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 py 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. INTRODUCTION Of the nearly two thousand offshore wind turbines (OWTs) installed globally, approximately 75% are supported by monopile foundations [1]. The lateral load demands on OWT monopiles from wind and wave loading is much larger than the axial demand from self-weight; as such, the calculation of lateral load capacity plays a significant role in design, particularly with respect to the cyclic wind and wave loads. OWT design guidelines (e.g. [2]) recommend the p-y curve method for analyzing laterally loaded piles, where soil-pile resistance is modelled by a series of nonlinear springs along the length of the pile per Winkler spring theory. The recommended p-y curves for cyclic load conditions were experimentally determined for small diameter piles assuming a wave-dominated load pattern for offshore platforms [3,4]. The experimental work consisted of displacementcontrolled cyclic loading on piles at constant amplitude, with limited information regarding the frequency of the loading or hysteretic behavior of the p-y cycles. Moreover, the cyclic loading was applied slowly such that inertial effects were negligible [5]. The resulting cyclic p-y curves were used to represent a lower-bound approximation of cyclic soil-pile response, assuming repeated loading of the design wave.