Boring below - tunnelling at hydro schemes11 February 2015
A number of hydro projects are calling for major tunnelling works. Report by Patrick Reynolds
A number of major TBM drives are underway on underground works on large hydro schemes around the world, with others due to start. Among the many interesting hydro tunnelling projects are two TBM drives - one to start in the near future in Lao PDR, and the other recently completed in Turkey.
In Lao PDR, the upcoming bore is for part of the headrace on the 410MW Xe-Pian Xe-Namnoy scheme.
In Turkey, tunnellers overcame significant challenges with severe ground conditions on the shield drive on the 102MW Kargi project.
Other interesting tunnelling works include further underground excavations in Norway, which has been seeing a re-awakening, perhaps, in hydro tunnelling if not a renewal. Tunnelling is scheduled to start in the near future on the Rosten project.
Lao PDR: Xe-Pian Xe-Namnoy HEP
The Xe-Pian Xe-Namnoy hydropower scheme is being developed in south of Lao PDR by Xe-Pian Xe-Namnoy Power Company Limited (PNPC) under a 32-year concession contract with the Government. The plant is due to be commissioned in 2018, and is expected to generate more than 1800GWh of electricity per year. Much of the output will be exported to Thailand, and the balance used domestically.
Xe-Pian Xe-Namnoy is located on the Bolaven Plateau, in Attapeu and Champasak provinces, and is near the border with Thailand. It is about 80km east of Pakse on the Mekong River and 35km northwest of Attapeu.
Two rockfill dams - Xe-Pian and Xe-Namnoy, respectively - will be constructed on the plateau, along with saddle dams and Houay Makchan dam, while the powerhouse will be located at the base, giving the scheme a head of more than 630m. Water will be discharged to the Xe Kong River.
The tunnel layout from the Xe-Namnoy main reservoir comprises a low pressure headrace tunnel extending towards the edge of the plateau, but shortly before it passes a surge shaft and then drops vertically, the shaft leading to the high pressure section of tunnel. Penstocks take the flow to the surface powerhouse, which will hold three Francis units an a Pelton turbine. From the switchyard, a 230kV line leads to Thailand and a 115kV line to the local grid.
The project developer, PNPC, is a venture of four partners - South Korean firm SK Engineering & Construction, Korea Western Power (Kowepo), Ratchaburi Electricity Generating Holding PCL, and Lao Holding State Enterprise.
The tri-national background of PNPC arises from the project being developed under a power cooperation deal between La PDR, Thailand and South Korea.
Studies for a hydro project on the site were undertaken in the mid-1990s, and the basic concept and layout of the present scheme is relatively similar to what was conceived then, except it is larger in power capacity.
The current project got going almost 10 years ago when a Memorandum of Understanding (MoU) was signed between SK Engineering & Construction and Kowepo, in 2005. The MoU began a process that lead to a chain of further agreements with the different parties that would subsequently form the venture, and others.
A project development agreement (PDA) was signed with the Government of Lao PDR in 2008.
A tariff MOU was agreed with Electricity Generating Authority of Thailand (EGAT) and domestic energy company Electricite du Laos (EDL) in 2010 and 2012, respectively. The concession agreement was also signed in 2012, and a power purchase agreement (PPA) was then signed with EGAT in 2013. About 90% of the project's output is to be generate by the Francis units and exported.
PNPC is developing the project on a 70:30 debt-to-equity basis.
Consultants working for PNPC include TEAM, AF-Colenco and DOHWA.
Main contractor for the engineering, procurement and construction (EPC) contract is SK Engineering & Construction. Kowepo will handle operation and maintenance during the concession period, and Ratchaburi will undertake supervision.
The upper (low pressure) headrace tunnel is 13.7km long and will be concrete-lined. Geology along the alignment comprises mudstone, siltstone and sandstone. The vertical shaft is 550m deep and also concrete-lined.
EPC contractor SK Engineering & Construction has hired s Italian specialist contractor Seli, under a subcontract, to perform the portion of the tunnelling works involving the TBM - which is the upper headrace.
Seli placed the TBM manufacturing contract with Terratec, and the Australian manufacturer produced parts at home as well as Japan before assembly the shield at its facilities in China. Factory acceptance of the TBM took place in early December 2014, and was attended by the main contractor, subcontractor and representatives of Lao government.
The TBM is a 5.74m diameter hard rock double shield with 17" disc cutters and four bucket opening on the cutterhead. The main drive power is 2000kW, maximum rotation speed of the cutterhead is 7rpm and torque is up to 8000kNm which Terratec says is to cope with fractured parts of the alignment.
Where geological problems are met, the TBM specification allows the machine to advance in single shield mode, and it is also fitted with high-pressure emergency thrust capability.
Currently being dismantled, the TBM is to be prepared for transport in early 2015 to Thailand Port. Following transport by road, site assembly of the shield is also to take place in early 2015, says Terratec.
The 80MW Rosten hydropower project is getting underway in Norway with contractor Skanska handling the main civils package, including a total of 5km of underground construction work and the powerhouse complex.
Rosten is being developed by energy utility Eidsiva Vannkraft, which expects the plant to generate more than 190GWh of electricity per year.
The tunnels on the scheme are to range in cross sectional area from 20m2 to 50m2, and
undertaken over 2015-17. The NKr409 million (US$) civils works package also includes a concrete dam.
The plant is to be commissioned by late 2017.
Rosten adds to the short number of new tunnel projects in Norway, and a list that includes Nedre Røssåga and Lysebotn II. Hydro tunnelling may be past its heyday, and prospects are ahead for major underground works for the country's strategic "Green Battery" project, but for now there is a resurgence in the volume of tunnelling, and some interesting projects are afoot.
Boring in tough ground conditions finally ended, and successfully so, last year on the 102MW Kargi hydropower project in Turkey when the 10m diameter Robbins TBM completed 7.8km of tunnel. The remainder of the tunnel was constructed by drill and blast.
Kargi is being built on the Kizilirmak River, in Corum province, by Kargi Kizilirmak Enerji A.S. which is a subsidiary of Statkraft Enerji A.S., itself wholly-owned by Statkraft. The project has a gross head of 75m, and is expected to produce more than 460GWh of electricity per year.
Construction started in early 2011, and the contractor is Gulermak. Key structures on the project include a 13m high by 500m long earthfill dam, an 11.8km long headrace tunnel, and the surface powerhouse near the village of Maksutlu. The plant will discharge into the Boyabat reservoir.
The plant was originally scheduled to be completed in late 2013. Weak volcanic rock, limestone and clay were anticipated in the first quarter of the headrace tunnel, but the majority was expected to be harder marble rock.
Given the distinctly different geology, the TBM was designed to allow for segmental lining in the early stage boring and then allow for switchover to mesh, bolts and ring beams, and a final lining of shotcrete. The TBM also had provision for ground conditioning. The TBM package also includes a continuous conveyor system. The shield was built at the project for the first time using Robbins' Onsite First Time Assembly (OFTA) system.
Soon after the TBM was launched in early 2012, tunnelling conditions were to become more challenging than anticipated with clay and unconsolidated rock, and then a fault zone - which trapped the shield - being encountered. After ground conditioning and digging a bypass tunnel to help free the TBM, the shield pushed ahead through the weak zone.
Tunnelling becomes especially interesting when the technical challenge of the ground is met and overcome, despite difficulties. By mid-2014 the TBM was able to complete its work on the tunnel which had 'complex geological conditions that proved to be a challenge nearly from the outset,' says Robbins.
The company adds that it was 'an impressive run through some of the most difficult conditions ever encountered by Robbins field teams.'
Seven bypass tunnels were needed in the first 2km of the drive to free the TBM each time. The highly variable ground saw the cutterhead facing, and being stopped by, the ground changing from good rock to running ground. Faults were another layer of complexity to the tunnelling efforts.
Having met with such difficult ground, it was decided that the TBM - the design of which was selected by the project owner, consultants, Robbins and the contractor - had to undergo 'major' modifications. With the project being delayed by the poor geology, innovative solutions had to be found, and changes made.
Probe drilling was more extensively used, enabling the contractor to find and size cavities, loose soil seams and fractured rock ahead of the face. Some voids were able to be filled to more than 30m above the level of the cutterhead.
Among the modifications, the contractor and Robbins installed a forepoling installation system that would provide extra support over the tunnel crown. They custom-built a canopy drill and positioner to allow pipe tube support installation through the forward shield. The 90mm diameter pipes extended up to 10m ahead of the face.
Ground conditioning with grout and resins was also employed to strengthen the crown zone when boring through soft ground.
With the range of modifications in place, the progress rates achieved by the TBM improved notably. Rates of 723m per month was obtained in early 2014, and a best day of almost 40m.
Progress rates achieved on the 4km long blasted length of tunnel were nearly 300m per month.
In addition to Kargi, Statkraft is developing a number of hydropower projects in Turkey, including the 517MW Cetin scheme on the Botan River. The scheme comprises plants of 401MW and 116MW capacities, respectively. Cetin was bought by Statkraft as part of a package along with Kargi and 20MW Cakit in 2009. The Cakit plant went into operation in 2010.
Turkey: Yamanli II
While the entire construction team faced, and overcame, significant tunnelling challenges at Kargi, the TBM manufacturer also had excavation challenges - though not as severe - on another hydro project in Turkey: 82MW Yamanli II.
The TBM drive involved a much smaller shield - 4.3m diameter - on a 9.3km long headrace for the project being developed by Enerji SA. The contractor, NTF Construction, bored through mainly limestone which had some clay-filled fissures, and met two fault zones. Ground treatment ahead of the face helped progress.
While the two tunnelling projects were underway at the same time, boring on the Yamanli II headrace was completed in late 2013, and soon after the TBM at Kargi was achieving its best progress rates.