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

TitleComparison of Cyclic P-Y Methods for Offshore Wind Turbine Monopiles Subjected to Extreme Storm Loading
Publication TypeConference Proceedings
Year of Publication2015
AuthorsCarswell W, Fontana CM, Arwade SR, Degroot DJ, Myers AT
Conference NameASME 2015 34th International Conference on Ocean, Offshore and Arctic Engineering
VolumeVolume 9: Ocean Renewable Energy
PaginationV009T09A060, 9 pages
Date Published05/2015
Conference LocationSt. John’s, Newfoundland, Canada
ISBN Number978-0-7918-5657-4
Other NumbersPaper No. OMAE2015-41312
KeywordsOffshore Wind Turbines, Storms

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 thep-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 cyclicp-y analysis. Deformed pile shapes as a result of the design load are compared for each of the cyclic p-ymethods as well as pile head displacement and rotation and degradation of soil-pile resistance with increasing number of cycles.