Seismic modelling of foundations supporting wind turbines in liquefiable soils

Offshore windfarms are now under development in seismically active regions and many of these locations exhibit soils that may liquefy when subject to design earthquakes. This paper presents a procedure to allow fully site-specific seismic assessments to be conducted for OWF foundations. This starts with the development of a site-specific generalised soil model describing the monotonic and cyclic soil response for any element of soil, in terms of excess pore pressure, accumulated shear strain and post-seismic reconsolidation strain. These soil models are incorporated into 3D finite element models of the soil continuum and are then subjected to a full dynamic analysis under the design seismic motions. An iterative procedure is used to ensure the predicted soil strains from the FEA model match the expected accumulated strains predicted from the generalised soil model, which ensures that the predicted foundation displacements and rotations are consistent with the site-specific soil data. Excess pore pressure fields are also established to pinpoint the risk of liquefaction. The proposed procedure is illustrated through a hypothetical design case with key features arising from these analyses highlighted.