According to political goals, electricity from offshore wind farms in Germany will amount to 15 GW by 2030. Most of the wind farms will be located far offshore in water depths larger than 30 m. For such water depths, lattice substructures like jackets or tripods are the preferred solution. The substructures are founded on piles and connected to these by submerged grouted joints. In lattice substructures grouted joints are predominantly loaded by cyclic axial loads. Even though, this connection is well known from the offshore oil and gas industry, comparatively few results on effects of attrition due to water for grouted joints are available. At the Institute for Steel Construction of the Leibniz University Hannover, Germany, within the joint research project ‘GROWup’ investigations focus on the fatigue performance of grouted joints under predominant axial loading. As part of this research project, cyclic loading tests on small scale grouted joints with shear keys were conducted. The specimens were filled with industrial grout products and tested in a water basin to evaluate the influence of water to the fatigue performance of the connection at small scale. These test results show that the water effect leads to a significant reduction of the fatigue performance of the connection compared to results from tests in dry conditions. Moreover, the impact of water is proportional to the applied test frequency. The results of the structural tests differ from results of pure material specimen tests, on which current code provisions are based. This paper presents results from the small scale tests on grouted joints with shear keys in dry and submerged conditions. Effects of attrition due to water are shown and their effect is quantified. The results are compared to other published findings. In conclusion the presented investigations underline the importance of the water effect to the fatigue performance of small scale grouted joints. In conclusion, this paper will stimulate discussions on the demand for research on large- and real-scale grouted joints.
With an increasing demand for renewable energy, offshore wind farms become more and more important. Within the next 15 years the German government intends to realize offshore wind farms with a capacity of 15 GW of electrical energy. This corresponds to approximately 3000 to 4000 new turbines. The grouted joint is a common structural detail for the connection between substructure and foundation piles in offshore wind turbine structures. For lattice substructures such as jackets, the connection is located just above the seabed and is permanently surrounded by water. Prior investigations by Schaumann et al. showed that the surrounding water may have an impact on the fatigue performance of grouted joint specimens. Thus far, very few results of submerged fatigue tests on grouted joint specimens are published and their statistical reliability is insecure. Within this paper, 24 individual test results are presented. Regarding test parameters, the focus is set on two different applied load levels, two different loading frequencies and two different grout materials. All parameters are varied in a factorial experiment and are statistically evaluated. The evaluation shows that load level and loading frequency have a significant effect on the fatigue performance of the connection. Moreover, both effects are significantly correlated. For the used grout materials no significant impact is visible, which can be explained by their similarity regarding mechanical properties and micro structure. Furthermore, the mean displacement and the stiffness degradation of the specimens during fatigue tests are discussed in detail in the paper. In conclusion, previously published results on the fatigue performance of submerged small scale grouted joint specimens can be confirmed. Load level as well as loading frequency can be stated as most relevant parameters for the fatigue performance.
Grouted connections represent a common joining technique between substructure and foundation piles of offshore oil & gas platforms as well as of offshore wind turbines. Due to cyclic loads arising from wind and wave actions the fatigue performance of the connection has to be considered. In lattice substructures like jackets the grouted connections are located at seabed level being fully submerged during their entire lifetime. Today’s fatigue design regulations are based on investigations neglecting any influence of the surrounding water since they were conducted in dry ambient conditions. So far, only Germanischer Lloyd gives additional recommendations for submerged grouted connections. At the Institute for Steel Construction, Leibniz Universität Hannover, Germany investigations of the joint research project ‘GROWup’ focus on the fatigue performance of axially loaded grouted connections. The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi, funding sign: 0325290) and is the third project in a row dealing with grouted connections. As part of this research project, cyclic loading tests on small-scale and large-scale grouted connections with shear keys are conducted. Small-scale fatigue tests showed a reduced number of endurable load cycles for connections when tested in wet ambient conditions. However, the transferability of these findings to a larger scale was still doubtful due to unknown scale effects. Therefore, the impact of water on the fatigue performance was tested recently at large-scale grouted connections. Previous to the submerged large-scale grouted connection fatigue tests, similar test specimens were exposed to alternating loads at dry ambient conditions. Comparison of both large-scale test results under wet and dry conditions enable to estimate the influence of water on the fatigue performance of grouted connections. Reflection of the small-scale test results gives hints on the scale effect. Test preparation, test results and design recommendations are presented in the paper.
Herrn Univ. Prof. Dr.‐Ing. Gerhard Hanswille zur Vollendung seines 65. Lebensjahres gewidmet Grout‐Verbindungen sind ein häufig eingesetztes Konstruktionsdetail in Offshore‐Windenergieanlagen und ‐Plattformen. Sie stellen eine lastabtragende Verbindung zwischen gerammten Pfählen und Tragstruktur her. Bei aufgelösten Unterstrukturen befindet sich diese Verbindung üblicherweise unter Wasser. Ein möglicher Wassereinfluss auf das Ermüdungsverhalten des Konstruktionsdetails wurde in früheren Versuchen und den daraus abgeleiteten Bemessungsverfahren nicht berücksichtigt. Die in dieser Veröffentlichung beschriebenen versuchsgestützten Untersuchungen zeigen an klein‐ und großskaligen Versuchskörpern eine deutliche Reduktion der Lebensdauer bei Ermüdungsbeanspruchung unter Wasser gegenüber trockenen Umgebungsbedingungen. Neben einer Beeinflussung der Lebensdauer verändert das umgebende Wasser auftretende Schadensmechanismen in der Verbindung. On the fatigue behaviour of grouted connections at varied ambient conditions. Grouted connections are a frequently used structural detail of offshore wind turbines and platforms for the load transferring connection between piles and support structure. At latticed substructures this connection is commonly located at mudline. However, a potential influence of the surrounding water on the connection's fatigue behaviour was neglected in earlier tests and consequential design methods. Herein described experimental investigations at small and large‐scale fatigue tests in submerged conditions, showed a significant reduction of endurable load cycles. In addition, the water impact caused varied damage mechanism in the connection.
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