TY - BOOK

T1 - Fatigue behaviour of submerged axially loaded grouted connections

AU - Raba, Alexander

N1 - Doctoral thesis

PY - 2018

Y1 - 2018

N2 - With conversion of the German supply system for electrical energy towards renewable resources, the demand for offshore wind farms significantly increases. For installation of wind turbines in the German North Sea, lattice substructures such as jackets are a possible design choice. These structures are usually fixed to their steel foundation piles via grouted connections, a structural detail consisting of one steel tube inserted into a second steel tube with larger diameter. The steel surfaces located inside the resulting annulus are equipped with shear keys and the annulus is filled with a grout material. The connection is located at mudline level in submerged conditions and is manufactured without sealing against water ingress. Due to the load bearing behaviour of the substructure, these grouted connections are predominantly axially loaded. All currently available design approaches for the Fatigue Limit State of grouted connections are based on experimental tests which were carried out in dry ambient conditions. However, experimental investigations on the fatigue behaviour of concrete specimens in submerged conditions show an obvious reduction of fatigue capacity caused by interactions between specimens and the surrounding water. Corresponding degradation phenomena were also observed at foundations of onshore wind turbines. Objective of this thesis is to investigate the influence of water on the fatigue behaviour of predominantly axially loaded grouted connections. In a first step, a set-up of tests on small-scale grouted connections in both, dry and wet ambient conditions, was realised. Besides the ambient condition, the influences of different grout layer thicknesses, different grout materials, the loading level as well as varied loading frequencies were investigated. In total, results from 78 small-scale specimens were evaluated. The tests showed a significant reduction of fatigue capacity caused by the interaction between connection and sur-rounding water. This effect was also influenced by the loading frequency. In order to reassess these results for larger grouted connections, in a second step a test set-up of large-scale specimens for fatigue tests in submerged conditions was developed and implemented. The results of 4 large-scale specimens with two different grout layer thicknesses, two comparable grout materials as well as alternating and compression-compression loading were evaluated. Via comparison of the results to published experiments of comparable specimens tested in dry ambient conditions, the previously observed influence of water was confirmed for the larger scale. In a third step, the experimental results were supplemented by numerical investigations. A modelling approach including detailed models with discrete shear keys was implemented. For the grout layer a non-linear material model, capable of tensile cracking by means of large plastic strains, was used. Furthermore, a fatigue design verification approach based on existing S-N curves for concrete was developed. The obtained results showed good agreement with the experiments. In summary, a significant reduction of the fatigue capacity of axially loaded grouted connections caused by interaction with surrounding water was ascertained and the relevant parameters were identified. In addition, a numerical fatigue verification approach for the design of reliable grouted connections for application in submerged conditions was introduced.

AB - With conversion of the German supply system for electrical energy towards renewable resources, the demand for offshore wind farms significantly increases. For installation of wind turbines in the German North Sea, lattice substructures such as jackets are a possible design choice. These structures are usually fixed to their steel foundation piles via grouted connections, a structural detail consisting of one steel tube inserted into a second steel tube with larger diameter. The steel surfaces located inside the resulting annulus are equipped with shear keys and the annulus is filled with a grout material. The connection is located at mudline level in submerged conditions and is manufactured without sealing against water ingress. Due to the load bearing behaviour of the substructure, these grouted connections are predominantly axially loaded. All currently available design approaches for the Fatigue Limit State of grouted connections are based on experimental tests which were carried out in dry ambient conditions. However, experimental investigations on the fatigue behaviour of concrete specimens in submerged conditions show an obvious reduction of fatigue capacity caused by interactions between specimens and the surrounding water. Corresponding degradation phenomena were also observed at foundations of onshore wind turbines. Objective of this thesis is to investigate the influence of water on the fatigue behaviour of predominantly axially loaded grouted connections. In a first step, a set-up of tests on small-scale grouted connections in both, dry and wet ambient conditions, was realised. Besides the ambient condition, the influences of different grout layer thicknesses, different grout materials, the loading level as well as varied loading frequencies were investigated. In total, results from 78 small-scale specimens were evaluated. The tests showed a significant reduction of fatigue capacity caused by the interaction between connection and sur-rounding water. This effect was also influenced by the loading frequency. In order to reassess these results for larger grouted connections, in a second step a test set-up of large-scale specimens for fatigue tests in submerged conditions was developed and implemented. The results of 4 large-scale specimens with two different grout layer thicknesses, two comparable grout materials as well as alternating and compression-compression loading were evaluated. Via comparison of the results to published experiments of comparable specimens tested in dry ambient conditions, the previously observed influence of water was confirmed for the larger scale. In a third step, the experimental results were supplemented by numerical investigations. A modelling approach including detailed models with discrete shear keys was implemented. For the grout layer a non-linear material model, capable of tensile cracking by means of large plastic strains, was used. Furthermore, a fatigue design verification approach based on existing S-N curves for concrete was developed. The obtained results showed good agreement with the experiments. In summary, a significant reduction of the fatigue capacity of axially loaded grouted connections caused by interaction with surrounding water was ascertained and the relevant parameters were identified. In addition, a numerical fatigue verification approach for the design of reliable grouted connections for application in submerged conditions was introduced.

U2 - 10.15488/3668

DO - 10.15488/3668

M3 - Doctoral thesis

CY - Hannover

ER -