Three-dimensional modeling of monopiles in sand subjected to lateral loading under static and cyclic conditions

Here, the results of a three-dimensional finite element study of the complex interaction of horizontal and moment loads (HM) on offshore monopiles as failure envelope, are reported. A new design criterion is described which is based on critical length, ultimate limit states, load characteristics and Eigen-frequency to ensure stable behavior of laterally loaded monopiles. Numerical analyses were performed to examine nonlinear interaction of a soil-pile system for 10,000 load cycles. The resulting framework can predict angular rotation due to cyclic loading. According to the loading level and duration of a load, elastic strains accumulate in the vicinity of a pile. Fairly intermediate two-way cyclic loading induced the largest rotations irrespective of the analysis performed (i.e., drained versus partially drained). Based on the regression coefficients of the non dimensional frameworks used, accumulating rocking deformations of a pile at seabed level appear to be dependent on cyclic load ratio, drainage condition, and duration of loading. For safe design, sensitivity of the natural frequency of offshore wind turbine (OWT) at a monopile critical length as well as shorter lengths were also examined. The analytical model proposed here for determining the natural frequency of an OWT considers that soil-structure interaction (SSI) can be represented by monopile head springs characterized by lateral stiffness, KL, rotational stiffness, KR, cross-coupling stiffness, KLR, and parabolic soil stiffness variation with depth.