Cyclic behavior of tripod suction bucket foundations for supporting offshore wind turbine using centrifuge tests

Abstract

vertical

and moment) loadings applied on each bucket lead to complicating mechanism of each bucket. A tripod suction bucket foundation consists of three individual buckets arranged in a triangular shape as shown. When horizontal cyclic loading is applied to the wind tower and blades at a certain height above the foundation, the foundation experiences large overturning moment loads that approximately acts as a ‘push–pull’ action at each bucket element. As the cyclic behaviour of the tripod foundation involves complicated mechanism, it is beneficial to investigate this behaviour based on the vertical up and down movement of each bucket element. This study, therefore, has investigated the cyclic behavior of the tripod suction bucket foundation system based on the understanding the up and down responses of each single bucket element. To achieve the purpose, this study focuses on the following topics. Firstly, the cyclic behaviors of the single bucket foundation were evaluated by applying the vertical loads via the geotechnical centrifuge. A series of centrifuge model tests were performed by applying various loading amplitudes and directions. The results were analyzed by focusing on the variance in vertical stiffness and permanent displacement of the bucket. Secondly, the behaviors of the tripod foundation under cyclic loadings were investigated via the centrifuge model tests. The moment– rotation response, the cyclic stiffness of the tripod foundation system, and the permanent displacement were assessed for various cyclic loading directions and levels. Thirdly, the method of simplifying the rotational stiffness of the tripod foundation based on the vertical stiffness of the single foundation was introduced and validated. The rotational behavior of tripod foundation structure and the push - pull behavior of the single bucket foundation were analyzed quantitatively. Lastly, the dynamic and cyclic responses of the offshore wind turbine installed on actual site were evaluated using the centrifuge model tests. To simulate the behavior of the demonstration turbine installed at the site, the ground condition

site environmental loads

and prototype OWT structure were modelled by scaled down modelling. The natural frequency of the structure considering SFSI and the long-term behavior of the structure were analyzed.

Offshore wind energy has been growing up as a promising renewable energy source. To be competitive in offshore wind energy production, safe and economical foundation design is crucial because foundation construction cost comprises a large portion of the total cost of offshore wind turbines. Monopiles are frequently used as a conventional foundation for the offshore wind turbine. In recent years, suction bucket foundations have been considered as an alternative to conventional foundations owing to their unique features suitable for offshore construction environments, such as easy installation, no heavy equipment for subgrade penetration, and no hammering noise from driving piles. Especially, tripod suction bucket foundation has been considered as an option to support offshore wind turbines due to having a potential to increase the bearing capacity and overturning resistance of the foundation. In an offshore environment, a wind turbine foundation should be designed considering cyclic loading, which can lead to the permanent deformation of structures, tilting, and the degradation of soil stiffness. However, there are several uncertainties in the prediction of the cyclic behaviour of the tripod foundation. Because the cyclic HVM (i.e., horizontal