Failure mode of monopile foundation for offshore wind turbine in soft clay under complex loads

Abstract

Offshore wind turbines (OWT) subjected to long-term complex marine loads are prone to foundation damages, affecting their regular operation. However, the failure mode of the large-diameter monopile foundation under complex marine loads is not well understood. Hence, failure modes of monopile foundation for OWT in soft-soil seabed under unidirectional and multidirectional loads are investigated. Firstly, the mechanical characteristics of soft soil are tested indoors. Subsequently, the monopile foundation failure modes under unidirectional loads are examined, which indicates capacity failures of monopile foundation are progressive, and two distinct soil failure mechanisms are identified. Then, the failure modes and failure envelopes under multidirectional loads (horizontal load, vertical load, and bending moment）are explored. Results show that the failure envelopes in H–V and M–V load space are oval-shaped, and failure modes in H–V and M–V load space are characterized by wedge failure. More critically, the service performance of the monopile foundation in soft soil in a full lifecycle can be estimated by the location of the actual loads in the failure envelope. Finally, the effect of pile diameter on the bearing capacity of the monopile foundation for OWT under unidirectional and multidirectional loads is discussed.