Wystan Carswell

Sensitivity of the Dynamic Response of Monopile-Supported Offshore Wind Turbines to Structural and Foundation Damping

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.

Reliability analysis of monopile offshore wind turbine support structures

We probe the reliability of monopile support stru ct res designed to support industrial scale turbines along the coastal United States using stochastic m odels for the wind and wave loadings, and representations of the uncertainty associated with soil propert ies. The turbine support structure investigated is that promulgated by the National Renewable Energy Laboratory as t pical of a monopile support structure designed for tens of meters of water depth and a characteristic wind/w ave environment. We investigate the structural reliability using structural finite element models develop ed in MATLAB and a commonly used industry tool, FAST, developed and distributed by NREL. Reliability investigations include the effect of spatial c orrelation of soil properties on reliability with respect t o serviceability and the combined effects of loading and soil property uncertainty on structural performance. We also comment on the interaction between the tower/pile desi gn space and the resulting reliability, allowing us to comment on the effect tower geometry may have on reliability . FAST uses reduced order structural models in the pursuit of c omputational efficiency, and we evaluate the efficacy of these models for structural behaviors which may e nter the nonlinear regime. These investigations include the ability of FAST to capture structural model shap es with large curvature gradients, and the effect of mode shape approximation on time-history dynamic analysis .

Comparison of Cyclic P-Y Methods for Offshore Wind Turbine Monopiles Subjected to Extreme Storm Loading

Approximately 75% of installed offshore wind turbines (OWTs) are supported by monopiles, a foundation whose design is dominated by lateral loading. Monopiles are typically designed using the p-y method which models soil-pile resistance using decoupled, nonlinear elastic Winkler springs. Because cyclic soil behavior is difficult to predict, the cyclic p-y method accounts for cyclic soil-pile interaction using a quasistatic analysis with cyclic p-y curves representing lower-bound soil resistance. This paper compares the Matlock (1970) and Dunnavant & O’Neill (1989) p-y curve methods, and the p-y degradation models from Rajashree & Sundaravadivelu (1996) and Dunnavant & O’Neill (1989) for a 6 m diameter monopile in stiff clay subjected to storm loading. Because the Matlock (1970) cyclic p-y curves are independent of the number of load cycles, the static p-y curves were used in conjunction with the Rajashree & Sundaravadivelu (1996) p-y degradation method in order to take number of cycles into account. All of the p-y methods were developed for small diameter piles, therefore it should be noted that the extrapolation of these methods for large diameter OWT monopiles may not be physically accurate; however, the Matlock (1970) curves are still the curves predominantly recommended in OWT design guidelines. The National Renewable Energy Laboratory wind turbine analysis program FAST was used to produce mudline design loads representative of extreme storm loading. These design loads were used as the load input to cyclic p-y analysis. Deformed pile shapes as a result of the design load are compared for each of the cyclic p-y methods as well as pile head displacement and rotation and degradation of soil-pile resistance with increasing number of cycles.Copyright

Dynamic Mudline Damping for Offshore Wind Turbine Monopiles

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.Copyright