By 2004, more than 485 suction anchors had been installed at over 50 different sites (Andersen et al., 2005). Most of these anchors are in clay, but some are in sand or layered soil. Examples of skirted foundations in sand are the offshore platforms at the Draupner E and Sleipner T sites in the North Sea (Tjelta, 1995). Recently there has arisen a salient trend in the construction of modern wind turbines with a slender design and more than 100 m in tower height; the natural frequencies of these rather novel structures are close to 0.2 Hz. Fig. 1 compares the average water depths for wind farms that are currently in the design phase. The transition to deeper water increases the span between the turbine superstructure and the seabed. Coupled with greater environmental loading from the highermagnitude wind and waves, the move to deeper water increases the moments applied to the foundations. While monopiles are attractive solutions for developers and designers alike, the increased water depth requires larger diameters with stiffer cross-sections. The monopiles used to date consists of a stiff pile of diameter of 4–۶ m and embedment depths ranging from 20 to 30 m. In 2008, Ibsen reported the performance of a bucket foundation installed in Frederikshavn, Denmark as an attractive alternative foundation which can be used to increase the moment capacity (Ibsen, 2008). The bucket foundation, also referred to as “suction caisson”, is a large cylindrical monopod foundation, typically made of steel (Fig. 2) which has the potential to be a cost effective option in certain soil conditions. A bucket foundation typically requires less steel compared to a monopile, but fabrication costs are slightly higher due to the complicated lid structure (Table 1). However, the total cost, steel and fabrication, of a bucket foundation is likely to be less than of a monopile. Typical loading conditions for an offshore bucket foundation are illustrated schematically in Table 2. The loads are shown to be acting at the interface level between the foundation and the turbine shaft. An axial load of approximately 264 t act at this point. This foundation is an upside-down bucket made of steel with diameter D, skirt length d and skirt wall thickness t. A standard realscale foundation for a 5 MW wind turbine has D = 12–۱۸ m, t ≈ ۳۰ mm and embedment ratio ranging between 0.5 and 1. Motion of a bucket foundation will induce forces transmitted through the foundation-soil contact elements into the underlying deformable ground, which produce cyclic strains in terms of displacements and rotation of the foundation. Under a moderate to high amplitude of cyclic loading, most soils change stiffness and damping. In order to study the long-term performance and the uncertainties related to the dynamic response of these structures, the soil stiffness due to these cyclic strains must be taken into consideration. The current codes of practice (API, ISO and DNV) for the design of offshore foundations provide limited guidance for predicting changes in the foundation stiffness and the resulting changes in damping, which are important design drivers for serviceability limit state (SLS) and fatigue limit state (FLS) requirements.