Suction caisson foundations for offshore wind turbines subjected to wave and earthquake loading: effect of soil–foundation interface

The response of wind turbines founded on suction caissons and subjected to lateral monotonic, cyclic and earthquake loading is studied with due consideration of the role of soil–sidewall adhesion, using non-linear three-dimensional finite-element analyses. In the case of monotonic and slow cyclic lateral loading it is shown that imperfect interface bonding could reduce the moment capacity and may lead to foundation detachment or even uplifting in the case of shallowly embedded caissons. A preliminary comparison of two caisson alternatives has shown that increasing the caisson diameter while maintaining the embedment ratio is more efficient in terms of material resources than increasing the skirt length while keeping the diameter constant. The second part of the study evaluates the response of a soil–foundation–wind turbine interacting system subjected to earthquake shaking. Contrary to an often prevailing impression that seismic effects are insignificant, apparently originating from evaluating the seismic behaviour on the basis of spectral characteristics, it is illustrated that the system kinematics may prove crucial for the response of large wind turbines subjected to simultaneous environmental and seismic loads. Although not instantly catastrophic, the accumulation of foundation rotation could lead to the turbine reaching serviceability limits early during its operation.