Investigation of foundation damping for monopile-supported offshore wind turbines

Abstract

Modern offshore wind turbines (OWTs) are characterized by their large, slender structures, with natural frequencies often lying in close proximity to the frequencies of environmental and mechanical loads, thereby significantly increasing the likelihood of resonance. As a result, it becomes crucial to evaluate the contributions of energy dissipation from various sources, as the overall damping ratio plays a key role in governing the dynamic amplification of the OWT response during resonance. One source of damping that is often overlooked is the contribution of the foundation to attenuate the dynamic response of OWTs through the nonlinear hysteretic behaviour of the surrounding soil. Therefore, the aim of this study is to evaluate the contribution of foundation damping for monopile-supported OWTs through finite element analyses adopting soil constitutive models calibrated with laboratory triaxial tests. To provide a comprehensive understanding of foundation damping, finite element simulations are conducted to investigate its variation with different monopile dimensions and soil relative densities. The study offers new insights into foundation damping, offering a deeper understanding of soil-structure interaction for OWTs and enabling the design of more efficient monopiles that extend the fatigue life-time of the support structure. To reasonably represent soil behaviour in the finite element analyses, the parameters of the HSsmall and SANISAND-MS constitutive models were calibrated using the results of an extensive campaign of monotonic and cyclic triaxial tests conducted on Blessington sand. The experiments were performed under varying mean effective stresses and relative densities to examine their influence on both monotonic and dynamic soil behaviour. In addition to informing the calibration of the constitutive models, the results of the cyclic triaxial tests were curve-fitted with hyperbolic functions, allowing the use of these functions to derive representative damping and shear modulus reduction curves for cohesionless materials when laboratory data is unavailable in the early stages of design. A comprehensive set of finite element analyses is conducted to assess the variation of foundation damping for monopiles of different dimensions installed in various soil relative densities. Dynamic free vibration simulations are performed to evaluate the foundation damping ratio, while cyclic (pseudo-dynamic) simulations are carried out to determine the foundation damping factors at the mudline level and along the monopile depth. The importance of the damping factors determined in this study is that they can be used in integrated wind turbine numerical analyses, enabling foundation damping effects to be incorporated.