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%.

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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

The prediction of ultimate and fatigue demands for the design of offshore wind turbines (OWTs) requires accurate simulation of the dynamic response of OWTs subject to time-varying wind and wave loads. The magnitude of damping in an OWT system significantly influences the dynamic response, however, some sources of damping, such as foundation damping, are not explicitly considered in design guidelines and may increase damping significantly compared to commonly assumed values in design. Experimental and analytical studies have estimated the magnitude of foundation damping to be between 0.17% and 1.5% of critical, and this paper investigates how increased damping within this range affects load maxima and fatigue damage for a hypothetical 5MW OWT subjected to a variety of wind, wave, and operational conditions. The paper shows that increased damping effects the greatest percentage reduction of ultimate moment demands and fatigue damage when the OWT rotor is parked and feathered. In such cases, the aerodynamic damping is relatively low, allowing for additional damping from the foundation to account for a relatively larger proportion of the total system damping. Incorporating foundation damping in design guidelines may lead to more efficient structures, which is a crucial factor in overcoming the high cost barrier associated with offshore wind development.

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Fatigue is often a design driver for large (e.g. 5–10 MW) offshore wind turbines (OWTs), necessitating a thorough examination of damping sources: aerodynamic, hydrodynamic, structural, and soil. Of these sources, soil damping has been least considered by researchers with respect to OWTs. Aeroelastic programs, such as the National Renewable Energy Laboratory (NREL) code FAST, are typically used for time history analysis of aerodynamic and hydrodynamic loads experienced by OWTs. To take into account foundation flexibility while minimizing computational expense, reduced-order foundation models such as the mudline stiffness matrix are often used. Mudline stiffness and damping matrices are derived here for the NREL 5MW reference turbine. By recompiling FAST with mudline stiffness and damping matrices, the contribution of soil damping to OWT dynamic behavior is then quantified by comparing time history analysis results including and excluding soil damping.

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