Monopile fatigue in geographical conditions and the effect of adding guyed lines

Abstract

The offshore wind industry is rapidly expanding, featuring larger turbines in deeper waters and new geographical locations, leading to increased uncertainties. These developments pose significant design challenges for maintaining the simple and structural robust monopile structures, which are currently the most popular foundations for offshore wind turbines.
This study aims to investigate major contributors to fatigue on XXL monopiles supporting 15 MW and 22 MW turbines based on metocean conditions across different geographical locations. Furthermore, the impact of guyed monopiles has been analysed based on numerous water depths and soil parameters. These aspects have been largely unexplored in the existing literature.
The research uses a frequency domain monopile fatigue estimation method that integrates aerodynamic effects with hydrodynamic excitations. The method assumes a uniform wind profile and white noise wave spectrum to compute the stress response spectrum. By applying a linear correlation between the stress response spectrum and hydrodynamic excitation, the stress is determined over a wave scatter diagram, considering the joint probability of wind-wave conditions. The approach uses time series loads, computed by the aero-hydro-servo-elastic load analysis tool OpenFAST. Additionally, a dimension scaling reduction is used to reduce the mass of the monopile when incorporating the guyed lines.
The findings reveal that fatigue is dominated by scenarios lacking aerodynamic damping, such as wind-wave misalignment and idling, where directional spreading of metocean conditions has lower influence. Furthermore, fatigue damage is significantly affected by the positioning of the system’s natural frequency relative to the peak wave period. A noted limitation to the model is the exclusion of turbulent wind effects.
Regarding the guyed monopile analysis, the dimension reduction strategy shows a significant mass reduction in deeper waters. The stiffness of the system is determined by the tendon parameters, where the envelope of the natural frequency is larger in clay conditions than for sand conditions, and it increases for increasing water depth. Using a feasible tendon set-up shows higher fatigue damages at the critical location when compared with the conventional monopile fatigue damage. However, lower fatigue damages are found at other locations along the monopile length. Additionally, it is concluded that using stiff tendons results in a high risk of snap loads especially when creep of the tendon lines is considered. The results show potential for guyed monopile systems especially in deeper waters, reducing the mass, whilst maintaining similar fatigue damages as conventional monopiles. These results encourage the need for extra research on the topic of guyed monopile systems.