An overview of offshore wind turbine (OWT) foundations is presented, focusing primarily on the monopile foundation. The uncertainty in offshore soil conditions as well as random wind and wave loading is currently treated with a deterministic design procedure, though some standards allow engineers to use a probability-based approach. Laterally loaded monopile foundations are typically designed using the American Petroleum Institute p-y method, which is problematic for large OWT pile diameters. Probabilistic methods are used to examine the reliability of OWT pile foundations under serviceability limit states using Euler-Bernoulli beam elements in a two-dimensional pile-spring model, non-linear with respect to the soil springs. The effects of soil property variation, pile design parameters, loading and large diameters on OWT pile reliability are presented.
a b s t r a c tThe contribution of foundation damping to offshore wind turbines (OWTs) is not well known, though researchers have back-calculated foundation damping from "rotor-stop" tests after estimating aerodynamic, hydrodynamic, and structural damping with numerical models. Because design guidelines do not currently recommend methods for determining foundation damping, it is typically neglected. This paper investigates the significance of foundation damping on monopile-supported OWTs subjected to extreme storm loading using a linear elastic two-dimensional finite element model. The effect of foundation damping primarily on the first natural frequency of the OWT was considered as OWT behavior is dominated by the first mode under storm loading. A simplified foundation model based on the soil-pile mudline stiffness matrix was used to represent the monopile, hydrodynamic effects were modeled via added hydrodynamic mass, and 1.00% Rayleigh structural damping was assumed. Hysteretic energy loss in the foundation was converted into a viscous, rotational dashpot at the mudline to represent foundation damping. Using the logarithmic decrement method on a finite element free vibration time history, 0.17%-0.28% of critical damping was attributed to foundation damping. Stochastic time history analysis of extreme storm conditions indicated that mudline OWT foundation damping decreases the maximum and standard deviation of mudline moment by 7e9%.
9Offshore wind turbine (OWTs) monopile foundations are subjected to cyclic loading from wind, waves, 10 and operational loads from rotating blades. Lateral monopile capacity can be significantly affected by 11 cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic 12 loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. 13 extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil 14 modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts 15 affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency 16 degradation and accumulated permanent rotation at the mudline, respectively. This paper introduced 17 novel combinations of existing p-y curve design methods and compared the impact of short-term cyclic 18 loading on monopiles in soft, medium, and stiff clay. The results of this paper indicate that short-term 19 cyclic loading from extreme storm conditions are unlikely to significantly affect natural frequency and 20 permanent accumulated rotation for OWT monopiles in stiff clays, but monopiles in soft clay may 21 experience significant degradation. Further consideration is required for medium clays, as load magnitude 22 played a strong role in both natural frequency and permanent rotation estimation. 23 Keywords 24 offshore wind turbines; monopiles; p-y curves; cyclic loading 25 Nomenclature 26 DE Delaware 27
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