Imagine you are a pilot flying in zero visibility, completely blind. Your mission is to hit a target only 900mm in diameter at the end of a 130m long tunnel. Peoples' lives depend on the accuracy of your aim.
This is not a training exercise for the RAF, although the 617 Squadron Dambusters once practised in the steep sided Peak District valley now closed off by Ladybower Dam. It is a real task faced by diving contractor DSND Subsea, which over the last few months has been carrying out the risky replacement of the dam's inlet valves as part of a £15M refurbishment project.
After 50 years of service the dam's owner and operator, Severn Trent Water, decided it was about time the structure had a thorough overhaul. This meant strengthening and raising the dam, refurbishing two pumping stations and constructing a new pumping main. Consultant for the scheme Babtie Group also identified that the sixteen major valves in the dam's two draw-off towers needed replacing.
With 8% of the East Midlands' water stored behind Ladybower's 363m long earth and rock bank, draining the reservoir was not an option. The job of removing the valves and installing new ones while keeping the reservoir serviceable looked fiendishly difficult and more than a little dangerous.
Realising that some special help was needed, Babtie called in offshore diving contractor DSND to carry out the work on a £3.7M design and construct contract.
DSND's solution was to use inflatable stoppers in the eight draw-off pipes to form a barrier between the valves and the millions of litres of water stored in the reservoir. But the big problem was how to install them.
'The bottom two scour inlets in particular were a real headache,' says DSND project manager Bob Macmillan. 'They are 43m deep - close to the limit for diving in air - and at the end of two 130m long tunnels. We couldn't get a diver up there because of all the safety issues.'
Instead, DSND adapted a Sea Eye Surveyor Plus remotely operated vehicle to carry the stoppers inside the pipes. This would be piloted from a control cabin on a barge at the surface of the reservoir, with divers located at the entrance to the tunnels to feed the power, air and communications lines in after the ROV.
All seemed well. But as soon as the ROV was lowered into the reservoir, it became apparent there would be serious visibility problems.
'Whenever we got close to the bottom we disturbed the fine silt, which stayed in suspension for a long time. Our visibility was reduced from a few metres to zero. We were basically having to fly blind,' says Macmillan.
DSND used high definition sonar and pitch and roll sensors to give the pilots an idea of where the ROV was going and how it was behaving. But even with these electronic aids it took two painstaking weeks to learn the technique.
Once the pilots were ready, the ROV was introduced into the inlet tunnel. Again silt proved a problem, reducing the tunnel's diameter in some places from 3.66m to just 1.2m. Further obstacles had been left behind by messy construction workers.
'At the end of the tunnel there were old bits of scaffold, wooden beams and metal plates sticking up through the floor. If the pilot had punctured the stoppers on the way up we wouldn't have known until we came to blow them up,' says Macmillan.
Fortunately this didn't happen, and all the stoppers were placed successfully. But what if the air supply to the stoppers failed?
To give workers confidence that they would hold in place a 'double-block and bleed' system was used. This comprised two stoppers, individually supplied with air, linked together with a universal joint.
Each stopper was inflated to 7 bar pressure and connected to a high pressure air cylinder at the surface capable of holding 217 bar. The high pressure air cylinders were in turn connected to a safety cylinder which would provide top up air in the case of a failure.
'We had to ensure absolute safety since failure of the stoppers when the valves had been removed would mean water rushing into the tower at 12 knots,' says Macmillan. 'To put that into context the average person can only swim against a current of 1 knot.'
The high pressure air cylinders were electronically monitored and set up to trigger an alarm. They also had the capability to automatically dial up emergency numbers if the pressure fell below 145 bar.
In the end, though, Macmillan is pleased to say that there were no accidents.
'We'd never done this before and I don't think anybody else has, so it's a relief to get this one under our belt,' he says.
The valves were eventually disconnected and lifted out of the draw-down towers without incident using a crane barge. It is hoped that their replacements will last for at least another 50 years.