Making waves in offshore wind power

In the middle of the Baltic Sea, specialists from Bilfinger Berger are placing more than 90 heavyweight foundations on the sea floor: the company is ensuring that the wind turbines at the Roedsand 2 offshore windpark stand straight.

(Text: Bernd Hauser, Photos: Rainer Kwiotek)

The sun has set. The ship’s spotlights illuminate the dark surface of the water. Suddenly, the yellow helmet of a professional diver rises from the depths below. He had been inspecting the gravel bed that had just been laid by a floating dredger on the sea floor. That gravel be will be the resting place of a 1,400-tonne giant that is now hanging on the floating crane: a heavyweight foundation for an offshore wind power generator. Based on the video images from the diver’s helmet camera, the people at the construction site already know that the gravel bed is perfect.

“Let’s get started”, says Nils Benecke, the 29-yearold project manager from Bilfinger Berger. He has already made several dozen notches with a pen on the inside of his construction helmet: one for each foundation that has been set. “I’ll be adding another one tonight”, says Benecke.

He and his staff won’t be finished until the 91st notch has been made. In the Roedsand offshore windpark between the Danish island of Lolland and the German island of Fehmarn, Bilfinger Berger and their Danish partners Per Aarsleff will set five rows with 18 foundations each for the wind turbines and an additional foundation for a transformer station.

The foundations have to be rock-solid because the wind turbines they support will be exposed to extraordinary burdens. They have a wing-span of 93 meters and at full-speed, the tips of the rotary blades reach speeds of over 250-kilometers an hour. And then each turbine generates 2.3 megawatts. All of the turbines combined are capable of providing enough electricity for 230,000 households.

Benecke’s radio starts to hiss. The project manager responsible for preparing the gravel beds wants to discuss how to proceed with the work on the next day: his dredger ships and Benecke’s floating crane should not get in each other’s way. The two men talk about the weather – which does not qualify as small talk around here. The wind decides how work will proceed. As soon as the waves are higher than 75 centimeters, all work has to stop.

The Fehmarn Belt is just 19 kilometers wide and water depths at Roedsand range between just six and eleven meters. Günter Grass once referred to the Baltic Sea as the ‘Baltic Puddle’, making it sound anything but dangerous. But anyone who sees how even the giant pontons begin to dance on the waves when the foundations are brought from Swinemünde in Poland understands the degree of respect these waters command from both engineers and sailors.

Although it may be difficult to build wind power stations on the sea, the potential is enormous. Stations with a total output of 1.5 gigawatt have been built on the North Sea and Baltic Sea since 2000. By 2020, Germany intends to generate ten gigawatts from seawind, the Netherlands is aiming for six and the United Kingdom 14 – all told, the equivalent of 75 windparks the size of Roedsand. Even if these plans turn out to be overlyambitious and only a portion of that total is actually built, Bilfinger Berger will still benefit from its expertise.

Leading position in laying foundations for offshore wind turbines

In November 2009, the Group won the order for its most important offshore project to date, thereby solidifying its leading position as a provider of foundations for offshore wind power stations. Power generation companies E.ON and Dong along with financial investor Masdar are building the vast London Array Windpark in the Thames estuary with an output of one gigawatt – enough to provide power to a quarter of metropolitan London. 175 wind turbines will be put up in the first phase of the project – the largest of its kind in the world.

Back at the Roedsand site, project manager Benecke’s team is lowering the 57th foundation into the water this evening. That afternoon, a tugboat was put to the test maneuvering the floating crane into position near the pontoon with the foundations – against the wind.Workers hang the slings, heavy-duty lifting cables made of plastic, on one of the foundations. They have already flooded the tanks in the pontoon under the foundation and at the same time the tanks in the rear of the floating crane in order to compensate for the weight of the foundation. The crane operator then pulls a lever and the foundation, as heavy as 1,000 Mercedes sedans, is in the air above the waves. At this point, the tugboat drags the crane 1.5 kilometers to the place the foundation is to be set. With the help of four anchors and satellite navigation, the captain can bring the foundation into exact position above the gravel bed on the sea floor below.

A story told in just a few sentences actually takes several hours and now night has settled on the sea. Aided by banks of high-powered spotlights, the crane operator slowly lowers the foundation while constantly checking the monitor with the navigation data. 20 centimeters above the gravel bed he stops the cables. The surveyor checks the position once more and then the crane operator again pulls the lever: slowly the foundation sinks to the ground with its top section reaching 3 meters out of the water. Now it resembles a giant candle stick in the middle of the sea.

The team is proud of the precision work they do

The surveyor shows project manager Nils Benecke his results on a data sheet. He must ensure that the foundation’s actual location does not deviate by more than 30 centimeters from the planned position. The team usually manages to put the foundation down within ten centimeters of the planned position. For foundation number 57, they missed the mark by a mere two centimeters. “That’ll cost a bottle of whiskey”, says the surveyor proudly – but only once they are on land, alcohol is strictly forbidden offshore.

Project manager Benecke just smiles. He’s more than happy to buy the bottle. Before he retires to his bunk in the container, he checks the weather forecast on the laptop once again. The waves are expected to remain below the critical level in the next 24 hours. “Might work”, mumbles Benecke. It looks as though he will be able to put another notch in his helmet tomorrow.