HBM Strain Gauges Measure the Loads on Offshore Wind Turbine Foundations

HBM strain gauges and data acquisition equipment were used in a unique experiment performed in the Westermeerwind wind farm in the Netherlands to investigate the damping effect of the sea bed on the loads on wind turbines.

In order to make wind energy more attractive than fossil power and also as a result of the effects of competition, providers are searching for new ways of supplying the required megawatts in as economical a way as possible when constructing offshore wind farms. Many technical innovations in the areas of masts, turbines, and rotors have therefore been introduced in recent years to achieve this. Siemens Windpower has started a project to gain more insight into the interaction between the sea bed and the wind turbine foundations.

HBM strain gauges and data acquisition equipment were used in a unique experiment performed in the Westermeerwind wind farm in the Netherlands to investigate the damping effect of the sea bed on the loads on wind turbines. The goal was to obtain reliable input parameters for a standardized design process for wind turbine foundations.

Siemens Windpower is a new company that has been an independent part of Siemens AG since 1 January, 2017, combining a number of existing Siemens divisions and acquisitions that deal with wind energy. Siemens Windpower B.V. in the Netherlands has grown, in the meantime, into an organization with around 120 staff. It maintains existing wind farms and also takes care of the engineering and project management for constructing new wind farms.

Thanks to its collaboration with the TU Delft, Siemens Windpower in the Netherlands has gradually evolved into a Centre of Competence for the development and construction of wind turbines. The emphasis is on calculating loads and designing the masts and foundations. Research into developing and manufacturing the turbines is carried out mainly by the company’s Danish branches.

"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.”

Siemens Windpower is also intensely involved in research into new technologies and construction methods for wind turbines. The Disstinct project, which investigates the interaction between the sea bed and the foundations of wind turbines, began in 2014. 

‘Disstinct’ stands for Dynamic Soil Structure Interaction. In addition to Siemens and the TU Delft, companies such as Fugro, Van Oord and DNV-GL are also involved in the project.

“As you can imagine, the foundations of wind turbines are extremely important. The rotor imposes enormous forces on the mast. In the case of offshore wind turbines, wave loads, in particular, play a role in addition to the wind", says Bongers, also the project leader for Disstinct.

"The installation’s natural vibration frequency is significant when engineering a supporting structure as it determines the loads it can bear.

The correct prediction of this frequency is very important, but the major uncertainty factor is the interaction between the structure and the sea bed.

The stiffness of the soil is generally underestimated in the case of current structures, with the result that structures are calculated conservatively, stronger foundations are designed and more steel is used. The logical consequence is that the price of the wind turbine increases, which is an undesirable occurrence in a highly-competitive market that is subject to high pressure on prices.”

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.

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.”

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.”

A shaker was attached to the monopile after it had been driven into the sea bed. This is a hydraulically-driven vibrating hammer that intentionally causes a mass imbalance in order to simulate the forces from the tower and the rotor, thereby giving an idea of their effect on the foundations. IHC tested and calibrated the shaker in advance on a reinforced concrete floor at WMC in Wieringerwerf. The strain gauge points in the monopile were connected to the HBM MGC Plus data acquisition system using special waterproof cables. Inclinometers and accelerometers were also connected to the data acquisition system, which was installed on a work vessel that was anchored next to the foundations. The test with the shaker lasted for three days.

After the shaker test, the measuring cabinet containing the HBM system and the catman® software was given a permanent place in the wind turbine to give continuous monitoring for some years to come. The cabinet is equipped with an industrial grade PC and an emergency power supply, so that data is not lost in the event of a power cut. More strain gauge measurement points have been added to the existing measurement positions, with one ring of four measurement points at a height of 37 meters in the tower. Siemens has connected the PC to its internal network, so that data can be collected remotely.