The rapidly expanding global offshore wind market and the declining number of high-quality, shallow-water sites are driving the industry to seek innovative and cost-effective foundation solutions for deep-water applications. This shift to deeper waters introduces new challenges to traditional foundation types such as conventional monopiles (CMP) and jacket structures, which face technical and economic challenges due to escalating material demands and increased installation complexity. Although monopile foundations have historically facilitated cost reduction because of their ease of manufacturability, transportability, and installability, they have been limited to relatively shallow water depths. Given that jacket-type and floating support structures remain costly, the possibility of scaling up or adapting monopile designs for water depths beyond 40–60 meters merits investigation. A promising solution is the guyed monopile (GMP) concept, which integrates a mooring system with a CMP to provide additional lateral restraint. This integration reduces the required structural dimensions and steel usage under deep-water loading conditions, thereby extending the feasibility of monopile structures into deeper waters.

The goal of this study is to investigate the use of a guyed monopile as a more favourable, cost-effective alternative to CMPs and jackets for deep-water applications with large wind turbine generators (rated at 15 MW) in water depths ranging from 60 to 100 meters, and to determine the specific water depth range at which this concept proves both technically and economically advantageous over the CMP and jackets. To ensure broad applicability, the Hollandse Kust West wind farm site in the North Sea is selected as the reference location.

Conceptual designs for the GMP, CMP, and jackets were developed and optimized for case study water depths of 60, 80, and 100 meters at the representative North Sea site. Using in-house Python tools and a comprehensive dataset covering environmental conditions, soil properties, and wind turbine characteristics, key design parameters such as diameter, wall thickness, and embedment depth were optimized. The designs are checked against and found to be in compliance with both ultimate limit state (ULS) and fatigue limit state (FLS) criteria, revealing that while CMP designs suffer from a steep increase in material requirements with depth, the GMP benefits from its fixed-cost mooring system and exhibits a more gradual cost evolution. In fact, comparative analysis indicates that the GMP reduces capital expenditures by roughly 20–30\% relative to CMPs and by around 10–25\% relative to jackets across the examined water depth range.

Lifecycle cost assessments are conducted using constructed cost models that incorporate fabrication, installation, maintenance, and decommissioning costs. The study shows that lifecycle costs for CMPs escalate dramatically with increasing depth due to their steel-intensive designs, whereas the GMP maintains a lower overall cost beyond approximately 70 to 75 meters. Moreover, as water depths approach 85 to 90 meters, it is found that jackets become competitive by exhibiting more moderated cost escalation. A sensitivity analysis conducted in this study further reveals that member thickness and diameter are the most critical cost drivers across all foundation types in this study.

Overall, the findings of this research establish the GMP as a technically robust and economically attractive foundation solution for deep-water offshore wind applications. Based on the assumptions and restrictions of this study, the GMP offers a promising alternative that successfully meets the load-bearing and stiffness requirements for large wind turbines while significantly reducing steel consumption, thereby supporting the extension of monopile use to water depths of up to 90 meters.
Future research should build upon these insights by conducting a more in-depth investigation of the GMP design, validating its performance through advanced dynamic analyses and targeted field trials. Expanding the applicability range of the GMP across varied offshore environments will further strengthen its appeal and unlock its full potential as a foundation concept for deep-water WTGs.