The worldwide potential for offshore wind energy is estimated at 71,000 GW, a value substantially larger than the current global electricity consumption, and it is generally seen as a critical technology for achieving environmentally and politically driven renewable energy targets. Despite this potential, only a minority of the offshore wind resource is located in shallower waters suited to fixed offshore turbines; the rest is only suitable for floating wind turbines. The Spar Floating Offshore Wind Turbine (spar FOWT) is the most common prototype the offshore industry deploys. Commercial projects based on spar FOWT include Hywind Scotland and Hywind Tampen, operating successfully in Europe. However, spar FOWT is only applied to water depths of more than 150 meters which restricts its further development worldwide. This paper proposes an innovative concept design scheme of a truncated spar FOWT with side columns to overcome the water depth limitation of the classic spar. The new design recovers the lost restoring moment of the truncated spar FOWT by implementing three columns around the Spar tower. Therefore, the new design scheme could be deployed at a sea area of 80 to 100 meters deep. Moreover, the paper develops and implements an optimization framework for designing new FOWT configurations, regarding stability, and investigates the global response of the new design schemes.


In recent decades, clean energy, especially wind energy, has been considered as a crucial solution to minimize climate change. The world electricity system has seen a sharp rise in wind power penetration; between 2009 and 2019, wind power installed capacity grew from 159 to 651 GW (Gomes et al., 2022). The majority of the capacity growth was verified in China, which is also expecting a sharp increase in offshore wind power capacity (Energy Research Institute and Natural Development and Commission Reform, 2015). Offshore wind is highlighted for the availability of space in the sea, consistent wind resource and less visual impact (Ren et al., 2021). However, while many offshore wind farm projects with bottom-fixed turbines are currently in development, particularly in near-shore areas, large parts of the world have limited shallow water areas suitable for bottom-fixed wind farms, leaving offshore floating as the only suitable option to harness offshore wind resources. Currently, three FOWTs are implemented in China. All three FOWTs are semi-submersible types implemented at water depths ranging from 30 m to 110 m. In China, the deployment of semi-submersible FOWTs (Roddier et al., 2010) has been prevalent due to most of the country's territorial waters being around 100 meters deep. While other floating structure types, such as spar-type FOWTs, are suitable for deeper waters, they are currently limited to depths of over 150 meters, rendering them infeasible for deployment in China. Nevertheless, spar type FOWT is a mature application, with several successful commercial projects. To reduce costs and expand the deployment potential of spar-type FOWTs in China, this paper proposes a novel hybrid design that combines the advantages of both semi-submersible and spar-type FOWTs. A brief literature review reveals that various studies have been conducted on optimizing FOWTs. However, despite these efforts, several gaps in the current literature persist, indicating a need for further research. In (Zhang et al., 2022) a novel method for optimizing both stability and hydrodynamic performance in a FOWT system is introduced. In the study, the FOWT analyzed consists of a hybridization of OC3 (Jonkman, 2010) and OC4 (Robertson et al., 2014). The study, however, does not verify the reliability of the structure's overall strength. In (Ahn and Shin, 2019), the motion characteristics of the OC3-Hywind spar-buoy type FOWT were investigated. The study's findings are crucial for categorizing the OC3 structure, and the threshold identified can be applied in the current study's analysis. In (Ferri et al., 2022), an optimization procedure was conducted for a semi-submersible FOWT to reduce the dynamic response and to identify its optimal size. Nevertheless, the study does not attend to minimize the probability of failure of the system given specific metocean conditions. In (Jiang et al., 2021), a solution for deploying spar-type FOWTs at 100 meters depths using a stepped short spar concept is proposed. While the stepped short spar concept has a construction-friendly configuration, it also exhibits a higher water displacement than spar-type FOWTs. In order to address the gaps in the analyzed literature, and overcome the water depth limitation to the implementation of classic spar design, in this study it is developed and optimized an innovative concept of a Semi-spar type floating offshore wind turbine. The new model proposes adding three columns around the spar central column. In this study it is also investigated the coherence between the configuration of the innovation design and its stability performance. To achieve this objective, the paper applies a novel method with hypothetical construction constraints to optimize the configuration of the innovation design. Additionally, a preliminary study on the hydrodynamic performance of the innovation design, mainly focusing on heave and pitch motion, is carried out (DNV, 2021). The paper is organized as follows: the section of METHODOLOGY introduces the semi-spar FOWT scheme and the optimization procedure, the section of RESULTS AND DISCUSSION shows and analyses the results. Finally, the main conclusions are presented in CONCLUSION.

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