Support structures and foundations

Experimental testing and validation

The Test Center Support Structures (TTH) in Hanover offers a unique infrastructure for testing all types of support structures (towers and foundations) on a scale of 1:10 and larger. The foundation test pit and the span can be used to investigate fatigue and extreme load behavior under multi-axial loading. The test center also offers four specially equipped laboratories to carry out scientific investigations. The “structural health monitoring laboratory”, the “soil mechanics laboratory”, the “concrete laboratory” and the “fiber composite laboratory” complete the infrastructure of the test center.

Clearly defined testing procedures up to extreme loads provide reproducible results and thus allow complex questions to be answered. Bringing together structural models, numerical analyses and large-scale experiments, simulation models can be validated, and wind turbines with improved availability and better cost effectiveness can be realized.

The structure’s dynamic and fatigue behavior under long-term cyclic loading by waves, wind and operation can be reproduced in short time, i.e., tests lasting three to four months can provide meaningful results.

System reserves can be determined, further optimization potential can be defined, and the structural design can be adapted accordingly. Constructing more slender structures while maintaining structural safety and reliability enables savings on materials and logistics. Besides, more environmentally acceptable and economical construction methods can be developed and tested at TTH.

The center is property of Leibniz University and ForWind. Fraunhofer IWES is the main user of the infrastructure.

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Virtual visit to the Test Center Support Structures

The foundation test pit (FTP) measures 14 m in length, 9 m in width and 10 m in depth, which makes it the world‘s largest testing facility of its kind. In it, realistic load tests with cyclic and static load application are carried out to examine the behavior of large-scale structural models (on a scale of up to 1:5). The abutment on the long side of the pit as well as steel frames and angle plates – the position of which can be varied – make it possible to test a wide variety of load scenarios: The structure can be subjected to both single and multi-axial loading via hydraulic cylinders. An analysis of the structural
behavior during the test allows conclusions to be drawn about the interactions between the sea floor and the support or foundation structure.

In addition, the FTP also facilitates the testing and assessment of all sorts of pile installation methods, such as impact-driving and vibratory-driving. Adjustable water level and sand filling level permit reproducible, realistic, homogeneous soil conditions during the test series.

The combination of structural models, numerical calculations, and large-scale experiments make it possible to test and validate new and existing foundation systems, installation methods, and simulation models.

© Photo Jan Meier

Offshore wind turbines are breaking records for power output and dimensions on an ongoing basis. This means that the bases of assessment, too, are nearing their limits when it comes to technical rules and standards. Large-scale tests are essential to verify the reliability of components. As the dimensions of the parts increase and load situations during operation become more complex, the test environments must become bigger and more sophisticated as well.
 

The clamping field (L 18.5 m x W 9.5 m) with its rigidly attached angled walls (L 9.5 m x W 10 m x H 8.0 m) in the Test Center for Support Structures represents a unique test environment in terms of size and variability. The fatigue and load-bearing behavior of components can be tested here. In addition to the massive abutments, welded angle constructions and/or portal frames can be placed anywhere in the clamping field to facilitate the multi-axial testing of any test specimens. A selection of 14 test cylinders in total with maximum loads of 250 kN to 2 MN makes it possible to simulate load scenarios realistically. In order to size a support structure, the focus is often on the transitions and connections between individual areas and elements of the construction.

This includes, for instance, grouted joints, which are a hybrid composite of steel and concrete and, as such, create a transition between the foundation pile and the substructure. The joints can be tested in the horizontal clamping field on a large-scale under bending, axial tensile, or compressive forces. Under alternating loads, it is possible to test the fatigue behavior and determine the current damage status. In what is called a jacket foundation, the nodes of the lattice structure are one of the details that are critical to fatigue.


These nodes are usually welded joints with hollow crosssections. The cyclic testing of a typical node with hollow cross-section in a double-K configuration in the horizontal clamping field of the Test Center for Support Structures is illustrated by way of example. For the test, the joint was scaled down to approximately 1:2.5 and subjected to cyclic loading. In addition to the structural behavior, a further aim of using this demonstrator was to analyze the resistance of a novel coating system to mechanical stresses.

Piles under axial loads

© Photo TenneT Offshore

Despite all their advantages, monopiles have their limitations in certain geotechnical and economic circumstances. For this reason, alternative foundations such as jackets are used at great water depths. These can be founded using individual piles or, as is usually the case with offshore platforms, using pile cluster-based systems. What these structures have in common is that horizontal loads acting on the support structure are transferred into vertical loads on the foundations; therefore, the key design parameter for the piles is their axial load capacity. These tensile and compressive load capacities are determined not only by the pile itself – that is, by its geometry and construction – but also to a large degree by the way in which the pile is installed into the seabed (as well as the properties of the seabed). The diameter of these piles is usually much smaller than that of monopiles, with the result that a whole array of installation methods is possible in addition to driving, for example jacking.

Investigating alternative foundation variants to the current impact driving is an important topic in the offshore industry. Apart from pure material costs, when implementing the foundation, it is crucial to consider factors such as installation time and effects on the pile load capacity.

For this reason, intensive research is being done in the area of pile-installation. For example, vibratory pile driving is being scrutinized. This offers a possible alternative to the current impact pile driving, because noise emissions and time consumption are much lower by comparison. However, a full picture of the effect of installation on pile load capacity – both on piles under lateral load and under vertical load – is not yet formed which is why vibrated piles are currently impact-driven afterwards. It is hoped that comparisons of vibratory-driven and impact-driven test piles will produce new findings in terms of their global axial and lateral load capacity.

Another new installation method, which is being studied at IWES in conjunction with an industry partner, is continuous-, jacked piling which demonstrates much higher axial load capacities compared to driven piles, and can therefore be used for jackets, for instance.

Piles under lateral loads

According to WindEurope’s findings, more than 80% of all wind turbine foundations that were installed in Europe at the end of 2016 were impact-driven monopiles*. This means that today, and also in the foreseeable future, this is the predominant type of foundation in sandy sea floors, as it offers an array of advantages over alternative systems: this method is a well-established and comparatively cost-effective process; there is an extensive database on load capacities and service life, and little or no groundwork is required on the sea floor. However, installation in greater water depths and wind turbines with capacities up to 10 MW are technically challenging, and require a greater diameter. Large diameters (already up to 8 m by the end of 2016) on the other hand place great demands on the driving technology, and, above all, the design models (e.g.-, the p-y curve method) currently available are, for the most part, no longer directly applicable to these extreme dimensions.

Fraunhofer IWES is collaborating with the industry to conduct in-depth research in the area of design and validation of monopiles. To this end, piles are scaled down to 1:5 to 1:13 based on scaling laws. Subsequently, these are installed in a model sand similar to the offshore construction site and subjected to either static or cyclic lateral loading. During the test, the support behavior is recorded by sensors on the pile (e.g., strain gauge, inclinometer, displacement transducers) and in the soil (e.g.-, earth pressure and pore pressure transducers). Using these, the pile deflection lines and load transfer curves can be reconstructed, analyzed, and compared later. This is used for the further development and validation of models, such as the p-y curve method.


* The European offshore wind industry / Key trends and statistics 2016, p.23.
Report by WindEurope asbl/vzw, available at: https://windeurope.org/wp-content/uploads/files/about-wind/statistics/WindEurope-Annual-Offshore-Statistics-2016.pdf

 

 

 

The reliability of offshore wind turbines is monitored non-stop during operation. To that end, measurements are analyzed, and changes are identified and assessed. Sensors are attached to selected points on the support structure, and sensor function is ensured to guarantee continual and consistent measurements. Benign changes caused by environmental influences and different operating conditions must be differentiated from changes that could, in the long-term, lead to damage; they must be localized and characterized. The systems that can do this are referred to as Structural Health Monitoring (SHM) systems. They should be able to operate automatically, as access is complicated. The analytical methods they use must be geared towards the damage to be expected and enable them to identify benign changes in later operation.

In particular, the ongoing functionality of the measuring technology and the clarity of the evaluation methodology for different types of damage and environmental conditions can be assessed and optimized with the aid of support structure models in the foundation test pit. Causing defined damage to individual support structure components in the clamping field furthermore facilitates the objective and targeted validation of the monitoring system.The combination of structural models, numerical calculations, and large-scale experiments make it possible to test and validate new and existing foundation systems, installation methods and simulation models.

© Photo Dr. Andreas Willecke, Abteilungsleiter, Ramboll Wind+Towers - Offshore Wind

„For Ramboll, the development and improvement of design methods is always of great importance in order to reduce the cost of renewable energy. The large-scale tests conducted within the framework of the TANDEM project at the Test Center for Support Structures are an important component for validating new geotechnical design approaches for offshore wind foundations.”

© Photo Dr. Elmar Wisotzki, Asset Manager Platforms - TenneT Offshore GmbH

„The verification of calculation approaches by realistic model testing is considered unavoidable. For the execution of such experiments, TenneT found an optimal partner in Fraunhofer IWES, which offered ideal testing conditions. The results of the large-scale tests gave us the required planning certainty for our offshore foundations.”

© Photo Andreas Lammers, Senior Projekt Manager Vallourec

„The prospect of industrial scale pile capacity testing at the Test Center for Support Structures will save both time and money for PREON®marines development. The planned testing program will continue to yield invaluable insight into the load bearing behavior of piles installed with realistic loading simulations in offshore environments, using the innovative pile installation technique. The close cooperation between Vallourec and IWES was essential in ensuring the successful completion of the first test phase.”

© Photo Dr. Herbert Friedmann, Leiter Forschung und Entwicklung Wölfel Engineering GmbH + Co. KG

„We investigated in the foundation test pit if damage at the grout connection of monopiles is detected and if it can be distinguished from nonhazardous changes with environmental and operational conditions that evoke very similar signals. Sensors at the tower generated measurement data that was evaluated by routines for signal processing. On the basis of the results, these routines will be improved for further automatization of the system.”

© Photo Prof. Peter Schaumann, Leiter Institut für Stahlbau, Leibniz Universität Hannover

„Safety and reliability are the main principles of structural design. Offshore, design often ranges at the proofed limits described in technical rules and standards. Thus, findings from large-scale experiments with dimensions and loading close to reality contributes to design support structures reliably and efficiently.”