Accuracy evaluation of API P-Y model for Offshore Piles in Clays

Piles are widely used as foundation structures supporting offshore wind turbines, with the p-y curve method commonly recommended for analysis of pile-soil interactions by current mainstream design specifications. Among these methods, the p-y curve model developed by the American Petroleum Institute (API) has gained extensive applications and been the key reference for development of analogous models in numerous countries and regions. Nevertheless, the accuracy of the API-specified p-y curve model remains to be quantitatively evaluated. This study first collected 402 sets of pile p-y curves from multiple offshore wind turbine projects in clays to establish a field-measured p-y curve database. The database was then used to statistically evaluate the prediction accuracy of the p-y curve model for clays recommended by the API specification. The model accuracy is represented by a model factor defined as the ratio of measured to predicted values of soil resistance around the pile. Results showed that the API p-y model underestimates the true soil resistance by about 50 % on average. The dispersions in prediction accuracy are high. Calibration terms considering statistic and physical information are introduced to calibrate the current API p-y model. The calibrated API model exhibits unbiased average prediction accuracy and have a significant reduction in dispersion of prediction accuracy. Finally, Gaussian functions were employed to fit the probability density functions of the API p-y curve model factors before and after calibration, respectively. The research outcomes of this study is of help towards reliability-based design of offshore piles.