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Energy NZ joins Gordon Fletcher (pictured) on the eve of his retirement, as he reflects on a career touching on half a century in hydro generation and the daily work routine at one of the country’s most interesting hydro projects.
It snakes wildly between the boulder-strewn Takaka River and the sides of the precipitous Cobb Valley. After passing the Cobb Power Station, the narrow road climbs steeply to the crest of the range to the Cobb Dam. “Proceed at your own risk,” a huge road sign warns. Should you meet a 4WD towing a fishing boat or, God forbid, a wide-eyed tourist wrestling a campervan around the hair-pin bends, you hope that it is on a section near one of the few lay-bys, or at least in backing distance of one. Gordon Fletcher drove along this tortuous road twice a day for over 18 years to get to and from work, and that doesn’t count the stint he did at the dam in the1960s when he was working through his apprenticeship, or in the 1980s when he worked at the NZED testing department in Nelson. He brushes the challenge aside. “You do have to be on the defensive, but you get used to it.” What you can never get used to is the location of his place of work – the Cobb Powerhouse, wedged into the beech-clad valley where the Cobb and Takaka Rivers meet. In the shadow of the Tasman Mountains, the glaciated, U-shape valley is on two levels. The generation is done on the lower level with twin steel penstocks soaring almost vertically up the rear of powerhouse and stretching two kilometres in a direct line to the tunnel portal (the dam stretches another two kilometres to the dam. With a fall of 595 metres, the highest of any hydro dam in the country, the water flow has such power it can generate 32MW from a water flow of just 7.25 cubic metres per second.
“One of our guys climbs those stairs once a month for an inspection, and also after any heavy rain or an earthquake,” says Gordon, adding quickly that he has been exempt from this duty for some time. “I was able to do it, but I would be puffing and blowing my way to the top now.” From outside the powerhouse, Gordon fills us in on the history of the project’s drawn-out construction. The original hydro-scheme was a private undertaking in 1935 by the Hume (Cobb River) Electric Power Company, which was looking for power to develop asbestos deposits in the valley. The snaking access road from Upper Takaka to the power station site was gouged from the hillsides and sheer rock-face by Downer and Company using the first earthmoving dozers in the country (in fact, the company’s first D8). Three concrete bridges were built over the meandering Takaka River, one over 60 metres long. Downer also constructed the original powerhouse. In the early days, there was no dam, just a small artificial lake created by a control weir at the tunnel intake some 13 kilometres away in the upper valley. A 900-metre tramline ran up the slopes from the power house site to the crest of the ridge (1070 metres) so workers and equipment could be winched up to the top for access to the two tunnel headings. The original 66kV transmission lines head the other way, across the Takaka River, up the valley slopes and over Barrons Gorge with a span of 1.3 kilometres, the longest in New Zealand, and a sag of 115 metres. Just thinking about the manual labour that would have gone into the construction of the pylons, tunnels and pipelines in those pre-helicopter days, is enough to make you break out in a sweat. Suffice to add, conditions were tough. The area has an annual rainfall of 2200mm and with frequent large floods and snow during winter.
In 1949, capacity was increased to 32MW and a compacted earth and rock dam 38 metres high and with a crest length of 204 metres was built to conserve the water previously wasted during floods. An associated concrete spillway was also built. This four-culvert spillway section discharges into a stilling basin and is able to cope with a flood of 850 cubic metres per second. Other extensions included a new penstock and two new (10MW) hydro generating machines in addition to the four, 3MW, original machines, and a larger powerhouse constructed over the old building while the station kept running. The early yearsGordon points to a flat area of land on the other side of the Takaka River that is connected by a sturdy bridge to the front of the powerhouse. “In the old days there were 12 houses, a hostel and a village hall to accommodate station staff. The staff turnover was very high. Mostly the wives couldn’t handle the isolation. Some people loved it and were upset when it closed down.”
For five years he moved around various departments until specialising in his fifth year when he chose ‘operations’ and in 1965 was transferred to Cobb Dam. “I didn’t like it and, in 1966, left to do an 18 month stint in Taumarunui with a company that had timber mill maintenance contracts. In 1968 I went back to Nelson to work in the NZED test room, which covered Golden Bay and involved trips back to Cobb.” Surprisingly, in such a remote location, with a dangerous road, swift flowing river, and isolated plant, noteworthy incidents have been few and far between, albeit well-remembered by locals. Returning back to the dam’s single men’s hostel from a Takaka pub, a van of four painters drove off the road and into the river. Two died. It was 1967. Two years before, a local woman drowned while taking pictures on the riverside. The station was shutdown and the river lowered to find her. Gordon remembers it well. The power station’s most dramatic incident also happened in the 1960s. One of the smaller turbines hadn’t been isolated properly and an automatic valve in-avertedly opened and empted the contents of the penstock into the station. The force of the water ripped a hole in the southern side of the station and took out a couple of local service transformers.
Gordon recalls a quiet two decades leading up to when the country “went mad” in 1987 with corporatisation and the splitting of the NZED into different business units. “When the axe fell, I was with Electricorp Marketing which became EMEC and that then became Power Mark at which point, in 1991, I got severance pay.” Between 1991 and 1992 he worked with a company helping Transpower to catalogue their spares at various sites around the South Island and relocate them at a central Transpowerstore located in Addington. “Then I got a call from Electricorp Production to come and work back at Cobb and I have been back since 1992.” Gordon has to jump all the way to 2005 when pressed for further incidents to flavour up the article. It was also the single most dramatic day in his 40-year career. “My pager went off in the middle of the night while I was on standby. I thought it was a communication fault and drove up to the station on my own. The access gates to the station had been forced open and the place smashed up.” A transient, high on booze and drugs, had taken a machete to mainly computer equipment, but included some relays of the two large machines, which they managed to trip, some how managing to avoid electrocution while hacking up live cables. “Fortunately he didn’t go up stairs into the control room,” says Gordon. “I passed a vehicle on the way up here and alerted staff in Takaka who got his registration.” Police found the culprit dosing down in an unoccupied school house. Towards retirement
An orchestrated machine hum fills the inside of the cavernous station, which went up a note when one of the larger machines is switched on from over 1000 kilometres away. On the lower level of the station floor are the original four, smaller, 4500HP, 3MW English Electric generators and governors built before WW11 and commissioned in 1944. On a higher level sit the twin, 14,000HP, 10MW Boving turbines and their English Bruce Pebbles alternators. The two big machines are basically original (rewound in 1977] and still on their original runners and only need to be pulled out for a manual inspection once a year and general maintenance about every three years. Their first upgrades is due in November when they will be fitted with more efficient governors and runners designed by HydroWorks and cast in Australia with finishing in New Zealand. The old control room above the offices was upgraded in 2002 along with new transformers and switchgear. “There’s always a day or two when we have to drive the station, usually because of a bottle neck at Stoke [regional substation] due to maintenance there,” says Gordon. This way, Cobb runs a split system. Pumping into the grid and, during down times at Stoke, keeping frequency and supplying all the power for Golden Bay and Motueka during the day – returning to automatic service at night.
It is the eve of Gordon’s retirement and Ian Lees, Gordon’s boss, had travelled from Christchurch for his farewell lunch; a delicious home-made spread provided by Gordon’s wife Ann. Waiting for Gordon in his well-deserved retirement are his hobbies; an investment lifestyle block with scrub to be cleared, a ham radio network to keep contact with the world, and a vintage 1947 Ford V8 coupe, waiting for restoration to be finished. “I will miss the camaraderie of my work mates,” he says pensively. “I wont miss the travelling, or the noise of the power house machines. Some days you didn’t notice it. Other days it would get right into you.” Cobb Dam factsThe 32MW Cobb power station lies 112 kilometres northwest of Nelson in mountainous country at the junction of the Takaka and Cobb rivers. The Cobb River runs from the Tasman Mountains into the Cobb Valley, a typically glaciated U-shaped area with a 70 square kilometre catchment area. The original hydro-scheme was a private undertaking in 1935 that, by 1940, hit financial problems and was taken over by the Government. Because of wartime shortages of men and materials, Cobb didn’t produce its first power (12 MW) until mid 1944. Capacity was increased to 32 MW in 1949 when the original weir and single penstock were replaced by a dam with a spillway, a new penstock, and two larger, six single-runner, double jets, Pelton type turbines 10MW machines in addition to the four, singe jet, 3MW originals in the power station.
The Cobb Dam project was finally completed in 1955. Up to this date it had worked on its own separate network, supplying the whole of the Nelson-Marlborough area with the help of a number of minor stations in the region. In 1956 Cobb was linked to the main South Island grid via a line to the West Coast and later was also linked to the 220kV lines via Kikiwa. The original owner was the Electricity Department and it has been owned and operated by Trustpower since 2003 after it paid $92.5 million to previous owner Natural Gas Corporation. Designed and built by Ministry of Works, Cobb was the first dam in the country to be built using modern soil control methods and the first incorporate instruments for measurement of internal water pressures. It is also the country’s highest hydro water storage, sitting at 783-metres above sea level. The site was unsuitable for a concrete dam as early surveys revealed that what looked like outcrops of rock in the valley floor were in fact huge boulders suspended in moraine, so Cobb Dam was constructed with silty gravel core with sandy gravel shoulders with a height over foundation of 35 metres and a crest length of 214 metres. The spillways (with its 857 cumecs capacity) and sluices are combined in a concrete structure built into the earth dam to the right of centre line. The definitive book of the dam project is ‘The Cobb - The History of Cobb River Hydro-electric Power Scheme’, by A.K Blair, a school teacher who taught in the valley during the late 1950s.
Energy NZ Vol.4 No.4 July-August 2010 |
The Cobb Dam Road that leads off from State Highway 60 at Upper Takaka in Golden Bay is only 15 kilometres, but it is a very long and anxious 15km.
A steel staircase hugging the penstocks presents a formidable challenge for maintenance of the penstocks.
Hume ran into trouble with the project and the Government took over in 1940, extending the road from the powerhouse to the upper valley with branch roads to the tunnel portal and pipeline. Cobb eventually produced its first power (12MW) in mid 1944.
The village was still there when Gordon started off his career with the NZED (January 1961) in Nelson as a hydro electrical apprentice.
“Took a bit of drying out but could have been a bit worse,” Gordon says.
Gordon was one of a crew of four, working Monday to Friday on general maintenance at Cobb, with one person on standby for night and weekend callouts. A larger staff contingent used to also operate the powerhouse but, in these days of computer automation, the power output and machinery is controlled by owner Trustpower from its operation centre in Tauranga.
“Cobb is also important for keeping the voltage at this end of the South Island. Currently, we are the only significant power station north of Coleridge.”
After passing through a tunnel system the dam water drops four kilometres through two steel penstocks to the power station. The gross head of 595.9-metres is the greatest of any power station in the country.