Characterization of model uncertainty of the p-y method for reliability-based design of offshore wind turbines under the serviceability limit state

Abstract

Recent studies show that the API p-y method may underestimate the pile-soil initial stiffness of offshore wind turbine (OWT) monopiles in clay while overestimate that in sand. To facilitate the reliability-based design of OWT monopiles with the API p-y design method, a two-stage evaluation method of the model bias factor for the p-y method (εpy) of monopiles under the serviceability limit state (SLS) was presented. The model bias factor εpy is derived by comparing field test data with high-fidelity 3D finite element model (FEM) results, and the p-y method predictions, addressing the issues of limited field test data for OWT monopiles. Results show that the mean of the model bias factor εpy depends on the characteristics of piles, the soil properties, and the composition of the layered ground. The systematic dependency is expressed by a trend function and the residual model bias factor follows a lognormal distribution with a mean of 1.01 and a COV of 0.14. The significance of the model uncertainty is illustrated using a case study. The results of the field example show that the design in clay sites may be considerably conservative and may be less conservative in sand sites if ignoring the model uncertainty. The result of this work can provide a basis for promoting reliability-based design for OWT monopiles.