Bonnington and Stonebyres are run-ofthe-river hydro-electric stations located on the River Clyde near the town of Lanark. Built in 1927, they are Scottish Power's oldest generating plants and are believed to be the oldest hydroelectric power stations in the UK.
Each station is fed with water via a pair of 2m internal diameter steel penstocks. Corrosion and general deterioration over many years have meant that extensive repairs were needed to the Bonnington penstocks along with a complete replacement of those at Stonebyres.
Work was complicated by the location. The Bonnington penstocks lie next to the boundary of the Falls of Clyde Site of Special Scientfiic Interest (SSSI). Physical access to them is also difficult.
'Although chnically straightforward, the project is logistically complicated and needs to be carried out with considerable environmental sensitivity', says Ronnie Robinson, project manager with client Scottish Power.
'As well as taking into account the SSSI, the team has had to divert a public footpath, avoid impact on local wildlife and deal with the repercussions of localised ooding in August 2006.' Bonnington is the larger of the two power stations, with a generating capacity of 11MW.
This compares with Stonebyres' 6MW of power. Bonnington also has the longer penstocks - 230m compared with Stonebyres' 21m.
To tackle the environmental constraints, main contractor DCT Engineering worked in conjunction with a number of agencies - including the Scottish Environmental Protection Agency (SEPA), Historic Scotland, Scottish Wildlife Trust (SWT), South Lanarkshire Council's access and development ofcer and Scottish Natural Heritage (SNH).
Extensive environmental mitigation measures were carried out in accordance with an Environmental Management Plan produced by Scottish Power.
To gain physical access to the Bonnington penstocks, DCT built a temporary access road running adjacent and parallel to the line of the western most one. It also built a second temporary access road to connect one of the site compounds with the power station access road.
Even with the access roads there are still challenges to overcome. Access to the higher sections of the penstocks at Bonnington is particularly difficult as the ground slopes steeply and the penstocks are in a deep cutting.
Refurbishing e penstocks has been relatively straightforward. Paint was removed from them by dry grit blasting. A detailed inspection of the pipeline was then carried out to establish whether any sections needed replacing before internal and external repainting was carried out.
A containment system was provided throughout the blasting and painting process and debris was taken to a licensed waste disposal site.
The 35 concrete saddles supporting the penstocks are also being refurbished with greasing points re-instated.
The edges of the saddles are being sealed to prevent moisture collecting between pipe and the saddle, reducing corrosion and future maintenance requirements.
Work started in June 2006 and is due to finish soon.