Load bearing mechanism and simplified design method of hybrid monopile foundation for offshore wind turbines

Abstract

The hybrid monopile foundation for offshore wind turbines demonstrates an enhancement of bearing capacities. This innovative foundation is composed of an embedded pile and a gravity footing (namely wheel). In this study, centrifuge tests are conducted to investigate the lateral capacity of hybrid monopile foundation in cohesionless soil. Pile-wheel-soil interactions are studied through validated finite element models. Load transfer mechanisms between the wheel and the pile and failure modes of each component in a hybrid monopile foundation are demonstrated. Functions of wheel are categorized into a friction resistance and additional rotational constraints at the pile head, which are quantitively assessed in terms of the vertical earth pressure generated beneath the wheel. A reduction factor is suggested to determine the effective contact area. The addition of the wheel effectively reduces bending moments at the embedded pile, and the maximum earth pressure of the equivalent monopile is similar to an original monopile. A simplified calculation method is proposed to estimate the lateral capacity of hybrid monopile foundation in ultimate condition. Previously reported tests were used to calibrate the analytical method, demonstrating good consistency. This method is applicable for performing an initial design for hybrid monopile foundation and enhancing calculation accuracy.