Steven's Croft biomass power station: Rising to the biomass challenge

The UK's largest biomass power station has demanded blue-sky thinking in order to meet ambitious feats of engineering and construction deadlines.

Based near Lockerbie, the Steven's Croft plant had to be built by a specialist range of contractors within just 25 months to a budget of £90m.

In October 2005, a consortium of Aker Kvaerner (Metso) and Siemens secured the contract for the turnkey delivery of the 44mW station on behalf of operator E.ON UK.

Working for Siemens, Alfred McAlpine Project Services (AMPS) was involved in the construction of the structural reinforced concrete works, drainage, pressurised pipework, hard standings and some structural steelwork and cladding.

Construction work started in January 2006, with the plant expected to produce green power for 70,000 homes in 2008. To date, consortium leader Siemens has handled all civil works on the site, and obtained building permits. Approval from the Scottish Environment Protection Agency (SEPA) was achieved by E.ON UK.

Siemens' work was to ensure the drainage for the local sustainable urban development system was not negatively affected. In addition to the co-ordination, engineering and project management, Siemens supplied and erected the steam turbine machinery, together with installing all electrical and instrumentation and control equipment.

In its entirety, the project required the earthworks and construction of a 45m-high boiler house, air-cooled condenser, turbine building, fuel storage building, switchgear building and an auxiliary transformer.

Dr Paul Golby, chief executive of E.ON UK, says: "This is a major project for Scotland because biomass is a carbon neutral fuel. It has huge potential for both electricity generation and for farmers growing the biofuel crops. We are committed to helping the UK government and the Scottish Executive meet tough green energy targets."

Rigorous deadlines

All contractors had to adhere to rigorous deadlines to ensure each phase ran as seamlessly as possible. The schedule demanded completion of all major foundations by December 2006, backfeed to the main transformer by February 2007, followed by reliability tests scheduled for October 2007.

As the UK's largest biomass power station, the construction phase has presented a steep learning curve for all the subcontractors involved. The project was new territory for AMPS, as civil engineering agent Mark Johnston explains. "The project was certainly a challenge from both a construction and design perspective. We had to work to very tight time scales to meet a gruelling build schedule and within a very congested site. Overall, we constructed about £1m-worth of steel and cladding, placing 7,000m³ of concrete and 900t of reinforcement."

Initial remediation, drainage and soil stabilisation of the 10,000m² site was undertaken by earthworks contractor Keller Ground Engineering.

Among the various soil types found at the site, the main material was clay. Construction demanded robust foundations with a high degree of settlement control, so careful attention was paid to ground conditions. Keller installed vibro stone columns as part of a £185,000 contract.

With the groundworks completed, AMPS started work on the structure's foundations in April 2006.

The structure posing the greatest engineering challenges to AMPS was the fuel storage building. Requiring 2,400m3 of concrete, the building was the largest single structure built by AMPS. The first pour took place in June last year, with the final one completed in December 2006.

Outside the building, the support columns were reinforced by 32mm steel bars, and inserted into a series of couplers positioned at different levels.

Johnston adds: "The construction of the fuel storage building began with the conveyor pit to the east side of the building, which was excavated to a depth of 4m. Meanwhile, 1,200mm x 1,000mm ground beams were established using back-blinding working from west to east. The ground beams connected the north and south cranked columns together."

For the foundation of the conveyor pit, the contractor back-filled with 500mm Type 1 granular fill, rolled tight and blinded on top with concrete.

Once the building format was set using the back-blinding, AMPS started on the gallery roof slabs and wall construction. The slabs would finally house the machinery to pull the biofuel from the central hopper on to the north and south conveyors.

Design aspects

The next challenge was to install the gallery roof slabs, which incorporate a drop-beam spanning a 48m length of the building. A key design aspect of the drop-beam was that it had to be unsupported, leaving a long slot for the fuel to be fed onto the conveyors. For this, AMPS incorporated a rolling soffit system.

Ahead of the gallery slabs, the cranked columns were packed full of 32mm-diameter bars and couplers set at different levels. Using three sets of raking shutters to support the columns and one set of closing shutters, AMPS was able to carefully construct each column.

"The entire team devised a unique technique to construct the sloping walls, which included a radical shutter design and concrete mix design," he explains.

"Once the wall had developed sufficient strength, the facing shutters were stripped and the raking shutters lowered away from the concrete face. They were then rolled on wheels onto a previously constructed stripping table."

Adjacent to the fuel storage building, AMPS excavated the water cooling basin sectioned into four open pits. These are designed to cool the water from the boiler before it is recirculated.

Adjoining the boiler, AMPS excavated the foundations and constructed a 1m-deep concrete slab to provide the base for the 70m2 water treatment building.

For the 45m-high boiler house, the contractor once again pooled its concrete engineering expertise, constructing 1,200mm-deep concrete slabs. These incorporated complex integral encast items and steel works.

Johnston says: "At the base of the boiler house, a cavity wall was constructed using concrete-filled hollow blocks. These were installed to provide a wall that can accommodate extreme temperature differentials on its opposing sides."

Engineering challenges

Further civil engineering challenges facing AMPS were in the construction of the flue gas fan building. Here, the contractor constructed three highly complex concrete plinths to support the fan motors. Isolation joints were fitted at the base of each plinth for the entire depth of the building slab to absorb excess vibration of the fan motors.

For the turbine building, AMPS was contracted to provide the concrete base, plinths and all encast fixtures, which required 400m2 of concrete.

The team instigated its first structural concrete pour on 6 June 2006, achieving the first project milestone. This included the 500m3 turbine block poured in homogenous temperature-monitored conditions.

In addition to all concrete pours, AMPS handled all structural and internal steelwork for the turbine building, and an 11m-high blast wall behind one side of the turbine building to protect the transformers. Further elements of AMPS' work included the construction of buildings for water treatment, switchgear and an auxiliary transformer, together with the concrete platform for the 85m-high exhaust stack.

Johnston sums up the overall scope of the scheme: "Ultimately, the project required a high level of joined-up thinking between all the various parties. The contract was a baptism of fire for many contractors, demanding hard work and commitment from everyone."

Project Fact File

Project: 44MW Biomass Power Station

Client: E.ON UK

Value: £90m

Engineering, procurement and construction contractor: Siemens and Metso

Civil engineering/infrastructure: Alfred McAlpine Project Services

Ground stabilisation: Keller Ground Engineering

Project length: 25 months