Cyclic behavior of unit bucket for tripod foundation system supporting offshore wind turbine via model tests

Offshore wind energy has been growing up as a promising renewable energy source. Recently, tripod suction bucket foundation is rapidly expanding as a foundation system supporting the offshore wind turbine. In offshore environment, wind turbine foundation structures should be designed considering cyclic loading which can lead to permanent deformation of structure, tilting problem, and overall degradation of soil stiffness. However, it is technically difficult to predict the cyclic behavior of the tripod accurately because the cyclic behaviors of the tripod bucket can be inferred from vertical pullout and compression behaviors of each single bucket elements. In this paper, a series of model tests was performed by applying cyclic vertical compression and extension loadings to a single bucket element that is one element of the tripod foundation. Loading directions, level, and rate were controlled for investigating of cyclic behavior of tripod foundation. On the basis of testing results, the permanent deformation and cyclic stiffness response of tripod suction caisson were discussed. Based on the test results, it was confirmed that the cyclic behavior of the single bucket is affected by the load level and rate. In addition, the behavior showed quite different trends with the loading directions: compression, pullout, and two-way.