In at the deep end

7 min read

The world’s first deep water offshore windfarm is in the pipeline. Jon Excell reports on an innovation that could have considerable advantages over current technology.

Late next year, a diverse group of engineers from across Europe will converge on a remote oil field 25km off the east coast of Scotland and begin construction of the world’s first deep water offshore windfarm.

The team will install two 5MW wind turbines in the 44m deep water alongside the Beatrice field oil platform in the Moray Firth. Standing 90m above sea level, yet barely visible from the distant shore, the turbine’s giant blades will sweep across an area roughly the size of two football pitches.

Expected to go live in October 2006, the massive structures will initially be used to provide electricity for the Beatrice installation. But Talisman, the oil and gas company that operates the oil field and is heading the project, has mooted the possibility of building a larger commercial windfarm that would use the existing infrastructure to pipe energy back to shore.

The £41m (£28m) demonstrator, while partly funded by Talisman, is also at the heart of Europe’s largest renewable energy R&D programme, the DOWNVind (Distant Offshore Windfarms with No Visual Impact in Deepwater) initiative. This EU-funded project aims to find ways of making deep water offshore windfarms economically viable.

Existing offshore farms, such as the Scroby Sands installation in Norfolk, tend to be closer to shore and in relatively shallow water — 20m deep at the most. No one has ever put them this far offshore or in such deep water. The advantages promise to be considerable.

By siting turbines so far away from land, accusations of unsightliness can be neatly swept aside and, perhaps more significantly, the wind resource is far more constant than that closer to shore or on land.

But the challenges thrown up by the project are huge in every sense. Not only are the core engineering components and equipment large in scale, but the collaborative effort also brings together a vast array of European designers, manufacturers and technologists from both the offshore oil and gas industry and the renewable energy arena.

According to Alan Macaskill, who is managing the project for Talisman, it is an unprecedented effort: ‘As an engineering challenge, it’s huge. We’re reinventing technology from one field and reapplying it to another.’

Plus, beyond the key engineering and construction challenges, those managing the project must also have one eye on the well-documented environmental and conservation issues associated with windfarms. Indeed, with many of the various engineering teams involved ready and raring to go, the status of the project now hangs on the favourable reception of an environmental assessment report that will be submitted to the DTI later this year.

But those involved are confident that this is a formality, a confidence backed up by the fact that a number of key contracts for the project are already in place.

Perhaps the most significant of these is the contract for the design and manufacture of the turbine itself, a task awarded to German wind power specialist RE Power.

While some aspects of this design are yet to be finalised, the turbine will, claimed RE Power’s Dietmar Gosch, essentially be a ‘marinised’ version of an onshore prototype that has been generating electricity for the German town of Brunsbüttel since February.

Standing on a 120m-high tubular steel tower and with a rated output of 5MW, this turbine, known as the REpower 5M, is currently the largest in existence. The huge turbine has a rotor diameter of 126m, and its three 61.5m-long blades each weigh 18 tonnes.

The turbine is equipped with a variable speed generator-inverter system and an individual electrical blade pitch system. It is also armed with a network of sensors that detect the wind direction and work with eight electric geared motors to turn the head (or nacelle) to face into the wind. Eight hydraulic brake calipers are used to hold the nacelle facing the wind in order to counter potentially damaging loads on the drives in turbulent wind conditions.

While the very latest condition monitoring systems allow much of the maintenance work to be caried out remotely, the top of the nacelle houses a helipad, allowing engineers easy access to the system. And should they be caught on the turbine in bad weather, there’s even a space further down the tower for them to have a cup of tea and ride out the storm in relative comfort.

Gosch said that, because the prototype was designed with offshore operation in mind, he’s confident that the technology demonstrated on the turbine will be able to go offshore. He said that the main differences in the design of the offshore system will be the use of improved coatings and a specially-designed cooling system to protect the internal workings of the turbine from the salty, corrosive marine environment.

Gosch believes that, if successful, the project will help kick-start the deep water turbine business: ‘If we make this a success, everybody will look at it and say “well that’s real offshore, not just wet feet!”’ He added that one of the key advantages of putting turbines in deep water is that they are rarely, if ever, exposed to breaking waves, potentially damaging conditions that are frequently experienced by shallower water wind turbines. His team hopes to begin assembly of the turbines early next year.

Another difference between the landbased prototype and the offshore version will be the height of the tower. While the existing turbine tower is 120m high, the offshore version will stand at around 90m above sea level. The reason for this is that much of the structure will lie beneath the surface of the water. For the design of this substructure, Talisman has turned to a concept developed by Norwegian marine specialist Owec Tower.

The company has developed a design for a quadropod jacket: a 700 tonne, 70m-high, four-legged steel structure that is attached to the sea bed using a pile-driven foundation and extends to around 25m above sea level.

This structure supports the tower above it and is designed in such a way that waves are able to pass through it without exerting damaging forces.

The company’s managing director, Per Bull Haugsøen, explained that during the design process of the jacket, computer models were exposed to simulations of the complex patterns of forces from both the wind turbine and the current and wave conditions. This analysis was further complicated by the eccentric forces encountered by a transition piece that links the four-legged jacket with a circular mounting piece for the turbine’s tower section. Haugsøen added that during the lifetime of the project a network of accelerometers and strain gauges will be attached to the structure to monitor its behaviour and the stresses that it is under.

Perhaps the trickiest part of the project will be the installation of the system. While the finer details are yet to be finalised, it appears likely that the turbines and subsea structures will be installed using specially-adapted versions of equipment commonly used by the offshore oil and gas industry.

Ken Bateman, construction manager with AMEC, the company charged with managing the project’s engineering effort, explained that while offshore turbines are usually installed piece by piece with coastal jack-up equipment, the water depth at the Beatrice field site is out of the range of this kind of equipment.

Instead, the team is considering a two-piece installation process where the jacket is installed in the seabed and then the tower, nacelle and rotors are installed in one piece from a crane barge equipped with an innovative ‘soft landing’ system.

While installation contracts are yet to be awarded, Bateman said that he is currently evaluating a subsea system developed by marine specialist The Engineering Business. This system uses a series of hydraulic cylinders to remove the momentum of the vessel during installation and provide a damping effect while the tower is attached to the transition piece. ‘If we are successful, we will change the way that we install offshore turbines,’ said Bateman.

But while confidence is high that the various engineering hurdles will be successfully overcome, some in the marine industry have suggested that the Beatrice Field project may not be addressing the biggest challenge facing the deep water concept — building, installing and operating such facilities at a reasonable cost.

Prof Nigel Barltrop, deputy head of Naval Architecture and Marine Engineering (NA-ME) in a department jointly owned by the University of Strathclyde and Glasgow University, warned that the challenges are economic rather than technical. ‘The main problem is a financial one — can you build the necessary substructures to support the turbines and do it all at a reasonable cost so that you end up making some money?’ he asked.

Barltrop suggested that the Beatrice demonstrator, while exciting and innovative, is not really typical of how such windfarms are likely to be constructed and operated. ‘In most places farms are likely to be independent of offshore platforms,’ he said. ‘Offshore operators have an understanding of working in the marine environment, which is helpful, but are more interested in the reliability of the operation and getting it done quickly than installation costs. With a typical windfarm installation you’re much more orientated towards keeping the costs down. The big crane barges typically used for offshore platforms are too expensive to use for installing windfarms.’

Dr Gordon Edge, head of offshore at the British Wind Energy Association (BWEA) also believes that the current cost of the required offshore technology argues against windfarms at this depth and at this distance from the coast. Edge said that while the wind resource is better the further out to sea you go, foundation issues, cabling issues and installation demands of the deep water environment require prohibitively expensive technology from the oil and gas industries.

However, an AMEC source who is closely involved in the project said that far from blowing the idea of deep sea windfarms out of the water, these economic challenges could provide a golden opportunity for the UK’s ailing offshore manufacturing industry to use their knowledge and experience to develop new low-cost techniques and technologies.

This optimism is echoed by Macaskill, who insisted that the project points the way forward for wind energy. He even suggested that deep water technology could operate in water up to 100m deep, although he conceded that the costs of installing pipes to get energy back to shore will probably mean that most systems are installed no more than 50km out.

‘The future for wind energy is largescale developments far from shore in deeper water,’ said Macaskill. ‘It’s a very different view from most other players in the wind industry — one of us is right and the other is wrong and I can tell you who’s right. Us.’

The wider picture

While the Beatrice Field project’s groundbreaking status is undisputed, some have argued that in terms of the wider wind energy industry it is a small part of a much bigger picture.

Dr Gordon Edge, head of offshore at the British Wind Energy Association, claimed that it will be some time before the UK needs deep water windfarms as a resource. He said the project is more likely to be of immediate use to Germany, which must use deep water farms as its entire northern coastline is a protected area.

The Beatrice Field demonstrator should, he said, be viewed against a wider backdrop of offshore wind activity that, while not taking place in such deep water, is rapidly turning the UK into a global centre of excellence.

The UK’s first offshore windfarm was commissioned in December 2000 off Blythe Harbour in Northumberland. Since then, 30 more turbines with a total capacity of 60MW have begun generating for the grid at North Hoyle off the coast of north Wales, and construction is now complete on a second windfarm at Scroby Sands, off Great Yarmouth in Norfolk. With consent granted for 360 more such turbines, the UK is expected to become the world leader in offshore generation at some point next year — not bad, claimed Edge, for an industry that is still in its infancy.

He also pointed to the plans submitted earlier this summer to build the world’s largest windfarm in the Thames estuary. The so-called London Array will use 270 turbines to generate up to 1,000MW of electricity, enough for about a quarter of London’s homes. If permission is granted, the consortium behind the £1.5bn project, made up of Shell Wind Energy, E.On Renewables and wind energy specialist CORE, is hoping to complete construction of the wind farm by 2010–11.