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Foundations of the future

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As wind turbines grow in size and the offshore farms they are installed in are located further from shore the challenges for finding suitable foundations to support them are also increasing. Ramboll, one of the biggest and oldest industry players explains what underlies foundation decisions and forecasts what the future holds.

The design of a wind turbine foundation depends first and foremost on the local site conditions and the chosen turbine plus the emergence of large wind farms which are playing an increasingly important role in the direction of industry solutions.

"These large offshore wind farms are setting the scene for foundations," says Henrik Carstens, Project & Technology Director, Ramboll Offshore Wind. Henrik has more than 10 years of offshore wind design experience, having implemented 15 projects. "The deeper the water, the more wave load and this means a different strategy to what we have seen up to now."

Within this timeframe wind turbine manufacturers like Vestas Wind Systems have doubled the diameter of rotors, further complicating the challenges for foundation design.

"When the rotor diameter increases you have to make the hub height higher," says Henrik Carstens. "The nacelles are getting bigger too which further increases the load. When the hub height increases, because of the increasing rotor diameter, the further up the turbine goes and the more stiffness in the foundation you need."

Ideal foundation

The ideal foundation takes into consideration these four aspects:

• Fabrication
• Installation
• Operation & Maintenance
• Decommissioning

Foundations cost about 21% of the total costs to set up an offshore wind turbine, which is quite a big chunk of the overall project, but water depth plays a decisive role in the final cost. This requires different solutions to those that have been pursued in land-based and close to shore installations up to now.

Different foundations

Monopile foundations are the most successful and widely used to date. These foundations are made out of steel. Around 70% of the world’s offshore wind foundations are monopile structures.

Another foundation used offshore, though much less frequently, is the gravity based structure. These foundations are made out of concrete and because of this they are not only heavy and thereby stable, they are also potentially cheaper to construct. However, there are only six foundations in the world like this so far for water depths in excess of 20 metres. But for areas besides the North Sea these might prove more relevant in the future.

"There will be some sites, for example maybe when France develops its wind farms where gravity based structures are the answer because it has a lot of rock in its seabed," says Henrik Carstens.

For shallow waters (i.e. 5, 10, 15 metres depth) gravity based and monopile foundations are mainly used. In deeper waters the wind turbines get larger and then you need a bigger diameter foundation. The downside to these foundations is that they attract bigger waves which cause stability and durability issues and due to their size and weight they are very heavy and difficult to install.

Instead, jacket foundations (three and four leg jackets) which have for many years been widely used in the offshore oil and gas industry, are recommended to support wind turbines. They can be as stiff and sturdy as monopiles but don’t attract the same amount of wave load.

"Our experience is that these structures scale best with the size of the turbine and the seabed depth," says Henrik Carstens. "At 20-35 metres of water depth you have a choice depending on the soil conditions and turbine’s rotor width. But in even deeper waters we believe mostly in jacket foundations."

There are also tripod and floating structures too. Tripods are not that commonly used.

"I don’t believe much in tripods," says Henrik Carstens. "It combines the high costs of the jacket and the high weight of the monopile and is not a winner."

Meanwhile, floating structures are still in their infancy. There is only one floating structure in the world which is located in Norway’s North Sea basin.

Increased degree of flexibility

Offshore wind farms today can range from 10-20 units to 100 or more and it is estimated that it won’t be long before we see sites with 300 or more. This further increases the challenge for foundation designers as the larger the area the greater the flexibility needed because the water depth and seabed can vary greatly within the designated site.

"You have to choose a concept that is flexible over the whole site," says Henrik Carstens. "If you have a wind farm with 200 wind turbines you want the same type of foundation in order to simplify installation, logistics and maintenance."

Weather’s role

Weather conditions also play a decisive role in the kind of foundations chosen as conditions further out to sea are more challenging than closer to shore. When installing a large wind farm this has to be considered, plus how many vessels are available. Maintenance is also an issue. Combined, this has an effect on the services and equipment offered by companies involved in offshore wind development. 

"If you have to install 20 units you can’t just do it in the three summer months.But if you want to install 200 units, you have to do it around the clock all year otherwise you would never finish," says Henrik Carstens.

"As there is a limit to which wave heights the various foundation concepts can be installed in, there is a drive for better equipment to make sure you can install all year round."

Storage/staging areas

To install such large wind farms great storage or staging areas are required to easily supply installation vessels. The size of the chosen structure determines how much storage area or "staging area" is needed. To avoid downtime in installation this area needs to be close to the servicing harbours.

Fabrication and installation methods and costs

Gravity based foundations are the cheapest to produce as concrete is cheaper than steel, which is also subject to market fluctuations. Installing such structures can be more costly though as concrete weighs more than steel and requires bigger cranes and vessels than for monopile or jacket installations.

Monopile foundations cost about 2 Euros per kilo steel to manufacture, which is less than half the price of jackets. Monopile parts have the advantage of being able to be welded by robots making it suitable for automated fabrication.

Jacket foundations cost around 5 Euros per kilo steel to manufacture. Jacket structures are more expensive because of the tubular joints which are welded manually.

Based on these differences it is apparent there’s room for optimisation of the manufacturing process.

"We expect that with the huge projects we are facing that there will be a lot of fabricators looking into automating the manufacturing process for jackets," says Henrik Carstens. "And if someone figures this out and automates the process then the jacket will become more and more competitive."

But for now it’s still cheaper to make a monopile. However in deeper waters the transportation and installing of such large amounts of steel becomes so great that a jacket then becomes more competitive. Calculations by Ramboll show that for a 5MW turbine in the Baltic Sea, at a depth of 35 metres the jacket and monopile are equally competitive. At even greater depths the jacket is the most cost effective solution.

"Therefore the majority of the structures we will see are going to be jackets," says Henrik Carstens.

Other considerations

The availability of installation vessels is also an important factor. If there’s only one vessel in the world to install then a developer’s negotiation position is not good.

The collision friendliness of a structure - so it collapses if hit by an oil tanker to avoid environmental damages - also influences the foundation chosen. This is particularly in focus in German offshore developments.

Decommissioning

No real decommissioning of wind farms has occurred yet but this process has to be taken into consideration too. Jacket and monopile structures are easy to remove whereas gravity based structures, which are also often filled with stones to provide further ballast, are more difficult.

"Ideal foundations take into consideration all of these factors," says Henrik Carstens. The best choice is site and turbine specific. Thankfully the global energy challenge is driving the cost down and making wind power more competitive with other power sources."

About Ramboll Offshore Wind and wind turbine foundation design

Ramboll handles foundation design, grid connections, power station specifications, scheduling, and timelines for geotechnical studies and basically assists wind farm developers all the way through projects. It optimises design using Ramboll in-house analysis software. Ramboll also does environmental impact assessments on some projects.

There are four to five stages of foundation design. It takes 3-5 years from pre-conceptual design to detailed design. After this long process, comes hopefully project sanction and financial close and that’s when contractors become involved and the wind power vision of the developer starts to become a reality.

Between 2009 and 2011 74% (2256MW out of 3058MW) of the offshore projects have been based on monopiles. Ramboll has designed the foundations for 54% (1646MW out of 3058MW) of these. To date Ramboll has carried out detailed design of more than 1,100 monopiles, corresponding to more than 50% of the world’s offshore wind foundations.

 
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