New beginning for St Germans
Providing flood protection for more than 25,000 homes and 70,000ha of Cambridgeshire farmland, together valued at more than £4-5B, is a major responsibility, and one which falls to the Middle Level Commissioners (MLC). The body was set up 150 year ago to provide water and land management for the Middle Level, an area of the Cambridgeshire Fens reclaimed during the l7th century
Lying between the Rivers Nene to the northwest and the Great Ouse to the east, it is an area of prime agricultural land that over the centuries has sunk to below sea level in many parts due to the effects of peat shrinkage and soil erosion. The largest of about 80 pumping stations (PS) feeding into the watercourse is at Wiggenhall St Germans, near Kings Lynn in Norfolk.
The original St Germans PS was completed in 1934 to pump land drainage water and flood flows from the Middle Level into the tidal River Ouse. It had a capacity of 70m3/sec, but by the turn of the millennium, the equipment had begun to show its age. Predictions about the effects of global warming and the threat of increasing tide levels also meant that a higher capacity station was needed.
For the £38M rebuild, six pumps were chosen in place of the previous four. Each of the six new pump sets can raise 16.66m3/sec to a static head of 4.25m, giving a total capacity of 100m3/ sec. This means the new station is built for a one-in-66 year event, handling extreme events, although no more than three pumps at a time are expected to run during a normal year. The belt-and-braces design of St Germans means that it could withstand flooding should an upstream embankment breach occur and can run off its own internal power generation, with 10-days full capacity diesel safely secured on site, should the mains supply fail.
As might be expected with a project of this scale, the contracting arrangement did not follow a traditional route. Unusually, the pump contract was let first, the client knew it wanted concrete volute pumps, considering them the most efficient and reliable on the market, but realised that the design and build route might not result in this specification. John Shepherd, project manager at consultancy, Atkins says: “It was a risky strategy, but it did work – we took the design data and fed it into the civils design.”
According to Don Lamont, Engineering Construction Contract (ECC) project manager and principal engineer for Atkins, the biggest challenge of the project was the temporary enabling works: “Anything below ground level is unknown,” he says.
It took a year to install a twin-walled cofferdam, built around the working area to create a dry zone unaffected by the tides. It consisted of 16 cells and was 10 x 15m. Piles for the outer wall were 21m long, while the inner wall piles were 19m long.
The internal area was 80m2 and included a causeway. A diversion channel was built around the new site to maintain the flow. This was finally removed in September 2010 and earthworks are expected to continue until July to reinstate the site.
MCC’s chief engineer David Thomas explains that the new station was situated in the best hydraulic position on the channel, with a long, straight drain. A splitter plate, ensures a linear flow, helping the pumps’ impellers. The foundations for the main structure included 469 continuous flight-augered reinforced concrete piles of 500-900mm diameter and 11-15m length. The 2,800m2 base slab was split into two 600mm thick sections.
The upstream slab positioned 2m lower than the downstream.
The formwork for the 2.1m concrete volute chamber was cast on site, using precision-cut polystyrene as a mould. The discharge channel for each pump is 13.5m high and 4m wide.
The superstructure included a steel frame with a fully-glazed southern elevation, displaying the giant pumps, and a louvred northern elevation.
According to pump supplier KSB, moving such a volume of water has required a rather special design and size of pump and a more unusual approach to the construction of both the pumps and the PS. Each pump weighs in at 19.5 tons, and the volutes and suction boxes had to be cast in situ, within the structure of the station being constructed around them.
KSB’s SEZ vertical tubular casing pump was brought together with Bosman’s concrete volute pump and the planning and project management of the pumps’ installation was undertaken jointly at KSB’s UK office.
Reimer Bojens, head of system solutions at KSB Aktiengesellschaft says: “Much of our success lay in ensuring that where possible all pump parts were pre-assembled and factory-tested and stored off site. In addition we ensured that all the fixing and fastening components right down to individual washers, together with purpose-designed tooling and lifting equipment was labelled and available from our stores when required.
“The KSB/Bosman variable speed pumps use SCADA control and enable the facility to pump 8 -100 tonnes of water per second. This enables the station operators to choose exactly where in that capacity range they want to pump. In effect, any number of pumps can be called up and set to the speed required enabling managers to respond to conditions and to know how much water is pumped at any time.
It is possible to choose the most appropriate mode for the operating conditions. The most suitable operating mode can be selected manually, automatically and from off-site remotely via SCADA control.
“With SCADA control, we can set the pumps to run at times of the day when we can take advantage of lower electricity tariffs,” says Thomas. “However, the sophistication of the SCADA programme enables pre-set operating modes to be over-ridden if circumstances change. What is special about this programme is that not only are we monitoring water levels at the pumping station, we are also referencing water level changes many miles away via a broadband link.
“If water levels increase in that location, the control systems will automatically set the pumps to work harder in order to reduce levels.”
The first phase of pump installation began in August 2008 starting with the anti-rotation plates, followed by the form work for the intake, removable wear-rings and suction bell.
The final stage was placing the volute designed and manufactured by Bosman.
The second phase of the pumps’ installation started in July 2009 once the main fabric and infrastructure of the building had been constructed and the building made watertight.
This mechanical work involved installing and aligning the pumps to the wear-ring, and installing the gear boxes, motors and all ancillary equipment.
The commissioning programme commenced in January 2010 and this was broken down into three stages. The first stage involved dry testing which involves testing all the instruments to make sure that the correct feedback was being obtained from all the protection devices.
The next stage was to run the ancillary equipment, including the oil pumps and the seal flushing pumps, and to turn the machine over at 5Hz, part of the control philosophy to ensure that the pumps never turbine backwards.
“We went through and tested every pump to ensure that they would rotate happily at 5Hz and then kept them running for five hours,” Bojens explains. “In real life, if the penstocks do fail to open, the default situation is for the pumps to remain at 5Hz until an engineer is in attendance to sort out the problem. The third stage was to recirculate water at full chat and get the pumps up to full speed for eight hours at duty point of 16.6m3/s at 4.25m static head.”
KSB handed the pumping station operation over to MLC in April 2010, following successful commissioning of four of the six pumps, thereby matching the pumping capacity of the old station. The new station went live at the end of April. The remaining two pumps were commissioned in December 2010 bringing the station capacity up to the design of 100 tonnes/ sec of water, at full load and against an average tide. Transfer of operation from old to new, which was performed in just four days, was critical as no pumping was available over this period.
As part of the community engagement activities, primary school children were invited to name the pumps. However, one entry that did not make it to the final ceremony was perhaps the most fitting – a boy wanted to name a pump ‘Jean’ after his mum, because she looked after him, just as the pumps would look after the fens.
The Middle Level Commissioners engaged Atkins to design an electrical system for the new station. Ben Staniland, an electrical engineer for the consultancy, says: “The pumps were tendered for first and the information we received from the pump manufacturer was the choice of ABB motors to drive them.
When it came to variable speed drives (VSDs), we met a lot of drives manufacturers to see what their drives needed.”
According to ABB, a number of options were examined, both low voltage and medium voltage, but a priority for MLC was to have low voltage equipment, partly because their staff were most familiar with this and partly because the security needed for high voltage equipment would not fit with the design of the station.
Using low voltage (LV) drives rated at 690V would also minimise the cable sizes and routing. The ultimate system chosen was to have an 11kV incomer and use a transformer to step down to 690V.
The drives were chosen jointly by contractors Birse and motor control centre (MCC) builders Technical Control Systems, with the final choice being six 1.2MW ABB industrial drives, to be fitted as a chassis mount in three MCC cabinets.
The drives are controlled via a SCADA system that is fed data from level meters.
This gives accurate control and adjustment of the flow. It is essential to avoid over pumping of water as the drains must be maintained at a minimum level to allow navigation by boats.
Another design consideration was the cooling arrangements for the new drives and both air and water cooling were considered. Although water cooling would mean a smaller drive and would give some advantages in the building design, the cooling systems themselves would take up too much space.
MLC considered that air cooling was a safe option as the drives would only be used at their top power level in exceptional circumstances of heavy rainfall.
The importance of reliability of equipment installed at St Germans was highlighted during an extreme event in April 1998, when the previous station was worked to its limits, pumping at full capacity for over 50 hours straight. The system was so stretched that mechanical failure of one or more of the diesel engines or electric motors that powered the pumps would have had disastrous consequences for the Middle Level catchment.
To ensure that the drive installation was successful, string tests were carried out on the VSDs. Brian Johnson, senior electrical engineer with installers Birse Water, says: “Testing, installation and commissioning of the drives went very well. The string tests were conducted with the actual motors and we had use of an ABB technician who helped us resolve a problem very quickly. This gave the customer confidence as they had not used VSDs before.”
Each of the six pumps has a generator and an electrical feed, so if one energy source fails, there is back-up. This also means that the operator can choose the cheapest fuel source at any point.
While it is usual to have a back-up generator, Atkins believes it is an innovation to have it as part of the usual system and a consideration for future projects in an energy insecure world. An 8MW electricity sub-station was also part of the works and the output is split between the PS and the nearby village.
The intake water for St Germans requires a sophisticated screening operation, to keep the channels clear, even in a flood event. Ovivo worked closely with Atkins in the specification and design of the screening requirements, two Brackett Bosker HD1000 trashrakes operate automatically on a 98m long monorail track.
They clean the 63m wide coarse bar screen, which protects the pumps in a system has echoes of a ‘fairground grabber’ – collected debris is scooped up, transferred, and deposited into skips on either side of the water intake.
The pumping station was officially opened on 20 April by Lord James Russell, brother of the Duke of Bedford, who started up the pumps. He recalled how his ancestors were involved with the original draining of the Fens in the 17th century.
Bringing the event up to the minute, MLC’s David Thomas, said: “The station’s design is dramatic without dominating surroundings. In planning, we considered the footprint we left behind, so all waste materials have been recycled and new shrubs and trees will be planted. This has been a project of a lifetime and we are confident it will run for the next 100 years.”
Project manager John Sheppard said: “This has been the project of a lifetime to work on from the moment we started. It is a beautiful example of the multidisciplinary approach that Atkins takes, covering all the bases from DEFRA grant application, environmental impact assessment and detailed design to implementation and project management.”
Don Lamont says: “I’ve lived and breathed it for four and a half years and it’s why I got into civil engineering – to build stuff – and build something spectacular.”