The drillers' challenge at Venda Nova

Drilling contractors using Atlas Copco equipment are hard at work helping Portugal to meet its future electricity needs.

In the far northern reaches of Portugal, some 75 km north-east of Porto, a city famous for its port wine, the country is about to make good use of another powerful liquid: water.

The Venda Nova Dam is situated on the river Cavado in Ruivães-Vieira do Minho, and in a country that imports about 80 per cent of its primary energy fuels, hydroelectric power has become the primary source of local energy.

As the need for electricity has increased, the utility that owns the dam, local electricity utility Companhia Portuguesa de Produção de Electricidade (CPPE) decided on a rebuilding project requiring the excavation of 300,000
cubic metres of rock.

The project includes a 2.8 km inlet tunnel, a 1.4 km discharge tunnel, one huge powerhouse cavern, a 625 m-long ventilation tunnel, a 1,210 m access tunnel, a 130 m water intake and two raisebored shafts of 415 and 110 metres.

Important project

The project is an important one, as it will bring on stream two new 93 MW turbines at the dam and provide much-needed power for the national grid.

What is more, the plant will be able to act as a pump storage station between Venda Nova and its companion dam Paradela further downstream. This means that when electricity demand is low, water that has passed through the powerhouse for storage at Paradela can be pumped back through 4,000 m of tunnels to the Venda Nova Lake to provide extra power at peak periods.

Rui Manuel Ferreira Carrito, Director General of the Venda Nova Consortium: "Atlas Copco is the only company that can provide a complete service."

A consortium of three Portuguese companies has been formed to complete the civil works of the Venda Nova upgrade - Somague, Moniz da Maia Serra & Fortunato (MSF) and Mota & Companhia - and its managing director Rui Manuel Ferreira Carrito, spoke about the rock conditions at the site.

"We have been aided by favourable local geology, as we have mainly granite and, although there were some areas of schist and zones with fractures and faults, geological problems were never a factor in the project," he says.

Reliable equipment

During drill and blast operations, the rock is classified round by round and the areas requiring the least support have three to nine bolts per metre. Steel arches are used to support the weakest rock.

The consortium is relying on equipment from Atlas Copco for most of the drilling and tunnelling work. For example, 3-4 m-long Swellex rock bolts are used in the tunnels. Rocket Boomer rigs are used for investigation and excavation drilling. Only 20 per cent of the tunnel requires a concrete lining as final support.

Towards a better power supply for Portugal: Checking the reinforced tunnel walls at Venda Nova.

At the face: The Atlas Copco Rocket Boomer L2 C in the tailrace tunnel.

Time savers

For drill and blast excavation of the tunnels, the consortium invested in Atlas Copco drill rigs. Somague has one Rocket Boomer L2 C with ABC Regular, while Mota & Companhia has another Rocket Boomer L2 C with ABC Total as well as a single boom Boomer 281 rig for tunnels with smaller sections.

"We have a new generation of machines with the Advanced Boom Control system for automatic drilling," says Mr Carrito. "It gives more accuracy, the overbreak is less and it saves the time and cost of spraying concrete."

He points out that the rigs can start the next job only when the current one is finished - and that's why Atlas Copco's ongoing maintenance support is so important.

Right from the start, Atlas Copco has been providing training support for operators who were not skilled in working with the rigs, and Mr Carrito adds: "At the start of the project, a drilling round was taking five or six hours. But by the end, the time was down to less than two-and-a-half hours."

The relationship between Somague and Atlas Copco goes back some 40 years. Mr Carrito comments: "We use a lot of Atlas Copco equipment. They are the main company in Portugal for drilling, mining and civil works and the only company that can provide a complete service from the beginning to the end of a project.

"We also use their Swellex bolts, grout equipment, grouting tools, grout registers, drilling materials and rock drilling tools - for which we have a contract with Atlas Copco that guarantees a stock of all the materials we need on site.

"We can always ask for advice on what equipment to use, what kind of lifetime figures we can expect from the rods, bits and shanks, how many metres they should last in different conditions, and so forth."

View enlargement of the illustration

The ROC D5 surface drilling rig gets a wash and brush up between shifts at the Venda Nova site.

Raising expectations

One of the most challenging aspects of the project was the development of a 110-metre escape and ventilation shaft, with a decline of only 26 degrees, from the turbine hall to the existing ventilation and escape tunnelling system.

Drillcon Iberia Lda, a subsidiary of Drillcon AB of Sweden, was responsible for the operation. Primarily a cor e drilling company, Drillcon is involved in a number of different projects in Portugal and Spain where raiseboring is being done.

Drillcon Iberia used a Robbins 73R raiseboring machine to develop the shaft. A Secoroc RRL 3.5 metre reamer was flown in from Australia especially for the job. A total of 18 new cutters were used on the reamer: nine 5-row cutters and nine 4-row cutters.

The raisedrill was mounted on a concrete platform in the escape tunnel. The pilot hole was drilled down to the turbine hall and then the tunnel was reamed upwards, back to the escape tunnel.

"The cutters performed perfectly," says Adriano Barros, Director General of Drillcon Iberia Lda. "The mucking was very easy. We put a little water in the bottom to alleviate the problem of dust and then used water pressure to assist with flushing the muck out during reaming."

The rock at the site was very good, fairly hard granite of 170 MPa. The raise was completed in May, 2001 and the deviation on the hole was 40 cm for the entire length of 110 metres - less than 0.5 per cent - which is described as an "outstanding" performance on a shaft of that length and at that incline.

José Manuel Cerqueira, Site Manager (left) and Zacarias Pereira da Silva, Mechanical Engineer from Drillcon Portugal in the 110 raise at Venda Nova, completed with a deviation of less than one per cent.

On-site support

"The support we received from Atlas Copco was the best part of our co-operation," says Mr Barros. "They have helped us with every step of the process, from training drillers on the Robbins unit, to helping us install the new cutters on the reamer. The service has been outstanding.

"When we were finished with the hole, Atlas Copco personnel were on hand to show us how to re-service the reamer, maintain all the cutters and
re-grease everything. We are more than satisfied with their help. It has been a great co-operative effort."

Drillcon Portugal is also involved in several other projects in Portugal where only Atlas Copco and Secoroc products are used. At the Somincor Mine, two Robbins raisedrills are permanently on-site, drilling approximately 2,500 m per year.

For bench drilling, Somague has a ROC 748 drill rig and MSF has a ROC D5. Both were used extensively in the turbine hall, which represents almost a third of the project, and excavation was completed in September.

Mr Carrito says: "We used three phases. We had a drift on the top gallery and two side drifts with a rock pillar in the middle for support.

"The cavern roof was bolted using one 9 m-long bolt per square metre - thanks to a BSH system on one of our Rocket Boomer rigs. Otherwise, we would have had to do it manually and the drilling would have been overhead."

Steady progress

Bolting was followed by shotcreting and the central pillar was removed. The rest of the cavern was blasted and mucked out via the access tunnel. Then the cavern walls, requiring less support, had a 6 m bolt installed in each 2x2 m area.

Meanwhile, with trained operators and reliable backup, the work is steadily progressing at three faces and averaging 80-90 m per heading and month with a site record of 110 m. The new plant and tunnel system is scheduled to be in operation in 2004.

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