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.

Offshore wind energy has become a important contributor to global renewable energy efforts, particularly as the industry seeks to expand into deeper waters where traditional fixed-bottom turbines are no longer feasible. In these deeper waters, floating offshore wind turbines (FOWTs) offer a promising solution, allowing for the generation of renewable energy in locations with stronger, more consistent winds. However, with the increasing use of FOWTs comes a set of logistical challenges that need to be addressed to ensure their commercial viability and large-scale deployment. One of these challenges is the large-scale production of floating foundations. One significant challenge to the commercial success of FOWTs is the slow fabrication rate of semi-submersible floaters, which require extensive welding and assembly. In contrast to faster monopile fabrication, these floaters are more complex and take longer to produce. Since installation campaigns are typically limited to favorable weather periods, especially in the summer months, many floaters need to be ready in advance, placing strain on fabrication yards and causing space constraints. Wet storage provides a viable solution by allowing temporary storage of floaters in coastal areas, freeing up space in fabrication yards for continuous production. This research explores the existing literature
on wet storage mooring systems and identifies key design criteria, environmental considerations, and components such as mooring lines, anchors, and auxiliary equipment.
To evaluate mooring system designs, a Multi-Criteria Decision Analysis (MCDA) framework was developed, facilitating structured comparisons and trade-offs between various design objectives. Although the MCDA enables informed decision making, dynamic analysis is essential to obtain deeper insights into system performance. In this study, frequency domain (FD) analysis was employed for rapid evaluations of the behavior of the mooring system. FD analysis proved sufficient for preliminary studies and is an ideal tool for exploring design variations. It efficiently captures first-order loading effects, which are typically dominant in expected wet storage conditions. However, for detailed design stages, to account for nonlinear behaviors such as slow drift and mooring line stiffness, time domain (TD) analysis are recommended to validate final configurations and ensure compliance with design codes.
Dynamic analysis of the mooring systems compared catenary and taut configurations under various environmental conditions. Concept-specific constraints such as maximum fairlead tension, uplift allowance, mooring line angles, and minimum anchor distances were incorporated into each mooring concept. Furthermore, diffraction analyzes were conducted with varying water depths and floater drafts to accurately define floater behavior under different conditions, and a mesh validation was performed to ensure that the mesh was fine enough for this study. Further validation of the full model was done by comparing the calculated Load Response Amplitude Operators (RAOs) and natural frequencies with
the reference turbine to ensure model accuracy. Tidal ranges and line diameters significantly influenced mooring performance, impacting tensions loads, and floater displacements. The results indicated that the catenary system consistently met these constraints and demonstrated resilience across the full tidal range, where a clear trade-off between tensions and displacements can be observed. However, the taut system experienced tension overloads, particularly at high tide, where it often exceeded line breaking strength, with only a single taut configuration yielding valid results. Despite being more spatially efficient, the taut system’s reliability was compromised compared to the catenary system. Several recommendations for future research are identified to further the understanding of mooring systems in wet storage applications. These include conducting TD analyzes to capture non-linear behaviors, exploring alternative mooring configurations such as Honeymooring or pile fields, refining design standards, and evaluating the economic and logistical feasibility of wet storage solutions. Additionally, investigating different storage locations is essential, as this study focused on a single location, and variations in environmental conditions could significantly influence mooring performance.