PERFORMANCE OF MONOPILE FOUNDATION IN MONOTONIC AND CYCLIC LOADING

Abstract

Monopiles are large open-ended steel piles of 3.5 m to 8 m in diameter, often used as foundations for offshore wind turbines. Due to the action of wind, waves, and rotation of the turbine blades, monopile foundations must withstand large lateral loads while meeting, serviceability conditions on pile head rotation, and natural frequency of the monopile. The monopole head rotation must not exceed 0.5 degrees, and the structure's natural frequency must not interact with the frequencies of cyclic lateral loading. To ensure this, engineers must predict the response of monopile foundations for monotonic and cyclic lateral loading. Monopile foundations are currently designed using the 𝑝-𝑦 method, assuming that the soil may be treated as a series of nonlinear Winkler springs. The stiffness of the 𝑝-𝑦 springs is based on empirical relationships from full-scale tests on laterally loaded piles. Research into the lateral response of monopile suggests that these empirical relationships overestimate the stiffness of stiff piles. This may result in interaction between the blade passing frequency and the natural frequency of the turbine, and they make it uneconomical. So in this research work, we predict the response of three monopile piles, 4 MW, 6 MW, and 10 MW, with diameters 5.05 m, 6.457 m, and 8.5 m and embedded lengths of 26 m, 30 m, and 34 m, in sandy soil respectively founded on undrained sand at Tamil Nadu offshore site as per the FOWIND Feasibility study to install a wind farm in Tamil Nadu with the help of FEM software PLAXIS. In this paper, we investigated the various response of monopile in increasing sets of monopile, monotonic loading in the hardening soil model, and cyclic loading hardening soil small strain model. This paper aims to improve the understanding of the behavior of rigid monopiles in cohesion-less soils when subjected to lateral monotonic and cyclic loading.