The exploration of bottom-founded offshore wind turbine development, focusing on monopiles, indicates that while the floating offshore wind sector is emerging to exploit deeper sites, there is a continued rationale for advancing XXL monopiles in the realm of bottom-founded offshore wind. Cost-effectiveness stands out as a pivotal factor among several compelling arguments.
This study aims to improve the economic viability of XXL monopiles by developing a more accurate methodology for calculating wave loads. The introduction of time-varying hydrodynamic coefficients is central to this effort. Building on an extensive literature review, a novel time-varying coefficient method was proposed. This method accounts for subperiodic fluctuations in wave force coefficients by introducing dependencies on the Keulegan-Carpenter and Reynolds numbers, which are determined using zero-crossing periods. This approach marks a significant methodological shift from previous attempts.
The newly developed method was implemented in a MATLAB environment and is compatible with both first and second-order wave theory, as well as the Morison and Rainey load models. Efforts were undertaken to replicate the results of previous experiments. The model successfully reproduced both regular and irregular waves, with particularly accurate results for the least steep regular waves. However, for steeper waves, the second-order wave theory did not fully capture all characteristics, leading to less accurate reproductions.
A simplified fatigue assessment was carried out using MonoPoly, an in-house software package developed by Sea and Land Project Engineering (SLPE). The North Sea Wind Farm, a reference project by SLPE, was used for site characterization and structural configuration.
The fatigue assessment evaluated various test cases to examine the impact of different analysis approaches in the time-varying coefficient method model. Overall, results showed that the time-varying coefficient method decreased the maximum Damage Equivalent Moment (DEM) values by 0.2 to 0.7 percent, depending on the analysis approach. The mean DEM values decreased by 0.2 to 0.5 percent. These findings apply to a more advanced constant coefficient method used in the industry, compared to the one recommended by ISO. A side study suggested that the decrease in DEM values would be even greater when comparing this new method to the ISO-recommended method.
For thin-walled cylinders like monopiles, the relationship between moment and thickness is essentially linear. Therefore, a 0.5 percent reduction in mean DEM allows for a 0.5 percent reduction in monopile thickness while maintaining stress levels. This reduction in material thickness has the potential to save several million euros for wind farms, demonstrating the significant economic benefits of the newly developed time-varying coefficient method.