Building Alpha Ventus wind farm (source: REpower)
Building Alpha Ventus wind farm (source: REpower)
Accessing wind turbines at Horns Rev 2, Denmark – currently the world's largest offshore wind farm. (Source – GWEC)
Accessing wind turbines at Horns Rev 2, Denmark – currently the world's largest offshore wind farm. (Source – GWEC)

The offshore wind industry is where onshore was 15-20 years ago, and several voices in the industry say that offshore wind is not even properly offshore yet. Norbert Giese, Vice President Business Unit Offshore at REpower tells Renewable Energy Focus: “If you compare the offshore industry with the onshore wind industry, you have to look back to the 1980s when we had 1500-1650 MW in operation worldwide

“Very few things are ready for this new business,” he says. “We are working with transportation and installation where we are using equipment already on the market which is not built for serial transportation and installation of offshore wind if we're talking about ‘real offshore’”.

By ‘real offshore’, Giese means projects with around 30 m water depths, 50-60 km from shore with larger turbines such as 5 MW ones.

Benjamin Sykes, Senior Technology Accelerations Manager for Innovations at the UK Carbon Trust, says: “Offshore wind is doable with current technology, but the challenge is to drive cost down.” Andy Williamson, Director of Business Development at the New and Renewable Energy Centre (Narec) agrees: “The capital cost needs to come down, probably about 40% for the offshore market.”

Truly offshore turbines

Wind turbines are an obvious area to look at for increasing efficiencies and drive costs down offshore, but at the moment, few turbines have been designed specifically for offshore use.

“Not all, but most of the turbines going offshore at the moment are based on onshore turbine design,” Sykes says, before adding: “I'm sure we'll see a new generation of turbines emerging over the next three to four years.”

However, with larger, more power wind turbines, come a new set of challenges: “There's opportunity for innovation around blade design and material, as blades approach 100 m in length there are strategic, logistics and manufacturing challenges that the industry has not really tackled before.”

"I was told by one major developer [in December] that 80% of their problems in offshore installation, is the cables. Because it's just very difficult to do."
- Williamson, Narec

If setting up the offshore wind farm of his choice, Sykes says that if he were to develop a Round 3 site today for example, he'd use today's turbines, “but I would hope that by 2014–2015, when I'm making my final technology choices to develop a Round 3 site, there will be new turbines on the market that have been demonstrated as reliable and effective offshore. So I will probably be looking for a next generation of turbines with perhaps 6 MW or 7 MW direct drive, permanent magnet, but I would also be looking for a turbine foundation system that I could construct onshore and deploy as a single unit.”

The offshore turbine manufacturer I: REpower

REpower is one of the turbine manufacturers that has designed a 5 MW turbine specifically for offshore purposes. The main shaft has a double bearing, and when repairs are needed, the rotor stern does not have to be rebuilt, Giese at REpower explains.

“You have to develop turbines for sites with 9.5–10 m/sec as an average wind speed, which is totally different – perhaps not to the north of Scotland – but to 90-95% of all continental onshore wind sites. They have to be stronger and, of course, also a little bit heavier.”

REpower's turbine has a 33 kV transformer in the nacelle. The turbine contains dehumidification technology to keep the air in the tower and nacelle dry. All of this is to reduce unexpected service visits, as these are “both tough and expensive offshore”, Giese says.

Asked whether REpower is working on any new offshore turbine designs, Giese says that currently the organisation is trying to improve the 5 MW offshore turbine. “We have designed an independent turbine, based on the same platform – a 6.15 MW turbine. We erected three onshore prototypes at the beginning of 2009 to test, assure and validate the turbine.”

Regarding turbine weight of these larger structures, Giese says it is not a “problem, but it's a question”. However, he admits that the 5 MW turbine is too heavy to be transported on normal highways.

Offshore wind turbine statistics

  • In terms of cumulative installed units, Siemens (386 turbines) and Vestas (349) have been the two largest suppliers to date;
  • In addition there are 18 WinWinD turbines operating, 14 GE turbines, 8 REpower turbines, 6 Multibrid turbines, two Nordex turbines and BARD and Enercon have one turbine operating;
  • Wind turbine capacity has been increasing year on year since 1991. In 2004, 3.6 MW and 4.5 MW turbines were installed. In 2005, one offshore wind farm went online using 3 MW turbines. Since 2005, the average turbine size has been slightly below the 3 MW mark, setting a new benchmark for the industry. In the coming years, average wind turbine size is expected to grow, as machines between 3 MW and 5 MW become standard. The average offshore wind turbine size is now 2.9 MW.
Source: EWEA

“I expect that in the range of, lets say approximately 5 MW +/−, we will see equipment in the market to solve this.” But he warns that this will take time as “we not only need bigger components for turbines, of which there are only few suppliers in the market, but also equipment for transportation and installation, which is very costly.

“The challenge today is to build a 5-6 MW turbine serially; to ship them out, to install them, and to have a good service. We are looking for new concepts for our 5 MW and 6 MW turbines in deep water, far from shore.”

Asked to estimate the market size for offshore wind, Giese says that the total German market is expected to be worth €20-25 billion up to 2020 based on permitted projects, and that UK Round 3 could be worth €50-65bn. According to Giese, REpower is well prepared to have a portion of these markets.

The turbine manufacturer II: Vestas

Over at Vestas, President of Vestas Offshore, Anders Søe-Jensen tells Renewable Energy Focus that although the offshore environment is harsh, the wind loads are far superior to onshore: “There are much more constant wind resources and that is a challenge to the machine, materials and components.”

Søe-Jensen adds that Vestas has a lot of experience in terms of component installation, materials selection and what happens to turbines operating offshore. “What are our learnings from yesterday will be our improvements for tomorrow,” he says.

One of these lessons was a gearbox problem at Kentish Flat, UK, where a third of the gearboxes had to be replaced at the beginning of 2009. “That was a learning where we saw some components that simply have to be re-designed to be able to stand there out in a harsh high-load wind regime.”

So far, Vestas has manufactured the V80, V90 and V112 wind turbines for the offshore market – all of which have gearboxes and generators. “We have a 6 MW platform that has been announced for offshore – whether that will be a geared or a non-geared direct drive machine, I cannot reveal,” Søe-Jensen says.

Looking ahead, Søe-Jensen tells Renewable Energy Focus: “I think you will see turbines with fewer components, it will be more like a wind collector. Today, all the turbines out there are individual power plants. Each wind turbine has its own control system and can basically be a stand-alone machine. When you're building offshore, you're building huge power plants where you would take out a lot of those control systems and deal with the electricity centrally.” In other words, a hub.

“Other than components, you need a machine you can easily repair on the spot with internal equipment and don't require specialist repair boats or anything like that. And then the whole scenario of predictability is also important for an offshore machine,” he adds.

The foundations for offshore wind turbines

As wind farms are moving further and further offshore, the question of foundations appear to become more and more complicated. Sykes at the Carbon Trust explains: “Monopiles are not going to be a realistic option in 40 m of water. Conventional jackets will work in 40 m water as demonstrated in Beatrice, but when that is said, there's a question of how much steel you need to build a conventional jacket – and they require a lot of steel – which means a lot of cost.

The Carbon Trust and Narec's offshore wind efforts

The Carbon Trust is working in partnership with five industry partners: Statoil, DONG Energy, SSE/Airtricity, Scottish Power Renewables and RWE npower in its Offshore Wind Accelerator.

Together, they look at foundations, electrical systems, marine access systems and yield effect and wake effect modelling, and the performance of large arrays in terms of the front turbines affecting the performance of those behind.

Part of the aim is to accelerate, de-risk and reduce the cost of specifically Round 3 developments.

Narec is about to build a large drive train test facility for nacelles up to 12 MW, Williamson says. “The ability to test major components at that scale is critical to prove out these new turbines, which are getting bigger, to meet the challenges of the offshore market. Drive train testing in particular onshore, will give you the ability to understand to a greater extent what potentially could go wrong offshore.”

Narec is also in the process of developing an offshore demonstration zone where turbines can be tested before Round 3. The intention is to develop up to 100 MW of cumulative capacity to test new and larger offshore turbines. The site will be off Blythe, Northumberland, UK.

“Deploying in 30-50 m of water is doable, but the cost – which I think needs to come down dramatically – and the foundation area, are key. Not just in terms of finding a foundation that's cheap to manufacture, but the whole process of manufacturing, transporting to site, installing and commissioning.

“Some of the designs we're looking at, for example, allow you to install the turbine and the blades at the quay side and then take the whole system out as a single unit and install it as a single unit offshore.”

In 2009, the Carbon Trust announced 7 offshore foundation designs that had been chosen for further research and development under the Offshore Wind Accelerator.

REpower used jacket foundations for its alpha ventus offshore wind farm project, which has water depths of around 30 m. However, this does not mean that jackets are always the best option, Giese explains. “The construction phase of the offshore substructures is very expensive, so you have to see what the best solution is for a particular site.”

Just as with turbines and installation and transportation, Giese says foundations need to be investigated further: “We need more projects to know what the best solution is – maybe we'll have different solutions for different projects depending on water depths, the soil, the sea, etc.”

Norwegian oil and gas giant Statoil is involved in a project for floating wind turbine foundations. Currently, its Hywind floating foundation can be found outside Karmøy on the west coast of Norway, with a Siemens SWT-2.3-82 turbine fixed on top.

The foundation is a cylinder concept, a solution similar to production platforms and offshore loading buoys in the oil and gas industry. The steel cylinder is 117 m long and the total structure with the turbine weighs 5300 tonnes. It was built by Finland's Tecnip. The foundation's three point anchoring system means it could be used in water depths of 120-700 m.

The pitch system of the rotor blades are used to stabilise movements controlled through an intelligent software system, which measures the success of previous adjustments and uses this information to fine-tune future adjustments.

How to access turbines for O&M

Vestas' Søe-Jensen says the organisation has looked at how to access turbines once offshore, as “typically, no matter what kind of machine it is, you will see a technical problem arising when the wind starts blowing.” Søe-Jensen explains that it is important to have a boat design that works for the local area in question.

Vestas has looked at other means of access than by boat – one of which is helipads, but in the end, Søe-Jensen says, “the main thing is to assure that you have reliability built into your [turbine], that you have predictability with the machine so you don't have the breakdowns when the wind blows, but that you can do your maintenance and components exchange while the weather is good.”

Narec has also looked into the challenge of accessing offshore wind farms. “If you're looking at Round 3, people are estimating that you could have 2500 guys out there at any one time servicing and maintaining thousands of turbines,” Williamson says.

"If you're looking at Round 3, people are estimating that you could have 2500 guys out there at any one time servicing and maintaining thousands of turbines."
- Wiliamson, Narec

“Helicopters are ok, but they're limited by the number you can get out there and the weather, so there seems to be a move towards offshore accommodation modules, which will act as service and maintenance hubs where you'll have hundreds of guys out there at any one time. They'll be transported by vessels as well as by helicopter.” In some ways this is similar to the accommodation modules at offshore oil and gas rigs. And some of the companies providing these, are starting to look at the offshore wind market, Williamson adds.

Sykes at the Carbon Trust says a lot can be learnt from the oil and gas sector in terms of offshore installation, operation and maintenance. “One of the key challenges we have, is taking the vast experience and knowledge of the oil and gas sector in terms of what's workable and not in the marine environment. What's safe and not, what's going to be durable and not.”

Connecting to the grid

Another big challenge for offshore wind is how to connect the offshore power plants to the onshore power grid.

The Carbon Trust is looking into how losses can be reduced in the electrical system, and how to improve the capital leveraging. Sykes, explains: “In Round 3, we will be deploying multiple hundreds of turbines – 200, potentially 300 turbines in each wind farm. At that scale you have an awful lot of opportunity to cut costs through finding smarter ways of doing intra-array cabling, both in terms of the technology and the operation of them.”

It is also a question of how much of the electrical equipment can be placed in the substations, perhaps in connection with an O&M accommodation module. Sykes says it's a question around how to re-engineer turbines so that the whole power station is not in the turbine.

Sykes believes high voltage direct current (HVDC) is the only realistic means of bringing the power to shore “because the losses with AC are just too great over any significant distance.” However, he says that “cable manufacturing and cable installation are two areas where there's a long way to go in terms of innovation. There's also a lot of opportunity there to be more efficient and to deliver more reliable, installed equipment.”

There is also a lot of risk involved in cable installation. Narec's Williamson tells Renewable Energy Focus that “I was told by one major developer [in December] that 80% of their problems in offshore installation, is the cables. Because it's just very difficult to do.”

Supplying the goods

It is not only technological challenges facing the offshore wind industry, but also how to get materials, components and required transport and installation vessels.

"One of the key challenges we have, is taking the vast experience and knowledge of the oil and gas sector in terms of what's workable and not in the marine environment."
- Sykes, The Carbon Trust

Sykes at the Carbon Trust says: “If you think about the 5-6000 turbines that are going out into the UK territorial waters over the next 10 years alone, the sheer scale of that is an immense challenge for the supply chain in terms of vessels, skilled engineers, people in the work force, etc.”

RWE Innogy is trying to avoid the problem of sourcing enough vessels for offshore wind construction. In December, the company announced it has contracted Korean shipyard Daewoo Shipbuilding & Marine Engineering Co Ltd (DSME) to build a specialist vessel for the construction of offshore wind farms. There is also the possibility of two further identical vessels at €100m each.

Professor Fritz Vahrenholt, Chairman of the Board of Directors at RWE Innogy, said at the time, “the building of our own construction vessels will overcome one of the most important supply bottlenecks we face in the construction of wind farms at sea. These ships will give us a decisive time and cost advantage in the North Sea and further afield in implementing our ambitious plans to expand wind energy.”

The vessels will be 109 m long and 40 m wide. They can simultaneously transport and install up to four multi-MW turbines, and can operate in water depths of over 40 m.

Looking ahead

According to Narec's Williamson, “what the industry needs overall is increased reliability and reduced downtime – and that means we have to have installation methods and foundations which offer a path of least resistance. They're easy to install, they're easy to construct and fabricate, they're lighter, and they're more integral.

“We have to have simple, easy to maintain turbine solutions which don't require regular service intervals because it's just too difficult an environment to get out to. If you have a bearing failure or a gearbox needs changing, and you're 100 km offshore – which is where some of the Round 3 projects are going to be – that's a very costly exercise. The best solutions will be the simplest solutions.”

REpower's Giese concludes that: “The real challenge is that we're at the beginning of a new renewable energy industry. This is something we have to recognise. It is not just to have onshore turbines in the water – it's a totally new industry.”