Army of workers opens up Whitelee Wind Farm

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As Europe's biggest inland wind farm takes shape, scale is not the only challenge. By Mark Alexander

ClientScottish Power Renewables
Joint venture partnersMorrison and Balfour Kilpatrick
Infrastructure contract £79.24 million
Contract duration 144 weeks
Turbine contractorSiemens Wind Power

Whitelee Wind Farm is a giant among onshore wind farms. It dwarfs everything that has come before – including the UK's biggest existing wind farm at Blacklaw in South Lanarkshire, which has 42 threebladed turbines generating 97 MW of electricity.

Whitelee, which is under construction in the damp and exposed landscape of Eaglesham Moor, just south of Glasgow, will have 140 2.3 MW turbines giving a total output of 322 MW. The scale is undeniably impressive.

When it is completed in July 2009, it will provide around 180,000 homes with electricity.

"It is the coming of age of the renewables industry," Gordon Brown said at a recent British Wind Energy Association conference.

But while Whitelee's contribution to the UK's renewable energy production will be huge, Whitelee's megawatt output tells only part of the story.

To properly gauge the size of Europe's biggest wind farm, you need to travel along its 17 km spine road and explore the 73 km of subsidiaries that lead past reservoirs and moorland to the giant windmills.

Staggering numbers

To get a feeling of what is involved, you have to witness the massive operation undertaken by Morrison Construction and Balfour Kilpatrick to prepare the infrastructure for the 91.5 m turbines.

At its height, six 15-seater minibuses were transporting 440 workers to various sites around the farm while 88, 31-tonne trucks moved earthworks from six on-site quarries. The figures are mind-boggling.

The man at the helm is Anthony Windle, project manager for Morrison, which is the principal contractor. He says Whitelee has been a learning experience for all involved.

"I've worked on a number of joint venture projects in the past but heading a site team on a project of this size is a different sort of challenge," he says.

"There are major quarrying operations and a lot of road building through difficult terrain, so there are plenty of technical challenges.

"I think everybody on the team has gone through a major learning curve since we started the job in October 2006."

The £80 million infrastructure contract began with a treeclearing exercise that covered 633 ha and enabled detailed survey work to assess the ground conditions.

Shortly afterwards, the road-building element of the scheme started. Mr Windle says that simply co-ordinating the work was a test in itself, particularly as most of the materials for the roads came from on-site excavations.

"The logistical challenges have been the most important hurdle to overcome," says Mr Windle.

"We're working across a project area of 57 sq km. While we were operating at peak times between May and October last year, we had up to 14 working fronts and two structures teams.

"The sheer logistics of getting labour, plant and materials out to the various locations was a huge challenge. At peak, we had up to six quarrying operations at borrow pits around the job and four crusher setups that we were controlling directly.

"I had a separate quarry manager for the majority of the project so we could make sure we had enough stone on the ground for the road building and processed stone for the turbine foundations."

Onsite supply

The site was certainly a great source of materials. In fact 2.5 million cu m of rock from Eaglesham Moor was used, primarily for road construction and hard standing in the form of blasted rock but also as processed stone for capping and engineered fill for turbine bases.

While haulage and secondary washing costs meant local rock could not be used for concrete aggregate, a commercial arrangement with Cemex meant a batching plant was located on site that produced concrete at a rate of 48 cu m/hr, resulting in around 50,000 cu m of the stuff being used during construction.

Eaglesham Moor offered plenty of raw materials, which temporarily made Whitelee one of the biggest quarrying operations in Scotland. The ground conditions also created a number of problems – especially for road construction.

With Morrison's base-line programme requiring 100 m of road construction every day and the displacement option taking up to four times longer to complete, the knock-on effects of the delays could have been serious.

Danger money

"The type of contract we have with Scottish Power meant this was a risk the joint venture carried," says Mr Windle, who says the fixed-budget approach was
undeniably risky.

"We tried to minimise delays in the quarries in terms of loading the trucks. The last thing we wanted was a dozen dump trucks queuing up to be loaded. It's dead money.

"The job was priced on a specific turnaround time for moving materials so if trucks are waiting for 10 minutes, that's out with what we bid on the job.

"With 2.5 million cu m of rock to shift, that translates into a big lump of money. From the client's perspective, a fixed-price contract is the way to go but from a contracting point of view, there is no doubt it's a high-risk project, especially with the ground conditions."

Whitelee may have been high risk but with the first phase of the project due to complete six weeks ahead of schedule, Morrison's emphasis on getting the logistics right seems to have paid off.

And that is in spite of difficult ground conditions that often meant building turbine bases on top of deep, fluid peat bogs.

With the additional headache of creating a hard standing area for Siemen's 500-tonne mobile crane, which was used to erect the turbines, the job of building 140 base structures was less than straight forward. In fact, the consultant engineers on the project, Donaldson Associates, had to verify the load capacity at each site before one of three base types could be used.

"Donaldson had to do a plate bearing test on every foundation to assess the bearing capacity available," Mr Windle explains.

"We've worked with three different base types which differed in plan footprint depending on the quality of foundation.

"Type One is generally founded on rock with a nominal engineered fill, while Type Three has a slightly lower allowable bearing capacity which will be typically found on poorer quality foundations.

"Type Two bases were somewhere in between but we used them very rarely – there's very little middle ground."

In the worst cases, a circular retaining wall was built to isolate fluid peat around the base site which was then removed.

The wall, or doughnut, was applied at various depths on 28 occasions, which gave the Morrison team the chance to fine tune the process.

"It's essentially a mass gravity structure that retains the fluid peat on the outside so that we can take it out from the inside," he says.

"We form a ring around the base location using heavy rock and then long-reach excavators dig out the peat. We've developed the process on the back of what we've done previously, when we used them up to 5 m deep.

"Here, with 9.5 m-deep foundations, the doughnuts have been up to 7 m deep. Also, when we've used them before, we've done so on an isolated basis. On this job, we've built 28 in various sizes so we've learnt a lot."

Mr Windle says the key lessons Whitelee has taught him is the importance of using experienced machine operators who can punch rock through fluid peat and then key it into the clay interface. So important is this that Morrison used the same four operators to build each doughnut.

Indeed, one of the benefits of a 144-week contract is the opportunity to learn and refine a variety of techniques.

Huge benefits

"In terms of the engineered fill and reinforced concrete," Mr Windle concludes, "we've definitely reaped the benefits of the scale of this project.

"For example, there's between 40 and 45 tonnes of reinforcement for each base. In the beginning, the guys would take up to five days to fix it.

"By the end of the project, they were turning them around in a day and a half. It was with the same squad, they just speeded up the process.

"Also, working in a location with significant peat volumes won't pose anything like the same concerns. It's always going to be difficult but I think we'll be far more confident about how we handle conditions of that nature in the future."

ROCKY ROAD

"The road building has been one of the biggest challenges," Anthony Windle says.

"Of the 90 km of road built, 60 km are on a floating construction which is a technique we've used in the Highlands on smaller wind farms.

"Essentially we lay a geotextile mat and geogrid reinforcements across the peat before creating the road. "If you break the surface, you've got a huge problem of the road sinking."

A 600 mm layer of graded rock is added to the geotextile mat and geogrid mix to shore up the structure. The construction is then topped off by 200 mm of crushed rock.

While this addressed most of peat-related problems, there were a few instances where more substantial measures were needed.

"There were a couple kilometres of road that no matter what type of geogrid re-enforcement was used, the road sunk because of the peat," says Mr Windle.

"So in these instances we had to build a displacement construction using rock as a base.

"Ideally we wanted to follow the floating road ethos because if we put in three to four meters of rock, we'd obviously use more rock but there was also a programme implication. It slowed the whole process down."