Natural frequency degradation and permanent accumulated rotation for offshore wind turbine monopiles in clay

Abstract

Offshore wind turbine (OWT) monopile foundations are subjected to cyclic loading from wind, waves, and operational loads from rotating blades. Lateral monopile capacity can be significantly affected by cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency degradation and accumulated permanent rotation at the mudline, respectively. This paper introduced novel combinations of existing p-y curve design methods and compared the impact of short-term cyclic loading on monopiles in soft, medium, and stiff clay. The results of this paper indicate that short-term cyclic loading from extreme storm conditions are unlikely to significantly affect natural frequency and permanent accumulated rotation for OWT monopiles in stiff clays, but monopiles in soft clay may experience significant degradation. Further consideration is required for medium clays, as load magnitude played a strong role in both natural frequency and permanent rotation estimation.