Simplified estimation of rotational stiffness of tripod foundation for offshore wind turbine under cyclic loadings

As the importance of safe and economical foundation design of offshore wind turbines has increased, suction bucket foundations have become increasingly common. A tripod suction bucket foundation consists of three individual buckets arranged in a triangular shape. The cyclic behaviour and soil-foundation-structure interaction of single suction bucket foundations has been a subject of active research. However, it is difficult to predict the behaviour of tripod bucket foundation because it is structurally complex and requires consideration of cyclic load effects. This study focuses on the behaviour of tripod suction foundations under cyclic loading. The rotational stiffness of the tripod foundation is estimated by combining equations depending on compression, pullout, and rotational stiffness of the single suction bucket with a series of centrifuge model test results. By presenting the theoretical stiffness values, it is verified that the rotational stiffness can be estimated only based on the cross-section of the foundation and the shear modulus of the soil. In addition, it is possible to predict the long-term rotational stiffness of the tripod by applying the vertical stiffness obtained from the cyclic loading test of the single bucket to the simplified method.