Competition, innovation and pressure on prices

"Wind power has become enormously popular all over the world in recent decades,” explains Jeroen Bongers from Siemens Windpower, Netherlands. Many wind turbines and wind farms have been constructed in the Netherlands in the last 25 years. More than five percent of the electricity generated in the Netherlands comes from wind turbines at the moment, and although the country is still a long way behind in international terms, we are on the right track. Wind power was given a prominent place in the new Energy Agreement in 2015. A total of some 4500 megawatts will be installed off the coast of the Netherlands at Borssele and IJmuiden in the next few years.” According to Bongers, one disadvantage of the rapid growth is the increasing competition. "More and more consortiums are bidding for new tenders, which means that prices are coming under pressure. That’s good for the government in any case. A few years ago, we assumed an average cost price of 100 euros per megawatt in 2020, but that has already fallen to 73 euros. Vattenfall has developed a new wind farm in Denmark with a cost price of less than 50 euros per megawatt. Top consortium Kennis en Innovatie Wind op Zee (TKI-WoZ) has calculated that a price reduction of 46 percent compared with the price level in 2010 is achievable by 2020, which would make wind energy a competitive source of energy, for which subsidies would no longer be necessary.”Competition and pressure on prices poses a considerable challenge for businesses to reduce the cost of wind turbines and wind farms, which is why a great deal of research is being carried out. This has already led to the output from wind turbines having doubled from 3.6 MW to 8 MW in just five years. On this, Bongers says,

“The price tag for a wind farm with an output of 100 MW that consists of 13 wind turbines of 8 MW each is naturally much more attractive than one with 25 wind turbines that deliver 4 MW each.”

Experiment in the IJsselmeer

We know a lot about the static and dynamic forces on wind turbines, but not so much about the damping effect of the sea bed”explains Bongers. “Our research was therefore directed mainly at the role of the sea bed. In broad terms, a stiffer floor absorbs loads more readily than a looser floor.

The condition of the sub-sea soil is therefore an important starting point when designing wind turbines, and an extensive soil survey yields better input parameters for designing the foundations.

We wanted to map out and validate this relationship in the Disstinct project, not just using computer models but also in practice, and that’s what we did when constructing the Westermeerwind wind farm.” This wind farm lies in the IJsselmeer, along the coast of the North-East Polder, just to the north of the town of Urk. It generates 144 MW, which is sufficient to provide power to 160,000 families. The 48 Siemens wind turbines are arranged in two rows and are spaced some 400 to 500 meters apart. They are 95 meters tall and the rotors are 108 meters in diameter. The water they stand in is 4 to 7 meters deep. Siemens was the turnkey contractor for the wind farm, which was constructed together with Van Oord, BM4Wind, and VMBS in a contract from Westermeer Wind B.V. The wind farm was officially commissioned by Minister Kamp of Economic Affairs on 21 June, 2016.

Foundations with strain gauges

A thorough examination, including seismic tests, of the bottom of the IJsselmeer under the Westermeerwind wind farm was carried out for the Disstinct project. The steel foundations were designed on the basis of these tests. The monopiles are five meters in diameter, weigh more than two hundred tonnes, and penetrate the sea bed to a depth of approximately 25 meters. One of the foundation piles was equipped with measuring equipment for the purpose of the experiment. The pile was fitted with rings of four strain gauges at seven levels on its inner side, so that the amount of stretch in the steel could be measured. The number of rings was deliberately chosen to be that large in order to be certain that the strain gauges would provide the desired information, even if one or more rings failed to function. “Installing the sensors was a complicated operation”,explains Marc van den Biggelaar, one of the certified strain gauge application engineers from HBM, North West Europe, which is involved in the project. “A foundation pile is some five meters in diameter, so we had to work with a small aerial work platform that could travel through it. The technicians wore personal protection equipment in accordance with health and safety legislation, and they used special low voltage apparatus. The steel pile had to be earthed because it is a conductive structure."

"There are two ways of attaching a strain gauge, gluing or spot welding; and gluing turned out to be the only option for this project because the monopile had already been certified.”

Project Realization

Strain gauges have been glued in place for many years, using particular types of glue and particular gluing techniques, depending on the material, the application, the temperature range and the ambient conditions. For the Disstinct project, in which the strain gauges were under water and even in the sea bed, a special type of glue and covering medium was used to make them waterproof. According to Van den Biggelaar, HBM engineers from Scandinavia have often used this technique, so it made sense to fly in a specialist team from Norway for the project. “The ambient conditions were not ideal during the work, which meant that the monopile had to be preheated in order to have the glue and the covering medium cure properly. The connection cables also had to be of a special waterproof type. To give them additional protection, they were laid in a cable duct that was installed by a third party and was also glued to the monopile. An interesting unplanned additional consequence was that the installed strain gauges gave us the facility of monitoring the behavior of the foundation pile while it was being driven. Eighty percent of the strain gauges eventually survived the pile driving, which was considerably more than what we had expected. The strain gauges will provide much more data in the coming years, and in Norway, we have similar measuring projects that have been operational since 2003.”

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