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The way the wind blows: The Rodsand 2 project

As the Crown Estate signs exclusive Zone Development Agreements for Round 3 of its offshore wind expansion programme and the developers gear up for the planning and consenting phase, the UK is looking ahead to an exciting future for renewables. Jo Stimpson reports.

With this latest mega round of investment, 32GW of new offshore wind capacity could be delivered around the country by 2020.

It is an exciting time to be in energy, says Grontmij head of renewable energy Chris Paddey. Grontmij is already heavily pursuing Round 3 work with the winning developers, he says − and it stands a fine chance.

Although the company is not so well known for renewables in the UK, it has plenty of experience of wind farm construction in Europe, says Paddey. Working with client Eon, Grontmij is seeing exciting progress on one such project in Denmark, just off of the coast of Lolland.

Named Rødsand 2, this wind farm is conceived as an extension of Rødsand 1 − a 160MW farm 3km to the east, which opened in 2004 and is better known as the Nysted Farm.

“The weather delay has been causing a lot of problems and there have been more wind delays than expected. It’s good to have wind, but not when you’re constructing a wind farm.”

Bjarne Haxgart, Eon

Rødsand 2 is being built by contractors Aarsleff and Bilfinger Berger (foundations), NSW (electrical grid), Siemens Wind Power (turbine manufacture) and A2sea (turbine installation).
The construction of the substation and the grid to the coast has been contracted separately by electricity and natural gas operator Energinet.dk.

The €430M (£384M) project comprises 90 wind turbines on concrete gravity foundations, arranged in five curved rows of 18 with an offshore substation at the farm’s northern edge. Submerged array cables run along the five rows, before being connected to an energy transformer substation by 33kV cables, and then to the shore by a 132kV cable.

Rodsand 4

33kv cables connect to an energy substation

The curved layout of Rødsand 2 differs from the straight lines of the Nysted farm. This is because wind investigations uncovered an opportunity for increased efficiency, says Eon site manager Bjarne Haxgart. “They realised you’re not having such a big wind effect if you set them up in [straight] rows rather than curves like this,” he says.

Dredging on this project is a sizeable task. Dredging along 75km is required for the installation of the array cables, whose inner diameters range from 120mm at the outer edges of the farm to 240mm at the central parts. A 90m long barge carries 40 men to install the cables and backfill the trenches.

Further dredging creates 90 gravel beds for each of the turbine foundations. The dredged material must be deposited elsewhere. A split barge takes the dredged material to a western area, past the nearby town of Rødby, for depositing.

An 18m2 steel frame with adjustable legs is then lowered into the bed by crane, before being filled with large stones and then removed. A gravel bed takes 14 to 16 hours to complete.

Rodsand 5

Barges will carry the final turbine components

Meanwhile, the 1,460t reinforced concrete gravity foundations are constructed at a port in Poland where, says Haxgart, “they have the space and the workers and are cheaper than in Denmark”. Further cost savings are made by constructing the foundations straight onto transport barges so they can be conveyed to site without the need for lifting equipment.

“They have a production line,” says Haxgart. The empty barges arrive at a harbour where steel reinforcement cages are prefabricated on the shore, then assembled on the barge. “We claim it’s a concrete foundation but it’s a lot of steelwork,” says Haxgart. The foundations are then cast in concrete, and secondary steel − including railings and an access ladder − is installed before the barges are shipped to Denmark, each carrying six foundations.

The varying seabed terrain means each foundation’s shaft height must be precisely determined to ensure each one protrudes 3.5m above sea level. The foundation’s bottom plates have a 16m diameter and contain ballast cells 3m deep.

“When we sent the picture [of the historical anchors and warship rudder] to the National Museum of Denmark they said: ‘We have been expecting that for 200 years.”

Bjarne Haxgart

Upon reaching the site, a neat trick comes into play. The barges are submersible, and they slowly sink to the seabed over eight to 10 hours, meaning less lifting height is needed when the foundations are moved into place. At this point, the load on the barge must be carefully managed to keep it submerged.

“The crane comes in and does the lift, and there are some guys on the barge to ballast the same weight as the foundation to avoid the barge rising,” says Haxgart.

With the foundation in place the cables can be installed, running through the ballast chambers and into the shaft. The foundation is then weighted with large ballast stones in the chambers, sand fill placed inside the shaft and smaller stones around the edges of the bottom plate. “All the stones are coming in from Norway,” says Haxgart.

While this type of foundation is complicated to manufacture and install, Haxgart says it has offered a 30% cost saving compared to the alternative of using monopile foundations. “Price is the biggest advantage,” he says, “even though the process is more complex”. The costs of the materials and pile drivers needed for monopiles could not compete with the relatively cheap Polish manufacture of the gravity foundations, and the savings made by using the submersible barge rather than heavy lifting equipment.

Rodsand 3

Getting it right: The seabed terrain means each foundation’s shaft height must be precise

Finally, the cable connections must be checked. Cable surveys are carried out using a remote underwater camera, which is cheaper and safer than using divers. The camera is operated from inside a tug boat, using a joystick to move the camera underwater and a monitor to see the seabed. It is just like a computer game, says Dan-Tug boat captain/owner Claus Drechsler, “but when this one says ‘game over’ it’s a little more expensive”.

The 2.3MW turbines are preassembled at a port around 100km away.

“They do as much as they can before coming out on site,” says Haxgart. “It’s almost plug and play.” The turbines, which have a hub height of 68.5m and a rotor blade diameter of 93m, are assembled on site in four pieces: bottom tower, top tower, cell and rotor.

“If it goes well it can be done in six hours. But it has been done in four,” says Haxgart.

The team from Eon − which part-owns the adjacent Nysted Farm − is benefitting from the experience of its past projects. However, the challenges faced on Rødsand 2 have been myriad.

The offshore environment has posed health and safety risks. “The guys are not sailors,” says Haxgart. “They are onshore guys working offshore.”

Rodsand 7

Working far offshore

 

It also brings environmental restrictions − all waste must be brought to an on site waste station and all oil spills must be reported to the relevant authorities.

Naturally, the site was chosen for its high levels of wind − something that has proven very challenging.

Heavy wind frequently makes the waves too high for construction, sometimes halting the project for whole days.

“The weather delay has been causing a lot of problems and there have been more wind delays than expected,” says Haxgart. “It’s good to have wind, but not when you’re constructing a wind farm.”

Negotiating for space with other vessels has also proven problematic. Ferries frequently sail nearby, shuttling between Lolland and the German coast and often getting in the way of vessels trying to get to the Rødsand 2 site. Haxgart says one occasion saw a construction vessel waiting for an hour because of the ferries.

“It gets very busy sometimes,” he says. “We have learned to live with it and they have learned to live with us.”

“The weather delay has been causing a lot of problems. It’s good to have wind, but not when you’re constructing a wind farm.”

Bjarne Haxgart

Eon supervision project manager Niels Kiersgaard says the global position system (GPS) has made it easier to keep track of the traffic, allowing the site to be mapped and a real time GPS record of the location of each of the 25 vessels on site to be kept.

“That’s very good for us to be able to see where all the vessels are,” he says. “With a lot of technology now it’s getting easier.”

Traffic is not the only thing getting in the way. Site investigations have uncovered considerable litter on the sea bed − including four historical anchors and a warship rudder, thought to belong to British ships that sank in storms in 1811 in the midst of the Napoleonic Wars.

Eon paid to have the artefacts salvaged for posterity, in an operation costing around 350,000DKK (£42,314). “That was extremely expensive,” says Haxgart.

“When we sent the picture to the National Museum of Denmark they said: ‘Thanks very much, we have been expecting that for 200 years.’”

Rodsand 6

A completed submerged section

Despite the weather hold-ups and unexpected costs for the archaeological work, Rødsand 2 is progressing very well. Work on the foundations began in January 2008, and the substation − weighing 900t − was installed by crane in mid December 2009. It will be energised at the end of this month. The turbines will be installed on site from mid March this year until September.

Now, 72 of the foundations have been completed, plus 21 array cables.

The area for turbine preassembly is also ready. If the weather co-operates and there are no more surprise dealings with the National Museum, says Haxgart, the team are confident they can meet their deadlines.

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