Feng Yu

The monopile foundation is a popular foundation type for offshore wind turbines; due to the harsh marine environment, there are lateral loads applied on the monopile foundation from winds and currents, and scouring also often occurs around the pile, reducing the bearing capacity and impacting the normal operation of offshore wind turbines. A series of 1 g model tests is conducted to investigate the lateral load response and scouring response of the monopile in sand. Based on the experimental results, the characteristics of the pile’s load-displacement curves, bending moments, and p-y curves under the effects of scour were analyzed. Particle Image Velocimetry technology was adopted to analyze the deformation development rules of soil particles around the pile. It is found that under the same lateral load, the maximum bending moment of the pile increases and the bearing capacity is reduced as the scour depth increases, the scour width increases, or the scour slope decreases. The effects of scour depth, slope, and width on pile bearing stability decrease successively. Soil displacements and strains in the passive zone in front of the pile develop gradually in both radial and vertical directions.

The wind turbine has become an efficient way for clean energy. For offshore wind turbine in a sandy area, the monopile is a widely used foundation type under proper water depths. During the life time, monopiles are subjected to millions of cyclic loads from winds and waves and scouring around the pile is a common phenomenon, both of which can influence the operation safety of the offshore turbine. To study the coupled effects of cyclic loading and scouring on the horizontally loaded pile, a series of horizontal monotonic and cyclic loading tests on model piles were carried out in sandy soil. The influence of pile diameter, cyclic loading amplitude, and scour depth on the long-term residual accumulated displacements of a monopile is investigated. The test results show that the deformation mode of pile and the dilatancy of sand result in different static responses of the pile. A quantised work is presented to estimate the accumulated residual displacements of the pile, in which the effects of the three factors can be expressed in a united expression. Since the lateral capacity is a crucial parameter for the normalisation in regression analysis, the confirmation of a lateral capacity should be related to the corresponding concern in practical projects.

Monopiles are widely used to support the upper structures of wind turbines in offshore engineering. Due to harsh environmental conditions, they are subjected to long-term lateral cyclic loading from waves and winds during the design life. A robust beam on the Winkler foundation model is presented to analyze the responses of monopile foundation under long-term lateral cyclic load, incorporating cyclic stiffness evolution of sand based on the results of drained cyclic triaxial tests. The reasonability of the model is proved by comparison with a published centrifuge model test. Furthermore, a parametric analysis is performed to study the effects of pile diameter, pile eccentricity, embedment depth and cyclic load amplitude to the accumulation of deformation at the pile head during the long-term lateral cyclic load, based on which an empirical expression for estimating the accumulated displacements of a long-term lateral loaded monopile under cyclic loads is given.