Modelling Multi-Directional Cyclic Loading on Foundations for Offshore Wind Turbines

Abstract

The offshore wind energy sector is growing rapidly as a means to achieving target net zero GHG (Greenhouse gas) emission by 2050. This technology is been expanded to the offshore wind turbines (OWTs) both bottom-fixed and floating foundations. The foundations of such structures have to be optimized geometrically such that they are robust and resilient against environmental loads from wind, waves and currents which are usually cyclic, multidirectional and complex in nature. Accurate modelling of the foundation behavior for bottom-fixed and floating OWTs is critical in predicting the global response of the system. This aspect of the integrated design of the entire OWT system is not very much understood in design software programs, typically resulting in empirical modelling to represent the constitutive behavior of the foundation or, and in extreme cases, models that assume the foundation is infinitely rigid (fixed) in many structural and geotechnical engineering applications. The foundation modeling for OWTs requires considering accumulated rotational deformations due to combined cyclic and sustained loading, which affects the foundation stiffness. Additionally, it needs to account for the coupling of loads from different directions. Such effects have been rarely accounted for comprehensively with simple numerical models. But, there exists certain specialized models like the Houlsby-Abadie Ratcheting Model (HARM) strongly rooted in the kinematic hardening principles within the hyperplasticity (thermo-mechanical) framework, which enables capturing of the accumulated deformations over many cycles (typically millions) of cyclic loadings. Then, there is the REDWIN model which is an acronym referring to “REDucing cost in offshore WINd by integrated structural and geotechnical design”, and it account for the multidirectional load coupling. This research focuses on programming, improving, and developing a novel constitutive model called CLAP an acronym for “Cyclic Loading & Analysis of Piles”. The output of this work can be further enhanced and used for various applications and case studies on complex multidirectional cyclic loading in the offshore industry.