Bhutan seeks bids for next major hydro tunnels 22 Nov 2018

Patrick Reynolds for TunnelTalk

Bids have been invited across three contract lots for major tunnel and underground power station works at the Kholongchhu hydro project in Bhutan.

A pre-bid meeting will be held on 6 December with Kholongchhu Hydro Energy Ltd (KHEP), the development company jointly established by India’s Satluj Jal Vidyut Nigam (SJVN) and local company Druk Green Power Corporation (DGPC) Bhutan.

Layout of Kholongchhu HEP in Bhutan
Layout of Kholongchhu HEP in Bhutan

Deadline is 17 January 2019 for the tenders, each of which must include some local contractor involvement. The bids will be opened on the same day as submission. Bids are allowed on up to two of the three lots.

Underground infrastructure for the Kholongchhu hydro project is comprised of: water inlet and sediment settling tunnels; a 15.8km long headrace conveyance tunnel; a surge shaft; penstock high pressure tunnels; the large powerhouse complex; and, the tailrace discharge tunnel. Numerous access tunnels are also required along the length of the project.

The underground works have been split across three lots:

Lot KC-1

The lot involves the upstream civil works. Key structures are the dam and a number of tunnels, including a 10m wide diversion tunnel, two 4.7m wide intake tunnels, two desilting chambers (each 350m long x 13m wide x 17.5m high) with their main and branch silt flushing tunnels, and also the upper 2.3km of the 5.7m wide, horseshoe-shaped headrace tunnel. The works call for a 6.8m wide access adit and must be completed within 58 months, including mobilisation time.

Lot KC-2

Located in the middle of the project, the lot comprises tunnel works entirely, principally to excavate the majority – 11.8km – of the headrace tunnel down to near the surge shaft. The lot also involves excavating four 6.8m wide adits, and a further inflow complex, at Chaplangchhu, with structures including a 10m wide diversion tunnel, a feeder tunnel, desilting chamber, and a 75m deep drop shaft with associated inlet and outflow tunnels. The completion period is 56 months.

Lot KC-3

Covering the downstream, the lot focuses on the powerhouse complex, featuring the 123m long by 21m wide by 42.5m high machine hall cavern, and the 130m long by 16m wide by 24m high transformer hall cavern. The contract package also includes tunnels feeding water into the powerhouse complex from upstream, such as the 153m deep by 9.5m diameter surge shaft, valve chamber, and pair of 1200m long by 3.3m diameter penstock tunnels. The tunnels taking the discharge flows from the cavern complex are four 605m long drainage tubes that join the main, 1521m long, 6m x 6m tailrace tunnel. Contract completion is 59 months.

Major contractors interested in lots KC-1 and KC-3 are to either partner with local firms or hire them to perform works of at least 20% of the contract value. The middle lot, KC-2, will see the winning bidder working with a local firm nominated by the Bhutanese Government.

The construction licence for Kholongchhu HEP was obtained in 2015, when the development company was established. The concession agreement is going through an approval process with the Bhutanese Government and while contract procurement was originally set for 2017 it was rescheduled. The power plant is expected to generate almost 2570 GWh of electricity annually and the developer hopes to gain carbon credits for the hydro scheme, helping to improve the financial profile of the 600MW high-head project which is expected to be funded with 70% debt borrowings and 30% equity input.

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