Offshore wind turbines supported by suction caisson jackets transfer the loading to the foundation primarily through a vertical push-pull mechanism. This leads to the foundation performance under vertical cyclic loading to be a critical design aspect, with recommended design practice usually limiting the tensile capacity in sandy soil to the drained frictional resistance. Recent research of suction caissons under cyclic vertical loading into tension in sand has revealed the complexity of the load transfer mechanisms and the importance of understanding the soil permeability governing the in-service foundation performance. Soil stratification, with sand and clay layers, adds further complexity. Numerical methods only provide rough estimates in the in-situ caisson performance. The current paucity of data of suction caisson in-service performance in layered soil under loading conditions relevant to offshore wind turbines calls for physical evidence to be gathered. This is addressed here through centrifuge tests on vertical cyclic loading, which unravel the actual impact of tensile loads below and above the drained frictional capacity. The vertical cyclic loading test results indicate that not just the development of suction pressure inside the caisson foundation, but also the interplay between tension and compression have a significant effect on the overall response of suction caisson under cyclic vertical loading in layered soils.
Marc Stapelfeldt, Britta Bienen and Jürgen Grabe
Keywords: offshore wind turbine, suction caisson, layered soil, cyclic loading, numerical modelling, centrifuge modelling
Publisher: DFI