The offshore wind industry is not really doing anything exceptional when it comes down to the basics. Wind turbine generators, WTGs, have been around onshore since one of the first electricity-generating wind turbines was built to power a holiday home in 1887 in Marykirk, Scotland. Offshore platforms were built in the early 20th century for forts to defend coast lines and later metal towers or legs were constructed that have supported offshore oil and gas exploration and production units since the late 1940’s. Putting the WTG onto legs in the offshore waters of North West Europe and China and South Korea is only the marriage of two well proven bits of engineering. What is surprising is that it took so long before the marriage was consummated in Denmark in 1991.

Since then, however, the two parties of the marriage have changed beyond all recognition and today there are single WTGs capable of producing more than the complete output of all the WTGs in that first wind farm. These new WTGs are understandably much larger and heavier than their predecessors and therefore need foundations that are far more robust and advanced than anything seen before. In this article we look at 3 types of foundations, the jacket, the monopile and the suction bucket.

Offshore WIND spoke to some of the leading players, working with each type to find out what is continuing to happen in the development of the foundations of the offshore wind industry.

Sif Group

Just as Offshore WIND started talking to Michel Kurstjens the CCO of the Sif Group he told us that he was preparing to make an announcement that would demonstrate the confidence that the company has in the offshore wind industry and that the industry will continue to use monopile foundations in the future. It was not possible to release the announcement at the time we were talking but it would be made before we go to press. If you have not heard it yet then you will have to wait to read it later in this article on foundations.

He started looking at the future by looking back 2 years when the diameter of an XL monopile was 6.5m, today it is 9m. The growth in turbine size has been matched by the size of monopiles, and the monopile still wins over the competition of other foundations on economic grounds. The monopile is growing with the industry but he cannot see what the conditions will be, the water depth, the soil conditions, the WTG size, and sea conditions, before monopiles give way to another – probably, Mr Kurstjens thinks, to move to floating foundations. He does concede, however, that jackets may well be used for a short set of conditions that prove hostile for monopiles and too expensive for floating foundations. But this transition is difficult to predict.

This growth in the size of the XL monopile has been led by the size of the turbine from 4MW to 6MW to 8MW and the length of blades. From a single plate or strip of steel being rolled and welded to form a pipe or tube they can now weld 2 or even 3 plates together to make the pipe. “It is a challenging industry but we have been able to meet the challenge”, he told Offshore WIND. These limits have been pushed and the targets have been met. There is a key challenge that remains however, and that is the production rate. How can you build enough of these 80 metre pipes to fill the requirements of the installation vessel which will be needing to load 4 or 5 sets per week? You need to meet the number, the quality and the economics.

We asked Mr Kurstjens whether a trend towards the tower and foundation without a transition piece is the way forward. Having made 1,100 transition pieces in cooperation with their partner Smulders he told us that they are able to, and will follow whichever way the market goes.

Foundations of 100 to 120 metres length, including sea depth, with a diameter of up to 11 metres and weighing 2,000 tonnes will be possible in the future, but not needed yet in the near future. The immediate future is a foundation of 9 metre diameter and 1,500 tonnes and 100 metres long, but if you were planning a new factory you would include the larger dimensions as being within its capability.
This leads us very neatly into the news we mentioned earlier. We can now tell you that more than €60 million will be invested in the headquarters in Roermond and in Rotterdam at Maasvlakte 2.

In Roermond, approximately €20 million will be invested in new production equipment and operational modifications to facilitate production expansion. In Rotterdam, an assembly hall and a coating hall will be built at Maasvlakte 2. In addition, a storage and logistical hub covering approximately 40 hectares will be constructed by Sif, making direct loading of the company’s offshore client vessels possible. This facility is planned to be fully operational early 2017. Certainly a statement of confidence in this industry!

Universal Foundation

Although the suction bucket is an alternative to piling and can be used with jackets and tripods, it is also possible to have a single tower fitted with a suction bucket which also makes it an alternative to both the monopile and the jacket. In Aalborg in Northern Denmark Kristian Ascanius Jacobsen is Head of Business Development at Universal Foundation, part of the Fred. Olsen offshore wind related group of companies. Universal Foundation have called their suction bucket the Mono Bucket to emphasize its ability to be an alternative to the monopile foundation and to steer clear of the misconception that suction is maintained after installation. The suction is only applied as a mean of installation.

With several met masts installed, Horns Rev 2 in 2009 built by Bladt Industries, and two locations at Dogger Bank in 2013 built by the Fred. Olsen owned Harland and Wolff shipyard, to date the Mono Bucket has only one turbine fitted on such a foundation. However, this prototype has been installed since 2002 off the Nordre Mole in 3 to 4 metres water depth outside the harbour of Frederikshavn, also in Northern Denmark. It supports, what was in 2002, one of the largest wind turbines, the 3MW Vestas V90 turbine weighing 264 tonnes.

The project was set up to monitor the turbine, also a prototype, in a project run by ELSAM and MBD Offshore Power which are known today as DONG Energy and Universal Foundation, respectively, in cooperation with Aarlborg University. The Mono Bucket and the turbine research programme continues today making it the most monitored project in the industry.

This 6 metre high bucket skirt has a diameter of 12 metres and weighs a total of 135 tonnes. Figures that have been verified by the Carbon Trust’s Large Rotor study led by the Offshore Wind Accelerator (OWA) programme now indicate that a similar design with a diameter of 18 metres weighing about 1,200 tonnes would support an 8MW WTG in 55 metre water depths. The length of the skirt and the diameter of the bucket are variable, depending on the type of sea bed conditions found on each WTG location.

Mr Jacobsen told Offshore WIND that key to the future of this type of foundation is to get a demonstration project set up with at least a 6MW wind turbine, no later than 2017. They have already been demonstrating installation capabilities with DONG Energy, Statoil, E.ON and Statkraft in the various types of seabed conditions on Dogger Bank (installed in 7 hours), Dudgeon and Hornsea This demonstration programme provided 29 successful installations in 24 days. A by-product of the installation programme was a clear demonstration of the decommissioning properties of the foundation.

Capable of being floated out to the location this is a self-levelling installation requiring no sea bed preparation and little or no noise during installation. It is proven to be a very cost effective and ecologically sound foundation able to make a great step towards lowering installation costs, demonstrating up to 30% reduction in fabrication and installation.

Three companies from Norway, Germany and Denmark have already shown interest in using the Mono Bucket. However, across the Atlantic, the interest from the Lake Erie Energy  Development Corporation (LEEDCo) great lakes offshore wind project has turned into an order, overturning LEEDCo’s original preference for a monopile design. There will be modifications made to the Universal Foundation design to protect the tower during the severe winter ice conditions encountered in the Great Lakes. A cone structure at the water line will be added to deflect the breaking ice either upwards and around, or downwards with an inverted cone. The order is expected to be for 6 foundations for 3 to 4MW WTGs.

Per Bull HaugsOen

Per Bull Haugs0en, one of the founders of offshore jacket designers OWEC Tower, was working in his Bergen office when Offshore WIND spoke to him. Before we got any further on details of the design of jackets he told me that the Dutch subsidiary of the Keppel Corporation, KV Ventus BV, had very recently bought the remaining 50.1% of their shares in the company to add to the 49.9% they had owned since 2012. Today Mr Haugs0en speaks not on behalf of OWEC Tower directly, but as an expert on behalf of his consultancy company Oceanvis AS.

OWEC Tower designs account for almost 90 WTG installed foundations, 2 on the Beatrice Demonstration project, 6 on the alpha ventus project in 2009, 32 on the Ormonde wind farm in 2010, and 48 WTG jackets plus 1 transformer platform jacket on the Thornton Bank II wind farm in 2011. The water depths vary between 12 and 48 metres. The premise that the jacket option is only for deeper waters is therefore immediately dispelled. At the deeper end of  the scale though there is a practical financial limit at 50 to 60 metres water depth. The jackets, Mr Haugs0en conceded, do take longer to construct, and one for 40 metres water depth would typically take a week to construct.

When 5 jackets would be an ideal number to be loaded on a barge or installation vessel for delivery to the wind farm site, the construction and marshalling sites would have to be greater than for the monopile. These sites would also have to be located close to the loading area on the coast as delivery by road or inland water ways would be restricted. Whether the recent news from the Keppel Corporation will make their Botlek yard, near Rotterdam, a good site for future mass construction projects remains to be seen. The yard is close to the North Sea mouth of the River Maas with deep water and heavy load bearing storage space, both of which are major factors for jacket construction.

Mr Haugs0en, this Guru of WTG jacket design, told Offshore WIND that the greatest advantage of the jacket when compared to the monopile is the weight. A 1,400 tonnes monopile compares to a 600 tonnes jacket (plus piles) in the same water. Quite simply a jacket uses less steel. OWEC Tower’s main design, the Quattropod, can easily support the larger and heavier, 5MW plus, WTGs in deeper waters. The Quattropod also has a 3 legged variant design available.

Piling remains an important part of the foundation, with preinstalled piles at the base of each leg, these jackets have and important advantage – the size and weight of each pile is smaller than a monopile and the power and therefore potential noise generated in the piling operation is considerably lower. It is also easier to ensure the critical precision needed to ensure vertical piles through a positioning template frame placed on the seabed foundation than for a single pile.

The experience gained in the offshore oil and gas industry sector’s deeper waters and the advantages for the wind sector will surely mean that jackets are going to be used in the deeper water offshore wind projects in the future. There are floating foundations and different designs for suction buckets, jackets and monopiles. All of them have their specific advantages in specific conditions. There will be hundreds of foundations used in the future, all of them requiring individual attention to detail and design. The recent news of investments, orders and acquisitions give a good indication that this part of the offshore wind industry is as firm as the foundations the companies make!

Dick Hill