Tough success at Dariali hydro scheme

14 April 2015

TBM boring was recently completed despite challenges on Dariali hydro scheme, in Georgia. Report by Patrick Reynolds

Tough underground construction for the 108MW Dariali hydropower project in Georgia was successfully completed recently when the TBM broke through to finish boring on the headrace drive.

The Robbins hard rock shield holed through in late October 2014, much of three years after its launch to bore the 5km long, 5.5m diameter headrace tunnel in north east Georgia.
Difficult conditions - from tunnelling to the region being landslip prone - led to many construction challenges for the project team. But with works below ground having achieved milestone completion of excavation, the project is advancing towards commissioning.

Separately, and unrelated to Dariali - and beyond excavation equipment such as TBMs and jumbos, developments in tunnelling support systems over the last year have included a couple of new loaders arriving on the market for mucking out.

While their primary market is mining, the large loaders - the LH204 from Sandvik, and the Scooptram ST18 and Haggloader from Atlas Copco - could offer fresh possibilities for major tunnelling projects, such as underground works on large hydropower schemes.

Georgia: Dariali HEP

The Dariali run-of-river scheme is under development on the Tergi River, near Stepantsminda in the Kazbegi region, in north east Georgia, close to the border with Russia.

With a static head of 390m, design discharge of 30m3/s-35m3/s and average discharge through the station of almost 22.5m3/s, the plant is expected to generate more than 500GWh of electricity per year when operational.

Key infrastructure on the Dariali project include:

  • headworks: small dam (overtopping concrete structure) impounding a 7000m3 pond;
  • headworks: design flood is 515m3/s (1-in-200 year event);
  • headworks: sand sluices (2 x [6.0m x 6.0m]), to also be used at gates spillway in floods;
  • pipe (326m long, 3.6m dia) to sand settling basin;
  • sand settling basin (112m long, 40m wide, 6m deep; structure will have two chambers);
  • pipe (1758m long, 3.6m dia; full and pressured) running from basin to power tunnel;
  • headrace tunnel (5042m long, 5.5m dia, grade 6.21%), plus adit tunnel (510m long);
  • surge shaft (315m height, 3.5m dia);
  • steel-lined pressure shaft (55m vertical length; 3.5m excavated dia; 2.9m steel lined dia)
  • underground powerhouse (71m long x 13.5m wide x 28m high);
  • cable tunnel (510m long, 5.5m dia)
  • tailrace tunnel (5.0m x 5.0m) and canal (125m long, 5.0m bottom width)

The power plant is to house three vertical Pelton turbines, each 36MW capacity, and three generators (45MVA). The underground facility is reached by a 330m long access tunnel (5.5m wide x 6.0m high)

Earlier concept plans for the scheme includes a 1.6km diversion canal rather than a pipeline, a slightly longer headrace tunnel and shorter tailrace tunnel.

About 50 people are to be employed in the scheme once it is in operation. The project lifespan is about 40 years.

The general area suffers from landslides, some related to the Devdoraki glacier.

Project development

Dariali is being developed as an independent power project (IPP) through Dariali Energy Ltd, a venture involving four firms - Georgian private companies Peri Ltd, Energy LLC and state-owned Georgian Energy Development Fund (GEDF).

In 2011, Dariali Energy Ltd signed agreements with Landsvirkjun Power - the international consultancy arm of Iceland's state power company Landsvirkjun - and another Icelandic firm, Verkis. They performed a feasibility study, tender design and document preparation, and the detailed design for the scheme. The consultants reported in 2012.

As part of the works, the Icelandic firms said they also concluded a sub-consultancy agreement with Peri, and that Tbilisi-based firm Geoengineering Ltd and Geographic Ltd were providing site investigation and topographical survey services, respectively.

The developer says the site offers a relatively stable, year-round generation, although about 70% of the power will be produced during the summer months (May-October). The consultants said that the river is fed by rain and groundwater, and meltwaters from glaciers and snow lead to the mid-year higher flows. Lowest flows are about February.

They add that the available run-off records covered 1928-40 and 1953-90. Key tributaries to the section of the Tergi feeding the plant include the Snostskali, Baidara, Mnaisi, Suatisi, Gimara, Desikami, Amali and Kistura. Mean river flow at the intake is almost 25m3/s.

The Icelandic firms' contract engagement on Dariali followed previous consultancy services on renewables in Georgia, and undertake on behalf of the UN Development Programme (UNDP) over 2010-11 as part of its Global Energy Fund Project. The work focused on small hydro, both for plant rehabilitation and pre-feasibility studies for new projects.

In 2011, the Environmental and Social Impact Assessment report for the proposed project - undertaken by other consultants, Gamma and Stucky, respectively - was submitted to the developer.

Other consultants involved in the project include: Fichtner, which submitted a technical due diligence report in 2013; and, Arup, which reported on aquatic biodiversity conservation, also in 2013.

Other project developments in Georgia
GEDF, one of the partners in Dariali Energy Ltd, was set up by the government to invest in and support the development of the Georgia's renewables sector. Looking beyond Dariali, therefore, the fund is promoting development of other project, such as Tsablari small hydro scheme.

In September 2013, GEDF hired Landsvirkjun Power and Verkis to provided consulting services to the Tsablari project which is being developed on the Tsablarislali River in the west of the country.

In terms of infrastructure, the scheme has been envisioned with a headworks, sand basin, a headrace tunnel at least 5km long. The powerhouse is expected to be about 15MW, take a design flow of approximately 6m3/s and produced an output of around 63GWh of electricity per year.

Landsvirkjun Power and Verkis are also working on plans for small hydro projects to be developed on the Machakhela River in south west Georgia. The projects are expected to be about 20MW-25MW capacity, and each requiring infrastructure that includes a river intake, concrete dam, headrace tunnel, and surface powerhouses with rated discharged of about 20m3/s and holding a pair of Francis units.

Funding support for Dariali HEP

In late 2014, the European Bank for Reconstruction and Development (EBRD) announced it was arranging a US$80 million syndicated loan to the developer, and noted that Dariali is 'one of the few privately-owned greenfield hydropower plants in Georgia.'

EBRD commented that the country's energy resources and demand centres are at opposite sides of the country - two-thirds of the generation possibilities are in the north west while two-thirds of domestic demand is in the east. Consequently, at peak hours of demand the grid network has stability problems.

Dariali, located in the east of the country, will help improve grid stability as well as provide energy. The developer said the scheme comprises the first project financing and first successful public private partnership (PPP) in Georgia's energy sector.

In a statement, Dariali Energy said the project 'provides a demonstration of new financing methods to other project developers in the market.'. It The adds that the project 'has the potential' to be the country's first energy project to deliver carbon neutral construction, and is investing in reforestation as part of the overall development to help do so.

The EBRD-led loan package is split between the bank, providing US$40 million, and the other half comes through a combination of contributions from Dutch development bank FMO (US$30 million) and, locally, the Green for Growth Fund, Southeast Europe (US$10 million). The funds are to support the entire development as well as operations of the project.

The funding was approved in May 2014, the loans were signed in August and the arrangement announced in November, shortly after TBM boring had been completed.

'This investment demonstrates our commitment to invest in renewable sources of energy to improve Georgia's energy security and mitigate the impact of climate change,' EBRD's director for power and energy, Nandita Parshad, said in a statement announcing the funding in November 2014.

Funding support from EBRD comes on top of the bank's previous backing for the Black Sea transmission line, which will link Dariali to the Georgian power system.

EBRD's director for the region (Caucasus, Moldova and Belarus), Bruno Balvanera, added: 'This will help to strengthen not only national but also regional energy security.'

Headrace tunnelling

Some 160km from Tbilisi and at an altitude of more than 1700m, the project site often gets blanketed by snow and temperatures in the mountains dive far below zero in winter, made worse by wind chill. The snow melts in spring onwards, and in the following months there can occasionally be landslides in the general area, or closer.

To reach the remote site, however, the Main Beam TBM had to start its journey from New Mexico, where it was stored in parts - the dry, hot climate being excellent for preserving the steel - and then some key parts were rebuilt (gearboxes, motors and clutches) to enable shipping to commence by August 2011.

The TBM was originally fabricated for another hydro scheme - the Vomano project, in Italy, in the late 1980s. Following the European contract, the TBM then underwent its first major refurbishment to then undertake its second tunnel drive, on the 79th Street Tunnel project, in Chicago.

Arriving at the port of Poti, in Georgia, the major TBM components were refurbished in Peri's workshop and then trucked along narrow winding roads, then dirt roads, to site for shield assembly. Early excavation was already underway on site, and the TBM was launched in February 2012. Spoil removal was by a conveyor system.

A spokeswoman for Robbins said: 'The diameter of the machine remained the same as that used on the Chicago tunnel, but all of the components underwent refurbishment.'

By April the shield was already 300m into the headrace bore, which would take it through slate, sandstone, limestone and malms. But there would also be weak and fault zones to cross, and tunnelling became a greater challenge.

'The geology was very difficult - fractured rock and fault zones slowed production.'

Approaching completion of the bore through 'incredibly difficult ground, a further challenge then arose when access to the exit portal area was blocked by a landslide. The large slip also blocked access to the main highway, and led to some weeks of additional delay.

However, after all the challenges, the TBM holed through on 23 October 2014 to finish the drive and reach open air once more.

Dariali 3 Robbins TBM prepared for launch at Dariali. Courtesy Dariali Energy
Dariali 1 TBM work underway at Dariali hydro project. Courtesy Dariali Energy
Dariali 2 Project area at Dariali HEP, Georgia. Courtesy Dariali Energy
Robbins 1 TBM breakthrough at Dariali. Courtesy Robbins
Robbins 2 Celebration at end of TBM bore. Courtesy Robbins

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