Modal damping ratio is an important parameter in the seismic design of offshore wind turbines. The modal damping ratio of monopile foundations is large, whereas that of jacket foundations is relatively small. In this study, a new offshore wind turbine foundation called Flexible Tripile (FTP) that uses the flexibility of the footing as a seismic isolator is proposed as an alternative to jacket foundation, and its seismic performance is investigated. First, 1/20 scale models are fabricated for shaking table tests. It is found that the FTP exhibits large modal damping and reduces seismic response compared with conventional steel pipe joint models as jackets. Next, the soil damping is set in the numerical model using Kagawa and Kraft formula, and the modal damping of the FTP is evaluated by complex eigenvalue analysis and simplified model analysis. The complex eigenvalue analysis agrees with the experimental results, and the simplified model analysis reveals that the large modal damping of the FTP is due to the rotational deformation at the footing center.

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