Centrifuge model tests and effective stress analyses of offshore wind turbine systems with a suction bucket foundation subject to seismic load

Abstract

The seismic behavior of a suction bucket foundation for offshore wind turbine systems is studied using centrifuge model tests with a scaling factor of 1/100 and an effective stress analysis based on the strain space multiple mechanism model implemented in the FLIP computer code (Finite element analysis program of Liquefaction Process). The primary dimensional parameters of the prototype scale suction bucket foundations selected for this study are as follows: skirt length L = 8 m, diameter D = 19 m, L = 9 m and D = 12 m. Placed on firmly compacted saturated sand, suction bucket foundations supporting an idealized wind turbine tower 100 m in height are subject to a recorded earthquake motion with a peak acceleration of 0.25g.

Results of the model tests and the analyses indicate that the constitutive model used in this study is capable of capturing the essential features of the seismic behavior of offshore wind turbine systems supported by suction bucket foundation. In particular, this model is capable of evaluating the confining effect of a suction bucket on the increase in excess pore water pressure inside the bucket in response to the vertical confining pressure applied from the tower and shaft structure of the wind turbine system. This model is also capable of evaluating the inertia effects of the tower and shaft structure above the foundation affecting the driving mudline moment and shear, which leads to the deformation of the suction bucket in terms of rotation. The residual inclination of the tower is less than 0.001 rad, which satisfies the design criteria, and has a rotation angle of 0.005 rad.

The strain space multiple mechanism model is applied to a general combination of static and cyclic loads in storm conditions. The computed results are consistent with those of a proposed cyclic load response diagram based on 1 g model tests at partially drained conditions in an earlier study by Nielsen et al. (2017), suggesting the applicability of this model to general load conditions is reasonable.