

By Jenny Baker

They did it ahead of schedule, under budget, and with such precision that the plant’s production capacity exceeds expectations. The contractors who built Mighty River Power’s new Kawerau geothermal power station turned out a world-class performance on a mammoth project.

Contractor, Sumitomo Corporation officially handed over the new 90MW Kawerau geothermal power station to owner Mighty River Power Limited in late August. The NZ$300 million renewable energy producer, outside the town of Kawerau in the Bay of Plenty, is the largest single geothermal development in New Zealand in more than 20 years.

It boosts New Zealand’s geothermal capacity by 25 per cent, significantly increases overall national generation capacity, and meets approximately one-third of residential and industrial demand in the Eastern Bay of Plenty region.

Kawerau connected and started supplying power into the national transmission grid in late June, six weeks ahead of schedule and under budget. In addition, during the generation testing and performance monitoring period it consistently generated electricity in excess of 100MW, compared with the 90MW specified.

Hawkins Construction project manager, Bernard Hill says as subcontractor to Sumitomo – responsible for civil and building work – it has been a major team effort for Hawkins to achieve this outcome. “Our commitment was to use as much local labour as possible. We had a team of about 130 who had to do most of their work over mid-winter … the hours were long and conditions were often harsh, but despite this and complex technical challenges our team achieved a high-quality result.

“The tight time constraints tested all the companies involved, and the successful outcome is the result of the close working relationships the contracting teams and the client working on the project developed,” he says.

The power station received resource consent in 2006. In July 2006 Mighty River signed an Engineering, Procurement and Construct contract with the Sumitomo Corporation of Japan to construct the plant and take responsibility for the works inside the boundary fence. Sumitomo subcontracted the majority of the design, supply, and installation of the generating equipment to Fuji Electric Systems.

In November 2006, Sumitomo awarded the subcontract for the design and construction of the architectural, building, civil, and structural services elements of the project, including consent, execution of site and building works, and procurement to Hawkins Construction. The job included the site development, foundations, civil works, and architectural and building services for the powerhouse and associated infrastructure, including the cooling towers.

How geothermal works
The Kawerau geothermal field is considered a world-class resource capable of sustaining further development. It is situated in the Kawerau geothermal field in the Taupo Volcanic Zone.

The first large-scale geothermal plant in the world was commissioned at the Larderello dry steam field in Italy in 1911. The second was commissioned in 1958 in Wairakei, outside Taupo. Wairakei was also the first plant in the world to exploit a wet geothermal resource.
The Kawerau geothermal dual flash plant uses geothermal fluids from six two-phase wells sunk 2000 metres into the reservoir to drive a single Fuji turbine to generate electricity. High pressure (HP) geothermal fluid travels cross-country from the wells in one three-kilometre two-phase production pipeline to the plant.

The two-phase fluid is flashed, or separated twice from residual water to produce HP and low pressure (LP) steam to feed the turbine, which in turn powers the generator. From there the steam is condensed in a condenser vessel and feed to a cooling tower. The residual water and some of the condensate is feed back into the reservoir via brine and condensate pipelines.

Cementing good relationships
The design, supply, and placement of approximately 9,000 cubic metres of concrete formed an important element of Hawkins’ involvement. This included 5,500m3 concrete containing fly ash, 1,000m3 off-site concrete, and 1,500m3 below ground level concrete containing xypex. The first 5,500m3 of concrete poured onsite, including piling, was placed in the first nine months.

Hawkins engaged Beca as the sub-consultant responsible for civil, structural, architectural, geotechnical and fire engineering design services for the site development, civil infrastructure and structures.

For specific advice on concrete mix designs, Hawkins engaged Michael Khrapko of CBE Consultancy, with Frank Papworth of Building & Construction Research & Consultancy (BCRC) also assisting with advice on thermal calculations and monitoring of large mass concrete foundations. Firth Concrete supplied the concrete and Tracks Concrete from Whakatane placed the majority of concrete on site.

Hill says the concrete works called for the team to think on its feet. Half a dozen key structures, including the cooling tower and all concrete elements 3m below ground, had to be water resistant. The 3,300m2 cooling tower slab posed several problems for the placement and finish of the concrete, given the potential for both plastic cracking and plastic shrinkage cracking and the need to monitor and control bleed water.

He says it took some time for the all the placers to identify the appropriate periods to complete the operations of bull floating, power floating, and placement of Sika anti-vap and curing compound.

“And then prior to completing the basin concrete Mighty River decided to epoxy coat the wall and floor surfaces to inhibit the possibility of microbiologically influenced corrosion. This necessitated the grit blasting of the completed concrete to achieve a bond, but well, it was all in a day’s work,” he says.

The majority of concrete was poured between April and December 2007, with an average of 50m3 concrete supplied daily. The team had to monitor the mixes closely, as simultaneous pours in various locations – with differing mixes – were common.

Team members also had to monitor and attend to the completed pours throughout the frequently cold, windy, and wet winter nights. “But both placers and supervisors came to respect the various mix designs throughout the project, as superior concrete finishes were achieved with well-compacted concrete – particularly ‘off-the form’ finishes,” Hill says.

Solid support from Beca
Beca’s scope of work included the architectural, civil and structural design elements. These included the design of the turbine hall building, including provision for the overhead crane and turbine erection loads, the turbine and generator table support structure, and a 160m-long by 20m-wide reinforced concrete cooling tower basin water retaining structure.
In addition, the Beca team designed various reinforced concrete shafts and pits up to 12m deep and the steam field equipment and balance of plant and pipe rack elevated support structures, including in ground cable and pipe trenches, pits and pad foundations.

The team also did extensive site development, including roads and general site drainage, the infrastructure necessary for safe access to equipment, storm water and thermal water retention ponds, potable water reticulation, and onsite sewage treatment systems.

The turbine hall comprises two separate structures. The tabletop supports the turbine and generator, and is separated from the main building for vibration reasons. The turbine hall is a large multi-level steel K braced structure supporting a 60-tonne overhead gantry crane.

The generator and turbine together weigh 325t. A substantial 10m-high tabletop structure supports this equipment and resists gravity and out of balance plant and seismic loads. A 1.6m-thick piled raft supports the turbine support table. In addition, 65 20m to 30 m deep piles, required to overcome site ground conditions including liquefaction effects, support both the turbine hall and tabletop structures.

To deal with the corrosive geothermal environment particular attention was paid to the durability of all concrete and steel building elements.

Beca project manager, Hamish Brookie says a key challenge was gaining an understanding of the plant processes in a very short time frame, in order to produce a fully integrated design that would address the complex needs of the generating equipment and balance of plant. Beca used the 3D design tool Revit to assist in the design process, enabling them to produce ten staged building consent packages, required to meet the agreed project deadlines.

“The close working relationship between the Hawkins Construction, Fuji and Beca teams helped us successfully deliver an exceptionally complex project in a world record time frame,” he says.

Cool work, flash work, coping with pressure
New Plymouth-based Meco Engineering built the station’s 10-cell cooling tower. Managing director, Ian Lind says the Meco team appreciates the level of commitment of staff and contractors involved in this project.

“The project holds national and international significance for Meco, proving our capability with major construction and being able to match extensive demands for labour, expertise, equipment and machinery,” he says.

Page Macrae designed, fabricated and installed the 50t, 12.5m-high 6.5m diameter flash tank, the 18t high-pressure scrubber vessel, and the 10t low-pressure demister vessel. The company fabricated and installed two 4t air receivers as well as 400t steam piping.

In addition it installed several major components of the plant, including the 170t each condenser and generator and the 180t steam turbine. At the peak of the 11-months job, the company had 95 workers onsite, who worked a collective 170,000 man-hours.

Robt Stone – a business of Tenix Alliance – supplied the main pressure vessels and designed and constructed the cross-country steam gathering pipelines. The team built the two LP and two HP strainers, the two 38t LP separator vessels, and the 80t HP separator in its yard in New Plymouth. Multi-Trans Ltd transported the separators to Kawerau.

Project manager, Ross Eden says the 80t colossus is the world’s second largest geothermal separator.

The pipeline project involved building and laying 7km of 24,” 16” and 14” brine line for re-injection of the remaining geothermal fluids and two-phase production line from the wells to the power station.

Eden says the team faced several construction challenges. One was the construction of a 1.8m-diameter 80m-long culvert across a rail yard and State Highway 34 and the installation of the 24” pipe within the culvert.

Another was the installation of the 24” pipe across McKee Road overbridge over State Highway 34. The 52m-long pipe was installed in one single lift after having been fully welded, insulated, and pipe shoes pre-installed. The pipe was installed with only two separate five minute road closures of State Highway 34. No underground services were damaged during the construction.

United Group from Auckland did the electrical installation works. A Mighty River spokesperson says as with many of these types of project the electrical works are the last trades to get access to do the installation. “In spite of the multiple trade activities on the site, the United team integrated well and through good cooperation were able to maintain the accelerated schedule,” he says.

The road forward
Mighty River Power believes geothermal generation plays a key role in New Zealand’s energy future.

Kawerau is part of the company’s geothermal exploration and development programme, focused on looking at a wide range of renewable generation options to meet New Zealand’s growing demand for electricity. Mighty River says it is the first of a possible four major geothermal projects it plans for the coming five years.

The company recently announced construction of a second $450m, 132MW plant at Rotokawa, called Nga Awa Purua, near Taupo.