Closed-form stiffnesses of multi-bucket foundations for OWT including group effect correction factors

Offshore Wind Turbine (OWT) support structures need to satisfy different Limit States (LS) such as Ultimate LS (ULS), Serviceability LS, Fatigue LS and Accidental LS. Furthermore, depending on the turbine rated power and the chosen design (all current designs are soft-stiff), target natural frequency requirements must also be met. Most of these calculations require the knowledge of the stiffnesses of the foundation which, especially in the case of large turbines in intermediate waters (30 to 60 meters), might need to be configured using multiple foundation elements. For this reason, this paper studies, for a homogeneous elastic halfspace, the static stiffnesses of groups of polygonally arranged non-slender suction bucket foundations in soft soils modeled as rigid solid embedded foundations. A set of formulas for correcting the stiffnesses obtained from isolated foundation formulation are proposed. It is shown through the study of several multi-megawatt OWTs that, as expected, group effects becomes more relevant as spacing decreases. Also, group effects are sensitive mainly to shear modulus of soil, foundation shape ratio and diameter, and the number of foundations. The results obtained from the soil-structure system show that ignoring group effects may add significant errors to the estimation of OWT fundamental frequencies and leads to either overestimating or underestimating it by 5%. This highlights the importance of adequately modeling the interaction between elements of closely-separated multi-bucket foundations in soft soils, when current guidelines specify the target fundamental frequency to be at least 10% away from operational 1P and blade passing frequencies (2P/3P frequencies)